Sample records for ice sheet configurations

  1. Validation of Modelled Ice Dynamics of the Greenland Ice Sheet using Historical Forcing

    NASA Astrophysics Data System (ADS)

    Hoffman, M. J.; Price, S. F.; Howat, I. M.; Bonin, J. A.; Chambers, D. P.; Tezaur, I.; Kennedy, J. H.; Lenaerts, J.; Lipscomb, W. H.; Neumann, T.; Nowicki, S.; Perego, M.; Saba, J. L.; Salinger, A.; Guerber, J. R.

    2015-12-01

    Although ice sheet models are used for sea level rise projections, the degree to which these models have been validated by observations is fairly limited, due in part to the limited duration of the satellite observation era and the long adjustment time scales of ice sheets. Here we describe a validation framework for the Greenland Ice Sheet applied to the Community Ice Sheet Model by forcing the model annually with flux anomalies at the major outlet glaciers (Enderlin et al., 2014, observed from Landsat/ASTER/Operation IceBridge) and surface mass balance (van Angelen et al., 2013, calculated from RACMO2) for the period 1991-2012. The ice sheet model output is compared to ice surface elevation observations from ICESat and ice sheet mass change observations from GRACE. Early results show promise for assessing the performance of different model configurations. Additionally, we explore the effect of ice sheet model resolution on validation skill.

  2. The build-up, configuration, and dynamical sensitivity of the Eurasian ice-sheet complex to Late Weichselian climatic and oceanic forcing

    NASA Astrophysics Data System (ADS)

    Patton, Henry; Hubbard, Alun; Andreassen, Karin; Winsborrow, Monica; Stroeven, Arjen P.

    2016-12-01

    The Eurasian ice-sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum (LGM), after the Antarctic and North American ice sheets. Despite its global significance, a comprehensive account of its evolution from independent nucleation centres to its maximum extent is conspicuously lacking. Here, a first-order, thermomechanical model, robustly constrained by empirical evidence, is used to investigate the dynamics of the EISC throughout its build-up to its maximum configuration. The ice flow model is coupled to a reference climate and applied at 10 km spatial resolution across a domain that includes the three main spreading centres of the Celtic, Fennoscandian and Barents Sea ice sheets. The model is forced with the NGRIP palaeo-isotope curve from 37 ka BP onwards and model skill is assessed against collated flowsets, marginal moraines, exposure ages and relative sea-level history. The evolution of the EISC to its LGM configuration was complex and asynchronous; the western, maritime margins of the Fennoscandian and Celtic ice sheets responded rapidly and advanced across their continental shelves by 29 ka BP, yet the maximum aerial extent (5.48 × 106 km2) and volume (7.18 × 106 km3) of the ice complex was attained some 6 ka later at c. 22.7 ka BP. This maximum stand was short-lived as the North Sea and Atlantic margins were already in retreat whilst eastern margins were still advancing up until c. 20 ka BP. High rates of basal erosion are modelled beneath ice streams and outlet glaciers draining the Celtic and Fennoscandian ice sheets with extensive preservation elsewhere due to frozen subglacial conditions, including much of the Barents and Kara seas. Here, and elsewhere across the Norwegian shelf and North Sea, high pressure subglacial conditions would have promoted localised gas hydrate formation.

  3. Ice stream activity scaled to ice sheet volume during Laurentide Ice Sheet deglaciation.

    PubMed

    Stokes, C R; Margold, M; Clark, C D; Tarasov, L

    2016-02-18

    The contribution of the Greenland and West Antarctic ice sheets to sea level has increased in recent decades, largely owing to the thinning and retreat of outlet glaciers and ice streams. This dynamic loss is a serious concern, with some modelling studies suggesting that the collapse of a major ice sheet could be imminent or potentially underway in West Antarctica, but others predicting a more limited response. A major problem is that observations used to initialize and calibrate models typically span only a few decades, and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves over longer timescales. This represents one of the largest sources of uncertainty when predicting the contributions of ice sheets to sea-level rise. A key question is whether ice streams might increase and sustain rates of mass loss over centuries or millennia, beyond those expected for a given ocean-climate forcing. Here we reconstruct the activity of 117 ice streams that operated at various times during deglaciation of the Laurentide Ice Sheet (from about 22,000 to 7,000 years ago) and show that as they activated and deactivated in different locations, their overall number decreased, they occupied a progressively smaller percentage of the ice sheet perimeter and their total discharge decreased. The underlying geology and topography clearly influenced ice stream activity, but--at the ice-sheet scale--their drainage network adjusted and was linked to changes in ice sheet volume. It is unclear whether these findings can be directly translated to modern ice sheets. However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet.

  4. LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models

    NASA Astrophysics Data System (ADS)

    Kennedy, Joseph H.; Bennett, Andrew R.; Evans, Katherine J.; Price, Stephen; Hoffman, Matthew; Lipscomb, William H.; Fyke, Jeremy; Vargo, Lauren; Boghozian, Adrianna; Norman, Matthew; Worley, Patrick H.

    2017-06-01

    To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptops to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Ultimately, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall.

  5. Reconstructing the last Irish Ice Sheet 2: a geomorphologically-driven model of ice sheet growth, retreat and dynamics

    NASA Astrophysics Data System (ADS)

    Greenwood, Sarah L.; Clark, Chris D.

    2009-12-01

    collapse would sever the ties between the British and Irish Ice Sheets and drive flow configuration changes in response. Enhanced calving and flow acceleration in response to rising relative sea level is speculated to have undermined the integrity of the ice stream system, precipitating its collapse and driving the reconstructed pattern of ice sheet evolution.

  6. LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kennedy, Joseph H.; Bennett, Andrew R.; Evans, Katherine J.

    To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptopsmore » to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Furthermore, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall.« less

  7. LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models

    DOE PAGES

    Kennedy, Joseph H.; Bennett, Andrew R.; Evans, Katherine J.; ...

    2017-03-23

    To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptopsmore » to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Furthermore, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall.« less

  8. Ice sheet margins and ice shelves

    NASA Technical Reports Server (NTRS)

    Thomas, R. H.

    1984-01-01

    The effect of climate warming on the size of ice sheet margins in polar regions is considered. Particular attention is given to the possibility of a rapid response to warming on the order of tens to hundreds of years. It is found that the early response of the polar regions to climate warming would be an increase in the area of summer melt on the ice sheets and ice shelves. For sufficiently large warming (5-10C) the delayed effects would include the breakup of the ice shelves by an increase in ice drainage rates, particularly from the ice sheets. On the basis of published data for periodic changes in the thickness and melting rates of the marine ice sheets and fjord glaciers in Greenland and Antarctica, it is shown that the rate of retreat (or advance) of an ice sheet is primarily determined by: bedrock topography; the basal conditions of the grounded ice sheet; and the ice shelf condition downstream of the grounding line. A program of satellite and ground measurements to monitor the state of ice sheet equilibrium is recommended.

  9. Response of the Antarctic ice sheet to ocean forcing using the POPSICLES coupled ice sheet-ocean model

    NASA Astrophysics Data System (ADS)

    Martin, D. F.; Asay-Davis, X.; Price, S. F.; Cornford, S. L.; Maltrud, M. E.; Ng, E. G.; Collins, W.

    2014-12-01

    We present the response of the continental Antarctic ice sheet to sub-shelf-melt forcing derived from POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010. Simulations are performed at 0.1 degree (~5 km) ocean resolution and ice sheet resolution as fine as 500 m using adaptive mesh refinement. A comparison of fully-coupled and comparable standalone ice-sheet model results demonstrates the importance of two-way coupling between the ice sheet and the ocean. The POPSICLES model couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). BISICLES makes use of adaptive mesh refinement to fully resolve dynamically-important regions like grounding lines and employs a momentum balance similar to the vertically-integrated formulation of Schoof and Hindmarsh (2009). Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests like MISMIP3D (Pattyn et al., 2013) and realistic configurations (Favier et al. 2014). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). A companion presentation, "Present-day circum-Antarctic simulations using the POPSICLES coupled land ice-ocean model" in session C027 describes the ocean-model perspective of this work, while we focus on the response of the ice sheet and on details of the model. The figure shows the BISICLES-computed vertically-integrated ice velocity field about 1 month into a 20-year coupled Antarctic run. Groundling lines are shown in green.

  10. The role of ice shelves in the Holocene evolution of the Antarctic ice sheet

    NASA Astrophysics Data System (ADS)

    Bernales, Jorge; Rogozhina, Irina; Thomas, Maik

    2014-05-01

    Using the continental-scale ice sheet-shelf model SICOPOLIS (Greve, 1997 [1]; Sato and Greve, 2012 [2]), we assess the influence of ice shelves on the Holocene evolution and present-day geometry of the Antarctic ice sheet. We have designed a series of paleoclimate simulations driven by a time-evolved climate forcing that couples the surface temperature record from the Vostok ice core with precipitation pattern using an empirical relation of Dahl-Jensen et al., (1998) [3]. Our numerical experiments show that the geometry of ice shelves is determined by the evolution of climate and ocean conditions over time scales of 15 to 25 kyr. This implies that the initial configuration of ice shelves at the Last Glacial Maximum (LGM, about 21 kyr before present) has a significant effect on the modelled Early Holocene volume of ice shelves (up to 20%) that gradually diminishes to a negligible level for the present-day ice shelf configuration. Thus, the present-day geometry of the Antarctic ice shelves can be attained even if an ice-shelf-free initial condition is chosen at the LGM. However, the grounded ice volume, thickness and dynamic states are found to be sensitive to the ice shelf dynamics over a longer history spanning several tens of thousands of years. A presence of extensive marine ice at the LGM, supported by sediment core reconstructions (e.g. Naish et al., 2009 [4]), has a clear buttressing effect on the grounded ice that remains significant over a period of 30 to 50 kyr. If ice-shelf-free conditions are prescribed at the LGM, the modelled Early Holocene and present-day grounded ice volumes are underestimated by up to 10%, as opposed to simulations incorporating ice shelf dynamics over longer periods. The use of ice-shelf-free LGM conditions thus results in 50 to over 200 meters thinner ice sheet across much of East Antarctica. References [1] Greve, R. (1997). Application of a polythermal three-dimensional ice sheet model to the Greenland ice sheet: response to

  11. Antarctic climate and ice-sheet configuration during the early Pliocene interglacial at 4.23 Ma

    NASA Astrophysics Data System (ADS)

    Golledge, Nicholas R.; Thomas, Zoë A.; Levy, Richard H.; Gasson, Edward G. W.; Naish, Timothy R.; McKay, Robert M.; Kowalewski, Douglas E.; Fogwill, Christopher J.

    2017-07-01

    The geometry of Antarctic ice sheets during warm periods of the geological past is difficult to determine from geological evidence, but is important to know because such reconstructions enable a more complete understanding of how the ice-sheet system responds to changes in climate. Here we investigate how Antarctica evolved under orbital and greenhouse gas conditions representative of an interglacial in the early Pliocene at 4.23 Ma, when Southern Hemisphere insolation reached a maximum. Using offline-coupled climate and ice-sheet models, together with a new synthesis of high-latitude palaeoenvironmental proxy data to define a likely climate envelope, we simulate a range of ice-sheet geometries and calculate their likely contribution to sea level. In addition, we use these simulations to investigate the processes by which the West and East Antarctic ice sheets respond to environmental forcings and the timescales over which these behaviours manifest. We conclude that the Antarctic ice sheet contributed 8.6 ± 2.8 m to global sea level at this time, under an atmospheric CO2 concentration identical to present (400 ppm). Warmer-than-present ocean temperatures led to the collapse of West Antarctica over centuries, whereas higher air temperatures initiated surface melting in parts of East Antarctica that over one to two millennia led to lowering of the ice-sheet surface, flotation of grounded margins in some areas, and retreat of the ice sheet into the Wilkes Subglacial Basin. The results show that regional variations in climate, ice-sheet geometry, and topography produce long-term sea-level contributions that are non-linear with respect to the applied forcings, and which under certain conditions exhibit threshold behaviour associated with behavioural tipping points.

  12. Comparing a thermo-mechanical Weichselian ice sheet reconstruction to GIA driven reconstructions: aspects of earth response and ice configuration

    NASA Astrophysics Data System (ADS)

    Schmidt, P.; Lund, B.; Näslund, J.-O.

    2013-12-01

    In this study we compare a recent reconstruction of the Weichselian ice-sheet as simulated by the University of Main ice-sheet model (UMISM) to two reconstructions commonly used in glacial isostatic adjustment (GIA) modeling: ICE-5G and ANU (also known as RSES). The UMISM reconstruction is carried out on a regional scale based on thermo-mechanical modelling whereas ANU and ICE-5G are global models based on the sea-level equation. The Weichselian ice-sheet in the three models are compared directly in terms of ice volume, extent and thickness, as well as in terms of predicted glacial isostatic adjustment in Fennoscandia. The three reconstructions display significant differences. UMISM and ANU includes phases of pronounced advance and retreat prior to the last glacial maximum (LGM), whereas the thickness and areal extent of the ICE-5G ice-sheet is more or less constant up until LGM. The final retreat of the ice-sheet initiates at earliest time in ICE-5G and latest in UMISM, while ice free conditions are reached earliest in UMISM and latest in ICE-5G. The post-LGM deglaciation style also differs notably between the ice models. While the UMISM simulation includes two temporary halts in the deglaciation, the later during the Younger Dryas, ANU only includes a decreased deglaciation rate during Younger Dryas and ICE-5G retreats at a relatively constant pace after an initial slow phase. Moreover, ANU and ICE-5G melt relatively uniformly over the entire ice-sheet in contrast to UMISM which melts preferentially from the edges. We find that all three reconstructions fit the present day uplift rates over Fennoscandia and the observed relative sea-level curve along the Ångerman river equally well, albeit with different optimal earth model parameters. Given identical earth models, ICE-5G predicts the fastest present day uplift rates and ANU the slowest, ANU also prefers the thinnest lithosphere. Moreover, only for ANU can a unique best fit model be determined. For UMISM and ICE

  13. The Impact of Geothermal Heat on the Scandinavian Ice Sheet's LGM Extent

    NASA Astrophysics Data System (ADS)

    Szuman, Izabela; Ewertowski, Marek W.; Kalita, Jakub Z.

    2016-04-01

    The last Scandinavian ice sheet attained its most southern extent over Poland and Germany, protruding c. 200 km south of the main ice sheet mass. There are number of factors that may control ice sheet dynamics and extent. One of the less recognised is geothermal heat, which is heat that is supplied to the base of the ice sheet. A heat at the ice/bed interface plays a crucial role in controlling ice sheet stability, as well as impacting basal temperatures, melting, and ice flow velocities. However, the influence of geothermal heat is still virtually neglected in reconstructions and modelling of paleo-ice sheets behaviour. Only in a few papers is geothermal heat recalled though often in the context of past climatic conditions. Thus, the major question is if and how spatial differences in geothermal heat had influenced paleo-ice sheet dynamics and in consequence their extent. Here, we assumed that the configuration of the ice sheet along its southern margin was moderately to strongly correlated with geothermal heat for Poland and non or negatively correlated for Germany.

  14. Dynamic Antarctic ice sheet during the early to mid-Miocene

    PubMed Central

    DeConto, Robert M.; Pollard, David; Levy, Richard H.

    2016-01-01

    Geological data indicate that there were major variations in Antarctic ice sheet volume and extent during the early to mid-Miocene. Simulating such large-scale changes is problematic because of a strong hysteresis effect, which results in stability once the ice sheets have reached continental size. A relatively narrow range of atmospheric CO2 concentrations indicated by proxy records exacerbates this problem. Here, we are able to simulate large-scale variability of the early to mid-Miocene Antarctic ice sheet because of three developments in our modeling approach. (i) We use a climate–ice sheet coupling method utilizing a high-resolution atmospheric component to account for ice sheet–climate feedbacks. (ii) The ice sheet model includes recently proposed mechanisms for retreat into deep subglacial basins caused by ice-cliff failure and ice-shelf hydrofracture. (iii) We account for changes in the oxygen isotopic composition of the ice sheet by using isotope-enabled climate and ice sheet models. We compare our modeling results with ice-proximal records emerging from a sedimentological drill core from the Ross Sea (Andrill-2A) that is presented in a companion article. The variability in Antarctic ice volume that we simulate is equivalent to a seawater oxygen isotope signal of 0.52–0.66‰, or a sea level equivalent change of 30–36 m, for a range of atmospheric CO2 between 280 and 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model data conflict of Miocene Antarctic ice sheet and sea level variability. PMID:26903645

  15. Ice sheet altimetry

    NASA Technical Reports Server (NTRS)

    Brooks, R. L.

    1981-01-01

    Generalized surface slopes were computed for the Antarctic and Greenland ice sheets by differencing plotted contour levels and dividing them by the distance between the contours. It was observed that more than 90% of the ice sheets have surface slopes less than 1%. Seasat test mode-1 Seasat altimeter measurements over Greenland were analyzed by comparisons with collinear and intersecting normal mode Seasat altimeter passes. Over the ice sheet, the computed surface elevations from test mode-1 measurements were consistently lower by about 45 m and the AGC levels were down by approximately 6 dB. No test mode-1 data were acquired over Antarctica. It is concluded that analysis of the existing altimeter data base over the two ice sheets is crucial in designing a future improved altimeter tracking capability. It is recommended that additional waveform retracking be performed to characterize ice sheet topography as a function of geographic area and elevation.

  16. Potential subglacial lake locations and meltwater drainage pathways beneath the Antarctic and Greenland ice sheets

    NASA Astrophysics Data System (ADS)

    Livingstone, S. J.; Clark, C. D.; Woodward, J.; Kingslake, J.

    2013-11-01

    We use the Shreve hydraulic potential equation as a simplified approach to investigate potential subglacial lake locations and meltwater drainage pathways beneath the Antarctic and Greenland ice sheets. We validate the method by demonstrating its ability to recall the locations of >60% of the known subglacial lakes beneath the Antarctic Ice Sheet. This is despite uncertainty in the ice-sheet bed elevation and our simplified modelling approach. However, we predict many more lakes than are observed. Hence we suggest that thousands of subglacial lakes remain to be found. Applying our technique to the Greenland Ice Sheet, where very few subglacial lakes have so far been observed, recalls 1607 potential lake locations, covering 1.2% of the bed. Our results will therefore provide suitable targets for geophysical surveys aimed at identifying lakes beneath Greenland. We also apply the technique to modelled past ice-sheet configurations and find that during deglaciation both ice sheets likely had more subglacial lakes at their beds. These lakes, inherited from past ice-sheet configurations, would not form under current surface conditions, but are able to persist, suggesting a retreating ice-sheet will have many more subglacial lakes than advancing ones. We also investigate subglacial drainage pathways of the present-day and former Greenland and Antarctic ice sheets. Key sectors of the ice sheets, such as the Siple Coast (Antarctica) and NE Greenland Ice Stream system, are suggested to have been susceptible to subglacial drainage switching. We discuss how our results impact our understanding of meltwater drainage, basal lubrication and ice-stream formation.

  17. Global ice-sheet system interlocked by sea level

    NASA Astrophysics Data System (ADS)

    Denton, George H.; Hughes, Terence J.; Karlén, Wibjörn

    1986-07-01

    permitted renewed formation of North Atlantic Deep Water, which could well have controlled atmospheric carbon dioxide ( W. S. Broecker, D. M. Peteet, and D. Rind, 1985, Nature ( London) 315, 21-26). Combined melting and consequent sea-level rise from the three warming factors initiated irreversible collapse of the interlocked global ice-sheet system, which was at its largest but most vulnerable configuration.

  18. Sensitivities of Greenland ice sheet volume inferred from an ice sheet adjoint model

    NASA Astrophysics Data System (ADS)

    Heimbach, P.; Bugnion, V.

    2009-04-01

    We present a new and original approach to understanding the sensitivity of the Greenland ice sheet to key model parameters and environmental conditions. At the heart of this approach is the use of an adjoint ice sheet model. Since its introduction by MacAyeal (1992), the adjoint method has become widespread to fit ice stream models to the increasing number and diversity of satellite observations, and to estimate uncertain model parameters such as basal conditions. However, no attempt has been made to extend this method to comprehensive ice sheet models. As a first step toward the use of adjoints of comprehensive three-dimensional ice sheet models we have generated an adjoint of the ice sheet model SICOPOLIS of Greve (1997). The adjoint was generated by means of the automatic differentiation (AD) tool TAF. The AD tool generates exact source code representing the tangent linear and adjoint model of the nonlinear parent model provided. Model sensitivities are given by the partial derivatives of a scalar-valued model diagnostic with respect to the controls, and can be efficiently calculated via the adjoint. By way of example, we determine the sensitivity of the total Greenland ice volume to various control variables, such as spatial fields of basal flow parameters, surface and basal forcings, and initial conditions. Reliability of the adjoint was tested through finite-difference perturbation calculations for various control variables and perturbation regions. Besides confirming qualitative aspects of ice sheet sensitivities, such as expected regional variations, we detect regions where model sensitivities are seemingly unexpected or counter-intuitive, albeit ``real'' in the sense of actual model behavior. An example is inferred regions where sensitivities of ice sheet volume to basal sliding coefficient are positive, i.e. where a local increase in basal sliding parameter increases the ice sheet volume. Similarly, positive ice temperature sensitivities in certain parts

  19. Modeling North American Ice Sheet Response to Changes in Precession and Obliquity

    NASA Astrophysics Data System (ADS)

    Tabor, C.; Poulsen, C. J.; Pollard, D.

    2012-12-01

    Milankovitch theory proposes that changes in insolation due to orbital perturbations dictate the waxing and waning of the ice sheets (Hays et al., 1976). However, variations in solar forcing alone are insufficient to produce the glacial oscillations observed in the climate record. Non-linear feedbacks in the Earth system likely work in concert with the orbital cycles to produce a modified signal (e.g. Berger and Loutre, 1996), but the nature of these feedbacks remain poorly understood. To gain a better understand of the ice dynamics and climate feedbacks associated with changes in orbital configuration, we use a complex Earth system model consisting of the GENESIS GCM and land surface model (Pollard and Thompson, 1997), the Pennsylvania State University ice sheet model (Pollard and DeConto, 2009), and the BIOME vegetation model (Kaplan et al., 2001). We began this study by investigating ice sheet sensitivity to a range of commonly used ice sheet model parameters, including mass balance and albedo, to optimize simulations for Pleistocene orbital cycles. Our tests indicate that choice of mass balance and albedo parameterizations can lead to significant differences in ice sheet behavior and volume. For instance, use of an insolation-temperature mass balance scheme (van den Berg, 2008) allows for a larger ice sheet response to orbital changes than the commonly employed positive degree-day method. Inclusion of a large temperature dependent ice albedo, representing phenomena such as melt ponds and dirty ice, also enhances ice sheet sensitivity. Careful tuning of mass balance and albedo parameterizations can help alleviate the problem of insufficient ice sheet retreat during periods of high summer insolation (Horton and Poulsen, 2007) while still accurately replicating the modern climate. Using our optimized configuration, we conducted a series of experiments with idealized transient orbits in an asynchronous coupling scheme to investigate the influence of obliquity and

  20. An ice sheet model validation framework for the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Price, Stephen F.; Hoffman, Matthew J.; Bonin, Jennifer A.; Howat, Ian M.; Neumann, Thomas; Saba, Jack; Tezaur, Irina; Guerber, Jeffrey; Chambers, Don P.; Evans, Katherine J.; Kennedy, Joseph H.; Lenaerts, Jan; Lipscomb, William H.; Perego, Mauro; Salinger, Andrew G.; Tuminaro, Raymond S.; van den Broeke, Michiel R.; Nowicki, Sophie M. J.

    2017-01-01

    We propose a new ice sheet model validation framework - the Cryospheric Model Comparison Tool (CmCt) - that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013, using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin-scale and whole-ice-sheet-scale metrics, we find that simulations using both idealized conceptual models and dynamic, numerical models provide an equally reasonable representation of the ice sheet surface (mean elevation differences of < 1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CmCt, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate a predictive skill with respect to observed dynamic changes that have occurred on

  1. An ice sheet model validation framework for the Greenland ice sheet

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Price, Stephen F.; Hoffman, Matthew J.; Bonin, Jennifer A.

    We propose a new ice sheet model validation framework the Cryospheric Model Comparison Tool (CMCT) that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quanti- tative metricsmore » for use in evaluating the different model simulations against the observations. We find 10 that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, the model initial condition as well as output from idealized and dynamic models all provide an equally reasonable representation of the ice sheet surface (mean elevation differences of <1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CMCT, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the past few

  2. An ice sheet model validation framework for the Greenland ice sheet

    PubMed Central

    Price, Stephen F.; Hoffman, Matthew J.; Bonin, Jennifer A.; Howat, Ian M.; Neumann, Thomas; Saba, Jack; Tezaur, Irina; Guerber, Jeffrey; Chambers, Don P.; Evans, Katherine J.; Kennedy, Joseph H.; Lenaerts, Jan; Lipscomb, William H.; Perego, Mauro; Salinger, Andrew G.; Tuminaro, Raymond S.; van den Broeke, Michiel R.; Nowicki, Sophie M. J.

    2018-01-01

    We propose a new ice sheet model validation framework – the Cryospheric Model Comparison Tool (CmCt) – that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, we find that simulations using both idealized conceptual models and dynamic, numerical models provide an equally reasonable representation of the ice sheet surface (mean elevation differences of <1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CmCt, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the

  3. An ice sheet model validation framework for the Greenland ice sheet.

    PubMed

    Price, Stephen F; Hoffman, Matthew J; Bonin, Jennifer A; Howat, Ian M; Neumann, Thomas; Saba, Jack; Tezaur, Irina; Guerber, Jeffrey; Chambers, Don P; Evans, Katherine J; Kennedy, Joseph H; Lenaerts, Jan; Lipscomb, William H; Perego, Mauro; Salinger, Andrew G; Tuminaro, Raymond S; van den Broeke, Michiel R; Nowicki, Sophie M J

    2017-01-01

    We propose a new ice sheet model validation framework - the Cryospheric Model Comparison Tool (CmCt) - that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, we find that simulations using both idealized conceptual models and dynamic, numerical models provide an equally reasonable representation of the ice sheet surface (mean elevation differences of <1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CmCt, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the past

  4. An ice sheet model validation framework for the Greenland ice sheet

    DOE PAGES

    Price, Stephen F.; Hoffman, Matthew J.; Bonin, Jennifer A.; ...

    2017-01-17

    We propose a new ice sheet model validation framework the Cryospheric Model Comparison Tool (CMCT) that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quanti- tative metricsmore » for use in evaluating the different model simulations against the observations. We find 10 that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, the model initial condition as well as output from idealized and dynamic models all provide an equally reasonable representation of the ice sheet surface (mean elevation differences of <1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CMCT, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the past few

  5. An Ice Sheet Model Validation Framework for the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Price, Stephen F.; Hoffman, Matthew J.; Bonin, Jennifer A.; Howat, Ian M.; Neumann, Thomas A.; Saba, Jack; Tezaur, Irina; Guerber, Jeffrey R.; Chambers, Don P.; Evans, Katherine J.; hide

    2017-01-01

    We propose a new ice sheet model validation framework - the Cryospheric Model Comparison Tool (CmCt) - that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013, using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin-scale and whole-ice-sheet-scale metrics, we find that simulations using both idealized conceptual models and dynamic, numerical models provide an equally reasonable representation of the ice sheet surface (mean elevation differences of less than 1 meter). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CmCt, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate a predictive skill with respect to observed dynamic changes that have occurred

  6. Coupled ice sheet-ocean modelling to investigate ocean driven melting of marine ice sheets in Antarctica

    NASA Astrophysics Data System (ADS)

    Jong, Lenneke; Gladstone, Rupert; Galton-Fenzi, Ben

    2017-04-01

    Ocean induced melting below the ice shelves of marine ice sheets is a major source of uncertainty for predictions of ice mass loss and Antarctica's resultant contribution to future sea level rise. The floating ice shelves provide a buttressing force against the flow of ice across the grounding line into the ocean. Thinning of these ice shelves due to an increase in melting reduces this force and can lead to an increase in the discharge of grounded ice. Fully coupled modelling of ice sheet-ocean interactions is key to improving understanding the influence of the Southern ocean on the evolution of the Antarctic ice sheet, and to predicting its future behaviour under changing climate conditions. Coupling of ocean and ice sheet models is needed to provide more realistic melt rates at the base of ice shelves and hence make better predictions of the behaviour of the grounding line and the shape of the ice-shelf cavity as the ice sheet evolves. The Framework for Ice Sheet - Ocean Coupling (FISOC) has been developed to provide a flexible platform for performing coupled ice sheet - ocean modelling experiments. We present preliminary results using FISOC to couple the Regional Ocean Modelling System (ROMS) with Elmer/Ice in idealised experiments Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP). These experiments use an idealised geometry motivated by that of Pine Island glacier and the adjacent Amundsen Sea in West Antarctica, a region which has shown shown signs of thinning ice and grounding line retreat.

  7. Ice sheet radar altimetry

    NASA Technical Reports Server (NTRS)

    Zwally, J.

    1988-01-01

    The surface topography of the Greenland and Antarctic ice sheets between 72 degrees north and south was mapped using radar altimetry data from the U.S. Navy GEOSAT. The glaciological objectives of this activity were to study the dynamics of the ice flow, changes in the position of floating ice-shelf fronts, and ultimately to measure temporal changes in ice surface elevation indicative of ice sheet mass balance.

  8. The frequency response of a coupled ice sheet-ice shelf-ocean system to climate forcing variability

    NASA Astrophysics Data System (ADS)

    Goldberg, D.; Snow, K.; Jordan, J. R.; Holland, P.; Arthern, R. J.

    2017-12-01

    Changes at the West Antarctic ice-ocean boundary in recent decades has triggered significant increases in the regions contribution to global sea-level rise, coincident with large scale, and in some cases potentially unstable, grounding line retreat. Much of the induced change is thought to be driven by fluctuations in the oceanic heat available at the ice-ocean boundary, transported on-shelf via warm Circumpolar Deep Water (CDW). However, the processes in which ocean heat drives ice-sheet loss remains poorly understood, with observational studies routinely hindered by the extreme environment notorious to the Antarctic region. In this study we apply a novel synchronous coupled ice-ocean model, developed within the MITgcm, and are thus able to provide detailed insight into the impacts of short time scale (interannual to decadal) climate variability and feedbacks within the ice-ocean system. Feedbacks and response are assessed in an idealised ice-sheet/ocean-cavity configuration in which the far field ocean condition is adjusted to emulate periodic climate variability patterns. We reveal a non-linear response of the ice-sheet to periodic variations in thermocline depth. These non-linearities illustrate the heightened sensitivity of fast flowing ice-shelves to periodic perturbations in heat fluxes occurring at interannual and decadal time scales. The results thus highlight how small perturbations in variable climate forcing, like that of ENSO, may trigger large changes in ice-sheet response.

  9. Greenland ice sheet retreat since the Little Ice Age

    NASA Astrophysics Data System (ADS)

    Beitch, Marci J.

    Late 20th century and 21st century satellite imagery of the perimeter of the Greenland Ice Sheet (GrIS) provide high resolution observations of the ice sheet margins. Examining changes in ice margin positions over time yield measurements of GrIS area change and rates of margin retreat. However, longer records of ice sheet margin change are needed to establish more accurate predictions of the ice sheet's future response to global conditions. In this study, the trimzone, the area of deglaciated terrain along the ice sheet edge that lacks mature vegetation cover, is used as a marker of the maximum extent of the ice from its most recent major advance during the Little Ice Age. We compile recently acquired Landsat ETM+ scenes covering the perimeter of the GrIS on which we map area loss on land-, lake-, and marine-terminating margins. We measure an area loss of 13,327 +/- 830 km2, which corresponds to 0.8% shrinkage of the ice sheet. This equates to an averaged horizontal retreat of 363 +/- 69 m across the entire GrIS margin. Mapping the areas exposed since the Little Ice Age maximum, circa 1900 C.E., yields a century-scale rate of change. On average the ice sheet lost an area of 120 +/- 16 km 2/yr, or retreated at a rate of 3.3 +/- 0.7 m/yr since the LIA maximum.

  10. A Transient Initialization Routine of the Community Ice Sheet Model for the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    van der Laan, Larissa; van den Broeke, Michiel; Noël, Brice; van de Wal, Roderik

    2017-04-01

    The Community Ice Sheet Model (CISM) is to be applied in future simulations of the Greenland Ice Sheet under a range of climate change scenarios, determining the sensitivity of the ice sheet to individual climatic forcings. In order to achieve reliable results regarding ice sheet stability and assess the probability of future occurrence of tipping points, a realistic initial ice sheet geometry is essential. The current work describes and evaluates the development of a transient initialization routine, using NGRIP 18O isotope data to create a temperature anomaly field. Based on the latter, surface mass balance components runoff and precipitation are perturbed for the past 125k years. The precipitation and runoff fields originate from a downscaled 1 km resolution version of the regional climate model RACMO2.3 for the period 1961-1990. The result of the initialization routine is a present-day ice sheet with a transient memory of the last glacial-interglacial cycle, which will serve as the future runs' initial condition.

  11. A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes.

    PubMed

    Young, Duncan A; Wright, Andrew P; Roberts, Jason L; Warner, Roland C; Young, Neal W; Greenbaum, Jamin S; Schroeder, Dustin M; Holt, John W; Sugden, David E; Blankenship, Donald D; van Ommen, Tas D; Siegert, Martin J

    2011-06-02

    The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ∼34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2-4.5 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.

  12. Decadal-Scale Response of the Antarctic Ice sheet to a Warming Ocean using the POPSICLES Coupled Ice Sheet-Ocean model

    NASA Astrophysics Data System (ADS)

    Martin, D. F.; Asay-Davis, X.; Cornford, S. L.; Price, S. F.; Ng, E. G.; Collins, W.

    2015-12-01

    We present POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period from 1990 to 2010. We use the CORE v. 2 interannual forcing data to force the ocean model. Simulations are performed at 0.1o(~5 km) ocean resolution with adaptive ice sheet resolution as fine as 500 m to adequately resolve the grounding line dynamics. We discuss the effect of improved ocean mixing and subshelf bathymetry (vs. the standard Bedmap2 bathymetry) on the behavior of the coupled system, comparing time-averaged melt rates below a number of major ice shelves with those reported in the literature. We also present seasonal variability and decadal melting trends from several Antarctic regions, along with the response of the ice shelves and the consequent dynamic response of the grounded ice sheet.POPSICLES couples the POP2x ocean model, a modified version of the Parallel Ocean Program, and the BISICLES ice-sheet model. POP2x includes sub-ice-shelf circulation using partial top cells and the commonly used three-equation boundary layer physics. Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP) and other continental-scale simulations and melt-rate observations. BISICLES makes use of adaptive mesh refinement and a 1st-order accurate momentum balance similar to the L1L2 model of Schoof and Hindmarsh to accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests (MISMIP-3d) and realistic configurations.The figure shows the BISICLES-computed vertically-integrated grounded ice velocity field 5 years into a 20-year coupled full-continent Antarctic-Southern-Ocean simulation. Submarine melt rates are painted onto the surface of the floating ice shelves. Grounding lines are shown in green.

  13. Active volcanism beneath the West Antarctic ice sheet and implications for ice-sheet stability

    USGS Publications Warehouse

    Blankenship, D.D.; Bell, R.E.; Hodge, S.M.; Brozena, J.M.; Behrendt, John C.; Finn, C.A.

    1993-01-01

    IT is widely understood that the collapse of the West Antarctic ice sheet (WAIS) would cause a global sea level rise of 6 m, yet there continues to be considerable debate about the detailed response of this ice sheet to climate change1-3. Because its bed is grounded well below sea level, the stability of the WAIS may depend on geologically controlled conditions at the base which are independent of climate. In particular, heat supplied to the base of the ice sheet could increase basal melting and thereby trigger ice streaming, by providing the water for a lubricating basal layer of till on which ice streams are thought to slide4,5. Ice streams act to protect the reservoir of slowly moving inland ice from exposure to oceanic degradation, thus enhancing ice-sheet stability. Here we present aerogeophysical evidence for active volcanism and associated elevated heat flow beneath the WAIS near the critical region where ice streaming begins. If this heat flow is indeed controlling ice-stream formation, then penetration of ocean waters inland of the thin hot crust of the active portion of the West Antarctic rift system could lead to the disappearance of ice streams, and possibly trigger a collapse of the inland ice reservoir.

  14. The Physics of Ice Sheets

    ERIC Educational Resources Information Center

    Bassis, J. N.

    2008-01-01

    The great ice sheets in Antarctica and Greenland are vast deposits of frozen freshwater that contain enough to raise sea level by approximately 70 m if they were to completely melt. Because of the potentially catastrophic impact that ice sheets can have, it is important that we understand how ice sheets have responded to past climate changes and…

  15. Large Ice Discharge From the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Rignot, Eric

    1999-01-01

    The objectives of this work are to measure the ice discharge of the Greenland Ice Sheet close to the grounding line and/or calving front, and compare the results with mass accumulation and ablation in the interior to estimate the ice sheet mass balance.

  16. Ice shelf fracture parameterization in an ice sheet model

    NASA Astrophysics Data System (ADS)

    Sun, Sainan; Cornford, Stephen L.; Moore, John C.; Gladstone, Rupert; Zhao, Liyun

    2017-11-01

    Floating ice shelves exert a stabilizing force onto the inland ice sheet. However, this buttressing effect is diminished by the fracture process, which on large scales effectively softens the ice, accelerating its flow, increasing calving, and potentially leading to ice shelf breakup. We add a continuum damage model (CDM) to the BISICLES ice sheet model, which is intended to model the localized opening of crevasses under stress, the transport of those crevasses through the ice sheet, and the coupling between crevasse depth and the ice flow field and to carry out idealized numerical experiments examining the broad impact on large-scale ice sheet and shelf dynamics. In each case we see a complex pattern of damage evolve over time, with an eventual loss of buttressing approximately equivalent to halving the thickness of the ice shelf. We find that it is possible to achieve a similar ice flow pattern using a simple rule of thumb: introducing an enhancement factor ˜ 10 everywhere in the model domain. However, spatially varying damage (or equivalently, enhancement factor) fields set at the start of prognostic calculations to match velocity observations, as is widely done in ice sheet simulations, ought to evolve in time, or grounding line retreat can be slowed by an order of magnitude.

  17. Design of the MISMIP+, ISOMIP+, and MISOMIP ice-sheet, ocean, and coupled ice sheet-ocean intercomparison projects

    NASA Astrophysics Data System (ADS)

    Asay-Davis, Xylar; Cornford, Stephen; Martin, Daniel; Gudmundsson, Hilmar; Holland, David; Holland, Denise

    2015-04-01

    The MISMIP and MISMIP3D marine ice sheet model intercomparison exercises have become popular benchmarks, and several modeling groups have used them to show how their models compare to both analytical results and other models. Similarly, the ISOMIP (Ice Shelf-Ocean Model Intercomparison Project) experiments have acted as a proving ground for ocean models with sub-ice-shelf cavities.As coupled ice sheet-ocean models become available, an updated set of benchmark experiments is needed. To this end, we propose sequel experiments, MISMIP+ and ISOMIP+, with an end goal of coupling the two in a third intercomparison exercise, MISOMIP (the Marine Ice Sheet-Ocean Model Intercomparison Project). Like MISMIP3D, the MISMIP+ experiments take place in an idealized, three-dimensional setting and compare full 3D (Stokes) and reduced, hydrostatic models. Unlike the earlier exercises, the primary focus will be the response of models to sub-shelf melting. The chosen configuration features an ice shelf that experiences substantial lateral shear and buttresses the upstream ice, and so is well suited to melting experiments. Differences between the steady states of each model are minor compared to the response to melt-rate perturbations, reflecting typical real-world applications where parameters are chosen so that the initial states of all models tend to match observations. The three ISOMIP+ experiments have been designed to to make use of the same bedrock topography as MISMIP+ and using ice-shelf geometries from MISMIP+ results produced by the BISICLES ice-sheet model. The first two experiments use static ice-shelf geometries to simulate the evolution of ocean dynamics and resulting melt rates to a quasi-steady state when far-field forcing changes in either from cold to warm or from warm to cold states. The third experiment prescribes 200 years of dynamic ice-shelf geometry (with both retreating and advancing ice) based on a BISICLES simulation along with similar flips between warm and

  18. Ice_Sheets_CCI: Essential Climate Variables for the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Forsberg, R.; Sørensen, L. S.; Khan, A.; Aas, C.; Evansberget, D.; Adalsteinsdottir, G.; Mottram, R.; Andersen, S. B.; Ahlstrøm, A.; Dall, J.; Kusk, A.; Merryman, J.; Hvidberg, C.; Khvorostovsky, K.; Nagler, T.; Rott, H.; Scharrer, M.; Shepard, A.; Ticconi, F.; Engdahl, M.

    2012-04-01

    As part of the ESA Climate Change Initiative (www.esa-cci.org) a long-term project "ice_sheets_cci" started January 1, 2012, in addition to the existing 11 projects already generating Essential Climate Variables (ECV) for the Global Climate Observing System (GCOS). The "ice_sheets_cci" goal is to generate a consistent, long-term and timely set of key climate parameters for the Greenland ice sheet, to maximize the impact of European satellite data on climate research, from missions such as ERS, Envisat and the future Sentinel satellites. The climate parameters to be provided, at first in a research context, and in the longer perspective by a routine production system, would be grids of Greenland ice sheet elevation changes from radar altimetry, ice velocity from repeat-pass SAR data, as well as time series of marine-terminating glacier calving front locations and grounding lines for floating-front glaciers. The ice_sheets_cci project will involve a broad interaction of the relevant cryosphere and climate communities, first through user consultations and specifications, and later in 2012 optional participation in "best" algorithm selection activities, where prototype climate parameter variables for selected regions and time frames will be produced and validated using an objective set of criteria ("Round-Robin intercomparison"). This comparative algorithm selection activity will be completely open, and we invite all interested scientific groups with relevant experience to participate. The results of the "Round Robin" exercise will form the algorithmic basis for the future ECV production system. First prototype results will be generated and validated by early 2014. The poster will show the planned outline of the project and some early prototype results.

  19. Sea-level and solid-Earth deformation feedbacks in ice sheet modelling

    NASA Astrophysics Data System (ADS)

    Konrad, Hannes; Sasgen, Ingo; Klemann, Volker; Thoma, Malte; Grosfeld, Klaus; Martinec, Zdeněk

    2014-05-01

    The interactions of ice sheets with the sea level and the solid Earth are important factors for the stability of the ice shelves and the tributary inland ice (e.g. Thomas and Bentley, 1978; Gomez et al, 2012). First, changes in ice extent and ice thickness induce viscoelastic deformation of the Earth surface and Earth's gravity field. In turn, global and local changes in sea level and bathymetry affect the grounding line and, subsequently, alter the ice dynamic behaviour. Here, we investigate these feedbacks for a synthetic ice sheet configuration as well as for the Antarctic ice sheet using a three-dimensional thermomechanical ice sheet and shelf model, coupled to a viscoelastic solid-Earth and gravitationally self-consistent sea-level model. The respective ice sheet undergoes a forcing from rising sea level, warming ocean, and/or changing surface mass balance. The coupling is realized by exchanging ice thickness, Earth surface deformation and sea level periodically. We apply several sets of viscoelastic Earth parameters to our coupled model, e.g. simulating a low-viscous upper mantle present at the Antarctic Peninsula (Ivins et al., 2011). Special focus of our study lies on the evolution of Earth surface deformation and local sea level changes, as well as on the accompanying grounding line evolution. N. Gomez, D. Pollard, J. X. Mitrovica, P. Huybers, and P. U. Clark 2012. Evolution of a coupled marine ice sheet-sea level model, J. Geophys. Res., 117, F01013, doi:10.1029/2011JF002128. E. R. Ivins, M. M. Watkins, D.-N. Yuan, R. Dietrich, G. Casassa, and A. Rülke 2011. On-land ice loss and glacial isostatic adjustment at the Drake Passage: 2003-2009, J. Geophys. Res. 116, B02403, doi: 10.1029/2010JB007607 R. H. Thomas and C. R. Bentley 1978. A model for Holocene retreat of the West Antarctic Ice Sheet, Quaternary Research, 10 (2), pages 150-170, doi: 10.1016/0033-5894(78)90098-4.

  20. Exposure age and ice-sheet model constraints on Pliocene East Antarctic ice sheet dynamics.

    PubMed

    Yamane, Masako; Yokoyama, Yusuke; Abe-Ouchi, Ayako; Obrochta, Stephen; Saito, Fuyuki; Moriwaki, Kiichi; Matsuzaki, Hiroyuki

    2015-04-24

    The Late Pliocene epoch is a potential analogue for future climate in a warming world. Here we reconstruct Plio-Pleistocene East Antarctic Ice Sheet (EAIS) variability using cosmogenic nuclide exposure ages and model simulations to better understand ice sheet behaviour under such warm conditions. New and previously published exposure ages indicate interior-thickening during the Pliocene. An ice sheet model with mid-Pliocene boundary conditions also results in interior thickening and suggests that both the Wilkes Subglacial and Aurora Basins largely melted, offsetting increased ice volume. Considering contributions from West Antarctica and Greenland, this is consistent with the most recent IPCC AR5 estimate, which indicates that the Pliocene sea level likely did not exceed +20 m on Milankovitch timescales. The inception of colder climate since ∼3 Myr has increased the sea ice cover and inhibited active moisture transport to Antarctica, resulting in reduced ice sheet thickness, at least in coastal areas.

  1. Modelling the Climate - Greenland Ice Sheet Interaction in the Coupled Ice-sheet/Climate Model EC-EARTH - PISM

    NASA Astrophysics Data System (ADS)

    Yang, S.; Madsen, M. S.; Rodehacke, C. B.; Svendsen, S. H.; Adalgeirsdottir, G.

    2014-12-01

    Recent observations show that the Greenland ice sheet (GrIS) has been losing mass with an increasing speed during the past decades. Predicting the GrIS changes and their climate consequences relies on the understanding of the interaction of the GrIS with the climate system on both global and local scales, and requires climate model systems with an explicit and physically consistent ice sheet module. A fully coupled global climate model with a dynamical ice sheet model for the GrIS has recently been developed. The model system, EC-EARTH - PISM, consists of the EC-EARTH, an atmosphere, ocean and sea ice model system, and the Parallel Ice Sheet Model (PISM). The coupling of PISM includes a modified surface physical parameterization in EC-EARTH adapted to the land ice surface over glaciated regions in Greenland. The PISM ice sheet model is forced with the surface mass balance (SMB) directly computed inside the EC-EARTH atmospheric module and accounting for the precipitation, the surface evaporation, and the melting of snow and ice over land ice. PISM returns the simulated basal melt, ice discharge and ice cover (extent and thickness) as boundary conditions to EC-EARTH. This coupled system is mass and energy conserving without being constrained by any anomaly correction or flux adjustment, and hence is suitable for investigation of ice sheet - climate feedbacks. Three multi-century experiments for warm climate scenarios under (1) the RCP85 climate forcing, (2) an abrupt 4xCO2 and (3) an idealized 1% per year CO2 increase are performed using the coupled model system. The experiments are compared with their counterparts of the standard CMIP5 simulations (without the interactive ice sheet) to evaluate the performance of the coupled system and to quantify the GrIS feedbacks. In particular, the evolution of the Greenland ice sheet under the warm climate and its impacts on the climate system are investigated. Freshwater fluxes from the Greenland ice sheet melt to the Arctic

  2. Modeling the evolution of the Laurentide Ice Sheet from MIS 3 to the Last Glacial Maximum: an approach using sea level modeling and ice flow dynamics

    NASA Astrophysics Data System (ADS)

    Weisenberg, J.; Pico, T.; Birch, L.; Mitrovica, J. X.

    2017-12-01

    The history of the Laurentide Ice Sheet since the Last Glacial Maximum ( 26 ka; LGM) is constrained by geological evidence of ice margin retreat in addition to relative sea-level (RSL) records in both the near and far field. Nonetheless, few observations exist constraining the ice sheet's extent across the glacial build-up phase preceding the LGM. Recent work correcting RSL records along the U.S. mid-Atlantic dated to mid-MIS 3 (50-35 ka) for glacial-isostatic adjustment (GIA) infer that the Laurentide Ice Sheet grew by more than three-fold in the 15 ky leading into the LGM. Here we test the plausibility of a late and extremely rapid glaciation by driving a high-resolution ice sheet model, based on a nonlinear diffusion equation for the ice thickness. We initialize this model at 44 ka with the mid-MIS 3 ice sheet configuration proposed by Pico et al. (2017), GIA-corrected basal topography, and mass balance representative of mid-MIS 3 conditions. These simulations predict rapid growth of the eastern Laurentide Ice Sheet, with rates consistent with achieving LGM ice volumes within 15 ky. We use these simulations to refine the initial ice configuration and present an improved and higher resolution model for North American ice cover during mid-MIS 3. In addition we show that assumptions of ice loads during the glacial phase, and the associated reconstructions of GIA-corrected basal topography, produce a bias that can underpredict ice growth rates in the late stages of the glaciation, which has important consequences for our understanding of the speed limit for ice growth on glacial timescales.

  3. Global ice sheet/RSL simulations using the higher-order Ice Sheet System Model.

    NASA Astrophysics Data System (ADS)

    Larour, E. Y.; Ivins, E. R.; Adhikari, S.; Schlegel, N.; Seroussi, H. L.; Morlighem, M.

    2017-12-01

    Relative sea-level rise is driven by processes that are intimately linked to the evolution ofglacial areas and ice sheets in particular. So far, most Earth System models capable of projecting theevolution of RSL on decadal to centennial time scales have relied on offline interactions between RSL andice sheets. In particular, grounding line and calving front dynamics have not been modeled in a way that istightly coupled with Elasto-Static Adjustment (ESA) and/or Glacial-Isostatic Adjustment (GIA). Here, we presenta new simulation of the entire Earth System in which both Greenland and Antarctica ice sheets are tightly coupledto an RSL model that includes both ESA and GIA at resolutions and time scales compatible with processes suchas grounding line dynamics for Antarctica ice shelves and calving front dynamics for Greenland marine-terminatingglaciers. The simulations rely on the Ice Sheet System Model (ISSM) and show the impact of higher-orderice flow dynamics and coupling feedbacks between ice flow and RSL. We quantify the exact impact of ESA andGIA inclusion on grounding line evolution for large ice shelves such as the Ronne and Ross ice shelves, as well asthe Agasea Embayment ice streams, and demonstate how offline vs online RSL simulations diverge in the long run,and the consequences for predictions of sea-level rise.This work was performed at the California Institute of Technology's Jet Propulsion Laboratory undera contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

  4. Comparing a thermo-mechanical Weichselian Ice Sheet reconstruction to reconstructions based on the sea level equation: aspects of ice configurations and glacial isostatic adjustment

    NASA Astrophysics Data System (ADS)

    Schmidt, P.; Lund, B.; Näslund, J.-O.; Fastook, J.

    2014-05-01

    In this study we compare a recent reconstruction of the Weichselian Ice Sheet as simulated by the University of Maine ice sheet model (UMISM) to two reconstructions commonly used in glacial isostatic adjustment (GIA) modelling: ICE-5G and ANU (Australian National University, also known as RSES). The UMISM reconstruction is carried out on a regional scale based on thermo-mechanical modelling, whereas ANU and ICE-5G are global models based on the sea level equation. The three models of the Weichselian Ice Sheet are compared directly in terms of ice volume, extent and thickness, as well as in terms of predicted glacial isostatic adjustment in Fennoscandia. The three reconstructions display significant differences. Whereas UMISM and ANU includes phases of pronounced advance and retreat prior to the last glacial maximum (LGM), the thickness and areal extent of the ICE-5G ice sheet is more or less constant up until the LGM. During the post-LGM deglaciation phase ANU and ICE-5G melt relatively uniformly over the entire ice sheet in contrast to UMISM, which melts preferentially from the edges, thus reflecting the fundamental difference in the reconstruction scheme. We find that all three reconstructions fit the present-day uplift rates over Fennoscandia equally well, albeit with different optimal earth model parameters. Given identical earth models, ICE-5G predicts the fastest present-day uplift rates, and ANU the slowest. Moreover, only for ANU can a unique best-fit model be determined. For UMISM and ICE-5G there is a range of earth models that can reproduce the present-day uplift rates equally well. This is understood from the higher present-day uplift rates predicted by ICE-5G and UMISM, which result in bifurcations in the best-fit upper- and lower-mantle viscosities. We study the areal distributions of present-day residual surface velocities in Fennoscandia and show that all three reconstructions generally over-predict velocities in southwestern Fennoscandia and that

  5. Constraining ice sheet history in the Weddell Sea, West Antarctica, using ice fabric at Korff Ice Rise

    NASA Astrophysics Data System (ADS)

    Brisbourne, A.; Smith, A.; Kendall, J. M.; Baird, A. F.; Martin, C.; Kingslake, J.

    2017-12-01

    The grounding history of ice rises (grounded area of independent flow regime within a floating ice shelf) can be used to constrain large scale ice sheet history: ice fabric, resulting from the preferred orientation of ice crystals due to the stress regime, can be used to infer this grounding history. With the aim of measuring the present day ice fabric at Korff Ice Rise, West Antarctica, a multi-azimuth wide-angle seismic experiment was undertaken. Three wide-angle common-midpoint gathers were acquired centred on the apex of the ice rise, at azimuths of 60 degrees to one another, to measure variation in seismic properties with offset and azimuth. Both vertical and horizontal receivers were used to record P and S arrivals including converted phases. Measurements of the variation with offset and azimuth of seismic traveltimes, seismic attenuation and shear wave splitting have been used to quantify seismic anisotropy in the ice column. The observations cannot be reproduced using an isotropic ice column model. Anisotropic ray tracing has been used to test likely models of ice fabric by comparison with the data. A model with a weak girdle fabric overlying a strong cluster fabric provides the best fit to the observations. Fabric of this nature is consistent with Korff Ice Rise having been stable for the order of 10,000 years without any ungrounding or significant change in the ice flow configuration across the ice rise for this period. This observation has significant implications for the ice sheet history of the Weddell Sea sector.

  6. Rewriting Ice Sheet "Glacier-ology"

    NASA Astrophysics Data System (ADS)

    Bindschadler, R.

    2006-12-01

    The revolution in glaciology driven by the suite of increasingly sophisticated satellite instruments has been no more extreme than in the area of ice dynamics. Years ago, glaciologists were (probably unwittingly) selective in what properties of mountain glaciers were also applied to ice sheets. This reinforced the view that they responded slowly to their environment. Notions of rapid response driven by the ideas of John Mercer, Bill Budd and Terry Hughes were politely rejected by the centrists of mainstream glaciological thought. How the tables have turned--and by the ice sheets themselves, captured in the act of rapidly changing by modern remote sensors! The saw-toothed record of sea-level change over past glacial-interglacial cycles required the existence of rapid ice loss processes. Satellite based observations, supported by hard-earned field observations have extended the time scale over which ice sheets can suddenly change to ever shorter intervals: from centuries, to decades, to years to even minutes. As changes continue to be observed, the scientific community is forced to consider new or previously ignored processes to explain these observations. The penultimate goal of ice-sheet dynamics is to credibly predict the future of both the Greenland and Antarctic ice sheets. In this important endeavor, there is no substitute for our ability to observe. Without the extensive data sets provided by remote sensing, numerical models can be neither tested nor improved. The impact of remote sensing on our existing ability to predict the future must be compared to our probable state of knowledge and ability were these data never collected. Among many satellite observed phenomena we would be largely or wholly ignorant of are the recent acceleration of ice throughout much of coastal Greenland; the sudden disintegration of multiple ice shelves along the Antarctic Peninsula; and the dramatic thinning and acceleration of the Amundsen Sea sector of West Antarctica. These

  7. Inception of the Laurentide Ice Sheet using asynchronous coupling of a regional atmospheric model and an ice model

    NASA Astrophysics Data System (ADS)

    Birch, L.; Cronin, T.; Tziperman, E.

    2017-12-01

    The climate over the past 0.8 million years has been dominated by ice ages. Ice sheets have grown about every 100 kyrs, starting from warm interglacials, until they spanned continents. State-of-the-art global climate models (GCMs) have difficulty simulating glacial inception, or the transition of Earth's climate from an interglacial to a glacial state. It has been suggested that this failure may be related to their poorly resolved local mountain topography, due to their coarse spatial resolution. We examine this idea as well as the possible role of ice flow dynamics missing in GCMs. We investigate the growth of the Laurentide Ice Sheet at 115 kya by focusing on the mountain glaciers of Canada's Baffin Island, where geologic evidence indicates the last inception occurred. We use the Weather Research and Forecasting model (WRF) in a regional, cloud-resolving configuration with resolved mountain terrain to explore how quickly Baffin Island could become glaciated with the favorable yet realizable conditions of 115 kya insolation, cool summers, and wet winters. Using the model-derived mountain glacier mass balance, we force an ice sheet model based on the shallow-ice approximation, capturing the ice flow that may be critical to the spread of ice sheets away from mountain ice caps. The ice sheet model calculates the surface area newly covered by ice and the change in the ice surface elevation, which we then use to run WRF again. Through this type of iterated asynchronous coupling, we investigate how the regional climate responds to both larger areas of ice cover and changes in ice surface elevation. In addition, we use the NOAH-MP Land model to characterize the importance of land processes, like refreezing. We find that initial ice growth on the Penny Ice Cap causes regional cooling that increases the accumulation on the Barnes Ice Cap. We investigate how ice and topography changes on Baffin Island may impact both the regional climate and the large-scale circulation.

  8. Evolution of a Greenland Ice sheet Including Shelves and Regional Sea Level Variations

    NASA Astrophysics Data System (ADS)

    Bradley, Sarah; Reerink, Thomas; van de Wal, Roderik S. W.; Helsen, Michiel; Goelzer, Heiko

    2016-04-01

    Observational evidence, including offshore moraines and marine sediment cores infer that at the Last Glacial maximum (LGM) the Greenland ice sheet (GIS) grounded out across the Davis Strait into Baffin Bay, with fast flowing ice streams extending out to the continental shelf break along the NW margin. These observations lead to a number of questions as to weather the GIS and Laurentide ice sheet (LIS) coalesced during glacial maximums, and if so, did a significant ice shelf develop across Baffin Bay and how would such a configuration impact on the relative contribution of these ice sheets to eustatic sea level (ESL). Most previous paleo ice sheet modelling simulations of the GIS recreated an ice sheet that either did not extend out onto the continental shelf or utilised a simplified marine ice parameterisation to recreate an extended GIS, and therefore did not fully include ice shelf dynamics. In this study we simulate the evolution of the GIS from 220 kyr BP to present day using IMAU-ice; a 3D thermodynamical ice sheet model which fully accounts for grounded and floating ice, calculates grounding line migration and ice shelf dynamics. As there are few observational estimates of the long-term (yrs) sub marine basal melting rates (mbm) for the GIS, we developed a mbm parameterization within IMAU-ice controlled primarily by changes in paleo water depth. We also investigate the influence of the LIS on the GIS evolution by including relative sea level forcing's derived from a Glacial Isostatic Adjustment model. We will present results of how changes in the mbm directly impacts on the ice sheet dynamics, timing and spatial extent of the GIS at the glacial maximums, but also on the rate of retreat and spatial extent at the Last interglacial (LIG) minimum. Results indicate that with the inclusion of ice shelf dynamics, a larger GIS is generated which is grounded out into Davis strait, up to a water depth of -750 m, but significantly reduces the GIS contribution to Last

  9. Evolution of a Greenland Ice sheet Including Shelves and Regional Sea Level Variations

    NASA Astrophysics Data System (ADS)

    Bradley, S.; Reerink, T.; Vandewal, R.; Helsen, M.

    2015-12-01

    Observational evidence, including offshore moraines and marine sediment cores infer that at the Last Glacial maximum (LGM) the Greenland ice sheet (GIS) grounded out across the Davis Strait into Baffin Bay, with fast flowing ice streams extending out to the continental shelf break along the NW margin. These observations lead to a number of questions as to weather the GIS and Laurentide ice sheet (LIS) coalesced during glacial maximums, and if so, did a significant ice shelf develop across Baffin Bay and how would such a configuration impact on the relative contribution of these ice sheets to eustatic sea level (ESL). Most previous paleo ice sheet modelling simulations of the GIS recreated an ice sheet that either did not extend out onto the continental shelf or utilised a simplified marine ice parameterisation to recreate an extended GIS, and therefore did not fully include ice shelf dynamics. In this study we simulate the evolution of the GIS from 220 kyr BP to present day using IMAU-ice; a 3D thermodynamical ice sheet model which fully accounts for grounded and floating ice, calculates grounding line migration and ice shelf dynamics. There is few observational estimates of long-term (yrs) sub marine basal melting rates (mbm) for the GIS. Therefore we investigate a range of relationships to constrain the spatial and temporal parameterisation of mbm within IMAU-ice related to changes in paleo water depth, driven by changes in relative sea level and ocean temperature. We will present results of how changes in the mbm directly impacts on the ice sheet dynamics, timing and spatial extent of the GIS at the glacial maximums, but also on the rate of retreat and spatial extent at the Last interglacial (LIG) minimum. Initial results indicate that with the inclusion of ice shelf dynamics, a larger GIS is generated which is grounded out into Davis strait, up to a water depth of -750 m, but the total contribution to LIG ESL is reduced by up to 0.6 m.

  10. Ice-sheet response to oceanic forcing.

    PubMed

    Joughin, Ian; Alley, Richard B; Holland, David M

    2012-11-30

    The ice sheets of Greenland and Antarctica are losing ice at accelerating rates, much of which is a response to oceanic forcing, especially of the floating ice shelves. Recent observations establish a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss. In Antarctica, most of these processes are reasonably well understood but have not been rigorously quantified. In Greenland, an understanding of the processes by which warmer ocean temperatures drive the observed retreat remains elusive. Experiments designed to identify the relevant processes are confounded by the logistical difficulties of instrumenting ice-choked fjords with actively calving glaciers. For both ice sheets, multiple challenges remain before the fully coupled ice-ocean-atmosphere models needed for rigorous sea-level projection are available.

  11. High-resolution coupled ice sheet-ocean modeling using the POPSICLES model

    NASA Astrophysics Data System (ADS)

    Ng, E. G.; Martin, D. F.; Asay-Davis, X.; Price, S. F.; Collins, W.

    2014-12-01

    It is expected that a primary driver of future change of the Antarctic ice sheet will be changes in submarine melting driven by incursions of warm ocean water into sub-ice shelf cavities. Correctly modeling this response on a continental scale will require high-resolution modeling of the coupled ice-ocean system. We describe the computational and modeling challenges in our simulations of the full Southern Ocean coupled to a continental-scale Antarctic ice sheet model at unprecedented spatial resolutions (0.1 degree for the ocean model and adaptive mesh refinement down to 500m in the ice sheet model). The POPSICLES model couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), with the BISICLES ice-sheet model (Cornford et al., 2012) using a synchronous offline-coupling scheme. Part of the PISCEES SciDAC project and built on the Chombo framework, BISICLES makes use of adaptive mesh refinement to fully resolve dynamically-important regions like grounding lines and employs a momentum balance similar to the vertically-integrated formulation of Schoof and Hindmarsh (2009). Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests like MISMIP3D (Pattyn et al., 2013) and realistic configurations (Favier et al. 2014). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). For the POPSICLES Antarctic-Southern Ocean simulations, ice sheet and ocean models communicate at one-month coupling intervals.

  12. Balance of the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    2002-01-01

    For several decades, measurements of the West Antarctic Ice Sheet showed it to be retreating rapidly. But new data derived from satellite-borne radar sensors show the ice sheet to be growing. Changing Antarctic ice sheets remains an area of high scientific interest, particularly in light of recent global warming concerns. These new findings are significant because scientists estimate that sea level would rise 5-6 meters (16-20 feet) if the ice sheet collapsed into the sea. Do these new measurements signal the end of the ice sheet's 10,000-year retreat? Or, are these new satellite data simply much more accurate than the sparse ice core and surface measurements that produced the previous estimates? Another possibility is that the ice accumulation may simply indicate that the ice sheet naturally expands and retreats in regular cycles. Cryologists will grapple with these questions, and many others, as they examine the new data. The image above depicts the region of West Antarctica where scientists measured ice speed. The fast-moving central ice streams are shown in red. Slower tributaries feeding the ice streams are shown in blue. Green areas depict slow-moving, stable areas. Thick black lines depict the areas that collect snowfall to feed their respective ice streams. Reference: Ian Joughin and Slawek Tulaczyk Science Jan 18 2002: 476-480. Image courtesy RADARSAT Antarctic Mapping Project

  13. Clouds enhance Greenland ice sheet meltwater runoff

    PubMed Central

    Van Tricht, K.; Lhermitte, S.; Lenaerts, J. T. M.; Gorodetskaya, I. V.; L'Ecuyer, T. S.; Noël, B.; van den Broeke, M. R.; Turner, D. D.; van Lipzig, N. P. M.

    2016-01-01

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m−2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise. PMID:26756470

  14. Clouds enhance Greenland ice sheet meltwater runoff.

    PubMed

    Van Tricht, K; Lhermitte, S; Lenaerts, J T M; Gorodetskaya, I V; L'Ecuyer, T S; Noël, B; van den Broeke, M R; Turner, D D; van Lipzig, N P M

    2016-01-12

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m(-2). Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

  15. History of the Greenland Ice Sheet: paleoclimatic insights

    USGS Publications Warehouse

    Alley, Richard B.; Andrews, John T.; Brigham-Grette, J.; Clarke, G.K.C.; Cuffey, Kurt M.; Fitzpatrick, J.J.; Funder, S.; Marshall, S.J.; Miller, G.H.; Mitrovica, J.X.; Muhs, D.R.; Otto-Bliesner, B. L.; Polyak, L.; White, J.W.C.

    2010-01-01

    Paleoclimatic records show that the GreenlandIce Sheet consistently has lost mass in response to warming, and grown in response to cooling. Such changes have occurred even at times of slow or zero sea-level change, so changing sea level cannot have been the cause of at least some of the ice-sheet changes. In contrast, there are no documented major ice-sheet changes that occurred independent of temperature changes. Moreover, snowfall has increased when the climate warmed, but the ice sheet lost mass nonetheless; increased accumulation in the ice sheet's center has not been sufficient to counteract increased melting and flow near the edges. Most documented forcings and ice-sheet responses spanned periods of several thousand years, but limited data also show rapid response to rapid forcings. In particular, regions near the ice margin have responded within decades. However, major changes of central regions of the ice sheet are thought to require centuries to millennia. The paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2 °C or more than 7 °C). Paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming, or changes may have been induced by factors other than temperature, without having been recorded.

  16. Capabilities and performance of Elmer/Ice, a new generation ice-sheet model

    NASA Astrophysics Data System (ADS)

    Gagliardini, O.; Zwinger, T.; Gillet-Chaulet, F.; Durand, G.; Favier, L.; de Fleurian, B.; Greve, R.; Malinen, M.; Martín, C.; Råback, P.; Ruokolainen, J.; Sacchettini, M.; Schäfer, M.; Seddik, H.; Thies, J.

    2013-03-01

    The Fourth IPCC Assessment Report concluded that ice-sheet flow models are unable to forecast the current increase of polar ice sheet discharge and the associated contribution to sea-level rise. Since then, the glaciological community has undertaken a huge effort to develop and improve a new generation of ice-flow models, and as a result, a significant number of new ice-sheet models have emerged. Among them is the parallel finite-element model Elmer/Ice, based on the open-source multi-physics code Elmer. It was one of the first full-Stokes models used to make projections for the evolution of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger scale problems, earning the status of an ice-sheet model. Here, we summarise almost 10 yr of development performed by different groups. We present the components already included in Elmer/Ice, its numerical performance, selected applications, as well as developments planned for the future.

  17. Potential Climatic Effects on the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Bindschadler, R. A.

    1984-01-01

    The Greenland Ice Sheet covers an area of 1,720,000 sq. km and contains approximately 2,600,000 cu km of ice. Most of the ice sheet receives an excess of snow accumulation over the amount of ice lost to wind, meltwater run-off or other ablative processes. The majority of mass loss occurs at the margin of the ice sheet as either surface melt, which flows into the sea or calving of icebergs from the tongues of outlet glaciers. Many estimates of these processes were published. An average of five published estimates is summarized. If these estimates are correct, then the Greenland Ice Sheet is in approximate equilibrium and contributes 490 cu km/a of fresh water to the North Atlantic and Arctic Oceans. Climate effects, ice sheet flow, and application of remote sensing to tracking of the ice sheet are discussed.

  18. Role of ice sheet dynamics in the collapse of the early-Holocene Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Matero, I. S. O.; Gregoire, L. J.; Cornford, S. L.; Ivanovic, R. F.

    2017-12-01

    The last stage of the deglaciation of the Laurentide Ice Sheet (LIS) during the early Holocene Thermal Maximum ( 9000 to 7000 years ago) provides an analogy and insight to the possible responses of contemporary ice sheets in a warming climate. What makes LIS particularly interesting is that meltwater from the collapse of an ice saddle over Hudson Bay was recently shown to be the primary forcing for the period of abrupt northern hemisphere cooling known as the 8.2 ka event. The evolution of the LIS during this period was likely influenced by its interaction with marginal lakes and the ocean, and its major ice stream, which exported ice towards Hudson Strait. Accurately simulating the early Holocene LIS evolution thus requires a model such as BISICLES, capable of accurately and efficiently resolving ice stream dynamics and grounding line migration thanks to the combined use of higher order physics and adaptive mesh refinement. We drive the BISICLES model using a positive degree day mass balance scheme with monthly precipitation and temperature from the HadCM3 climate model under climatic conditions from 10,000 to 8,000 years ago. We test the effect of varying the initial topographies and ice thicknesses from different timeslices in the ICE-6Gc reconstruction. We also test different parameterisations for the basal friction based on the thicknesses of the underlying sediments. These simulations evaluate the role of the Hudson Strait ice stream, ice sheet dynamics and interactions with the adjacent proglacial Lake Agassiz and North Atlantic Ocean in the collapse of the LIS. Our results highlight that the choice of parameterisation for basal friction has major effects on ice sheet dynamics and evolution.

  19. Capabilities and performance of the new generation ice-sheet model Elmer/Ice

    NASA Astrophysics Data System (ADS)

    Gagliardini, O.; Zwinger, T.; Durand, G.; Favier, L.; de Fleurian, B.; Gillet-chaulet, F.; Seddik, H.; Greve, R.; Mallinen, M.; Martin, C.; Raback, P.; Ruokolainen, J.; Schäfer, M.; Thies, J.

    2012-12-01

    Since the Fourth IPCC Assessment Report, and its conclusion about the inability of ice-sheet flow models to forecast the current increase of polar ice sheet discharge and associated contribution to sea-level rise, a huge development effort has been undertaken by the glaciological community. All around the world, models have been improved and, interestingly, a significant number of new ice-sheet models have emerged. Among them, the parallel finite-element model Elmer/Ice (based on the open-source multi-physics code Elmer) was one of the first full-Stokes models used to make projections of the future of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve dedicated local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger scale problems, earning the status of an ice-sheet model. In this presentation, we summarise the almost 10 years of development performed by different groups. We present the components already included in Elmer/Ice, its numerical performance, selected applications, as well as developments planed for the future.

  20. Holocene deceleration of the Greenland Ice Sheet.

    PubMed

    MacGregor, Joseph A; Colgan, William T; Fahnestock, Mark A; Morlighem, Mathieu; Catania, Ginny A; Paden, John D; Gogineni, S Prasad

    2016-02-05

    Recent peripheral thinning of the Greenland Ice Sheet is partly offset by interior thickening and is overprinted on its poorly constrained Holocene evolution. On the basis of the ice sheet's radiostratigraphy, ice flow in its interior is slower now than the average speed over the past nine millennia. Generally higher Holocene accumulation rates relative to modern estimates can only partially explain this millennial-scale deceleration. The ice sheet's dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across Nares Strait also contributed to this pattern. Thus, recent interior thickening of the Greenland Ice Sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry. Copyright © 2016, American Association for the Advancement of Science.

  1. Evaluation of Ice sheet evolution and coastline changes from 1960s in Amery Ice Shelf using multi-source remote sensing images

    NASA Astrophysics Data System (ADS)

    Qiao, G.; Ye, W.; Scaioni, M.; Liu, S.; Feng, T.; Liu, Y.; Tong, X.; Li, R.

    2013-12-01

    Global change is one of the major challenges that all the nations are commonly facing, and the Antarctica ice sheet changes have been playing a critical role in the global change research field during the past years. Long time-series of ice sheet observations in Antarctica would contribute to the quantitative evaluation and precise prediction of the effects on global change induced by the ice sheet, of which the remote sensing technology would make critical contributions. As the biggest ice shelf and one of the dominant drainage systems in East Antarctic, the Amery Ice Shelf has been making significant contributions to the mass balance of the Antarctic. Study of Amery Ice shelf changes would advance the understanding of Antarctic ice shelf evolution as well as the overall mass balance. At the same time, as one of the important indicators of Antarctica ice sheet characteristics, coastlines that can be detected from remote sensing imagery can help reveal the nature of the changes of ice sheet evolution. Most of the scientific research on Antarctica with satellite remote sensing dated from 1970s after LANDSAT satellite was brought into operation. It was the declassification of the cold war satellite reconnaissance photographs in 1995, known as Declassified Intelligence Satellite Photograph (DISP) that provided a direct overall view of the Antarctica ice-sheet's configuration in 1960s, greatly extending the time span of Antarctica surface observations. This paper will present the evaluation of ice-sheet evolution and coastline changes in Amery Ice Shelf from 1960s, by using multi-source remote sensing images including the DISP images and the modern optical satellite images. The DISP images scanned from negatives were first interior-oriented with the associated parameters, and then bundle block adjustment technology was employed based on the tie points and control points, to derive the mosaic image of the research region. Experimental results of coastlines generated

  2. Simulation of the Greenland Ice Sheet over two glacial-interglacial cycles: investigating a sub-ice-shelf melt parameterization and relative sea level forcing in an ice-sheet-ice-shelf model

    NASA Astrophysics Data System (ADS)

    Bradley, Sarah L.; Reerink, Thomas J.; van de Wal, Roderik S. W.; Helsen, Michiel M.

    2018-05-01

    Observational evidence, including offshore moraines and sediment cores, confirm that at the Last Glacial Maximum (LGM) the Greenland ice sheet (GrIS) expanded to a significantly larger spatial extent than seen at present, grounding into Baffin Bay and out onto the continental shelf break. Given this larger spatial extent and its close proximity to the neighbouring Laurentide Ice Sheet (LIS) and Innuitian Ice Sheet (IIS), it is likely these ice sheets will have had a strong non-local influence on the spatial and temporal behaviour of the GrIS. Most previous paleo ice-sheet modelling simulations recreated an ice sheet that either did not extend out onto the continental shelf or utilized a simplified marine ice parameterization which did not fully include the effect of ice shelves or neglected the sensitivity of the GrIS to this non-local bedrock signal from the surrounding ice sheets. In this paper, we investigated the evolution of the GrIS over the two most recent glacial-interglacial cycles (240 ka BP to the present day) using the ice-sheet-ice-shelf model IMAU-ICE. We investigated the solid earth influence of the LIS and IIS via an offline relative sea level (RSL) forcing generated by a glacial isostatic adjustment (GIA) model. The RSL forcing governed the spatial and temporal pattern of sub-ice-shelf melting via changes in the water depth below the ice shelves. In the ensemble of simulations, at the glacial maximums, the GrIS coalesced with the IIS to the north and expanded to the continental shelf break to the southwest but remained too restricted to the northeast. In terms of the global mean sea level contribution, at the Last Interglacial (LIG) and LGM the ice sheet added 1.46 and -2.59 m, respectively. This LGM contribution by the GrIS is considerably higher (˜ 1.26 m) than most previous studies whereas the contribution to the LIG highstand is lower (˜ 0.7 m). The spatial and temporal behaviour of the northern margin was highly variable in all simulations

  3. Ice sheet topography by satellite altimetry

    USGS Publications Warehouse

    Brooks, R.L.; Campbell, W.J.; Ramseier, R.O.; Stanley, H.R.; Zwally, H.J.

    1978-01-01

    The surface elevation of the southern Greenland ice sheet and surface features of the ice flow are obtained from the radar altimeter on the GEOS 3 satellite. The achieved accuracy in surface elevation is ???2 m. As changes in surface elevation are indicative of changes in ice volume, the mass balance of the present ice sheets could be determined by repetitive mapping of the surface elevation and the surface could be monitored to detect surging or significant changes in ice flow. ?? 1978 Nature Publishing Group.

  4. Enhanced ice sheet growth in Eurasia owing to adjacent ice-dammed lakes.

    PubMed

    Krinner, G; Mangerud, J; Jakobsson, M; Crucifix, M; Ritz, C; Svendsen, J I

    2004-01-29

    Large proglacial lakes cool regional summer climate because of their large heat capacity, and have been shown to modify precipitation through mesoscale atmospheric feedbacks, as in the case of Lake Agassiz. Several large ice-dammed lakes, with a combined area twice that of the Caspian Sea, were formed in northern Eurasia about 90,000 years ago, during the last glacial period when an ice sheet centred over the Barents and Kara seas blocked the large northbound Russian rivers. Here we present high-resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet. We show that the main influence of the Eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the Barents-Kara ice sheet through strong regional summer cooling over large parts of Russia. In our simulations, the summer melt reduction clearly outweighs lake-induced decreases in moisture and hence snowfall, such as has been reported earlier for Lake Agassiz. We conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in Eurasia and probably also in North America.

  5. The Response of Ice Sheets to Climate Variability

    NASA Astrophysics Data System (ADS)

    Snow, K.; Goldberg, D. N.; Holland, P. R.; Jordan, J. R.; Arthern, R. J.; Jenkins, A.

    2017-12-01

    West Antarctic Ice Sheet loss is a significant contributor to sea level rise. While the ice loss is thought to be triggered by fluctuations in oceanic heat at the ice shelf bases, ice sheet response to ocean variability remains poorly understood. Using a synchronously coupled ice-ocean model permitting grounding line migration, this study evaluates the response of an ice sheet to periodic variations in ocean forcing. Resulting oscillations in grounded ice volume amplitude is shown to grow as a nonlinear function of ocean forcing period. This implies that slower oscillations in climatic forcing are disproportionately important to ice sheets. The ice shelf residence time offers a critical time scale, above which the ice response amplitude is a linear function of ocean forcing period and below which it is quadratic. These results highlight the sensitivity of West Antarctic ice streams to perturbations in heat fluxes occurring at decadal time scales.

  6. Antarctic ice-sheet loss driven by basal melting of ice shelves.

    PubMed

    Pritchard, H D; Ligtenberg, S R M; Fricker, H A; Vaughan, D G; van den Broeke, M R; Padman, L

    2012-04-25

    Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.

  7. Extensive massive basal-ice structures in West Antarctica relate to ice-sheet anisotropy and ice-flow

    NASA Astrophysics Data System (ADS)

    Ross, N.; Bingham, R. G.; Corr, H. F. J.; Siegert, M. J.

    2016-12-01

    Complex structures identified within both the East Antarctic and Greenland ice sheets are thought to be generated by the action of basal water freezing to the ice-sheet base, evolving under ice flow. Here, we use ice-penetrating radar to image an extensive series of similarly complex basal ice facies in West Antarctica, revealing a thick (>500 m) tectonised unit in an area of cold-based and relatively slow-flowing ice. We show that major folding and overturning of the unit perpendicular to ice flow elevates deep, warm ice into the mid ice-sheet column. Fold axes align with present ice flow, and axis amplitudes increase down-ice, suggesting long-term consistency in the direction and convergence of flow. In the absence of basal water, and the draping of the tectonised unit over major subglacial mountain ranges, the formation of the unit must be solely through the deformation of meteoric ice. Internal layer radar reflectivity is consistently greater parallel to flow compared with the perpendicular direction, revealing ice-sheet crystal anisotropy is associated with the folding. By linking layers to the Byrd ice-core site, we show the basal ice dates to at least the last glacial cycle and may be as old as the last interglacial. Deformation of deep-ice in this sector of WAIS, and potentially elsewhere in Antarctica, may be caused by differential shearing at interglacial-glacial boundaries, in a process analogous to that proposed for interior Greenland. The scale and heterogeneity of the englacial structures, and their subsequent impact on ice sheet rheology, means that the nature of ice flow across the bulk of West Antarctica must be far more complex that is currently accounted for by any numerical ice sheet model.

  8. The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet

    NASA Astrophysics Data System (ADS)

    Martin, M. A.; Winkelmann, R.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

    2010-08-01

    We present a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). The simulation is initialized with present-day conditions for topography and ice thickness and then run to steady state with constant present-day surface mass balance. Surface temperature and basal melt distribution are parameterized. Grounding lines and calving fronts are free to evolve, and their modeled equilibrium state is compared to observational data. A physically-motivated dynamic calving law based on horizontal spreading rates allows for realistic calving fronts for various types of shelves. Steady-state dynamics including surface velocity and ice flux are analyzed for whole Antarctica and the Ronne-Filchner and Ross ice shelf areas in particular. The results show that the different flow regimes in sheet and shelves, and the transition zone between them, are captured reasonably well, supporting the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice. This approach also leads to a natural emergence of streams in this new 3-D marine ice sheet model.

  9. Programme for Monitoring of the Greenland Ice Sheet - Ice Surface Velocities

    NASA Astrophysics Data System (ADS)

    Andersen, S. B.; Ahlstrom, A. P.; Boncori, J. M.; Dall, J.

    2011-12-01

    In 2007, the Danish Ministry of Climate and Energy launched the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) as an ongoing effort to assess changes in the mass budget of the Greenland Ice Sheet. Iceberg calving from the outlet glaciers of the Greenland Ice Sheet, often termed the ice-dynamic mass loss, is responsible for an important part of the mass loss during the last decade. To quantify this part of the mass loss, we combine airborne surveys yielding ice-sheet thickness along the entire margin, with surface velocities derived from satellite synthetic-aperture radar (SAR). In order to derive ice sheet surface velocities from SAR a processing chain has been developed for GEUS by DTU Space based on a commercial software package distributed by GAMMA Remote Sensing. The processor, named SUSIE (Scripts and Utilities for SAR Ice-motion Estimation), can use both differential SAR interferometry and offset-tracking techniques to measure the horizontal velocity components, providing also an estimate of the corresponding measurement error. So far surface velocities have been derived for a number of sites including Nioghalvfjerdsfjord Glacier, the Kangerlussuaq region, the Nuuk region, Helheim Glacier and Daugaard-Jensen Glacier using data from ERS-1/ERS-2, ENVISAT ASAR and ALOS Palsar. Here we will present these first results.

  10. Probability based hydrologic catchments of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hudson, B. D.

    2015-12-01

    Greenland Ice Sheet melt water impacts ice sheet flow dynamics, fjord and coastal circulation, and sediment and biogeochemical fluxes. Melt water exiting the ice sheet also is a key term in its mass balance. Because of this, knowledge of the area of the ice sheet that contributes melt water to a given outlet (its hydrologic catchment) is important to many ice sheet studies and is especially critical to methods using river runoff to assess ice sheet mass balance. Yet uncertainty in delineating ice sheet hydrologic catchments is a problem that is rarely acknowledged. Ice sheet catchments are delineated as a function of both basal and surface topography. While surface topography is well known, basal topography is less certain because it is dependent on radar surveys. Here, I a present a Monte Carlo based approach to delineating ice sheet catchments that quantifies the impact of uncertain basal topography. In this scheme, over many iterations I randomly vary the ice sheet bed elevation within published error bounds (using Morlighem et al., 2014 bed and bed error datasets). For each iteration of ice sheet bed elevation, I calculate the hydraulic potentiometric surface and route water over its path of 'steepest' descent to delineate the catchment. I then use all realizations of the catchment to arrive at a probability map of all major melt water outlets in Greenland. I often find that catchment size is uncertain, with small, random perturbations in basal topography leading to large variations in catchments size. While some catchments are well defined, others can double or halve in size within published basal topography error bars. While some uncertainty will likely always remain, this work points to locations where studies of ice sheet hydrology would be the most successful, allows reinterpretation of past results, and points to where future radar surveys would be most advantageous.

  11. Ice Sheet Roughness Estimation Based on Impulse Responses Acquired in the Global Ice Sheet Mapping Orbiter Mission

    NASA Astrophysics Data System (ADS)

    Niamsuwan, N.; Johnson, J. T.; Jezek, K. C.; Gogineni, P.

    2008-12-01

    The Global Ice Sheet Mapping Orbiter (GISMO) mission was developed to address scientific needs to understand the polar ice subsurface structure. This NASA Instrument Incubator Program project is a collaboration between Ohio State University, the University of Kansas, Vexcel Corporation and NASA. The GISMO design utilizes an interferometric SAR (InSAR) strategy in which ice sheet reflected signals received by a dual-antenna system are used to produce an interference pattern. The resulting interferogram can be used to filter out surface clutter so as to reveal the signals scattered from the base of the ice sheet. These signals are further processed to produce 3D-images representing basal topography of the ice sheet. In the past three years, the GISMO airborne field campaigns that have been conducted provide a set of useful data for studying geophysical properties of the Greenland ice sheet. While topography information can be obtained using interferometric SAR processing techniques, ice sheet roughness statistics can also be derived by a relatively simple procedure that involves analyzing power levels and the shape of the radar impulse response waveforms. An electromagnetic scattering model describing GISMO impulse responses has previously been proposed and validated. This model suggested that rms-heights and correlation lengths of the upper surface profile can be determined from the peak power and the decay rate of the pulse return waveform, respectively. This presentation will demonstrate a procedure for estimating the roughness of ice surfaces by fitting the GISMO impulse response model to retrieved waveforms from selected GISMO flights. Furthermore, an extension of this procedure to estimate the scattering coefficient of the glacier bed will be addressed as well. Planned future applications involving the classification of glacier bed conditions based on the derived scattering coefficients will also be described.

  12. Algae Drive Enhanced Darkening of Bare Ice on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Stibal, Marek; Box, Jason E.; Cameron, Karen A.; Langen, Peter L.; Yallop, Marian L.; Mottram, Ruth H.; Khan, Alia L.; Molotch, Noah P.; Chrismas, Nathan A. M.; Calı Quaglia, Filippo; Remias, Daniel; Smeets, C. J. P. Paul; van den Broeke, Michiel R.; Ryan, Jonathan C.; Hubbard, Alun; Tranter, Martyn; van As, Dirk; Ahlstrøm, Andreas P.

    2017-11-01

    Surface ablation of the Greenland ice sheet is amplified by surface darkening caused by light-absorbing impurities such as mineral dust, black carbon, and pigmented microbial cells. We present the first quantitative assessment of the microbial contribution to the ice sheet surface darkening, based on field measurements of surface reflectance and concentrations of light-absorbing impurities, including pigmented algae, during the 2014 melt season in the southwestern part of the ice sheet. The impact of algae on bare ice darkening in the study area was greater than that of nonalgal impurities and yielded a net albedo reduction of 0.038 ± 0.0035 for each algal population doubling. We argue that algal growth is a crucial control of bare ice darkening, and incorporating the algal darkening effect will improve mass balance and sea level projections of the Greenland ice sheet and ice masses elsewhere.

  13. Ice sheets play important role in climate change

    NASA Astrophysics Data System (ADS)

    Clark, Peter U.; MacAyeal, Douglas R.; Andrews, John T.; Bartlein, Patrick J.

    Ice sheets once were viewed as passive elements in the climate system enslaved to orbitally generated variations in solar radiation. Today, modeling results and new geologic records suggest that ice sheets actively participated in late-Pleistocene climate change, amplifying or driving significant variability at millennial as well as orbital timescales. Although large changes in global ice volume were ultimately caused by orbital variations (the Milankovitch hypothesis), once in existence, the former ice sheets behaved dynamically and strongly influenced regional and perhaps even global climate by altering atmospheric and oceanic circulation and temperature.Experiments with General Circulation Models (GCMs) yielded the first inklings of ice sheets' climatic significance. Manabe and Broccoli [1985], for example, found that the topographic and albedo effects of ice sheets alone explain much of the Northern Hemisphere cooling identified in paleoclimatic records of the last glacial maximum (˜21 ka).

  14. Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Weber, M. E.; Clark, P. U.; Ricken, W.; Mitrovica, J. X.; Hostetler, S. W.; Kuhn, G.

    2012-04-01

    The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood because only a few findings with robust chronologies exist for Antarctic ice sheets. We developed a chronology for the Weddell Sea sector of the East Antarctic ice sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates the advance to their maximum extent at 29 -28 ka, and retreat from their maximum extent at 19 ka was nearly synchronous with Northern Hemisphere ice sheets (Weber, M.E., Clark, P. U., Ricken, W., Mitrovica, J. X., Hostetler, S. W., and Kuhn, G. (2011): Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum. - Science, 334, 1265-1269, doi: 10.1126:science.1209299). As for the deglaciation, modeling studies suggest a late ice-sheet retreat starting around 14 ka BP and ending around 7 ka BP with a large impact of an unstable West Antarctic Ice Sheet (WAIS) and a small impact of a stable East Antarctic Ice Sheet (EAIS). However, the Weddell Sea sites studied here, as well as sites from the Scotia Sea, provide evidence that specifically the EAIS responded much earlier, possibly provided a significant contribution to the last sea-level rise, and was much more dynamic than previously thought. Using the results of an atmospheric general circulation we conclude that surface climate forcing of Antarctic ice mass balance would likely cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Furthermore, our new data support teleconnections involving a sea-level fingerprint forced from Northern Hemisphere ice sheets as indicated by gravitational modeling. Also, changes in North Atlantic Deepwater formation and attendant heat flux to Antarctic grounding lines may have contributed to synchronizing the hemispheric ice sheets.

  15. The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet

    NASA Astrophysics Data System (ADS)

    Martin, M. A.; Winkelmann, R.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

    2011-09-01

    We present a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). The simulation is initialized with present-day conditions for bed topography and ice thickness and then run to steady state with constant present-day surface mass balance. Surface temperature and sub-shelf basal melt distribution are parameterized. Grounding lines and calving fronts are free to evolve, and their modeled equilibrium state is compared to observational data. A physically-motivated calving law based on horizontal spreading rates allows for realistic calving fronts for various types of shelves. Steady-state dynamics including surface velocity and ice flux are analyzed for whole Antarctica and the Ronne-Filchner and Ross ice shelf areas in particular. The results show that the different flow regimes in sheet and shelves, and the transition zone between them, are captured reasonably well, supporting the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice. This approach also leads to a natural emergence of sliding-dominated flow in stream-like features in this new 3-D marine ice sheet model.

  16. Greenland ice sheet motion insensitive to exceptional meltwater forcing.

    PubMed

    Tedstone, Andrew J; Nienow, Peter W; Sole, Andrew J; Mair, Douglas W F; Cowton, Thomas R; Bartholomew, Ian D; King, Matt A

    2013-12-03

    Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt-induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt-induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ~3.9 σ above the 1958-2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt-induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.

  17. Ice sheets on plastically-yielding beds

    NASA Astrophysics Data System (ADS)

    Hewitt, Ian

    2016-11-01

    Many fast flowing regions of ice sheets are underlain by a layer of water-saturated sediments, or till. The rheology of the till has been the subject of some controversy, with laboratory tests suggesting almost perfectly plastic behaviour (stress independent of strain rate), but many models adopting a pseudo-viscous description. In this work, we consider the behaviour of glaciers underlain by a plastic bed. The ice is treated as a viscous gravity current, on a bed that allows unconstrained slip above a critical yield stress. This simplified description allows rapid sliding, and aims to investigate 'worst-case' scenarios of possible ice-sheet disintegration. The plastic bed results in an approximate ice-sheet geometry that is primarily controlled by force balance, whilst ice velocity is determined from mass conservation (rather than the other way around, as standard models would hold). The stability of various states is considered, and particular attention is given to the pace at which transitions between unstable states can occur. Finally, we observe that the strength of basal tills depends strongly on pore pressure, and combine the model with a description of subglacial hydrology. Implications for the present-day ice sheets in Greenland and Antarctica will be discussed. Funding: ERC Marie Curie FP7 Career Integration Grant.

  18. Interhemispheric ice-sheet synchronicity during the last glacial maximum

    USGS Publications Warehouse

    Weber, Michael E.; Clark, Peter U.; Ricken, Werner; Mitrovica, Jerry X.; Hostetler, Steven W.; Kuhn, Gerhard

    2011-01-01

    The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood. We develop a chronology for the Weddell Sea sector of the East Antarctic Ice Sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of Northern Hemisphere ice sheets. Surface climate forcing of Antarctic mass balance would probably cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Our new data support teleconnections involving sea-level forcing from Northern Hemisphere ice sheets and changes in North Atlantic deep-water formation and attendant heat flux to Antarctic grounding lines to synchronize the hemispheric ice sheets.

  19. Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum.

    PubMed

    Weber, Michael E; Clark, Peter U; Ricken, Werner; Mitrovica, Jerry X; Hostetler, Steven W; Kuhn, Gerhard

    2011-12-02

    The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood. We develop a chronology for the Weddell Sea sector of the East Antarctic Ice Sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of Northern Hemisphere ice sheets. Surface climate forcing of Antarctic mass balance would probably cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Our new data support teleconnections involving sea-level forcing from Northern Hemisphere ice sheets and changes in North Atlantic deep-water formation and attendant heat flux to Antarctic grounding lines to synchronize the hemispheric ice sheets.

  20. Using the glacial geomorphology of palaeo-ice streams to understand mechanisms of ice sheet collapse

    NASA Astrophysics Data System (ADS)

    Stokes, Chris R.; Margold, Martin; Clark, Chris; Tarasov, Lev

    2017-04-01

    Processes which bring about ice sheet deglaciation are critical to our understanding of glacial-interglacial cycles and ice sheet sensitivity to climate change. The precise mechanisms of deglaciation are also relevant to our understanding of modern-day ice sheet stability and concerns over global sea level rise. Mass loss from ice sheets can be broadly partitioned between melting and a 'dynamic' component whereby rapidly-flowing ice streams/outlet glaciers transfer ice from the interior to the oceans. Surface and basal melting (e.g. of ice shelves) are closely linked to atmospheric and oceanic conditions, but the mechanisms that drive dynamic changes in ice stream discharge are more complex, which generates much larger uncertainties about their future contribution to ice sheet mass loss and sea level rise. A major problem is that observations of modern-day ice streams typically span just a few decades and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves during deglaciation. A key question is whether ice streams might increase and sustain rates of mass loss over centuries or millennia, beyond those expected for a given ocean-climate forcing. To address this issue, numerous workers have sought to understand ice stream dynamics over longer time-scales using their glacial geomorphology in the palaeo-record. Indeed, our understanding of their geomorphology has grown rapidly in the last three decades, from almost complete ignorance to a detailed knowledge of their geomorphological products. Building on this body of work, this paper uses the glacial geomorphology of 117 ice streams in the North American Laurentide Ice Sheet to reconstruct their activity during its deglaciation ( 22,000 to 7,000 years ago). Ice stream activity was characterised by high variability in both time and space, with ice streams switching on and off in different locations. During deglaciation, we find that their overall number decreased, they occupied a

  1. Challenges faced by ice sheet projections: lessons from the SeaRISE effort

    NASA Astrophysics Data System (ADS)

    Nowicki, S.

    2013-12-01

    Projecting the future evolution of the Greenland and Antarctic ice sheets is a problem of enormous societal importance, as ice sheet influence our future sea levels. This crucial issue is however a non trivial task, as demonstrated by the Sea level Response to Ice Sheet Evolution (SeaRISE) effort: prescribing simple external forcings to a group of ice sheet models results in a spread in responses. Understanding the source of the diversity in the model results is therefore crucial in order to reduce the uncertainty in the projection. Just as in any future climate simulation, the analysis presented here demonstrates that the model spread in the SeaRISE effort is due to a number of factors. First is the problem of obtaining an initial configuration for the projection. The two commonly used methods, interglacial spin-up or data assimilation, have both advantages and drawbacks, and will affect the determination of fields that cannot be measured (such as basal slipperiness). Second is the uncertainty in actual observations, which includes but is not limited to surface mass balance, basal topography, ice thickness, and surface velocities. An additional issue with these observations is that they can be transient quantities which are not measured at the same time, but ice sheet models require them to be simultaneous. Third is the uncertainty in the models' physics and discretization, which is limited by our understanding (or lack of understanding) of crucial processes that often occur at subgrid scale relative to the resolution used by continental ice sheet models, and thus require parameterization. Grounding line migration and sliding laws are such an example. Fourth is the determination of the future forcing scenarios and their implementation as the external forcing. Unfortunately, as demonstrated in this analysis, all ice sheet models face these limitations to some degree, so that it is extremely difficult to identify a set of models and projections that should be

  2. Impacts of polar ice sheets on the East Asian monsoon during the MIS-13 interglacial

    NASA Astrophysics Data System (ADS)

    Shi, Feng; Yin, Qiuzhen; Nikolova, Irina; Guo, Zhengtang; Berger, Andre

    2017-04-01

    Among all the interglacials of the last one million years, Marine Isotope Stage (MIS) 13 has the highest δ18O value over the past 800 ka in the deep-sea sediments. This would indicate that MIS-13 is the coolest interglacial if assuming δ18O mainly represents global ice volume. The Antarctic ice core records show also that MIS-13 is the coolest interglacial over Antarctica with almost the lowest greenhouse gases concentrations (GHG). However, many proxy records from the northern hemisphere (NH) indicate that MIS-13 is at least as warm as or even warmer than the recent interglacials, with extremely strong summer monsoon and a possible melting of Greenland ice sheet. In this study, based on proxy reconstructions, different scenarios regarding the size of the Greenland and Antarctic ice sheets are made, and the response of the East Asian summer monsoon to these scenarios are tested by using the models HadCM3 and LOVECLIM as well as factor separation analysis and under the astronomical and GHG configurations of MIS-13. The results show that the influence of the disappearance of Greenland ice sheet on the surface temperature is quite localized, mainly over the northern high latitudinal regions, however, the influence of the bigger southern Hemisphere (SH) ice sheet on the surface temperature is very global, especially in the southern hemisphere. This ice sheet condition has an impact on the precipitation pattern over tropical-subtropical regions. It causes much more summer precipitation over all the East Asian monsoon region, in consistent with the paleosol record from southern China. The scenario of melted Greenland ice sheet and of larger SH ice sheets provides one of the explanations of the strong monsoon rainfall documented by the proxy data.

  3. Sensitivity of Pliocene ice sheets to orbital forcing

    USGS Publications Warehouse

    Dolan, A.M.; Haywood, A.M.; Hill, D.J.; Dowsett, H.J.; Hunter, S.J.; Lunt, D.J.; Pickering, S.J.

    2011-01-01

    The stability of the Earth's major ice sheets is a critical uncertainty in predictions of future climate and sea level change. One method of investigating the behaviour of the Greenland and the Antarctic ice sheets in a warmer-than-modern climate is to look back at past warm periods of Earth history, for example the Pliocene. This paper presents climate and ice sheet modelling results for the mid-Pliocene warm period (mPWP; 3.3 to 3.0 million years ago), which has been identified as a key interval for understanding warmer-than-modern climates (Jansen et al., 2007). Using boundary conditions supplied by the United States Geological Survey PRISM Group (Pliocene Research, Interpretation and Synoptic Mapping), the Hadley Centre coupled ocean–atmosphere climate model (HadCM3) and the British Antarctic Survey Ice Sheet Model (BASISM), we show large reductions in the Greenland and East Antarctic Ice Sheets (GrIS and EAIS) compared to modern in standard mPWP experiments. We also present the first results illustrating the variability of the ice sheets due to realistic orbital forcing during the mid-Pliocene. While GrIS volumes are lower than modern under even the most extreme (cold) mid-Pliocene orbit (losing at least 35% of its ice mass), the EAIS can both grow and shrink, losing up to 20% or gaining up to 10% of its present-day volume. The changes in ice sheet volume incurred by altering orbital forcing alone means that global sea level can vary by more than 25 m during the mid-Pliocene. However, we have also shown that the response of the ice sheets to mPWP orbital hemispheric forcing can be in anti-phase, whereby the greatest reductions in EAIS volume are concurrent with the smallest reductions of the GrIS. If this anti-phase relationship is in operation throughout the mPWP, then the total eustatic sea level response would be dampened compared to the ice sheet fluctuations that are theoretically possible. This suggests that maximum eustatic sea level rise does not

  4. Capabilities and performance of Elmer/Ice, a new-generation ice sheet model

    NASA Astrophysics Data System (ADS)

    Gagliardini, O.; Zwinger, T.; Gillet-Chaulet, F.; Durand, G.; Favier, L.; de Fleurian, B.; Greve, R.; Malinen, M.; Martín, C.; Råback, P.; Ruokolainen, J.; Sacchettini, M.; Schäfer, M.; Seddik, H.; Thies, J.

    2013-08-01

    The Fourth IPCC Assessment Report concluded that ice sheet flow models, in their current state, were unable to provide accurate forecast for the increase of polar ice sheet discharge and the associated contribution to sea level rise. Since then, the glaciological community has undertaken a huge effort to develop and improve a new generation of ice flow models, and as a result a significant number of new ice sheet models have emerged. Among them is the parallel finite-element model Elmer/Ice, based on the open-source multi-physics code Elmer. It was one of the first full-Stokes models used to make projections for the evolution of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger-scale problems, earning the status of an ice sheet model. Here, we summarise almost 10 yr of development performed by different groups. Elmer/Ice solves the full-Stokes equations, for isotropic but also anisotropic ice rheology, resolves the grounding line dynamics as a contact problem, and contains various basal friction laws. Derived fields, like the age of the ice, the strain rate or stress, can also be computed. Elmer/Ice includes two recently proposed inverse methods to infer badly known parameters. Elmer is a highly parallelised code thanks to recent developments and the implementation of a block preconditioned solver for the Stokes system. In this paper, all these components are presented in detail, as well as the numerical performance of the Stokes solver and developments planned for the future.

  5. Reconciliation of Antarctic marine and terrestrial geologic records: climate and ice-sheet variability in the mid-Miocene

    NASA Astrophysics Data System (ADS)

    Halberstadt, A. R. W.; DeConto, R.; Gasson, E.; Kowalewski, D. E.; Levy, R. H.; Naish, T.; Chorley, H.

    2017-12-01

    The mid-Miocene Climatic Optimum ( 17-15 Ma) serves as a possible analog for future Antarctic conditions, as atmospheric CO2 concentrations were similar to those projected for the next few decades. During the subsequent mid-Miocene Climatic Transition, the Antarctic Ice Sheet (AIS) developed from a more variable ice sheet to a continental, marine-terminating ice sheet resembling the modern configuration. Near-shore marine records from the Ross Sea (ANDRILL-2A; Levy et al., 2016) imply highly dynamic AIS behavior in the mid-Miocene. Reconstructed environmental conditions during this time period range from full glaciation of the area to a warm interglacial environment. Multiple AIS expansions during the mid-Miocene are interpreted from geophysical evidence including seismic surveys correlated to drill core data (Chow & Bart, 2003). These marine records are seemingly at odds with sedimentary and geomorphic studies in the McMurdo Dry Valleys (MDVs) that suggest the East Antarctic Ice Sheet was mostly invariable since the mid-Miocene (Sugden & Denton, 2004). Well-preserved landforms, observed by Marchant et al. (2013) and others, lack any indication of surface modification from glacial advance or wet cryoturbation, suggesting that hyper-arid cold-desert conditions have persisted in the MDVs since the mid-Miocene. This long-term landform stability in the MDVs implying a stable ice sheet is seemingly inconsistent with the highly dynamic AIS behavior reconstructed by Levy et al. (2016). Here, we use a Regional Climate Model (cf. Gasson et al., 2016) with a range of greenhouse gas concentrations, orbital configurations, ice sheet and shelf geometries, and sea surface conditions to reconcile the apparent dichotomy between marine and terrestrial records. Preliminary results reveal lapse-rate-corrected temperatures in the MDVs that generally remained below freezing in the austral summer, even under the warmest Miocene simulations (840 ppmv atmospheric CO2, `warm' austral

  6. SPH non-Newtonian Model for Ice Sheet and Ice Shelf Dynamics

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Tartakovsky, Alexandre M.; Pan, Wenxiao; Monaghan, Joseph J.

    2012-07-07

    We propose a new three-dimensional smoothed particle hydrodynamics (SPH) non-Newtonian model to study coupled ice sheet and ice shelf dynamics. Most existing ice sheet numerical models use a grid-based Eulerian approach, and are usually restricted to shallow ice sheet and ice shelf approximations of the momentum conservation equation. SPH, a fully Lagrangian particle method, solves the full momentum conservation equation. SPH method also allows modeling of free-surface flows, large material deformation, and material fragmentation without employing complex front-tracking schemes, and does not require re-meshing. As a result, SPH codes are highly scalable. Numerical accuracy of the proposed SPH model ismore » first verified by simulating a plane shear flow with a free surface and the propagation of a blob of ice along a horizontal surface. Next, the SPH model is used to investigate the grounding line dynamics of ice sheet/shelf. The steady position of the grounding line, obtained from our SPH simulations, is in good agreement with laboratory observations for a wide range of bedrock slopes, ice-to-fluid density ratios, and flux. We examine the effect of non-Newtonian behavior of ice on the grounding line dynamics. The non-Newtonian constitutive model is based on Glen's law for a creeping flow of a polycrystalline ice. Finally, we investigate the effect of a bedrock geometry on a steady-state position of the grounding line.« less

  7. Ice-sheet thinning and acceleration at Camp Century, Greenlan

    NASA Astrophysics Data System (ADS)

    Colgan, W. T.

    2017-12-01

    Camp Century, Greenland (77.18 °N, 61.12 °W, 1900 m), is located approximately 150 km inland from the ice-sheet margin in Northwest Greenland. In-situ and remotely-sensed measurements of ice-sheet elevation at Camp Century exhibit a thinning trend between 1964 and the present. A comparison of 1966 and 2017 firn density profiles indicates that a portion of this ice-sheet thinning is attributable to increased firn compaction rate. In-situ measurements of increasing ice surface velocity over the 1977-2017 period indicate that enhanced horizontal divergence of ice flux is also contributing to ice dynamic thinning at Camp Century. This apparent ice dynamic thinning could potentially result from a migrating local flow divide or decreasing effective ice viscosity. In a shorter-term context, observations of decadal-scale ice-sheet thinning and acceleration at Camp Century highlights underappreciated transience in inland ice form and flow during the satellite era. In a longer-term context, these multi-decadal observations contrast with inferences of millennial-scale ice-sheet thickening and deceleration at Camp Century.

  8. Ice core evidence for extensive melting of the greenland ice sheet in the last interglacial.

    PubMed

    Koerner, R M

    1989-05-26

    Evidence from ice at the bottom of ice cores from the Canadian Arctic Islands and Camp Century and Dye-3 in Greenland suggests that the Greenland ice sheet melted extensively or completely during the last interglacial period more than 100 ka (thousand years ago), in contrast to earlier interpretations. The presence of dirt particles in the basal ice has previously been thought to indicate that the base of the ice sheets had melted and that the evidence for the time of original growth of these ice masses had been destroyed. However, the particles most likely blew onto the ice when the dimensions of the ice caps and ice sheets were much smaller. Ice texture, gas content, and other evidence also suggest that the basal ice at each drill site is superimposed ice, a type of ice typical of the early growth stages of an ice cap or ice sheet. If the present-day ice masses began their growth during the last interglacial, the ice sheet from the earlier (Illinoian) glacial period must have competely or largely melted during the early part of the same interglacial period. If such melting did occur, the 6-meter higher-than-present sea level during the Sangamon cannot be attributed to disintegration of the West Antarctic ice sheet, as has been suggested.

  9. Pleistocene hydrology of North America: The role of ice sheets in reorganizing groundwater flow systems

    NASA Astrophysics Data System (ADS)

    Person, Mark; McIntosh, Jennifer; Bense, Victor; Remenda, V. H.

    2007-09-01

    While the geomorphic consequences of Pleistocene megafloods have been known for some time, it has been only in the past 2 decades that hydrogeologists and glaciologists alike have begun to appreciate the important impact that ice sheet-aquifer interactions have had in controlling subsurface flow patterns, recharge rates, and the distribution of fresh water in confined aquifer systems across North America. In this paper, we document the numerous lines of geochemical, isotopic, and geomechanical evidence of ice sheet hydrogeology across North America. We also review the mechanical, thermal, and hydrologic processes that control subsurface fluid migration beneath ice sheets. Finite element models of subsurface fluid flow, permafrost formation, and ice sheet loading are presented to investigate the coupled nature of transport processes during glaciation/deglaciation. These indicate that recharge rates as high as 10 times modern values occurred as the Laurentide Ice Sheet overran the margins of sedimentary basins. The effects of ice sheet loading and permafrost formation result in complex transient flow patterns within aquifers and confining units alike. Using geochemical and environmental isotopic data, we estimate that the volume of glacial meltwater emplaced at the margins of sedimentary basins overrun by the Laurentide Ice Sheet totals about 3.7 × 104 km3, which is about 0.2% of the volume of the Laurentide Ice Sheet. Subglacial infiltration estimates based on continental-scale hydrologic models are even higher (5-10% of meltwater generated). These studies in sum call into question the widely held notion that groundwater flow patterns within confined aquifer systems are controlled primarily by the water table configuration during the Pleistocene. Rather, groundwater flow patterns were likely much more complex and transient in nature than has previously been thought. Because Pleistocene recharge rates are believed to be highly variable, these studies have profound

  10. Reconstructions of the Weichselian ice sheet, a comparative study of a thermo-mechanical approach to GIA driven models.

    NASA Astrophysics Data System (ADS)

    Schmidt, Peter; Lund, Björn; Näslund, Jens-Ove; Fastook, James

    2014-05-01

    Observations of glacial isostatic adjustment (GIA) have been used both to study the mechanical properties of the Earth and to invert for Northern Hemisphere palaeo-ice-sheets. This is typically done by solving the sea-level equation using simplified scaling laws to control ice-sheet thickness. However, past ice-sheets can also be reconstructed based on thermo-mechanical modelling driven by palaeo-climate data, invoking simple analytical models to account for the Earth's response. Commonly, both approaches use dated geological markers to constrain the ice-sheet margin location. Irrespective of the approach, the resulting ice-sheet reconstruction depends on the earth response, although the interdependence between the ice model and the earth model differs and therefore the two types of reconstructions could provide complementary information on Earth properties. We compare a thermo-mechanical reconstruction of the Weichselian ice-sheet using the UMISM model (Näslund, 2010) to two GIA driven reconstructions, ANU (Lambeck et al., 2010) and ICE-5G (Peltier & Fairbanks, 2006), commonly used in GIA modelling. We evaluate the three reconstructions both in terms of ice-sheet configurations and predicted Fennoscandian surface deformation ICE-5G comprise the largest reconstructed ice-sheet whereas ANU and UMISM are more similar in volume and areal extent. Significant differences still exists between ANU and UMISM, especially during the final deglaciation phase. Prior to the final retreat of the ice-sheet, ICE-5G is displays a massive and more or less constant ice-sheet configuration, while both ANU and UMISM fluctuates with at times almost ice-free conditions, such as during MIS3. This results in ICE-5G being close to isostatic equilibrium at LGM, whereas ANU and UMISM are not. Hence, the pre-LGM evolution of the Weichselian ice-sheet needs to be considered in GIA studies. For example, perturbing the ANU or UMISM reconstructions we find that changes more recent than 36 kyr BP

  11. Glaciological constraints on current ice mass changes from modelling the ice sheets over the glacial cycles

    NASA Astrophysics Data System (ADS)

    Huybrechts, P.

    2003-04-01

    The evolution of continental ice sheets introduces a long time scale in the climate system. Large ice sheets have a memory of millenia, hence the present-day ice sheets of Greenland and Antarctica are still adjusting to climatic variations extending back to the last glacial period. This trend is separate from the direct response to mass-balance changes on decadal time scales and needs to be correctly accounted for when assessing current and future contributions to sea level. One way to obtain estimates of current ice mass changes is to model the past history of the ice sheets and their underlying beds over the glacial cycles. Such calculations assist to distinguish between the longer-term ice-dynamic evolution and short-term mass-balance changes when interpreting altimetry data, and are helpful to isolate the effects of postglacial rebound from gravity and altimetry trends. The presentation will discuss results obtained from 3-D thermomechanical ice-sheet/lithosphere/bedrock models applied to the Antarctic and Greenland ice sheets. The simulations are forced by time-dependent boundary conditions derived from sediment and ice core records and are constrained by geomorphological and glacial-geological data of past ice sheet and sea-level stands. Current simulations suggest that the Greenland ice sheet is close to balance, while the Antarctic ice sheet is still losing mass, mainly due to incomplete grounding-line retreat of the West Antarctic ice sheet since the LGM. The results indicate that altimetry trends are likely dominated by ice thickness changes but that the gravitational signal mainly reflects postglacial rebound.

  12. Sensitivity of an Antarctic Ice Sheet Model to Sub-Ice-Shelf Melting

    NASA Astrophysics Data System (ADS)

    Lipscomb, W. H.; Leguy, G.; Urban, N. M.; Berdahl, M.

    2017-12-01

    Theory and observations suggest that marine-based sectors of the Antarctic ice sheet could retreat rapidly under ocean warming and increased melting beneath ice shelves. Numerical models of marine ice sheets vary widely in sensitivity, depending on grid resolution and the parameterization of key processes (e.g., calving and hydrofracture). Here we study the sensitivity of the Antarctic ice sheet to ocean warming and sub-shelf melting in standalone simulations of the Community Ice Sheet Model (CISM). Melt rates either are prescribed based on observations and high-resolution ocean model output, or are derived from a plume model forced by idealized ocean temperature profiles. In CISM, we vary the model resolution (between 1 and 8 km), Stokes approximation (shallow-shelf, depth-integrated higher-order, or 3D higher-order) and calving scheme to create an ensemble of plausible responses to sub-shelf melting. This work supports a broader goal of building statistical and reduced models that can translate large-scale Earth-system model projections to changes in Antarctic ocean temperatures and ice sheet discharge, thus better quantifying uncertainty in Antarctic-sourced sea-level rise.

  13. Bacteria beneath the West Antarctic ice sheet.

    PubMed

    Lanoil, Brian; Skidmore, Mark; Priscu, John C; Han, Sukkyun; Foo, Wilson; Vogel, Stefan W; Tulaczyk, Slawek; Engelhardt, Hermann

    2009-03-01

    Subglacial environments, particularly those that lie beneath polar ice sheets, are beginning to be recognized as an important part of Earth's biosphere. However, except for indirect indications of microbial assemblages in subglacial Lake Vostok, Antarctica, no sub-ice sheet environments have been shown to support microbial ecosystems. Here we report 16S rRNA gene and isolate diversity in sediments collected from beneath the Kamb Ice Stream, West Antarctic Ice Sheet and stored for 15 months at 4 degrees C. This is the first report of microbes in samples from the sediment environment beneath the Antarctic Ice Sheet. The cells were abundant ( approximately 10(7) cells g(-1)) but displayed low diversity (only five phylotypes), likely as a result of enrichment during storage. Isolates were cold tolerant and the 16S rRNA gene diversity was a simplified version of that found in subglacial alpine and Arctic sediments and water. Although in situ cell abundance and the extent of wet sediments beneath the Antarctic ice sheet can only be roughly extrapolated on the basis of this sample, it is clear that the subglacial ecosystem contains a significant and previously unrecognized pool of microbial cells and associated organic carbon that could potentially have significant implications for global geochemical processes.

  14. Obliquity-paced Pliocene West Antarctic ice sheet oscillations.

    PubMed

    Naish, T; Powell, R; Levy, R; Wilson, G; Scherer, R; Talarico, F; Krissek, L; Niessen, F; Pompilio, M; Wilson, T; Carter, L; DeConto, R; Huybers, P; McKay, R; Pollard, D; Ross, J; Winter, D; Barrett, P; Browne, G; Cody, R; Cowan, E; Crampton, J; Dunbar, G; Dunbar, N; Florindo, F; Gebhardt, C; Graham, I; Hannah, M; Hansaraj, D; Harwood, D; Helling, D; Henrys, S; Hinnov, L; Kuhn, G; Kyle, P; Läufer, A; Maffioli, P; Magens, D; Mandernack, K; McIntosh, W; Millan, C; Morin, R; Ohneiser, C; Paulsen, T; Persico, D; Raine, I; Reed, J; Riesselman, C; Sagnotti, L; Schmitt, D; Sjunneskog, C; Strong, P; Taviani, M; Vogel, S; Wilch, T; Williams, T

    2009-03-19

    Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch ( approximately 5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, approximately 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to approximately 3 degrees C warmer than today and atmospheric CO(2) concentration was as high as approximately 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO(2).

  15. Ice-sheet contributions to future sea-level change.

    PubMed

    Gregory, J M; Huybrechts, P

    2006-07-15

    Accurate simulation of ice-sheet surface mass balance requires higher spatial resolution than is afforded by typical atmosphere-ocean general circulation models (AOGCMs), owing, in particular, to the need to resolve the narrow and steep margins where the majority of precipitation and ablation occurs. We have developed a method for calculating mass-balance changes by combining ice-sheet average time-series from AOGCM projections for future centuries, both with information from high-resolution climate models run for short periods and with a 20km ice-sheet mass-balance model. Antarctica contributes negatively to sea level on account of increased accumulation, while Greenland contributes positively because ablation increases more rapidly. The uncertainty in the results is about 20% for Antarctica and 35% for Greenland. Changes in ice-sheet topography and dynamics are not included, but we discuss their possible effects. For an annual- and area-average warming exceeding 4.5+/-0.9K in Greenland and 3.1+/-0.8K in the global average, the net surface mass balance of the Greenland ice sheet becomes negative, in which case it is likely that the ice sheet would eventually be eliminated, raising global-average sea level by 7m.

  16. Surface water hydrology and the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Smith, L. C.; Yang, K.; Pitcher, L. H.; Overstreet, B. T.; Chu, V. W.; Rennermalm, A. K.; Cooper, M. G.; Gleason, C. J.; Ryan, J.; Hubbard, A.; Tedesco, M.; Behar, A.

    2016-12-01

    Mass loss from the Greenland Ice Sheet now exceeds 260 Gt/year, raising global sea level by >0.7 mm annually. Approximately two-thirds of this total mass loss is now driven by negative ice sheet surface mass balance (SMB), attributed mainly to production and runoff of meltwater from the ice sheet surface. This new dominance of runoff as a driver of GrIS total mass loss will likely persist owing to anticipated further increases in surface melting, reduced meltwater storage in firn, and the waning importance of dynamical mass losses (ice calving) as the ice sheets retreat from their marine-terminating margins. It also creates the need and opportunity for integrative research pairing traditional surface water hydrology approaches with glaciology. As one example, we present a way to measure supraglacial "runoff" (i.e. specific discharge) at the supraglacial catchment scale ( 101-102 km2), using in situ measurements of supraglacial river discharge and high-resolution satellite/drone mapping of upstream catchment area. This approach, which is standard in terrestrial hydrology but novel for ice sheet science, enables independent verification and improvement of modeled SMB runoff estimates used to project sea level rise. Furthermore, because current SMB models do not consider the role of fluvial watershed processes operating on the ice surface, inclusion of even a simple surface routing model materially improves simulations of runoff delivered to moulins, the critical pathways for meltwater entry into the ice sheet. Incorporating principles of surface water hydrology and fluvial geomorphology and into glaciological models will thus aid estimates of Greenland meltwater runoff to the global ocean as well as connections to subglacial hydrology and ice sheet dynamics.

  17. Growth of Greenland ice sheet - Interpretation

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay

    1989-01-01

    An observed 0.23 m/year thickening of the Greenland ice sheet indicates a 25 percent to 45 percent excess ice accumulation over the amount required to balance the outward ice flow. The implied global sea-level depletion is 0.2 to 0.4 mm/year, depending on whether the thickening is only recent (5 to 10 years) or longer term (less than 100 years). If there is a similar imbalance in the northern 60 percent of the ice-sheet area, the depletion is 0.35 to 0.7 mm/year. Increasing ice thickness suggests that the precipitation is higher than the long-term average; higher precipitation may be a characteristic of warmer climates in polar regions.

  18. ISMIP6 - initMIP: Greenland ice sheet model initialisation experiments

    NASA Astrophysics Data System (ADS)

    Goelzer, Heiko; Nowicki, Sophie; Payne, Tony; Larour, Eric; Abe Ouchi, Ayako; Gregory, Jonathan; Lipscomb, William; Seroussi, Helene; Shepherd, Andrew; Edwards, Tamsin

    2016-04-01

    Earlier large-scale Greenland ice sheet sea-level projections e.g. those run during ice2sea and SeaRISE initiatives have shown that ice sheet initialisation can have a large effect on the projections and gives rise to important uncertainties. This intercomparison exercise (initMIP) aims at comparing, evaluating and improving the initialization techniques used in the ice sheet modeling community and to estimate the associated uncertainties. It is the first in a series of ice sheet model intercomparison activities within ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). The experiments are conceived for the large-scale Greenland ice sheet and are designed to allow intercomparison between participating models of 1) the initial present-day state of the ice sheet and 2) the response in two schematic forward experiments. The latter experiments serve to evaluate the initialisation in terms of model drift (forward run without any forcing) and response to a large perturbation (prescribed surface mass balance anomaly). We present and discuss first results of the intercomparison and highlight important uncertainties with respect to projections of the Greenland ice sheet sea-level contribution.

  19. Extraction of Ice Sheet Layers from Two Intersected Radar Echograms Near Neem Ice Core in Greenland

    NASA Astrophysics Data System (ADS)

    Xiong, S.; Muller, J.-P.

    2016-06-01

    Accumulation of snow and ice over time result in ice sheet layers. These can be remotely sensed where there is a contrast in electromagnetic properties, which reflect variations of the ice density, acidity and fabric orientation. Internal ice layers are assumed to be isochronous, deep beneath the ice surface, and parallel to the direction of ice flow. The distribution of internal layers is related to ice sheet dynamics, such as the basal melt rate, basal elevation variation and changes in ice flow mode, which are important parameters to model the ice sheet. Radar echo sounder is an effective instrument used to study the sedimentology of the Earth and planets. Ice Penetrating Radar (IPR) is specific kind of radar echo sounder, which extends studies of ice sheets from surface to subsurface to deep internal ice sheets depending on the frequency utilised. In this study, we examine a study site where folded ice occurs in the internal ice sheet south of the North Greenland Eemian ice drilling (NEEM) station, where two intersected radar echograms acquired by the Multi-channel Coherent Radar Depth Sounder (MCoRDS) employed in the NASA's Operation IceBridge (OIB) mission imaged this folded ice. We propose a slice processing flow based on a Radon Transform to trace and extract these two sets of curved ice sheet layers, which can then be viewed in 3-D, demonstrating the 3-D structure of the ice folds.

  20. Laurentide ice-sheet instability during the last deglaciation

    NASA Astrophysics Data System (ADS)

    Ullman, David J.; Carlson, Anders E.; Anslow, Faron S.; Legrande, Allegra N.; Licciardi, Joseph M.

    2015-07-01

    Changes in the amount of summer incoming solar radiation (insolation) reaching the Northern Hemisphere are the underlying pacemaker of glacial cycles. However, not all rises in boreal summer insolation over the past 800,000 years resulted in deglaciation to present-day ice volumes, suggesting that there may be a climatic threshold for the disappearance of land-based ice. Here we assess the surface mass balance stability of the Laurentide ice sheet--the largest glacial ice mass in the Northern Hemisphere--during the last deglaciation (24,000 to 9,000 years ago). We run a surface energy balance model with climate data from simulations with a fully coupled atmosphere-ocean general circulation model for key time slices during the last deglaciation. We find that the surface mass balance of the Laurentide ice sheet was positive throughout much of the deglaciation, and suggest that dynamic discharge was mainly responsible for mass loss during this time. Total surface mass balance became negative only in the early Holocene, indicating the transition to a new state where ice loss occurred primarily by surface ablation. We conclude that the Laurentide ice sheet remained a viable ice sheet before the Holocene and began to fully deglaciate only once summer temperatures and radiative forcing over the ice sheet increased by 6-7 °C and 16-20 W m-2, respectively, relative to full glacial conditions.

  1. Improving Surface Mass Balance Over Ice Sheets and Snow Depth on Sea Ice

    NASA Technical Reports Server (NTRS)

    Koenig, Lora Suzanne; Box, Jason; Kurtz, Nathan

    2013-01-01

    Surface mass balance (SMB) over ice sheets and snow on sea ice (SOSI) are important components of the cryosphere. Large knowledge gaps remain in scientists' abilities to monitor SMB and SOSI, including insufficient measurements and difficulties with satellite retrievals. On ice sheets, snow accumulation is the sole mass gain to SMB, and meltwater runoff can be the dominant single loss factor in extremely warm years such as 2012. SOSI affects the growth and melt cycle of the Earth's polar sea ice cover. The summer of 2012 saw the largest satellite-recorded melt area over the Greenland ice sheet and the smallest satellite-recorded Arctic sea ice extent, making this meeting both timely and relevant.

  2. Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

    NASA Astrophysics Data System (ADS)

    Hein, Andrew S.; Woodward, John; Marrero, Shasta M.; Dunning, Stuart A.; Steig, Eric J.; Freeman, Stewart P. H. T.; Stuart, Finlay M.; Winter, Kate; Westoby, Matthew J.; Sugden, David E.

    2016-02-01

    Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial-interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present.

  3. Mass Balance Changes and Ice Dynamics of Greenland and Antarctic Ice Sheets from Laser Altimetry

    NASA Astrophysics Data System (ADS)

    Babonis, G. S.; Csatho, B.; Schenk, T.

    2016-06-01

    During the past few decades the Greenland and Antarctic ice sheets have lost ice at accelerating rates, caused by increasing surface temperature. The melting of the two big ice sheets has a big impact on global sea level rise. If the ice sheets would melt down entirely, the sea level would rise more than 60 m. Even a much smaller rise would cause dramatic damage along coastal regions. In this paper we report about a major upgrade of surface elevation changes derived from laser altimetry data, acquired by NASA's Ice, Cloud and land Elevation Satellite mission (ICESat) and airborne laser campaigns, such as Airborne Topographic Mapper (ATM) and Land, Vegetation and Ice Sensor (LVIS). For detecting changes in ice sheet elevations we have developed the Surface Elevation Reconstruction And Change detection (SERAC) method. It computes elevation changes of small surface patches by keeping the surface shape constant and considering the absolute values as surface elevations. We report about important upgrades of earlier results, for example the inclusion of local ice caps and the temporal extension from 1993 to 2014 for the Greenland Ice Sheet and for a comprehensive reconstruction of ice thickness and mass changes for the Antarctic Ice Sheets.

  4. The microbiome of glaciers and ice sheets.

    PubMed

    Anesio, Alexandre M; Lutz, Stefanie; Chrismas, Nathan A M; Benning, Liane G

    2017-01-01

    Glaciers and ice sheets, like other biomes, occupy a significant area of the planet and harbour biological communities with distinct interactions and feedbacks with their physical and chemical environment. In the case of the glacial biome, the biological processes are dominated almost exclusively by microbial communities. Habitats on glaciers and ice sheets with enough liquid water to sustain microbial activity include snow, surface ice, cryoconite holes, englacial systems and the interface between ice and overridden rock/soil. There is a remarkable similarity between the different specific glacial habitats across glaciers and ice sheets worldwide, particularly regarding their main primary producers and ecosystem engineers. At the surface, cyanobacteria dominate the carbon production in aquatic/sediment systems such as cryoconite holes, while eukaryotic Zygnematales and Chlamydomonadales dominate ice surfaces and snow dynamics, respectively. Microbially driven chemolithotrophic processes associated with sulphur and iron cycle and C transformations in subglacial ecosystems provide the basis for chemical transformations at the rock interface under the ice that underpin an important mechanism for the delivery of nutrients to downstream ecosystems. In this review, we focus on the main ecosystem engineers of glaciers and ice sheets and how they interact with their chemical and physical environment. We then discuss the implications of this microbial activity on the icy microbiome to the biogeochemistry of downstream ecosystems.

  5. Present-day Circum-Antarctic Simulations using the POPSICLES Coupled Ice Sheet-Ocean Model

    NASA Astrophysics Data System (ADS)

    Asay-Davis, X.; Martin, D. F.; Price, S. F.; Maltrud, M. E.; Collins, W.

    2014-12-01

    We present POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010. Simulations are performed at 0.1o (~5 km) ocean resolution and with adaptive ice-sheet model resolution as fine as 500 m. We compare time-averaged melt rates below a number of major ice shelves with those reported by Rignot et al. (2013) as well as other recent studies. We also present seasonal variability and decadal trends in submarine melting from several Antarctic regions. Finally, we explore the influence on basal melting and system dynamics resulting from two different choices of climate forcing: a "normal-year" climatology and the CORE v. 2 forcing data (Large and Yeager 2008).POPSICLES couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). BISICLES makes use of adaptive mesh refinement and a 1st-order accurate momentum balance similar to the L1L2 model of Schoof and Hindmarsh (2009) to accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests (MISMIP-3D; Pattyn et al., 2013) and realistic configurations (Favier et al. 2014).A companion presentation, "Response of the Antarctic Ice Sheet to ocean forcing using the POPSICLES coupled ice sheet-ocean model" in session C024 covers the ice-sheet response to these melt rates in the coupled simulation

  6. Marine record of late quaternary glacial-interglacial fluctuations in the Ross Sea and evidence for rapid, episodic sea level change due to marine ice sheet collapse

    NASA Technical Reports Server (NTRS)

    Anderson, John B.

    1991-01-01

    Some of the questions to be addressed by SeaRISE include: (1) what was the configuration of the West Antarctic ice sheet during the last glacial maximum; (2) What is its configuration during a glacial minimum; and (3) has it, or any marine ice sheet, undergone episodic rapid mass wasting. These questions are addressed in terms of what is known about the history of the marine ice sheet, specifically in Ross Sea, and what further studies are required to resolve these problems. A second question concerns the extent to which disintegration of marine ice sheets may result in rises in sea level that are episodic in nature and extremely rapid, as suggested by several glaciologists. Evidence that rapid, episodic sea level changes have occurred during the Holocene is also reviewed.

  7. Ice sheet systems and sea level change.

    NASA Astrophysics Data System (ADS)

    Rignot, E. J.

    2015-12-01

    Modern views of ice sheets provided by satellites, airborne surveys, in situ data and paleoclimate records while transformative of glaciology have not fundamentally changed concerns about ice sheet stability and collapse that emerged in the 1970's. Motivated by the desire to learn more about ice sheets using new technologies, we stumbled on an unexplored field of science and witnessed surprising changes before realizing that most were coming too fast, soon and large. Ice sheets are integrant part of the Earth system; they interact vigorously with the atmosphere and the oceans, yet most of this interaction is not part of current global climate models. Since we have never witnessed the collapse of a marine ice sheet, observations and exploration remain critical sentinels. At present, these observations suggest that Antarctica and Greenland have been launched into a path of multi-meter sea level rise caused by rapid climate warming. While the current loss of ice sheet mass to the ocean remains a trickle, every mm of sea level change will take centuries of climate reversal to get back, several major marine-terminating sectors have been pushed out of equilibrium, and ice shelves are irremediably being lost. As glaciers retreat from their salty, warm, oceanic margins, they will melt away and retreat slower, but concerns remain about sea level change from vastly marine-based sectors: 2-m sea level equivalent in Greenland and 23-m in Antarctica. Significant changes affect 2/4 marine-based sectors in Greenland - Jakobshavn Isb. and the northeast stream - with Petermann Gl. not far behind. Major changes have affected the Amundsen Sea sector of West Antarctica since the 1980s. Smaller yet significant changes affect the marine-based Wilkes Land sector of East Antarctica, a reminder that not all marine-based ice is in West Antarctica. Major advances in reducing uncertainties in sea level projections will require massive, interdisciplinary efforts that are not currently in place

  8. The Ice Sheet Mass Balance Inter-comparison Exercise

    NASA Astrophysics Data System (ADS)

    Shepherd, A.; Ivins, E. R.

    2015-12-01

    Fluctuations in the mass of ice stored in Antarctica and Greenland are of considerable societal importance. The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) is a joint-initiative of ESA and NASA aimed at producing a single estimate of the global sea level contribution to polar ice sheet losses. Within IMBIE, estimates of ice sheet mass balance are developed from a variety of satellite geodetic techniques using a common spatial and temporal reference frame and a common appreciation of the contributions due to external signals. The project brings together the laboratories and space agencies that have been instrumental in developing independent estimates of ice sheet mass balance to date. In its first phase, IMBIE involved 27 science teams, and delivered a first community assessment of ice sheet mass imbalance to replace 40 individual estimates. The project established that (i) there is good agreement between the three main satellite-based techniques for estimating ice sheet mass balance, (ii) combining satellite data sets leads to significant improvement in certainty, (iii) the polar ice sheets contributed 11 ± 4 mm to global sea levels between 1992 and 2012, and (iv) that combined ice losses from Antarctica and Greenland have increased over time, rising from 10% of the global trend in the early 1990's to 30% in the late 2000's. Demand for an updated assessment has grown, and there are now new satellite missions, new geophysical corrections, new techniques, and new teams producing data. The period of overlap between independent satellite techniques has increased from 5 to 12 years, and the full period of satellite data over which an assessment can be performed has increased from 19 to 40 years. It is also clear that multiple satellite techniques are required to confidently separate mass changes associated with snowfall and ice dynamical imbalance - information that is of critical importance for climate modelling. This presentation outlines the approach

  9. Ice Sheet Model Intercomparison Project (ISMIP6) contribution to CMIP6

    PubMed Central

    Nowicki, Sophie M.J.; Payne, Tony; Larour, Eric; Seroussi, Helene; Goelzer, Heiko; Lipscomb, William; Gregory, Jonathan; Abe-Ouchi, Ayako; Shepherd, Andrew

    2018-01-01

    Reducing the uncertainty in the past, present and future contribution of ice sheets to sea-level change requires a coordinated effort between the climate and glaciology communities. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) is the primary activity within the Coupled Model Intercomparison Project – phase 6 (CMIP6) focusing on the Greenland and Antarctic Ice Sheets. In this paper, we describe the framework for ISMIP6 and its relationship to other activities within CMIP6. The ISMIP6 experimental design relies on CMIP6 climate models and includes, for the first time within CMIP, coupled ice sheet – climate models as well as standalone ice sheet models. To facilitate analysis of the multi-model ensemble and to generate a set of standard climate inputs for standalone ice sheet models, ISMIP6 defines a protocol for all variables related to ice sheets. ISMIP6 will provide a basis for investigating the feedbacks, impacts, and sea-level changes associated with dynamic ice sheets and for quantifying the uncertainty in ice-sheet-sourced global sea-level change. PMID:29697697

  10. Preservation of Midlatitude Ice Sheets on Mars

    NASA Astrophysics Data System (ADS)

    Bramson, A. M.; Byrne, S.; Bapst, J.

    2017-11-01

    Excess ice with a minimum age of tens of millions of years is widespread in Arcadia Planitia on Mars, and a similar deposit has been found in Utopia Planitia. The conditions that led to the formation and preservation of these midlatitude ice sheets hold clues to past climate and subsurface structure on Mars. We simulate the thermal stability and retreat of buried excess ice sheets over 21 Myr of Martian orbital solutions and find that the ice sheets can be orders of magnitude older than the obliquity cycles that are typically thought to drive midlatitude ice deposition and sublimation. Retreat of this ice in the last 4 Myr could have contributed 6% of the volume of the north polar layered deposits (NPLD) and more than 10% if the NPLD are older than 4 Myr. Matching the measured dielectric constants of the Arcadia and Utopia Planitia deposits requires ice porosities of 25-35%. We model geothermally driven vapor migration through porous ice under Martian temperatures and find that Martian firn may be able to maintain porosity for timescales longer than we predict for retreat of the ice.

  11. Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models

    NASA Astrophysics Data System (ADS)

    Levermann, A.; Winkelmann, R.; Nowicki, S.; Fastook, J. L.; Frieler, K.; Greve, R.; Hellmer, H. H.; Martin, M. A.; Meinshausen, M.; Mengel, M.; Payne, A. J.; Pollard, D.; Sato, T.; Timmermann, R.; Wang, W. L.; Bindschadler, R. A.

    2014-08-01

    The largest uncertainty in projections of future sea-level change results from the potentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting induced by a warming ocean has been identified as a major cause for additional ice flow across the grounding line. Here we attempt to estimate the uncertainty range of future ice discharge from Antarctica by combining uncertainty in the climatic forcing, the oceanic response and the ice-sheet model response. The uncertainty in the global mean temperature increase is obtained from historically constrained emulations with the MAGICC-6.0 (Model for the Assessment of Greenhouse gas Induced Climate Change) model. The oceanic forcing is derived from scaling of the subsurface with the atmospheric warming from 19 comprehensive climate models of the Coupled Model Intercomparison Project (CMIP-5) and two ocean models from the EU-project Ice2Sea. The dynamic ice-sheet response is derived from linear response functions for basal ice-shelf melting for four different Antarctic drainage regions using experiments from the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. The resulting uncertainty range for the historic Antarctic contribution to global sea-level rise from 1992 to 2011 agrees with the observed contribution for this period if we use the three ice-sheet models with an explicit representation of ice-shelf dynamics and account for the time-delayed warming of the oceanic subsurface compared to the surface air temperature. The median of the additional ice loss for the 21st century is computed to 0.07 m (66% range: 0.02-0.14 m; 90% range: 0.0-0.23 m) of global sea-level equivalent for the low-emission RCP-2.6 (Representative Concentration Pathway) scenario and 0.09 m (66% range: 0.04-0.21 m; 90% range: 0.01-0.37 m) for the strongest RCP-8.5. Assuming no time delay between the atmospheric warming and the oceanic subsurface, these

  12. Geoengineering Marine Ice Sheets

    NASA Astrophysics Data System (ADS)

    Wolovick, M.

    2017-12-01

    Mass loss from Greenland and Antarctica is highly sensitive to the presence of warm ocean water that drives melting at the grounding line. Rapid melting near the grounding line causes ice shelf thinning, loss of buttressing, flow acceleration, grounding line retreat, and ultimately mass loss and sea-level rise. If the grounding line enters a section of overdeepened bed the ice sheet may even enter a runaway collapse via the marine ice sheet instability. The warm water that triggers this process resides offshore at depth and accesses the grounding line through deep troughs in the continental shelf. In Greenland, warm water transport is further constricted through narrow fjords. Here, I propose blocking warm water transport through these choke points with an artificial sill. Using a simple width- and depth-averaged model of ice stream flow coupled to a buoyant-plume model of ocean melting, I find that grounding line retreat and sea level rise can be delayed or reversed for hundreds of years if warm water is prevented from accessing the grounding line at depth. Blocking of warm water from the sub-ice cavity causes ice shelf thickening, increased buttressing, and grounding line readvance. The increase in buttressing is greatly magnified if the thickened ice shelf regrounds on a bathymetric high or on the artificial sill itself. In some experiments for Thwaites Glacier the grounding line is able to recover from a severely retreated state over 100 km behind its present-day position. Such a dramatic recovery demonstrates that it is possible, at least in principle, to stop and reverse an ongoing marine ice sheet collapse. If the ice shelf regrounds on the artificial sill itself, erosion of the sill beneath the grounded ice could reduce the effectiveness of the intervention. However, experiments including sill erosion suggest that even a very weak sill (1 kPa) could delay a collapse for centuries. The scale of the artificial sills in Greenlandic fjords is comparable to

  13. Ice-sheet mass balance and climate change.

    PubMed

    Hanna, Edward; Navarro, Francisco J; Pattyn, Frank; Domingues, Catia M; Fettweis, Xavier; Ivins, Erik R; Nicholls, Robert J; Ritz, Catherine; Smith, Ben; Tulaczyk, Slawek; Whitehouse, Pippa L; Zwally, H Jay

    2013-06-06

    Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain.

  14. Obliquity-paced Pliocene West Antarctic ice sheet oscillations

    USGS Publications Warehouse

    Naish, T.; Powell, R.; Levy, R.; Wilson, G.; Scherer, R.; Talarico, F.; Krissek, L.; Niessen, F.; Pompilio, M.; Wilson, T.; Carter, L.; DeConto, R.; Huybers, P.; McKay, R.; Pollard, D.; Ross, J.; Winter, D.; Barrett, P.; Browne, G.; Cody, R.; Cowan, E.; Crampton, J.; Dunbar, G.; Dunbar, N.; Florindo, F.; Gebhardt, C.; Graham, I.; Hannah, M.; Hansaraj, D.; Harwood, D.; Helling, D.; Henrys, S.; Hinnov, L.; Kuhn, G.; Kyle, P.; Laufer, A.; Maffioli, P.; Magens, D.; Mandernack, K.; McIntosh, W.; Millan, C.; Morin, R.; Ohneiser, C.; Paulsen, T.; Persico, D.; Raine, I.; Reed, J.; Riesselman, C.; Sagnotti, L.; Schmitt, D.; Sjunneskog, C.; Strong, P.; Taviani, M.; Vogel, S.; Wilch, T.; Williams, T.

    2009-01-01

    Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages1, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles2. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch (5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming3. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to 3 °C warmer than today4 and atmospheric CO2 concentration was as high as 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model7 that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt8 under conditions of elevated CO2.

  15. Ice sheets and nitrogen.

    PubMed

    Wolff, Eric W

    2013-07-05

    Snow and ice play their most important role in the nitrogen cycle as a barrier to land-atmosphere and ocean-atmosphere exchanges that would otherwise occur. The inventory of nitrogen compounds in the polar ice sheets is approximately 260 Tg N, dominated by nitrate in the much larger Antarctic ice sheet. Ice cores help to inform us about the natural variability of the nitrogen cycle at global and regional scale, and about the extent of disturbance in recent decades. Nitrous oxide concentrations have risen about 20 per cent in the last 200 years and are now almost certainly higher than at any time in the last 800 000 years. Nitrate concentrations recorded in Greenland ice rose by a factor of 2-3, particularly between the 1950s and 1980s, reflecting a major change in NOx emissions reaching the background atmosphere. Increases in ice cores drilled at lower latitudes can be used to validate or constrain regional emission inventories. Background ammonium concentrations in Greenland ice show no significant recent trend, although the record is very noisy, being dominated by spikes of input from biomass burning events. Neither nitrate nor ammonium shows significant recent trends in Antarctica, although their natural variations are of biogeochemical and atmospheric chemical interest. Finally, it has been found that photolysis of nitrate in the snowpack leads to significant re-emissions of NOx that can strongly impact the regional atmosphere in snow-covered areas.

  16. Ice sheets and nitrogen

    PubMed Central

    Wolff, Eric W.

    2013-01-01

    Snow and ice play their most important role in the nitrogen cycle as a barrier to land–atmosphere and ocean–atmosphere exchanges that would otherwise occur. The inventory of nitrogen compounds in the polar ice sheets is approximately 260 Tg N, dominated by nitrate in the much larger Antarctic ice sheet. Ice cores help to inform us about the natural variability of the nitrogen cycle at global and regional scale, and about the extent of disturbance in recent decades. Nitrous oxide concentrations have risen about 20 per cent in the last 200 years and are now almost certainly higher than at any time in the last 800 000 years. Nitrate concentrations recorded in Greenland ice rose by a factor of 2–3, particularly between the 1950s and 1980s, reflecting a major change in NOx emissions reaching the background atmosphere. Increases in ice cores drilled at lower latitudes can be used to validate or constrain regional emission inventories. Background ammonium concentrations in Greenland ice show no significant recent trend, although the record is very noisy, being dominated by spikes of input from biomass burning events. Neither nitrate nor ammonium shows significant recent trends in Antarctica, although their natural variations are of biogeochemical and atmospheric chemical interest. Finally, it has been found that photolysis of nitrate in the snowpack leads to significant re-emissions of NOx that can strongly impact the regional atmosphere in snow-covered areas. PMID:23713125

  17. Leakage of the Greenland Ice Sheet through accelerated ice flow

    NASA Astrophysics Data System (ADS)

    Rignot, E.

    2005-12-01

    A map of coastal velocities of the Greenland ice sheet was produced from Radarsat-1 acquired during the background mission of 2000 and combined with radio echo sounding data to estimate the ice discharge from the ice sheet. On individual glaciers, ice discharge was compared with snow input from the interior and melt above the flux gate to determine the glacier mass balance. Time series of velocities on several glaciers at different latitudes reveal seasonal fluctuations of only 7-8 percent so that winter velocities are only 2 percent less than the yearly mean. The results show the northern Greenland glaciers to be close to balance yet losing mass. No change in ice flow is detected on Petermann, 79north and Zachariae Isstrom in 2000-2004. East Greenland glaciers are in balance and flowing steadily north of Kangerdlussuaq, but Kangerdlussuaq, Helheim and all the southeastern glaciers are thinning dramatically. All these glaciers accelerated, Kangerdlussuaq in 2000, Helheim prior to 2004, and southeast Greenland glaciers accelerated 10 to 50 percent in 2000-2004. Glacier acceleration is generally brutal, probably once the glacier reached a threshold, and sustained. In the northwest, most glaciers are largely out of balance. Jakobshavn accelerated significantly in 2002, and glaciers in its immediate vicinity accelerated more than 50 percent in 2000-2004. Less is known about southwest Greenland glaciers due to a lack of ice thickness data but the glaciers have accelerated there as well and are likely to be strongly out of balance despite thickening of the interior. Overall, I estimate the mass balance of the Greenland ice sheet to be about -80 +/-10 cubic km of ice per year in 2000 and -110 +/-15 cubic km of ice per year in 2004, i.e. more negative than based on partial altimetry surveys of the outlet glaciers. As climate continues to warm, more glaciers will accelerate, and the mass balance will become increasingly negative, regardless of the evolution of the ice sheet

  18. Ice Sheet Model Intercomparison Project (ISMIP6) Contribution to CMIP6

    NASA Technical Reports Server (NTRS)

    Nowicki, Sophie M. J.; Payne, Tony; Larour, Eric; Seroussi, Helene; Goelzer, Heiko; Lipscomb, William; Gregory, Jonathan; Abe-Ouchi, Ayako; Shepherd, Andrew

    2016-01-01

    Reducing the uncertainty in the past, present, and future contribution of ice sheets to sea-level change requires a coordinated effort between the climate and glaciology communities. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) is the primary activity within the Coupled Model Intercomparison Project phase 6 (CMIP6) focusing on the Greenland and Antarctic ice sheets. In this paper, we describe the framework for ISMIP6 and its relationship with other activities within CMIP6. The ISMIP6 experimental design relies on CMIP6 climate models and includes, for the first time within CMIP, coupled ice-sheetclimate models as well as standalone ice-sheet models. To facilitate analysis of the multi-model ensemble and to generate a set of standard climate inputs for standalone ice-sheet models, ISMIP6 defines a protocol for all variables related to ice sheets. ISMIP6 will provide a basis for investigating the feedbacks, impacts, and sea-level changes associated with dynamic ice sheets and for quantifying the uncertainty in ice-sheet-sourced global sea-level change.

  19. How might the North American ice sheet influence the northwestern Eurasian climate?

    NASA Astrophysics Data System (ADS)

    Beghin, P.; Charbit, S.; Dumas, C.; Kageyama, M.; Ritz, C.

    2015-10-01

    It is now widely acknowledged that past Northern Hemisphere ice sheets covering Canada and northern Europe at the Last Glacial Maximum (LGM) exerted a strong influence on climate by causing changes in atmospheric and oceanic circulations. In turn, these changes may have impacted the development of the ice sheets themselves through a combination of different feedback mechanisms. The present study is designed to investigate the potential impact of the North American ice sheet on the surface mass balance (SMB) of the Eurasian ice sheet driven by simulated changes in the past glacial atmospheric circulation. Using the LMDZ5 atmospheric circulation model, we carried out 12 experiments under constant LGM conditions for insolation, greenhouse gases and ocean. In these experiments, the Eurasian ice sheet is removed. The 12 experiments differ in the North American ice-sheet topography, ranging from a white and flat (present-day topography) ice sheet to a full-size LGM ice sheet. This experimental design allows the albedo and the topographic impacts of the North American ice sheet onto the climate to be disentangled. The results are compared to our baseline experiment where both the North American and the Eurasian ice sheets have been removed. In summer, the sole albedo effect of the American ice sheet modifies the pattern of planetary waves with respect to the no-ice-sheet case, resulting in a cooling of the northwestern Eurasian region. By contrast, the atmospheric circulation changes induced by the topography of the North American ice sheet lead to a strong decrease of this cooling. In winter, the Scandinavian and the Barents-Kara regions respond differently to the American ice-sheet albedo effect: in response to atmospheric circulation changes, Scandinavia becomes warmer and total precipitation is more abundant, whereas the Barents-Kara area becomes cooler with a decrease of convective processes, causing a decrease of total precipitation. The gradual increase of the

  20. Ocean Tide Influences on the Antarctic and Greenland Ice Sheets

    NASA Astrophysics Data System (ADS)

    Padman, Laurie; Siegfried, Matthew R.; Fricker, Helen A.

    2018-03-01

    Ocean tides are the main source of high-frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide-induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite-based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice-ocean models and analytical glacier models that quantify the effect of ocean tides on lower-frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice-ocean interface, and higher-resolution models with improved representation of tidal energy sinks.

  1. Monitoring southwest Greenland's ice sheet melt with ambient seismic noise.

    PubMed

    Mordret, Aurélien; Mikesell, T Dylan; Harig, Christopher; Lipovsky, Bradley P; Prieto, Germán A

    2016-05-01

    The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth's crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations.

  2. Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

    PubMed Central

    Hein, Andrew S.; Woodward, John; Marrero, Shasta M.; Dunning, Stuart A.; Steig, Eric J.; Freeman, Stewart P. H. T.; Stuart, Finlay M.; Winter, Kate; Westoby, Matthew J.; Sugden, David E.

    2016-01-01

    Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial–interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present. PMID:26838462

  3. Phased occupation and retreat of the last British-Irish Ice Sheet in the southern North Sea; geomorphic and seismostratigraphic evidence of a dynamic ice lobe

    NASA Astrophysics Data System (ADS)

    Dove, Dayton; Evans, David J. A.; Lee, Jonathan R.; Roberts, David H.; Tappin, David R.; Mellett, Claire L.; Long, David; Callard, S. Louise

    2017-05-01

    Along the terrestrial margin of the southern North Sea, previous studies of the MIS 2 glaciation impacting eastern Britain have played a significant role in the development of principles relating to ice sheet dynamics (e.g. deformable beds), and the practice of reconstructing the style, timing, and spatial configuration of palaeo-ice sheets. These detailed terrestrially-based findings have however relied on observations made from only the outer edges of the former ice mass, as the North Sea Lobe (NSL) of the British-Irish Ice Sheet (BIIS) occupied an area that is now almost entirely submarine (c.21-15 ka). Compounded by the fact that marine-acquired data have been primarily of insufficient quality and density, the configuration and behaviour of the last BIIS in the southern North Sea remains surprisingly poorly constrained. This paper presents analysis of a new, integrated set of extensive seabed geomorphological and seismo-stratigraphic observations that both advances the principles developed previously onshore (e.g. multiple advance and retreat cycles), and provides a more detailed and accurate reconstruction of the BIIS at its southern-most extent in the North Sea. A new bathymetry compilation of the region reveals a series of broad sedimentary wedges and associated moraines that represent several terminal positions of the NSL. These former still-stand ice margins (1-4) are also found to relate to newly-identified architectural patterns (shallow stacked sedimentary wedges) in the region's seismic stratigraphy (previously mapped singularly as the Bolders Bank Formation). With ground-truthing constraint provided by sediment cores, these wedges are interpreted as sub-marginal till wedges, formed by complex subglacial accretionary processes that resulted in till thickening towards the former ice-sheet margins. The newly sub-divided shallow seismic stratigraphy (at least five units) also provides an indication of the relative event chronology of the NSL. While there

  4. Radiostratigraphy and age structure of the Greenland Ice Sheet

    PubMed Central

    MacGregor, Joseph A; Fahnestock, Mark A; Catania, Ginny A; Paden, John D; Prasad Gogineni, S; Young, S Keith; Rybarski, Susan C; Mabrey, Alexandria N; Wagman, Benjamin M; Morlighem, Mathieu

    2015-01-01

    Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along track, these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing. Core-intersecting reflections were dated using synchronized depth-age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth-age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet. Key Points Phase information predicts reflection slope and simplifies reflection tracing Reflections can be dated away from ice cores using a simple ice flow model Radiostratigraphy is often disrupted near the onset of fast ice flow PMID:26213664

  5. Long-term record of Barents Sea Ice Sheet advance to the shelf edge from a 140,000 year record

    NASA Astrophysics Data System (ADS)

    Pope, Ed L.; Talling, Peter J.; Hunt, James E.; Dowdeswell, Julian A.; Allin, Joshua R.; Cartigny, Matthieu J. B.; Long, David; Mozzato, Alessandro; Stanford, Jennifer D.; Tappin, David R.; Watts, Millie

    2016-10-01

    The full-glacial extent and deglacial behaviour of marine-based ice sheets, such as the Barents Sea Ice Sheet, is well documented since the Last Glacial Maximum about 20,000 years ago. However, reworking of older sea-floor sediments and landforms during repeated Quaternary advances across the shelf typically obscures their longer-term behaviour, which hampers our understanding. Here, we provide the first detailed long-term record of Barents Sea Ice Sheet advances, using the timing of debris-flows on the Bear Island Trough-Mouth Fan. Ice advanced to the shelf edge during four distinct periods over the last 140,000 years. By far the largest sediment volumes were delivered during the oldest advance more than 128,000 years ago. Later advances occurred from 68,000 to 60,000, 39,400 to 36,000 and 26,000 to 20,900 years before present. The debris-flows indicate that the dynamics of the Saalian and the Weichselian Barents Sea Ice Sheet were very different. The repeated ice advance and retreat cycles during the Weichselian were shorter lived than those seen in the Saalian. Sediment composition shows the configuration of the ice sheet was also different between the two glacial periods, implying that the ice feeding the Bear Island Ice stream came predominantly from Scandinavia during the Saalian, whilst it drained more ice from east of Svalbard during the Weichselian.

  6. BRITICE-CHRONO: Constraining rates and style of marine-influenced ice sheet decay to provide a data-rich playground for ice sheet modellers

    NASA Astrophysics Data System (ADS)

    Clark, Chris

    2014-05-01

    Uncertainty exists regarding the fate of the Antarctic and Greenland ice sheets and how they will respond to forcings from sea level and atmospheric and ocean temperatures. If we want to know more about the mechanisms and rate of change of shrinking ice sheets, then why not examine an ice sheet that has fully disappeared and track its retreat through time? If achieved in enough detail such information could become a data-rich playground for improving the next breed of numerical ice sheet models to be used in ice and sea level forecasting. We regard that the last British-Irish Ice Sheet is a good target for this work, on account of its small size, density of information and with its numerous researchers already investigating it. BRITICE-CHRONO is a large (>45 researchers) NERC-funded consortium project comprising Quaternary scientists and glaciologists who will search the seafloor around Britain and Ireland and parts of the landmass in order to find and extract samples of sand, rock and organic matter that can be dated (OSL; Cosmogenic; 14C) to reveal the timing and rate of change of the collapsing British-Irish Ice Sheet. The purpose is to produce a high resolution dataset on the demise on an ice sheet - from the continental shelf edge and across the marine to terrestrial transition. Some 800 new date assessments will be added to those that already exist. This poster reports on the hypotheses that underpin the work. Data on retreat will be collected by focusing on 8 transects running from the continental shelf edge to a short distance (10s km) onshore and acquiring marine and terrestrial samples for geochronometric dating. The project includes funding for 587 radiocarbon, 140 OSL and 158 TCN samples for surface exposure dating; with sampling accomplished by two research cruises and 16 fieldwork campaigns. Results will reveal the timing and rate of change of ice margin recession for each transect, and combined with existing landform and dating databases, will be

  7. Modeling of Firn Compaction for Estimating Ice-Sheet Mass Change from Observed Ice-Sheet Elevation Change

    NASA Technical Reports Server (NTRS)

    Li, Jun; Zwally, H. Jay

    2011-01-01

    Changes in ice-sheet surface elevation are caused by a combination of ice-dynamic imbalance, ablation, temporal variations in accumulation rate, firn compaction and underlying bedrock motion. Thus, deriving the rate of ice-sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion, which do not involve changes in mass, and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability. We use a 25 year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model. We apply this formulation to ICESat measurements of surface elevation change at three locations on the Greenland ice sheet in order to separate the accumulation-driven changes from the ice-dynamic/ablation-driven changes, and thus to derive the corresponding mass change. Our calculated densities for the accumulation-driven changes range from 410 to 610 kg/cu m, which along with 900 kg/cu m for the dynamic/ablation-driven changes gives average densities ranging from 680 to 790 kg/cu m. We show that using an average (or "effective") density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign.

  8. Ice Sheet and Sea Ice Observations from Unmanned Aircraft Systems

    NASA Astrophysics Data System (ADS)

    Crocker, R. I.; Maslanik, J. A.

    2011-12-01

    A suite of sensors has been assembled to map ice sheet and sea ice surface topography with fine-resolution from small unmanned aircraft systems (UAS). This payload is optimized to provide coincident surface elevation and imagery data, and with its low cost and ease of reproduction, it has the potential to become a widely-distributed observational resource to complement polar manned-aircraft and satellite missions. To date, it has been deployed to map ice sheet elevations near Jakobshavn Isbræ in Greenland, and to measure sea ice freeboard and roughness in Fram Strait off the coast of Svalbard. Data collected during these campaigns have facilitate a detailed assessment of the system's surface elevation measurement accuracy, and provide a glimpse of the summer 2009 Fram Strait sea ice conditions. These findings are presented, along with a brief overview of our future Arctic UAS operations.

  9. Mass balance of the Antarctic ice sheet.

    PubMed

    Wingham, D J; Shepherd, A; Muir, A; Marshall, G J

    2006-07-15

    The Antarctic contribution to sea-level rise has long been uncertain. While regional variability in ice dynamics has been revealed, a picture of mass changes throughout the continental ice sheet is lacking. Here, we use satellite radar altimetry to measure the elevation change of 72% of the grounded ice sheet during the period 1992-2003. Depending on the density of the snow giving rise to the observed elevation fluctuations, the ice sheet mass trend falls in the range -5-+85Gtyr-1. We find that data from climate model reanalyses are not able to characterise the contemporary snowfall fluctuation with useful accuracy and our best estimate of the overall mass trend-growth of 27+/-29Gtyr-1-is based on an assessment of the expected snowfall variability. Mass gains from accumulating snow, particularly on the Antarctic Peninsula and within East Antarctica, exceed the ice dynamic mass loss from West Antarctica. The result exacerbates the difficulty of explaining twentieth century sea-level rise.

  10. ISMIP6: Ice Sheet Model Intercomparison Project for CMIP6

    NASA Technical Reports Server (NTRS)

    Nowicki, S.

    2015-01-01

    ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6) targets the Cryosphere in a Changing Climate and the Future Sea Level Grand Challenges of the WCRP (World Climate Research Program). Primary goal is to provide future sea level contribution from the Greenland and Antarctic ice sheets, along with associated uncertainty. Secondary goal is to investigate feedback due to dynamic ice sheet models. Experiment design uses and augment the existing CMIP6 (Coupled Model Intercomparison Project Phase 6) DECK (Diagnosis, Evaluation, and Characterization of Klima) experiments. Additonal MIP (Model Intercomparison Project)- specific experiments will be designed for ISM (Ice Sheet Model). Effort builds on the Ice2sea, SeaRISE (Sea-level Response to Ice Sheet Evolution) and COMBINE (Comprehensive Modelling of the Earth System for Better Climate Prediction and Projection) efforts.

  11. On the long-term memory of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Rogozhina, I.; Martinec, Z.; Hagedoorn, J. M.; Thomas, M.; Fleming, K.

    2011-03-01

    In this study, the memory of the Greenland Ice Sheet (GIS) with respect to its past states is analyzed. According to ice core reconstructions, the present-day GIS reflects former climatic conditions dating back to at least 250 thousand years before the present (kyr BP). This fact must be considered when initializing an ice sheet model. The common initialization techniques are paleoclimatic simulations driven by atmospheric forcing inferred from ice core records and steady state simulations driven by the present-day or past climatic conditions. When paleoclimatic simulations are used, the information about the past climatic conditions is partly reflected in the resulting present-day state of the GIS. However, there are several important questions that need to be clarified. First, for how long does the model remember its initial state? Second, it is generally acknowledged that, prior to 100 kyr BP, the longest Greenland ice core record (GRIP) is distorted by ice-flow irregularities. The question arises as to what extent do the uncertainties inherent in the GRIP-based forcing influence the resulting GIS? Finally, how is the modeled thermodynamic state affected by the choice of initialization technique (paleo or steady state)? To answer these questions, a series of paleoclimatic and steady state simulations is carried out. We conclude that (1) the choice of an ice-covered initial configuration shortens the initialization simulation time to 100 kyr, (2) the uncertainties in the GRIP-based forcing affect present-day modeled ice-surface topographies and temperatures only slightly, and (3) the GIS forced by present-day climatic conditions is overall warmer than that resulting from a paleoclimatic simulation.

  12. Eastern Ross Ice Sheet Deglacial History inferred from the Roosevelt Island Ice Core

    NASA Astrophysics Data System (ADS)

    Fudge, T. J.; Buizert, C.; Lee, J.; Waddington, E. D.; Bertler, N. A. N.; Conway, H.; Brook, E.; Severinghaus, J. P.

    2017-12-01

    The Ross Ice Sheet drains large portions of both West and East Antarctica. Understanding the retreat of the Ross Ice Sheet following the Last Glacial Maximum is particularly difficult in the eastern Ross area where there is no exposed rock and the Ross Ice Shelf prevents extensive bathymetric mapping. Coastal domes, by preserving old ice, can be used to infer the establishment of grounded ice and be used to infer past ice thickness. Here we focus on Roosevelt Island, in the eastern Ross Sea, where the Roosevelt Island Climate Evolution project recently completed an ice core to bedrock. Using ice-flow modeling constrained by the depth-age relationship and an independent estimate of accumulation rate from firn-densification measurements and modeling, we infer ice thickness histories for the LGM (20ka) to present. Preliminary results indicate thinning of 300m between 15ka and 12ka is required. This is similar to the amount and timing of thinning inferred at Siple Dome, in the central Ross Sea (Waddington et al., 2005; Price et al., 2007) and supports the presence of active ice streams throughout the Ross Ice Sheet advance during the LGM.

  13. Geological and geomorphological insights into Antarctic ice sheet evolution.

    PubMed

    Sugden, David E; Bentley, Michael J; O Cofaigh, Colm

    2006-07-15

    Technical advances in the study of ice-free parts of Antarctica can provide quantitative records that are useful for constraining and refining models of ice sheet evolution and behaviour. Such records improve our understanding of system trajectory, influence the questions we ask about system stability and help to define the ice-sheet processes that are relevant on different time-scales. Here, we illustrate the contribution of cosmogenic isotope analysis of exposed bedrock surfaces and marine geophysical surveying to the understanding of Antarctic ice sheet evolution on a range of time-scales. In the Dry Valleys of East Antarctica, 3He dating of subglacial flood deposits that are now exposed on mountain summits provide evidence of an expanded and thicker Mid-Miocene ice sheet. The survival of surface boulders for approximately 14Myr, the oldest yet measured, demonstrates exceptionally low rates of subsequent erosion and points to the persistence and stability of the dry polar desert climate since that time. Increasingly, there are constraints on West Antarctic ice sheet fluctuations during Quaternary glacial cycles. In the Sarnoff Mountains of Marie Byrd Land in West Antarctica, 10Be and 26Al cosmogenic isotope analysis of glacial erratics and bedrock reveal steady thinning of the ice sheet from 10400 years ago to the present, probably as a result of grounding line retreat. In the Antarctic Peninsula, offshore analysis reveals an extensive ice sheet at the last glacial maximum. Based on radiocarbon dating, deglaciation began by 17000cal yr BP and was complete by 9500cal yr BP. Deglaciation of the west and east sides of the Antarctic Peninsula ice sheet occurred at different times and rates, but was largely complete by the Early Holocene. At that time ice shelves were less extensive on the west side of the Antarctic Peninsula than they are today. The message from the past is that individual glacier drainage basins in Antarctica respond in different and distinctive

  14. Radar attenuation and temperature within the Greenland Ice Sheet

    USGS Publications Warehouse

    MacGregor, Joseph A; Li, Jilu; Paden, John D; Catania, Ginny A; Clow, Gary D.; Fahnestock, Mark A; Gogineni, Prasad S.; Grimm, Robert E.; Morlighem, Mathieu; Nandi, Soumyaroop; Seroussi, Helene; Stillman, David E

    2015-01-01

    The flow of ice is temperature-dependent, but direct measurements of englacial temperature are sparse. The dielectric attenuation of radio waves through ice is also temperature-dependent, and radar sounding of ice sheets is sensitive to this attenuation. Here we estimate depth-averaged radar-attenuation rates within the Greenland Ice Sheet from airborne radar-sounding data and its associated radiostratigraphy. Using existing empirical relationships between temperature, chemistry, and radar attenuation, we then infer the depth-averaged englacial temperature. The dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry. Where radar transects intersect boreholes, radar-inferred temperature is consistently higher than that measured directly. We attribute this discrepancy to the poorly recognized frequency dependence of the radar-attenuation rate and correct for this effect empirically, resulting in a robust relationship between radar-inferred and borehole-measured depth-averaged temperature. Radar-inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice-sheet model, particularly in southern Greenland. This pattern suggests that past changes in surface boundary conditions (temperature and accumulation rate) affect the ice sheet's present temperature structure over a much larger area than previously recognized. This radar-inferred temperature structure provides a new constraint for thermomechanical models of the Greenland Ice Sheet.

  15. Data assimilation and prognostic whole ice sheet modelling with the variationally derived, higher order, open source, and fully parallel ice sheet model VarGlaS

    NASA Astrophysics Data System (ADS)

    Brinkerhoff, D. J.; Johnson, J. V.

    2013-07-01

    We introduce a novel, higher order, finite element ice sheet model called VarGlaS (Variational Glacier Simulator), which is built on the finite element framework FEniCS. Contrary to standard procedure in ice sheet modelling, VarGlaS formulates ice sheet motion as the minimization of an energy functional, conferring advantages such as a consistent platform for making numerical approximations, a coherent relationship between motion and heat generation, and implicit boundary treatment. VarGlaS also solves the equations of enthalpy rather than temperature, avoiding the solution of a contact problem. Rather than include a lengthy model spin-up procedure, VarGlaS possesses an automated framework for model inversion. These capabilities are brought to bear on several benchmark problems in ice sheet modelling, as well as a 500 yr simulation of the Greenland ice sheet at high resolution. VarGlaS performs well in benchmarking experiments and, given a constant climate and a 100 yr relaxation period, predicts a mass evolution of the Greenland ice sheet that matches present-day observations of mass loss. VarGlaS predicts a thinning in the interior and thickening of the margins of the ice sheet.

  16. Modeling the fracture of ice sheets on parallel computers.

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Waisman, Haim; Bell, Robin; Keyes, David

    2010-03-01

    The objective of this project is to investigate the complex fracture of ice and understand its role within larger ice sheet simulations and global climate change. At the present time, ice fracture is not explicitly considered within ice sheet models due in part to large computational costs associated with the accurate modeling of this complex phenomena. However, fracture not only plays an extremely important role in regional behavior but also influences ice dynamics over much larger zones in ways that are currently not well understood. Dramatic illustrations of fracture-induced phenomena most notably include the recent collapse of ice shelves inmore » Antarctica (e.g. partial collapse of the Wilkins shelf in March of 2008 and the diminishing extent of the Larsen B shelf from 1998 to 2002). Other fracture examples include ice calving (fracture of icebergs) which is presently approximated in simplistic ways within ice sheet models, and the draining of supraglacial lakes through a complex network of cracks, a so called ice sheet plumbing system, that is believed to cause accelerated ice sheet flows due essentially to lubrication of the contact surface with the ground. These dramatic changes are emblematic of the ongoing change in the Earth's polar regions and highlight the important role of fracturing ice. To model ice fracture, a simulation capability will be designed centered around extended finite elements and solved by specialized multigrid methods on parallel computers. In addition, appropriate dynamic load balancing techniques will be employed to ensure an approximate equal amount of work for each processor.« less

  17. Enhanced ice sheet melting driven by volcanic eruptions during the last deglaciation.

    PubMed

    Muschitiello, Francesco; Pausata, Francesco S R; Lea, James M; Mair, Douglas W F; Wohlfarth, Barbara

    2017-10-24

    Volcanic eruptions can impact the mass balance of ice sheets through changes in climate and the radiative properties of the ice. Yet, empirical evidence highlighting the sensitivity of ancient ice sheets to volcanism is scarce. Here we present an exceptionally well-dated annual glacial varve chronology recording the melting history of the Fennoscandian Ice Sheet at the end of the last deglaciation (∼13,200-12,000 years ago). Our data indicate that abrupt ice melting events coincide with volcanogenic aerosol emissions recorded in Greenland ice cores. We suggest that enhanced ice sheet runoff is primarily associated with albedo effects due to deposition of ash sourced from high-latitude volcanic eruptions. Climate and snowpack mass-balance simulations show evidence for enhanced ice sheet runoff under volcanically forced conditions despite atmospheric cooling. The sensitivity of past ice sheets to volcanic ashfall highlights the need for an accurate coupling between atmosphere and ice sheet components in climate models.

  18. Ice sheet climate modeling: past achievements, ongoing challenges, and future endeavors

    NASA Astrophysics Data System (ADS)

    Lenaerts, J.

    2017-12-01

    Fluctuations in surface mass balance (SMB) mask out a substantial portion of contemporary Greenland and Antarctic ice sheet mass loss. That implies that we need accurate, consistent, and long-term SMB time series to isolate the mass loss signal. This in turn requires understanding of the processes driving SMB, and how they interplay. The primary controls on present-day ice sheet SMB are snowfall, which is regulated by large-scale atmospheric variability, and surface meltwater production at the ice sheet's edges, which is a complex result of atmosphere-surface interactions. Additionally, wind-driven snow redistribution and sublimation are large SMB contributors on the downslope areas of the ice sheets. Climate models provide an integrated framework to simulate all these individual ice sheet components. Recent developments in RACMO2, a regional climate model bound by atmospheric reanalyses, have focused on enhancing horizontal resolution, including blowing snow, snow albedo, and meltwater processes. Including these physics not only enhanced our understanding of the ice sheet climate system, but also enabled to obtain increasingly accurate estimates of ice sheet SMB. However, regional models are not suitable to capture the mutual interactions between ice sheet and the remainder of the global climate system in transient climates. To take that next step, global climate models are essential. In this talk, I will highlight our present work on improving ice sheet climate in the Community Earth System Model (CESM). In particular, we focus on an improved representation of polar firn, ice sheet clouds, and precipitation. For this exercise, we extensively use field observations, remote sensing data, as well as RACMO2. Next, I will highlight how CESM is used to enhance our understanding of ice sheet SMB, its drivers, and past and present changes.

  19. Polar ice-sheet contributions to sea level during past warm periods

    NASA Astrophysics Data System (ADS)

    Dutton, A.

    2015-12-01

    Recent sea-level rise has been dominated by thermal expansion and glacier loss, but the contribution from mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions under future sustained warming. Due to limitations of existing ice sheet models and the lack of relevant analogues in the historical record, projecting the timing and magnitude of polar ice sheet mass loss in the future remains challenging. One approach to improving our understanding of how polar ice-sheet retreat will unfold is to integrate observations and models of sea level, ice sheets, and climate during past intervals of warmth when the polar ice sheets contributed to higher sea levels. A recent review evaluated the evidence of polar ice sheet mass loss during several warm periods, including interglacials during the mid-Pliocene warm period, Marine Isotope Stage (MIS) 11, 5e (Last Interglacial), and 1 (Holocene). Sea-level benchmarks of ice-sheet retreat during the first of these three periods, when global mean climate was ~1 to 3 deg. C warmer than preindustrial, are useful for understanding the long-term potential for future sea-level rise. Despite existing uncertainties in these reconstructions, it is clear that our present climate is warming to a level associated with significant polar ice-sheet loss in the past, resulting in a conservative estimate for a global mean sea-level rise of 6 meters above present (or more). This presentation will focus on identifying the approaches that have yielded significant advances in terms of past sea level and ice sheet reconstruction as well as outstanding challenges. A key element of recent advances in sea-level reconstructions is the ability to recognize and quantify the imprint of geophysical processes, such as glacial isostatic adjustment (GIA) and dynamic topography, that lead to significant spatial variability in sea level reconstructions. Identifying specific ice-sheet sources that contributed to higher sea levels

  20. Ice streams of the Late Wisconsin Cordilleran Ice Sheet in western North America

    NASA Astrophysics Data System (ADS)

    Eyles, Nick; Arbelaez Moreno, Lina; Sookhan, Shane

    2018-01-01

    The Late Wisconsin Cordilleran Ice Sheet (CIS) of western North America is thought to have reached its maximum extent (∼2.5 × 106 km2) as late at c. 14.5 ka. Most (80%) of the ice sheet's bed consists of high mountains but its 'core zone' sited on plateaux of the Intermontane Belt of British Columbia and coterminous parts of the USA, shows broad swaths of subglacially-streamlined rock and sediment. Broad scale mapping from new digital imagery data identifies three subglacial bed types: 1) 'hard beds' of variably streamlined bedrock; 2) drumlinized 'soft beds' of deformation till reworked from antecedent sediment, and 3) 'mixed beds' of variably-streamlined bedrock protruding through drumlinized sediment. Drumlins on soft beds appear to be erosional features cut into till and antecedent sediments, and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations (MSGLs). 'Grooved' and 'cloned' drumlins appear to record the transition from drumlins to MSGLs. The location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow; rock-cut MSGLs are also present locally on the floors of outlet valleys. CIS disintegrated in <1000 years shortly after c. 13.0 ka releasing very large volumes of meltwater and sediment to the Pacific coast. Abrupt deglaciation may reflect unsustainable calving of marine-based ice streams along the glacio-isostatically depressed coast; large deep 'fiord lakes' in the ice sheet's interior may have played an analogous role. Mapping of the broad scale distribution of bed types across the Cordilleran Ice Sheet provides key information for paleoglaciological modelling and also for understanding the beds of modern ice masses such as the Greenland Ice Sheet which is of a comparable topographic setting.

  1. Collapse of polar ice sheets during the stage 11 interglacial.

    PubMed

    Raymo, Maureen E; Mitrovica, Jerry X

    2012-03-14

    Contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 metres higher than it is today. Geochronologic and geomorphic evidence indicates that these features formed during interglacial marine isotope stage (MIS) 11, an unusually long interval of warmth during the ice age. Previous work has advanced two divergent hypotheses for these shoreline features: first, significant melting of the East Antarctic Ice Sheet, in addition to the collapse of the West Antarctic Ice Sheet and the Greenland Ice Sheet; or second, emplacement by a mega-tsunami during MIS 11 (ref. 4, 5). Here we show that the elevations of these features are corrected downwards by ∼10 metres when we account for post-glacial crustal subsidence of these sites over the course of the anomalously long interglacial. On the basis of this correction, we estimate that eustatic sea level rose to ∼6-13 m above the present-day value in the second half of MIS 11. This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor, thereby resolving the long-standing controversy over the stability of the East Antarctic Ice Sheet during MIS 11.

  2. connecting the dots between Greenland ice sheet surface melting and ice flow dynamics (Invited)

    NASA Astrophysics Data System (ADS)

    Box, J. E.; Colgan, W. T.; Fettweis, X.; Phillips, T. P.; Stober, M.

    2013-12-01

    This presentation is of a 'unified theory' in glaciology that first identifies surface albedo as a key factor explaining total ice sheet mass balance and then surveys a mechanistic self-reinforcing interaction between melt water and ice flow dynamics. The theory is applied in a near-real time total Greenland mass balance retrieval based on surface albedo, a powerful integrator of the competing effects of accumulation and ablation. New snowfall reduces sunlight absorption and increases meltwater retention. Melting amplifies absorbed sunlight through thermal metamorphism and bare ice expansion in space and time. By ';following the melt'; we reveal mechanisms linking existing science into a unified theory. Increasing meltwater softens the ice sheet in three ways: 1.) sensible heating given the water temperature exceeds that of the ice sheet interior; 2.) Some infiltrating water refreezes, transferring latent heat to the ice; 3.) Friction from water turbulence heats the ice. It has been shown that for a point on the ice sheet, basal lubrication increases ice flow speed to a time when an efficient sub-glacial drainage network develops that reduces this effect. Yet, with an increasing melt duration the point where the ice sheet glides on a wet bed increases inland to a larger area. This effect draws down the ice surface elevation, contributing to the ';elevation feedback'. In a perpetual warming scenario, the elevation feedback ultimately leads to ice sheet loss reversible only through much slower ice sheet growth in an ice age environment. As the inland ice sheet accelerates, the horizontal extension pulls cracks and crevasses open, trapping more sunlight, amplifying the effect of melt accelerated ice. As the bare ice area increases, the direct sun-exposed crevassed and infiltration area increases further allowing the ice warming process to occur more broadly. Considering hydrofracture [a.k.a. hydrofracking]; surface meltwater fills cracks, attacking the ice integrity

  3. Final Laurentide ice-sheet deglaciation and Holocene climate-sea level change

    USGS Publications Warehouse

    Ullman, David J.; Carlson, Anders E.; Hostetler, Steven W.; Clark, Peter U.; Cuzzone, Joshua; Milne, Glenn A.; Winsor, Kelsey; Caffee, Marc A.

    2016-01-01

    Despite elevated summer insolation forcing during the early Holocene, global ice sheets retained nearly half of their volume from the Last Glacial Maximum, as indicated by deglacial records of global mean sea level (GMSL). Partitioning the GMSL rise among potential sources requires accurate dating of ice-sheet extent to estimate ice-sheet volume. Here, we date the final retreat of the Laurentide Ice Sheet with 10Be surface exposure ages for the Labrador Dome, the largest of the remnant Laurentide ice domes during the Holocene. We show that the Labrador Dome deposited moraines during North Atlantic cold events at ∼10.3 ka, 9.3 ka and 8.2 ka, suggesting that these regional climate events helped stabilize the retreating Labrador Dome in the early Holocene. After Hudson Bay became seasonally ice free at ∼8.2 ka, the majority of Laurentide ice-sheet melted abruptly within a few centuries. We demonstrate through high-resolution regional climate model simulations that the thermal properties of a seasonally ice-free Hudson Bay would have increased Laurentide ice-sheet ablation and thus contributed to the subsequent rapid Labrador Dome retreat. Finally, our new 10Be chronology indicates full Laurentide ice-sheet had completely deglaciated by 6.7 ± 0.4 ka, which re quires that Antarctic ice sheets contributed 3.6–6.5 m to GMSL rise since 6.3–7.1 ka.

  4. Deciphering the evolution of the last Eurasian ice sheets

    NASA Astrophysics Data System (ADS)

    Hughes, Anna; Gyllencreutz, Richard; Mangerud, Jan; Svendsen, John Inge

    2016-04-01

    Glacial geologists need ice sheet-scale chronological reconstructions of former ice extent to set individual records in a wider context and compare interpretations of ice sheet response to records of past environmental changes. Ice sheet modellers require empirical reconstructions on size and volume of past ice sheets that are fully documented, specified in time and include uncertainty estimates for model validation or constraints. Motivated by these demands, in 2005 we started a project (Database of the Eurasian Deglaciation, DATED) to compile and archive all published dates relevant to constraining the build-up and retreat of the last Eurasian ice sheets, including the British-Irish, Scandinavian and Svalbard-Barents-Kara Seas ice sheets (BIIS, SIS and SBKIS respectively). Over 5000 dates were assessed for reliability and used together with published ice-sheet margin positions to reconstruct time-slice maps of the ice sheets' extent, with uncertainty bounds, every 1000 years between 25-10 kyr ago and at four additional periods back to 40 kyr ago. Ten years after the idea for a database was conceived, the first version of results (DATED-1) has now been released (Hughes et al. 2016). We observe that: i) both the BIIS and SBKIS achieve maximum extent, and commence retreat earlier than the larger SIS; ii) the eastern terrestrial margin of the SIS reached its maximum extent up to 7000 years later than the westernmost marine margin; iii) the combined maximum ice volume (~24 m sea-level equivalent) was reached c. 21 ka; iv) large uncertainties exist; predominantly across marine sectors (e.g. the timing of coalescence and separation of the SIS and BKIS) but also in well-studied areas due to conflicting yet equally robust data. In just three years since the DATED-1 census (1 January 2013), the volume of new information (from both dates and mapped glacial geomorphology) has grown significantly (~1000 new dates). Here, we present the DATED-1 results in the context of the

  5. A model of the Greenland ice sheet deglaciation

    NASA Astrophysics Data System (ADS)

    Lecavalier, Benoit

    The goal of this thesis is to improve our understanding of the Greenland ice sheet (GrIS) and how it responds to climate change. This was achieved using ice core records to infer elevation changes of the GrIS during the Holocene (11.7 ka BP to Present). The inferred elevation changes show the response of the ice sheet interior to the Holocene Thermal Maximum (HTM; 9-5 ka BP) when temperatures across Greenland were warmer than present. These ice-core derived thinning curves act as a new set of key constraints on the deglacial history of the GrIS. Furthermore, a calibration was conducted on a three-dimensional thermomechanical ice sheet, glacial isostatic adjustment, and relative sea-level model of GrIS evolution during the most recent deglaciation (21 ka BP to present). The model was data-constrained to a variety of proxy records from paleoclimate archives and present-day observations of ice thickness and extent.

  6. Hypsometric amplification and routing moderation of Greenland ice sheet meltwater release

    NASA Astrophysics Data System (ADS)

    van As, Dirk; Mikkelsen, Andreas Bech; Holtegaard Nielsen, Morten; Box, Jason E.; Claesson Liljedahl, Lillemor; Lindbäck, Katrin; Pitcher, Lincoln; Hasholt, Bent

    2017-06-01

    Concurrent ice sheet surface runoff and proglacial discharge monitoring are essential for understanding Greenland ice sheet meltwater release. We use an updated, well-constrained river discharge time series from the Watson River in southwest Greenland, with an accurate, observation-based ice sheet surface mass balance model of the ˜ 12 000 km2 ice sheet area feeding the river. For the 2006-2015 decade, we find a large range of a factor of 3 in interannual variability in discharge. The amount of discharge is amplified ˜ 56 % by the ice sheet's hypsometry, i.e., area increase with elevation. A good match between river discharge and ice sheet surface meltwater production is found after introducing elevation-dependent transit delays that moderate diurnal variability in meltwater release by a factor of 10-20. The routing lag time increases with ice sheet elevation and attains values in excess of 1 week for the upper reaches of the runoff area at ˜ 1800 m above sea level. These multi-day routing delays ensure that the highest proglacial discharge levels and thus overbank flooding events are more likely to occur after multi-day melt episodes. Finally, for the Watson River ice sheet catchment, we find no evidence of meltwater storage in or release from the en- and subglacial environments in quantities exceeding our methodological uncertainty, based on the good match between ice sheet runoff and proglacial discharge.

  7. Greenland Ice Sheet Surface Temperature, Melt, and Mass Loss: 2000-2006

    NASA Technical Reports Server (NTRS)

    Hall, Dorothy K.; Williams, Richard S., Jr.; Luthcke, Scott B.; DiGirolamo, Nocolo

    2007-01-01

    Extensive melt on the Greenland Ice Sheet has been documented by a variety of ground and satellite measurements in recent years. If the well-documented warming continues in the Arctic, melting of the Greenland Ice Sheet will likely accelerate, contributing to sea-level rise. Modeling studies indicate that an annual or summer temperature rise of 1 C on the ice sheet will increase melt by 20-50% therefore, surface temperature is one of the most important ice-sheet parameters to study for analysis of changes in the mass balance of the ice-sheet. The Greenland Ice Sheet contains enough water to produce a rise in eustatic sea level of up to 7.0 m if the ice were to melt completely. However, even small changes (centimeters) in sea level would cause important economic and societal consequences in the world's major coastal cities thus it is extremely important to monitor changes in the ice-sheet surface temperature and to ultimately quantify these changes in terms of amount of sea-level rise. We have compiled a high-resolution, daily time series of surface temperature of the Greenland Ice Sheet, using the I-km resolution, clear-sky land-surface temperature (LST) standard product from the Moderate-Resolution Imaging Spectroradiometer (MODIS), from 2000 - 2006. We also use Gravity Recovery and Climate Experiment (GRACE) data, averaged over 10-day periods, to measure change in mass of the ice sheet as it melt and snow accumulates. Surface temperature can be used to determine frequency of surface melt, timing of the start and the end of the melt season, and duration of melt. In conjunction with GRACE data, it can also be used to analyze timing of ice-sheet mass loss and gain.

  8. Determining Greenland Ice Sheet Accumulation Rates from Radar Remote Sensing

    NASA Technical Reports Server (NTRS)

    Jezek, Kenneth C.

    2001-01-01

    An important component of NASA's Program for Arctic Regional Climate Assessment (PARCA) is a mass balance investigation of the Greenland Ice Sheet. The mass balance is calculated by taking the difference between the snow accumulation and the ice discharge of the ice sheet. Uncertainties in this calculation include the snow accumulation rate, which has traditionally been determined by interpolating data from ice core samples taken throughout the ice sheet. The sparse data associated with ice cores, coupled with the high spatial and temporal resolution provided by remote sensing, have motivated scientists to investigate relationships between accumulation rate and microwave observations.

  9. Recent Changes in the Greenland Ice Sheet as Seen from Space

    NASA Technical Reports Server (NTRS)

    Hall, Dorothy K.

    2011-01-01

    Many changes in the Greenland Ice Sheet have been reported in the recent scientific literature and have been attributed to various responses of the ice sheet due to regional (and global) warming. Because melting of the ice sheet would contribute approximately 7 m to sea-level rise, the lives and habitat of hundreds of millions of people worldwide would be directly and indirectly affected if continued ice-sheet melting occurs. As mean-annual global temperatures have increased, there has been an increasing focus on studying the Greenland Ice Sheet using available satellite data, and numerous expeditions have been undertaken. Regional "clear-sky" surface temperature increases since the early 1980s in the Arctic, measured using Advanced Very High Resolution Radiometer (AVHRR) infrared data, range from 0.57+/-0.02 C to 0.72+/-0.10 C per decade. Arctic warming has important implications for ice-sheet mass balance because much of the periphery of the Greenland Ice Sheet is already near O C during the melt season, and is thus vulnerable to more extensive melting if temperatures continue to increase. An increase in melting of the ice sheet would accelerate sea-level rise, an issue of increasing concern to billions of people worldwide. The surface temperature of the ice sheet has been studied in even greater detail using Moderate-Resolution Imaging Spectroradiometer (MODIS) data in the six individual drainage basins as well as for the ice sheet as a whole. Surface temperature trends in the decade of the 2000s have not been strong, according to the MODIS measurements. In addition to surface-temperature increases over the last few decades as measured by AVHRR, other changes have been observed such as accelerated movement of many of Greenland's outlet glaciers and sudden draining of supraglacial lakes. Decreasing mass of the ice sheet since (at least) 2002 has been measured using Gravity Recovery and Climate Experiment (GRACE) data, along with an build-up of ice at the higher

  10. Sea-level response to ice sheet evolution: An ocean perspective

    NASA Technical Reports Server (NTRS)

    Jacobs, Stanley S.

    1991-01-01

    The ocean's influence upon and response to Antarctic ice sheet changes is considered in relation to sea level rise over recent and future decades. Assuming present day ice fronts are in approximate equilibrium, a preliminary budget for the ice sheet is estimated from accumulation vs. iceberg calving and the basal melting that occurs beneath floating ice shelves. Iceberg calving is derived from the volume of large bergs identified and tracked by the Navy/NOAA Joint Ice Center and from shipboard observations. Basal melting exceeds 600 cu km/yr and is concentrated near the ice fronts and ice shelf grounding lines. An apparent negative mass balance for the Antarctic ice sheet may result from an anomalous calving rate during the past decade, but there are large uncertainties associated with all components of the ice budget. The results from general circulation models are noted in the context of projected precipitation increases and ocean temperature changes on and near the continent. An ocean research program that could help refine budget estimates is consistent with goals of the West Antarctic Ice Sheet Initiative.

  11. The Svalbard-Barents Sea ice-sheet - Historical, current and future perspectives

    NASA Astrophysics Data System (ADS)

    Ingólfsson, Ólafur; Landvik, Jon Y.

    2013-03-01

    The history of research on the Late Quaternary Svalbard-Barents Sea ice sheet mirrors the developments of ideas and the shifts of paradigms in glacial theory over the past 150 years. Since the onset of scientific research there in the early 19th Century, Svalbard has been a natural laboratory where ideas and concepts have been tested, and played an important (but rarely acknowledged) role in the break-through of the Ice Age theory in the 1870's. The history of how the scientific perception of the Svalbard-Barents sea ice sheet developed in the mid-20th Century also tells a story of how a combination of fairly scattered and often contradictory observational data, and through both deductive and inductive reasoning, could outline a major ice sheet that had left but few tangible fingerprints. Since the 1980's, with increased terrestrial stratigraphical data, ever more marine geological evidence and better chronological control of glacial events, our perception of the Svalbard-Barents Sea ice sheet has changed. The first reconstructions depicted it as a static, concentric, single-domed ice sheet, with ice flowing from an ice divide over the central northern Barents Sea that expanded and declined in response to large-scale, Late Quaternary climate fluctuations, and which was more or less in tune with other major Northern Hemisphere ice sheets. We now increasingly perceive it as a very dynamic, multidomed ice sheet, controlled by climate fluctuations, relative sea-level change, as well as subglacial topography, substrate properties and basal temperature. In this respect, the Svalbard-Barents Sea ice sheet will increasingly hold the key for understanding the dynamics and processes of how marine-based ice sheets build-up and decay.

  12. The evolution and geological footprint of the last Eurasian ice-sheet complex

    NASA Astrophysics Data System (ADS)

    Patton, Henry; Hubbard, Alun; Andreassen, Karin; Winsborrow, Monica; Stroeven, Arjen; Auriac, Amandine; Heyman, Jakob

    2017-04-01

    with its isostatic footprint had a major impact on fluvial hydrology of western Eurasia, damming the Baltic and White Sea proglacial lakes from c. 17.8 ka BP through to the Holocene and diverting many river systems. Acknowledegments This project is funded by CAGE (Centre for Arctic Gas Hydrate, Environment and Climate), Norwegian Research Council grant no. 223259. Clark, P.U., Dyke, A.S., Shakun, J.D., Carlson, A.E., Clark, J., Wohlfarth, B., Mitrovica, J.X., Hostetler, S.W., McCabe, a M., 2009. The Last Glacial Maximum. Science 325, 710-714. doi:10.1126/science.1172873 Hughes, A.L.C., Gyllencreutz, R., Lohne, Ø.S., Mangerud, J., Svendsen, J.I., 2016. The last Eurasian ice sheets - a chronological database and time-slice reconstruction, DATED-1. Boreas 45, 1-45. doi:10.1111/bor.12142 Patton, H., Hubbard, A., Andreassen, K., Winsborrow, M., Stroeven, A.P., 2016. The build-up, configuration, and dynamical sensitivity of the Eurasian ice-sheet complex to Late Weichselian climatic and oceanic forcing. Quat. Sci. Rev. 153, 97-121. doi:10.1016/j.quascirev.2016.10.009

  13. Greenland Ice Sheet flow response to runoff variability

    NASA Astrophysics Data System (ADS)

    Stevens, Laura A.; Behn, Mark D.; Das, Sarah B.; Joughin, Ian; Noël, Brice P. Y.; Broeke, Michiel R.; Herring, Thomas

    2016-11-01

    We use observations of ice sheet surface motion from a Global Positioning System network operating from 2006 to 2014 around North Lake in west Greenland to investigate the dynamical response of the Greenland Ice Sheet's ablation area to interannual variability in surface melting. We find no statistically significant relationship between runoff season characteristics and ice flow velocities within a given year or season. Over the 7 year time series, annual velocities at North Lake decrease at an average rate of -0.9 ± 1.1 m yr-2, consistent with the negative trend in annual velocities observed in neighboring regions over recent decades. We find that net runoff integrated over several preceding years has a negative correlation with annual velocities, similar to findings from the two other available decadal records of ice velocity in western Greenland. However, we argue that this correlation is not necessarily evidence for a direct hydrologic mechanism acting on the timescale of multiple years but could be a statistical construct. Finally, we stress that neither the decadal slowdown trend nor the negative correlation between velocity and integrated runoff is predicted by current ice-sheet models, underscoring that these models do not yet capture all the relevant feedbacks between runoff and ice dynamics needed to predict long-term trends in ice sheet flow.

  14. Current state and future perspectives on coupled ice-sheet - sea-level modelling

    NASA Astrophysics Data System (ADS)

    de Boer, Bas; Stocchi, Paolo; Whitehouse, Pippa L.; van de Wal, Roderik S. W.

    2017-08-01

    The interaction between ice-sheet growth and retreat and sea-level change has been an established field of research for many years. However, recent advances in numerical modelling have shed new light on the precise interaction of marine ice sheets with the change in near-field sea level, and the related stability of the grounding line position. Studies using fully coupled ice-sheet - sea-level models have shown that accounting for gravitationally self-consistent sea-level change will act to slow down the retreat and advance of marine ice-sheet grounding lines. Moreover, by simultaneously solving the 'sea-level equation' and modelling ice-sheet flow, coupled models provide a global field of relative sea-level change that is consistent with dynamic changes in ice-sheet extent. In this paper we present an overview of recent advances, possible caveats, methodologies and challenges involved in coupled ice-sheet - sea-level modelling. We conclude by presenting a first-order comparison between a suite of relative sea-level data and output from a coupled ice-sheet - sea-level model.

  15. Clouds enhance Greenland ice sheet mass loss

    NASA Astrophysics Data System (ADS)

    Van Tricht, Kristof; Gorodetskaya, Irina V.; L'Ecuyer, Tristan; Lenaerts, Jan T. M.; Lhermitte, Stef; Noel, Brice; Turner, David D.; van den Broeke, Michiel R.; van Lipzig, Nicole P. M.

    2015-04-01

    Clouds have a profound influence on both the Arctic and global climate, while they still represent one of the key uncertainties in climate models, limiting the fidelity of future climate projections. The potentially important role of thin liquid-containing clouds over Greenland in enhancing ice sheet melt has recently gained interest, yet current research is spatially and temporally limited, focusing on particular events, and their large scale impact on the surface mass balance remains unknown. We used a combination of satellite remote sensing (CloudSat - CALIPSO), ground-based observations and climate model (RACMO) data to show that liquid-containing clouds warm the Greenland ice sheet 94% of the time. High surface reflectivity (albedo) for shortwave radiation reduces the cloud shortwave cooling effect on the absorbed fluxes, while not influencing the absorption of longwave radiation. Cloud warming over the ice sheet therefore dominates year-round. Only when albedo values drop below ~0.6 in the coastal areas during summer, the cooling effect starts to overcome the warming effect. The year-round excess of energy due to the presence of liquid-containing clouds has an extensive influence on the mass balance of the ice sheet. Simulations using the SNOWPACK snow model showed not only a strong influence of these liquid-containing clouds on melt increase, but also on the increased sublimation mass loss. Simulations with the Community Earth System Climate Model for the end of the 21st century (2080-2099) show that Greenland clouds contain more liquid water path and less ice water path. This implies that cloud radiative forcing will be further enhanced in the future. Our results therefore urge the need for improving cloud microphysics in climate models, to improve future projections of ice sheet mass balance and global sea level rise.

  16. The future of ice sheets and sea ice: between reversible retreat and unstoppable loss.

    PubMed

    Notz, Dirk

    2009-12-08

    We discuss the existence of cryospheric "tipping points" in the Earth's climate system. Such critical thresholds have been suggested to exist for the disappearance of Arctic sea ice and the retreat of ice sheets: Once these ice masses have shrunk below an anticipated critical extent, the ice-albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. We here give an overview of our current understanding of such threshold behavior. By using conceptual arguments, we review the recent findings that such a tipping point probably does not exist for the loss of Arctic summer sea ice. Hence, in a cooler climate, sea ice could recover rapidly from the loss it has experienced in recent years. In addition, we discuss why this recent rapid retreat of Arctic summer sea ice might largely be a consequence of a slow shift in ice-thickness distribution, which will lead to strongly increased year-to-year variability of the Arctic summer sea-ice extent. This variability will render seasonal forecasts of the Arctic summer sea-ice extent increasingly difficult. We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet.

  17. Reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Margold, Martin; Stokes, Chris R.; Clark, Chris D.

    2018-06-01

    This paper reconstructs the deglaciation of the Laurentide Ice Sheet (LIS; including the Innuitian Ice Sheet) from the Last Glacial Maximum (LGM), with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides. We build on a recent inventory of Laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the LIS, which we depict in a series of palaeogeographic maps. Results show that the drainage network at the LGM was similar to modern-day Antarctica. The majority of the ice streams were marine terminating and topographically-controlled and many of these continued to function late into the deglaciation, until the ice sheet lost its marine margin. Ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build-up, peak-phase and collapse of the Cordilleran-Laurentide ice saddle. The south-eastern marine margin in Atlantic Canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the LGM. In contrast, the ice streams draining towards the north-western and north-eastern marine margins in the Beaufort Sea and in Baffin Bay appear to have remained stable throughout most of the Late Glacial, and some of them continued to function until after the Younger Dryas (YD). The YD influenced the dynamics of the deglaciation, but there remains uncertainty about the response of the ice sheet in several sectors. We tentatively ascribe the switching-on of some major ice streams during this period (e.g. M'Clintock Channel Ice Stream at the north-west margin), but for other large ice streams whose timing partially overlaps with the YD, the drivers are less clear and ice-dynamical processes, rather than effects of climate and surface mass balance are viewed as more likely drivers. Retreat

  18. Coupled energy-balance/ice-sheet model simulations of the glacial cycle: A possible connection between terminations and terrigenous dust

    NASA Astrophysics Data System (ADS)

    Peltier, W. Richard; Marshall, Shawn

    1995-07-01

    We apply a coupled energy-balance/ice-sheet climate model in an investigation of northern hemisphere ice-sheet advance and retreat over the last glacial cycle. When driven only by orbital insolation variations, the model predicts ice-sheet advances over the continents of North America and Eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses. The orbital forcing alone, however, is unable to induce the observed rapid ice-sheet retreat, and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting. In the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination. These include marine instability, thermohaline circulation effects, carbon dioxide variations, and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration. For the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records. Of these various influences, our analyses suggest that the albedo variations in the ice-sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism. Using our parameterization of "dirty" snow in the ablation zone we find glacial retreat to be strongly accelerated, such that complete collapse of the otherwise stable Laurentide ice sheet ensues. The last glacial maximum configurations of the Laurentide and Fennoscandian complexes are also brought into much closer accord with the ICE-3G reconstruction of Tushingham and Peltier (1991,1992) and the ICE-4G reconstruction of Peltier (1994) when this effect is reasonably introduced.

  19. Dating Antarctic ice sheet collapse: Proposing a molecular genetic approach

    NASA Astrophysics Data System (ADS)

    Strugnell, Jan M.; Pedro, Joel B.; Wilson, Nerida G.

    2018-01-01

    Sea levels at the end of this century are projected to be 0.26-0.98 m higher than today. The upper end of this range, and even higher estimates, cannot be ruled out because of major uncertainties in the dynamic response of polar ice sheets to a warming climate. Here, we propose an ecological genetics approach that can provide insight into the past stability and configuration of the West Antarctic Ice Sheet (WAIS). We propose independent testing of the hypothesis that a trans-Antarctic seaway occurred at the last interglacial. Examination of the genomic signatures of bottom-dwelling marine species using the latest methods can provide an independent window into the integrity of the WAIS more than 100,000 years ago. Periods of connectivity facilitated by trans-Antarctic seaways could be revealed by dating coalescent events recorded in DNA. These methods allow alternative scenarios to be tested against a fit to genomic data. Ideal candidate taxa for this work would need to possess a circumpolar distribution, a benthic habitat, and some level of genetic structure indicated by phylogeographical investigation. The purpose of this perspective piece is to set out an ecological genetics method to help resolve when the West Antarctic Ice Shelf last collapsed.

  20. Could a new ice core offer an insight into the stability of the West Antarctic Ice Sheet during the last interglacial?

    NASA Astrophysics Data System (ADS)

    Mulvaney, R.; Hindmarsh, R. C.

    2013-12-01

    Vaughan et al., in their 2011 paper 'Potential Seaways across West Antarctica' (Geochem. Geophys. Geosyst., 12, Q10004, doi:10.1029/2011GC003688), offer the intriguing prospect that substantial ice loss from the West Antarctic Ice Sheet during the previous interglacial period might have resulted in the opening of a seaway between the Weddell Sea and the Amundsen Sea. One of their potential seaways passes between the south western corner of the present Ronne Ice Shelf and the Pine Island Bay, through what is currently the course of the Rutford Ice Stream, between the Ellsworth Mountains and the Fletcher Promontory. To investigate whether this seaway could have existed (and to recover a paleoclimate and ice sheet history from the Weddell Sea), a team from the British Antarctic Survey and the Laboratoire de Glaciologie et Géophysique de l'Environnement drilled an ice core from a close to a topographic dome in the ice surface on the Fletcher Promontory in January 2012, reaching the bedrock at 654.3m depth from the surface. The site was selected to penetrate directly through the centre of a Raymond cupola observed in internal radar reflections from the ice sheet, with the intention that this would ensure we obtained the oldest ice available from the Fletcher Promontory. The basal ice sheet temperature measured was -18°C, implying the oldest ice would not have melted away from the base, while the configuration of the Raymond cupola in the radar horizons suggested stability in the ice dome topography during the majority of the Holocene. Our hypothesis is that chemical analysis of the ice core will reveal whether the site was ever relatively close to open sea water or ice shelf in the Rutford channel 20 km distant, rather than the current 700 km distance to sea ice/open water in either the Weddell Sea or the Amundsen Sea. While we do not yet have the chemistry data to test this hypothesis, in this poster we will discuss whether there is in reality any potential local

  1. Radar Detection of Layering in Ice: Experiments on a Constructed Layered Ice Sheet

    NASA Astrophysics Data System (ADS)

    Carter, L. M.; Koenig, L.; Courville, Z.; Ghent, R. R.; Koutnik, M. R.

    2016-12-01

    The polar caps and glaciers of both Earth and Mars display internal layering that preserves a record of past climate. These layers are apparent both in optical datasets (high resolution images, core samples) and in ground penetrating radar (GPR) data. On Mars, the SHARAD (Shallow Radar) radar on the Mars Reconnaissance Orbiter shows fine layering that changes spatially and with depth across the polar caps. This internal layering has been attributed to changes in fractional dust contamination due to obliquity-induced climate variations, but there are other processes that can lead to internal layers visible in radar data. In particular, terrestrial sounding of ice sheets compared with core samples have revealed that ice density and composition differences account for the majority of the radar reflectors. The large cold rooms and ice laboratory facility at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) provide us a unique opportunity to construct experimental ice sheets in a controlled setting and measure them with radar. In a CRREL laboratory, we constructed a layered ice sheet that is 3-m deep with a various snow and ice layers with known dust concentrations (using JSC Mars-1 basaltic simulant) and density differences. These ice sheets were profiled using a commercial GPR, at frequencies of 200, 400 and 900 MHz, to determine how the radar profile changes due to systematic and known changes in snow and ice layers, including layers with sub-wavelength spacing. We will report results from these experiments and implications for interpreting radar-detected layering in ice on Earth and Mars.

  2. NASA: First Map Of Thawed Areas Under Greenland Ice Sheet

    NASA Image and Video Library

    2017-12-08

    NASA researchers have helped produce the first map showing what parts of the bottom of the massive Greenland Ice Sheet are thawed – key information in better predicting how the ice sheet will react to a warming climate. Greenland’s thick ice sheet insulates the bedrock below from the cold temperatures at the surface, so the bottom of the ice is often tens of degrees warmer than at the top, because the ice bottom is slowly warmed by heat coming from the Earth’s depths. Knowing whether Greenland’s ice lies on wet, slippery ground or is anchored to dry, frozen bedrock is essential for predicting how this ice will flow in the future, But scientists have very few direct observations of the thermal conditions beneath the ice sheet, obtained through fewer than two dozen boreholes that have reached the bottom. Now, a new study synthesizes several methods to infer the Greenland Ice Sheet’s basal thermal state –whether the bottom of the ice is melted or not– leading to the first map that identifies frozen and thawed areas across the whole ice sheet. Map caption: This first-of-a-kind map, showing which parts of the bottom of the Greenland Ice Sheet are likely thawed (red), frozen (blue) or still uncertain (gray), will help scientists better predict how the ice will flow in a warming climate. Credit: NASA Earth Observatory/Jesse Allen Read more: go.nasa.gov/2avKgl2 NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  3. Improving Climate Literacy Using The Ice Sheet System Model (ISSM): A Prototype Virtual Ice Sheet Laboratory For Use In K-12 Classrooms

    NASA Astrophysics Data System (ADS)

    Halkides, D. J.; Larour, E. Y.; Perez, G.; Petrie, K.; Nguyen, L.

    2013-12-01

    Statistics indicate that most Americans learn what they will know about science within the confines of our public K-12 education system and the media. Next Generation Science Standards (NGSS) aim to remedy science illiteracy and provide guidelines to exceed the Common Core State Standards that most U.S. state governments have adopted, by integrating disciplinary cores with crosscutting ideas and real life practices. In this vein, we present a prototype ';Virtual Ice Sheet Laboratory' (I-Lab), geared to K-12 students, educators and interested members of the general public. I-Lab will allow users to perform experiments using a state-of-the-art dynamical ice sheet model and provide detailed downloadable lesson plans, which incorporate this model and are consistent with NGSS Physical Science criteria for different grade bands (K-2, 3-5, 6-8, and 9-12). The ultimate goal of this website is to improve public climate science literacy, especially in regards to the crucial role of the polar ice sheets in Earth's climate and sea level. The model used will be the Ice Sheet System Model (ISSM), an ice flow model developed at NASA's Jet Propulsion Laboratory and UC Irvine, that simulates the near-term evolution of polar ice sheets (Greenland and Antarctica) and includes high spatial resolution capabilities and data assimilation to produce realistic simulations of ice sheet dynamics at the continental scale. Open sourced since 2011, ISSM is used in cutting edge cryosphere research around the globe. Thru I-Lab, students will be able to access ISSM using a simple, online graphical interface that can be launched from a web browser on a computer, tablet or smart phone. The interface will allow users to select different climate conditions and watch how the polar ice sheets evolve in time under those conditions. Lesson contents will include links to background material and activities that teach observation recording, concept articulation, hypothesis formulation and testing, and

  4. Monitoring Antarctic ice sheet surface melting with TIMESAT algorithm

    NASA Astrophysics Data System (ADS)

    Ye, Y.; Cheng, X.; Li, X.; Liang, L.

    2011-12-01

    Antarctic ice sheet contributes significantly to the global heat budget by controlling the exchange of heat, moisture, and momentum at the surface-atmosphere interface, which directly influence the global atmospheric circulation and climate change. Ice sheet melting will cause snow humidity increase, which will accelerate the disintegration and movement of ice sheet. As a result, detecting Antarctic ice sheet melting is essential for global climate change research. In the past decades, various methods have been proposed for extracting snowmelt information from multi-channel satellite passive microwave data. Some methods are based on brightness temperature values or a composite index of them, and others are based on edge detection. TIMESAT (Time-series of Satellite sensor data) is an algorithm for extracting seasonality information from time-series of satellite sensor data. With TIMESAT long-time series brightness temperature (SSM/I 19H) is simulated by Double Logistic function. Snow is classified to wet and dry snow with generalized Gaussian model. The results were compared with those from a wavelet algorithm. On this basis, Antarctic automatic weather station data were used for ground verification. It shows that this algorithm is effective in ice sheet melting detection. The spatial distribution of melting areas(Fig.1) shows that, the majority of melting areas are located on the edge of Antarctic ice shelf region. It is affected by land cover type, surface elevation and geographic location (latitude). In addition, the Antarctic ice sheet melting varies with seasons. It is particularly acute in summer, peaking at December and January, staying low in March. In summary, from 1988 to 2008, Ross Ice Shelf and Ronnie Ice Shelf have the greatest interannual variability in amount of melting, which largely determines the overall interannual variability in Antarctica. Other regions, especially Larsen Ice Shelf and Wilkins Ice Shelf, which is in the Antarctic Peninsula

  5. The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss

    PubMed Central

    Notz, Dirk

    2009-01-01

    We discuss the existence of cryospheric “tipping points” in the Earth's climate system. Such critical thresholds have been suggested to exist for the disappearance of Arctic sea ice and the retreat of ice sheets: Once these ice masses have shrunk below an anticipated critical extent, the ice–albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. We here give an overview of our current understanding of such threshold behavior. By using conceptual arguments, we review the recent findings that such a tipping point probably does not exist for the loss of Arctic summer sea ice. Hence, in a cooler climate, sea ice could recover rapidly from the loss it has experienced in recent years. In addition, we discuss why this recent rapid retreat of Arctic summer sea ice might largely be a consequence of a slow shift in ice-thickness distribution, which will lead to strongly increased year-to-year variability of the Arctic summer sea-ice extent. This variability will render seasonal forecasts of the Arctic summer sea-ice extent increasingly difficult. We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet. PMID:19884496

  6. Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning.

    PubMed

    Krabill; Abdalati; Frederick; Manizade; Martin; Sonntag; Swift; Thomas; Wright; Yungel

    2000-07-21

    Aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999 have been coupled with previously reported data from southern Greenland to analyze the recent mass-balance of the Greenland Ice Sheet. Above 2000 meters elevation, the ice sheet is in balance on average but has some regions of local thickening or thinning. Thinning predominates at lower elevations, with rates exceeding 1 meter per year close to the coast. Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeter per year-approximately 7% of the observed rise.

  7. Dynamic Inland Propagation of Thinning Due to Ice Loss at the Margins of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Wang, Wei Li; Li, Jun J.; Zwally, H. Jay

    2012-01-01

    Mass-balance analysis of the Greenland ice sheet based on surface elevation changes observed by the European Remote-sensing Satellite (ERS) (1992-2002) and Ice, Cloud and land Elevation Satellite (ICESat) (2003-07) indicates that the strongly increased mass loss at lower elevations (<2000 m) of the ice sheet, as observed during 2003-07, appears to induce interior ice thinning at higher elevations. In this paper, we perform a perturbation experiment with a three-dimensional anisotropic ice-flow model (AIF model) to investigate this upstream propagation. Observed thinning rates in the regions below 2000m elevation are used as perturbation inputs. The model runs with perturbation for 10 years show that the extensive mass loss at the ice-sheet margins does in fact cause interior thinning on short timescales (i.e. decadal). The modeled pattern of thinning over the ice sheet agrees with the observations, which implies that the strong mass loss since the early 2000s at low elevations has had a dynamic impact on the entire ice sheet. The modeling results also suggest that even if the large mass loss at the margins stopped, the interior ice sheet would continue thinning for 300 years and would take thousands of years for full dynamic recovery.

  8. Oceanic Forcing of Ice-Sheet Retreat: West Antarctica and More

    NASA Astrophysics Data System (ADS)

    Alley, Richard B.; Anandakrishnan, Sridhar; Christianson, Knut; Horgan, Huw J.; Muto, Atsu; Parizek, Byron R.; Pollard, David; Walker, Ryan T.

    2015-05-01

    Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence.

  9. The Rapid Ice Sheet Change Observatory (RISCO)

    NASA Astrophysics Data System (ADS)

    Morin, P.; Howat, I. M.; Ahn, Y.; Porter, C.; McFadden, E. M.

    2010-12-01

    The recent expansion of observational capacity from space has revealed dramatic, rapid changes in the Earth’s ice cover. These discoveries have fundamentally altered how scientists view ice-sheet change. Instead of just slow changes in snow accumulation and melting over centuries or millennia, important changes can occur in sudden events lasting only months, weeks, or even a single day. Our understanding of these short time- and space-scale processes, which hold important implications for future global sea level rise, has been impeded by the low temporal and spatial resolution, delayed sensor tasking, incomplete coverage, inaccessibility and/or high cost of data available to investigators. New cross-agency partnerships and data access policies provide the opportunity to dramatically improve the resolution of ice sheet observations by an order of magnitude, from timescales of months and distances of 10’s of meters, to days and meters or less. Advances in image processing technology also enable application of currently under-utilized datasets. The infrastructure for systematically gathering, processing, analyzing and distributing these data does not currently exist. Here we present the development of a multi-institutional, multi-platform observatory for rapid ice change with the ultimate objective of helping to elucidate the relevant timescales and processes of ice sheet dynamics and response to climate change. The Rapid Ice Sheet Observatory (RISCO) gathers observations of short time- and space-scale Cryosphere events and makes them easily accessible to investigators, media and general public. As opposed to existing data centers, which are structured to archive and distribute diverse types of raw data to end users with the specialized software and skills to analyze them, RISCO focuses on three types of geo-referenced raster (image) data products in a format immediately viewable with commonly available software. These three products are (1) sequences of images

  10. The response of grounded ice to ocean temperature forcing in a coupled ice sheet-ice shelf-ocean cavity model

    NASA Astrophysics Data System (ADS)

    Goldberg, D. N.; Little, C. M.; Sergienko, O. V.; Gnanadesikan, A.

    2010-12-01

    Ice shelves provide a pathway for the heat content of the ocean to influence continental ice sheets. Changes in the rate or location of basal melting can alter their geometry and effect changes in stress conditions at the grounding line, leading to a grounded ice response. Recent observations of ice streams and ice shelves in the Amundsen Sea sector of West Antarctica have been consistent with this story. On the other hand, ice dynamics in the grounding zone control flux into the shelf and thus ice shelf geometry, which has a strong influence on the circulation in the cavity beneath the shelf. Thus the coupling between the two systems, ocean and ice sheet-ice shelf, can be quite strong. We examine the response of the ice sheet-ice shelf-ocean cavity system to changes in ocean temperature using a recently developed coupled model. The coupled model consists a 3-D ocean model (GFDL's Generalized Ocean Layered Dynamics model, or GOLD) to a two-dimensional ice sheet-ice shelf model (Goldberg et al, 2009), and allows for changing cavity geometry and a migrating grounding line. Steady states of the coupled system are found even under considerable forcing. The ice shelf morphology and basal melt rate patterns of the steady states exhibit detailed structure, and furthermore seem to be unique and robust. The relationship between temperature forcing and area-averaged melt rate is influenced by the response of ice shelf morphology to thermal forcing, and is found to be sublinear in the range of forcing considered. However, results suggest that area-averaged melt rate is not the best predictor of overall system response, as grounding line stability depends on local aspects of the basal melt field. Goldberg, D N, D M Holland and C G Schoof, 2009. Grounding line movement and ice shelf buttressing in marine ice sheets, Journal of Geophysical Research-Earth Surfaces, 114, F04026.

  11. Future Antarctic bed topography and its implications for ice sheet dynamics

    NASA Astrophysics Data System (ADS)

    Adhikari, S.; Ivins, E. R.; Larour, E.; Seroussi, H.; Morlighem, M.; Nowicki, S.

    2014-06-01

    The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45 mm yr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.

  12. Future Antarctic bed topography and its implications for ice sheet dynamics

    NASA Astrophysics Data System (ADS)

    Adhikari, S.; Ivins, E.; Larour, E.; Seroussi, H.; Morlighem, M.; Nowicki, S.

    2014-01-01

    The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A~recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) is generally losing its mass since the last glacial maximum (LGM). In a sustained warming climate, the AIS is predicted to retreat at a greater pace primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that the past loading is relatively less important than future loading on the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years 2100 and 2500 AD, respectively, and that the East Antarctic Ice Sheet (EAIS) is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay approaches roughly 45 mm yr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is associated with the flattening of reverse bed, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote the stability to marine portions of the ice sheet in future.

  13. Future Antarctic Bed Topography and Its Implications for Ice Sheet Dynamics

    NASA Technical Reports Server (NTRS)

    Adhikari, Surendra; Ivins, Erik R.; Larour, Eric Y.; Seroussi, Helene L.; Morlighem, Mathieu; Nowicki, S.

    2014-01-01

    The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves.We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS.We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45mmyr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.

  14. Investigation of Controls on Ice Dynamics in Northeast Greenland from Ice-Thickness Change Record Using Ice Sheet System Model (ISSM)

    NASA Astrophysics Data System (ADS)

    Csatho, B. M.; Larour, E. Y.; Schenk, A. F.; Schlegel, N.; Duncan, K.

    2015-12-01

    We present a new, complete ice thickness change reconstruction of the NE sector of the Greenland Ice Sheet for 1978-2014, partitioned into changes due to surface processes and ice dynamics. Elevation changes are computed from all available stereoscopic DEMs, and laser altimetry data (ICESat, ATM, LVIS). Surface Mass Balance and firn-compaction estimates are from RACMO2.3. Originating nearly at the divide of the Greenland Ice Sheet (GrIS), the dynamically active North East Ice Stream (NEGIS) is capable of rapidly transmitting ice-marginal forcing far inland. Thus, NEGIS provides a possible mechanism for a rapid drawdown of ice from the ice sheet interior as marginal warming, thinning and retreat continues. Our altimetry record shows accelerating dynamic thinning of Zachariæ Isstrom, initially limited to the deepest part of the fjord near the calving front (1978-2000) and then extending at least 75 km inland. At the same time, changes over the Nioghalvfjerdsfjorden (N79) Glacier are negligible. We also detect localized large dynamic changes at higher elevations on the ice sheet. These thickness changes, often occurring at the onset of fast flow, could indicate rapid variations of basal lubrication due to rerouting of subglacial drainage. We investigate the possible causes of the observed spatiotemporal pattern of ice sheet elevation changes using the Ice Sheet System Model (ISSM). This work build on our previous studies examining the sensitivity of ice flow within the Northeast Greenland Ice Stream (NEGIS) to key fields, including ice viscosity, basal drag. We assimilate the new altimetry record into ISSM to improve the reconstruction of basal friction and ice viscosity. Finally, airborne geophysical (gravity, magnetic) and ice-penetrating radar data is examined to identify the potential geologic controls on the ice thickness change pattern. Our study provides the first comprehensive reconstruction of ice thickness changes for the entire NEGIS drainage basin during

  15. An Imminent Revolution in Modeling Interactions of Ice Sheets With Climate

    NASA Astrophysics Data System (ADS)

    Hughes, T.

    2008-12-01

    Modeling continental ice sheets was inaugurated by meteorologists William Budd and Uwe Radok, with mathematician Richard Jenssen, in 1971. Their model calculated the thermal and mechanical regime using measured surface accumulation rates, temperatures, and elevations, and bed topography. This top-down approach delivered a basal thermal regime of temperatures or melting rates for an assumed basal geothermal heat flux. When Philippe Huybrechts and others incorporated time, largely unknownpast surface conditions had a major effect on present basal thermal conditions. This approach produced ice-sheet models with only a slow response to external forcing, whereas the glacial geological record and climate records from ice and ocean cores show that ice sheets can have rapid changes in size and shape independent of external forcing. These top-down models were wholly inadequate for reconstructing former ice sheets at the LGM for CLIMAP in 1981. Ice-sheet areas,elevations, and volumes provided the albedo, surface topography, and sea-surface area as input to climate models. A bottom-up model based on dated glacial geology was developed to provide the areal extent and basal thermal regime of ice sheets at the LGM. Basal thermal conditions determined ice-bed coupling and therefore the elevation of ice sheets. High convex ice surfaces for slow sheet flow lower about 20 percent when a frozen bed becomes thawed. As further basal melting drowns bedrock bumps that "pin" basal ice, the ice surface becomes concave in fast stream flow that ends as low floating ice shelves at marine ice margins. A revolution in modeling interactions between glaciation, climate, and sea level is driven by new Greenland and Antarctic data from Earth-orbiting satellites, airborne and surface traverses, and deep drilling. We anticipate continuous data acquisition of surface albedo, accumulation/ablation rates, elevations, velocities, and temperatures over a whole ice sheet, mapping basal thermal conditions

  16. Estimating the impact of internal climate variability on ice sheet model simulations

    NASA Astrophysics Data System (ADS)

    Tsai, C. Y.; Forest, C. E.; Pollard, D.

    2016-12-01

    Rising sea level threatens human societies and coastal habitats and melting ice sheets are a major contributor to sea level rise (SLR). Thus, understanding uncertainty of both forcing and variability within the climate system is essential for assessing long-term risk of SLR given their impact on ice sheet evolution. The predictability of polar climate is limited by uncertainties from the given forcing, the climate model response to this forcing, and the internal variability from feedbacks within the fully coupled climate system. Among those sources of uncertainty, the impact of internal climate variability on ice sheet changes has not yet been robustly assessed. Here we investigate how internal variability affects ice sheet projections using climate fields from two Community Earth System Model (CESM) large-ensemble (LE) experiments to force a three-dimensional ice sheet model. Each ensemble member in an LE experiment undergoes the same external forcings but with unique initial conditions. We find that for both LEs, 2m air temperature variability over Greenland ice sheet (GrIS) can lead to significantly different ice sheet responses. Our results show that the internal variability from two fully coupled CESM LEs can cause about 25 35 mm differences of GrIS's contribution to SLR in 2100 compared to present day (about 20% of the total change), and 100m differences of SLR in 2300. Moreover, only using ensemble-mean climate fields as the forcing in ice sheet model can significantly underestimate the melt of GrIS. As the Arctic region becomes warmer, the role of internal variability is critical given the complex nonlinear interactions between surface temperature and ice sheet. Our results demonstrate that internal variability from coupled atmosphere-ocean general circulation model can affect ice sheet simulations and the resulting sea-level projections. This study highlights an urgent need to reassess associated uncertainties of projecting ice sheet loss over the next few

  17. Centennial-scale Holocene climate variations amplified by Antarctic Ice Sheet discharge

    NASA Astrophysics Data System (ADS)

    Bakker, Pepijn; Clark, Peter U.; Golledge, Nicholas R.; Schmittner, Andreas; Weber, Michael E.

    2017-01-01

    Proxy-based indicators of past climate change show that current global climate models systematically underestimate Holocene-epoch climate variability on centennial to multi-millennial timescales, with the mismatch increasing for longer periods. Proposed explanations for the discrepancy include ocean-atmosphere coupling that is too weak in models, insufficient energy cascades from smaller to larger spatial and temporal scales, or that global climate models do not consider slow climate feedbacks related to the carbon cycle or interactions between ice sheets and climate. Such interactions, however, are known to have strongly affected centennial- to orbital-scale climate variability during past glaciations, and are likely to be important in future climate change. Here we show that fluctuations in Antarctic Ice Sheet discharge caused by relatively small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally, suggesting that a dynamic Antarctic Ice Sheet may have driven climate fluctuations during the Holocene. We analysed high-temporal-resolution records of iceberg-rafted debris derived from the Antarctic Ice Sheet, and performed both high-spatial-resolution ice-sheet modelling of the Antarctic Ice Sheet and multi-millennial global climate model simulations. Ice-sheet responses to decadal-scale ocean forcing appear to be less important, possibly indicating that the future response of the Antarctic Ice Sheet will be governed more by long-term anthropogenic warming combined with multi-centennial natural variability than by annual or decadal climate oscillations.

  18. Landform Formation Under Ice Sheets

    NASA Astrophysics Data System (ADS)

    Schoof, C. G.; Ng, F. S.; Hallet, B.

    2004-12-01

    We present a new mathematical model for the formation of subglacial landforms such as drumlins under a warm-based, soft-bedded ice sheet. At the heart of the model is a channelized drainage system in which smaller channels grow at the expense of larger ones, leading to the continuous creation and extinction of drainage paths, and to a spatially distributed imprint on the landscape. We demonstrate how interactions between such a drainage system, bed topography and ice flow can lead to the spontaneous formation of subglacial landforms, and discuss the effect of different sediment transport characteristics in the drainage system on the shape and migration of these landforms. This mathematical model is the first component of a study of landscape/ice-sheet self-organization, which is inspired and guided, in part, by new digital topographic data (LIDAR) that are revealing with unprecedented detail the striking grain of glacially scoured topography on length scales ranging from 0.5 to 20 km.

  19. ICESat's First Year of Measurements Over the Polar Ice Sheets

    NASA Astrophysics Data System (ADS)

    Shuman, C. A.

    2004-05-01

    NASA's Ice, Cloud and Land Elevation Satellite (ICESat) mission was developed to measure changes in elevation of the Greenland and Antarctic ice sheets. Its primary mission goal is to significantly refine estimates of polar ice sheet mass balance. Obtaining precise, spatially dense, ice sheet elevations through time is the first step towards this goal. ICESat data will then enable study of associations between observed ice changes and dynamic or climatic forcing factors, and thus enable improved estimation of the present and future contributions of the ice sheets to global sea level rise. ICESat was launched on January 12, 2003 and acquired science data from February 20th to March 29th with the first of the three lasers of the Geoscience Laser Altimeter System (GLAS). Data acquisition with the second laser began on September 25th and continued until November 18th, 2003. For one-year change detection, the second laser is scheduled for operation from approximately February 17th to March 20th, 2004. Additional operational periods will be selected to 1) enable periodic measurements through the year, and 2) to support of other NASA Earth Science Enterprise missions and activities. To obtain these precise ice sheet elevations, GLAS has a 1064 nm wavelength laser operating at 40 Hz with a designed range precision of about 10 cm. The laser footprints are about 70 m in diameter on the Earth's surface and are spaced every 172 m along-track. The on-board GPS receiver enables radial orbit determinations to an accuracy better than 5 cm. The star-tracking attitude-determination system will enable laser footprints to be located to 6 m horizontally when attitude calibration is completed. The orbital altitude averages 600 km at an inclination of 94 degrees with coverage extending from 86 degrees N and S latitude. The spacecraft attitude can be controlled to point the laser beam to within 50 m of surface reference tracks over the ice sheets and to point off-nadir up to 5 degrees to

  20. Results from ISOMIP+ and MISOMIP1, two interrelated marine ice sheet and ocean model intercomparison projects

    NASA Astrophysics Data System (ADS)

    Asay-Davis, X.; Galton-Fenzi, B.; Gwyther, D.; Jourdain, N.; Martin, D. F.; Nakayama, Y.; Seroussi, H. L.

    2016-12-01

    MISMIP+ (the third Marine Ice Sheet MIP), ISOMIP+ (the second Ice Shelf-Ocean MIP) and MISOMIP1 (the first Marine Ice Sheet-Ocean MIP) prescribe a set of idealized experiments for marine ice-sheet models, ocean models with ice-shelf cavities, and coupled ice sheet-ocean models, respectively. Here, we present results from ISOMIP+ and MISOMIP1 experiments using several ocean-only and coupled ice sheet-ocean models. Among the ocean models, we show that differences in model behavior are significant enough that similar results can only be achieved by tuning model parameters (the heat- and salt-transfer coefficients across the sub-ice-shelf boundary layer) for each model. This tuning is constrained by a desired mean melt rate in quasi-steady state under specified forcing conditions, akin to tuning the models to match observed melt rates. We compare the evolution of ocean temperature transects, melt rate, friction velocity and thermal driving between ocean models for the five ISOMIP+ experiments (Ocean0-4), which have prescribed ice-shelf topography. We find that melt patterns differ between models based on the relative importance of overturning strength and vertical mixing of temperature even when the models have been tuned to achieve similar melt rates near the grounding line. For the two MISOMIP1 experiments (IceOcean1 without dynamic calving and IceOcean2 with a simple calving parameterization), we compare temperature transects, melt rate, ice-shelf topography and grounded area across models and for several model configurations. Consistent with preliminary results from MISMIP+, we find that for a given coupled model, the use of a Coulomb-limited basal friction parameterization below grounded ice and the application of dynamic calving both significantly increase the rate of grounding-line retreat, whereas the rate of retreat appears to be less sensitive to the ice stress approximation (shallow-shelf approximation, higher-order, etc.). We show that models with similar

  1. Advances in Measuring Antarctic Sea-Ice Thickness and Ice-Sheet Elevations with ICESat Laser Altimetry

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay

    2004-01-01

    NASA's Ice, Cloud and Land Elevation Satellite (ICESat) has been measuring elevations of the Antarctic ice sheet and sea-ice freeboard elevations with unprecedented accuracy. Since February 20,2003, data has been acquired during three periods of laser operation varying from 36 to 54 days, which is less than the continuous operation of 3 to 5 years planned for the mission. The primary purpose of ICESat is to measure time-series of ice-sheet elevation changes for determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. ICESat data will continue to be acquired for approximately 33 days periods at 3 to 6 month intervals with the second of ICESat's three lasers, and eventually with the third laser. The laser footprints are about 70 m on the surface and are spaced at 172 m along-track. The on-board GPS receiver enables radial orbit determinations to an accuracy better than 5 cm. The orbital altitude is around 600 km at an inclination of 94 degrees with a 8-day repeat pattern for the calibration and validation period, followed by a 91 -day repeat period for the rest of the mission. The expected range precision of single footprint measurements was 10 cm, but the actual range precision of the data has been shown to be much better at 2 to 3 cm. The star-tracking attitude-determination system should enable footprints to be located to 6 m horizontally when attitude calibrations are completed. With the present attitude calibration, the elevation accuracy over the ice sheets ranges from about 30 cm over the low-slope areas to about 80 cm over areas with slopes of 1 to 2 degrees, which is much better than radar altimetry. After the first period of data collection, the spacecraft attitude was controlled to point the laser beam to within 50 m of reference surface tracks over the ice sheets. Detection of ice

  2. Using paleoclimate data to improve models of the Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    King, M. A.; Phipps, S. J.; Roberts, J. L.; White, D.

    2016-12-01

    Ice sheet models are the most descriptive tools available to simulate the future evolution of the Antarctic Ice Sheet (AIS), including its contribution towards changes in global sea level. However, our knowledge of the dynamics of the coupled ice-ocean-lithosphere system is inevitably limited, in part due to a lack of observations. Furthemore, to build computationally efficient models that can be run for multiple millennia, it is necessary to use simplified descriptions of ice dynamics. Ice sheet modeling is therefore an inherently uncertain exercise. The past evolution of the AIS provides an opportunity to constrain the description of physical processes within ice sheet models and, therefore, to constrain our understanding of the role of the AIS in driving changes in global sea level. We use the Parallel Ice Sheet Model (PISM) to demonstrate how paleoclimate data can improve our ability to predict the future evolution of the AIS. A large, perturbed-physics ensemble is generated, spanning uncertainty in the parameterizations of four key physical processes within ice sheet models: ice rheology, ice shelf calving, and the stress balances within ice sheets and ice shelves. A Latin hypercube approach is used to optimally sample the range of uncertainty in parameter values. This perturbed-physics ensemble is used to simulate the evolution of the AIS from the Last Glacial Maximum ( 21,000 years ago) to present. Paleoclimate records are then used to determine which ensemble members are the most realistic. This allows us to use data on past climates to directly constrain our understanding of the past contribution of the AIS towards changes in global sea level. Critically, it also allows us to determine which ensemble members are likely to generate the most realistic projections of the future evolution of the AIS.

  3. Balance Velocities of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Joughin, Ian; Fahnestock, Mark; Ekholm, Simon; Kwok, Ron

    1997-01-01

    We present a map of balance velocities for the Greenland ice sheet. The resolution of the underlying DEM, which was derived primarily from radar altimetry data, yields far greater detail than earlier balance velocity estimates for Greenland. The velocity contours reveal in striking detail the location of an ice stream in northeastern Greenland, which was only recently discovered using satellite imagery. Enhanced flow associated with all of the major outlets is clearly visible, although small errors in the source data result in less accurate estimates of the absolute flow speeds. Nevertheless, the balance map is useful for ice-sheet modelling, mass balance studies, and field planning.

  4. The influence of ice sheets on temperature during the past 38 million years inferred from a one-dimensional ice sheet-climate model

    NASA Astrophysics Data System (ADS)

    Stap, Lennert B.; van de Wal, Roderik S. W.; de Boer, Bas; Bintanja, Richard; Lourens, Lucas J.

    2017-09-01

    Since the inception of the Antarctic ice sheet at the Eocene-Oligocene transition (˜ 34 Myr ago), land ice has played a crucial role in Earth's climate. Through feedbacks in the climate system, land ice variability modifies atmospheric temperature changes induced by orbital, topographical, and greenhouse gas variations. Quantification of these feedbacks on long timescales has hitherto scarcely been undertaken. In this study, we use a zonally averaged energy balance climate model bidirectionally coupled to a one-dimensional ice sheet model, capturing the ice-albedo and surface-height-temperature feedbacks. Potentially important transient changes in topographic boundary conditions by tectonics and erosion are not taken into account but are briefly discussed. The relative simplicity of the coupled model allows us to perform integrations over the past 38 Myr in a fully transient fashion using a benthic oxygen isotope record as forcing to inversely simulate CO2. Firstly, we find that the results of the simulations over the past 5 Myr are dependent on whether the model run is started at 5 or 38 Myr ago. This is because the relation between CO2 and temperature is subject to hysteresis. When the climate cools from very high CO2 levels, as in the longer transient 38 Myr run, temperatures in the lower CO2 range of the past 5 Myr are higher than when the climate is initialised at low temperatures. Consequently, the modelled CO2 concentrations depend on the initial state. Taking the realistic warm initialisation into account, we come to a best estimate of CO2, temperature, ice-volume-equivalent sea level, and benthic δ18O over the past 38 Myr. Secondly, we study the influence of ice sheets on the evolution of global temperature and polar amplification by comparing runs with ice sheet-climate interaction switched on and off. By passing only albedo or surface height changes to the climate model, we can distinguish the separate effects of the ice-albedo and surface

  5. The geomorphic signature of past ice sheets in the marine record

    NASA Astrophysics Data System (ADS)

    Dowdeswell, J. A.

    2016-12-01

    The deglaciation of high-latitude continental shelves since the Last Glacial Maximum has revealed suites of subglacial and ice-contact landforms that have remained well-preserved beneath tens to hundreds of metres of water. Once ice has retreated, sedimentation is generally low on polar shelves during interglacials and the submarine landforms have not, therefore, been buried by subsequent sedimentation. By contrast, the beds of modern ice sheets are hidden by several thousand metres of ice, which is much more difficult than water to penetrate using geophysical methods. These submarine glacial landforms provide insights into past ice-sheet form and flow, and information on the processes that have taken place beneath former ice sheets. Examples will be shown of streamlined subglacial landforms that indicate the distribution and dimensions of former ice streams on high-latitde continental margins. Distinctive landform assemblages characterise ice stream and inter-ice stream areas. Landforms, including subglacially formed channel systems in inner- and mid-shelf areas, and the lack of them on sedimentary outer shelves, allow inferences to be made about subglacial hydrology. The distribution of grounding-zone wedges and other transverse moraine ridges also provides evidence on the nature of ice-sheet retreat - whether by rapid collapse, episodic retreat or by the slow retreat of grounded ice. Such information can be used to test the predictive capability of ice-sheet numerical models. These marine geophysical and geological observations of submarine glacial landforms enhance our understanding of the form and flow of past ice masses at scales ranging from ice sheets (1000s of km in flow-line and margin length), through ice streams (100s of km long), to surge-type glaciers (10s of km long).

  6. Observationally constrained projections of Antarctic ice sheet instability

    NASA Astrophysics Data System (ADS)

    Edwards, Tamsin; Ritz, Catherine; Durand, Gael; Payne, Anthony; Peyaud, Vincent; Hindmarsh, Richard

    2015-04-01

    Large parts of the Antarctic ice sheet lie on bedrock below sea level and may be vulnerable to a positive feedback known as Marine Ice Sheet Instability (MISI), a self-sustaining retreat of the grounding line triggered by oceanic or atmospheric changes. There is growing evidence MISI may be underway throughout the Amundsen Sea Embayment (ASE) of West Antarctica, induced by circulation of warm Circumpolar Deep Water. If this retreat is sustained the region could contribute up to 1-2 m to global mean sea level, and if triggered in other areas the potential contribution to sea level on centennial to millennial timescales could be two to three times greater. However, physically plausible projections of Antarctic MISI are challenging: numerical ice sheet models are too low in spatial resolution to resolve grounding line processes or else too computationally expensive to assess modelling uncertainties, and no dynamical models exist of the ocean-atmosphere-ice sheet system. Furthermore, previous numerical ice sheet model projections for Antarctica have not been calibrated with observations, which can reduce uncertainties. Here we estimate the probability of dynamic mass loss in the event of MISI under a medium climate scenario, assessing 16 modelling uncertainties and calibrating the projections with observed mass losses in the ASE from 1992-2011. We project losses of up to 30 cm sea level equivalent (SLE) by 2100 and 72 cm SLE by 2200 (95% credibility interval: CI). Our results are substantially lower than previous estimates. The ASE sustains substantial losses, 83% of the continental total by 2100 and 67% by 2200 (95% CI), but in other regions losses are limited by ice dynamical theory, observations, or a lack of projected triggers.

  7. Phreatomagmatic eruptions under the West Antarctic Ice Sheet: potential hazard for ice sheet stability

    NASA Astrophysics Data System (ADS)

    Iverson, N. A.; Dunbar, N. W.; Lieb-Lappen, R.; Kim, E. J.; Golden, E. J.; Obbard, R. W.

    2014-12-01

    Volcanic tephra layers have been seen in most ice cores in Antarctica. These tephra layers are deposited almost instantaneously across wide areas of ice sheets, creating horizons that can provide "pinning points" to adjust ice time scales that may otherwise be lacking detailed chronology. A combination of traditional particle morphology characterization by SEM with new non-destructive X-ray micro-computed tomography (Micro-CT) has been used to analyze selected coarse grained tephra in the West Antarctica Ice Sheet (WAIS) Divide WDC06A ice core. Micro-CT has the ability to image particles as small as 50µm in length (15µm resolution), quantifying both particle shape and size. The WDC06A contains hundreds of dusty layers of which 36 have so far been identified as primary tephra layers. Two of these tephra layers have been characterized as phreatomagmatic eruptions based on SEM imagery and are blocky and platy in nature, with rare magmatic particles. These layers are strikingly different in composition from the typical phonolitic and trachytic tephra produced from West Antarctic volcanoes. These two layers are coarser in grain size, with many particles (including feldspar crystals) exceeding 100µm in length. One tephra layer found at 3149.138m deep in the ice core is a coarse ~1mm thick basanitic tephra layer with a WDC06-7 ice core age of 45,381±2000yrs. The second layer is a ~1.3 cm thick zoned trachyandesite to trachydacite tephra found at 2569.205m deep with an ice core age 22,470±835yrs. Micro-CT analysis shows that WDC06A-3149.138 has normal grading with the largest particles at the bottom of the sample (~160μm). WDC06A-2569.205 has a bimodal distribution of particles with large particles at the top and bottom of the layer. These large particles are more spherical in shape at the base and become more irregular and finer grained higher in the layer, likely showing changes in eruption dynamics. The distinct chemistry as well as the blocky and large grain size

  8. Unusual radar echoes from the Greenland ice sheet

    NASA Technical Reports Server (NTRS)

    Rignot, E. J.; Vanzyl, J. J.; Ostro, S. J.; Jezek, K. C.

    1993-01-01

    In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR) instrument collected the first calibrated data set of multifrequency, polarimetric, radar observations of the Greenland ice sheet. At the time of the AIRSAR overflight, ground teams recorded the snow and firn (old snow) stratigraphy, grain size, density, and temperature at ice camps in three of the four snow zones identified by glaciologists to characterize four different degrees of summer melting of the Greenland ice sheet. The four snow zones are: (1) the dry-snow zone, at high elevation, where melting rarely occurs; (2) the percolation zone, where summer melting generates water that percolates down through the cold, porous, dry snow and then refreezes in place to form massive layers and pipes of solid ice; (3) the soaked-snow zone where melting saturates the snow with liquid water and forms standing lakes; and (4) the ablation zone, at the lowest elevations, where melting is vigorous enough to remove the seasonal snow cover and ablate the glacier ice. There is interest in mapping the spatial extent and temporal variability of these different snow zones repeatedly by using remote sensing techniques. The objectives of the 1991 experiment were to study changes in radar scattering properties across the different melting zones of the Greenland ice sheet, and relate the radar properties of the ice sheet to the snow and firn physical properties via relevant scattering mechanisms. Here, we present an analysis of the unusual radar echoes measured from the percolation zone.

  9. Characteristics of basal ice and subglacial water at Dome Fuji, Antarctica ice sheet

    NASA Astrophysics Data System (ADS)

    Motoyama, H.; Uemura, R.; Hirabayashi, M.; Miyake, T.; Kuramoto, T.; Tanaka, Y.; Dome Fuji Ice Core Project, M.

    2008-12-01

    (Introduction): The second deep ice coring project at Dome Fuji, Antarctica reached a depth of 3035.22 m during the austral summer season in 2006/2007. The recovered ice cores contain records of global environmental changes going back about 720,000 years. (Estimation of basal ice melt): The borehole measurement was carried out on January 2nd in 2007 when the temperature disturbance in the borehole calmed down by the rest of drilling for 2 days. Temperature measurement was performed after 0 C thermometer test was done in the ground. The temperature sensor of pt100 installed in the skate-like anti-torque was used. We did not have the enough time until the temperature of thermometer was matched with the temperature of ice sheet. Some error was included in ice temperature data. The resistance of pt100 sensor was converted to temperature in the borehole measurement machine. But we used only two electrical lines for pt100 sensor. Rate of heat flow in the ice sheet was calculated using the vertical temperature gradient of the ice sheet and rate of heat conductivity of ice. The deepest part of heat flux using temperatures at 3000m and 3030m was about 45mW/m2. We assumed that this value was the heat flux from the bedrock in the ice sheet. Heat flux to the bedrock surface in the ground was assumed 54.6mW/m2 adopted by ice sheet model (P. Huybrechts, 2006). Then the heat flux for basal ice melt was about 10mW/m2. This value was equaled to melting of 1.1mm of ice thickness per year. On the other hand, the annual layer thickness under 2500m was not changed so much and its average was 1.3mm of ice thickness. So the annual layer thickness and melting rate of basal ice was the same in ordering way. Or ice equivalent in annual layer is melting every year. The age of the deepest part of ice core is guessed at 720,000 years old and the ice older than basal ice has melted away. (The state of basal ice): When the ice core drilling depth passed 3031.44m, amount of ice chip more abundant

  10. Understanding Recent Mass Balance Changes of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    vanderVeen, Cornelius

    2003-01-01

    The ultimate goal of this project is to better understand the current transfer of mass between the Greenland Ice Sheet, the world's oceans and the atmosphere, and to identify processes controlling the rate of this transfer, to be able to predict with greater confidence future contributions to global sea level rise. During the first year of this project, we focused on establishing longer-term records of change of selected outlet glaciers, reevaluation of mass input to the ice sheet and analysis of climate records derived from ice cores, and modeling meltwater production and runoff from the margins of the ice sheet.

  11. Buttressing and stability of marine Ice sheets

    NASA Astrophysics Data System (ADS)

    Goldberg, D.; Holland, D. M.; Schoof, C.

    2009-04-01

    The West Antarctic Ice Sheet is marine in nature, meaning most of its base is below sea level. At the grounding line (where it becomes thin enough to float), its outlet streams flow into large ice shelves. Gravitational stress in the shelf is transmitted back to the grounding line, and largely balanced by basal friction in the transition zone. The details of this force balance control the evolution of both the thickness and grounded extent of the ice sheet, and can lead to Weertman's (1974) Marine Instability for a foredeepened bedrock (one that deepens inland). However, the presence of rigid sidewalls and locally grounded regions in the shelf can reduce the longitudinal stresses felt at the grounding line (a phenomenon called buttressing). Thomas (1979) and others pointed out that Marine Instability may be lessened or reversed by ice shelf buttressing. When modelling marine ice sheets numerically, the physics of the grounded-to-floating transition must be represented and the associated small length scales must be resolved (Schoof, 2007). Failing to do so can result in nonphysical or numerically inconsistent behavior (Vieli and Payne, 2005). While several methods have been developed to treat these issues (Vieli and Payne, 2005; Pattyn et al, 2006; Schoof, 2007) they are limited to flowline models. We present a model that represents the physics of the grounded-to-floating transition in a time-dependent three-dimensional marine ice sheet, using mesh adaption to resolve the transition zone. We show that in the special case of a two-dimensional sheet our model reproduces the theoretical results of the MISMIP experiments, and that it produces robust results when both horizontal dimensions are resolved. In idealized experiments in a channel with rigid sidewalls and a foredeepened bed, we narrow the channel to determine whether buttressing is sufficient to reverse instability. We find that for strong beds (high friction coefficients), while the timescales and dynamics are

  12. Deglaciation of the Eurasian ice sheet complex

    NASA Astrophysics Data System (ADS)

    Patton, Henry; Hubbard, Alun; Andreassen, Karin; Auriac, Amandine; Whitehouse, Pippa L.; Stroeven, Arjen P.; Shackleton, Calvin; Winsborrow, Monica; Heyman, Jakob; Hall, Adrian M.

    2017-08-01

    The Eurasian ice sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea-level lowering. Whilst recent terrestrial and marine empirical insights have improved understanding of the chronology, pattern and rates of retreat of this vast ice sheet, a concerted attempt to model the deglaciation of the EISC honouring these new constraints is conspicuously lacking. Here, we apply a first-order, thermomechanical ice sheet model, validated against a diverse suite of empirical data, to investigate the retreat of the EISC after 23 ka BP, directly extending the work of Patton et al. (2016) who modelled the build-up to its maximum extent. Retreat of the ice sheet complex was highly asynchronous, reflecting contrasting regional sensitivities to climate forcing, oceanic influence, and internal dynamics. Most rapid retreat was experienced across the Barents Sea sector after 17.8 ka BP when this marine-based ice sheet disintegrated at a rate of ∼670 gigatonnes per year (Gt a-1) through enhanced calving and interior dynamic thinning, driven by oceanic/atmospheric warming and exacerbated by eustatic sea-level rise. From 14.9 to 12.9 ka BP the EISC lost on average 750 Gt a-1, peaking at rates >3000 Gt a-1, roughly equally partitioned between surface melt and dynamic losses, and potentially contributing up to 2.5 m to global sea-level rise during Meltwater Pulse 1A. Independent glacio-isostatic modelling constrained by an extensive inventory of relative sea-level change corroborates our ice sheet loading history of the Barents Sea sector. Subglacial conditions were predominately temperate during deglaciation, with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network. Moreover, the maximum EISC and its isostatic footprint had a profound impact on the proglacial hydrological network, forming the Fleuve Manche mega-catchment which had an area of

  13. Issues in subsurface exploration of ice sheets

    NASA Technical Reports Server (NTRS)

    French, L.; Carsey, F.; Zimmerman, W.

    2000-01-01

    Exploration of the deep subsurface ice sheets of Earth, Mars, Europa, and Titan has become a major consideration in addressing scientific objectives in climate change, extremophile biology, exobiology,chemical weathering, planetary evolution and ice dynamics.

  14. Discharge of debris from ice at the margin of the Greenland ice sheet

    USGS Publications Warehouse

    Knight, P.G.; Waller, R.I.; Patterson, C.J.; Jones, A.P.; Robinson, Z.P.

    2002-01-01

    Sediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12-45 m3 m-1 a-1. This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glacio-fluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

  15. A sensitivity analysis for a thermomechanical model of the Antarctic ice sheet and ice shelves

    NASA Astrophysics Data System (ADS)

    Baratelli, F.; Castellani, G.; Vassena, C.; Giudici, M.

    2012-04-01

    The outcomes of an ice sheet model depend on a number of parameters and physical quantities which are often estimated with large uncertainty, because of lack of sufficient experimental measurements in such remote environments. Therefore, the efforts to improve the accuracy of the predictions of ice sheet models by including more physical processes and interactions with atmosphere, hydrosphere and lithosphere can be affected by the inaccuracy of the fundamental input data. A sensitivity analysis can help to understand which are the input data that most affect the different predictions of the model. In this context, a finite difference thermomechanical ice sheet model based on the Shallow-Ice Approximation (SIA) and on the Shallow-Shelf Approximation (SSA) has been developed and applied for the simulation of the evolution of the Antarctic ice sheet and ice shelves for the last 200 000 years. The sensitivity analysis of the model outcomes (e.g., the volume of the ice sheet and of the ice shelves, the basal melt rate of the ice sheet, the mean velocity of the Ross and Ronne-Filchner ice shelves, the wet area at the base of the ice sheet) with respect to the model parameters (e.g., the basal sliding coefficient, the geothermal heat flux, the present-day surface accumulation and temperature, the mean ice shelves viscosity, the melt rate at the base of the ice shelves) has been performed by computing three synthetic numerical indices: two local sensitivity indices and a global sensitivity index. Local sensitivity indices imply a linearization of the model and neglect both non-linear and joint effects of the parameters. The global variance-based sensitivity index, instead, takes into account the complete variability of the input parameters but is usually conducted with a Monte Carlo approach which is computationally very demanding for non-linear complex models. Therefore, the global sensitivity index has been computed using a development of the model outputs in a

  16. Influence of temperature fluctuations on equilibrium
    ice sheet volume

    NASA Astrophysics Data System (ADS)

    Bøgeholm Mikkelsen, Troels; Grinsted, Aslak; Ditlevsen, Peter

    2018-01-01

    Forecasting the future sea level relies on accurate modeling of the response of the Greenland and Antarctic ice sheets to changing temperatures. The surface mass balance (SMB) of the Greenland Ice Sheet (GrIS) has a nonlinear response to warming. Cold and warm anomalies of equal size do not cancel out and it is therefore important to consider the effect of interannual fluctuations in temperature. We find that the steady-state volume of an ice sheet is biased toward larger size if interannual temperature fluctuations are not taken into account in numerical modeling of the ice sheet. We illustrate this in a simple ice sheet model and find that the equilibrium ice volume is approximately 1 m SLE (meters sea level equivalent) smaller when the simple model is forced with fluctuating temperatures as opposed to a stable climate. It is therefore important to consider the effect of interannual temperature fluctuations when designing long experiments such as paleo-spin-ups. We show how the magnitude of the potential bias can be quantified statistically. For recent simulations of the Greenland Ice Sheet, we estimate the bias to be 30 Gt yr-1 (24-59 Gt yr-1, 95 % credibility) for a warming of 3 °C above preindustrial values, or 13 % (10-25, 95 % credibility) of the present-day rate of ice loss. Models of the Greenland Ice Sheet show a collapse threshold beyond which the ice sheet becomes unsustainable. The proximity of the threshold will be underestimated if temperature fluctuations are not taken into account. We estimate the bias to be 0.12 °C (0.10-0.18 °C, 95 % credibility) for a recent estimate of the threshold. In light of our findings it is important to gauge the extent to which this increased variability will influence the mass balance of the ice sheets.

  17. A Synthesis of the Basal Thermal State of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Macgregor, J. A.; Fahnestock, M. A.; Catania, G. A.; Aschwanden, A.; Clow, G. D.; Colgan, W. T.; Gogineni, S. P.; Morlighem, M.; Nowicki, S. M. J.; Paden, J. D.; hide

    2016-01-01

    Greenland's thick ice sheet insulates the bedrock below from the cold temperatures at the surface, so the bottom of the ice is often tens of degrees warmer than at the top, because the ice bottom is slowly warmed by heat coming from the Earth's depths. Knowing whether Greenland's ice lies on wet, slippery ground or is anchored to dry, frozen bedrock is essential for predicting how this ice will flow in the future. But scientists have very few direct observations of the thermal conditions beneath the ice sheet, obtained through fewer than two dozen boreholes that have reached the bottom. Our study synthesizes several independent methods to infer the Greenland Ice Sheet's basal thermal state -whether the bottom of the ice is melted or not-leading to the first map that identifies frozen and thawed areas across the whole ice sheet. This map will guide targets for future investigations of the Greenland Ice Sheet toward the most vulnerable and poorly understood regions, ultimately improving our understanding of its dynamics and contribution to future sea-level rise. It is of particular relevance to ongoing Operation IceBridge activities and future large-scale airborne missions over Greenland.

  18. The East Antarctic Ice Sheet and the Gamburtsev Subglacial Mountains (Invited)

    NASA Astrophysics Data System (ADS)

    Bell, R. E.; Studinger, M.; Ferraccioli, F.; Damaske, D.; Finn, C.; Braaten, D. A.; Fahnestock, M. A.; Jordan, T. A.; Corr, H.; Elieff, S.; Frearson, N.; Block, A. E.; Rose, K.

    2009-12-01

    Models of the onset of glaciation in Antarctica routinely document the early growth of the ice sheet on the summit of the Gamburtsev Subglacial Mountains in the center of the East Antarctic Craton. While ice sheet models replicate the formation of the East Antarctic ice sheet 35 million years ago, the age, evolution and structure of the Gamburtsev Mountains remain completely unresolved. During the International Polar Year scientists from seven nations have launched a major collaborative program (AGAP) to explore the Gamburtsev Subglacial Mountains buried by the East Antarctic ice sheet and bounded by numerous subglacial lakes. The AGAP umbrella is a multi-national, multi-disciplinary effort and includes aerogeophysics, passive seismology, traverse programs and will be complimented by future ice core and bedrock drilling. A major new airborne data set including gravity; magnetics; ice thickness; SAR images of the ice-bed interface; near-surface and deep internal layers; and ice surface elevation is providing insights into a more dynamic East Antarctica. More than 120,000 km of aerogeophysical data have been acquired from two remote field camps during the 2008/09 field season. AGAP effort was designed to address several fundamental questions including: 1) What role does topography play in the nucleation of continental ice sheets? 2) How do tectonic processes control the formation, distribution, and stability of subglacial lakes? The preliminary analysis of this major new data set indicated these 3000m high mountains are deeply dissected by a dendritic system. The northern margin of the mountain range terminates against the inland extent of the Lambert Graben. Evidence of the onset of glaciation is preserved as cirques and U shaped valleys along the axis of the uplifted massifs. The geomorphology reflects the interaction between the ice sheet and the Gamburtsev Mountains. Bright reflectors in the radar data in the deep valleys indicate the presence of water that has

  19. Future Antarctic bed topography and its implications for ice sheet dynamics

    NASA Astrophysics Data System (ADS)

    Adhikari, Surendra; Ivins, Erik; Larour, Eric; Seroussi, Helene; Morlighem, Mathieu; Nowicki, Sophie

    2014-05-01

    A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has been generally losing its mass since the last glacial maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that the past loading is relatively less important than future loading on the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years 2100 and 2500 AD, respectively, and that the East Antarctic Ice Sheet (EAIS) is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector of WAIS in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay approaches roughly 45 mm/yr in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is associated with the flattening of reverse bed, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote the stability to marine portions of the ice sheet in the future.

  20. Greenland Ice Sheet in 3D Cutaway

    NASA Image and Video Library

    2017-12-08

    Peering into the thousands of frozen layers inside Greenland’s ice sheet is like looking back in time. Each layer provides a record of what Earth’s climate was like at the dawn of civilization, or during the last ice age, or during an ancient period of warmth similar to the one we experience today. Scientists using ice-penetrating radar data collected by NASA’s Operation IceBridge and earlier airborne campaigns have built the first-ever comprehensive map of layers deep inside the Greenland Ice Sheet. View the full video: youtu.be/u0VbPE0TOtQ Credit: NASA’s Goddard Space Flight Center NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  1. Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Bierman, Paul R.; Corbett, Lee B.; Graly, Joseph A.; Neumann, Thomas Allen; Lini, Andrea; Crosby, Benjamin T.; Rood, Dylan H.

    2014-01-01

    Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been non-erosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets

  2. West Antarctic Ice Sheet retreat driven by Holocene warm water incursions

    PubMed Central

    Hillenbrand, Claus-Dieter; Smith, James A.; Hodell, David A.; Greaves, Mervyn; Poole, Christopher R.; Kender, Sev; Williams, Mark; Andersen, Thorbjørn Joest; Jernas, Patrycja E.; Klages, Johann P.; Roberts, Stephen J.; Gohl, Karsten; Larter, Robert D.; Kuhn, Gerhard

    2017-01-01

    Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) upwelling onto the West Antarctic continental shelf causes melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet loss today. Here we present the first multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the last 11,000 years. The chemical composition of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector both until 7,500 years ago, when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream, and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models. PMID:28682333

  3. Modelling the climate and ice sheets of the mid-Pliocene warm period: a test of model dependency

    NASA Astrophysics Data System (ADS)

    Dolan, Aisling; Haywood, Alan; Lunt, Daniel; Hill, Daniel

    2010-05-01

    British Antarctic Survey thermomechanically coupled ice sheet model (BASISM) to test the extent to which equilibrium state ice sheets in the Northern Hemisphere are GCM dependent. Initial results which do not use GCM-specific topography suggest that employing different GCM climatologies with only small differences in surface air temperature and precipitation has a dramatic effect on the resultant Greenland ice sheet, where the end-member ice sheets vary from near modern to almost zero ice volume. As an extension of this analysis, we will also present results using a second ice sheet model (Glimmer), with a view to testing the degree to which end-member ice sheets are ice sheet model dependent, something which has not previously been addressed. Initially, BASISM and Glimmer will be internally optimised for performance, but we will also present a comparison where BASISM will be configured to the Glimmer model setup in a further test of ice sheet model dependency.

  4. SPICE: Sentinel-3 Performance Improvement for Ice Sheets

    NASA Astrophysics Data System (ADS)

    McMillan, M.; Escola, R.; Roca, M.; Thibaut, P.; Aublanc, J.; Shepherd, A.; Remy, F.; Benveniste, J.; Ambrózio, A.; Restano, M.

    2017-12-01

    For the past 25 years, polar-orbiting satellite radar altimeters have provided a valuable record of ice sheet elevation change and mass balance. One of the principle challenges associated with radar altimetry comes from the relatively large ground footprint of conventional pulse-limited radars, which reduces their capacity to make measurements in areas of complex topographic terrain. In recent years, progress has been made towards improving ground resolution, through the implementation of Synthetic Aperture Radar (SAR), or Delay-Doppler, techniques. In 2010, the launch of CryoSat-2 heralded the start of a new era of SAR Interferometric (SARIn) altimetry. However, because the satellite operated in SARIn and LRM mode over the ice sheets, many of the non-interferometric SAR altimeter processing techniques have been optimized for water and sea ice surfaces only. The launch of Sentinel-3, which provides full non-interferometric SAR coverage of the ice sheets, therefore presents the opportunity to further develop these SAR processing methodologies over ice sheets. Here we present results from SPICE, a 2 year study that focuses on (1) developing and evaluating Sentinel-3 SAR altimetry processing methodologies over the Polar ice sheets, and (2) investigating radar wave penetration through comparisons of Ku- and Ka-band satellite measurements. The project, which is funded by ESA's SEOM (Scientific Exploitation of Operational Missions) programme, has worked in advance of the operational phase of Sentinel-3, to emulate Sentinel-3 SAR and pseudo-LRM data from dedicated CryoSat-2 SAR acquisitions made at the Lake Vostok, Dome C and Spirit sites in East Antarctica, and from reprocessed SARIn data in Greenland. In Phase 1 of the project we have evaluated existing processing methodologies, and in Phase 2 we are investigating new evolutions to the Delay-Doppler Processing (DDP) and retracking chains. In this presentation we (1) evaluate the existing Sentinel-3 processing chain by

  5. A reconciled estimate of ice-sheet mass balance.

    PubMed

    Shepherd, Andrew; Ivins, Erik R; A, Geruo; Barletta, Valentina R; Bentley, Mike J; Bettadpur, Srinivas; Briggs, Kate H; Bromwich, David H; Forsberg, René; Galin, Natalia; Horwath, Martin; Jacobs, Stan; Joughin, Ian; King, Matt A; Lenaerts, Jan T M; Li, Jilu; Ligtenberg, Stefan R M; Luckman, Adrian; Luthcke, Scott B; McMillan, Malcolm; Meister, Rakia; Milne, Glenn; Mouginot, Jeremie; Muir, Alan; Nicolas, Julien P; Paden, John; Payne, Antony J; Pritchard, Hamish; Rignot, Eric; Rott, Helmut; Sørensen, Louise Sandberg; Scambos, Ted A; Scheuchl, Bernd; Schrama, Ernst J O; Smith, Ben; Sundal, Aud V; van Angelen, Jan H; van de Berg, Willem J; van den Broeke, Michiel R; Vaughan, David G; Velicogna, Isabella; Wahr, John; Whitehouse, Pippa L; Wingham, Duncan J; Yi, Donghui; Young, Duncan; Zwally, H Jay

    2012-11-30

    We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods--especially in Greenland and West Antarctica--and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 ± 49, +14 ± 43, -65 ± 26, and -20 ± 14 gigatonnes year(-1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year(-1) to the rate of global sea-level rise.

  6. Widespread Refreezing of Both Surface and Basal Melt Water Beneath the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Bell, R. E.; Tinto, K. J.; Das, I.; Wolovick, M.; Chu, W.; Creyts, T. T.; Frearson, N.

    2013-12-01

    The isotopically and chemically distinct, bubble-free ice observed along the Greenland Ice Sheet margin both in the Russell Glacier and north of Jacobshavn must have formed when water froze from subglacial networks. Where this refreezing occurs and what impact it has on ice sheet processes remain unclear. We use airborne radar data to demonstrate that freeze-on to the ice sheet base and associated deformation produce large ice units up to 700 m thick throughout northern Greenland. Along the ice sheet margin, in the ablation zone, surface meltwater, delivered via moulins, refreezes to the ice sheet base over rugged topography. In the interior, water melted from the ice sheet base is refrozen and surrounded by folded ice. A significant fraction of the ice sheet is modified by basal freeze-on and associated deformation. For the Eqip and Petermann catchments, representing the ice sheet margin and interior respectively, extensive airborne radar datasets show that 10%-13% of the base of the ice sheet and up to a third of the catchment width is modified by basal freeze-on. The interior units develop over relatively subdued topography with modest water flux from basal melt where conductive cooling likely dominates. Steps in the bed topography associated with subglacial valley networks may foster glaciohydraulic supercooling. The ablation zone units develop where both surface melt and crevassing are widespread and large volumes of surface meltwater will reach the base of the ice sheet. The relatively steep topography at the upslope edge of the ablation zone units combined with the larger water flux suggests that supercooling plays a greater role in their formation. The ice qualities of the ablation zone units should reflect the relatively fresh surface melt whereas the chemistry of the interior units should reflect solute-rich basal melt. Changes in basal conditions such as the presence of till patches may contribute to the formation of the large basal units near the

  7. Recent Changes in Arctic Glaciers, Ice Caps, and the Greenland Ice Sheet: Cold Facts About Warm Ice

    NASA Astrophysics Data System (ADS)

    Abdalati, W.

    2005-12-01

    One of the major manifestations of Arctic change can be observed in the state of balance of Arctic glaciers and ice caps and the Greenland ice sheet. These ice masses are estimated to contain nearly 3 million cubic kilometers of ice, which is more than six times greater than all the water stored in the Earth's lakes, rivers, and snow combined and is the equivalent of over 7 meters of sea level. Most of these ice masses have been shrinking in recent in years, but their mass balance is highly variable on a wide range of spatial and temporal scales. On the Greenland ice sheet most of the coastal regions have thinned substantially as melt has increased and some of its outlet glaciers have accelerated. Near the equilibrium line in West Greenland, we have seen evidence of summer acceleration that is linked to surface meltwater production, suggesting a relatively rapid response mechanism of the ice sheet change to a warming climate. At the same time, however, the vast interior regions of the Greenland ice sheet have shown little change or slight growth, as accumulation in these areas may have increased. Throughout much of the rest of the Arctic, many glaciers and ice caps have been shrinking in the past few decades, and in Canada and Alaska, the rate of ice loss seems to have accelerated during the late 1990s. These recent observations offer only a snapshot in time of the long-term behavior, but they are providing crucial information about the current state of ice mass balance and the mechanisms that control it in one of the most climatically sensitive regions on Earth. As we continue to learn more through a combination of remote sensing observations, in situ measurements and improved modeling capabilities, it is important that we coordinate and integrate these approaches effectively in order to predict future changes and their impact on sea level, freshwater discharge, and ocean circulation.

  8. Reconstructing the post-LGM decay of the Eurasian Ice Sheets with Ice Sheet Models; data-model comparison and focus on the Storfjorden (Svalbard) ice stream dynamics history

    NASA Astrophysics Data System (ADS)

    Petrini, Michele; Kirchner, Nina; Colleoni, Florence; Camerlenghi, Angelo; Rebesco, Michele; Lucchi, Renata G.; Forte, Emanuele; Colucci, Renato R.

    2017-04-01

    The challenge of reconstructing palaeo-ice sheets past growth and decay represent a critical task to better understand mechanisms of present and future global climate change. Last Glacial Maximum (LGM), and the subsequent deglaciation until Pre-Industrial time (PI) represent an excellent testing ground for numerical Ice Sheet Models (ISMs), due to the abundant data available that can be used in an ISM as boundary conditions, forcings or constraints to test the ISMs results. In our study, we simulate with ISMs the post-LGM decay of the Eurasian Ice Sheets, with a focus on the marine-based Svalbard-Barents Sea-Kara Sea Ice Sheet. In particular, we aim to reconstruct the Storfjorden ice stream dynamics history by comparing the model results with the marine geological data (MSGLs, GZWs, sediment cores analysis) available from the area, e.g., Pedrosa et al. 2011, Rebesco et al. 2011, 2013, Lucchi et al. 2013. Two hybrid SIA/SSA ISMs are employed, GRISLI, Ritz et al. 2001, and PSU, Pollard&DeConto 2012. These models differ mainly in the complexity with which grounding line migration is treated. Climate forcing is interpolated by means of climate indexes between LGM and PI climate. Regional climate indexes are constructed based on the non-accelerated deglaciation transient experiment carried out with CCSM3, Liu et al. 2009. Indexes representative of the climate evolution over Siberia, Svalbard and Scandinavia are employed. The impact of such refined representation as opposed to the common use of the NGRIP δ18O index for transient experiments is analysed. In this study, the ice-ocean interaction is crucial to reconstruct the Storfjorden ice stream dynamics history. To investigate the sensitivity of the ice shelf/stream retreat to ocean temperature, we allow for a space-time variation of basal melting under the ice shelves by testing two-equations implementations based on Martin et al. 2011 forced with simulated ocean temperature and salinity from the TraCE-21ka coupled

  9. Interaction of ice sheets and climate during the past 800 000 years

    NASA Astrophysics Data System (ADS)

    Stap, L. B.; van de Wal, R. S. W.; de Boer, B.; Bintanja, R.; Lourens, L. J.

    2014-12-01

    During the Cenozoic, land ice and climate interacted on many different timescales. On long timescales, the effect of land ice on global climate and sea level is mainly set by large ice sheets in North America, Eurasia, Greenland and Antarctica. The climatic forcing of these ice sheets is largely determined by the meridional temperature profile resulting from radiation and greenhouse gas (GHG) forcing. As a response, the ice sheets cause an increase in albedo and surface elevation, which operates as a feedback in the climate system. To quantify the importance of these climate-land ice processes, a zonally averaged energy balance climate model is coupled to five one-dimensional ice sheet models, representing the major ice sheets. In this study, we focus on the transient simulation of the past 800 000 years, where a high-confidence CO2 record from ice core samples is used as input in combination with Milankovitch radiation changes. We obtain simulations of atmospheric temperature, ice volume and sea level that are in good agreement with recent proxy-data reconstructions. We examine long-term climate-ice-sheet interactions by a comparison of simulations with uncoupled and coupled ice sheets. We show that these interactions amplify global temperature anomalies by up to a factor of 2.6, and that they increase polar amplification by 94%. We demonstrate that, on these long timescales, the ice-albedo feedback has a larger and more global influence on the meridional atmospheric temperature profile than the surface-height-temperature feedback. Furthermore, we assess the influence of CO2 and insolation by performing runs with one or both of these variables held constant. We find that atmospheric temperature is controlled by a complex interaction of CO2 and insolation, and both variables serve as thresholds for northern hemispheric glaciation.

  10. Limited Impact of Subglacial Supercooling Freeze-on for Greenland Ice Sheet Stratigraphy

    NASA Astrophysics Data System (ADS)

    Dow, Christine F.; Karlsson, Nanna B.; Werder, Mauro A.

    2018-02-01

    Large units of disrupted radiostratigraphy (UDR) are visible in many radio-echo sounding data sets from the Greenland Ice Sheet. This study investigates whether supercooling freeze-on rates at the bed can cause the observed UDR. We use a subglacial hydrology model to calculate both freezing and melting rates at the base of the ice sheet in a distributed sheet and within basal channels. We find that while supercooling freeze-on is a phenomenon that occurs in many areas of the ice sheet, there is no discernible correlation with the occurrence of UDR. The supercooling freeze-on rates are so low that it would require tens of thousands of years with minimal downstream ice motion to form the hundreds of meters of disrupted radiostratigraphy. Overall, the melt rates at the base of the ice sheet greatly overwhelm the freeze-on rates, which has implications for mass balance calculations of Greenland ice.

  11. Studies of ice sheet hydrology using SAR

    NASA Technical Reports Server (NTRS)

    Bindschadler, R. A.; Vornberger, P. L.

    1989-01-01

    Analysis of SAR data of the Greenland ice sheet in summer and winter suggest the use of SAR to monitor the temporal hydrology of ice sheets. Comparisons of each SAR data set with summer Landsat TM imagery show an areal-positive correlation with summer SAR data and a negative correlation with winter SAR data. It is proposed that the summer SAR data are most sensitive to the variable concentrations of free water in the surface snow and that the winter SAR data indicate variations in snow grain size.

  12. Improvements in Ice-Sheet Sea-Level Projections

    NASA Technical Reports Server (NTRS)

    Shepherd, Andrew; Nowicki, Sophie

    2017-01-01

    Ice losses from Antarctica and Greenland are the largest uncertainty in sea-level projections. Nevertheless, improvements in ice-sheet models over recent decades have led to closer agreement with satellite observations, keeping track with their increasing contribution to global sea-level rise.

  13. Study of elevation changes along a profile crossing the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hvidegaard, S. M.; Sandberg, L.

    2009-04-01

    In recent years much research has focused on determining how the Greenland Ice Sheet is responding to the observed climate changes. There is wide agreement on the fact that the Ice Sheet is currently loosing mass, and studies have shown that the mass loss is found near the ice edge and that no significant changes are found in the central part of the Ice Sheet. As a part of European Space Agency's CryoSat Validation Experiment (CryoVEx) running from 2004 to 2008, the National Space Institute (DTU Space) measured the elevations along a profile crossing the Greenland Ice Sheet. The elevation observations were carried out in 2004, 2006 and 2008 using airborne laser altimetry from a Twin Otter aircraft. The observed profile follows the old EGIG line (Expédition Glaciologique au Groenland, measured in the 1950's) situated between 69-71N, heading nearly east-west. This unique dataset gives the opportunity to study elevation changes along the profile crossing the ice sheet. With this work, we outline the observed elevation changes from the different zones of the ice sheet. We furthermore compare elevation changes based on coincident ICESat and airborne laser altimeter data.

  14. A 25-year Record of Antarctic Ice Sheet Elevation and Mass Change

    NASA Astrophysics Data System (ADS)

    Shepherd, A.; Muir, A. S.; Sundal, A.; McMillan, M.; Briggs, K.; Hogg, A.; Engdahl, M.; Gilbert, L.

    2017-12-01

    Since 1992, the European Remote-Sensing (ERS-1 and ERS-2), ENVISAT, and CryoSat-2 satellite radar altimeters have measured the Antarctic ice sheet surface elevation, repeatedly, at approximately monthly intervals. These data constitute the longest continuous record of ice sheet wide change. In this paper, we use these observations to determine changes in the elevation, volume and mass of the East Antarctic and West Antarctic ice sheets, and of parts of the Antarctic Peninsula ice sheet, over a 25-year period. The root mean square difference between elevation rates computed from our survey and 257,296 estimates determined from airborne laser measurements is 54 cm/yr. The longevity of the satellite altimeter data record allows to identify and chart the evolution of changes associated with meteorology and ice flow, and we estimate that 3.6 % of the continental ice sheet, and 21.7 % of West Antarctica, is in a state of dynamical imbalance. Based on this partitioning, we estimate the mass balance of the East and West Antarctic ice sheet drainage basins and the root mean square difference between these and independent estimates derived from satellite gravimetry is less than 5 Gt yr-1.

  15. The sea-level fingerprints of ice-sheet collapse during interglacial periods

    NASA Astrophysics Data System (ADS)

    Hay, Carling; Mitrovica, Jerry X.; Gomez, Natalya; Creveling, Jessica R.; Austermann, Jacqueline; E. Kopp, Robert

    2014-03-01

    Studies of sea level during previous interglacials provide insight into the stability of polar ice sheets in the face of global climate change. Commonly, these studies correct ancient sea-level highstands for the contaminating effect of isostatic adjustment associated with past ice age cycles, and interpret the residuals as being equivalent to the peak eustatic sea level associated with excess melting, relative to present day, of ancient polar ice sheets. However, the collapse of polar ice sheets produces a distinct geometry, or fingerprint, of sea-level change, which must be accounted for to accurately infer peak eustatic sea level from site-specific residual highstands. To explore this issue, we compute fingerprints associated with the collapse of the Greenland Ice Sheet, West Antarctic Ice Sheet, and marine sectors of the East Antarctic Ice Sheet in order to isolate regions that would have been subject to greater-than-eustatic sea-level change for all three cases. These fingerprints are more robust than those associated with modern melting events, when applied to infer eustatic sea level, because: (1) a significant collapse of polar ice sheets reduces the sensitivity of the computed fingerprints to uncertainties in the geometry of the melt regions; and (2) the sea-level signal associated with the collapse will dominate the signal from steric effects. We evaluate these fingerprints at a suite of sites where sea-level records from interglacial marine isotopes stages (MIS) 5e and 11 have been obtained. Using these results, we demonstrate that previously discrepant estimates of peak eustatic sea level during MIS5e based on sea-level markers in Australia and the Seychelles are brought into closer accord.

  16. Conditions for a steady ice sheet ice shelf junction

    NASA Astrophysics Data System (ADS)

    Nowicki, S. M. J.; Wingham, D. J.

    2008-01-01

    This paper investigates the conditions under which a marine ice sheet may adopt a steady profile. The ice is treated as a linear viscous fluid caused to flow from a rigid base to and over water, treated as a denser but inviscid fluid. The solutions in the region around the point of flotation, or 'transition' zone, are calculated numerically. In-flow and out-flow conditions appropriate to ice sheet and ice shelf flow are applied at the ends of the transition zone and the rigid base is specified; the flow and steady free surfaces are determined as part of the solutions. The basal stress upstream, and the basal deflection downstream, of the flotation point are examined to determine which of these steady solutions satisfy 'contact' conditions that would prevent (i) the steady downstream basal deflection contacting the downstream base, and (ii) the upstream ice commencing to float in the event it was melted at the base. In the case that the upstream bed is allowed to slide, we find only one mass flux that satisfies the contact conditions. When no sliding is allowed at the bed, however, we find a range of mass fluxes satisfy the contact conditions. The effect of 'backpressure' on the solutions is investigated, and is found to have no affect on the qualitative behaviour of the junctions. To the extent that the numerical, linearly viscous treatment may be applied to the case of ice flowing out over the ocean, we conclude that when sliding is present, Weertman's 'instability' hypothesis holds.

  17. A Reconciled Estimate of Ice-Sheet Mass Balance

    NASA Technical Reports Server (NTRS)

    Shepherd, Andrew; Ivins, Erik R.; Geruo, A.; Barletta, Valentia R.; Bentley, Mike J.; Bettadpur, Srinivas; Briggs, Kate H.; Bromwich, David H.; Forsberg, Rene; Galin, Natalia; hide

    2012-01-01

    We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods-especially in Greenland and West Antarctica-and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 plus or minus 49, +14 plus or minus 43, -65 plus or minus 26, and -20 plus or minus 14 gigatonnes year(sup -1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 plus or minus 0.20 millimeter year(sup -1) to the rate of global sea-level rise.

  18. Satellite radar interferometry for monitoring ice sheet motion: application to an antarctic ice stream.

    PubMed

    Goldstein, R M; Engelhardt, H; Kamb, B; Frolich, R M

    1993-12-03

    Satellite radar interferometry (SRI) provides a sensitive means of monitoring the flow velocities and grounding-line positions of ice streams, which are indicators of response of the ice sheets to climatic change or internal instability. The detection limit is about 1.5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction. The grounding line, detected by tidal motions where the ice goes afloat, can be mapped at a resolution of approximately 0.5 kilometer. The SRI velocities and grounding line of the Rutford Ice Stream, Antarctica, agree fairly well with earlier ground-based data. The combined use of SRI and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior.

  19. Formation and interpretation of eskers beneath retreating ice sheets

    NASA Astrophysics Data System (ADS)

    Creyts, T. T.; Hewitt, I.

    2017-12-01

    The retreat of the ice sheets during the Pleistocene left large and spectacular subglacial features exposed. Understanding these features gives us insight into how the ice sheets retreated, how meltwater influenced retreat, and can help inform our understanding of potential future rates of ice sheet retreat. Among these features, eskers, long sinuous ridges primarily composed of clastic sediments, lack a detailed explanation of how surface melt rates and ice sheet retreat rates influence their growth and spatial distribution. Here, we develop a theory for esker formation based on the initial work of Rothlisberger modified for sediment transport and inclusion of surface meltwater forcing. The primary subglacial ingredients include water flow through subglacial tunnels with the addition of mass balances for sediment transport. We show how eskers when water flow slows below a critical stress for sediment motion. This implies that eskers are deposited in a localized region near the snout of the ice sheet. Our findings suggest that very long eskers form sequentially as the ice front retreats. The position of the esker follows the path of the channel mouth through time, which does not necessarily coincide with the instantaneous route of the feeding channel. However, in most cases, we expect those locations to be similar. The role of surface meltwater and the climatology associated with the forcing is crucial to the lateral spacing of the eskers. We predict that high surface melt rates lead to narrower catchments but that the greater extent of the ablation area means that channels are likely larger. At the same time, for a given channel size (and hence sediment flux), the size of a deposited esker depends on a margin retreat rate. Hence, the size of the eskers is related delicately to the balance between surface melt rates and margin retreat rates. We discuss how our theory can be combined with observed esker distributions to infer the relationship between these two rates

  20. Results of the Greenland Ice Sheet Model Initialisation Experiments ISMIP6 - initMIP-Greenland

    NASA Astrophysics Data System (ADS)

    Goelzer, H.; Nowicki, S.; Edwards, T.; Beckley, M.; Abe-Ouchi, A.; Aschwanden, A.; Calov, R.; Gagliardini, O.; Gillet-chaulet, F.; Golledge, N. R.; Gregory, J. M.; Greve, R.; Humbert, A.; Huybrechts, P.; Larour, E. Y.; Lipscomb, W. H.; Le ´h, S.; Lee, V.; Kennedy, J. H.; Pattyn, F.; Payne, A. J.; Rodehacke, C. B.; Rückamp, M.; Saito, F.; Schlegel, N.; Seroussi, H. L.; Shepherd, A.; Sun, S.; Vandewal, R.; Ziemen, F. A.

    2016-12-01

    Earlier large-scale Greenland ice sheet sea-level projections e.g. those run during ice2sea and SeaRISE initiatives have shown that ice sheet initialisation can have a large effect on the projections and gives rise to important uncertainties. The goal of this intercomparison exercise (initMIP-Greenland) is to compare, evaluate and improve the initialization techniques used in the ice sheet modeling community and to estimate the associated uncertainties. It is the first in a series of ice sheet model intercomparison activities within ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of 1) the initial present-day state of the ice sheet and 2) the response in two schematic forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without any forcing) and response to a large perturbation (prescribed surface mass balance anomaly). We present and discuss final results of the intercomparison and highlight important uncertainties with respect to projections of the Greenland ice sheet sea-level contribution.

  1. West Antarctic Ice Sheet retreat driven by Holocene warm water incursions.

    PubMed

    Hillenbrand, Claus-Dieter; Smith, James A; Hodell, David A; Greaves, Mervyn; Poole, Christopher R; Kender, Sev; Williams, Mark; Andersen, Thorbjørn Joest; Jernas, Patrycja E; Elderfield, Henry; Klages, Johann P; Roberts, Stephen J; Gohl, Karsten; Larter, Robert D; Kuhn, Gerhard

    2017-07-05

    Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) incursions onto the West Antarctic continental shelf cause melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet retreat today. Here we present a multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the Holocene epoch (from 11.7 thousand years ago to the present). The chemical compositions of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector from at least 10,400 years ago until 7,500 years ago-when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream-and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models.

  2. West Antarctic Ice Sheet Initiative. Volume 2: Discipline Reviews

    NASA Technical Reports Server (NTRS)

    Bindschadler, Robert A. (Editor)

    1991-01-01

    Seven discipline review papers are presented on the state of the knowledge of West Antarctica and opinions on how that knowledge must be increased to predict the future behavior of this ice sheet and to assess its potential to collapse, rapidly raising the global sea level. These are the goals of the West Antarctic Ice Sheet Initiative (WAIS).

  3. A Detailed Geophysical Investigation of the Grounding of Henry Ice Rise, with Implications for Holocene Ice-Sheet Extent.

    NASA Astrophysics Data System (ADS)

    Wearing, M.; Kingslake, J.

    2017-12-01

    It is generally assumed that since the Last Glacial Maximum the West Antarctic Ice Sheet (WAIS) has experienced monotonic retreat of the grounding line (GL). However, recent studies have cast doubt on this assumption, suggesting that the retreat of the WAIS grounding line may have been followed by a significant advance during the Holocene in the Weddell and Ross Sea sectors. Constraining this evolution is important as reconstructions of past ice-sheet extent are used to spin-up predictive ice-sheet models and correct mass-balance observations for glacial isostatic adjustment. Here we examine in detail the formation of the Henry Ice Rise (HIR), which ice-sheet model simulations suggest played a key role in Holocene ice-mass changes in the Weddell Sea sector. Observations from a high-resolution ground-based, ice-penetrating radar survey are best explained if the ice rise formed when the Ronne Ice Shelf grounded on a submarine high, underwent a period of ice-rumple flow, before the GL migrated outwards to form the present-day ice rise. We constrain the relative chronology of this evolution by comparing the alignment and intersection of isochronal internal layers, relic crevasses, surface features and investigating the dynamic processes leading to their complex structure. We also draw analogies between HIR and the neighbouring Doake Ice Rumples. The date of formation is estimated using vertical velocities derived with a phase-sensitive radio-echo sounder (pRES). Ice-sheet models suggest that the formation of the HIR and other ice rises may have halted and reversed large-scale GL retreat. Hence the small-scale dynamics of these crucial regions could have wide-reaching consequences for future ice-sheet mass changes and constraining their formation and evolution further would be beneficial. One stringent test of our geophysics-based conclusions would be to drill to the bed of HIR to sample the ice for isotopic analysis and the bed for radiocarbon analysis.

  4. Sensitivity of grounding line dynamics to viscoelastic deformation of the solid Earth: Inferences from a fully coupled ice sheet - solid Earth model

    NASA Astrophysics Data System (ADS)

    Konrad, H.; Sasgen, I.; Thoma, M.; Klemann, V.; Grosfeld, K.; Martinec, Z.

    2013-12-01

    The interactions of ice sheets with the sea level and the solid Earth are important factors for the stability of the ice shelves and the tributary inland ice (e.g. Thomas and Bentley, 1978; Gomez et al, 2012). First, changes in ice extent and ice thickness induce viscoelastic deformation of the Earth surface and Earth's gravity field. In turn, global and local changes in sea level and bathymetry affect the grounding line and, subsequently, alter the ice dynamic behaviour. Here, we investigate these feedbacks for a synthetic ice sheet configuration as well as for the Antarctic ice sheet using a three-dimensional thermomechanical ice sheet and shelf model, coupled to a viscoelastic solid-Earth and gravitationally self-consistent sea-level model. The respective ice sheet undergoes a forcing from rising sea level, warming ocean, and/or changing surface mass balance. The coupling is realized by exchanging ice thickness, Earth surface deformation, and sea level periodically. We apply several sets of viscoelastic Earth parameters to our coupled model, e.g. simulating a low-viscous upper mantle present at the Antarctic Peninsula (Ivins et al., 2011). Special focus of our study lies on the evolution of Earth surface deformation and local sea level changes, as well as on the accompanying grounding line evolution. N. Gomez, D. Pollard, J. X. Mitrovica, P. Huybers, and P. U. Clark 2012. Evolution of a coupled marine ice sheet-sea level model, J. Geophys. Res., 117, F01013, doi:10.1029/2011JF002128. E. R. Ivins, M. M. Watkins, D.-N. Yuan, R. Dietrich, G. Casassa, and A. Rülke 2011. On-land ice loss and glacial isostatic adjustment at the Drake Passage: 2003-2009, J. Geophys. Res. 116, B02403, doi: 10.1029/2010JB007607 R. H. Thomas and C. R. Bentley 1978. A model for Holocene retreat of the West Antarctic Ice Sheet, Quaternary Research, 10 (2), pages 150-170, doi: 10.1016/0033-5894(78)90098-4.

  5. Meltwater storage in low-density near-surface bare ice in the Greenland ice sheet ablation zone

    NASA Astrophysics Data System (ADS)

    Cooper, Matthew G.; Smith, Laurence C.; Rennermalm, Asa K.; Miège, Clément; Pitcher, Lincoln H.; Ryan, Jonathan C.; Yang, Kang; Cooley, Sarah W.

    2018-03-01

    We document the density and hydrologic properties of bare, ablating ice in a mid-elevation (1215 m a.s.l.) supraglacial internally drained catchment in the Kangerlussuaq sector of the western Greenland ice sheet. We find low-density (0.43-0.91 g cm-3, μ = 0.69 g cm-3) ice to at least 1.1 m depth below the ice sheet surface. This near-surface, low-density ice consists of alternating layers of water-saturated, porous ice and clear solid ice lenses, overlain by a thin (< 0.5 m), even lower density (0.33-0.56 g cm-3, μ = 0.45 g cm-3) unsaturated weathering crust. Ice density data from 10 shallow (0.9-1.1 m) ice cores along an 800 m transect suggest an average 14-18 cm of specific meltwater storage within this low-density ice. Water saturation of this ice is confirmed through measurable water levels (1-29 cm above hole bottoms, μ = 10 cm) in 84 % of cryoconite holes and rapid refilling of 83 % of 1 m drilled holes sampled along the transect. These findings are consistent with descriptions of shallow, depth-limited aquifers on the weathered surface of glaciers worldwide and confirm the potential for substantial transient meltwater storage within porous low-density ice on the Greenland ice sheet ablation zone surface. A conservative estimate for the ˜ 63 km2 supraglacial catchment yields 0.009-0.012 km3 of liquid meltwater storage in near-surface, porous ice. Further work is required to determine if these findings are representative of broader areas of the Greenland ice sheet ablation zone, and to assess the implications for sub-seasonal mass balance processes, surface lowering observations from airborne and satellite altimetry, and supraglacial runoff processes.

  6. Greenland ice sheet beyond 2100: Simulating its evolution and influence using the coupled climate-ice sheet model EC-Earth - PISM

    NASA Astrophysics Data System (ADS)

    Yang, S.; Christensen, J. H.; Madsen, M. S.; Ringgaard, I. M.; Petersen, R. A.; Langen, P. P.

    2017-12-01

    Greenland ice sheet (GrIS) is observed undergoing a rapid change in the recent decades, with an increasing area of surface melting and ablation and a speeding mass loss. Predicting the GrIS changes and their climate consequences relies on the understanding of the interaction of the GrIS with the climate system on both global and local scales, and requires climate model systems incorporating with an explicit and physically consistent ice sheet module. In this work we study the GrIS evolution and its interaction with the climate system using a fully coupled global climate model with a dynamical ice sheet model for the GrIS. The coupled model system, EC-EARTH - PISM, consisting of the atmosphere-ocean-sea ice model system EC-EARTH, and the Parallel Ice Sheet Model (PISM), has been employed for a 1400-year simulation forced by CMIP5 historical forcing from 1850 to 2005 and continued along an extended RCP8.5 scenario with the forcing peaking at 2200 and stabilized hereafter. The simulation reveals that, following the anthropogenic forcing increase, the global mean surface temperature rapidly rises about 10 °C in the 21st and 22nd century. After the forcing stops increasing after 2200, the temperature change slows down and eventually stabilizes at about 12.5 °C above the preindustrial level. In response to the climate warming, the GrIS starts losing mass slowly in the 21st century, but the ice retreat accelerates substantially after 2100 and ice mass loss continues hereafter at a constant rate of approximately 0.5 m sea level rise equivalence per 100 years, even as the warming rate gradually levels off. Ultimately the volume and extent of GrIS reduce to less than half of its preindustrial value. To understand the interaction of GrIS with the climate system, the characteristics of atmospheric and oceanic circulation in the warm climate are analyzed. The circulation patterns associated with the negative surface mass balance that leads to GrIS retreat are investigated

  7. Implementation of higher-order vertical finite elements in ISSM v4.13 for improved ice sheet flow modeling over paleoclimate timescales

    NASA Astrophysics Data System (ADS)

    Cuzzone, Joshua K.; Morlighem, Mathieu; Larour, Eric; Schlegel, Nicole; Seroussi, Helene

    2018-05-01

    Paleoclimate proxies are being used in conjunction with ice sheet modeling experiments to determine how the Greenland ice sheet responded to past changes, particularly during the last deglaciation. Although these comparisons have been a critical component in our understanding of the Greenland ice sheet sensitivity to past warming, they often rely on modeling experiments that favor minimizing computational expense over increased model physics. Over Paleoclimate timescales, simulating the thermal structure of the ice sheet has large implications on the modeled ice viscosity, which can feedback onto the basal sliding and ice flow. To accurately capture the thermal field, models often require a high number of vertical layers. This is not the case for the stress balance computation, however, where a high vertical resolution is not necessary. Consequently, since stress balance and thermal equations are generally performed on the same mesh, more time is spent on the stress balance computation than is otherwise necessary. For these reasons, running a higher-order ice sheet model (e.g., Blatter-Pattyn) over timescales equivalent to the paleoclimate record has not been possible without incurring a large computational expense. To mitigate this issue, we propose a method that can be implemented within ice sheet models, whereby the vertical interpolation along the z axis relies on higher-order polynomials, rather than the traditional linear interpolation. This method is tested within the Ice Sheet System Model (ISSM) using quadratic and cubic finite elements for the vertical interpolation on an idealized case and a realistic Greenland configuration. A transient experiment for the ice thickness evolution of a single-dome ice sheet demonstrates improved accuracy using the higher-order vertical interpolation compared to models using the linear vertical interpolation, despite having fewer degrees of freedom. This method is also shown to improve a model's ability to capture sharp

  8. Uncertainty Quantification for Ice Sheet Science and Sea Level Projections

    NASA Astrophysics Data System (ADS)

    Boening, C.; Schlegel, N.; Limonadi, D.; Schodlok, M.; Seroussi, H. L.; Larour, E. Y.; Watkins, M. M.

    2017-12-01

    In order to better quantify uncertainties in global mean sea level rise projections and in particular upper bounds, we aim at systematically evaluating the contributions from ice sheets and potential for extreme sea level rise due to sudden ice mass loss. Here, we take advantage of established uncertainty quantification tools embedded within the Ice Sheet System Model (ISSM) as well as sensitivities to ice/ocean interactions using melt rates and melt potential derived from MITgcm/ECCO2. With the use of these tools, we conduct Monte-Carlo style sampling experiments on forward simulations of the Antarctic ice sheet, by varying internal parameters and boundary conditions of the system over both extreme and credible worst-case ranges. Uncertainty bounds for climate forcing are informed by CMIP5 ensemble precipitation and ice melt estimates for year 2100, and uncertainty bounds for ocean melt rates are derived from a suite of regional sensitivity experiments using MITgcm. Resulting statistics allow us to assess how regional uncertainty in various parameters affect model estimates of century-scale sea level rise projections. The results inform efforts to a) isolate the processes and inputs that are most responsible for determining ice sheet contribution to sea level; b) redefine uncertainty brackets for century-scale projections; and c) provide a prioritized list of measurements, along with quantitative information on spatial and temporal resolution, required for reducing uncertainty in future sea level rise projections. Results indicate that ice sheet mass loss is dependent on the spatial resolution of key boundary conditions - such as bedrock topography and melt rates at the ice-ocean interface. This work is performed at and supported by the California Institute of Technology's Jet Propulsion Laboratory. Supercomputing time is also supported through a contract with the National Aeronautics and Space Administration's Cryosphere program.

  9. Combustion of available fossil-fuel resources sufficient to eliminate the Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Winkelmann, R.; Levermann, A.; Ridgwell, A.; Caldeira, K.

    2015-12-01

    The Antarctic Ice Sheet stores water equivalent to 58 meters in global sea-level rise. Here we show in simulations with the Parallel Ice Sheet Model that burning the currently attainable fossil-fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil-fuel emissions of 10 000 GtC, Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 meters per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West- and East Antarctica results in a threshold-increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources.

  10. Surface Energy and Mass Balance Model for Greenland Ice Sheet and Future Projections

    NASA Astrophysics Data System (ADS)

    Liu, Xiaojian

    The Greenland Ice Sheet contains nearly 3 million cubic kilometers of glacial ice. If the entire ice sheet completely melted, sea level would raise by nearly 7 meters. There is thus considerable interest in monitoring the mass balance of the Greenland Ice Sheet. Each year, the ice sheet gains ice from snowfall and loses ice through iceberg calving and surface melting. In this thesis, we develop, validate and apply a physics based numerical model to estimate current and future surface mass balance of the Greenland Ice Sheet. The numerical model consists of a coupled surface energy balance and englacial model that is simple enough that it can be used for long time scale model runs, but unlike previous empirical parameterizations, has a physical basis. The surface energy balance model predicts ice sheet surface temperature and melt production. The englacial model predicts the evolution of temperature and meltwater within the ice sheet. These two models can be combined with estimates of precipitation (snowfall) to estimate the mass balance over the Greenland Ice Sheet. We first compare model performance with in-situ observations to demonstrate that the model works well. We next evaluate how predictions are degraded when we statistically downscale global climate data. We find that a simple, nearest neighbor interpolation scheme with a lapse rate correction is able to adequately reproduce melt patterns on the Greenland Ice Sheet. These results are comparable to those obtained using empirical Positive Degree Day (PDD) methods. Having validated the model, we next drove the ice sheet model using the suite of atmospheric model runs available through the CMIP5 atmospheric model inter-comparison, which in turn built upon the RCP 8.5 (business as usual) scenarios. From this exercise we predict how much surface melt production will increase in the coming century. This results in 4-10 cm sea level equivalent, depending on the CMIP5 models. Finally, we try to bound melt water

  11. Ice sheet-ocean interactions and sea level change

    NASA Astrophysics Data System (ADS)

    Heimbach, Patrick

    2014-03-01

    Mass loss from the Greenland and Antarctic ice sheets has increased rapidly since the mid-1990s. Their combined loss now accounts for about one-third of global sea level rise. In Greenland, a growing body of evidence points to the marine margins of these glaciers as the region from which this dynamic response originated. Similarly, ice streams in West Antarctica that feed vast floating ice shelves have exhibited large decadal changes. We review observational evidence and present physical mechanisms that might explain the observed changes, in particular in the context of ice sheet-ocean interactions. Processes involve cover 7 orders of magnitudes of scales, ranging from mm boundary-layer processes to basin-scale coupled atmosphere-ocean variability. We discuss observational needs to fill the gap in our mechanistic understanding.

  12. Seismic evidence for complex sedimentary control of Greenland Ice Sheet flow

    PubMed Central

    Kulessa, Bernd; Hubbard, Alun L.; Booth, Adam D.; Bougamont, Marion; Dow, Christine F.; Doyle, Samuel H.; Christoffersen, Poul; Lindbäck, Katrin; Pettersson, Rickard; Fitzpatrick, Andrew A. W.; Jones, Glenn A.

    2017-01-01

    The land-terminating margin of the Greenland Ice Sheet has slowed down in recent decades, although the causes and implications for future ice flow are unclear. Explained originally by a self-regulating mechanism where basal slip reduces as drainage evolves from low to high efficiency, recent numerical modeling invokes a sedimentary control of ice sheet flow as an alternative hypothesis. Although both hypotheses can explain the recent slowdown, their respective forecasts of a long-term deceleration versus an acceleration of ice flow are contradictory. We present amplitude-versus-angle seismic data as the first observational test of the alternative hypothesis. We document transient modifications of basal sediment strengths by rapid subglacial drainages of supraglacial lakes, the primary current control on summer ice sheet flow according to our numerical model. Our observations agree with simulations of initial postdrainage sediment weakening and ice flow accelerations, and subsequent sediment restrengthening and ice flow decelerations, and thus confirm the alternative hypothesis. Although simulated melt season acceleration of ice flow due to weakening of subglacial sediments does not currently outweigh winter slowdown forced by self-regulation, they could dominate over the longer term. Subglacial sediments beneath the Greenland Ice Sheet must therefore be mapped and characterized, and a sedimentary control of ice flow must be evaluated against competing self-regulation mechanisms. PMID:28835915

  13. Seismic evidence for complex sedimentary control of Greenland Ice Sheet flow.

    PubMed

    Kulessa, Bernd; Hubbard, Alun L; Booth, Adam D; Bougamont, Marion; Dow, Christine F; Doyle, Samuel H; Christoffersen, Poul; Lindbäck, Katrin; Pettersson, Rickard; Fitzpatrick, Andrew A W; Jones, Glenn A

    2017-08-01

    The land-terminating margin of the Greenland Ice Sheet has slowed down in recent decades, although the causes and implications for future ice flow are unclear. Explained originally by a self-regulating mechanism where basal slip reduces as drainage evolves from low to high efficiency, recent numerical modeling invokes a sedimentary control of ice sheet flow as an alternative hypothesis. Although both hypotheses can explain the recent slowdown, their respective forecasts of a long-term deceleration versus an acceleration of ice flow are contradictory. We present amplitude-versus-angle seismic data as the first observational test of the alternative hypothesis. We document transient modifications of basal sediment strengths by rapid subglacial drainages of supraglacial lakes, the primary current control on summer ice sheet flow according to our numerical model. Our observations agree with simulations of initial postdrainage sediment weakening and ice flow accelerations, and subsequent sediment restrengthening and ice flow decelerations, and thus confirm the alternative hypothesis. Although simulated melt season acceleration of ice flow due to weakening of subglacial sediments does not currently outweigh winter slowdown forced by self-regulation, they could dominate over the longer term. Subglacial sediments beneath the Greenland Ice Sheet must therefore be mapped and characterized, and a sedimentary control of ice flow must be evaluated against competing self-regulation mechanisms.

  14. Modelled Growth and Decay of the Cordilleran Ice Sheet Through the Last Glacial Cycle

    NASA Astrophysics Data System (ADS)

    Marshall, S. J.; Banwell, A.

    2015-12-01

    The Cordilleran Ice Sheet in western North America had an enigmatic evolution during the last glacial cycle, developing out of sync with the larger Laurentide and global glaciation. The geological record suggests that the ice sheet emerged late, ca. 45 ka, growing to be a fully-established ice sheet in isotope stages 3 and 2 and deglaciating late in the glacial cycle. This has been a challenge to model, and is a paleoclimatic curiosity, because the western Cordillera of North America is heavily glacierized today, and one would intuitively expect it to act as an inception centre for the Pleistocene ice sheets. The region receives heavy precipitation, and modest cooling should induce large-scale glacier expansion. Indeed, a Cordilleran Ice Sheet quickly nucleates in isotope substage 5d in most ice sheet modeling studies to date, and is a resilient feature throughout the glaciation. The fact that a full-scale Cordilleran Ice Sheet did not develop until relatively late argues for either: (a) ice sheet models that have been inadequate in resolving the process of alpine-style glaciation, i.e., the coalescence of alpine icefields, or (b) a climatic history in western North America that deviated strongly from the hemispheric-scale cooling which drove the growth of the Laurentide and Scandinavian Ice Sheets, as recorded in Greenland. We argue that reasonable reconstructions of Cordilleran Ice Sheet growth and decay implicate a combination of these two considerations. Sufficient model resolution is required to capture the valley-bottom melt that suppresses icefield coalescence, while early-glacial cooling must have been modest in the Pacific sector of North America. We argue for a persistent warm, dry climate relative to that in eastern North America and the Atlantic sector, likely associated with positive feedbacks between atmospheric circulation and the nascent Laurentide Ice Sheet (i.e., peristent circulation patterns similar to those of 2014-2015). This must have been

  15. Rapid Access Ice Drill: A New Tool for Exploration of the Deep Antarctic Ice Sheets and Subglacial Geology

    NASA Astrophysics Data System (ADS)

    Goodge, J. W.; Severinghaus, J. P.

    2014-12-01

    The Rapid Access Ice Drill (RAID) will penetrate the Antarctic ice sheets in order to core through deep ice, the glacial bed, and into bedrock below. This new technology will provide a critical first look at the interface between major ice caps and their subglacial geology. Currently in construction, RAID is a mobile drilling system capable of making several long boreholes in a single field season in Antarctica. RAID is interdisciplinary and will allow access to polar paleoclimate records in ice >1 Ma, direct observation at the base of the ice sheets, and recovery of rock cores from the ice-covered East Antarctic craton. RAID uses a diamond rock-coring system as in mineral exploration. Threaded drill-pipe with hardened metal bits will cut through ice using reverse circulation of Estisol for pressure-compensation, maintenance of temperature, and removal of ice cuttings. Near the bottom of the ice sheet, a wireline bottom-hole assembly will enable diamond coring of ice, the glacial bed, and bedrock below. Once complete, boreholes will be kept open with fluid, capped, and made available for future down-hole measurement of thermal gradient, heat flow, ice chronology, and ice deformation. RAID will also sample for extremophile microorganisms. RAID is designed to penetrate up to 3,300 meters of ice and take sample cores in less than 200 hours. This rapid performance will allow completion of a borehole in about 10 days before moving to the next drilling site. RAID is unique because it can provide fast borehole access through thick ice; take short ice cores for paleoclimate study; sample the glacial bed to determine ice-flow conditions; take cores of subglacial bedrock for age dating and crustal history; and create boreholes for use as an observatory in the ice sheets. Together, the rapid drilling capability and mobility of the drilling system, along with ice-penetrating imaging methods, will provide a unique 3D picture of the interior Antarctic ice sheets.

  16. Recent Changes in High-Latitude Glaciers, Ice Caps, and Ice Sheets

    NASA Technical Reports Server (NTRS)

    Abdalati, Waleed

    2006-01-01

    The glaciers and ice sheets of the world contain enough ice to raise sea level by approximately 70 meters if they were to disappear entirely, and most of this ice is located in the climatically sensitive polar regions. Fortunately changes of this magnitude would probably take many thousands of years to occur, but recent discoveries indicate that these ice masses are responding to changes in today s climate more rapidly than previously thought. These responses are likely to be of great societal significance, primarily in terms of their implications for sea level, but also in terms of how their discharge of freshwater, through melting or calving, may impact ocean circulation. For millions of years, oceans have risen and fallen as the Earth has warmed and cooled, and ice on land has shrunk and grown. Today is no different in that respect, as sea levels have been rising at a rate of nearly 2 m per year during the last century (Miller and Douglas 2004), and 3 mm/yr in the last 12 years (Leuliette et al. 2004). What is different today, however, is that tens - perhaps hundreds - of millions of people live in coastal areas that are vulnerable to changes in sea level. Rising seas erode beaches, increase flood potential, and reduce the ability of barrier islands and coastal wetlands to mitigate the effects of major storms and hurricanes. The costs associated with a one-meter rise in sea level are estimated to be in the hundreds of billions of dollars in the United States alone. The worldwide costs in human terms would be far greater as some vulnerable low-lying coastal regions would become inundated, especially in poorer nations that do not have the resources to deal with such changes. Such considerations are particularly important in light of the fact that a one meter sea level rise is not significantly outside the 0.09 to 0.88 range of predictions for this century (IPCC 2001), and rises of this magnitude have occurred in the past in as little as 20 years (Fairbanks 1989

  17. The Antarctic Ice Sheet during the last Interglaciation: Insights from my Thesis

    NASA Astrophysics Data System (ADS)

    Whipple, Matthew; Lunt, Dan; Singarayer, Joy; Bradley, Sarah; Milne, Glenn; Wolff, Eric; Siddall, Mark

    2015-04-01

    The last interglaciation represents a period of warmer climates and higher sea levels, and a useful analogue to future climate. While many studies have focussed on the response of the Greenland Ice sheet, far less is known about the response of the Antarctic ice sheet. Here, I present the summarised results of my PhD thesis "Constraints on the minimum extent of the Antarctic ice sheet during the last interglaciation". Firstly, I cover the timings of interglaciation in Antarctica, and their differences with respect to the Northern Hemisphere timings, based on paleo sea level indicators, and oceanic temperature records. I move on to cover climate forcings, and how they influence the ice sheet, relative to present, and early Holocene. Secondly, I present thesis results, from looking at ice core stable water isotopes. These are compared with Isostatic and Climatic modelling results, for various different Ice sheet scenarios, as to the resulting Climate, from changes in Elevation, Temperature, Precipitation, and Sublimation, all contributing to the recorded stable water isotope record. Thirdly, I move on to looking at the mid-field relative sea level records, from Australia and Argentina. Using isostatic modelling, these are used to assess the relative contribution of the Eastern and Western Antarctic Ice sheets. Although data uncertainties result in us being to identify the contribution from West Antarctica. Overall, using model-data comparison, we find a lack of evidence for a substantial retreat of the Wilkes Subglacial basin. No data location is close enough to determine the existence of the marine based West Antarctic Ice sheet. Model uncertainty is unable to constrain evidence of variations in ice thickness in East Antarctica.

  18. Using palaeoclimate data to improve models of the Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Phipps, Steven; King, Matt; Roberts, Jason; White, Duanne

    2017-04-01

    Ice sheet models are the most descriptive tools available to simulate the future evolution of the Antarctic Ice Sheet (AIS), including its contribution towards changes in global sea level. However, our knowledge of the dynamics of the coupled ice-ocean-lithosphere system is inevitably limited, in part due to a lack of observations. Furthemore, to build computationally efficient models that can be run for multiple millennia, it is necessary to use simplified descriptions of ice dynamics. Ice sheet modelling is therefore an inherently uncertain exercise. The past evolution of the AIS provides an opportunity to constrain the description of physical processes within ice sheet models and, therefore, to constrain our understanding of the role of the AIS in driving changes in global sea level. We use the Parallel Ice Sheet Model (PISM) to demonstrate how palaeoclimate data can improve our ability to predict the future evolution of the AIS. A 50-member perturbed-physics ensemble is generated, spanning uncertainty in the parameterisations of three key physical processes within the model: (i) the stress balance within the ice sheet, (ii) basal sliding and (iii) calving of ice shelves. A Latin hypercube approach is used to optimally sample the range of uncertainty in parameter values. This perturbed-physics ensemble is used to simulate the evolution of the AIS from the Last Glacial Maximum ( 21,000 years ago) to present. Palaeoclimate records are then used to determine which ensemble members are the most realistic. This allows us to use data on past climates to directly constrain our understanding of the past contribution of the AIS towards changes in global sea level. Critically, it also allows us to determine which ensemble members are likely to generate the most realistic projections of the future evolution of the AIS.

  19. Understanding Greenland ice sheet hydrology using an integrated multi-scale approach

    NASA Astrophysics Data System (ADS)

    Rennermalm, A. K.; Moustafa, S. E.; Mioduszewski, J.; Chu, V. W.; Forster, R. R.; Hagedorn, B.; Harper, J. T.; Mote, T. L.; Robinson, D. A.; Shuman, C. A.; Smith, L. C.; Tedesco, M.

    2013-03-01

    Improved understanding of Greenland ice sheet hydrology is critically important for assessing its impact on current and future ice sheet dynamics and global sea level rise. This has motivated the collection and integration of in situ observations, model development, and remote sensing efforts to quantify meltwater production, as well as its phase changes, transport, and export. Particularly urgent is a better understanding of albedo feedbacks leading to enhanced surface melt, potential positive feedbacks between ice sheet hydrology and dynamics, and meltwater retention in firn. These processes are not isolated, but must be understood as part of a continuum of processes within an integrated system. This letter describes a systems approach to the study of Greenland ice sheet hydrology, emphasizing component interconnections and feedbacks, and highlighting research and observational needs.

  20. A Tale of Two Forcings: Present-Day Coupled Antarctic Ice-sheet/Southern Ocean dynamics using the POPSICLES model.

    NASA Astrophysics Data System (ADS)

    Martin, Daniel; Asay-Davis, Xylar; Cornford, Stephen; Price, Stephen; Ng, Esmond; Collins, William

    2015-04-01

    We present POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010 resulting from two different choices of climate forcing: a 'normal-year' climatology and the CORE v. 2 interannual forcing data (Large and Yeager 2008). Simulations are performed at 0.1o (~5 km) ocean resolution and adaptive ice sheet resolution as fine as 500 m. We compare time-averaged melt rates below a number of major ice shelves with those reported by Rignot et al. (2013) as well as other recent studies. We also present seasonal variability and decadal melting trends from several Antarctic regions, along with the response of the ice shelves and consequent dynamics of the grounded ice sheet. POPSICLES couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). BISICLES makes use of adaptive mesh refinement and a 1st-order accurate momentum balance similar to the L1L2 model of Schoof and Hindmarsh (2009) to accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests (MISMIP-3d; Pattyn et al., 2013) and realistic configurations (Favier et al. 2014).

  1. Spatiotemporal Variability of Meltwater Refreezing in Southwest Greenland Ice Sheet Firn

    NASA Astrophysics Data System (ADS)

    Rennermalm, A. K.; Hock, R.; Tedesco, M.; Corti, G.; Covi, F.; Miège, C.; Kingslake, J.; Leidman, S. Z.; Munsell, S.

    2017-12-01

    A substantial fraction of the summer meltwater formed on the surface of the Greenland ice sheet is retained in firn, while the remaining portion runs to the ocean through surface and subsurface channels. Refreezing of meltwater in firn can create impenetrable ice lenses, hence being a crucial process in the redistribution of surface runoff. To quantify the impact of refreezing on runoff and current and future Greenland surface mass balance, a three year National Science Foundation funded project titled "Refreezing in the firn of the Greenland ice sheet: Spatiotemporal variability and implications for ice sheet mass balance" started this past year. Here we present an overview of the project and some initial results from the first field season in May 2017 conducted in proximity of the DYE-2 site in the percolation zone of the Southwest Greenland ice sheet at elevations between 1963 and 2355 m a.s.l.. During this fieldwork two automatic weather stations were deployed, outfitted with surface energy balance sensors and 16 m long thermistor strings, over 300 km of ground penetrating radar data were collected, and five 20-26 m deep firn cores were extracted and analyzed for density and stratigraphy. Winter snow accumulation was measured along the radar tracks. Preliminary work on the firn-core data reveals increasing frequency and thickness of ice lenses at lower ice-sheet elevations, in agreement with other recent work in the area. Data collected within this project will facilitate advances in our understanding of the spatiotemporal variability of firn refreezing and its role in the hydrology and surface mass balance of the Greenland Ice Sheet.

  2. Demonstration of Sparse Signal Reconstruction for Radar Imaging of Ice Sheets

    NASA Astrophysics Data System (ADS)

    Heister, Anton; Scheiber, Rolf

    2017-04-01

    Conventional processing of ice-sounder data produces 2-D images of the ice sheet and bed, where the two dimensions are along-track and depth, while the across-track direction is fixed to nadir. The 2-D images contain information about the topography and radar reflectivity of the ice sheet's surface, bed, and internal layers in the along-track direction. Having multiple antenna phase centers in the across-track direction enables the production of 3-D images of the ice sheet and bed. Compared to conventional 2-D images, these contain additional information about the surface and bed topography, and orientation of the internal layers over a swath in the across-track direction. We apply a 3-D SAR tomographic ice-sounding method based on sparse signal reconstruction [1] to the data collected by Center for Remote Sensing of Ice Sheets (CReSIS) in 2008 in Greenland [2] using their multichannel coherent radar depth sounder (MCoRDS). The MCoRDS data have 16 effective phase centers which allows us to better understand the performance of the method. Lastly we offer sparsity improvement by including wavelet dictionaries into the reconstruction.The results show improved scene feature resolvability in across-track direction compared to MVDR beamformer. References: [1] A. Heister, R. Scheiber, "First Analysis of Sparse Signal Reconstruction for Radar Imaging of Ice Sheets". In: Proceedings of EUSAR, pp. 788-791, June 2016. [2] X. Wu, K. C. Jezek, E. Rodriguez, S. Gogineni, F. Rodriguez-Morales, and A. Freeman, "Ice sheet bed mapping with airborne SAR tomography". IEEE Transactions on Geoscience and Remote Sensing, vol. 49, no. 10 Part 1, pp. 3791-3802, 2011.

  3. Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet

    PubMed Central

    Winkelmann, Ricarda; Levermann, Anders; Ridgwell, Andy; Caldeira, Ken

    2015-01-01

    The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources. PMID:26601273

  4. Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet.

    PubMed

    Winkelmann, Ricarda; Levermann, Anders; Ridgwell, Andy; Caldeira, Ken

    2015-09-01

    The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources.

  5. Modelling large-scale ice-sheet-climate interactions at the last glacial inception

    NASA Astrophysics Data System (ADS)

    Browne, O. J. H.; Gregory, J. M.; Payne, A. J.; Ridley, J. K.; Rutt, I. C.

    2010-05-01

    In order to investigate the interactions between coevolving climate and ice-sheets on multimillenial timescales, a low-resolution atmosphere-ocean general circulation model (AOGCM) has been coupled to a three-dimensional thermomechanical ice-sheet model. We use the FAMOUS AOGCM, which is almost identical in formulation to the widely used HadCM3 AOGCM, but on account of its lower resolution (7.5° longitude × 5° latitude in the atmosphere, 3.75°× 2.5° in the ocean) it runs about ten times faster. We use the community ice-sheet model Glimmer at 20 km resolution, with the shallow ice approximation and an annual degree-day scheme for surface mass balance. With the FAMOUS-Glimmer coupled model, we have simulated the growth of the Laurentide and Fennoscandian ice sheets at the last glacial inception, under constant orbital forcing and atmospheric composition for 116 ka BP. Ice grows in both regions, totalling 5.8 m of sea-level equivalent in 10 ka, slower than proxy records suggest. Positive climate feedbacks reinforce this growth at local scales (order hundreds of kilometres), where changes are an order of magnitude larger than on the global average. The albedo feedback (higher local albedo means a cooler climate) is important in the initial expansion of the ice-sheet area. The topography feedback (higher surface means a cooler climate) affects ice-sheet thickness and is not noticeable for the first 1 ka. These two feedbacks reinforce each other. Without them, the ice volume is ~90% less after 10 ka. In Laurentia, ice expands initially on the Canadian Arctic islands. The glaciation of the islands eventually cools the nearby mainland climate sufficiently to produce a positive mass balance there. Adjacent to the ice-sheets, cloud feedbacks tend to reduce the surface mass balance and restrain ice growth; this is an example of a local feedback whose simulation requires a model that includes detailed atmospheric physics.

  6. From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model

    NASA Astrophysics Data System (ADS)

    Feldmann, Johannes; Levermann, Anders

    2017-08-01

    Here we report on a cyclic, physical ice-discharge instability in the Parallel Ice Sheet Model, simulating the flow of a three-dimensional, inherently buttressed ice-sheet-shelf system which periodically surges on a millennial timescale. The thermomechanically coupled model on 1 km horizontal resolution includes an enthalpy-based formulation of the thermodynamics, a nonlinear stress-balance-based sliding law and a very simple subglacial hydrology. The simulated unforced surging is characterized by rapid ice streaming through a bed trough, resulting in abrupt discharge of ice across the grounding line which is eventually calved into the ocean. We visualize the central feedbacks that dominate the subsequent phases of ice buildup, surge and stabilization which emerge from the interaction between ice dynamics, thermodynamics and the subglacial till layer. Results from the variation of surface mass balance and basal roughness suggest that ice sheets of medium thickness may be more susceptible to surging than relatively thin or thick ones for which the surge feedback loop is damped. We also investigate the influence of different basal sliding laws (ranging from purely plastic to nonlinear to linear) on possible surging. The presented mechanisms underlying our simulations of self-maintained, periodic ice growth and destabilization may play a role in large-scale ice-sheet surging, such as the surging of the Laurentide Ice Sheet, which is associated with Heinrich events, and ice-stream shutdown and reactivation, such as observed in the Siple Coast region of West Antarctica.

  7. Dynamic behaviour of the East Antarctic ice sheet during Pliocene warmth

    NASA Astrophysics Data System (ADS)

    Cook, Carys P.; van de Flierdt, Tina; Williams, Trevor; Hemming, Sidney R.; Iwai, Masao; Kobayashi, Munemasa; Jimenez-Espejo, Francisco J.; Escutia, Carlota; González, Jhon Jairo; Khim, Boo-Keun; McKay, Robert M.; Passchier, Sandra; Bohaty, Steven M.; Riesselman, Christina R.; Tauxe, Lisa; Sugisaki, Saiko; Galindo, Alberto Lopez; Patterson, Molly O.; Sangiorgi, Francesca; Pierce, Elizabeth L.; Brinkhuis, Henk; Klaus, Adam; Fehr, Annick; Bendle, James A. P.; Bijl, Peter K.; Carr, Stephanie A.; Dunbar, Robert B.; Flores, José Abel; Hayden, Travis G.; Katsuki, Kota; Kong, Gee Soo; Nakai, Mutsumi; Olney, Matthew P.; Pekar, Stephen F.; Pross, Jörg; Röhl, Ursula; Sakai, Toyosaburo; Shrivastava, Prakash K.; Stickley, Catherine E.; Tuo, Shouting; Welsh, Kevin; Yamane, Masako

    2013-09-01

    Warm intervals within the Pliocene epoch (5.33-2.58 million years ago) were characterized by global temperatures comparable to those predicted for the end of this century and atmospheric CO2 concentrations similar to today. Estimates for global sea level highstands during these times imply possible retreat of the East Antarctic ice sheet, but ice-proximal evidence from the Antarctic margin is scarce. Here we present new data from Pliocene marine sediments recovered offshore of Adélie Land, East Antarctica, that reveal dynamic behaviour of the East Antarctic ice sheet in the vicinity of the low-lying Wilkes Subglacial Basin during times of past climatic warmth. Sedimentary sequences deposited between 5.3 and 3.3 million years ago indicate increases in Southern Ocean surface water productivity, associated with elevated circum-Antarctic temperatures. The geochemical provenance of detrital material deposited during these warm intervals suggests active erosion of continental bedrock from within the Wilkes Subglacial Basin, an area today buried beneath the East Antarctic ice sheet. We interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the East Antarctic ice sheet was sensitive to climatic warmth during the Pliocene.

  8. Trends in ice sheet mass balance, 1992 to 2017

    NASA Astrophysics Data System (ADS)

    Shepherd, A.; Ivins, E. R.; Smith, B.; Velicogna, I.; Whitehouse, P. L.; Rignot, E. J.; van den Broeke, M. R.; Briggs, K.; Hogg, A.; Krinner, G.; Joughin, I. R.; Nowicki, S.; Payne, A. J.; Scambos, T.; Schlegel, N.; Moyano, G.; Konrad, H.

    2017-12-01

    The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) is a community effort, jointly supported by ESA and NASA, that aims to provide a consensus estimate of ice sheet mass balance from satellite gravimetry, altimetry and mass budget assessments, on an annual basis. The project has five experiment groups, one for each of the satellite techniques and two others to analyse surface mass balance (SMB) and glacial isostatic adjustment (GIA). The basic premise for the exercise is that individual ice sheet mass balance datasets are generated by project participants using common spatial and temporal domains to allow meaningful inter-comparison, and this controlled comparison in turn supports aggregation of the individual datasets over their full period. Participation is open to the full community, and the quality and consistency of submissions is regulated through a series of data standards and documentation requirements. The second phase of IMBIE commenced in 2015, with participant data submitted in 2016 and a combined estimate due for public release in 2017. Data from 48 participant groups were submitted to one of the three satellite mass balance technique groups or to the ancillary dataset groups. The individual mass balance estimates and ancillary datasets have been compared and combined within the respective groups. Following this, estimates of ice sheet mass balance derived from the individual techniques were then compared and combined. The result is single estimates of ice sheet mass balance for Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula. The participants, methodology and results of the exercise will be presented in this paper.

  9. Changes in ice dynamics and mass balance of the Antarctic ice sheet.

    PubMed

    Rignot, Eric

    2006-07-15

    The concept that the Antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites. Pronounced regional warming in the Antarctic Peninsula triggered ice shelf collapse, which led to a 10-fold increase in glacier flow and rapid ice sheet retreat. This chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of Antarctica. In West Antarctica, the Pine Island Bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation. This sector could raise sea level by 1m and trigger widespread retreat of ice in West Antarctica. Pine Island Glacier accelerated 38% since 1975, and most of the speed up took place over the last decade. Its neighbour Thwaites Glacier is widening up and may double its width when its weakened eastern ice shelf breaks up. Widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0.3 degrees C. In contrast, glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level. In East Antarctica, many glaciers are close to a state of mass balance, but sectors grounded well below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes.

  10. Possible contribution of ice-sheet/lithosphere interactions to past glaciological changes in Greenland

    NASA Astrophysics Data System (ADS)

    Alley, R. B.; Parizek, B. R.; Anandakrishnan, S.; Pollard, D.; Stevens, N. T.; Pourpoint, M.

    2017-12-01

    Ice-lithosphere interactions may have influenced the history of ice-sheet sensitivity to climate change. The Greenland ice sheet (GIS) is sensitive to warming, and is likely to be largely removed if subjected to relatively small additional temperature increases. The recent report (Schaefer et al., 2016, Nature) of near-complete GIS removal under modest Pleistocene forcing suggests that GIS sensitivity may be even greater than generally modeled, but lack of major Holocene retreat is more consistent with existing models. As shown by Stevens et al. (2016, JGR), peak lithospheric flexural stresses associated with ice-age GIS cycling are of the same order as dike-driving stresses in plutonic systems, and migrate over ice-age cycles. The full analysis by Stevens et al. suggests the possibility that the onset of cyclic ice-sheet loading allowed deep melt associated with the passage of the Icelandic hot spot beneath Greenland to work up though the crust to or near the base of the ice sheet, helping explain the anomalous geothermal heat fluxes observed at the head of the Northeast Greenland Ice Stream and elsewhere in the northern part of GIS. If ice-age cycling aided extraction of an existing reservoir of melted rock, then geothermal heat flux would have risen with the onset of extraction and migration, but with a subsequent fall associated with reservoir depletion. Simple parameterized flow-model simulations confirm intuition that a higher geothermal flux makes deglaciation easier, with the northern part of the ice sheet especially important. Large uncertainties remain in quantification, but we suggest the hypothesis that, following the onset of ice-age cycling, deglaciation of the GIS first became easier and then more difficult in response to feedbacks involving the ice sheet and the geological system beneath. In turn, this suggests that evidence of past deglaciation under moderate forcing is consistent with existing ice-sheet models.

  11. The extreme melt across the Greenland ice sheet in 2012

    NASA Astrophysics Data System (ADS)

    Nghiem, S. V.; Hall, D. K.; Mote, T. L.; Tedesco, M.; Albert, M. R.; Keegan, K.; Shuman, C. A.; DiGirolamo, N. E.; Neumann, G.

    2012-10-01

    The discovery of the 2012 extreme melt event across almost the entire surface of the Greenland ice sheet is presented. Data from three different satellite sensors - including the Oceansat-2 scatterometer, the Moderate-resolution Imaging Spectroradiometer, and the Special Sensor Microwave Imager/Sounder - are combined to obtain composite melt maps, representing the most complete melt conditions detectable across the ice sheet. Satellite observations reveal that melt occurred at or near the surface of the Greenland ice sheet across 98.6% of its entire extent on 12 July 2012, including the usually cold polar areas at high altitudes like Summit in the dry snow facies of the ice sheet. This melt event coincided with an anomalous ridge of warm air that became stagnant over Greenland. As seen in melt occurrences from multiple ice core records at Summit reported in the published literature, such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the Medieval Warm Period. Given its rarity, the 2012 extreme melt across Greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research.

  12. Results of the Greenland ice sheet model initialisation experiments: ISMIP6 - initMIP-Greenland

    NASA Astrophysics Data System (ADS)

    Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin; Beckley, Matthew

    2017-04-01

    Ice sheet model initialisation has a large effect on projected future sea-level contributions and gives rise to important uncertainties. The goal of this intercomparison exercise for the continental-scale Greenland ice sheet is therefore to compare, evaluate and improve the initialisation techniques used in the ice sheet modelling community. The initMIP-Greenland project is the first in a series of ice sheet model intercomparison activities within ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). The experimental set-up has been designed to allow comparison of the initial present-day state of the Greenland ice sheet between participating models and against observations. Furthermore, the initial states are tested with two schematic forward experiments to evaluate the initialisation in terms of model drift (forward run without any forcing) and response to a large perturbation (prescribed surface mass balance anomaly). We present and discuss results that highlight the wide diversity of data sets, boundary conditions and initialisation techniques used in the community to generate initial states of the Greenland ice sheet.

  13. ISSM: Ice Sheet System Model

    NASA Technical Reports Server (NTRS)

    Larour, Eric; Schiermeier, John E.; Seroussi, Helene; Morlinghem, Mathieu

    2013-01-01

    In order to have the capability to use satellite data from its own missions to inform future sea-level rise projections, JPL needed a full-fledged ice-sheet/iceshelf flow model, capable of modeling the mass balance of Antarctica and Greenland into the near future. ISSM was developed with such a goal in mind, as a massively parallelized, multi-purpose finite-element framework dedicated to ice-sheet modeling. ISSM features unstructured meshes (Tria in 2D, and Penta in 3D) along with corresponding finite elements for both types of meshes. Each finite element can carry out diagnostic, prognostic, transient, thermal 3D, surface, and bed slope simulations. Anisotropic meshing enables adaptation of meshes to a certain metric, and the 2D Shelfy-Stream, 3D Blatter/Pattyn, and 3D Full-Stokes formulations capture the bulk of the ice-flow physics. These elements can be coupled together, based on the Arlequin method, so that on a large scale model such as Antarctica, each type of finite element is used in the most efficient manner. For each finite element referenced above, ISSM implements an adjoint. This adjoint can be used to carry out model inversions of unknown model parameters, typically ice rheology and basal drag at the ice/bedrock interface, using a metric such as the observed InSAR surface velocity. This data assimilation capability is crucial to allow spinning up of ice flow models using available satellite data. ISSM relies on the PETSc library for its vectors, matrices, and solvers. This allows ISSM to run efficiently on any parallel platform, whether shared or distrib- ISSM: Ice Sheet System Model NASA's Jet Propulsion Laboratory, Pasadena, California uted. It can run on the largest clusters, and is fully scalable. This allows ISSM to tackle models the size of continents. ISSM is embedded into MATLAB and Python, both open scientific platforms. This improves its outreach within the science community. It is entirely written in C/C++, which gives it flexibility in its

  14. The sensitivity of the Greenland Ice Sheet to glacial-interglacial oceanic forcing

    NASA Astrophysics Data System (ADS)

    Tabone, Ilaria; Blasco, Javier; Robinson, Alexander; Alvarez-Solas, Jorge; Montoya, Marisa

    2018-04-01

    Observations suggest that during the last decades the Greenland Ice Sheet (GrIS) has experienced a gradually accelerating mass loss, in part due to the observed speed-up of several of Greenland's marine-terminating glaciers. Recent studies directly attribute this to warming North Atlantic temperatures, which have triggered melting of the outlet glaciers of the GrIS, grounding-line retreat and enhanced ice discharge into the ocean, contributing to an acceleration of sea-level rise. Reconstructions suggest that the influence of the ocean has been of primary importance in the past as well. This was the case not only in interglacial periods, when warmer climates led to a rapid retreat of the GrIS to land above sea level, but also in glacial periods, when the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes. However, the GrIS response to palaeo-oceanic variations has yet to be investigated in detail from a mechanistic modelling perspective. In this work, the evolution of the GrIS over the past two glacial cycles is studied using a three-dimensional hybrid ice-sheet-shelf model. We assess the effect of the variation of oceanic temperatures on the GrIS evolution on glacial-interglacial timescales through changes in submarine melting. The results show a very high sensitivity of the GrIS to changing oceanic conditions. Oceanic forcing is found to be a primary driver of GrIS expansion in glacial times and of retreat in interglacial periods. If switched off, palaeo-atmospheric variations alone are not able to yield a reliable glacial configuration of the GrIS. This work therefore suggests that considering the ocean as an active forcing should become standard practice in palaeo-ice-sheet modelling.

  15. The Sensitivity of the Greenland Ice Sheet to Glacial-Interglacial Oceanic Forcing

    NASA Astrophysics Data System (ADS)

    Tabone, I.; Blasco Navarro, J.; Robinson, A.; Alvarez-Solas, J.; Montoya, M.

    2017-12-01

    Up to now, the scientific community has mainly focused on the sensitivity of the Greenland Ice Sheet (GrIS) to atmospheric variations. However, several studies suggest that the enhanced ice mass loss experienced by the GrIS in the past decades is directly connected to the increasing North Atlantic temperatures. Melting of GrIS outlet glaciers triggers grounding-line retreat increasing ice discharge into the ocean. This new evidence leads to consider the ocean as a relevant driver to be taken into account when modeling the evolution of the GrIS. The ice-ocean interaction is a primary factor controling not only the likely future retreat of GrIS outlet glaciers, or the huge ice loss in past warming climates, but also, and more strongly, the past GrIS glacial expansion. The latter assumption is supported by reconstructions which propose the GrIS to be fully marine-based during glacials, and thus more exposed to the influence of the ocean. Here, for the first time, we investigate the response of the GrIS to past oceanic changes using a three-dimensional hybrid ice-sheet/ice-shelf model, which combines the Shallow Ice Approximation (SIA) for slow grounded ice sheets and the Shallow Shelf Approximation (SSA) in ice shelves and ice streams. The model accounts for a time-dependent parametrisation of the marine basal melting rate, which is used to reproduce past oceanic variations. In this work simulations of the last two glacial cycles are performed. Our results show that the GrIS is very sensitive to the ocean-triggered submarine melting (freezing). Mild oceanic temperature variations lead to a rapid retreat (expansion) of the GrIS margins, which, inducing a dynamic adjustment of the grounded ice sheet, drive the evolution of the whole ice sheet. Our results strongly suggest the need to consider the ocean as an active forcing in paleo ice sheet models.

  16. From Outlet Glacier Changes to Ice Sheet Mass Balance - Evolution of Greenland Ice Sheet from Laser Altimetry Data

    NASA Astrophysics Data System (ADS)

    Csatho, B. M.; Schenk, A.; Nagarajan, S.; Babonis, G. S.

    2010-12-01

    Investigations of ice sheet mass balance and the changing dynamics of outlet glaciers have been hampered by the lack of comprehensive data. In recent years, this situation has been remedied. Satellite laser altimetry data from the Ice Cloud and land Elevation Satellite mission (ICESat), combined with airborne laser altimetry, provide accurate measurements of surface elevation changes, and surface velocities derived from various satellite platforms yield crucial information on changing glacier dynamics. Taken together, a rich and diverse data set is emerging that allows for characterizing the spatial and temporal evolution of ice sheets and outlet glaciers. In particular, it enables quantitative studies of outlet glaciers undergoing rapid and complex changes. Although airborne and laser altimetry have been providing precise measurements of ice sheet topography since the early 1990s, determining detailed and accurate spatial and temporal distribution of surface changes remains a challenging problem. We have developed a new, comprehensive method, called Surface Elevation Reconstruction And Change detection (SERAC), which estimates surface changes by a simultaneous reconstruction of surface topography from fused multisensor data. The mathematical model is based on the assumption that for a small surface area, only the absolute elevation changes over time but not the shape of the surface patch. Therefore, laser points of all time epochs contribute to the shape parameters; points of each time period determine the absolute elevation of the surface patch at that period. This method provides high-resolution surface topography, precise changes and a rigorous error estimate of the quantities. By using SERAC we combined ICESat and ATM laser altimetry data to determine the evolution of surface change rates of the whole Greenland Ice Sheet between 2003 and 2009 on a high-resolution grid. Our reconstruction, consistent with GRACE results, shows ice sheet thinning propagating

  17. Minimum and Maximum Potential Contributions to Future Sea Level Rise from Polar Ice Sheets

    NASA Astrophysics Data System (ADS)

    Deconto, R. M.; Pollard, D.

    2017-12-01

    New climate and ice-sheet modeling, calibrated to past changes in sea-level, is painting a stark picture of the future fate of the great polar ice sheets if greenhouse gas emissions continue unabated. This is especially true for Antarctica, where a substantial fraction of the ice sheet rests on bedrock more than 500-meters below sea level. Here, we explore the sensitivity of the polar ice sheets to a warming atmosphere and ocean under a range of future greenhouse gas emissions scenarios. The ice sheet-climate-ocean model used here considers time-evolving changes in surface mass balance and sub-ice oceanic melting, ice deformation, grounding line retreat on reverse-sloped bedrock (Marine Ice Sheet Instability), and newly added processes including hydrofracturing of ice shelves in response to surface meltwater and rain, and structural collapse of thick, marine-terminating ice margins with tall ice-cliff faces (Marine Ice Cliff Instability). The simulations improve on previous work by using 1) improved atmospheric forcing from a Regional Climate Model and 2) a much wider range of model physical parameters within the bounds of modern observations of ice dynamical processes (particularly calving rates) and paleo constraints on past ice-sheet response to warming. Approaches to more precisely define the climatic thresholds capable of triggering rapid and potentially irreversible ice-sheet retreat are also discussed, as is the potential for aggressive mitigation strategies like those discussed at the 2015 Paris Climate Conference (COP21) to substantially reduce the risk of extreme sea-level rise. These results, including physics that consider both ice deformation (creep) and calving (mechanical failure of marine terminating ice) expand on previously estimated limits of maximum rates of future sea level rise based solely on kinematic constraints of glacier flow. At the high end, the new results show the potential for more than 2m of global mean sea level rise by 2100

  18. Acoustic Gravity Waves Generated by an Oscillating Ice Sheet in Arctic Zone

    NASA Astrophysics Data System (ADS)

    Abdolali, A.; Kadri, U.; Kirby, J. T., Jr.

    2016-12-01

    We investigate the formation of acoustic-gravity waves due to oscillations of large ice blocks, possibly triggered by atmospheric and ocean currents, ice block shrinkage or storms and ice-quakes.For the idealized case of a homogeneous weakly compressible water bounded at the surface by ice sheet and a rigid bed, the description of the infinite family of acoustic modes is characterized by the water depth h and angular frequency of oscillating ice sheet ω ; The acoustic wave field is governed by the leading mode given by: Nmax=\\floor {(ω h)/(π c)} where c is the sound speed in water and the special brackets represent the floor function (Fig1). Unlike the free-surface setting, the higher acoustic modes might exhibit a larger contribution and therefore all progressive acoustic modes have to be considered.This study focuses on the characteristics of acoustic-gravity waves generated by an oscillating elastic ice sheet in a weakly compressible fluid coupled with a free surface model [Abdolali et al. 2015] representing shrinking ice blocks in realistic sea state, where the randomly oriented ice sheets cause inter modal transition and multidirectional reflections. A theoretical solution and a 3D numerical model have been developed for the study purposes. The model is first validated against the theoretical solution [Kadri, 2016]. To overcome the computational difficulties of 3D models, we derive a depth-integrated equation valid for spatially varying ice sheet thickness and water depth. We show that the generated acoustic-gravity waves contribute significantly to deep ocean currents compared to other mechanisms. In addition, these waves travel at the sound speed in water carrying information on ice sheet motion, providing various implications for ocean monitoring and detection of ice-quakes. Fig1:Snapshots of dynamic pressure given by an oscillating ice sheet; h=4500m, c=1500m/s, semi-length b=10km, ζ =1m, omega=π rad/s. Abdolali, A., Kirby, J. T. and Bellotti, G

  19. Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss

    PubMed Central

    Gomez, Natalya; Pollard, David; Holland, David

    2015-01-01

    The stability of marine sectors of the Antarctic Ice Sheet (AIS) in a warming climate has been identified as the largest source of uncertainty in projections of future sea-level rise. Sea-level fall near the grounding line of a retreating marine ice sheet has a stabilizing influence on the ice sheets, and previous studies have established the importance of this feedback on ice age AIS evolution. Here we use a coupled ice sheet–sea-level model to investigate the impact of the feedback mechanism on future AIS retreat over centennial and millennial timescales for a range of emission scenarios. We show that the combination of bedrock uplift and sea-surface drop associated with ice-sheet retreat significantly reduces AIS mass loss relative to a simulation without these effects included. Sensitivity analyses show that the stabilization tends to be greatest for lower emission scenarios and Earth models characterized by a thin elastic lithosphere and low-viscosity upper mantle, as is the case for West Antarctica. PMID:26554381

  20. Assessing the Impact of Laurentide Ice-sheet Topography on Glacial Climate

    NASA Technical Reports Server (NTRS)

    Ullman, D. J.; LeGrande, A. N.; Carlson, A. E.; Anslow, F. S.; Licciardi, J. M.

    2014-01-01

    Simulations of past climates require altered boundary conditions to account for known shifts in the Earth system. For the Last Glacial Maximum (LGM) and subsequent deglaciation, the existence of large Northern Hemisphere ice sheets caused profound changes in surface topography and albedo. While ice-sheet extent is fairly well known, numerous conflicting reconstructions of ice-sheet topography suggest that precision in this boundary condition is lacking. Here we use a high-resolution and oxygen-isotopeenabled fully coupled global circulation model (GCM) (GISS ModelE2-R), along with two different reconstructions of the Laurentide Ice Sheet (LIS) that provide maximum and minimum estimates of LIS elevation, to assess the range of climate variability in response to uncertainty in this boundary condition.We present this comparison at two equilibrium time slices: the LGM, when differences in ice-sheet topography are maximized, and 14 ka, when differences in maximum ice-sheet height are smaller but still exist. Overall, we find significant differences in the climate response to LIS topography, with the larger LIS resulting in enhanced Atlantic Meridional Overturning Circulation and warmer surface air temperatures, particularly over northeastern Asia and the North Pacific. These up- and downstream effects are associated with differences in the development of planetary waves in the upper atmosphere, with the larger LIS resulting in a weaker trough over northeastern Asia that leads to the warmer temperatures and decreased albedo from snow and sea-ice cover. Differences between the 14 ka simulations are similar in spatial extent but smaller in magnitude, suggesting that climate is responding primarily to the larger difference in maximum LIS elevation in the LGM simulations. These results suggest that such uncertainty in ice-sheet boundary conditions alone may significantly impact the results of paleoclimate simulations and their ability to successfully simulate past climates

  1. Frustules to fragments, diatoms to dust: How degradation of microfossil shape and microstructures can teach us how ice sheets work

    USGS Publications Warehouse

    Scherer, R.P.; Sjunneskog, C.M.; Iverson, M.R.; Hooyer, T.S.

    2005-01-01

    In a laboratory experiment we investigated micro- and nanoscale changes in fossil diatom valves and in the texture of diatomaceous sediments that result from ice sheet overburden and subglacial shearing. Our experiment included compression and shearing of Antarctic diatom-rich sediments in a ring shear device and comparison of experimental samples with natural glacial sediments from the Antarctic continental shelf. The purpose of the experiment is to establish objective criteria for analyzing subglacial processes and interpreting the origin of glacial-geologic features on the Antarctic continental shelf. We find distinct changes resulting from different glacial settings, with respect to whole diatom frustules, diatom micromorphology, and microtextural properties of sedimentary units. By providing constraints on subglacial shearing, these observations of genetically controlled micro- and nanoscale diatom structures and architecture are contributing to the understanding of large-scale glacial processes, aiding the development of models of modern ice sheet processes, and guiding interpretation of past ice sheet configurations. Copyright ?? 2005 American Scientific Publishers. All rights reserved.

  2. Outreach/education interface for Cryosphere models using the Virtual Ice Sheet Laboratory

    NASA Astrophysics Data System (ADS)

    Larour, E. Y.; Halkides, D. J.; Romero, V.; Cheng, D. L.; Perez, G.

    2014-12-01

    In the past decade, great strides have been made in the development of models capable of projecting the future evolution of glaciers and the polar ice sheets in a changing climate. These models are now capable of replicating some of the trends apparent in satellite observations. However, because this field is just now maturing, very few efforts have been dedicated to adapting these capabilities to education. Technologies that have been used in outreach efforts in Atmospheric and Oceanic sciences still have not been extended to Cryospheric Science. We present a cutting-edge, technologically driven virtual laboratory, geared towards outreach and k-12 education, dedicated to the polar ice sheets on Antarctica and Greenland, and their role as major contributors to sea level rise in coming decades. VISL (Virtual Ice Sheet Laboratory) relies on state-of-the art Web GL rendering of polar ice sheets, Android/iPhone and web portability using Javascript, as well as C++ simulations (back-end) based on the Ice Sheet System Model, the NASA model for simulating the evolution of polar ice sheets. Using VISL, educators and students can have an immersive experience into the world of polar ice sheets, while at the same exercising the capabilities of a state-of-the-art climate model, all of it embedded into an education experience that follows the new STEM standards for education.This work was performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

  3. Simulating a Dynamic Antarctic Ice Sheet in the Early to Middle Miocene

    NASA Astrophysics Data System (ADS)

    Gasson, E.; DeConto, R.; Pollard, D.; Levy, R. H.

    2015-12-01

    There are a variety of sources of geological data that suggest major variations in the volume and extent of the Antarctic ice sheet during the early to middle Miocene. Simulating such variability using coupled climate-ice sheet models is problematic due to a strong hysteresis effect caused by height-mass balance feedback and albedo feedback. This results in limited retreat of the ice sheet once it has reached the continental size, as likely occurred prior to the Miocene. Proxy records suggest a relatively narrow range of atmospheric CO2 during the early to middle Miocene, which exacerbates this problem. We use a new climate forcing which accounts for ice sheet-climate feedbacks through an asynchronous GCM-RCM coupling, which is able to better resolve the narrow Antarctic ablation zone in warm climate simulations. When combined with recently suggested mechanisms for retreat into subglacial basins due to ice shelf hydrofracture and ice cliff failure, we are able to simulate large-scale variability of the Antarctic ice sheet in the Miocene. This variability is equivalent to a seawater oxygen isotope signal of ~0.5 ‰, or a sea level equivalent change of ~35 m, for a range of atmospheric CO2 between 280 - 500 ppm.

  4. Extensive retreat and re-advance of the West Antarctic Ice Sheet during the Holocene.

    PubMed

    Kingslake, J; Scherer, R P; Albrecht, T; Coenen, J; Powell, R D; Reese, R; Stansell, N D; Tulaczyk, S; Wearing, M G; Whitehouse, P L

    2018-06-01

    To predict the future contributions of the Antarctic ice sheets to sea-level rise, numerical models use reconstructions of past ice-sheet retreat after the Last Glacial Maximum to tune model parameters 1 . Reconstructions of the West Antarctic Ice Sheet have assumed that it retreated progressively throughout the Holocene epoch (the past 11,500 years or so) 2-4 . Here we show, however, that over this period the grounding line of the West Antarctic Ice Sheet (which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf) retreated several hundred kilometres inland of today's grounding line, before isostatic rebound caused it to re-advance to its present position. Our evidence includes, first, radiocarbon dating of sediment cores recovered from beneath the ice streams of the Ross Sea sector, indicating widespread Holocene marine exposure; and second, ice-penetrating radar observations of englacial structure in the Weddell Sea sector, indicating ice-shelf grounding. We explore the implications of these findings with an ice-sheet model. Modelled re-advance of the grounding line in the Holocene requires ice-shelf grounding caused by isostatic rebound. Our findings overturn the assumption of progressive retreat of the grounding line during the Holocene in West Antarctica, and corroborate previous suggestions of ice-sheet re-advance 5 . Rebound-driven stabilizing processes were apparently able to halt and reverse climate-initiated ice loss. Whether these processes can reverse present-day ice loss 6 on millennial timescales will depend on bedrock topography and mantle viscosity-parameters that are difficult to measure and to incorporate into ice-sheet models.

  5. Isostasy as a Driver of Paleo Retreat of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Robinson, A.; Tabone, I.; Alvarez-Solas, J.; Montoya, M.

    2016-12-01

    During glacial times, the Greenland ice sheet (GrIS) extended onto the continental shelf, and thus was much more directly affected by changing ocean temperatures through basal melt of the marine ice margins than it is today. The larger glacial ice sheet also induced lithospheric depression of several hundred meters in regions that are near sea level today. As the ice sheet retreated inland under interglacial climatic forcing, the regions significantly affected by local isostatic changes in elevation were exposed to much higher basal melt rates than they would have been given the present-day topography. Here we explore this effect using a hybrid ice sheet model that represents both grounded and floating ice, as well as local isostatic effects, and is driven by both atmospheric and oceanic temperature anomalies. We find that when transient oceanic forcing is included in the model, isostasy plays an important role in allowing oceanic melting to drive GrIS retreat in some regions. During the last interglacial, for example, this effect can account for a significant additional sea-level contribution, as well as an increase in the rate of sea-level rise. Our results highlight the importance of accounting for ice-ocean-lithosphere interactions in the past, in order to be able to properly reconstruct the evolution of the ice sheet, and for estimating its sensitivity to potential changes in climate in the future.

  6. Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume.

    PubMed

    Abe-Ouchi, Ayako; Saito, Fuyuki; Kawamura, Kenji; Raymo, Maureen E; Okuno, Jun'ichi; Takahashi, Kunio; Blatter, Heinz

    2013-08-08

    The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere 'excess ice', but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere-asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere

  7. Modeling the growth and decay of the Antarctic Peninsula Ice Sheet

    NASA Astrophysics Data System (ADS)

    Payne, A. J.; Sugden, D. E.; Clapperton, C. M.

    1989-03-01

    A model of the growth and decay of the Antarctic Peninsula Ice Sheet during the last glacial/interglacial cycle is used to identify the main controls on ice sheet behavior. Using as input glaciological assumptions derived by W. F. Budd and I. N. Smith (1982, Annals of Glaciology3, 42-49), bedrock topography, isostatic compensation, and mass balance relationships, the model is driven by sea-level change over the last 40,000 yr in association with assumed changes in the rate of melting beneath ice shelves. An ice sheet dome over 3.5 km thick grows on the offshore shelf and straits west of the Antarctic Peninsula and reaches a maximum at 18,000 yr B.P. Collapse begins at 14,000 yr B.P. but becomes rapid and continuous after 10,000 yr B.P. The present stable ice cover is achieved at 6500 yr B.P. Ice growth and decay are characterized by thresholds which separate periods of steady state from periods of rapid transition; the thresholds usually relate to topography. Tests show that ice sheet behavior is most sensitive to sea-level change, basal marine melting, and accumulation and is less sensitive to isostasy, spatial variation in accumulation, calving rates, and ice flow parameterization. Tests of the model against field evidence show good agreement in places, as well as discrepancies which require further work.

  8. Thinning of the ice sheet in northwest Greenland over the past forty years.

    PubMed

    Paterson, W S; Reeh, N

    2001-11-01

    Thermal expansion of the oceans, as well as melting of glaciers, ice sheets and ice caps have been the main contributors to global sea level rise over the past century. The greatest uncertainty in predicting future sea level changes lies with our estimates of the mass balance of the ice sheets in Greenland and Antarctica. Satellite measurements have been used to determine changes in these ice sheets on short timescales, demonstrating that surface-elevation changes on timescales of decades or less result mainly from variations in snow accumulation. Here we present direct measurements of the changes in surface elevation between 1954 and 1995 on a traverse across the north Greenland ice sheet. Measurements over a time interval of this length should reflect changes in ice flow-the important quantity for predicting changes in sea level-relatively unperturbed by short-term fluctuations in snow accumulation. We find only small changes in the eastern part of the transect, except for some thickening of the north ice stream. On the west side, however, the thinning rates of the ice sheet are significantly higher and thinning extends to higher elevations than had been anticipated from previous studies.

  9. Numerical simulations of the Cordilleran ice sheet through the last glacial cycle

    NASA Astrophysics Data System (ADS)

    Seguinot, Julien; Rogozhina, Irina; Stroeven, Arjen P.; Margold, Martin; Kleman, Johan

    2016-03-01

    After more than a century of geological research, the Cordilleran ice sheet of North America remains among the least understood in terms of its former extent, volume, and dynamics. Because of the mountainous topography on which the ice sheet formed, geological studies have often had only local or regional relevance and shown such a complexity that ice-sheet-wide spatial reconstructions of advance and retreat patterns are lacking. Here we use a numerical ice sheet model calibrated against field-based evidence to attempt a quantitative reconstruction of the Cordilleran ice sheet history through the last glacial cycle. A series of simulations is driven by time-dependent temperature offsets from six proxy records located around the globe. Although this approach reveals large variations in model response to evolving climate forcing, all simulations produce two major glaciations during marine oxygen isotope stages 4 (62.2-56.9 ka) and 2 (23.2-16.9 ka). The timing of glaciation is better reproduced using temperature reconstructions from Greenland and Antarctic ice cores than from regional oceanic sediment cores. During most of the last glacial cycle, the modelled ice cover is discontinuous and restricted to high mountain areas. However, widespread precipitation over the Skeena Mountains favours the persistence of a central ice dome throughout the glacial cycle. It acts as a nucleation centre before the Last Glacial Maximum and hosts the last remains of Cordilleran ice until the middle Holocene (6.7 ka).

  10. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets.

    PubMed

    Pritchard, Hamish D; Arthern, Robert J; Vaughan, David G; Edwards, Laura A

    2009-10-15

    Many glaciers along the margins of the Greenland and Antarctic ice sheets are accelerating and, for this reason, contribute increasingly to global sea-level rise. Globally, ice losses contribute approximately 1.8 mm yr(-1) (ref. 8), but this could increase if the retreat of ice shelves and tidewater glaciers further enhances the loss of grounded ice or initiates the large-scale collapse of vulnerable parts of the ice sheets. Ice loss as a result of accelerated flow, known as dynamic thinning, is so poorly understood that its potential contribution to sea level over the twenty-first century remains unpredictable. Thinning on the ice-sheet scale has been monitored by using repeat satellite altimetry observations to track small changes in surface elevation, but previous sensors could not resolve most fast-flowing coastal glaciers. Here we report the use of high-resolution ICESat (Ice, Cloud and land Elevation Satellite) laser altimetry to map change along the entire grounded margins of the Greenland and Antarctic ice sheets. To isolate the dynamic signal, we compare rates of elevation change from both fast-flowing and slow-flowing ice with those expected from surface mass-balance fluctuations. We find that dynamic thinning of glaciers now reaches all latitudes in Greenland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates far into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt. In Greenland, glaciers flowing faster than 100 m yr(-1) thinned at an average rate of 0.84 m yr(-1), and in the Amundsen Sea embayment of Antarctica, thinning exceeded 9.0 m yr(-1) for some glaciers. Our results show that the most profound changes in the ice sheets currently result from glacier dynamics at ocean margins.

  11. On the Reconstruction of Palaeo-Ice Sheets: Recent Advances and Future Challenges

    NASA Technical Reports Server (NTRS)

    Stokes, Chris R.; Tarasov, Lev; Blomdin, Robin; Cronin, Thomas M.; Fisher, Timothy G.; Gyllencreutz, Richard; Hattestrand, Clas; Heyman, Jacob; Hindmarsh, Richard C. A.; Hughes, Anna L. C.; hide

    2015-01-01

    Reconstructing the growth and decay of palaeo-ice sheets is critical to understanding mechanisms of global climate change and associated sea-level fluctuations in the past, present and future. The significance of palaeo-ice sheets is further underlined by the broad range of disciplines concerned with reconstructing their behaviour, many of which have undergone a rapid expansion since the 1980s. In particular, there has been a major increase in the size and qualitative diversity of empirical data used to reconstruct and date ice sheets, and major improvements in our ability to simulate their dynamics in numerical ice sheet models. These developments have made it increasingly necessary to forge interdisciplinary links between sub-disciplines and to link numerical modelling with observations and dating of proxy records. The aim of this paper is to evaluate recent developments in the methods used to reconstruct ice sheets and outline some key challenges that remain, with an emphasis on how future work might integrate terrestrial and marine evidence together with numerical modelling. Our focus is on pan-ice sheet reconstructions of the last deglaciation, but regional case studies are used to illustrate methodological achievements, challenges and opportunities. Whilst various disciplines have made important progress in our understanding of ice-sheet dynamics, it is clear that data-model integration remains under-used, and that uncertainties remain poorly quantified in both empirically-based and numerical ice-sheet reconstructions. The representation of past climate will continue to be the largest source of uncertainty for numerical modelling. As such, palaeo-observations are critical to constrain and validate modelling. State-of-the-art numerical models will continue to improve both in model resolution and in the breadth of inclusion of relevant processes, thereby enabling more accurate and more direct comparison with the increasing range of palaeo-observations. Thus

  12. On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

    USGS Publications Warehouse

    Stokes, Chris R.; Tarasov, Lev; Blomdin, Robin; Cronin, Thomas M.; Fisher, Timothy G.; Gyllencreutz, Richard; Hattestrand, Clas; Heyman, Jakob; Hindmarsh, Richard C. A.; Hughes, Anna L. C.; Jakobsson, Martin; Kirchner, Nina; Livingstone, Stephen J.; Margold, Martin; Murton, Julian B.; Noormets, Riko; Peltier, W. Richard; Peteet, Dorothy M.; Piper, David J. W.; Preusser, Frank; Renssen, Hans; Roberts, David H.; Roche, Didier M.; Saint-Ange, Francky; Stroeven, Arjen P.; Teller, James T.

    2015-01-01

    Reconstructing the growth and decay of palaeo-ice sheets is critical to understanding mechanisms of global climate change and associated sea-level fluctuations in the past, present and future. The significance of palaeo-ice sheets is further underlined by the broad range of disciplines concerned with reconstructing their behaviour, many of which have undergone a rapid expansion since the 1980s. In particular, there has been a major increase in the size and qualitative diversity of empirical data used to reconstruct and date ice sheets, and major improvements in our ability to simulate their dynamics in numerical ice sheet models. These developments have made it increasingly necessary to forge interdisciplinary links between sub-disciplines and to link numerical modelling with observations and dating of proxy records. The aim of this paper is to evaluate recent developments in the methods used to reconstruct ice sheets and outline some key challenges that remain, with an emphasis on how future work might integrate terrestrial and marine evidence together with numerical modelling. Our focus is on pan-ice sheet reconstructions of the last deglaciation, but regional case studies are used to illustrate methodological achievements, challenges and opportunities. Whilst various disciplines have made important progress in our understanding of ice-sheet dynamics, it is clear that data-model integration remains under-used, and that uncertainties remain poorly quantified in both empirically-based and numerical ice-sheet reconstructions. The representation of past climate will continue to be the largest source of uncertainty for numerical modelling. As such, palaeo-observations are critical to constrain and validate modelling. State-of-the-art numerical models will continue to improve both in model resolution and in the breadth of inclusion of relevant processes, thereby enabling more accurate and more direct comparison with the increasing range of palaeo-observations. Thus

  13. Damage Mechanics in the Community Ice Sheet Model

    NASA Astrophysics Data System (ADS)

    Whitcomb, R.; Cathles, L. M. M., IV; Bassis, J. N.; Lipscomb, W. H.; Price, S. F.

    2016-12-01

    Half of the mass that floating ice shelves lose to the ocean comes from iceberg calving, which is a difficult process to simulate accurately. This is especially true in the large-scale ice dynamics models that couple changes in the cryosphere to climate projections. Damage mechanics provide a powerful technique with the potential to overcome this obstacle by describing how fractures in ice evolve over time. Here, we demonstrate the application of a damage model to ice shelves that predicts realistic geometries. We incorporated this solver into the Community Ice Sheet Model, a three dimensional ice sheet model developed at Los Alamos National Laboratory. The damage mechanics formulation that we use comes from a first principles-based evolution law for the depth of basal and surface crevasses and depends on the large scale strain rate, stress state, and basal melt. We show that under idealized conditions it produces ice tongue lengths that match well with observations for a selection of natural ice tongues, including Erebus, Drygalski, and Pine Island in Antarctica, as well as Petermann in Greenland. We also apply the model to more generalized ideal ice shelf geometries and show that it produces realistic calving front positions. Although our results are preliminary, the damage mechanics model that we developed provides a promising first principles method for predicting ice shelf extent and how the calving margins of ice shelves respond to climate change.

  14. MIS-11 duration key to disappearance of the Greenland ice sheet

    PubMed Central

    Robinson, Alexander; Alvarez-Solas, Jorge; Calov, Reinhard; Ganopolski, Andrey; Montoya, Marisa

    2017-01-01

    Palaeo data suggest that Greenland must have been largely ice free during Marine Isotope Stage 11 (MIS-11). However, regional summer insolation anomalies were modest during this time compared to MIS-5e, when the Greenland ice sheet likely lost less volume. Thus it remains unclear how such conditions led to an almost complete disappearance of the ice sheet. Here we use transient climate–ice sheet simulations to simultaneously constrain estimates of regional temperature anomalies and Greenland’s contribution to the MIS-11 sea-level highstand. We find that Greenland contributed 6.1 m (3.9–7.0 m, 95% credible interval) to sea level, ∼7 kyr after the peak in regional summer temperature anomalies of 2.8 °C (2.1–3.4 °C). The moderate warming produced a mean rate of mass loss in sea-level equivalent of only around 0.4 m per kyr, which means the long duration of MIS-11 interglacial conditions around Greenland was a necessary condition for the ice sheet to disappear almost completely. PMID:28681860

  15. MIS-11 duration key to disappearance of the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Robinson, Alexander; Alvarez-Solas, Jorge; Calov, Reinhard; Ganopolski, Andrey; Montoya, Marisa

    2017-07-01

    Palaeo data suggest that Greenland must have been largely ice free during Marine Isotope Stage 11 (MIS-11). However, regional summer insolation anomalies were modest during this time compared to MIS-5e, when the Greenland ice sheet likely lost less volume. Thus it remains unclear how such conditions led to an almost complete disappearance of the ice sheet. Here we use transient climate-ice sheet simulations to simultaneously constrain estimates of regional temperature anomalies and Greenland's contribution to the MIS-11 sea-level highstand. We find that Greenland contributed 6.1 m (3.9-7.0 m, 95% credible interval) to sea level, ~7 kyr after the peak in regional summer temperature anomalies of 2.8 °C (2.1-3.4 °C). The moderate warming produced a mean rate of mass loss in sea-level equivalent of only around 0.4 m per kyr, which means the long duration of MIS-11 interglacial conditions around Greenland was a necessary condition for the ice sheet to disappear almost completely.

  16. MIS-11 duration key to disappearance of the Greenland ice sheet.

    PubMed

    Robinson, Alexander; Alvarez-Solas, Jorge; Calov, Reinhard; Ganopolski, Andrey; Montoya, Marisa

    2017-07-06

    Palaeo data suggest that Greenland must have been largely ice free during Marine Isotope Stage 11 (MIS-11). However, regional summer insolation anomalies were modest during this time compared to MIS-5e, when the Greenland ice sheet likely lost less volume. Thus it remains unclear how such conditions led to an almost complete disappearance of the ice sheet. Here we use transient climate-ice sheet simulations to simultaneously constrain estimates of regional temperature anomalies and Greenland's contribution to the MIS-11 sea-level highstand. We find that Greenland contributed 6.1 m (3.9-7.0 m, 95% credible interval) to sea level, ∼7 kyr after the peak in regional summer temperature anomalies of 2.8 °C (2.1-3.4 °C). The moderate warming produced a mean rate of mass loss in sea-level equivalent of only around 0.4 m per kyr, which means the long duration of MIS-11 interglacial conditions around Greenland was a necessary condition for the ice sheet to disappear almost completely.

  17. Modelling water flow under glaciers and ice sheets.

    PubMed

    Flowers, Gwenn E

    2015-04-08

    Recent observations of dynamic water systems beneath the Greenland and Antarctic ice sheets have sparked renewed interest in modelling subglacial drainage. The foundations of today's models were laid decades ago, inspired by measurements from mountain glaciers, discovery of the modern ice streams and the study of landscapes evacuated by former ice sheets. Models have progressed from strict adherence to the principles of groundwater flow, to the incorporation of flow 'elements' specific to the subglacial environment, to sophisticated two-dimensional representations of interacting distributed and channelized drainage. Although presently in a state of rapid development, subglacial drainage models, when coupled to models of ice flow, are now able to reproduce many of the canonical phenomena that characterize this coupled system. Model calibration remains generally out of reach, whereas widespread application of these models to large problems and real geometries awaits the next level of development.

  18. Modelling water flow under glaciers and ice sheets

    PubMed Central

    Flowers, Gwenn E.

    2015-01-01

    Recent observations of dynamic water systems beneath the Greenland and Antarctic ice sheets have sparked renewed interest in modelling subglacial drainage. The foundations of today's models were laid decades ago, inspired by measurements from mountain glaciers, discovery of the modern ice streams and the study of landscapes evacuated by former ice sheets. Models have progressed from strict adherence to the principles of groundwater flow, to the incorporation of flow ‘elements’ specific to the subglacial environment, to sophisticated two-dimensional representations of interacting distributed and channelized drainage. Although presently in a state of rapid development, subglacial drainage models, when coupled to models of ice flow, are now able to reproduce many of the canonical phenomena that characterize this coupled system. Model calibration remains generally out of reach, whereas widespread application of these models to large problems and real geometries awaits the next level of development. PMID:27547082

  19. The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 1: Model description

    NASA Astrophysics Data System (ADS)

    Winkelmann, R.; Martin, M. A.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

    2011-09-01

    We present the Potsdam Parallel Ice Sheet Model (PISM-PIK), developed at the Potsdam Institute for Climate Impact Research to be used for simulations of large-scale ice sheet-shelf systems. It is derived from the Parallel Ice Sheet Model (Bueler and Brown, 2009). Velocities are calculated by superposition of two shallow stress balance approximations within the entire ice covered region: the shallow ice approximation (SIA) is dominant in grounded regions and accounts for shear deformation parallel to the geoid. The plug-flow type shallow shelf approximation (SSA) dominates the velocity field in ice shelf regions and serves as a basal sliding velocity in grounded regions. Ice streams can be identified diagnostically as regions with a significant contribution of membrane stresses to the local momentum balance. All lateral boundaries in PISM-PIK are free to evolve, including the grounding line and ice fronts. Ice shelf margins in particular are modeled using Neumann boundary conditions for the SSA equations, reflecting a hydrostatic stress imbalance along the vertical calving face. The ice front position is modeled using a subgrid-scale representation of calving front motion (Albrecht et al., 2011) and a physically-motivated calving law based on horizontal spreading rates. The model is tested in experiments from the Marine Ice Sheet Model Intercomparison Project (MISMIP). A dynamic equilibrium simulation of Antarctica under present-day conditions is presented in Martin et al. (2011).

  20. The Last Interglacial History of the Antarctic Ice sheet

    NASA Astrophysics Data System (ADS)

    Bradley, Sarah; Siddall, Mark; Milne, Glenn A.; Masson-Delmotte, Valerie; Wolff, Eric; Hindmarsh, Richard C. A.

    2014-05-01

    In this paper we present a summary of the work which was conducted as part of the 'PAST4FUTURE -WP4.1: Sea Level and Ice sheets' project. The overall aim of this study was to understand the response of the Antarctic Ice sheet (AIS) to climate forcing during the Last interglacial (LIG) and its contribution to the observed higher than present sea level during this period. The study involved the application and development of a novel technique which combined East Antarctic stable isotope ice core data with the output from a Glacial Isostatic Adjustment (GIA) model [Bradley et al., 2012]. We investigated if the stable isotope ice core data are sensitive to detecting isostatically driven changes in the surface elevation driven by changes in the ice-loading history of the AIS and if so, could we address some key questions relating to the LIG history of the AIS. Although it is believed that the West Antarctic Ice sheet (WAIS) reduced in size during the LIG compared to the Holocene, major uncertainties and unknowns remain unresolved: Did the WAIS collapse? What would the contribution of such a collapse be the higher than present LIG eustatic sea level (ESL)? We will show that a simulated collapse of the WAIS does not generate a significant elevation driven signal at the EAIS LIG ice core sites, and as such, these ice core records cannot be used to assess WAIS stability over this period. However, we will present 'treasure maps' [Bradley et al., 2012] to identify regions of the AIS where results from geological studies and/or new paleoclimate data may be sensitive to detecting a WAIS collapse. These maps can act as a useful tool for the wider science community/field scientists as a guide to highlight sites suitable to constrain the evolution of the WAIS during the LIG. Studies have proposed that the surface temperature across the East Antarctic Ice Sheet (EAIS) was significantly warmer, 2-5°C during the LIG compared to present [Lang and Wolff, 2011]. These higher

  1. Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica

    NASA Astrophysics Data System (ADS)

    Halberstadt, Anna Ruth W.; Simkins, Lauren M.; Greenwood, Sarah L.; Anderson, John B.

    2016-05-01

    Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

  2. Bedrock structure and the interpretation of palaeo ice stream footprints: examples from the Pleistocene British Ice Sheet

    NASA Astrophysics Data System (ADS)

    Krabbendam, M.; Bradwell, T.

    2009-04-01

    To model past and future behaviour of ice sheets, a good understanding of both modern and ancient ice streams is required. The study of present-day ice streams provides detailed data of short-term dynamic changes, whilst the study of Pleistocene palaeo-ice streams can provide crucial constraints on the longer-term evolution of ice sheets. To date, palaeo-ice streams, such as the classical Dubawnt Lake palaeo-ice stream of the former Laurentide Ice Sheet, have been recognised largely on the basis of extremely elongate drumlins and megascale glacial lineations; all soft-sediment features. Whilst it appears that topographically unconstrained ice streams (eg. within the West Antarctic Ice Sheet) are generally underlain by deformable till, topographically constrained ice streams such as Jakobshavn Isbrae do not require deformable sediment and may occur on a bedrock-dominated bed. Analysis of DEM data and geomorphology and structural geology fieldwork in Northern Scotland and Northern England has shown the occurrence of highly streamlined bedforms in bedrock of the former base of topographically controlled palaeo-ice streams, which drained parts of the British Ice Sheet. The bedforms are predominantly bedrock megagrooves with asymmetric cross-profiles. In the Ullapool tributary of the Minch palaeo ice stream, bedrock megagrooves form the dominant evidence for ice streaming. The megagrooves are typically 5-15 m deep, 10-30 m wide and 500 - 3000 m long. Spacing of megagrooves is typically 100 - 200 m. In both study areas, the bedrock is strongly anisotropic, either consisting of thin-bedded strata or strongly foliated metasedimentary rocks, with the strata or foliation having a gentle dip. Megagrooves are best developed where the strike of the anisotropy is sub-parallel (within 10 - 20°) with palaeo ice flow. The bedrock in both areas has a well-developed, relatively densely spaced (< 1m), conjugate joint system. We suggest that asymmetric megagrooves are formed by

  3. Greenland Regional and Ice Sheet-wide Geometry Sensitivity to Boundary and Initial conditions

    NASA Astrophysics Data System (ADS)

    Logan, L. C.; Narayanan, S. H. K.; Greve, R.; Heimbach, P.

    2017-12-01

    Ice sheet and glacier model outputs require inputs from uncertainly known initial and boundary conditions, and other parameters. Conservation and constitutive equations formalize the relationship between model inputs and outputs, and the sensitivity of model-derived quantities of interest (e.g., ice sheet volume above floatation) to model variables can be obtained via the adjoint model of an ice sheet. We show how one particular ice sheet model, SICOPOLIS (SImulation COde for POLythermal Ice Sheets), depends on these inputs through comprehensive adjoint-based sensitivity analyses. SICOPOLIS discretizes the shallow-ice and shallow-shelf approximations for ice flow, and is well-suited for paleo-studies of Greenland and Antarctica, among other computational domains. The adjoint model of SICOPOLIS was developed via algorithmic differentiation, facilitated by the source transformation tool OpenAD (developed at Argonne National Lab). While model sensitivity to various inputs can be computed by costly methods involving input perturbation simulations, the time-dependent adjoint model of SICOPOLIS delivers model sensitivities to initial and boundary conditions throughout time at lower cost. Here, we explore both the sensitivities of the Greenland Ice Sheet's entire and regional volumes to: initial ice thickness, precipitation, basal sliding, and geothermal flux over the Holocene epoch. Sensitivity studies such as described here are now accessible to the modeling community, based on the latest version of SICOPOLIS that has been adapted for OpenAD to generate correct and efficient adjoint code.

  4. Abrupt shift in the observed runoff from the southwestern Greenland ice sheet

    PubMed Central

    Ahlstrøm, Andreas P.; Petersen, Dorthe; Langen, Peter L.; Citterio, Michele; Box, Jason E.

    2017-01-01

    The recent decades of accelerating mass loss of the Greenland ice sheet have arisen from an increase in both surface meltwater runoff and ice flow discharge from tidewater glaciers. Despite the role of the Greenland ice sheet as the dominant individual cryospheric contributor to sea level rise in recent decades, no observational record of its mass loss spans the 30-year period needed to assess its climatological state. We present for the first time a 40-year (1975–2014) time series of observed meltwater discharge from a >6500-km2 catchment of the southwestern Greenland ice sheet. We find that an abrupt 80% increase in runoff occurring between the 1976–2002 and 2003–2014 periods is due to a shift in atmospheric circulation, with meridional exchange events occurring more frequently over Greenland, establishing the first observation-based connection between ice sheet runoff and climate change. PMID:29242827

  5. Significance of Thermal Fluvial Incision and Bedrock Transfer due to Ice Advection on Greenland Ice Sheet Topography

    NASA Astrophysics Data System (ADS)

    Crozier, J. A.; Karlstrom, L.; Yang, K.

    2017-12-01

    Ice sheet surface topography reflects a complicated combination of processes that act directly upon the surface and that are products of ice advection. Using recently-available high resolution ice velocity, imagery, ice surface elevation, and bedrock elevation data sets, we seek to determine the domain of significance of two important processes - thermal fluvial incision and transfer of bedrock topography through the ice sheet - on controlling surface topography in the ablation zone. Evaluating such controls is important for understanding how melting of the GIS surface during the melt season may be directly imprinted in topography through supraglacial drainage networks, and indirectly imprinted through its contribution to basal sliding that affects bedrock transfer. We use methods developed by (Karlstrom and Yang, 2016) to identify supraglacial stream networks on the GIS, and use high resolution surface digital elevation models as well as gridded ice velocity and melt rate models to quantify surface processes. We implement a numerically efficient Fourier domain bedrock transfer function (Gudmundsson, 2003) to predict surface topography due to ice advection over bedrock topography obtained from radar. Despite a number of simplifying assumptions, the bedrock transfer function predicts the observed ice sheet surface in most regions of the GIS with ˜90% accuracy, regardless of the presence or absence of supraglacial drainage networks. This supports the hypothesis that bedrock is the most significant driver of ice surface topography on wavelengths similar to ice thickness. Ice surface topographic asymmetry on the GIS is common, with slopes in the direction of ice flow steeper than those faced opposite to ice flow, consistent with bedrock transfer theory. At smaller wavelengths, topography consistent with fluvial erosion by surface hydrologic features is evident. We quantify the effect of ice advection versus fluvial thermal erosion on supraglacial longitudinal stream

  6. Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing

    PubMed Central

    Golledge, Nicholas R.; Fogwill, Christopher J.; Mackintosh, Andrew N.; Buckley, Kevin M.

    2012-01-01

    Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments—a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets. PMID:22988078

  7. Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing.

    PubMed

    Golledge, Nicholas R; Fogwill, Christopher J; Mackintosh, Andrew N; Buckley, Kevin M

    2012-10-02

    Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments-a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets.

  8. Ice Sheet System Model as Educational Entertainment

    NASA Astrophysics Data System (ADS)

    Perez, G.

    2013-12-01

    Understanding the importance of polar ice sheets and their role in the evolution of Sea Level Rise (SLR), as well as Climate Change, is of paramount importance for policy makers as well as the public and schools at large. For example, polar ice sheets and glaciers currently account for 1/3 of the SLR signal, a ratio that will increase in the near to long-term future, which has tremendous societal ramifications. Consequently, it is important to increase awareness about our changing planet. In our increasingly digital society, mobile and web applications are burgeoning venues for such outreach. The Ice Sheet System Model (ISSM) is a software that was developed at the Jet Propulsion Laboratory/CalTech/NASA, in collaboration with University of California Irvine (UCI), with the goal of better understanding the evolution of polar ice sheets. It is a state-of-the-art framework, which relies on higher-end cluster-computing to address some of the aforementioned challenges. In addition, it is a flexible framework that can be deployed on any hardware; in particular, on mobile platforms such as Android or iOS smart phones. Here, we look at how the ISSM development team managed to port their model to these platforms, what the implications are for improving how scientists disseminate their results, and how a broader audience may familiarize themselves with running complex climate models in simplified scenarios which are highly educational and entertaining in content. We also look at the future plans toward a web portal fully integrated with mobile technologies to deliver the best content to the public, and to provide educational plans/lessons that can be used in grades K-12 as well as collegiate under-graduate and graduate programs.

  9. An investigation of the astronomical theory of the ice ages using a simple climate-ice sheet model

    NASA Technical Reports Server (NTRS)

    Pollard, D.

    1978-01-01

    The astronomical theory of the Quaternary ice ages is incorporated into a simple climate model for global weather; important features of the model include the albedo feedback, topography and dynamics of the ice sheets. For various parameterizations of the orbital elements, the model yields realistic assessments of the northern ice sheet. Lack of a land-sea heat capacity contrast represents one of the chief difficulties of the model.

  10. Greenland ice sheet surface temperature, melt and mass loss: 2000-06

    USGS Publications Warehouse

    Hall, D.K.; Williams, R.S.; Luthcke, S.B.; DiGirolamo, N.E.

    2008-01-01

    A daily time series of 'clear-sky' surface temperature has been compiled of the Greenland ice sheet (GIS) using 1 km resolution moderate-resolution imaging spectroradiometer (MODIS) land-surface temperature (LST) maps from 2000 to 2006. We also used mass-concentration data from the Gravity Recovery and Climate Experiment (GRACE) to study mass change in relationship to surface melt from 2003 to 2006. The mean LST of the GIS increased during the study period by ???0.27??Ca-1. The increase was especially notable in the northern half of the ice sheet during the winter months. Melt-season length and timing were also studied in each of the six major drainage basins. Rapid (<15 days) and sustained mass loss below 2000 m elevation was triggered in 2004 and 2005 as recorded by GRACE when surface melt begins. Initiation of large-scale surface melt was followed rapidly by mass loss. This indicates that surface meltwater is flowing rapidly to the base of the ice sheet, causing acceleration of outlet glaciers, thus highlighting the metastability of parts of the GIS and the vulnerability of the ice sheet to air-temperature increases. If air temperatures continue to rise over Greenland, increased surface melt will play a large role in ice-sheet mass loss.

  11. The influence of atmospheric grid resolution in a climate model-forced ice sheet simulation

    NASA Astrophysics Data System (ADS)

    Lofverstrom, Marcus; Liakka, Johan

    2018-04-01

    Coupled climate-ice sheet simulations have been growing in popularity in recent years. Experiments of this type are however challenging as ice sheets evolve over multi-millennial timescales, which is beyond the practical integration limit of most Earth system models. A common method to increase model throughput is to trade resolution for computational efficiency (compromise accuracy for speed). Here we analyze how the resolution of an atmospheric general circulation model (AGCM) influences the simulation quality in a stand-alone ice sheet model. Four identical AGCM simulations of the Last Glacial Maximum (LGM) were run at different horizontal resolutions: T85 (1.4°), T42 (2.8°), T31 (3.8°), and T21 (5.6°). These simulations were subsequently used as forcing of an ice sheet model. While the T85 climate forcing reproduces the LGM ice sheets to a high accuracy, the intermediate resolution cases (T42 and T31) fail to build the Eurasian ice sheet. The T21 case fails in both Eurasia and North America. Sensitivity experiments using different surface mass balance parameterizations improve the simulations of the Eurasian ice sheet in the T42 case, but the compromise is a substantial ice buildup in Siberia. The T31 and T21 cases do not improve in the same way in Eurasia, though the latter simulates the continent-wide Laurentide ice sheet in North America. The difficulty to reproduce the LGM ice sheets in the T21 case is in broad agreement with previous studies using low-resolution atmospheric models, and is caused by a substantial deterioration of the model climate between the T31 and T21 resolutions. It is speculated that this deficiency may demonstrate a fundamental problem with using low-resolution atmospheric models in these types of experiments.

  12. Long term ice sheet mass change rates and inter-annual variability from GRACE gravimetry.

    NASA Astrophysics Data System (ADS)

    Harig, C.

    2017-12-01

    The GRACE time series of gravimetry now stretches 15 years since its launch in 2002. Here we use Slepian functions to estimate the long term ice mass trends of Greenland, Antarctica, and several glaciated regions. The spatial representation shows multi-year to decadal regional shifts in accelerations, in agreement with increases in radar derived ice velocity. Interannual variations in ice mass are of particular interest since they can directly link changes in ice sheets to the drivers of change in the polar ocean and atmosphere. The spatial information retained in Slepian functions provides a tool to determine how this link varies in different regions within an ice sheet. We present GRACE observations of the 2013-2014 slowdown in mass loss of the Greenland ice sheet, which was concentrated in specific parts of the ice sheet and in certain months of the year. We also discuss estimating the relative importance of climate factors that control ice mass balance, as a function of location of the glacier/ice cap as well as the spatial variation within an ice sheet by comparing gravimetry with observations of surface air temperature, ocean temperature, etc. as well as model data from climate reanalysis products.

  13. Abrupt drainage cycles of the Fennoscandian Ice Sheet

    PubMed Central

    Soulet, Guillaume; Ménot, Guillemette; Bayon, Germain; Rostek, Frauke; Ponzevera, Emmanuel; Toucanne, Samuel; Lericolais, Gilles; Bard, Edouard

    2013-01-01

    Continental ice sheets are a key component of the Earth’s climate system, but their internal dynamics need to be further studied. Since the last deglaciation, the northern Eurasian Fennoscandian Ice Sheet (FIS) has been connected to the Black Sea (BS) watershed, making this basin a suitable location to investigate former ice-sheet dynamics. Here, from a core retrieved in the BS, we combine the use of neodymium isotopes, high-resolution elemental analysis, and biomarkers to trace changes in sediment provenance and river runoff. We reveal cyclic releases of meltwater originating from Lake Disna, a proglacial lake linked to the FIS during Heinrich Stadial 1. Regional interactions within the climate–lake–FIS system, linked to changes in the availability of subglacial water, led to abrupt drainage cycles of the FIS into the BS watershed. This phenomenon raised the BS water level by ∼100 m until the sill of the Bosphorus Strait was reached, flooding the vast northwestern BS shelf and deeply affecting the hydrology and circulation of the BS and, probably, of the Marmara and Aegean Seas. PMID:23569264

  14. Abrupt drainage cycles of the Fennoscandian Ice Sheet.

    PubMed

    Soulet, Guillaume; Ménot, Guillemette; Bayon, Germain; Rostek, Frauke; Ponzevera, Emmanuel; Toucanne, Samuel; Lericolais, Gilles; Bard, Edouard

    2013-04-23

    Continental ice sheets are a key component of the Earth's climate system, but their internal dynamics need to be further studied. Since the last deglaciation, the northern Eurasian Fennoscandian Ice Sheet (FIS) has been connected to the Black Sea (BS) watershed, making this basin a suitable location to investigate former ice-sheet dynamics. Here, from a core retrieved in the BS, we combine the use of neodymium isotopes, high-resolution elemental analysis, and biomarkers to trace changes in sediment provenance and river runoff. We reveal cyclic releases of meltwater originating from Lake Disna, a proglacial lake linked to the FIS during Heinrich Stadial 1. Regional interactions within the climate-lake-FIS system, linked to changes in the availability of subglacial water, led to abrupt drainage cycles of the FIS into the BS watershed. This phenomenon raised the BS water level by ∼100 m until the sill of the Bosphorus Strait was reached, flooding the vast northwestern BS shelf and deeply affecting the hydrology and circulation of the BS and, probably, of the Marmara and Aegean Seas.

  15. Determining Greenland Ice Sheet Accumulation Rates from Radar Remote Sensing

    NASA Technical Reports Server (NTRS)

    Jezek, Kenneth C.

    2002-01-01

    An important component of NASA's Program for Arctic Regional Climate Assessment (PARCA) is a mass balance investigation of the Greenland Ice Sheet. The mass balance is calculated by taking the difference between the areally Integrated snow accumulation and the net ice discharge of the ice sheet. Uncertainties in this calculation Include the snow accumulation rate, which has traditionally been determined by interpolating data from ice core samples taken from isolated spots across the ice sheet. The sparse data associated with ice cores juxtaposed against the high spatial and temporal resolution provided by remote sensing , has motivated scientists to investigate relationships between accumulation rate and microwave observations as an option for obtaining spatially contiguous estimates. The objective of this PARCA continuation proposal was to complete an estimate of surface accumulation rate on the Greenland Ice Sheet derived from C-band radar backscatter data compiled in the ERS-1 SAR mosaic of data acquired during, September-November, 1992. An empirical equation, based on elevation and latitude, is used to determine the mean annual temperature. We examine the influence of accumulation rate, and mean annual temperature on C-band radar backscatter using a forward model, which incorporates snow metamorphosis and radar backscatter components. Our model is run over a range of accumulation and temperature conditions. Based on the model results, we generate a look-up table, which uniquely maps the measured radar backscatter, and mean annual temperature to accumulation rate. Our results compare favorably with in situ accumulation rate measurements falling within our study area.

  16. The last forests in Greenland, and the age of the ice sheet

    NASA Astrophysics Data System (ADS)

    Funder, Svend; Schmidt, Astrid M. Z.; Dahl-Jensen, Dorthe; Steffensen, Jørgen Peder; Willerslev, Eske

    2014-05-01

    Recently ancient DNA (aDNA) studies of the basal ice in the Camp Century ice core, northern Greenland, have shown that mixed coniferous-deciduous forest grew here before the area was invaded and permanently covered by the ice sheet. The coring site is situated only 100 km from the present ice margin and more than 500 km from the ice divide, indicating that since this last inception the northern part of the ice sheet never receded more than 100 km from its present margin. Dating of the basal ice and obtaining an age for the forest and for the beginning of the ice sheet's permanency has been attempted by analyzing for optically stimulated luminescence (OSL), meteoric 10Be/36Cl cosmogenic nuclides, 234U/238U recoil. These methods all provide only minimum ages and show that the forest at Cap Century is older than 500 ka. Comparison with other Pleistocene "forest sites" in Greenland - the Kap København Formation in northernmost Greenland, the DYE-3 ice core in the south, the ODP boring 646 south of Greenland, as well as results from basal ice in the GRIP ice core - extends the minimum age to c. 1 ma. The maximum age is provided by the Kap København Formation, which must be older - or contemporaneous. The formation has recently been confirmed to date within the interval 2-2.5 ma, with a preferred age of 2.3-2.4 ma. Surprisingly, application of the molecular clock of insect COI sequences on the Camp Century aDNA now seem to push the minimum age just as far back - to 2.4 ma, suggesting that the timberline boreal forest at Kap København is contemporaneous with the mixed forest at Camp Century, 600 km to the south. From this we conclude that the northern ice sheet dome, which today contains 85% of the total ice sheet volume, has remained within 100 km of its present margin for at least 1 ma, and possibly may go back as far as 2.4 ma. The ice sheet has therefore survived both interglacials and "super interglacials" that were both warmer and longer than the present. This

  17. Interactive Ice Sheet Flowline Model for High School and College Students

    NASA Astrophysics Data System (ADS)

    Stearns, L. A.; Rezvanbehbahani, S.; Shankar, S.

    2017-12-01

    Teaching about climate and climate change is conceptually challenging. While teaching tools and lesson plans are rapidly evolving to help teachers and students improve their understanding of climate processes, there are very few tools targeting ice sheet and glacier dynamics. We have built an interactive ice sheet model that allows students to explore how Antarctic glaciers respond to different climate perturbations. Interactive models offer advantages that are hard to obtain in traditional classroom settings; users can systematically investigate hypothetical situations, explore the effects of modifying systems, and repeatedly observe how systems interrelate. As a result, this project provides a much-needed bridge between the data and models used by the scientific community and students in high school and college. We target our instructional and assessment activities to three high school and college students with the overall aim of increasing understanding of ice sheet dynamics and the different ways that ice sheets are impacted by climate change, while also improving their fundamental math skills.

  18. Propagation of acoustic-gravity waves in arctic zones with elastic ice-sheets

    NASA Astrophysics Data System (ADS)

    Kadri, Usama; Abdolali, Ali; Kirby, James T.

    2017-04-01

    We present an analytical solution of the boundary value problem of propagating acoustic-gravity waves generated in the ocean by earthquakes or ice-quakes in arctic zones. At the surface, we assume elastic ice-sheets of a variable thickness, and show that the propagating acoustic-gravity modes have different mode shape than originally derived by Ref. [1] for a rigid ice-sheet settings. Computationally, we couple the ice-sheet problem with the free surface model by Ref. [2] representing shrinking ice blocks in realistic sea state, where the randomly oriented ice-sheets cause inter modal transition at the edges and multidirectional reflections. We then derive a depth-integrated equation valid for spatially slowly varying thickness of ice-sheet and water depth. Surprisingly, and unlike the free-surface setting, here it is found that the higher acoustic-gravity modes exhibit a larger contribution. These modes travel at the speed of sound in water carrying information on their source, e.g. ice-sheet motion or submarine earthquake, providing various implications for ocean monitoring and detection of quakes. In addition, we found that the propagating acoustic-gravity modes can result in orbital displacements of fluid parcels sufficiently high that may contribute to deep ocean currents and circulation, as postulated by Refs. [1, 3]. References [1] U. Kadri, 2016. Generation of Hydroacoustic Waves by an Oscillating Ice Block in Arctic Zones. Advances in Acoustics and Vibration, 2016, Article ID 8076108, 7 pages http://dx.doi.org/10.1155/2016/8076108 [2] A. Abdolali, J. T. Kirby and G. Bellotti, 2015, Depth-integrated equation for hydro-acoustic waves with bottom damping, J. Fluid Mech., 766, R1 doi:10.1017/jfm.2015.37 [3] U. Kadri, 2014. Deep ocean water transportation by acoustic?gravity waves. J. Geophys. Res. Oceans, 119, doi:10.1002/ 2014JC010234

  19. The Distribution of Basal Water Beneath the Greenland Ice Sheet from Radio-Echo Sounding

    NASA Astrophysics Data System (ADS)

    Jordan, T.; Williams, C.; Schroeder, D. M.; Martos, Y. M.; Cooper, M.; Siegert, M. J.; Paden, J. D.; Huybrechts, P.; Bamber, J. L.

    2017-12-01

    There is widespread, but often indirect, evidence that a significant fraction of the Greenland Ice Sheet is thawed at the bed. This includes major outlet glaciers and around the NorthGRIP ice-core in the interior. However, the ice-sheet-wide distribution of basal water is poorly constrained by existing observations, and the spatial relationship between basal water and other ice-sheet and subglacial properties is therefore largely unexplored. In principle, airborne radio-echo sounding (RES) surveys provide the necessary information and spatial coverage to infer the presence of basal water at the ice-sheet scale. However, due to uncertainty and spatial variation in radar signal attenuation, the commonly used water diagnostic, bed-echo reflectivity, is highly ambiguous and prone to spatial bias. Here we introduce a new RES diagnostic for the presence of basal water which incorporates both sharp step-transitions and rapid fluctuations in bed-echo reflectivity. This has the advantage of being (near) independent of attenuation model, and enables a decade of recent Operation Ice Bride RES survey data to be combined in a single map for basal water. The ice-sheet-wide water predictions are compared with: bed topography and drainage network structure, existing knowledge of the thermal state and geothermal heat flux, and ice velocity. In addition to the fast flowing ice-sheet margins, we also demonstrate widespread water routing and storage in parts of the slow-flowing northern interior. Notably, this includes a quasi-linear `corridor' of basal water, extending from NorthGRIP to Petermann glacier, which spatially correlates with a region of locally high (magnetic-derived) geothermal heat flux. The predicted water distribution places a new constraint upon the basal thermal state of the Greenland Ice Sheet, and could be used as an input for ice-sheet model simulations.

  20. Variability of Surface Temperature and Melt on the Greenland Ice Sheet, 2000-2011

    NASA Technical Reports Server (NTRS)

    Hall, Dorothy K.; Comiso, Josefino, C.; Shuman, Christopher A.; Koenig, Lora S.; DiGirolamo, Nicolo E.

    2012-01-01

    Enhanced melting along with surface-temperature increases measured using infrared satellite data, have been documented for the Greenland Ice Sheet. Recently we developed a climate-quality data record of ice-surface temperature (IST) of the Greenland Ice Sheet using the Moderate-Resolution Imaging Spectroradiometer (MODIS) 1ST product -- http://modis-snow-ice.gsfc.nasa.gov. Using daily and mean monthly MODIS 1ST maps from the data record we show maximum extent of melt for the ice sheet and its six major drainage basins for a 12-year period extending from March of 2000 through December of 2011. The duration of the melt season on the ice sheet varies in different drainage basins with some basins melting progressively earlier over the study period. Some (but not all) of the basins also show a progressively-longer duration of melt. The short time of the study period (approximately 12 years) precludes an evaluation of statistically-significant trends. However the dataset provides valuable information on natural variability of IST, and on the ability of the MODIS instrument to capture changes in IST and melt conditions indifferent drainage basins of the ice sheet.

  1. Airborne Laser Altimetry Mapping of the Greenland Ice Sheet: Application to Mass Balance Assessment

    NASA Technical Reports Server (NTRS)

    Abdalati, W.; Krabill, W.; Frederick, E.; Manizade, S.; Martin, C.; Sonntag, J.; Swift, R.; Thomas, R.; Wright, W.; Yungel, J.

    2000-01-01

    In 1998 and '99, the Arctic Ice Mapping (AIM) program completed resurveys of lines occupied 5 years earlier revealing elevation changes of the Greenland ice sheet and identifying areas of significant thinning, thickening and balance. In planning these surveys, consideration had to be given to the spatial constraints associated with aircraft operation, the spatial nature of ice sheet behavior, and limited resources, as well as temporal issues, such as seasonal and interannual variability in the context of measurement accuracy. This paper examines the extent to which the sampling and survey strategy is valid for drawing conclusions on the current state of balance of the Greenland ice sheet. The surveys covered the entire ice sheet with an average distance of 21.4 km between each location on the ice sheet and the nearest flight line. For most of the ice sheet, the elevation changes show relatively little spatial variability, and their magnitudes are significantly smaller than the observed elevation change signal. As a result, we conclude that the density of the sampling and the accuracy of the measurements are sufficient to draw meaningful conclusions on the state of balance of the entire ice sheet over the five-year survey period. Outlet glaciers, however, show far more spatial and temporal variability, and each of the major ones is likely to require individual surveys in order to determine its balance.

  2. Multi-channel Ice Penetrating Radar Traverse for Estimates of Firn Density in the Percolation Zone, Western Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Meehan, T.; Osterberg, E. C.; Lewis, G.; Overly, T. B.; Hawley, R. L.; Bradford, J.; Marshall, H. P.

    2016-12-01

    To better predict the response of the Greenland Ice Sheet (GrIS) to future warming, leading edge Regional Climate Models (RCM) must be calibrated with in situ measurements of recent accumulation and melt. Mass balance estimates averaged across the entire Greenland Ice Sheet (GrIS) vary between models by more than 30 percent, and regional comparisons of mass balance reconstructions in Greenland vary by 100 percent or more. Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) is a multi-year and multi-disciplinary 1700 km science traverse from Raven/Dye2 in SW Greenland, to Summit Station. Multi-offset radar measurements can provide high accuracy electromagnetic (EM) velocity estimates of the firn to within (+-) 0.002 to 0.003 m/ns. EM velocity, in turn, can be used to estimate bulk firn density. Using a mixing equation such as the CRIM Equation we use the measured EM velocity, along with the known EM velocity in air and ice, to estimate bulk density. During spring 2016, we used multi-channel 500MHz radar in a multi-offset configuration to survey more than 800 km from Raven towards summit. Preliminary radar-derived snow density estimates agree with density estimates from a firn core measurement ( 50 kg/m3), despite the lateral heterogeneity of the firn across the length of the antenna array (12 m).

  3. Antarctic Ice-Sheet Mass Balance from Satellite Altimetry 1992 to 2001

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Brenner, Anita C.; Cornejo, Helen; Giovinetto, Mario; Saba, Jack L.; Yi, Donghui

    2003-01-01

    A major uncertainty in understanding the causes of the current rate of sea level rise is the potential contributions from mass imbalances of the Greenland and Antarctic ice sheets. Estimates of the current mass balance of the Antarctic ice sheet are derived from surface- elevation changes obtained from 9 years of ERS - 1 & 2 radar altimeter data. Elevation time-series are created from altimeter crossovers among 90-day data periods on a 50 km grid to 81.5 S. The time series are fit with a multivariate linear/sinusoidal function to give the average rate of elevation change (dH/dt). On the major Rome-Filchner, Ross, and Amery ice shelves, the W d t are small or near zero. In contrast, the ice shelves of the Antarctic Peninsula and along the West Antarctic coast appear to be thinning significantly, with a 23 +/- 3 cm per year surface elevation decrease on the Larsen ice shelf and a 65 +/- 4 cm per year decrease on the Dotson ice shelf. On the grounded ice, significant elevation decreases are obtained over most of the drainage basins of the Pine Island and Thwaites glaciers in West Antarctica and inland of Law Dome in East Antarctica. Significant elevation increases are observed within about 200 km of the coast around much of the rest of the ice sheet. Farther inland, the changes are a mixed pattern of increases and decreases with increases of a few centimeters per year at the highest elevations of the East Antarctic plateau. The derived elevation changes are combined with estimates of the bedrock uplift from several models to provide maps of ice thickness change. The ice thickness changes enable estimates of the ice mass balances for the major drainage basins, the overall mass balance, and the current contribution of the ice sheet to global sea level change.

  4. Surface Mass Balance of the Greenland Ice Sheet Derived from Paleoclimate Reanalysis

    NASA Astrophysics Data System (ADS)

    Badgeley, J.; Steig, E. J.; Hakim, G. J.; Anderson, J.; Tardif, R.

    2017-12-01

    Modeling past ice-sheet behavior requires independent knowledge of past surface mass balance. Though models provide useful insight into ice-sheet response to climate forcing, if past climate is unknown, then ascertaining the rate and extent of past ice-sheet change is limited to geological and geophysical constraints. We use a novel data-assimilation framework developed under the Last Millennium Reanalysis Project (Hakim et al., 2016) to reconstruct past climate over ice sheets with the intent of creating an independent surface mass balance record for paleo ice-sheet modeling. Paleoclimate data assimilation combines the physics of climate models and the time series evidence of proxy records in an offline, ensemble-based approach. This framework allows for the assimilation of numerous proxy records and archive types while maintaining spatial consistency with known climate dynamics and physics captured by the models. In our reconstruction, we use the Community Climate System Model version 4, CMIP5 last millennium simulation (Taylor et al., 2012; Landrum et al., 2013) and a nearly complete database of ice core oxygen isotope records to reconstruct Holocene surface temperature and precipitation over the Greenland Ice Sheet on a decadal timescale. By applying a seasonality to this reconstruction (from the TraCE-21ka simulation; Liu et al., 2009), our reanalysis can be used in seasonally-based surface mass balance models. Here we discuss the methods behind our reanalysis and the performance of our reconstruction through prediction of unassimilated proxy records and comparison to paleoclimate reconstructions and reanalysis products.

  5. Analogue modelling of the influence of ice shelf collapse on the flow of ice sheets grounded below sea-level

    NASA Astrophysics Data System (ADS)

    Corti, Giacomo; Zeoli, Antonio

    2016-04-01

    The sudden breakup of ice shelves is expected to result in significant acceleration of inland glaciers, a process related to the removal of the buttressing effect exerted by the ice shelf on the tributary glaciers. This effect has been tested in previous analogue models, which however applied to ice sheets grounded above sea level (e.g., East Antarctic Ice Sheet; Antarctic Peninsula and the Larsen Ice Shelf). In this work we expand these previous results by performing small-scale laboratory models that analyse the influence of ice shelf collapse on the flow of ice streams draining an ice sheet grounded below sea level (e.g., the West Antarctic Ice Sheet). The analogue models, with dimensions (width, length, thickness) of 120x70x1.5cm were performed at the Tectonic Modelling Laboratory of CNR-IGG of Florence, Italy, by using Polydimethilsyloxane (PDMS) as analogue for the flowing ice. This transparent, Newtonian silicone has been shown to well approximate the rheology of natural ice. The silicone was allowed to flow into a water reservoir simulating natural conditions in which ice streams flow into the sea, terminating in extensive ice shelves which act as a buttress for their glaciers and slow their flow. The geometric scaling ratio was 10(-5), such that 1cm in the models simulated 1km in nature; velocity of PDMS (a few mm per hour) simulated natural velocities of 100-1000 m/year. Instability of glacier flow was induced by manually removing a basal silicone platform (floating on water) exerting backstresses to the flowing analogue glacier: the simple set-up adopted in the experiments isolates the effect of the removal of the buttressing effect that the floating platform exerts on the flowing glaciers, thus offering insights into the influence of this parameter on the flow perturbations resulting from a collapse event. The experimental results showed a significant increase in glacier velocity close to its outlet following ice shelf breakup, a process similar to what

  6. The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 1: Model description

    NASA Astrophysics Data System (ADS)

    Winkelmann, R.; Martin, M. A.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

    2010-08-01

    We present the Potsdam Parallel Ice Sheet Model (PISM-PIK), developed at the Potsdam Institute for Climate Impact Research to be used for simulations of large-scale ice sheet-shelf systems. It is derived from the Parallel Ice Sheet Model (Bueler and Brown, 2009). Velocities are calculated by superposition of two shallow stress balance approximations within the entire ice covered region: the shallow ice approximation (SIA) is dominant in grounded regions and accounts for shear deformation parallel to the geoid. The plug-flow type shallow shelf approximation (SSA) dominates the velocity field in ice shelf regions and serves as a basal sliding velocity in grounded regions. Ice streams naturally emerge through this approach and can be identified diagnostically as regions with a significant contribution of membrane stresses to the local momentum balance. All lateral boundaries in PISM-PIK are free to evolve, including the grounding line and ice fronts. Ice shelf margins in particular are modeled using Neumann boundary conditions for the SSA equations, reflecting a hydrostatic stress imbalance along the vertical calving face. The ice front position is modeled using a subgrid scale representation of calving front motion (Albrecht et al., 2010) and a physically motivated dynamic calving law based on horizontal spreading rates. The model is validated within the Marine Ice Sheet Model Intercomparison Project (MISMIP) and is used for a dynamic equilibrium simulation of Antarctica under present-day conditions in the second part of this paper (Martin et al., 2010).

  7. Retrieving Ice Basal Motion Using the Hydrologically Coupled JPL/UCI Ice Sheet System Model (ISSM)

    NASA Astrophysics Data System (ADS)

    Khakbaz, B.; Morlighem, M.; Seroussi, H. L.; Larour, E. Y.

    2011-12-01

    The study of basal sliding in ice sheets requires coupling ice-flow models with subglacial water flow. In fact, subglacial hydrology models can be used to model basal water-pressure explicitly and to generate basal sliding velocities. This study addresses the addition of a thin-film-based subglacial hydrologic module to the Ice Sheet System Model (ISSM) developed by JPL in collaboration with the University of California Irvine (UCI). The subglacial hydrology model follows the study of J. Johnson (2002) who assumed a non-arborscent distributed drainage system in the form of a thin film beneath ice sheets. The differential equation that arises from conservation of mass in the water system is solved numerically with the finite element method in order to obtain the spatial distribution of basal water over the study domain. The resulting sheet water thickness is then used to model the basal water-pressure and subsequently the basal sliding velocity. In this study, an introduction and preliminary results of the subglacial water flow and basal sliding velocity will be presented for the Pine Island Glacier west Antarctica.This work was performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Modeling, Analysis and Prediction (MAP) Program.

  8. Nature and History of Cenozoic Polar Ice Covers: The Case of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Spielhagen, R.; Thiede, J.

    2009-04-01

    The nature of the modern climate System is characterized by steep temperature gradients between the tropical and polar climatic zones and finds its most spectacular expression in the formation of ice caps in high Northern and Southern latitudes. While polar regions of Planet Earth have been glaciated repeatedly in the long course of their geological history, the Cenozoic transition from a „greenhouse" to an „icehouse" has in fact produced a unique climatic scenario with bipolar glacation, different from all previous glacial events. The Greenland ice sheet is a remainder of the Northern Hemisphere last glacial maximum ice sheets and represents hence a spectacular anomaly. Geological records from Tertiary and Quaternary terrestrial and oceanic sections have documented the presence of ice caps and sea ice covers both on the Southern as well on the Northern hemisphere since Eocene times, aqpprox. 45 Mio. years ago. While this was well known in the case of Antarctica already for some time, previous ideas about the origin of Northern hemisphere glaciation during Pliocene times (approx. 2-3 Mio. years ago) have been superceded by the dramatic findings of coarse, terrigenous ice rafted detritus in Eocene sediments from Lomonosov Ridge (close to the North Pole) apparently slightly older than the oldest Antarctic records of ice rafting.The histories of the onset of Cenozoic glaciation in high Northern and Southern latitudes remain enigmatic and are presently subjects of international geological drilling projects, with prospects to reveal some of their secrets over the coming decades. By virtue of the physical porperties of ice and the processes controlling the dynamics of the turn-over of the ice-sheets only young records of glacial ice caps on Antarctica and on Greemnland have been preserved, on Greenland with ice probably not older than a few hundred thousand years, on Antarctica potentially as old as 1.5-2 Mio. years. Deep-sea cores with their records od ice

  9. Holocene Accumulation and Ice Flow near the West Antarctic Ice Sheet Divide Ice Core Site

    NASA Technical Reports Server (NTRS)

    Koutnik, Michelle R.; Fudge, T.J.; Conway, Howard; Waddington, Edwin D.; Neumann, Thomas A.; Cuffey, Kurt M.; Buizert, Christo; Taylor, Kendrick C.

    2016-01-01

    The West Antarctic Ice Sheet Divide Core (WDC) provided a high-resolution climate record from near the Ross-Amundsen Divide in Central West Antarctica. In addition, radar-detected internal layers in the vicinity of the WDC site have been dated directly from the ice core to provide spatial variations in the age structure of the region. Using these two data sets together, we first infer a high-resolution Holocene accumulation-rate history from 9.2 thousand years of the ice-core timescale and then confirm that this climate history is consistent with internal layers upstream of the core site. Even though the WDC was drilled only 24 kilometers from the modern ice divide, advection of ice from upstream must be taken into account. We evaluate histories of accumulation rate by using a flowband model to generate internal layers that we compare to observed layers. Results show that the centennially averaged accumulation rate was over 20 percent lower than modern at 9.2 thousand years before present (B.P.), increased by 40 percent from 9.2 to 2.3 thousand years B.P., and decreased by at least 10 percent over the past 2 thousand years B.P. to the modern values; these Holocene accumulation-rate changes in Central West Antarctica are larger than changes inferred from East Antarctic ice-core records. Despite significant changes in accumulation rate, throughout the Holocene the regional accumulation pattern has likely remained similar to today, and the ice-divide position has likely remained on average within 5 kilometers of its modern position. Continent-scale ice-sheet models used for reconstructions of West Antarctic ice volume should incorporate this accumulation history.

  10. Polar Ice Caps: a Canary for the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Honsaker, W.; Lowell, T. V.; Sagredo, E.; Kelly, M. A.; Hall, B. L.

    2010-12-01

    Ice caps are glacier masses that are highly sensitive to climate change. Because of their hypsometry they can have a binary state. When relatively slight changes in the equilibrium line altitude (ELA) either intersect or rise above the land the ice can become established or disappear. Thus these upland ice masses have a fast response time. Here we consider a way to extract the ELA signal from independent ice caps adjacent to the Greenland Ice Sheet margin. It may be that these ice caps are sensitive trackers of climate change that also impact the ice sheet margin. One example is the Istorvet Ice Cap located in Liverpool Land, East Greenland (70.881°N, 22.156°W). The ice cap topography and the underlying bedrock surface dips to the north, with peak elevation of the current ice ranging in elevation from 1050 to 745 m.a.s.l. On the eastern side of the ice mass the outlet glaciers extending down to sea level. The western margin has several small lobes in topographic depressions, with the margin reaching down to 300 m.a.s.l. Topographic highs separate the ice cap into at least 5 main catchments, each having a pair of outlet lobes toward either side of the ice cap. Because of the regional bedrock slope each catchment has its own elevation range. Therefore, as the ELA changes it is possible for some catchments of the ice cap to experience positive mass balance while others have a negative balance. Based on weather observations we estimate the present day ELA to be ~1000 m.a.s.l, meaning mass balance is negative for the majority of the ice cap. By tracking glacier presence/absence in these different catchments, we can reconstruct small changes in the ELA. Another example is the High Ice Cap (informal name) in Milne Land (70.903°N, 25.626°W, 1080 m), East Greenland. Here at least 4 unconformities in ice layers found near the southern margin of the ice cap record changing intervals of accumulation and ablation. Therefore, this location may also be sensitive to slight

  11. Implementing an empirical scalar constitutive relation for ice with flow-induced polycrystalline anisotropy in large-scale ice sheet models

    NASA Astrophysics Data System (ADS)

    Graham, Felicity S.; Morlighem, Mathieu; Warner, Roland C.; Treverrow, Adam

    2018-03-01

    The microstructure of polycrystalline ice evolves under prolonged deformation, leading to anisotropic patterns of crystal orientations. The response of this material to applied stresses is not adequately described by the ice flow relation most commonly used in large-scale ice sheet models - the Glen flow relation. We present a preliminary assessment of the implementation in the Ice Sheet System Model (ISSM) of a computationally efficient, empirical, scalar, constitutive relation which addresses the influence of the dynamically steady-state flow-compatible induced anisotropic crystal orientation patterns that develop when ice is subjected to the same stress regime for a prolonged period - sometimes termed tertiary flow. We call this the ESTAR flow relation. The effect on ice flow dynamics is investigated by comparing idealised simulations using ESTAR and Glen flow relations, where we include in the latter an overall flow enhancement factor. For an idealised embayed ice shelf, the Glen flow relation overestimates velocities by up to 17 % when using an enhancement factor equivalent to the maximum value prescribed in the ESTAR relation. Importantly, no single Glen enhancement factor can accurately capture the spatial variations in flow across the ice shelf generated by the ESTAR flow relation. For flow line studies of idealised grounded flow over varying topography or variable basal friction - both scenarios dominated at depth by bed-parallel shear - the differences between simulated velocities using ESTAR and Glen flow relations depend on the value of the enhancement factor used to calibrate the Glen flow relation. These results demonstrate the importance of describing the deformation of anisotropic ice in a physically realistic manner, and have implications for simulations of ice sheet evolution used to reconstruct paleo-ice sheet extent and predict future ice sheet contributions to sea level.

  12. Extensive Holocene ice sheet grounding line retreat and uplift-driven readvance in West Antarctica

    NASA Astrophysics Data System (ADS)

    Kingslake, J.; Scherer, R. P.; Albrecht, T.; Coenen, J. J.; Powell, R. D.; Reese, R.; Stansell, N.; Tulaczyk, S. M.; Whitehouse, P. L.

    2017-12-01

    The West Antarctic Ice Sheet (WAIS) reached its Last Glacial Maximum (LGM) extent 29-14 kyr before present. Numerical models used to project future ice-sheet contributions to sea-level rise exploit reconstructions of post-LGM ice mass loss to tune model parameterizations. Ice-sheet reconstructions are poorly constrained in areas where floating ice shelves or a lack of exposed geology obstruct conventional glacial-geological techniques. In the Weddell and Ross Sea sectors, ice-sheet reconstructions have traditionally assumed progressive grounding line (GL) retreat throughout the Holocene. Contrasting this view, using three distinct lines of evidence, we show that the GL retreated hundreds of kilometers inland of its present position, before glacial isostatic rebound during the Mid to Late Holocene caused the GL to readvance to its current position. Evidence for retreat and readvance during the last glacial termination includes (1) widespread radiocarbon in sediment cores recovered from beneath ice streams along the Siple and Gould Coasts, indicating marine exposure at least 200 km inland of the current GL, (2) ice-penetrating radar observations of relic crevasses and other englacial structures preserved in slow-moving grounded ice, indicating ice-shelf grounding and (3) an ensemble of new ice-sheet simulations showing widespread post-LGM retreat of the GL inland of its current location and later readvance. The model indicates that GL readvance across low slope ice-stream troughs requires uplift-driven grounding of the ice shelf on topographic highs (ice rises). Our findings highlight ice-shelf pinning points and lithospheric response to unloading as drivers of major ice-sheet fluctuations. Full WAIS collapse likely requires GL retreat well beyond its current position in the Ronne and Ross Sectors and linkage via Amundsen Sea sector glaciers.

  13. Latest Word on Retreat of the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    Bindschadler, R.

    2000-01-01

    The West Antarctic ice sheet during the Last Glacial Maximum (LGM) is estimated to have been three times its present volume and to have extended close to the edge of the continental shelf Holocene retreat of this ice sheet in the Ross Sea began between 11,000 and 12,000 years ago. This history implies an average contribution of this ice sheet to sea level of 0.9 mm/a. Evidence of dateable past grounding line positions in the Ross sector are broadly consistent with a linear retreat model. However, inferred rates of retreat for some of these grounding line positions are not consistent with a linear retreat model. More rapid retreat approximately 7600 years ago and possible near-stability in the Ross Sea sector at present suggest a slow rate of initial retreat followed by a more rapid-than-average retreat during the late Holocene, returning to a near-zero rate of retreat currently. This model is also consistent with the mid-Holocene high stand observations of eustatic sea level. Recent compilation of Antarctic bed elevations (BEDMAP) illustrates that the LGM and present grounding lines occur in the shallowest waters, further supporting the model of a middle phase of rapid retreat bracketed by an older and a more recent phase of modest retreat. Extension of these hypotheses into the future make subsequent behavior of the West Antarctic ice sheet more difficult to predict but suggest that if it loses its hold on the present shallow bed, the final retreat of the ice sheet could be very rapid.

  14. Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage.

    PubMed

    Sundal, Aud Venke; Shepherd, Andrew; Nienow, Peter; Hanna, Edward; Palmer, Steven; Huybrechts, Philippe

    2011-01-27

    Fluctuations in surface melting are known to affect the speed of glaciers and ice sheets, but their impact on the Greenland ice sheet in a warming climate remains uncertain. Although some studies suggest that greater melting produces greater ice-sheet acceleration, others have identified a long-term decrease in Greenland's flow despite increased melting. Here we use satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting. Although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slowdown occurs, on average, when a critical run-off threshold of about 1.4 centimetres a day is exceeded. In contrast to the first half of summer, when flow is similar in all years, speed-up during the latter half is 62 ± 16 per cent less in warmer years. Consequently, in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower. This behaviour is at odds with that expected from basal lubrication alone. Instead, it mirrors that of mountain glaciers, where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient. A model of ice-sheet flow that captures switching between cavity and channel drainage modes is consistent with the run-off threshold, fast-flow periods, and later-summer speeds we have observed. Simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage; a simple model of basal lubrication alone misses key aspects of the ice sheet's response to climate warming.

  15. Holocene temperature history at the west Greenland Ice Sheet margin reconstructed from lake sediments

    NASA Astrophysics Data System (ADS)

    Axford, Y.; Losee, S.; Briner, J. P.; Francis, D.; Langdon, P. G.; Walker, I.

    2011-12-01

    Paleoclimate proxy data can help reduce uncertainties regarding how the Greenland Ice Sheet, and thus global sea level, will respond to future climate change. Studies of terrestrial deposits along Greenland's margins offer opportunities to reconstruct both past temperature changes and the associated changes in Greenland Ice Sheet extent, thus empirically characterizing the ice sheet's response to temperature change. Here we present Holocene paleoclimate reconstructions developed from sediment records of five lakes along the western ice sheet margin, near Jakobshavn Isbræ and Disko Bugt. Insect (Chironomidae, or non-biting midge) remains from North Lake provide quantitative estimates of summer temperatures over the past ca. 7500 years at multi-centennial resolution, and changes in sediment composition at all five lakes offer evidence for glacier fluctuations, changes in lake productivity, and other environmental changes throughout the Holocene. Aims of this study include quantification of warmth in the early to mid Holocene, when summer solar insolation forcing exceeded present-day values at northern latitudes and the local Greenland Ice Sheet margin receded inboard of its present position, and the magnitude of subsequent Neoglacial and Little Ice Age cooling that drove ice sheet expansion. We find that the Jakobshavn Isbrae region experienced the warmest temperatures of the Holocene (with summers 2 to 3.5 degrees C warmer than present) between ~6000 and 4000 years ago. Neoglacial cooling began rather abruptly ~4000 years ago and intensified 3000 years ago. Our proxy data suggest that the coldest summers of the Holocene occurred during the 18th and 19th centuries in the Jakobshavn region. These results agree well with previous glacial geologic studies reconstructing local ice margin positions through the Holocene. Such reconstructions of paleoclimate and past ice sheet extent provide targets for testing and improving ice sheet models.

  16. Holocene thinning of the Greenland ice sheet.

    PubMed

    Vinther, B M; Buchardt, S L; Clausen, H B; Dahl-Jensen, D; Johnsen, S J; Fisher, D A; Koerner, R M; Raynaud, D; Lipenkov, V; Andersen, K K; Blunier, T; Rasmussen, S O; Steffensen, J P; Svensson, A M

    2009-09-17

    On entering an era of global warming, the stability of the Greenland ice sheet (GIS) is an important concern, especially in the light of new evidence of rapidly changing flow and melt conditions at the GIS margins. Studying the response of the GIS to past climatic change may help to advance our understanding of GIS dynamics. The previous interpretation of evidence from stable isotopes (delta(18)O) in water from GIS ice cores was that Holocene climate variability on the GIS differed spatially and that a consistent Holocene climate optimum-the unusually warm period from about 9,000 to 6,000 years ago found in many northern-latitude palaeoclimate records-did not exist. Here we extract both the Greenland Holocene temperature history and the evolution of GIS surface elevation at four GIS locations. We achieve this by comparing delta(18)O from GIS ice cores with delta(18)O from ice cores from small marginal icecaps. Contrary to the earlier interpretation of delta(18)O evidence from ice cores, our new temperature history reveals a pronounced Holocene climatic optimum in Greenland coinciding with maximum thinning near the GIS margins. Our delta(18)O-based results are corroborated by the air content of ice cores, a proxy for surface elevation. State-of-the-art ice sheet models are generally found to be underestimating the extent and changes in GIS elevation and area; our findings may help to improve the ability of models to reproduce the GIS response to Holocene climate.

  17. Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

    PubMed Central

    Yallop, Marian L; Anesio, Alexandre M; Perkins, Rupert G; Cook, Joseph; Telling, Jon; Fagan, Daniel; MacFarlane, James; Stibal, Marek; Barker, Gary; Bellas, Chris; Hodson, Andy; Tranter, Martyn; Wadham, Jemma; Roberts, Nicholas W

    2012-01-01

    Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet. PMID:23018772

  18. Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet.

    PubMed

    Yallop, Marian L; Anesio, Alexandre M; Perkins, Rupert G; Cook, Joseph; Telling, Jon; Fagan, Daniel; MacFarlane, James; Stibal, Marek; Barker, Gary; Bellas, Chris; Hodson, Andy; Tranter, Martyn; Wadham, Jemma; Roberts, Nicholas W

    2012-12-01

    Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet.

  19. Channelized melting drives thinning under Dotson ice shelf, Western Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Gourmelen, N.; Goldberg, D.; Snow, K.; Henley, S. F.; Bingham, R. G.; Kimura, S.; Hogg, A.; Shepherd, A.; Mouginot, J.; Lenaerts, J.; Ligtenberg, S.; Van De Berg, W. J.

    2017-12-01

    The majority of meteoric ice that forms in West Antarctica leaves the ice sheet through floating ice shelves, many of which have been thinning substantially over the last 25 years. A significant proportion of ice-shelf thinning has been driven by submarine melting facilitated by increased access of relatively warm (>0.6oC) modified Circumpolar Deep Water to sub-shelf cavities. Ice shelves play a significant role in stabilising the ice sheet from runaway retreat and regulating its contribution to sea level change. Ice-shelf melting has also been implicated in sustaining high primary productivity in Antarctica's coastal seas. However, these processes vary regionally and are not fully understood. Under some ice shelves, concentrated melting leads to the formation of inverted channels. These channels guide buoyant melt-laden outflow, which can lead to localised melting of the sea ice cover. The channels may also potentially lead to heightened crevassing, which in turn affects ice-shelf stability. Meanwhile, numerical studies suggest that buttressing loss is sensitive to the location of ice removal within an ice-shelf. Thus it is important that we observe spatial patterns, as well as magnitudes, of ice-shelf thinning, in order to improve understanding of the ocean drivers of thinning and of their impacts on ice-shelf stability. Here we show from high-resolution altimetry measurements acquired between 2010 to 2016 that Dotson Ice Shelf, West Antarctica, thins in response to basal melting focussed along a single 5 km-wide and 60 km-long channel extending from the ice shelf's grounding zone to its calving front. The coupled effect of geostrophic circulation and ice-shelf topography leads to the observed concentration of basal melting. Analysis of previous datasets suggests that this process has been ongoing for at least the last 25 years. If focused thinning continues at present rates, the channel would melt through within 40-50 years, almost two centuries before it is

  20. A Newly Updated Database of Elevation-changes of the Greenand Ice Sheet to Study Surface Processes and Ice Dynamics

    NASA Astrophysics Data System (ADS)

    Schenk, A. F.; Csatho, B. M.; van den Broeke, M.; Kuipers Munneke, P.

    2015-12-01

    This paper reports about important upgrades of the Greenland Ice Sheet (GrIS) surface elevation and elevation-change database obtained with our Surface Elevation And Change detection (SERAC) software suite. We have developed SERAC to derive information from laser altimetry data, particularly time series of elevation changes and their partitioning into changes caused by ice dynamics. This allows direct investigation of ice dynamic processes that is much needed for improving the predictive power of ice sheet models. SERAC is different from most other change detection methods. It is based on detecting changes of surface patches, about 1 km by 1 km in size, rather than deriving elevation changes from individual laser points. The current database consists of ~100,000 time series with satellite laser altimetry data from ICESat, airborne laser observations obtained by NASA's Airborne Topographic Mapper (ATM) and the Land, Vegetation and Ice Sensor (LVIS). The upgrade is significant, because not only new observations from 2013 and 2014 have been added but also a number of improvements lead to a more comprehensive and consistent record of elevation-changes. First, we used the model that gives in addition to ice sheet also information about ice caps and glaciers (Rastner et al., 2012) for deciding if a laser point is on the ice sheet or ice cap. Then we added small gaps that exist in the ICESat GLA12 data set because the ice sheet mask is not wide enough. The new database is now more complete and will facilitate more accurate comparisons of mass balance studies obtained from the Gravity Recovery and Climate Experiment system (GRACE). For determining the part of a time series caused by ice dynamics we used the new firn compaction model and Surface Mass Balance (SMB) estimates from RACMO2.3. The new database spans the time period from 1993 to 2014. Adding new observations amounts to a spatial densification of the old record and at the same time extends the time domain by two

  1. Mass Balance of the Greenland Ice Sheet at High Elevations.

    PubMed

    Thomas; Akins; Csatho; Fahnestock; Gogineni; Kim; Sonntag

    2000-07-21

    Comparison of ice discharge from higher elevation areas of the entire Greenland Ice Sheet with total snow accumulation gives estimates of ice thickening rates over the past few decades. On average, the region has been in balance, but with thickening of 21 centimeters per year in the southwest and thinning of 30 centimeters per year in the southeast. The north of the ice sheet shows less variability, with average thickening of 2 centimeters per year in the northeast and thinning of about 5 centimeters per year in the northwest. These results agree well with those from repeated altimeter surveys, except in the extreme south, where we find substantially higher rates of both thickening and thinning.

  2. Moulin Migration and Development on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Chu, V. W.; Yang, L.

    2017-12-01

    Extensive river networks that terminate into moulins efficiently drain the surface of the Greenland ice sheet. These river moulins connect surface meltwater to englacial and subglacial drainage networks, where increased meltwater can enhance ice sliding dynamics. Previous moulin studies were limited to small geographic areas using field observations and/or high-resolution aerial/satellite imagery, or to medium-resolution satellite imagery for larger areas. In this study, high-resolution moulin maps created from WorldView-1/2/3 imagery near Russell Glacier in southwest Greenland show development of moulins and their migration between 2012 and 2015. Moulins are mapped and categorized as being located: in crevasse fields, along a single ice fracture, within drained lake basins, or having no visible formation mechanism. A majority of moulins mapped in 2015 (73%) are linked to moulins in 2012 and are analysed for their movement patterns and compared to ice velocity and strain rates. New moulins most commonly form in crevassed, thinner ice near the ice sheet edge, but significant quantities also develop at higher elevations (22% above 1300 m elevation).

  3. The Gamburtsev mountains and the origin and early evolution of the Antarctic Ice Sheet.

    PubMed

    Bo, Sun; Siegert, Martin J; Mudd, Simon M; Sugden, David; Fujita, Shuji; Xiangbin, Cui; Yunyun, Jiang; Xueyuan, Tang; Yuansheng, Li

    2009-06-04

    Ice-sheet development in Antarctica was a result of significant and rapid global climate change about 34 million years ago. Ice-sheet and climate modelling suggest reductions in atmospheric carbon dioxide (less than three times the pre-industrial level of 280 parts per million by volume) that, in conjunction with the development of the Antarctic Circumpolar Current, led to cooling and glaciation paced by changes in Earth's orbit. Based on the present subglacial topography, numerical models point to ice-sheet genesis on mountain massifs of Antarctica, including the Gamburtsev mountains at Dome A, the centre of the present ice sheet. Our lack of knowledge of the present-day topography of the Gamburtsev mountains means, however, that the nature of early glaciation and subsequent development of a continental-sized ice sheet are uncertain. Here we present radar information about the base of the ice at Dome A, revealing classic Alpine topography with pre-existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 degrees C. This landscape is likely to have developed during the initial phases of Antarctic glaciation. According to Antarctic climate history (estimated from offshore sediment records) the Gamburtsev mountains are probably older than 34 million years and were the main centre for ice-sheet growth. Moreover, the landscape has most probably been preserved beneath the present ice sheet for around 14 million years.

  4. Capturing total chronological and spatial uncertainties in palaeo-ice sheet reconstructions: the DATED example

    NASA Astrophysics Data System (ADS)

    Hughes, Anna; Gyllencreutz, Richard; Mangerud, Jan; Svendsen, John Inge

    2017-04-01

    Glacial geologists generate empirical reconstructions of former ice-sheet dynamics by combining evidence from the preserved record of glacial landforms (e.g. end moraines, lineations) and sediments with chronological evidence (mainly numerical dates derived predominantly from radiocarbon, exposure and luminescence techniques). However the geomorphological and sedimentological footprints and chronological data are both incomplete records in both space and time, and all have multiple types of uncertainty associated with them. To understand ice sheets' response to climate we need numerical models of ice-sheet dynamics based on physical principles. To test and/or constrain such models, empirical reconstructions of past ice sheets that capture and acknowledge all uncertainties are required. In 2005 we started a project (Database of the Eurasian Deglaciation, DATED) to produce an empirical reconstruction of the evolution of the last Eurasian ice sheets, (including the British-Irish, Scandinavian and Svalbard-Barents-Kara Seas ice sheets) that is fully documented, specified in time, and includes uncertainty estimates. Over 5000 dates relevant to constraining ice build-up and retreat were assessed for reliability and used together with published ice-sheet margin positions based on glacial geomorphology to reconstruct time-slice maps of the ice sheets' extent. The DATED maps show synchronous ice margins with maximum-minimum uncertainty bounds for every 1000 years between 25-10 kyr ago. In the first version of results (DATED-1; Hughes et al. 2016) all uncertainties (both quantitative and qualitative, e.g. precision and accuracy of numerical dates, correlation of moraines, stratigraphic interpretations) were combined based on our best glaciological-geological assessment and expressed in terms of distance as a 'fuzzy' margin. Large uncertainties (>100 km) exist; predominantly across marine sectors and other locations where there are spatial gaps in the dating record (e.g. the

  5. Development and applications of a radar-attenuation model for polar ice sheets

    NASA Astrophysics Data System (ADS)

    MacGregor, Joseph A.

    Modern ice sheets are currently responding to significant climatic forcings and undergoing ice-dynamics changes that are not yet well understood. Ice-penetrating radar surveys are often used to infer their basal condition (e.g., is the bed wet or dry?) and internal properties. However, such inferences typically require a model of the electromagnetic attenuation through the ice sheet. Here I first develop and test a radar-attenuation model that is based on a synthesis of existing laboratory measurements of the dielectric properties of ice. This synthesis shows that radar attenuation in polar ice has a strong non-linear temperature dependence and a weaker linear dependence on the concentrations of acid and sea-salt chloride. This model was tested at Siple Dome, West Antarctica, using ice-core-chemistry and borehole-temperature data, and the model agreed well with an existing radar-attenuation measurement. I then use this model to investigate the nature of radar detection of accreted ice over Lake Vostok, East Antarctica. My analysis of ice-core and radar data found that the observed reflection is likely due to a fabric contrast near the boundary between the dirty and clean accreted ices. This reflection mechanism is also consistent with the spatial pattern of detection of the reflection. In anticipation of the requirements of a thermomechanical ice-sheet model to predict the spatial variation of attenuation over Lake Vostok, I develop an accumulation-rate map for the Lake Vostok region using radar data, a steady-state flow-band model, and inverse methods. I found that accumulation rates there are not inversely correlated with surface elevation, that there is a broad maximum above the lake's northwestern corner, and a minimum above most of its eastern shoreline. Finally, I investigate the spatial variability of attenuation in an ice sheet, using the flowline that crosses through the Vostok ice core as an example. I use radar layers and ice-velocity and temperature

  6. Ice sheet runoff and Dansgaard-Oeschger Cycles

    NASA Astrophysics Data System (ADS)

    Hewitt, Ian; Wolff, Eric; Fowler, Andrew; Clark, Chris; Evatt, Geoff; Johnson, Helen; Munday, David; Rickaby, Ros; Stokes, Chris

    2016-04-01

    Many northern hemisphere climate records, particularly those from around the North Atlantic, show a series of rapid climate changes that recurred on centennial to millennial timescales throughout most of the last glacial period. These Dansgaard-Oeschger (D-O) sequences are observed most prominently in Greenland ice cores, although they have a global signature, including an out of phase Antarctic signal. They consist of warming jumps of order 10°C, occurring in typically 40 years, followed generally by a slow cooling (Greenland Interstadial, GI) lasting between a few centuries and a few millennia, and then a final rapid temperature drop into a cold Greenland Stadial (GS) that lasts for a similar period. Most explanations for D-O events call on changes in Atlantic meridional overturning circulation strength, and the majority of such explanations use changes in freshwater delivery from ice sheets as a trigger. Many have relied on large inputs of freshwater from singular events (such as lake outbursts or iceberg armadas) to push the AMOC into its cold state. However the evidence for such events at the right time in each cycle is sparse. Here we investigate mechanisms that would arise from a change in the rate of ice sheet runoff, which would be a natural feedback from each rapid warming or cooling event. Recent work has suggested that AMOC is most easily disrupted by freshwater delivered through the Arctic. We investigate whether the proposed AMOC changes could have occurred as part of a natural oscillation, in which runoff from the Laurentide ice sheet into the Arctic is controlled by temperature around the North Atlantic. The Arctic buffers the salinity changes, but under warm conditions, high runoff eventually leads to water entering the North Atlantic with low enough salinity to switch AMOC into its weaker state. Under the colder conditions now prevailing, the Arctic is starved of runoff, and the salinity rises until a further switch occurs. Contrary to many

  7. On the scalability of the Albany/FELIX first-order Stokes approximation ice sheet solver for large-scale simulations of the Greenland and Antarctic ice sheets

    DOE PAGES

    Tezaur, Irina K.; Tuminaro, Raymond S.; Perego, Mauro; ...

    2015-01-01

    We examine the scalability of the recently developed Albany/FELIX finite-element based code for the first-order Stokes momentum balance equations for ice flow. We focus our analysis on the performance of two possible preconditioners for the iterative solution of the sparse linear systems that arise from the discretization of the governing equations: (1) a preconditioner based on the incomplete LU (ILU) factorization, and (2) a recently-developed algebraic multigrid (AMG) preconditioner, constructed using the idea of semi-coarsening. A strong scalability study on a realistic, high resolution Greenland ice sheet problem reveals that, for a given number of processor cores, the AMG preconditionermore » results in faster linear solve times but the ILU preconditioner exhibits better scalability. In addition, a weak scalability study is performed on a realistic, moderate resolution Antarctic ice sheet problem, a substantial fraction of which contains floating ice shelves, making it fundamentally different from the Greenland ice sheet problem. We show that as the problem size increases, the performance of the ILU preconditioner deteriorates whereas the AMG preconditioner maintains scalability. This is because the linear systems are extremely ill-conditioned in the presence of floating ice shelves, and the ill-conditioning has a greater negative effect on the ILU preconditioner than on the AMG preconditioner.« less

  8. Antarctic and Greenland ice sheet mass balance products from satellite gravimetry

    NASA Astrophysics Data System (ADS)

    Horwath, Martin; Groh, Andreas; Horvath, Alexander; Forsberg, René; Meister, Rakia; Barletta, Valentina R.; Shepherd, Andrew

    2017-04-01

    Because of their important role in the Earth's climate system, ESA's Climate Change Initiative (CCI) has identified both the Antarctic Ice Sheet (AIS) and the Greenland Ice Sheet (GIS) as Essential Climate Variables (ECV). Since respondents of a user survey indicated that the ice sheet mass balance is one of the most important ECV data products needed to better understand climate change, the AIS_cci and the GIS_cci project provide Gravimetric Mass Balance (GMB) products based on satellite gravimetry data. The GMB products are derived from GRACE (Gravity Recovery and Climate Experiment) monthly solutions of release ITSG-Grace2016 produced at TU Graz. GMB basin products (i.e. time series of monthly mass changes for the entire ice sheets and selected drainage basins) and GMB gridded products (e.g. mass balance estimates with a formal resolution of about 50km, covering the entire ice sheets) are generated for the period from 2002 until present. The first GMB product was released in mid 2016. Here we present an extended and updated version of the ESA CCI GMB products, which are freely available through data portals hosted by the projects (https://data1.geo.tu-dresden.de/ais_gmb, http://products.esa-icesheets-cci.org/products/downloadlist/GMB). Since the initial product release, the applied processing strategies have been improved in order to further reduce GRACE errors and to enhance the separation of signals super-imposed to the ice mass changes. While a regional integration approach is used by the AIS_cci project, the GMB products of the GIS_cci project are derived using a point mass inversion. The differences between both approaches are investigated through the example of the GIS, where an alternative GMB product was generated using the regional integration approach implemented by the AIS_cci. Finally, we present the latest mass balance estimates for both ice sheets as well as their corresponding contributions to global sea level rise.

  9. The Antarctic Ice Sheet, Sea Ice, and the Ozone Hole: Satellite Observations of how they are Changing

    NASA Technical Reports Server (NTRS)

    Parkinson, Claire L.

    2012-01-01

    Antarctica is the Earth's coldest and highest continent and has major impacts on the climate and life of the south polar vicinity. It is covered almost entirely by the Earth's largest ice sheet by far, with a volume of ice so great that if all the Antarctic ice were to go into the ocean (as ice or liquid water), this would produce a global sea level rise of about 60 meters (197 feet). The continent is surrounded by sea ice that in the wintertime is even more expansive than the continent itself and in the summertime reduces to only about a sixth of its wintertime extent. Like the continent, the expansive sea ice cover has major impacts, reflecting the sun's radiation back to space, blocking exchanges between the ocean and the atmosphere, and providing a platform for some animal species while impeding other species. Far above the continent, the Antarctic ozone hole is a major atmospheric phenomenon recognized as human-caused and potentially quite serious to many different life forms. Satellites are providing us with remarkable information about the ice sheet, the sea ice, and the ozone hole. Satellite visible and radar imagery are providing views of the large scale structure of the ice sheet never seen before; satellite laser altimetry has produced detailed maps of the topography of the ice sheet; and an innovative gravity-measuring two-part satellite has allowed mapping of regions of mass loss and mass gain on the ice sheet. The surrounding sea ice cover has a satellite record that goes back to the 1970s, allowing trend studies that show a decreasing sea ice presence in the region of the Bellingshausen and Amundsen seas, to the west of the prominent Antarctic Peninsula, but increasing sea ice presence around much of the rest of the continent. Overall, sea ice extent around Antarctica has increased at an average rate of about 17,000 square kilometers per year since the late 1970s, as determined from satellite microwave data that can be collected under both light and

  10. Records of past ice sheet fluctuations in interior East Antarctica

    USGS Publications Warehouse

    Liu, Xiaohan; Huang, Feixin; Kong, Ping; Fang, Aimin; Li, Xiaoli

    2007-01-01

    The results of a land-based multi-disciplinary study of the past ice surface elevation in the Grove Mountains of interior East Antarctica support a dynamic evolution of the East Antarctic Ice Sheet (EAIS). Moraine boulders of sedimentary rocks and spore pollen assemblage imply a significant shrinkage of the EAIS, with its margin retreating south of the Grove Mountains (~450 km south of recent coast line) before the middle Pliocene. The exposure ages indicate that the ice sheet subsequently re-advanced, with the ice surface rising locally at least 450 m higher than today. It then went back down constantly from before 2.3 Ma to 1.6 Ma. The glacial topography and existence of soil show that the ice surface fluctuation continued since the early Quaternary, but with highest levels never exceeding ~100 m higher than today.

  11. Why Europa's icy shell may convect, but ice sheets do not: a glaciological perspective

    NASA Astrophysics Data System (ADS)

    Bassis, J. N.

    2016-12-01

    Jupiter's moon Europa is covered in an icy shell that lies over a liquid ocean. Geological evidence and numerical models suggest that Europa's icy shell convects, providing the possibility that Europa may experience a form of plate tectonics and could even harbor life in its subsurface ocean. The hypothesis that Europa convects is supported by both models and geological evidence. Surprisingly, when we apply similar calculations and (assumptions) used by planetary scientists to infer convection in icy moons like Europa we find that these models also predict that vigorous convection should also occur in portions of our own terrestrial ice sheets and ice shelves where we have firm evidence to the contrary. We can explain the lack of convection within our own ice sheets by recognizing that instead of the diffusion creep limited rheology frequently invoked by planetary scientists, terrestrial ice undergoes power-law creep down to very low strain rates. Glaciological studies find that power-law creep is required to explain the structure of vertical strain rate near ice sheet divides and shape of the ice sheets near an ice divide. However, when we now apply a rheology that is consistent with terrestrial ice sheet dynamics to icy moon conditions, we find conditions are far less favorable for convection in icy moons, with only a very limited parameter regime where convection can occur. Given the many unknowns (grain size, impurities etc.) it is challenging to draw strong conclusions about the behavior of icy moons . Nonetheless, the lack of convection in terrestrial ice sheets provides an important constraint on the dynamics of icy moons and models that explain convection of icy moons should also explain the lack of convection on terrestrial ice sheets.

  12. Extensive Liquid Meltwater Storage in Firn Within the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Forster, Richard R.; Box, Jason E.; vandenBroeke, Michael R.; Miege, Clement; Burgess, Evan W.; vanAngelen, Jan H.; Lenaerts, Jan T. M.; Koenig, Lora S.; Paden, John; Lewis, Cameron; hide

    2013-01-01

    The accelerating loss of mass from the Greenland ice sheet is a major contribution to current sea level rise. Increased melt water runoff is responsible for half of Greenlands mass loss increase. Surface melt has been increasing in extent and intensity, setting a record for surface area melt and runoff in 2012. The mechanisms and timescales involved in allowing surface melt water to reach the ocean where it can contribute to sea level rise are poorly understood. The potential capacity to store this water in liquid or frozen form in the firn (multi-year snow layer) is significant, and could delay its sea-level contribution. Here we describe direct observation of water within a perennial firn aquifer persisting throughout the winter in the southern ice sheet,where snow accumulation and melt rates are high. This represents a previously unknown storagemode for water within the ice sheet. Ice cores, groundairborne radar and a regional climatemodel are used to estimate aquifer area (70 plue or minus 10 x 10(exp 3) square kilometers ) and water table depth (5-50 m). The perennial firn aquifer represents a new glacier facies to be considered 29 in future ice sheet mass 30 and energy budget calculations.

  13. Assessing the accuracy of Greenland ice sheet ice ablation measurements by pressure transducer

    NASA Astrophysics Data System (ADS)

    Fausto, R. S.; van As, D.; Ahlstrøm, A. P.

    2012-04-01

    In the glaciological community there is a need for reliable mass balance measurements of glaciers and ice sheets, ranging from daily to yearly time scales. Here we present a method to measure ice ablation using a pressure transducer. The pressure transducer is drilled into the ice, en-closed in a hose filled with a liquid that is non-freezable at common Greenlandic temperatures. The pressure signal registered by the transducer is that of the vertical column of liquid over the sensor, which can be translated in depth knowing the density of the liquid. As the free-standing AWS moves down with the ablating surface and the hose melts out of the ice, an increasingly large part of the hose will lay flat on the ice surface, and the hydrostatic pressure from the vertical column of liquid in the hose will get smaller. This reduction in pressure provides us with the ablation rate. By measuring at (sub-) daily timescales this assembly is well-suited to monitor ice ablation in remote regions, with clear advantages over other well-established methods of measuring ice ablation in the field. The pressure transducer system has the potential to monitor ice ablation for several years without re-drilling and the system is suitable for high ablation areas. A routine to transform raw measurements into ablation values will also be presented, including a physically based method to remove air pressure variability from the signal. The pressure transducer time-series is compared to that recorded by a sonic ranger for the climatically hostile setting on the Greenland ice sheet.

  14. Quaternary evolution of the Fennoscandian Ice Sheet from 3D seismic data

    NASA Astrophysics Data System (ADS)

    Montelli, A.; Dowdeswell, J. A.; Ottesen, D.; Johansen, S. E.

    2016-12-01

    The Quaternary seismic stratigraphy and architecture of the mid-Norwegian continental shelf and slope are investigated using extensive grids of marine 2D and 3D seismic reflection data that cover more than 100,000 km2 of the continental margin. At least 26 distinct regional palaeo-surfaces have been interpreted within the stratigraphy of the Quaternary Naust Formation on the mid-Norwegian margin. Multiple assemblages of buried glacigenic landforms are preserved within the Naust Formation across most of the study area, facilitating detailed palaeo-glaciological reconstructions. We document a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the Norwegian shelf since at least the beginning of the Quaternary. Elongate, streamlined landforms interpreted as mega-scale glacial lineations (MSGLs) have been found within the upper part of the Naust sequence N ( 1.9-1.6 Ma), sugesting the development of fast-flowing ice streams since that time. Shifts in the location of depocentres and direction of features indicative of fast ice-flow suggest that several reorganisations in the FIS drainage have occurred since 1.5 Ma. Subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and internal ice-sheet structure. Lack of subglacial meltwater channels suggests a largely distributed, low-volume meltwater system that drained the FIS through permeable subglacial till without leaving much erosional evidence. This regional palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.

  15. An object-oriented, coprocessor-accelerated model for ice sheet simulations

    NASA Astrophysics Data System (ADS)

    Seddik, H.; Greve, R.

    2013-12-01

    Recently, numerous models capable of modeling the thermo-dynamics of ice sheets have been developed within the ice sheet modeling community. Their capabilities have been characterized by a wide range of features with different numerical methods (finite difference or finite element), different implementations of the ice flow mechanics (shallow-ice, higher-order, full Stokes) and different treatments for the basal and coastal areas (basal hydrology, basal sliding, ice shelves). Shallow-ice models (SICOPOLIS, IcIES, PISM, etc) have been widely used for modeling whole ice sheets (Greenland and Antarctica) due to the relatively low computational cost of the shallow-ice approximation but higher order (ISSM, AIF) and full Stokes (Elmer/Ice) models have been recently used to model the Greenland ice sheet. The advance in processor speed and the decrease in cost for accessing large amount of memory and storage have undoubtedly been the driving force in the commoditization of models with higher capabilities, and the popularity of Elmer/Ice (http://elmerice.elmerfem.com) with an active user base is a notable representation of this trend. Elmer/Ice is a full Stokes model built on top of the multi-physics package Elmer (http://www.csc.fi/english/pages/elmer) which provides the full machinery for the complex finite element procedure and is fully parallel (mesh partitioning with OpenMPI communication). Elmer is mainly written in Fortran 90 and targets essentially traditional processors as the code base was not initially written to run on modern coprocessors (yet adding support for the recently introduced x86 based coprocessors is possible). Furthermore, a truly modular and object-oriented implementation is required for quick adaptation to fast evolving capabilities in hardware (Fortran 2003 provides an object-oriented programming model while not being clean and requiring a tricky refactoring of Elmer code). In this work, the object-oriented, coprocessor-accelerated finite element

  16. Evaluation of a 12-km Satellite-Era Reanalysis of Surface Mass Balance for the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Cullather, R. I.; Nowicki, S.; Zhao, B.; Max, S.

    2016-12-01

    The recent contribution to sea level change from the Greenland Ice Sheet is thought to be strongly driven by surface processes including melt and runoff. Global reanalyses are potential means of reconstructing the historical time series of ice sheet surface mass balance (SMB), but lack spatial resolution needed to resolve ablation areas along the periphery of the ice sheet. In this work, the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) is used to examine the spatial and temporal variability of surface melt over the Greenland Ice Sheet. MERRA-2 is produced for the period 1980 to the present at a grid spacing of ½° latitude by ⅝° longitude, and includes snow hydrology processes including compaction, meltwater percolation and refreezing, runoff, and a prognostic surface albedo. The configuration of the MERRA-2 system allows for the background model - the Goddard Earth Observing System model, version 5 (GEOS-5) - to be carried in phase space through analyzed states via the computation of analysis increments, a capability referred to as "replay". Here, a MERRA-2 replay integration is conducted in which atmospheric forcing fields are interpolated and adjusted to sub- atmospheric grid-scale resolution. These adjustments include lapse-rate effects on temperature, humidity, precipitation, and other atmospheric variables that are known to have a strong elevation dependency over ice sheets. The surface coupling is performed such that mass and energy are conserved. The atmospheric forcing influences the surface representation, which operates on land surface tiles with an approximate 12-km spacing. This produces a high-resolution, downscaled SMB which is interactively coupled to the reanalysis model. We compare the downscaled SMB product with other reanalyses, regional climate model values, and a second MERRA-2 replay in which the background model has been replaced with a 12-km, non-hydrostatic version of GEOS-5. The assessment

  17. Physical Limits on Hmax, the Maximum Height of Glaciers and Ice Sheets

    NASA Astrophysics Data System (ADS)

    Lipovsky, B. P.

    2017-12-01

    The longest glaciers and ice sheets on Earth never achieve a topographic relief, or height, greater than about Hmax = 4 km. What laws govern this apparent maximum height to which a glacier or ice sheet may rise? Two types of answer appear possible: one relating to geological process and the other to ice dynamics. In the first type of answer, one might suppose that if Earth had 100 km tall mountains then there would be many 20 km tall glaciers. The counterpoint to this argument is that recent evidence suggests that glaciers themselves limit the maximum height of mountain ranges. We turn, then, to ice dynamical explanations for Hmax. The classical ice dynamical theory of Nye (1951), however, does not predict any break in scaling to give rise to a maximum height, Hmax. I present a simple model for the height of glaciers and ice sheets. The expression is derived from a simplified representation of a thermomechanically coupled ice sheet that experiences a basal shear stress governed by Coulomb friction (i.e., a stress proportional to the overburden pressure minus the water pressure). I compare this model to satellite-derived digital elevation map measurements of glacier surface height profiles for the 200,000 glaciers in the Randolph Glacier Inventory (Pfeffer et al., 2014) as well as flowlines from the Greenland and Antarctic Ice Sheets. The simplified model provides a surprisingly good fit to these global observations. Small glaciers less than 1 km in length are characterized by having negligible influence of basal melt water, cold ( -15C) beds, and high surface slopes ( 30 deg). Glaciers longer than a critical distance 30km are characterized by having an ice-bed interface that is weakened by the presence of meltwater and is therefore not capable of supporting steep surface slopes. The simplified model makes predictions of ice volume change as a function of surface temperature, accumulation rate, and geothermal heat flux. For this reason, it provides insights into

  18. Last Interglacial climate and sea-level evolution from a coupled ice sheet-climate model

    NASA Astrophysics Data System (ADS)

    Goelzer, Heiko; Huybrechts, Philippe; Loutre, Marie-France; Fichefet, Thierry

    2016-12-01

    As the most recent warm period in Earth's history with a sea-level stand higher than present, the Last Interglacial (LIG, ˜ 130 to 115 kyr BP) is often considered a prime example to study the impact of a warmer climate on the two polar ice sheets remaining today. Here we simulate the Last Interglacial climate, ice sheet, and sea-level evolution with the Earth system model of intermediate complexity LOVECLIM v.1.3, which includes dynamic and fully coupled components representing the atmosphere, the ocean and sea ice, the terrestrial biosphere, and the Greenland and Antarctic ice sheets. In this setup, sea-level evolution and climate-ice sheet interactions are modelled in a consistent framework.Surface mass balance change governed by changes in surface meltwater runoff is the dominant forcing for the Greenland ice sheet, which shows a peak sea-level contribution of 1.4 m at 123 kyr BP in the reference experiment. Our results indicate that ice sheet-climate feedbacks play an important role to amplify climate and sea-level changes in the Northern Hemisphere. The sensitivity of the Greenland ice sheet to surface temperature changes considerably increases when interactive albedo changes are considered. Southern Hemisphere polar and sub-polar ocean warming is limited throughout the Last Interglacial, and surface and sub-shelf melting exerts only a minor control on the Antarctic sea-level contribution with a peak of 4.4 m at 125 kyr BP. Retreat of the Antarctic ice sheet at the onset of the LIG is mainly forced by rising sea level and to a lesser extent by reduced ice shelf viscosity as the surface temperature increases. Global sea level shows a peak of 5.3 m at 124.5 kyr BP, which includes a minor contribution of 0.35 m from oceanic thermal expansion. Neither the individual contributions nor the total modelled sea-level stand show fast multi-millennial timescale variations as indicated by some reconstructions.

  19. Elevation Change of the Southern Greenland Ice Sheet from Satellite Radar Altimeter Data

    NASA Technical Reports Server (NTRS)

    Haines, Bruce J.

    1999-01-01

    Long-term changes in the thickness of the polar ice sheets are important indicators of climate change. Understanding the contributions to the global water mass balance from the accumulation or ablation of grounded ice in Greenland and Antarctica is considered crucial for determining the source of the about 2 mm/yr sea-level rise in the last century. Though the Antarctic ice sheet is much larger than its northern counterpart, the Greenland ice sheet is more likely to undergo dramatic changes in response to a warming trend. This can be attributed to the warmer Greenland climate, as well as a potential for amplification of a global warming trend in the polar regions of the Northern Hemisphere. In collaboration with Drs. Curt Davis and Craig Kluever of the University of Missouri, we are using data from satellite radar altimeters to measure changes in the elevation of the Southern Greenland ice sheet from 1978 to the present. Difficulties with systematic altimeter measurement errors, particularly in intersatellite comparisons, beset earlier studies of the Greenland ice sheet thickness. We use altimeter data collected contemporaneously over the global ocean to establish a reference for correcting ice-sheet data. In addition, the waveform data from the ice-sheet radar returns are reprocessed to better determine the range from the satellite to the ice surface. At JPL, we are focusing our efforts principally on the reduction of orbit errors and range biases in the measurement systems on the various altimeter missions. Our approach emphasizes global characterization and reduction of the long-period orbit errors and range biases using altimeter data from NASA's Ocean Pathfinder program. Along-track sea-height residuals are sequentially filtered and backwards smoothed, and the radial orbit errors are modeled as sinusoids with a wavelength equal to one revolution of the satellite. The amplitudes of the sinusoids are treated as exponentially-correlated noise processes with a

  20. Antarctic Ice Sheet Discharge Driven by Atmosphere-Ocean Feedbacks Across the Last Glacial Termination

    NASA Astrophysics Data System (ADS)

    Fogwill, C. J.; Turney, C. S.; Golledge, N. R.; Etheridge, D. M.; Rubino, M.; Thornton, D.; Baker, A.; Weber, M. E.; Woodward, J.; van Ommen, T. D.; Moy, A. D.; Davies, S. M.; Bird, M. I.; Winter, K.; Munksgaard, N.; Menviel, L.; Rootes, C.; Vohra, J.; Rivera, A.; Cooper, A.

    2016-12-01

    Reconstructing the dynamic response of the Antarctic ice sheets to warming during the Last Glacial Termination (LGT; 18,000-11,650 yrs ago) allows us to identify ice-climate feedbacks that could improve future projections1,2. Whilst the sequence of events during this period are reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records2, making it difficult to assess relationships between Antarctic ice-sheet dynamics, climate change and sea-level rise3-5. Here we present results from a highly-resolved `horizontal ice core'6,7 from the Weddell Sea Embayment, which records millennial-scale ice-sheet dynamics across this extensive sector of Antarctica. Counterintuitively, we find ice-sheet surface drawdown of 600 m across the Antarctic Cold Reversal (ACR; 14,600-12,700 yrs ago)5, with stabilisation during the subsequent millennia of atmospheric warming. Earth system and ice-sheet modelling highlights that this response was likely sustained by strong ocean-ice feedbacks4,8; however, the drivers remain uncertain. Given the coincidence of the ice-sheet changes recorded with marked shifts in atmospheric circulation9,10,11we suggest that millennial-scale Antarctic ice-sheet behaviour was initiated and sustained by global atmospheric teleconnections across the LGT. This has important ramifications ice-sheet stability under contemporary climate change, with changing atmospheric and oceanic circulation patterns. 1 Collins, M. et al. in Climate Change 2013: The Physical Science Basis. 2 Weber, M. E. et al. Nature 510, 134-138, (2014). 3 Weaver, A. J., et al., Science 299, 1709-1713, (2003). 4 Golledge, N. R. et al. Nat Commun 5, (2014). 5 Pedro, J. B. et al. Nature Geosci9. 51-55 (2015). 6 Turney, C. S. M. et al. Journal of Quaternary Science 28, 697-704 (2013). 7 Winter, K. et al. Geophys. Res. Lett.43. 5. 2019-2026 (2016). 8 Menviel, L., A. et al., Quaternary Science Reviews 30, 1155-1172 (2011). 9 Hogg

  1. Climate Variability, Melt-Flow Acceleration, and Ice Quakes at the Western Slope of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Steffen, K.; Zwally, J. H.; Rial, J. A.; Behar, A.; Huff, R.

    2006-12-01

    The Greenland ice sheet experienced surface melt increase over the past 15 years with record melt years in 1987, 1991, 1998, 2002 and 2005. For the western part of the ice sheet the melt area increased by 30 percent (1979-2005). Monthly mean air temperatures increased in spring and fall by 0.23 deg. C per year since 1990, extending the length of melt and total ablation. Winter air temperatures increased by as much as 0.5 deg. C per year during the past 15 years. The equilibrium line altitude ranged between 400 and 1530 m above sea level at 70 deg. north along the western slope of the ice sheet for the past 15 years, equaling a horizontal distance of 100 km. The ELA has been below the Swiss Camp (1100 m elevation) in the nineties, and since 1997 moved above the Swiss Camp height. An increase in ELA leads to an increase in melt water run-off which has been verified by regional model studies (high-resolution re-analysis). Interannual variability of snow accumulation varies from 0.3 to 2.0 m, whereas snow and ice ablation ranges from 0 to 1.5 m water equivalent at Swiss Camp during 1990-2005. A GPS network (10 stations) monitors ice velocity, acceleration, and surface height change at high temporal resolution throughout the year. The network covers a range of 500 and 1500 m above sea level, close to the Ilulissat Icefjord World Heritage region. The ice sheet continued to accelerate during the height of the melt season with short-term velocity increases up to 100 percent, and vertical uplift rates of 0.5 m. There seems to be a good correlation between the change in ice velocity and total surface melt, suggesting that melt water penetrates to great depth through moulins and cracks, lubricating the bottom of the ice sheet. A new bore-hole video movie will be shown from a 110 m deep moulin close to Swiss Camp. A PASSCAL array of 10 portable, 3-component seismic stations deployed around Swiss Camp from May to August 2006 detected numerous microearthquakes within the ice

  2. The Greenland Ice Sheet's surface mass balance in a seasonally sea ice-free Arctic

    NASA Astrophysics Data System (ADS)

    Day, J. J.; Bamber, J. L.; Valdes, P. J.

    2013-09-01

    General circulation models predict a rapid decrease in sea ice extent with concurrent increases in near-surface air temperature and precipitation in the Arctic over the 21st century. This has led to suggestions that some Arctic land ice masses may experience an increase in accumulation due to enhanced evaporation from a seasonally sea ice-free Arctic Ocean. To investigate the impact of this phenomenon on Greenland Ice Sheet climate and surface mass balance (SMB), a regional climate model, HadRM3, was used to force an insolation-temperature melt SMB model. A set of experiments designed to investigate the role of sea ice independently from sea surface temperature (SST) forcing are described. In the warmer and wetter SI + SST simulation, Greenland experiences a 23% increase in winter SMB but 65% reduced summer SMB, resulting in a net decrease in the annual value. This study shows that sea ice decline contributes to the increased winter balance, causing 25% of the increase in winter accumulation; this is largest in eastern Greenland as the result of increased evaporation in the Greenland Sea. These results indicate that the seasonal cycle of Greenland's SMB will increase dramatically as global temperatures increase, with the largest changes in temperature and precipitation occurring in winter. This demonstrates that the accurate prediction of changes in sea ice cover is important for predicting Greenland SMB and ice sheet evolution.

  3. Recent Ice Sheet and Glacier Elevation Changes in Greenland from Aircraft Laser Altimetry

    NASA Technical Reports Server (NTRS)

    Krabill, William B.; Thomas, R.; Sonntag, J.; Manizade, S.; Yungel, J.

    2008-01-01

    The Arctic Ice Mapping group (Project AIM) at the NASA Goddard Space Flight Center Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet (GIS) since 1993, using scanning airborne laser altimeters combined with Global Positioning System (UPS) technology. Earlier surveys showed the ice sheet above 2000-rn elevation to be in balance, but with localized regions of thickening or thinning. Thinning predominates at lower elevations and thinning rates have recently increased, resulting in a negative mass balance for the entire ice sheet. Recently, critical segments of near-coastal flight lines in Greenland were resurveyed. Results from the new data will be presented.

  4. Troughs on Martian Ice Sheets: Analysis of Their Closure and Mass Balance

    NASA Technical Reports Server (NTRS)

    Fountain, A.; Kargel, J.; Lewis, K.; MacAyeal, D.; Pfeffer, T.; Zwally, J.

    2000-01-01

    At the Copenhagen workshop on Martian polar processes, Ralf Greve commented that the flow regime surrounding scarps and troughs of the Martian polar ice sheets cannot be modeled using traditional "plan view" ice-sheet models. Such models are inadequate because they typically use reduced equations that embody certain simplifications applicable only to terrestrial ice sheets where the upper ice sheet surface is smooth. In response to this suggestion, we have constructed a 2-dimensional, time dependent "side view" (two spatial dimensions: one horizontal, one vertical) model of scarp closure that is designed to overcome the difficulties described by Greve. The purpose of the model is to evaluate the scales of stress variation and styles of flow closure so as to estimate errors that may be encountered by "plan view" models. We show that there may be avenues whereby the complications associated with scarp closure can be overcome in "plan view" models through appropriate parameterizations of 3-dimensional effects. Following this, we apply the flow model to simulate the evolution of a typical scarp on the North Polar Cap of Mars. Our simulations investigate: (a) the role of "radiation trapping" (see our companion abstract) in creating and maintaining "spiral-like" scarps on the ice sheet, (b) the consequences of different flowlaws and ice compositions on scarp evolution and, in particular, scarp age, and (c) the role of dust and debris in scarp evolution.

  5. Frozen-bed Fennoscandian and Laurentide ice sheets during the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Kleman, Johan; Hättestrand, Clas

    1999-11-01

    The areal extents of the Laurentide and Fennoscandian ice sheets during the Last Glacial Maximum (about 20,000 years ago) are well known, but thickness estimates range widely, from high-domed to thin, with large implications for our reconstruction of the climate system regarding, for example, Northern Hemisphere atmospheric circulation and global sea levels. This uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials, a knowledge of which would better constrain reconstructions of ice-sheet thickness. Here we show that, in the absence of direct data, the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen- and thawed-bed conditions. We argue that ribbed moraines were formed by brittle fracture of subglacial sediments, induced by the excessive stress at the boundary between frozen- and thawed-bed conditions resulting from the across-boundary difference in basal ice velocity. Maps of glacial landforms from aerial photographs of Canada and Scandinavia reveal a concentration of ribbed moraines around the ice-sheet retreat centres of Quebec, Keewatin, Newfoundland and west-central Fennoscandia. Together with the evidence from relict landscapes that mark glacial areas with frozen-bed conditions, the distribution of ribbed moraines on both continents suggest that a large area of the Laurentide and Fennoscandian ice sheets was frozen-based-and therefore high-domed and stable-during the Last Glacial Maximum.

  6. Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

    NASA Astrophysics Data System (ADS)

    Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin; Beckley, Matthew; Abe-Ouchi, Ayako; Aschwanden, Andy; Calov, Reinhard; Gagliardini, Olivier; Gillet-Chaulet, Fabien; Golledge, Nicholas R.; Gregory, Jonathan; Greve, Ralf; Humbert, Angelika; Huybrechts, Philippe; Kennedy, Joseph H.; Larour, Eric; Lipscomb, William H.; Le clec'h, Sébastien; Lee, Victoria; Morlighem, Mathieu; Pattyn, Frank; Payne, Antony J.; Rodehacke, Christian; Rückamp, Martin; Saito, Fuyuki; Schlegel, Nicole; Seroussi, Helene; Shepherd, Andrew; Sun, Sainan; van de Wal, Roderik; Ziemen, Florian A.

    2018-04-01

    Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. The goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within the Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.

  7. Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

    DOE PAGES

    Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin; ...

    2018-04-19

    Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. Here, the goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within themore » Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.« less

  8. Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin

    Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. Here, the goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within themore » Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.« less

  9. Evolution of supra-glacial lakes across the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Sundal, A. V.; Shepherd, A.; Nienow, P.; Hanna, E.; Palmer, S.; Huybrechts, P.

    2009-04-01

    We have used 268 cloud-free Moderate-resolution Imaging Spectroradiometer (MODIS) images spanning the 2003 and 2005-2007 melt seasons to study the seasonal evolution of supra-glacial lakes in three different regions of the Greenland Ice Sheet. Lake area estimates were obtained by developing an automated classification method for their identification based on 250 m resolution MODIS surface reflectance observations. Widespread supra-glacial lake formation and drainage is observed across the ice sheet, with a 2-3 weeks delay in the evolution of total supra-glacial lake area in the northern areas compared to the south-west. The onset of lake growth varies by up to one month inter-annually, and lakes form and drain at progressively higher altitudes during the melt season. A correlation was found between the annual peak in total lake area and modelled annual runoff across all study areas. Our results indicate that, in a future warmer climate (Meehl et al., 2007), Greenland supra-glacial lakes can be expected to form at higher altitudes and over a longer time period than is presently the case, expanding the area and time period over which connections between the ice sheet surface and base may be established (Das et al., 2008) with potential consequences for ice sheet discharge (Zwally et al., 2002). Das, S., Joughin, M., Behn, M., Howat, I., King, M., Lizarralde, D., & Bhatia, M. (2008). Fracture propagation to the base of the Greenland Ice Sheet during supra-glacial lake drainage. Science, 5877, 778-781. Meehl, G.A., Stocker, T.F., Collins W.D., Friedlingstein, P., Gaye, A.T., Gregory, J.M., Kitoh, A., Knutti, R., Murphy, J.M., Noda, A., Raper, S.C.B., Watterson, I.G., Weaver, A.J. & Zhao, Z.C. (2007). Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor

  10. Mapping Solid and Liquid Meltwater Retention on the Greenland and Antarctic Ice Sheets from Space

    NASA Astrophysics Data System (ADS)

    Miller, J.; Bringer, A.; Jezek, K. C.; Johnson, J. T.; Scambos, T.; Forster, R. R.; Long, D. G.

    2017-12-01

    We use satellite and airborne microwave radiometry to explore the potential for mapping both solid (infiltration ice) and liquid (firn aquifers) meltwater retention on ice sheets. Meltwater retention in firn is currently poorly understood, especially on an ice sheet-scale, however, critical to understanding the ultimate fate of liquid meltwater produced at the surface of ice sheets. Is it contributing to sea level? Or, is it being buffered prior to escaping into the ocean? We previously developed a simple satellite retrieval technique to map firn aquifers on the Greenland ice sheet using distinct L-band brightness temperature signatures that decrease on timescales of months following surface freeze-up, however, similar L-band brightness temperature signatures that decrease on timescales ranging from weeks to days are also present throughout the percolation facies of both the Greenland and Antarctic ice sheets. We hypothesize this characteristic family of temporal signatures represents meltwater retention within firn, where slowly decreasing signatures are characteristic of meltwater retention within perennial firn aquifers, and rapidly decreasing signatures are characteristic of meltwater retention as superimposed ice. Decreasing signatures on timescales between likely represent a continuum of firn characteristics, such as transient firn aquifers, perched firn aquifers, ice layers, ice pipes and lenses, and iced firn. To investigate these temporal signatures, we use L-band (1.4 GHz) brightness temperature observations collected over the Greenland and Antarctic ice sheets by the interferometric MIRAS instrument aboard ESA's Soil Moisture and Ocean Salinity (SMOS) satellite, and the radiometer aboard NASA's Soil Moisture Active Passive (SMAP) satellite. We will also investigate spectral signatures using multi-frequency L-band brightness temperature data (0.5-2 GHz) to be collected over several firn aquifer areas on the Greenland ice sheet by the Ohio State University

  11. The Research on Elevation Change of Antarctic Ice Sheet Based on CRYOSAT-2 Alimeter

    NASA Astrophysics Data System (ADS)

    Sun, Q.; Wan, J.; Liu, S.; Li, Y.

    2018-04-01

    In this paper, the Cryosat-2 altimeter data distributed by the ESA, and these data are processed to extract the information of the elevation change of the Antarctic ice sheet from 2010 to 2017. Firstly, the main pretreatment preprocessing for Cryosat-2 altimetry data is crossover adjustment and elimination of rough difference. Then the grid DEM of the Antarctic ice sheet was constructed by using the kriging interpolation method,and analyzed the spatial characteristic time characteristics of the Antarctic ice sheet. The latitude-weighted elevation can be obtained by using the elevation data of each cycle, and then the general trend of the Antarctic ice sheet elevation variation can be seen roughly.

  12. A Historical Forcing Ice Sheet Model Validation Framework for Greenland

    NASA Astrophysics Data System (ADS)

    Price, S. F.; Hoffman, M. J.; Howat, I. M.; Bonin, J. A.; Chambers, D. P.; Kalashnikova, I.; Neumann, T.; Nowicki, S.; Perego, M.; Salinger, A.

    2014-12-01

    We propose an ice sheet model testing and validation framework for Greenland for the years 2000 to the present. Following Perego et al. (2014), we start with a realistic ice sheet initial condition that is in quasi-equilibrium with climate forcing from the late 1990's. This initial condition is integrated forward in time while simultaneously applying (1) surface mass balance forcing (van Angelen et al., 2013) and (2) outlet glacier flux anomalies, defined using a new dataset of Greenland outlet glacier flux for the past decade (Enderlin et al., 2014). Modeled rates of mass and elevation change are compared directly to remote sensing observations obtained from GRACE and ICESat. Here, we present a detailed description of the proposed validation framework including the ice sheet model and model forcing approach, the model-to-observation comparison process, and initial results comparing model output and observations for the time period 2000-2013.

  13. High export of dissolved silica from the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Meire, L.; Meire, P.; Struyf, E.; Krawczyk, D. W.; Arendt, K. E.; Yde, J. C.; Juul Pedersen, T.; Hopwood, M. J.; Rysgaard, S.; Meysman, F. J. R.

    2016-09-01

    Silica is an essential element for marine life and plays a key role in the biogeochemistry of the ocean. Glacial activity stimulates rock weathering, generating dissolved silica that is exported to coastal areas along with meltwater. The magnitude of the dissolved silica export from large glacial areas such as the Greenland Ice Sheet is presently poorly quantified and not accounted for in global budgets. Here we present data from two fjord systems adjacent to the Greenland Ice Sheet which reveal a large export of dissolved silica by glacial meltwater relative to other macronutrients. Upscaled to the entire Greenland Ice Sheet, the export of dissolved silica equals 22 ± 10 Gmol Si yr-1. When the silicate-rich meltwater mixes with upwelled deep water, either inside or outside Greenland's fjords, primary production takes place at increased silicate to nitrate ratios. This likely stimulates the growth of diatoms relative to other phytoplankton groups.

  14. Bedrock Erosion Surfaces Record Former East Antarctic Ice Sheet Extent

    NASA Astrophysics Data System (ADS)

    Paxman, Guy J. G.; Jamieson, Stewart S. R.; Ferraccioli, Fausto; Bentley, Michael J.; Ross, Neil; Armadillo, Egidio; Gasson, Edward G. W.; Leitchenkov, German; DeConto, Robert M.

    2018-05-01

    East Antarctica hosts large subglacial basins into which the East Antarctic Ice Sheet (EAIS) likely retreated during past warmer climates. However, the extent of retreat remains poorly constrained, making quantifying past and predicted future contributions to global sea level rise from these marine basins challenging. Geomorphological analysis and flexural modeling within the Wilkes Subglacial Basin are used to reconstruct the ice margin during warm intervals of the Oligocene-Miocene. Flat-lying bedrock plateaus are indicative of an ice sheet margin positioned >400-500 km inland of the modern grounding zone for extended periods of the Oligocene-Miocene, equivalent to a 2-m rise in global sea level. Our findings imply that if major EAIS retreat occurs in the future, isostatic rebound will enable the plateau surfaces to act as seeding points for extensive ice rises, thus limiting extensive ice margin retreat of the scale seen during the early EAIS.

  15. Entrainment, transport and concentration of meteorites in polar ice sheets

    NASA Technical Reports Server (NTRS)

    Drewry, D. J.

    1986-01-01

    Glaciers and ice sheets act as slow-moving conveyancing systems for material added to both their upper and lower surfaces. Because the transit time for most materials is extremely long the ice acts as a major global storage facility. The effects of horizontal and vertical motions on the flow patterns of Antarctic ice sheets are summarized. The determination of the source areas of meteorites and their transport paths is a problem of central importance since it relates not only directly to concentration mechanisms but also to the wider issues in glaciology and meteorites. The ice and snow into which a meteorite falls, and which moves with it to the concentration area, encodes information about the infall area. The principle environmental conditions being former elevation, temperature (also related to elevation), and age of the ice. This encoded information could be used to identify the infall area.

  16. Cryosphere Science Outreach using the Ice Sheet System Model and a Virtual Ice Sheet Laboratory

    NASA Astrophysics Data System (ADS)

    Cheng, D. L. C.; Halkides, D. J.; Larour, E. Y.

    2015-12-01

    Understanding the role of Cryosphere Science within the larger context of Sea Level Rise is both a technical and educational challenge that needs to be addressed if the public at large is to trulyunderstand the implications and consequences of Climate Change. Within this context, we propose a new approach in which scientific tools are used directly inside a mobile/website platform geared towards Education/Outreach. Here, we apply this approach by using the Ice Sheet System Model, a state of the art Cryosphere model developed at NASA, and integrated within a Virtual Ice Sheet Laboratory, with the goal is to outreach Cryospherescience to K-12 and College level students. The approach mixes laboratory experiments, interactive classes/lessons on a website, and a simplified interface to a full-fledged instance of ISSM to validate the classes/lessons. This novel approach leverages new insights from the Outreach/Educational community and the interest of new generations in web based technologies and simulation tools, all of it delivered in a seamlessly integrated web platform. This work was performed at the California Institute of Technology's Jet Propulsion Laboratory undera contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

  17. Future ice ages and the challenges related to final disposal of nuclear waste: The Greenland Ice Sheet Hydrology Project

    NASA Astrophysics Data System (ADS)

    Lehtinen, A.; Claesson-Liljedahl, L.; Näslund, J.-O.; Ruskeeniemi, T.

    2009-04-01

    A deep geological repository for nuclear waste is designed to keep radiotoxic material separated from mankind and the environment for several hundreds of thousands of years. Within this time perspective glacial conditions are expected in high latitudes/Canada and North Europe. Climate induced changes such as the growth of ice sheets and permafrost will influence and alter the ground surface and subsurface environment, which may impact repository safety. In order to understand how climate change, particularly cooling and glaciation, might affect a repository in the long term, the use of present-day analogues helps to reduce the uncertainties and support the assumptions made in safety assessments. There are major uncertainties concerning hydrological processes related to glacial conditions. The impact of glaciations on any planned repository is a key consideration when performing safety assessments as it is one of the strongest perturbations related to climate change in the long term. The main aspects that need to be further investigated include: 1) to what extent does the meltwater produced by an ice sheet penetrates into the bedrock; 2) what is the pressure situation under an ice sheet, driving ground water flow; 3) how much oxygenated water will reach repository depth; 4) to what depth does glacial meltwater penetrate into the bedrock ; 5)what chemical composition does such water has when and if it reaches repository depth; and 6) can taliks (unfrozen ground in a permafrost area) act as concentrated discharge points of deep groundwater potentially transporting radionuclides in case of repository failure? Field data is needed in order to achieve a better and integrated understanding of the problems discussed above. Thus, research in a natural analogue site in Greenland has been planned and initiated by the Finnish (Posiva), Swedish (SKB) and Canadian (NWMO) nuclear waste management companies. The Greenland ice sheet and the Kangerlussuaq area (west Greenland

  18. Holocene lowering of the Laurentide ice sheet affects North Atlantic gyre circulation and climate

    NASA Astrophysics Data System (ADS)

    Ivanovic, R. F.; Gregoire, L. J.; Maycock, A.; Valdes, P. J.

    2017-12-01

    The Laurentide ice sheet, which covered Canada during glacial periods, had a major influence on atmospheric circulation and surface climate, but its role in climate during the early Holocene (9-7 ka), when it was thinner and confined around Hudson Bay, is unclear. It has been suggested that the demise of the ice sheet played a role in the 8.2 ka event (an abrupt 1-3 °C Northern Hemisphere cooling lasting 160 years) through the influence of changing topography on atmospheric circulation. To test this hypothesis, and to investigate the broader implications of changing ice sheet topography for climate, we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9.5 ka to 8.0 ka. Between 9.5 and 8.0 ka, our simulations show a 2 °C cooling south of Iceland and a 1 °C warming between 40-50° N in the North Atlantic. These surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid-North Atlantic as the ice sheet lowers. The climate response is strongest during the period of peak ice volume change (9.5 ka - 8.5 ka), but becomes negligible after 8.5 ka. The climatic effects of the Laurentide ice sheet lowering are restricted to the North Atlantic sector. Thus, topographic forcing did not play a significant role in the 8.2 ka event and had only a small effect on Holocene climate change compared to the effects of changes in greenhouse gases, insolation and ice sheet meltwater.

  19. Holocene lowering of the Laurentide ice sheet affects North Atlantic gyre circulation and climate

    NASA Astrophysics Data System (ADS)

    Gregoire, Lauren J.; Ivanovic, Ruza F.; Maycock, Amanda C.; Valdes, Paul J.; Stevenson, Samantha

    2018-02-01

    The Laurentide ice sheet, which covered Canada during glacial periods, had a major influence on atmospheric circulation and surface climate, but its role in climate during the early Holocene (9-7 ka), when it was thinner and confined around Hudson Bay, is unclear. It has been suggested that the demise of the ice sheet played a role in the 8.2 ka event (an abrupt 1-3 °C Northern Hemisphere cooling lasting 160 years) through the influence of changing topography on atmospheric circulation. To test this hypothesis, and to investigate the broader implications of changing ice sheet topography for climate, we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9.5 to 8.0 ka. Between 9.5 and 8.0 ka, our simulations show a 2 °C cooling south of Iceland and a 1 °C warming between 40° and 50°N in the North Atlantic. These surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid-North Atlantic as the ice sheet lowers. The climate response is strongest during the period of peak ice volume change (9.5-8.5 ka), but becomes negligible after 8.5 ka. The climatic effects of the Laurentide ice sheet lowering during the Holocene are restricted to the North Atlantic sector. Thus, topographic forcing is unlikely to have played a major role in the 8.2 ka event and had only a small effect on Holocene climate change compared to the effects of changes in greenhouse gases, insolation and ice sheet meltwater.

  20. Proglacial River Reveals Substantial Greenland Ice Sheet Climate Sensitivity and Meltwater Routing Delays

    NASA Astrophysics Data System (ADS)

    van As, D.; Mikkelsen, A. B.; Holtegaard Nielsen, M.; Claesson Liljedahl, L.; Lindback, K.; Pitcher, L. H.; Hasholt, B.

    2016-12-01

    A 12.000 km2 area of the Greenland ice sheet discharges meltwater via the proglacial Watson River in west Greenland. In a ten-year time span of continuous monitoring (2006-2015), the river discharged 3.8 km3 to 11.2 km3 yr-1. The large interannual variability is for an important part explained by hypsometric amplification: the flattening of the ice sheet with elevation adds 70% meltwater discharge sensitivity to atmospheric temperature. Comparing river discharge with ice sheet surface meltwater production from an observation-based surface mass balance model we quantify multiple-day routing delays for meltwater transit through the supra-, en-, sub- and proglacial system. This delay increases with ice sheet surface elevation: on average five days for surface water at the previous-known equilibrium line altitude (ELA) of ca. 1550 m, and seven days at the 2009-2015 ELA of ca. 1800 m above sea level. A flooding of the Kangerlussuaq bridge as in July 2012 thus requires a multi-day high-melt episode and can therefore be anticipated by in-situ monitoring of ice sheet melt. No evidence of significant en- or subglacial meltwater retention is found.

  1. Improving Altimetry Height-change Retrieval on the Fringes of the Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Paolo, F. S.; Nilsson, J.; Gardner, A. S.

    2017-12-01

    Projections of sea-level change over the next century are highly uncertain, in part, due to insufficient understanding of ice-sheet sensitivity to changes in oceanic and atmospheric circulation. This limitation is, to a large degree, related to the lack of long and continuous observational records covering critical regions along the ice-sheet margins where the ice interacts with the ocean. Of particular importance are accurate records of changes in ice thickness that provide information on how mass fluctuates on the floating extensions of ice streams and glaciers through which the ice-sheet drains. These changes can modify the stability of the grounded ice sheet through changing back-stress, for example, through loss of ice-shelf buttressing. Here, we synthetize 25+ years of satellite altimetry observations to extend the time span and improve the resolution and accuracy of the existing record of Antarctic floating ice thickness. We incorporate data from ESA's ERS-1, ERS-2, Envisat and Cryosat-2 radar altimeters (1992-present) and NASA's ICESat laser altimeter (2003-2009) and Operation IceBridge surveys (2009-present); with plans to include ICESat-2 data soon after its launch in September 2018. Towards this effort, we revisit some of the main corrections applied to altimeter data, such as minimization of the difference between measurements from radar and laser systems; and we improve the approach for the synthesis of heterogeneous measurements of ice-surface topography and uncertainty estimation. We report on our progress in constructing this long-term and homogeneous record, with a particular focus on the floating ice shelves.

  2. Coupled Northern Hemisphere permafrost-ice-sheet evolution over the last glacial cycle

    NASA Astrophysics Data System (ADS)

    Willeit, M.; Ganopolski, A.

    2015-09-01

    Permafrost influences a number of processes which are relevant for local and global climate. For example, it is well known that permafrost plays an important role in global carbon and methane cycles. Less is known about the interaction between permafrost and ice sheets. In this study a permafrost module is included in the Earth system model CLIMBER-2, and the coupled Northern Hemisphere (NH) permafrost-ice-sheet evolution over the last glacial cycle is explored. The model performs generally well at reproducing present-day permafrost extent and thickness. Modeled permafrost thickness is sensitive to the values of ground porosity, thermal conductivity and geothermal heat flux. Permafrost extent at the Last Glacial Maximum (LGM) agrees well with reconstructions and previous modeling estimates. Present-day permafrost thickness is far from equilibrium over deep permafrost regions. Over central Siberia and the Arctic Archipelago permafrost is presently up to 200-500 m thicker than it would be at equilibrium. In these areas, present-day permafrost depth strongly depends on the past climate history and simulations indicate that deep permafrost has a memory of surface temperature variations going back to at least 800 ka. Over the last glacial cycle permafrost has a relatively modest impact on simulated NH ice sheet volume except at LGM, when including permafrost increases ice volume by about 15 m sea level equivalent in our model. This is explained by a delayed melting of the ice base from below by the geothermal heat flux when the ice sheet sits on a porous sediment layer and permafrost has to be melted first. Permafrost affects ice sheet dynamics only when ice extends over areas covered by thick sediments, which is the case at LGM.

  3. Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin

    PubMed Central

    Feldmann, Johannes; Levermann, Anders

    2015-01-01

    The future evolution of the Antarctic Ice Sheet represents the largest uncertainty in sea-level projections of this and upcoming centuries. Recently, satellite observations and high-resolution simulations have suggested the initiation of an ice-sheet instability in the Amundsen Sea sector of West Antarctica, caused by the last decades’ enhanced basal ice-shelf melting. Whether this localized destabilization will yield a full discharge of marine ice from West Antarctica, associated with a global sea-level rise of more than 3 m, or whether the ice loss is limited by ice dynamics and topographic features, is unclear. Here we show that in the Parallel Ice Sheet Model, a local destabilization causes a complete disintegration of the marine ice in West Antarctica. In our simulations, at 5-km horizontal resolution, the region disequilibrates after 60 y of currently observed melt rates. Thereafter, the marine ice-sheet instability fully unfolds and is not halted by topographic features. In fact, the ice loss in Amundsen Sea sector shifts the catchment's ice divide toward the Filchner–Ronne and Ross ice shelves, which initiates grounding-line retreat there. Our simulations suggest that if a destabilization of Amundsen Sea sector has indeed been initiated, Antarctica will irrevocably contribute at least 3 m to global sea-level rise during the coming centuries to millennia. PMID:26578762

  4. Ku/Ka band observations over polar ice sheets

    NASA Astrophysics Data System (ADS)

    Thibaut, Pierre; Lasne, Yannick; Guillot, Amandine; Picot, Nicolas; Rémy, Frédérique

    2015-04-01

    For the first time, comparisons between Ku and Ka altimeter measurements are possible thanks to the new AltiKa instrument embarked onboard the Saral mission launched on February 25, 2013. This comparison is of particular interest when dealing with ice sheet observations because both frequencies have different penetration characteristics. We propose in this paper to revisit the estimation of the ice sheet topography (and other related parameters) with altimeter systems and to present illustrations of the differences observed in Ku and Ka bands using AltiKa, Envisat/RA-2 but also Cryosat-2 measurements. Working on AltiKa waveforms in the frame of the PEACHI project has allowed us to better understand the impact of the penetration depth on the echo shape, to improve the estimation algorithm and to compare its output with historical results obtained on Envisat and ERS missions. In particular, analyses at cross-overs of the Cryosat-2 and Saral data will be presented. Sentinel-3 mission should be launch during 2015. Operating in Ku band and in delay/doppler mode, it will be crucial to account for penetration effects in order to accurately derive the ice sheet heights and trends. The results of the work presented here, will benefit to the Sentinel-3 mission.

  5. Bed roughness of palaeo-ice streams: insights and implications for contemporary ice sheet dynamics

    NASA Astrophysics Data System (ADS)

    Falcini, Francesca; Rippin, David; Selby, Katherine; Krabbendam, Maarten

    2017-04-01

    Bed roughness is the vertical variation of elevation along a horizontal transect. It is an important control on ice stream location and dynamics, with a correspondingly important role in determining the behaviour of ice sheets. Previous studies of bed roughness have been limited to insights derived from Radio Echo Sounding (RES) profiles across parts of Antarctica and Greenland. Such an approach has been necessary due to the inaccessibility of the underlying bed. This approach has led to important insights, such as identifying a general link between smooth beds and fast ice flow, as well as rough beds and slow ice flow. However, these insights are mainly derived from relatively coarse datasets, so that links between roughness and flow are generalised and rather simplistic. Here, we explore the use of DTMs from the well-preserved footprints of palaeo-ice streams, coupled with high resolution models of palaeo-ice flow, as a tool for investigating basal controls on the behaviour of contemporary, active ice streams in much greater detail. Initially, artificial transects were set up across the Minch palaeo-ice stream (NW Scotland) to mimic RES flight lines from past studies in Antarctica. We then explored how increasing data-resolution impacted upon the roughness measurements that were derived. Our work on the Minch palaeo-ice stream indicates that different roughness signatures are associated with different glacial landforms, and we discuss the potential for using these insights to infer, from RES-based roughness measurements, the occurrence of particular landform assemblages that may exist beneath contemporary ice sheets.

  6. How Will Sea Ice Loss Affect the Greenland Ice Sheet? On the Puzzling Features of Greenland Ice-Core Isotopic Composition

    NASA Technical Reports Server (NTRS)

    Pausata, Francesco S. R.; Legrande, Allegra N.; Roberts, William H. G.

    2016-01-01

    The modern cryosphere, Earth's frozen water regime, is in fast transition. Greenland ice cores show how fast theses changes can be, presenting evidence of up to 15 C warming events over timescales of less than a decade. These events, called Dansgaard/Oeschger (D/O) events, are believed to be associated with rapid changes in Arctic sea ice, although the underlying mechanisms are still unclear. The modern demise of Arctic sea ice may, in turn, instigate abrupt changes on the Greenland Ice Sheet. The Arctic Sea Ice and Greenland Ice Sheet Sensitivity (Ice2Ice Chttps://ice2ice.b.uib.noD) initiative, sponsored by the European Research Council, seeks to quantify these past rapid changes to improve our understanding of what the future may hold for the Arctic. Twenty scientists gathered in Copenhagen as part of this initiative to discuss the most recent observational, technological, and model developments toward quantifying the mechanisms behind past climate changes in Greenland. Much of the discussion focused on the causes behind the changes in stable water isotopes recorded in ice cores. The participants discussed sources of variability for stable water isotopes and framed ways that new studies could improve understanding of modern climate. The participants also discussed how climate models could provide insights into the relative roles of local and nonlocal processes in affecting stable water isotopes within the Greenland Ice Sheet. Presentations of modeling results showed how a change in the source or seasonality of precipitation could occur not only between glacial and modern climates but also between abrupt events. Recent fieldwork campaigns illustrate an important role of stable isotopes in atmospheric vapor and diffusion in the final stable isotope signal in ice. Further, indications from recent fieldwork campaigns illustrate an important role of stable isotopes in atmospheric vapor and diffusion in the final stable isotope signal in ice. This feature complicates

  7. A transient fully coupled climate-ice-sheet simulation of the last glacial inception

    NASA Astrophysics Data System (ADS)

    Lofverstrom, M.; Otto-Bliesner, B. L.; Lipscomb, W. H.; Fyke, J. G.; Marshall, S.; Sacks, B.; Brady, E. C.

    2017-12-01

    The last glacial inception occurred around 115 ka, following a relative minimum in the Northern Hemisphere summer insolation. It is believed that small and spatially separated ice caps initially formed in the high elevation regions of northern Canada, Scandinavia, and along the Siberian Arctic coast. These ice caps subsequently migrated down in the valleys where they coalesced and formed the initial seeds of the large coherent ice masses that covered the northern parts of the North American and Eurasian continents over most of the last glacial cycle. Sea level records show that the initial growth period lasted for about 10 kyrs, and the resulting ice sheets may have lowered the global sea level by as much as 30 to 50 meters. Here we examine the transient climate system evolution over the period between 118 and 110 ka, using the fully coupled Community Earth System Model, version 2 (CESM2). This model features a two-way coupled high-resolution (4x4 km) ice-sheet component (Community Ice Sheet model, version 2; CISM2) that simulates ice sheets as an interactive component of the climate system. We impose a transient forcing protocol where the greenhouse gas concentrations and the orbital parameters follow the nominal year in the simulation; the model topography is also dynamically evolving in order to reflect changes in ice elevation throughout the simulation. The analysis focuses on how the climate system evolves over this time interval, with a special focus on glacial inception in the high-latitude continents. Results will highlight how the evolving ice sheets compare to data and previous model based reconstructions.

  8. E-tracers: A New Technique for Wireless Sensing Under Ice Sheets

    NASA Astrophysics Data System (ADS)

    Burrow, S.; Wadham, J. L.; Salter, M.; Barnes, R.

    2009-12-01

    A significant hurdle to the understanding of ice sheet basal hydrology and its coupling with ice motion is the difficulty in making in-situ measurements along a flow path. While dye tracing techniques may be used in small glaciers to determine transit times of surface melt water through the sub-glacial system, they provide no information on in situ conditions (e.g. pressure) and are ineffective at ice-sheet scale where dilution is high. The use of tethered sensor packages is complicated by the long lengths (~100’s m) and torturous path of the moulins and conduits within ice sheets. Recent attempts to pass solid objects (rubber ducks) and other sensor packages through glacial moulins have confirmed the difficultly in deploying sensors into the sub glacial environment. Here, we report the first successful deployment and recovery of compact, electronic units to moulins up to 7 km from the margin of a large land-terminating Greenland outlet. The technique uses RF (Radio Frequency) location to create an electronic tracer (an ‘e-tracer’) enabling a data-logging sensor package to be located in the pro-glacial flood plain once it has passed through the ice sheet. A number of individual packages are used in each deployment mitigating for the risk that some may become stuck within the moulin or lodge in an inaccessible part of the floodplain. In preliminary tests on the Leverett glacier in West Greenland during August 2009 we have demonstrated that this technique can be used to locate and retrieve dummy sensor packages: 50% and 20% of the dummy sensor packages introduced to moulins at 1 and 7 km from the ice sheet terminus respectively, emerged in the sub-glacial stream. It was possible to effectively detect the e-tracer units (which broadcast on 151MHz with 10mW of power) over a horizontal range of up to 5km across the pro-glacial floodplain and locate them to a high accuracy, allowing visual recognition and manual recovery. These performance statistics give this

  9. An Antarctic stratigraphic record of step-wise ice-sheet growth through the Eocene-Oligocene transition

    NASA Astrophysics Data System (ADS)

    Passchier, S.; Ciarletta, D. J.; Miriagos, T.; Bijl, P.; Bohaty, S. M.

    2016-12-01

    The Antarctic cryosphere plays a critical role in the ocean-atmosphere system, but its early evolution is still poorly known. With a near-field record from Prydz Bay, Antarctica, we conclude that Antarctic continental ice-sheet growth commenced with the EOT-1 "precursor" glaciation, during a time of Subantarctic surface ocean cooling and a decline in atmospheric pCO2. Prydz Bay lies downstream of a major East Antarctic ice-sheet drainage system and the Gamburtsev Mountains, a likely nucleation point for the first ice sheets. Its sedimentary records uniquely constrain the timing of ice-sheet advance onto the continental shelf. We investigate a detrital record extracted from three Ocean Drilling Program drill holes in Prydz Bay within a new depositional and chronological framework spanning the late Eocene to early Oligocene ( 36-33 Ma). The chemical index of alteration (CIA) and the S-index, calculated from the major element geochemistry of bulk samples, yield estimates of chemical weathering intensities and mean annual temperature (MAT) on the East Antarctic continent. We document evidence for late Eocene mountain glaciation along with transient warm events at 35.8-34.8 Ma. These data and our sedimentological analyses confirm the presence of ephemeral mountain glaciers on East Antarctica during the late Eocene between 35.9 and 34.4 Ma. Furthermore, we document the stepwise climate cooling of the Antarctic hinterland from 34.4 Ma as the ice sheet advanced towards the edges of the continent during EOT-1. The youngest part of our data set correlates to the time interval of the Oi-1 glaciation, when the ice-sheet in Prydz Bay extended to the outer shelf. Cooling and ice growth on Antarctica was spatially variable and ice sheets formed under declining pCO2. These results point to complex ice sheet - atmosphere - ocean - solid-earth feedbacks.

  10. Antarctica and Its Ice Sheet: Knowledge Gained During the IGY/IGC

    NASA Astrophysics Data System (ADS)

    Bentley, C. R.

    2006-12-01

    At the end of World War II, the interior of Antarctica, with the exception of the mountains south of the Ross Ice Shelf, was still terra incognita. It was described simply as a nearly continuous high plateau. Even less was known about the ice thickness; the eminent glacial geologist, Richard Foster Flint, believed it "unlikely that the ice thickness exceeds 2000 feet except very locally; probably its average thickness is considerably less." Then in the late 1940's and early 1950's, seismic sounding in Greenland by the Expéditions Polaires Françaises and in Queen Maud Land by the Norwegian-British-Swedish Antarctic Expedition, 1949-52, revealed that, inland of the coastal mountains, the beds in both regions lie close to sea level. This led to a reappraisal of the Antarctic ice sheet, such that the prescient glaciologist, Robert P. Sharp, could predict, on the eve of the IGY, that "between 3000 and 4000 meters of ice will be found" in East Antarctica and that "work during IGY will establish an average thickness for Antarctic inland ice in excess of 1600 m." Seismic and gravity soundings on oversnow traverses conducted by eight countries during the IGY and the succeeding IGC showed Sharp to be basically correct, but there were major surprises, such as the vast Gamburtsev Subglacial Mountains, completely hidden by the ice in central East Antarctica, and the equally vast Byrd Subglacial Basin beneath much of the West Antarctic ice sheet, so deep that roughly half the ice in the region lies below sea level. There were major discoveries on and above the surface too, such as the huge size of the Filchner/Ronne Ice Shelf, and the very existence of the Ellsworth and Pensacola Mountains, the former including the highest peak on the continent. Further, the fundamental difference between the crustal structures of East and West Antarctica became clear. A summary paper published in 1960, looking primarily at West Antarctica where the main U.S. activity lay, could conclude that

  11. Advances in modelling subglacial lakes and their interaction with the Antarctic ice sheet.

    PubMed

    Pattyn, Frank; Carter, Sasha P; Thoma, Malte

    2016-01-28

    Subglacial lakes have long been considered hydraulically isolated water bodies underneath ice sheets. This view changed radically with the advent of repeat-pass satellite altimetry and the discovery of multiple lake discharges and water infill, associated with water transfer over distances of more than 200 km. The presence of subglacial lakes also influences ice dynamics, leading to glacier acceleration. Furthermore, subglacial melting under the Antarctic ice sheet is more widespread than previously thought, and subglacial melt rates may explain the availability for water storage in subglacial lakes and water transport. Modelling of subglacial water discharge in subglacial lakes essentially follows hydraulics of subglacial channels on a hard bed, where ice sheet surface slope is a major control on triggering subglacial lake discharge. Recent evidence also points to the development of channels in deformable sediment in West Antarctica, with significant water exchanges between till and ice. Most active lakes drain over short time scales and respond rapidly to upstream variations. Several Antarctic subglacial lakes exhibit complex interactions with the ice sheet due to water circulation. Subglacial lakes can therefore-from a modelling point of view-be seen as confined small oceans underneath an imbedded ice shelf. © 2015 The Author(s).

  12. Evaluation of changes in atmospheric and oceanic fluxes during continental ice sheet retreat

    NASA Astrophysics Data System (ADS)

    Martin, J.; Martin, E. E.; Deuerling, K. M.

    2017-12-01

    Extensive land areas were exposed across North America, Eurasia, and to a lesser extent Greenland as continental ice sheets retreated following the last glacial maximum. A transect of watersheds from the coast to the western Greenland Ice Sheet (GrIS) provides an opportunity to evaluate possible changes in oceanic solute fluxes and atmospheric CO2 exchange as ice sheets retreat. We evaluate these fluxes in one proglacial watershed (draining ice sheet runoff) and four deglaciated watersheds (draining local precipitation and permafrost melt). Sr isotope ratios indicate bedrock near the coast has experienced greater weathering than near the ice sheet. A mass balance model of the major element composition of stream water indicates weathering in deglaciated watersheds is dominated by carbonic acid dissolution of carbonate minerals near the ice sheet that switches to carbonic acid alteration of silicate minerals near the coast. In addition, weathering by sulfuric acid, derived from oxidative dissolution of sulfide minerals, increases from the ice sheet to the coast. These changes in the weathered minerals and weathering acids impact CO2 sequestration associated with weathering. Weathering consumes 350 to 550 µmol CO2/L in watersheds near the ice sheet, but close to the coast, consumes only 15 µmol CO2/L in one watershed and sources 140 µmol CO2/L to the atmosphere at another coastal watershed. The decreasing CO2 weathering sink from the GrIS to coast reflects decreased carbonic acid weathering and increased sulfuric acid weathering of carbonate minerals. The proglacial stream shows downstream variations in composition from mixing of two water sources, with only minor in-stream weathering, which consumes < 0.1 µmol CO2/L. Discharge from the deglaciated watersheds is currently unknown but their higher solute concentrations and CO2 exchange than proglacial systems suggest deglaciated watersheds dominate atmospheric fluxes of CO2 and oceanic solute fluxes. These results

  13. Ice-sheet modelling accelerated by graphics cards

    NASA Astrophysics Data System (ADS)

    Brædstrup, Christian Fredborg; Damsgaard, Anders; Egholm, David Lundbek

    2014-11-01

    Studies of glaciers and ice sheets have increased the demand for high performance numerical ice flow models over the past decades. When exploring the highly non-linear dynamics of fast flowing glaciers and ice streams, or when coupling multiple flow processes for ice, water, and sediment, researchers are often forced to use super-computing clusters. As an alternative to conventional high-performance computing hardware, the Graphical Processing Unit (GPU) is capable of massively parallel computing while retaining a compact design and low cost. In this study, we present a strategy for accelerating a higher-order ice flow model using a GPU. By applying the newest GPU hardware, we achieve up to 180× speedup compared to a similar but serial CPU implementation. Our results suggest that GPU acceleration is a competitive option for ice-flow modelling when compared to CPU-optimised algorithms parallelised by the OpenMP or Message Passing Interface (MPI) protocols.

  14. ICESat-2, its retrievals of ice sheet elevation change and sea ice freeboard, and potential synergies with CryoSat-2

    NASA Astrophysics Data System (ADS)

    Neumann, Thomas; Markus, Thorsten; Smith, Benjamin; Kwok, Ron

    2017-04-01

    Understanding the causes and magnitudes of changes in the cryosphere remains a priority for Earth science research. Over the past decade, NASA's and ESA's Earth-observing satellites have documented a decrease in both the areal extent and thickness of Arctic sea ice, and an ongoing loss of grounded ice from the Greenland and Antarctic ice sheets. Understanding the pace and mechanisms of these changes requires long-term observations of ice-sheet mass, sea-ice thickness, and sea-ice extent. NASA's ICESat-2 mission is the next-generation space-borne laser altimeter mission and will use three pairs of beams, each pair separated by about 3 km across-track with a pair spacing of 90 m. The spot size is 17 m with an along-track sampling interval of 0.7 m. This measurement concept is a result of the lessons learned from the original ICESat mission. The multi-beam approach is critical for removing the effects of ice sheet surface slope from the elevation change measurements of most interest. For sea ice, the dense spatial sampling (eliminating along-track gaps) and the small footprint size are especially useful for sea surface height measurements in the, often narrow, leads needed for sea ice freeboard and ice thickness retrievals. Currently, algorithms are being developed to calculate ice sheet elevation change and sea ice freeboard from ICESat-2 data. The orbits of ICESat-2 and Cryosat-2 both converge at 88 degrees of latitude, though the orbit altitude differences result in different ground track patterns between the two missions. This presentation will present an overview of algorithm approaches and how ICESat-2 and Cryosat-2 data may augment each other.

  15. Overview of Ice-Sheet Mass Balance and Dynamics from ICESat Measurements

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay

    2010-01-01

    The primary purpose of the ICESat mission was to determine the present-day mass balance of the Greenland and Antarctic ice sheets, identify changes that may be occurring in the surface-mass flux and ice dynamics, and estimate their contributions to global sea-level rise. Although ICESat's three lasers were planned to make continuous measurements for 3 to 5 years, the mission was re-planned to operate in 33-day campaigns 2 to 3 times each year following failure of the first laser after 36 days. Seventeen campaigns were conducted with the last one in the Fall of 2009. Mass balance maps derived from measured ice-sheet elevation changes show that the mass loss from Greenland has increased significantly to about 170 Gt/yr for 2003 to 2007 from a state of near balance in the 1990's. Increased losses (189 Gt/yr) from melting and dynamic thinning are over seven times larger'than increased gains (25 gt/yr) from precipitation. Parts of the West Antarctic ice sheet and the Antarctic Peninsula are losing mass at an increasing rate, but other parts of West Antarctica and the East Antarctic ice sheet are gaining mass at an increasing rate. Increased losses of 35 Gt/yr in Pine Island, Thwaites-Smith, and Marie-Bryd.Coast are more than balanced by gains in base of Peninsula and ice stream C, D, & E systems. From the 1992-2002 to 2003-2007 period, the overall mass balance for Antarctica changed from a loss of about 60 Gt/yr to near balance or slightly positive.

  16. Modeled Variations of Precipitation over the Greenland Ice Sheet.

    NASA Astrophysics Data System (ADS)

    Bromwich, David H.; Robasky, Frank M.; Keen, Richard A.; Bolzan, John F.

    1993-07-01

    A parameterization of the synoptic activity at 500 hPa and a simple orographic scheme are used to model the spatial and temporal variations of precipitation over the Greenland Ice Sheet for 1963-88 from analyzed geopotential height fields produced by the National Meteorological Center (NMC). Model coefficients are fitted to observed accumulation data, primarily from the summit area of the ice sheet. All major spatial characteristics of the observed accumulation distribution are reproduced apart from the orographic accumulation maximum over the northwestern coastal slopes. The modeled time-averaged total precipitation amount over Greenland is within the range of values determined by other investigators from surface-based observations. A realistic degree of interannual variability in precipitation is also simulated.A downward trend in simulated ice sheet precipitation over the 26 years is found. This is supported by a number of lines of evidence. It matches the accumulation trends during this period from ice cores drilled in south-central Greenland. The lower tropospheric specific humidifies at two south coastal radiosonde stations also decrease over this interval. A systematic shift away from Greenland and a decrease in activity of the dominant storm track are found for relatively low precipitation periods as compared to relatively high precipitation periods. This negative precipitation trend would mean that the Greenland Ice Sheet, depending on its 1963 mass balance state, has over the 1963-88 period either decreased its negative, or increased its positive, contribution to recently observed global sea level rise.Superimposed on the declining simulated precipitation rate for the entire ice sheet is a pronounced 3-5-yr periodicity. This is prominent in the observed and modeled precipitation time series from Summit, Greenland. This cycle shows some aspects in common with the Southern Oscillation.Some deficiencies in the NMC analysts were highlighted by this work. A

  17. The geomicrobiology of the Greenland Ice Sheet: impact on DOC export (Invited)

    NASA Astrophysics Data System (ADS)

    Wadham, J. L.; Stibal, M.; Lawson, E. C.; Barnett, M. J.; Hasan, F.; Telling, J.; Anesio, A.; Lis, G.; Cullen, D.; Butler, C.; Tranter, M.; Nienow, P. W.

    2010-12-01

    The Greenland Ice Sheet (GrIS) is the largest mass of ice in the northern hemisphere, and contributes ~370 km3 in runoff annually to the Arctic Ocean. While recent work has highlighted runoff increases of up to 100% from the GrIS over the next century, very little is known about the associated impacts upon rates of sediment-bound and dissolved organic carbon export from the ice sheet to the coastal ocean. This is relevant given recent work that has suggested that the high proportion of labile dissolved organic carbon (DOC) present in glacial runoff may be important in sustaining the productivity of ecosystems downstream. Here we report the phylogenetic and functional diversity of micro-organisms inhabiting the surface and basal regions of the Greenland Ice Sheet (at Leverett Glacier, SW Greenland), and whose activity influences the biogeochemical composition of runoff. Real time PCR data on runoff, together with 16S-rRNA bacterial clone libraries on sediments, demonstrate a subglacial microbial community that contrasts phylogenetically and functionally with the ice sheet surface ecosystem. We envisage that large sectors of the subglacial environment are microbially active, with overridden paleosols and in-washed surface organic matter providing a carbon substrate for a range of metabolic pathways. This includes methanogenesis which proceeds at rates similar to deep ocean sediments and via a CO2/H2 pathway. These subglacial microbial communities serve to chemically modify the DOC composition of meltwater inputs from the ice sheet surface and modulate the reactivity of bulk DOC exported in runoff. Evidence for subglacial microbial influences on DOC in runoff includes elevated concentrations of dissolved carbohydrates (e.g. glucose and fructose of up to 1 μmol/L), which are preferentially exported during subglacial outburst events. We examine the temporal changes in DOC export in runoff from the ice sheet over a full melt season, and consider how changes in total

  18. Abrupt glacial climate shifts controlled by ice sheet changes.

    PubMed

    Zhang, Xu; Lohmann, Gerrit; Knorr, Gregor; Purcell, Conor

    2014-08-21

    During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.

  19. Modeling Antarctic Ice Sheet retreat in warm climates: a historical perspective.

    NASA Astrophysics Data System (ADS)

    Pollard, D.; Deconto, R. M.; Gasson, E.

    2016-12-01

    Early modeling of Antarctic Ice Sheet size vs. climate focused on asymmetry between retreat and growth, with much greater warming needed to cause retreat from full ice cover, due to Height Mass Balance Feedback and albedo feedback. This led to a long-standing model-data conflict, with models needing 1000 to2000 ppmv atmospheric CO2 to produce retreat from full size, vs. proxy data of large ice fluctuations despite much lower CO2 since the Miocene.Subsequent modeling with marine ice physics found that the West Antarctic Ice Sheet could undergo repeated warm-period collapses with realistic past forcing. However, that yields only 3 to 7 m equivalent sea-level rise above modern, compared to 10 to 20 m or more suggested by some geologic data. Large subglacial basins in East Antarctica could be vulnerable to the same processes,but did not retreat in most models due to narrower and shallower sills.After recent modifications, some ice sheet models were able to produce warm-period collapse of major East Antarctic basins, with sea-level rise of up to 15 m. The modifications are (i) hydrofracturing by surface melt, and structural failure of ice cliffs, or (ii) numerical treatment at the grounding line. In these models, large retreat occurs both for past warmintervals, and also for future business-as-usual scenarios.Some interpretations of data in the late Oligocene and Miocene suggest yet larger fluctuations, between 50 to 100% of modern Antarctic size. That would require surface-melt driven retreat of some terrestrial East Antarctic ice, despite the hysteresis issue raised above. A recent study using a coupled climate-ice sheet model found that with a finer climate gridand more frequent coupling exchange, substantial retreat of terrestrial Antarctica can occur with 500 to 840 ppmv CO2, much lower than in earlier models. This will allow meaningful interactions between modeling and deeper-time geologic interpretations since the late Oligocene.

  20. Cosmogenic nuclide age estimate for Laurentide Ice Sheet recession from the terminal moraine, New Jersey, USA, and constraints on latest Pleistocene ice sheet history

    USGS Publications Warehouse

    Corbett, Lee B.; Bierman, Paul R.; Stone, Byron D.; Caffee, Marc W.; Larsen, Patrick L.

    2017-01-01

    The time at which the Laurentide Ice Sheet reached its maximum extent and subsequently retreated from its terminal moraine in New Jersey has been constrained by bracketing radiocarbon ages on preglacial and postglacial sediments. Here, we present measurements of in situ produced 10Be and 26Al in 16 quartz-bearing samples collected from bedrock outcrops and glacial erratics just north of the terminal moraine in north-central New Jersey; as such, our ages represent a minimum limit on the timing of ice recession from the moraine. The data set includes field and laboratory replicates, as well as replication of the entire data set five years after initial measurement. We find that recession of the Laurentide Ice Sheet from the terminal moraine in New Jersey began before 25.2±2.1 ka (10Be, n=16, average, 1 standard deviation). This cosmogenic nuclide exposure age is consistent with existing limiting radiocarbon ages in the study area and cosmogenic nuclide exposure ages from the terminal moraine on Martha’s Vineyard ~300 km to the northeast. The age we propose for Laurentide Ice Sheet retreat from the New Jersey terminal position is broadly consistent with regional and global climate records of the last glacial maximum termination and records of fluvial incision.

  1. The dynamics of climate-induced deglacial ice stream acceleration

    NASA Astrophysics Data System (ADS)

    Robel, A.; Tziperman, E.

    2015-12-01

    Geological observations indicate that ice streams were a significant contributor to ice flow in the Laurentide Ice Sheet during the Last Glacial Maximum. Conceptual and simple model studies have also argued that the gradual development of ice streams increases the sensitivity of large ice sheets to weak climate forcing. In this study, we use an idealized configuration of the Parallel Ice Sheet Model to explore the role of ice streams in rapid deglaciation. In a growing ice sheet, ice streams develop gradually as the bed warms and the margin expands outward onto the continental shelf. Then, a weak change in equilibrium line altitude commensurate with Milankovitch forcing results in a rapid deglacial response, as ice stream acceleration leads to enhanced calving and surface melting at low elevations. We explain the dynamical mechanism that drives this ice stream acceleration and its broader applicability as a feedback for enhancing ice sheet decay in response to climate forcing. We show how our idealized ice sheet simulations match geomorphological observations of deglacial ice stream variability and previous model-data analyses. We conclude with observations on the potential for interaction between ice streams and other feedback mechanisms within the earth system.

  2. Antarctic ice sheet thickness estimation using the horizontal-to-vertical spectral ratio method with single-station seismic ambient noise

    NASA Astrophysics Data System (ADS)

    Yan, Peng; Li, Zhiwei; Li, Fei; Yang, Yuande; Hao, Weifeng; Bao, Feng

    2018-03-01

    We report on a successful application of the horizontal-to-vertical spectral ratio (H / V) method, generally used to investigate the subsurface velocity structures of the shallow crust, to estimate the Antarctic ice sheet thickness for the first time. Using three-component, five-day long, seismic ambient noise records gathered from more than 60 temporary seismic stations located on the Antarctic ice sheet, the ice thickness measured at each station has comparable accuracy to the Bedmap2 database. Preliminary analysis revealed that 60 out of 65 seismic stations on the ice sheet obtained clear peak frequencies (f0) related to the ice sheet thickness in the H / V spectrum. Thus, assuming that the isotropic ice layer lies atop a high velocity half-space bedrock, the ice sheet thickness can be calculated by a simple approximation formula. About half of the calculated ice sheet thicknesses were consistent with the Bedmap2 ice thickness values. To further improve the reliability of ice thickness measurements, two-type models were built to fit the observed H / V spectrum through non-linear inversion. The two-type models represent the isotropic structures of single- and two-layer ice sheets, and the latter depicts the non-uniform, layered characteristics of the ice sheet widely distributed in Antarctica. The inversion results suggest that the ice thicknesses derived from the two-layer ice models were in good concurrence with the Bedmap2 ice thickness database, and that ice thickness differences between the two were within 300 m at almost all stations. Our results support previous finding that the Antarctic ice sheet is stratified. Extensive data processing indicates that the time length of seismic ambient noise records can be shortened to two hours for reliable ice sheet thickness estimation using the H / V method. This study extends the application fields of the H / V method and provides an effective and independent way to measure ice sheet thickness in Antarctica.

  3. West Antarctic Ice Sheet Initiative. Volume 1: Science and Implementation Plan

    NASA Technical Reports Server (NTRS)

    Bindschadler, Robert A. (Editor)

    1990-01-01

    The Science and Implementation Plan of the West Antarctic Ice Sheet Initiative (WAIS) is described. The goal of this initiative is the prediction of the future behavior of this ice sheet and an assessment of its potential to collapse, rapidly raising global sea level. The multidisciplinary nature of WAIS reflects the complexity of the polar ice sheet environment. The project builds upon past and current polar studies in many fields and meshes with future programs of both the U.S. and other countries. Important tasks in each discipline are described and a coordinated schedule by which the majority of these tasks can be accomplished in 5 years is presented. The companion report (Volume 2) contains seven discipline review papers on the state of knowledge of Antarctica and opinions on how that knowledge must be increased to attain the WAIS goal.

  4. The abandoned ice sheet base at Camp Century, Greenland, in a warming climate

    NASA Astrophysics Data System (ADS)

    Colgan, William; Machguth, Horst; MacFerrin, Mike; Colgan, Jeff D.; As, Dirk; MacGregor, Joseph A.

    2016-08-01

    In 1959 the U.S. Army Corps of Engineers built Camp Century beneath the surface of the northwestern Greenland Ice Sheet. There they studied the feasibility of deploying ballistic missiles within the ice sheet. The base and its wastes were abandoned with minimal decommissioning in 1967, under the assumption they would be preserved for eternity by perpetually accumulating snowfall. Here we show that a transition in ice sheet surface mass balance at Camp Century from net accumulation to net ablation is plausible within the next 75 years, under a business-as-usual anthropogenic emissions scenario (Representative Concentration Pathway 8.5). Net ablation would guarantee the eventual remobilization of physical, chemical, biological, and radiological wastes abandoned at the site. While Camp Century and four other contemporaneous ice sheet bases were legally established under a Danish-U.S. treaty, the potential remobilization of their abandoned wastes, previously regarded as sequestered, represents an entirely new pathway of political dispute resulting from climate change.

  5. The Abandoned Ice Sheet Base at Camp Century, Greenland, in a Warming Climate

    NASA Technical Reports Server (NTRS)

    Colgan, William; Machguth, Horst; Macferrin, Mike; Colgan, Jeff D.; Van As, Dirk; Macgregor, Joseph A.

    2016-01-01

    In 1959 the U.S. Army Corps of Engineers built Camp Century beneath the surface of the northwestern Greenland Ice Sheet. There they studied the feasibility of deploying ballistic missiles within the ice sheet. The base and its wastes were abandoned with minimal decommissioning in 1967, under the assumption they would be preserved for eternity by perpetually accumulating snowfall. Here we show that a transition in ice sheet surface mass balance at Camp Century from net accumulation to net ablation is plausible within the next 75years, under a business-as-usual anthropogenic emissions scenario (Representative Concentration Pathway 8.5). Net ablation would guarantee the eventual remobilization of physical, chemical, biological, and radiological wastes abandoned at the site. While Camp Century and four other contemporaneous ice sheet bases were legally established under a Danish-U.S. treaty, the potential remobilization of their abandoned wastes, previously regarded as sequestered, represents an entirely new pathway of political dispute resulting from climate change.

  6. Evidence of unfrozen liquids and seismic anisotropy at the base of the polar ice sheets

    NASA Astrophysics Data System (ADS)

    Wittlinger, Gérard; Farra, Véronique

    2015-03-01

    We analyze seismic data from broadband stations located on the Antarctic and Greenland ice sheets to determine polar ice seismic velocities. P-to-S converted waves at the ice/rock interface and inside the ice sheets and their multiples (the P-receiver functions) are used to estimate in-situ P-wave velocity (Vp) and P-to-S velocity ratio (Vp/Vs) of polar ice. We find that the polar ice sheets have a two-layer structure; an upper layer of variable thickness (about 2/3 of the total thickness) with seismic velocities close to the standard ice values, and a lower layer of approximately constant thickness with standard Vp but ∼25% smaller Vs. The lower layer ceiling corresponds approximately to the -30 °C isotherm. Synthetic modeling of P-receiver functions shows that strong seismic anisotropy and low vertical S velocity are needed in the lower layer. The seismic anisotropy results from the preferred orientation of ice crystal c-axes toward the vertical. The low vertical S velocity may be due to the presence of unfrozen liquids resulting from premelting at grain joints and/or melting of chemical solutions buried in the ice. The strongly preferred ice crystal orientation fabric and the unfrozen fluids may facilitate polar ice sheet basal flow.

  7. Terrestrial Ice Sheets: Studies of Climate History, Internal Structure, Surface, and Bedrock

    NASA Astrophysics Data System (ADS)

    Thorsteinsson, Th.; Kipfstuhl, J.; Nixdorf, U.; Oerter, H.; Miller, H.; Fritsche, D.; Jung-Rothenhaeusler, F.; Mayer, C.; Schwager, M.; Wilhelms, F.; Steinhage, D.; Goektas, F.

    1998-01-01

    Recently drilled deep ice cores from Central Greenland (GRIP and GISP2) provide the most detailed results available on climatic variation in the northern hemisphere during the last 100,000 years, a period that includes the Holocene (0-11.5 ka) and most of the Wisconsin glacial period. Summer-winter variation in various physical and chemical properties of polar ice allows dating of ice cores by annual layer counting. Several such methods are currently being employed on an ice core drilled by the new North Greenland Ice Core Project (NGRIP), which is aimed at extending the Greenland ice palaeoclimatic record through the last interglacial, the Eemian. Two examples will be presented: (1) visual and photographic studies of seasonal variation in stratigraphic layering, crystal size, air bubble and clathrate concentration, and (2) studies of electric stratigraphy, using the method of dielectric profiling (DEP). This method records the AC conductivity of ice cores, which is negatively correlated with the concentration of airborne dust in the ice but positively correlated with volcanic and marine aerosols. Comprehensive surface traverse programs, which include shallow coring and ice velocity measurements, have recently been carried out by the Alfred Wegener Institute in previously little-investigated regions of Greenland and Antarctica. Serving partly as reconnaissance prior to deep drilling projects, such studies also help to reduce considerable uncertainties in the mass balance of the two large polar ice sheets and thus in their estimated response to climate change. Main results of a recent traverse in North Greenland include the following: (1) A new map of the accumulation distribution on the ice sheet indicates a large low-accumulation region in Northeast-Greenland; (2) North Greenland records show significantly greater climatic variability during the last 500 yr than corresponding records from the southern part of the ice sheet; and (3) data on variation in

  8. A synthesis of the basal thermal state of the Greenland Ice Sheet

    PubMed Central

    MacGregor, Joseph A.; Fahnestock, Mark A.; Catania, Ginny A.; Aschwanden, Andy; Clow, Gary D.; Colgan, William T.; Gogineni, S. Prasad; Morlighem, Mathieu; Nowicki, Sophie M. J.; Paden, John D.; Price, Stephen F.; Seroussi, Hélène

    2017-01-01

    The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state. PMID:28163988

  9. A synthesis of the basal thermal state of the Greenland Ice Sheet.

    PubMed

    MacGregor, Joseph A; Fahnestock, Mark A; Catania, Ginny A; Aschwanden, Andy; Clow, Gary D; Colgan, William T; Gogineni, S Prasad; Morlighem, Mathieu; Nowicki, Sophie M J; Paden, John D; Price, Stephen F; Seroussi, Hélène

    2016-08-10

    The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state.

  10. A synthesis of the basal thermal state of the Greenland Ice Sheet

    USGS Publications Warehouse

    MacGregor, Joseph A; Fahnestock, Mark A; Catania, Ginny A; Aschwanden, Andy; Clow, Gary D.; Colgan, William T.; Gogineni, Prasad S.; Morlighem, Mathieu; Nowicki, Sophie M .J.; Paden, John D; Price, Stephen F.; Seroussi, Helene

    2016-01-01

    The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state.

  11. Radar measurements of melt zones on the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Jezek, Kenneth C.; Gogineni, Prasad; Shanableh, M.

    1994-01-01

    Surface-based microwave radar measurements were performed at a location on the western flank of the Greenland Ice Sheet. Here, firn metamorphasis is dominated by seasonal melt, which leads to marked contrasts in the vertical structure of winter and summer firn. This snow regime is also one of the brightest radar targets on Earth with an average backscatter coefficient of 0 dB at 5.3 GHz and an incidence angle of 25 deg. By combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winter's snow pack. This observation has important implications for monitoring and understanding changes in ice sheet volume using spaceborne microwave sensors.

  12. A balanced water layer concept for subglacial hydrology in large scale ice sheet models

    NASA Astrophysics Data System (ADS)

    Goeller, S.; Thoma, M.; Grosfeld, K.; Miller, H.

    2012-12-01

    There is currently no doubt about the existence of a wide-spread hydrological network under the Antarctic ice sheet, which lubricates the ice base and thus leads to increased ice velocities. Consequently, ice models should incorporate basal hydrology to obtain meaningful results for future ice dynamics and their contribution to global sea level rise. Here, we introduce the balanced water layer concept, covering two prominent subglacial hydrological features for ice sheet modeling on a continental scale: the evolution of subglacial lakes and balance water fluxes. We couple it to the thermomechanical ice-flow model RIMBAY and apply it to a synthetic model domain inspired by the Gamburtsev Mountains, Antarctica. In our experiments we demonstrate the dynamic generation of subglacial lakes and their impact on the velocity field of the overlaying ice sheet, resulting in a negative ice mass balance. Furthermore, we introduce an elementary parametrization of the water flux-basal sliding coupling and reveal the predominance of the ice loss through the resulting ice streams against the stabilizing influence of less hydrologically active areas. We point out, that established balance flux schemes quantify these effects only partially as their ability to store subglacial water is lacking.

  13. Inferring unknow boundary conditions of the Greenland Ice Sheet by assimilating ICESat-1 and IceBridge altimetry intothe Ice Sheet System Model.

    NASA Astrophysics Data System (ADS)

    Larour, E. Y.; Khazendar, A.; Seroussi, H. L.; Schlegel, N.; Csatho, B. M.; Schenk, A. F.; Rignot, E. J.; Morlighem, M.

    2014-12-01

    Altimetry signals from missions such as ICESat-1, CryoSat, EnviSat, as well as altimeters onboard Operation IceBridge provide vital insights into processes such as surface mass balance, mass transport and ice-flow dynamics. Historically however, ice-flow models have been focused on assimilating surface velocities from satellite-based radar observations, to infer properties such as basal friction or the position of the bedrock. Here, we leverage a new methodology based on automatic differentation of the Ice Sheet System Model to assimilate surface altimetry data into a reconstruction of the past decade of ice flow on the North Greenland area. We infer corrections to boundary conditions such as basal friction and surface mass balance, as well as corrections to the ice hardness, to best-match the observed altimetry record. We compare these corrections between glaciers such as Petermann Glacier, 79 North and Zacchariae Isstrom. The altimetry signals exhibit very different patterns between East and West, which translate into very different signatures for the inverted boundary conditions. This study gives us greater insights into what differentiates different basins, both in terms of mass transport and ice-flow dynamics, and what could bethe controlling mechanisms behind the very different evolutions of these basins.

  14. Ice-sheet dynamics through the Quaternary on the mid-Norwegian continental margin inferred from 3D seismic data.

    PubMed

    Montelli, A; Dowdeswell, J A; Ottesen, D; Johansen, S E

    2017-02-01

    Reconstructing the evolution of ice sheets is critical to our understanding of the global environmental system, but most detailed palaeo-glaciological reconstructions have hitherto focused on the very recent history of ice sheets. Here, we present a three-dimensional (3D) reconstruction of the changing nature of ice-sheet derived sedimentary architecture through the Quaternary Ice Age of almost 3 Ma. An extensive geophysical record documents a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the mid-Norwegian shelf since the beginning of the Quaternary. Spatial and temporal variability of the FIS is illustrated by the gradual development of fast-flowing ice streams and associated intensification of focused glacial erosion and sedimentation since that time. Buried subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and basal thermal regime. Lack of major subglacial meltwater channels suggests a largely distributed drainage system beneath the marine-terminating part of the FIS. This palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.

  15. Predicting Ice Sheet and Climate Evolution at Extreme Scales

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Heimbach, Patrick

    2016-02-06

    A main research objectives of PISCEES is the development of formal methods for quantifying uncertainties in ice sheet modeling. Uncertainties in simulating and projecting mass loss from the polar ice sheets arise primarily from initial conditions, surface and basal boundary conditions, and model parameters. In general terms, two main chains of uncertainty propagation may be identified: 1. inverse propagation of observation and/or prior onto posterior control variable uncertainties; 2. forward propagation of prior or posterior control variable uncertainties onto those of target output quantities of interest (e.g., climate indices or ice sheet mass loss). A related goal is the developmentmore » of computationally efficient methods for producing initial conditions for an ice sheet that are close to available present-day observations and essentially free of artificial model drift, which is required in order to be useful for model projections (“initialization problem”). To be of maximum value, such optimal initial states should be accompanied by “useful” uncertainty estimates that account for the different sources of uncerainties, as well as the degree to which the optimum state is constrained by available observations. The PISCEES proposal outlined two approaches for quantifying uncertainties. The first targets the full exploration of the uncertainty in model projections with sampling-based methods and a workflow managed by DAKOTA (the main delivery vehicle for software developed under QUEST). This is feasible for low-dimensional problems, e.g., those with a handful of global parameters to be inferred. This approach can benefit from derivative/adjoint information, but it is not necessary, which is why it often referred to as “non-intrusive”. The second approach makes heavy use of derivative information from model adjoints to address quantifying uncertainty in high-dimensions (e.g., basal boundary conditions in ice sheet models). The use of local gradient

  16. Annual accumulation over the Greenland ice sheet interpolated from historical and newly compiled observation data

    USGS Publications Warehouse

    Shen, Dayong; Liu, Yuling; Huang, Shengli

    2012-01-01

    The estimation of ice/snow accumulation is of great significance in quantifying the mass balance of ice sheets and variation in water resources. Improving the accuracy and reducing uncertainty has been a challenge for the estimation of annual accumulation over the Greenland ice sheet. In this study, we kriged and analyzed the spatial pattern of accumulation based on an observation data series including 315 points used in a recent research, plus 101 ice cores and snow pits and newly compiled 23 coastal weather station data. The estimated annual accumulation over the Greenland ice sheet is 31.2 g cm−2 yr−1, with a standard error of 0.9 g cm−2 yr−1. The main differences between the improved map developed in this study and the recently published accumulation maps are in the coastal areas, especially southeast and southwest regions. The analysis of accumulations versus elevation reveals the distribution patterns of accumulation over the Greenland ice sheet.

  17. Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments

    USGS Publications Warehouse

    Escutia, C.; De Santis, L.; Donda, F.; Dunbar, R.B.; Cooper, A. K.; Brancolini, Giuliano; Eittreim, S.L.

    2005-01-01

    The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene "climatic optimum" (14-10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14-10 Ma). Sequence WL-S9 forms under a polar

  18. Effect of Mantle Rheology on Viscous Heating induced during Ice Sheet Cycles

    NASA Astrophysics Data System (ADS)

    Huang, Pingping; Wu, Patrick; van der Wal, Wouter

    2017-04-01

    Hanyk et al. (2005) studied the viscous shear heating in the mantle induced by the surface loading and unloading of a parabolic-shaped Laurentide-size ice sheet. They found that for linear rheology, viscous heating is mainly concentrated below the ice sheet. The depth extent of the heating in the mantle is determined by the viscosity distribution. Also, the magnitude of viscous heating is significantly affected by the rate of ice thickness change. However, only one ice sheet has been considered in their work and the interactions between ice sheets and ocean loading have been neglected. Furthermore, only linear rheology has been considered, although they suggested that non-Newtonian rheology may have a stronger effect. Here we follow Hanyk et al. (2005) and computed the viscous dissipation for viscoelastic models using the finite element methodology of Wu (2004) and van der Wal et al. (2010). However, the global ICE6G model (Peltier et al. 2015) with realistic oceans is used here to provide the surface loading. In addition, viscous heating in non-linear rheology, composite rheology, in addition to linear rheology with uniform or VM5a profile are computed and compared. Our results for linear rheology mainly confirm the findings of Hanyk et al. (2005). For both non-linear and composite rheologies, viscous heating is also mainly distributed near and under the ice sheets, but, more concentrated; depending on the horizontal dimension of the ice sheet, it can extend into the lower mantle, but for some of the time, not as deep as that for linear rheology. For composite rheology, the viscous heating is dominated by the effect of non-linear relation between the stress and the strain. The ice history controls the time when the local maximum in viscous heating appears. However, the magnitude of the viscous heating is affected by mantle rheology as well as the ice loading. Due to viscosity stratification, the shape of the region with high viscous heating in model VM5a is a

  19. Insights into Spatial Sensitivities of Ice Mass Response to Environmental Change from the SeaRISE Ice Sheet Modeling Project I: Antarctica

    NASA Technical Reports Server (NTRS)

    Nowicki, Sophie; Bindschadler, Robert A.; Abe-Ouchi, Ayako; Aschwanden, Andy; Bueler, Ed; Choi, Hyengu; Fastook, Jim; Granzow, Glen; Greve, Ralf; Gutowski, Gail; hide

    2013-01-01

    Atmospheric, oceanic, and subglacial forcing scenarios from the Sea-level Response to Ice Sheet Evolution (SeaRISE) project are applied to six three-dimensional thermomechanical ice-sheet models to assess Antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies. Results indicate (i) growth with warming, except within low-latitude basins (where inland thickening is outpaced by marginal thinning); (ii) mass loss with enhanced sliding (with basins dominated by high driving stresses affected more than basins with low-surface-slope streaming ice); and (iii) mass loss with enhanced ice shelf melting (with changes in West Antarctica dominating the signal due to its marine setting and extensive ice shelves; cf. minimal impact in the Terre Adelie, George V, Oates, and Victoria Land region of East Antarctica). Ice loss due to dynamic changes associated with enhanced sliding and/or sub-shelf melting exceeds the gain due to increased precipitation. Furthermore, differences in results between and within basins as well as the controlling impact of sub-shelf melting on ice dynamics highlight the need for improved understanding of basal conditions, grounding-zone processes, ocean-ice interactions, and the numerical representation of all three.

  20. Free oscillations in a climate model with ice-sheet dynamics

    NASA Technical Reports Server (NTRS)

    Kallen, E.; Crafoord, C.; Ghil, M.

    1979-01-01

    A study of stable periodic solutions to a simple nonlinear model of the ocean-atmosphere-ice system is presented. The model has two dependent variables: ocean-atmosphere temperature and latitudinal extent of the ice cover. No explicit dependence on latitude is considered in the model. Hence all variables depend only on time and the model consists of a coupled set of nonlinear ordinary differential equations. The globally averaged ocean-atmosphere temperature in the model is governed by the radiation balance. The reflectivity to incoming solar radiation, i.e., the planetary albedo, includes separate contributions from sea ice and from continental ice sheets. The major physical mechanisms active in the model are (1) albedo-temperature feedback, (2) continental ice-sheet dynamics and (3) precipitation-rate variations. The model has three-equilibrium solutions, two of which are linearly unstable, while one is linearly stable. For some choices of parameters, the stability picture changes and sustained, finite-amplitude oscillations obtain around the previously stable equilibrium solution. The physical interpretation of these oscillations points to the possibility of internal mechanisms playing a role in glaciation cycles.

  1. Formation of a wave on an ice-sheet above the dipole, moving in a fluid

    NASA Astrophysics Data System (ADS)

    Il'ichev, A. T.; Savin, A. A.; Savin, A. S.

    2012-05-01

    Theory of wave motions of a fluid with an ice-sheet was developed due to the necessity of solving of a number of problems of marine and land physics. The main attention in these investigations was focused on propagation and interaction of free waves, and also on appearance of waves under action of different loadings on the ice-sheet. From the other side, the problems dealing with waves on the fluid surface, free from the ice due to motion in the mass of the fluid of rigid bodies, has the known solutions. In this connection, it seems natural to disserminate the formulation and methods of such problems to the case of the fluid with the ice-sheet. In the present note we describe the character of formation of waves from the singularity, localized in the fluid of infinite depth beneath the ice-sheet. We use the example of the dipole, which models a cylinder in the infinite mass of the fluid. The character of the formation does not depend on the type of singularity. The ice-sheet is considered as a thin elastic plate of a constant width, floating on the water surface.

  2. Continuous, Pulsed Export of Methane-Supersaturated Meltwaters from the Bed of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Lamarche-Gagnon, G.; Wadham, J.; Beaton, A.; Fietzek, P.; Stanley, K. M.; Tedstone, A.; Sherwood Lollar, B.; Lacrampe Couloume, G.; Telling, J.; Liz, B.; Hawkings, J.; Kohler, T. J.; Zarsky, J. D.; Stibal, M.; Mowlem, M. C.

    2016-12-01

    Both past and present ice sheets have been proposed to cap large quantities of methane (CH4), on orders of magnitude significant enough to impact global greenhouse gas concentrations during periods of rapid ice retreat. However, to date most evidence for sub-ice sheet methane has been indirect, derived from calculations of the methanogenic potential of basal-ice microbial communities and biogeochemical models; field-based empirical measurements are lacking from large ice sheet catchments. Here, we present the first continuous, in situ record of dissolved methane export from a large catchment of the Greenland Ice Sheet (GrIS) in South West Greenland from May-July 2015. Our results indicate that glacial runoff was continuously supersaturated with methane over the observation period (dissolved CH4 concentrations of 30-700 nM), with total methane flux rising as subglacial discharge increased. Periodic subglacial drainage events, characterised by rapid changes (i.e. pulses) in meltwater hydrochemistry, also coincided with a rise in methane concentrations. We argue that these are likely indicative of the flushing of subglacial reservoirs of CH4 beneath the ice sheet. Total methane export was relatively modest when compared to global methane budgets, but too high to be explained by previously determined methanogenic rates from Greenland basal ice. Discrepancies between estimated Greenland methane reserves and observed fluxes stress the need to further investigate GrIS methane fluxes and sources, and suggest a more biogeochemically active subglacial environment than previously considered. Results indicate that future warming, and a coincident increase in ice melt rates, would likely make the GrIS, and by extension the Antarctic Ice Sheet, more significant sources of atmospheric methane, consequently acting as a positive feedback to a warming climate.

  3. Marine evidence of a deconvolving Antarctic Ice Sheet during post-LGM retreat of the Ross Sea sector

    NASA Astrophysics Data System (ADS)

    Prothro, L. O.; Yokoyama, Y.; Simkins, L. M.; Anderson, J. B.; Majewski, W.; Yamane, M.; Ohkouchi, N.

    2017-12-01

    Predictive models of ice sheet and sea level change are dependent on observational data of ice-sheet behavior for model testing and tuning. The geologic record contains a wealth of information about ice-sheet dynamics, with fewer logistical, spatial, and temporal limitations than are involved in data acquisition along contemporary ice margins. However, past ice-sheet behavior is still largely uncertain or contested due to issues with obtaining meaningful radiocarbon dates. We minimize bias from glacially-reworked carbon and limitations from unknown geomorphic context and uncertainty in sediment facies identification by using careful sedimentary analyses within a geomorphic framework, as well as selection of appropriate dating methods. Our study area, the Ross Sea sector of Antarctica, is the primary drainage outlet for 25% of the continent's grounded ice. During the Last Glacial Maximum, the low-profile, marine-based West Antarctic Ice Sheet (WAIS) and the steeper profile, largely land-based East Antarctic Ice Sheet (EAIS) converged in the Ross Sea to flow out to or near the continental shelf edge. Geomorphic and sedimentary data reveal that during their subsequent retreat to form the Ross Sea Embayment, the two ice sheets behaved differently, with the WAIS rapidly retreating tens of kilometers followed by extended pauses, while the EAIS retreated steadily, with shorter (decadal- to century-long) pauses. This behavior leads us to believe that the two ice sheets may have contributed diachronously to sea level. By acquiring accurate timing of grounding line retreat, we are able to calculate volumes of ice lost throughout deglaciation, as well as associated sea level contributions. In addition, we attempt to rectify the contradicting marine and terrestrial interpretations of retreat patterns from the Ross Sea continental shelf.

  4. Retreat of the Southwest Labrador Sector of the Laurentide Ice Sheet During the Last Termination

    NASA Astrophysics Data System (ADS)

    Lowell, T. V.; Kelly, M. A.; Fisher, T. G.; Barnett, P. J.; Howley, J. A.; Zimmerman, S. R. H.

    2016-12-01

    Large ice sheets are suspected to have played a major role in forcing the transitions from glacial to interglacial conditions, known as terminations. To improve the understanding of the role of the Laurentide Ice Sheet in the last termination, we present a chronology of ice sheet recession from just subsequent to end of the Last Glacial Maximum (LGM) to the early Holocene. We focus on the retreat of the southwest Labrador Sector of the ice sheet in northern Minnesota and adjacent Ontario. Multiple moraines in this region mark an overall pattern of ice recession interrupted by stillstands and/or minor readvances. Radiocarbon and 10Be ages from 50 sites along this 400 km-long transect indicate that the oldest moraine complex, the Vermillion moraine, formed at 17.0 ka. Subsequently, the ice margin retreated with minor standstills until the Dog Lake moraine was deposited between 12.7 and 12.3 ka. Recession from the Dog Lake moraine began by 12.3 ka the ice margin receded 150 km to the north-northeast by 10.7 ka. In general, the radiocarbon and 10Be ages define a pattern of near-continuous ice sheet retreat. Deposition of the Vermillion and Dog Lake moraines occurred at the beginning of Heinrich stadials 1 ( 17.5-14.5 ka) and 0 ( 12.9-11.7 ka), respectively, but ice recession occurred throughout the remainder of these stadials. This pattern is different from climate conditions registered by Greenland ice cores which show cold conditions from the end of the LGM until the Bølling warming at 14.5 ka, and throughout the Younger Dryas ( 12.9-11.7 ka). We suggest that the pattern of ice sheet recession is more similar to mountain glaciers in the southern mid-latitudes and tropics, and that Heinrich stadials may have been characterized by warming at least in the summertime that influenced near global ice recession.

  5. A microbial ecosystem beneath the West Antarctic ice sheet.

    PubMed

    Christner, Brent C; Priscu, John C; Achberger, Amanda M; Barbante, Carlo; Carter, Sasha P; Christianson, Knut; Michaud, Alexander B; Mikucki, Jill A; Mitchell, Andrew C; Skidmore, Mark L; Vick-Majors, Trista J

    2014-08-21

    Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years, but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale. Here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial Antarctic lake. Subglacial Lake Whillans (SLW) lies beneath approximately 800 m of ice on the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and evolving subglacial drainage network. The water column of SLW contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component. Heterotrophic and autotrophic production data together with small subunit ribosomal RNA gene sequencing and biogeochemical data indicate that SLW is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea. Our results confirm that aquatic environments beneath the Antarctic ice sheet support viable microbial ecosystems, corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes that can mobilize elements from the lithosphere and influence Southern Ocean geochemical and biological systems.

  6. Initiation and long-term instability of the East Antarctic Ice Sheet.

    PubMed

    Gulick, Sean P S; Shevenell, Amelia E; Montelli, Aleksandr; Fernandez, Rodrigo; Smith, Catherine; Warny, Sophie; Bohaty, Steven M; Sjunneskog, Charlotte; Leventer, Amy; Frederick, Bruce; Blankenship, Donald D

    2017-12-13

    Antarctica's continental-scale ice sheets have evolved over the past 50 million years. However, the dearth of ice-proximal geological records limits our understanding of past East Antarctic Ice Sheet (EAIS) behaviour and thus our ability to evaluate its response to ongoing environmental change. The EAIS is marine-terminating and grounded below sea level within the Aurora subglacial basin, indicating that this catchment, which drains ice to the Sabrina Coast, may be sensitive to climate perturbations. Here we show, using marine geological and geophysical data from the continental shelf seaward of the Aurora subglacial basin, that marine-terminating glaciers existed at the Sabrina Coast by the early to middle Eocene epoch. This finding implies the existence of substantial ice volume in the Aurora subglacial basin before continental-scale ice sheets were established about 34 million years ago. Subsequently, ice advanced across and retreated from the Sabrina Coast continental shelf at least 11 times during the Oligocene and Miocene epochs. Tunnel valleys associated with half of these glaciations indicate that a surface-meltwater-rich sub-polar glacial system existed under climate conditions similar to those anticipated with continued anthropogenic warming. Cooling since the late Miocene resulted in an expanded polar EAIS and a limited glacial response to Pliocene warmth in the Aurora subglacial basin catchment. Geological records from the Sabrina Coast shelf indicate that, in addition to ocean temperature, atmospheric temperature and surface-derived meltwater influenced East Antarctic ice mass balance under warmer-than-present climate conditions. Our results imply a dynamic EAIS response with continued anthropogenic warming and suggest that the EAIS contribution to future global sea-level projections may be under-estimated.

  7. Initiation and long-term instability of the East Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Gulick, Sean P. S.; Shevenell, Amelia E.; Montelli, Aleksandr; Fernandez, Rodrigo; Smith, Catherine; Warny, Sophie; Bohaty, Steven M.; Sjunneskog, Charlotte; Leventer, Amy; Frederick, Bruce; Blankenship, Donald D.

    2017-12-01

    Antarctica’s continental-scale ice sheets have evolved over the past 50 million years. However, the dearth of ice-proximal geological records limits our understanding of past East Antarctic Ice Sheet (EAIS) behaviour and thus our ability to evaluate its response to ongoing environmental change. The EAIS is marine-terminating and grounded below sea level within the Aurora subglacial basin, indicating that this catchment, which drains ice to the Sabrina Coast, may be sensitive to climate perturbations. Here we show, using marine geological and geophysical data from the continental shelf seaward of the Aurora subglacial basin, that marine-terminating glaciers existed at the Sabrina Coast by the early to middle Eocene epoch. This finding implies the existence of substantial ice volume in the Aurora subglacial basin before continental-scale ice sheets were established about 34 million years ago. Subsequently, ice advanced across and retreated from the Sabrina Coast continental shelf at least 11 times during the Oligocene and Miocene epochs. Tunnel valleys associated with half of these glaciations indicate that a surface-meltwater-rich sub-polar glacial system existed under climate conditions similar to those anticipated with continued anthropogenic warming. Cooling since the late Miocene resulted in an expanded polar EAIS and a limited glacial response to Pliocene warmth in the Aurora subglacial basin catchment. Geological records from the Sabrina Coast shelf indicate that, in addition to ocean temperature, atmospheric temperature and surface-derived meltwater influenced East Antarctic ice mass balance under warmer-than-present climate conditions. Our results imply a dynamic EAIS response with continued anthropogenic warming and suggest that the EAIS contribution to future global sea-level projections may be under-estimated.

  8. Gradual demise of a thin southern Laurentide ice sheet recorded by Mississippi drainage.

    PubMed

    Wickert, Andrew D; Mitrovica, Jerry X; Williams, Carlie; Anderson, Robert S

    2013-10-31

    At the Last Glacial Maximum (LGM), about 21,000 years before present, land-based ice sheets held enough water to reduce global mean sea level by 130 metres. Yet after decades of study, major uncertainties remain as to the distribution of that ice. Here we test four reconstructions of North American deglacial ice-sheet history by quantitatively connecting them to high-resolution oxygen isotope (δ(18)O) records from the Gulf of Mexico using a water mixing model. For each reconstruction, we route meltwater and seasonal runoff through the time-evolving Mississippi drainage basin, which co-evolves with ice geometry and changing topography as ice loads deform the solid Earth and produce spatially variable sea level in a process known as glacial isostatic adjustment. The δ(18)O records show that the Mississippi-drained southern Laurentide ice sheet contributed only 5.4 ± 2.1 metres to global sea level rise, of which 0.66 ± 0.07 metres were released during the meltwater pulse 1A event 14,650-14,310 years before present, far less water than previously thought. In contrast, the three reconstructions based on glacial isostatic adjustment overpredict the δ(18)O-based post-LGM meltwater volume by a factor of 1.6 to 3.6. The fourth reconstruction, which is based on ice physics, has a low enough Mississippi-routed meltwater discharge to be consistent with δ(18)O constraints, but also contains the largest LGM North American ice volume. This suggests that modelling based on ice physics may be the best way of matching isotopic records while also sequestering enough water in the North American ice sheets to match the observed LGM sea level fall.

  9. Whillans Ice Stream Subglacial Access Research Drilling (WISSARD): Integrative Study of Marine Ice Sheet Stability and Subglacial Life Habitats (Invited)

    NASA Astrophysics Data System (ADS)

    Tulaczyk, S. M.; Anandakrishnan, S.; Behar, A. E.; Christner, B. C.; Fisher, A. T.; Fricker, H. A.; Holland, D. M.; Jacobel, R. W.; Mikucki, J.; Mitchell, A. C.; Powell, R. D.; Priscu, J. C.; Scherer, R. P.; Severinghaus, J. P.

    2009-12-01

    The WISSARD project is a large, NSF-funded, interdisciplinary initiative focused on scientific drilling, exploration, and investigation of Antarctic subglacial aquatic environments. The project consists of three interrelated components: (1) LISSARD - Lake and Ice Stream Subglacial Access Research Drilling, (2) RAGES - Robotic Access to Grounding-zones for Exploration and Science, and (3) GBASE - GeomicroBiology of Antarctic Subglacial Environments). A number of previous studies in West Antarctica highlighted the importance of understanding ice sheet interactions with water, either at the basal boundary where ice streams come in contact with active subglacial hydrologic and geological systems or at the marine margin where the ice sheet is exposed to forcing from the global ocean and sedimentation. Recent biological investigations of Antarctic subglacial environments show that they provide a significant habitat for life and source of bacterial carbon in a setting that was previously thought to be inhospitable. Subglacial microbial ecosystems also enhance biogeochemical weathering, mobilizing elements from long term geological storage. The overarching scientific objective of WISSARD is to examine the subglacial hydrological system of West Antarctica in glaciological, geological, microbiological, geochemical, and oceanographic contexts. Direct sampling will yield seminal information on these systems and test the overarching hypothesis that active hydrological systems connect various subglacial environments and exert major control on ice sheet dynamics, subglacial sediment transfer, geochemistry, metabolic and phylogenetic diversity, and biogeochemical transformations and geological records of ice sheet history. Technological advances during WISSARD will provide the US-science community with a capability to access and study sub-ice sheet environments. Developing this technological infrastructure will benefit the broader science community and it will be available for

  10. Isochronal Ice Sheet Model: a New Approach to Tracer Transport by Explicitly Tracing Accumulation Layers

    NASA Astrophysics Data System (ADS)

    Born, A.; Stocker, T. F.

    2014-12-01

    The long, high-resolution and largely undisturbed depositional record of polar ice sheets is one of the greatest resources in paleoclimate research. The vertical profile of isotopic and other geochemical tracers provides a full history of depositional and dynamical variations. Numerical simulations of this archive could afford great advances both in the interpretation of these tracers as well as to help improve ice sheet models themselves, as show successful implementations in oceanography and atmospheric dynamics. However, due to the slow advection velocities, tracer modeling in ice sheets is particularly prone to numerical diffusion, thwarting efforts that employ straightforward solutions. Previous attemps to circumvent this issue follow conceptually and computationally extensive approaches that augment traditional Eulerian models of ice flow with a semi-Lagrangian tracer scheme (e.g. Clarke et al., QSR, 2005). Here, we propose a new vertical discretization for ice sheet models that eliminates numerical diffusion entirely. Vertical motion through the model mesh is avoided by mimicking the real-world ice flow as a thinning of underlying layers (see figure). A new layer is added to the surface at equidistant time intervals (isochronally). Therefore, each layer is uniquely identified with an age. Horizontal motion follows the shallow ice approximation using an implicit numerical scheme. Vertical diffusion of heat which is physically desirable is also solved implicitly. A simulation of a two-dimensional section through the Greenland ice sheet will be discussed.

  11. Ice Streams as the Critical Link Between the Interior Ice Reservoir of the Antarctic Ice Sheet and the Global Climate System - a WISSARD Perspective (Invited)

    NASA Astrophysics Data System (ADS)

    Tulaczyk, S. M.; Beem, L.; Walter, J. I.; Hossainzadeh, S.; Mankoff, K. D.

    2010-12-01

    Fast flowing ice streams represent crucial features of the Antarctic ice sheet because they provide discharge ‘valves’ for the interior ice reservoir and because their grounding lines are exposed to ocean thermal forcing. Even with no/little topographic control ice flow near the perimeter of a polar ice sheet self-organizes into discrete, fast-flowing ice streams. Within these features basal melting (i.e. lubrication for ice sliding) is sustained through elevated basal shear heating in a region of thin ice that would otherwise be characterized by basal freezing and slow ice motion. Because faster basal ice motion is typically associated with faster subglacial erosion, ice streams tend to localize themselves over time by carving troughs into underlying rocks and sediments. Debris generated by this erosional activity is carried to the continental shelf and/or continental slope where it may be deposited at very high rates, rivaling these associated with deposition by some of the largest rivers on Earth. In terms of their hydrologic and geological functions, Antarctic ice streams play pretty much the same role as rivers do on non-glaciated continents. However, understanding of their dynamics is still quite rudimentary, largely because of the relative inaccessibility of the key basal and marine boundaries of ice streams where pertinent measurements need to be made. The present elevated interest in predicting future contribution of Antarctica to global sea level changes is driving ambitious research programs aimed at scientific exploration of these poorly investigated environments that will play a key role in defining the response of the ice sheet to near future climate changes. We will review one of these programs, the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) with particular focus on its planned contributions to understanding of ice stream dynamics.

  12. Inception and variability of the Antarctic ice sheet across the Eocene-Oligocene transition

    NASA Astrophysics Data System (ADS)

    Stocchi, Paolo; Galeotti, Simone; Ladant, Jan-Baptiste; DeConto, Robert; Vermeersen, Bert; Rugenstein, Maria

    2014-05-01

    Climate cooling throughout middle to late Eocene (~48 - 34 Million years ago, Ma) triggered the transition from hot-house to ice-house conditions. Based on deep-sea marine δ18O values, a continental-scale Antarctic Ice Sheet (AIS) rapidly developed across the Eocene-Oligocene transition (EOT) in two ~200 kyr-spaced phases between 34.0 - 33.5 Ma. Regardless of the geographical configuration of southern ocean gateways, geochemical data and ice-sheet modelling show that AIS glaciation initiated as atmospheric CO2 fell below ~2.5 times pre-industrial values. AIS likely reached or even exceeded present-day dimensions. Quantifying the magnitude and timing of AIS volume variations by means of δ18O records is hampered by the fact that the latter reflect a coupled signal of temperature and ice-sheet volume. Besides, bathymetric variations based on marine geologic sections are affected by large uncertainties and, most importantly, reflect the local response of relative sea level (rsl) to ice volume fluctuations rather than the global eustatic signal. AIS proximal and Northern Hemisphere (NH) marine settings show an opposite trend of rsl change across the EOT. In fact, consistently with central values based on δ18O records, an 60 ± 20m rsl drop is estimated from NH low-latitude shallow marine sequences. Conversely, sedimentary facies from shallow shelfal areas in the proximity of the AIS witness an 50 - 150m rsl rise across the EOT. Accounting for ice-load-induced crustal and geoidal deformations and for the mutual gravitational attraction between the growing AIS and the ocean water is a necessary requirement to reconcile near- and far-field rsl sites, regardless of tectonics and of any other possible local contamination. In this work we investigate the AIS inception and variability across the EOT by combining the observed rsl changes with predictions based on numerical modeling of Glacial Isostatic Adjustment (GIA). We solve the gravitationally self-consistent Sea Level

  13. Influence of glacial ice sheets on the Atlantic meridional overturning circulation through surface wind change

    NASA Astrophysics Data System (ADS)

    Sherriff-Tadano, Sam; Abe-Ouchi, Ayako; Yoshimori, Masakazu; Oka, Akira; Chan, Wing-Le

    2018-04-01

    Coupled modeling studies have recently shown that the existence of the glacial ice sheets intensifies the Atlantic meridional overturning circulation (AMOC). However, most models show a strong AMOC in their simulations of the Last Glacial Maximum (LGM), which is biased compared to reconstructions that indicate both a weaker and stronger AMOC during the LGM. Therefore, a detailed investigation of the mechanism behind this intensification of the AMOC is important for a better understanding of the glacial climate and the LGM AMOC. Here, various numerical simulations are conducted to focus on the effect of wind changes due to glacial ice sheets on the AMOC and the crucial region where the wind modifies the AMOC. First, from atmospheric general circulation model experiments, the effect of glacial ice sheets on the surface wind is evaluated. Second, from ocean general circulation model experiments, the influence of the wind stress change on the AMOC is evaluated by applying wind stress anomalies regionally or at different magnitudes as a boundary condition. These experiments demonstrate that glacial ice sheets intensify the AMOC through an increase in the wind stress at the North Atlantic mid-latitudes, which is induced by the North American ice sheet. This intensification of the AMOC is caused by the increased oceanic horizontal and vertical transport of salt, while the change in sea ice transport has an opposite, though minor, effect. Experiments further show that the Eurasian ice sheet intensifies the AMOC by directly affecting the deep-water formation in the Norwegian Sea.

  14. Tropical tales of polar ice: evidence of Last Interglacial polar ice sheet retreat recorded by fossil reefs of the granitic Seychelles islands

    NASA Astrophysics Data System (ADS)

    Dutton, Andrea; Webster, Jody M.; Zwartz, Dan; Lambeck, Kurt; Wohlfarth, Barbara

    2015-01-01

    In the search for a record of eustatic sea level change on glacial-interglacial timescales, the Seychelles ranks as one of the best places on the planet to study. Owing to its location with respect to the former margins of Northern Hemisphere ice sheets that wax and wane on orbital cycles, the local-or relative-sea level history is predicted to lie within a few meters of the globally averaged eustatic signal during the Last Interglacial period. We have surveyed and dated Last Interglacial fossil corals to ascertain peak sea level and hence infer maximum retreat of polar ice sheets during this time interval. We observe a pattern of gradually rising sea level in the Seychelles between ˜129 and 125 thousand years ago (ka), with peak eustatic sea level attained after 125 ka at 7.6 ± 1.7 m higher than present. After accounting for thermal expansion and loss of mountain glaciers, this sea-level budget would require ˜5-8 m of polar ice sheet contribution, relative to today's volume, of which only ˜2 m came from the Greenland ice sheet. This result clearly identifies the Antarctic ice sheet as a significant source of melt water, most likely derived from one of the unstable, marine-based sectors in the West and/or East Antarctic ice sheet. Furthermore, the establishment of a +5.9 ± 1.7 m eustatic sea level position by 128.6 ± 0.8 ka would require that partial AIS collapse was coincident with the onset of the sea level highstand.

  15. Aerogeophysical evidence for active volcanism beneath the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    Blankenship, Donald D.; Bell, Robin E.; Hodge, Steven M.; Brozena, John M.; Behrendt, John C.

    1993-01-01

    Although it is widely understood that the collapse of the West Antarctic Ice Sheet (WAIS) would cause a global sea-level rise of 6 m, there continues to be considerable debate about the response of this ice sheet to climate change. The stability of the WAIS, which is characterized by a bed grounded well below sea level, may depend on geologically controlled conditions at the base, which are independent of climate. Ice streams moving up to 750 m/yr disperse material from the interior through to the oceans. As these ice streams tend to buffer the reservoir of slow-moving inland ice from exposure to oceanic degradation, understanding the ice-streaming process is important for evaluating WAIS stability. There is strong evidence that ice streams slide on a lubricating layer of water-saturated till. Development of this basal layer requires both water and easily eroded sediments. Active lithospheric extension may elevate regional heat flux, increase basal melting, and trigger ice streaming. If a geologically defined boundary with a sharp contrast in geothermal flux exists beneath the WAIS, ice streams may only be capable of operating as a buffer over a restricted region. Should ocean waters penetrate beyond this boundary, the ice-stream buffer would disappear, possibly triggering a collapse of the inland ice reservoir. Aerogeophysical evidence for active volcanism and elevated heat flux beneath the WAIS near the critical region where ice streaming begins is presented.

  16. Documenting Melting Features of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Tedesco, M.

    2011-12-01

    There is an increasing interest in studying the Greenland Ice Sheet, its hydrology and dynamics over the short term and longer term because of the potential impact of a warming Arctic. Major studies concern about whether increased surface melting will lead to changes in production of supraglacial lakes and subglacial water pressures and hence , potentially, rates of ice movement. In this talk I will show movies recorded over the past three years form fieldwork activities carried out over the West Greenland ice sheet. In particular, I will project and comment movies concerning surface streams and supraglacial lakes, as the one at http://www.youtube.com/watch?v=QbuFphwJn4c. I will discuss the importance of observing such phenomena and how the recorded videos can be used to summarize scientific studies and communicate the relevance of scientific findings. I will also show, for the first time, the video of the drainage of a supraglacial lake, an event during which a lake ~ 6 m deep and ~ 1 km drained in ~ 1.5 hours. This section of the movie is under development as video material was collected during our latest expedition in June 2011.

  17. Greenland ice sheet mass balance: a review.

    PubMed

    Khan, Shfaqat A; Aschwanden, Andy; Bjørk, Anders A; Wahr, John; Kjeldsen, Kristian K; Kjær, Kurt H

    2015-04-01

    Over the past quarter of a century the Arctic has warmed more than any other region on Earth, causing a profound impact on the Greenland ice sheet (GrIS) and its contribution to the rise in global sea level. The loss of ice can be partitioned into processes related to surface mass balance and to ice discharge, which are forced by internal or external (atmospheric/oceanic/basal) fluctuations. Regardless of the measurement method, observations over the last two decades show an increase in ice loss rate, associated with speeding up of glaciers and enhanced melting. However, both ice discharge and melt-induced mass losses exhibit rapid short-term fluctuations that, when extrapolated into the future, could yield erroneous long-term trends. In this paper we review the GrIS mass loss over more than a century by combining satellite altimetry, airborne altimetry, interferometry, aerial photographs and gravimetry data sets together with modelling studies. We revisit the mass loss of different sectors and show that they manifest quite different sensitivities to atmospheric and oceanic forcing. In addition, we discuss recent progress in constructing coupled ice-ocean-atmosphere models required to project realistic future sea-level changes.

  18. Ross Sea paleo-ice sheet drainage and deglacial history during and since the LGM

    NASA Astrophysics Data System (ADS)

    Anderson, John B.; Conway, Howard; Bart, Philip J.; Witus, Alexandra E.; Greenwood, Sarah L.; McKay, Robert M.; Hall, Brenda L.; Ackert, Robert P.; Licht, Kathy; Jakobsson, Martin; Stone, John O.

    2014-09-01

    Onshore and offshore studies show that an expanded, grounded ice sheet occupied the Ross Sea Embayment during the Last Glacial Maximum (LGM). Results from studies of till provenance and the orientation of geomorphic features on the continental shelf show that more than half of the grounded ice sheet consisted of East Antarctic ice flowing through Transantarctic Mountain (TAM) outlet glaciers; the remainder came from West Antarctica. Terrestrial data indicate little or no thickening in the upper catchment regions in both West and East Antarctica during the LGM. In contrast, evidence from the mouths of the southern and central TAM outlet glaciers indicate surface elevations between 1000 m and 1100 m (above present-day sea level). Farther north along the western margin of the Ross Ice Sheet, surface elevations reached 720 m on Ross Island, and 400 m at Terra Nova Bay. Evidence from Marie Byrd Land at the eastern margin of the ice sheet indicates that the elevation near the present-day grounding line was more than 800 m asl, while at Siple Dome in the central Ross Embayment, the surface elevation was about 950 m asl. Farther north, evidence that the ice sheet was grounded on the middle and the outer continental shelf during the LGM implies that surface elevations had to be at least 100 m above the LGM sea level. The apparent low surface profile and implied low basal shear stress in the central and eastern embayment suggests that although the ice streams may have slowed during the LGM, they remained active. Ice-sheet retreat from the western Ross Embayment during the Holocene is constrained by marine and terrestrial data. Ages from marine sediments suggest that the grounding line had retreated from its LGM outer shelf location only a few tens of kilometer to a location south of Coulman Island by ˜13 ka BP. The ice sheet margin was located in the vicinity of the Drygalski Ice Tongue by ˜11 ka BP, just north of Ross Island by ˜7.8 ka BP, and near Hatherton Glacier by

  19. Microbial oxidation as a methane sink beneath the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Michaud, Alexander B.; Dore, John E.; Achberger, Amanda M.; Christner, Brent C.; Mitchell, Andrew C.; Skidmore, Mark L.; Vick-Majors, Trista J.; Priscu, John C.

    2017-08-01

    Aquatic habitats beneath ice masses contain active microbial ecosystems capable of cycling important greenhouse gases, such as methane (CH4). A large methane reservoir is thought to exist beneath the West Antarctic Ice Sheet, but its quantity, source and ultimate fate are poorly understood. For instance, O2 supplied by basal melting should result in conditions favourable for aerobic methane oxidation. Here we use measurements of methane concentrations and stable isotope compositions along with genomic analyses to assess the sources and cycling of methane in Subglacial Lake Whillans (SLW) in West Antarctica. We show that sub-ice-sheet methane is produced through the biological reduction of CO2 using H2. This methane pool is subsequently consumed by aerobic, bacterial methane oxidation at the SLW sediment-water interface. Bacterial oxidation consumes >99% of the methane and represents a significant methane sink, and source of biomass carbon and metabolic energy to the surficial SLW sediments. We conclude that aerobic methanotrophy may mitigate the release of methane to the atmosphere upon subglacial water drainage to ice sheet margins and during periods of deglaciation.

  20. Sustained High Basal Motion of the Greenland Ice Sheet Revealed by Borehole Deformation

    NASA Technical Reports Server (NTRS)

    Ryser, Claudia; Luthi, Martin P.; Andrews, Lauren C.; Hoffman, Matthew, J.; Catania, Ginny A.; Hawley, Robert L.; Neumann, Thomas A.; Kristensen, Steen S.

    2014-01-01

    Ice deformation and basal motion characterize the dynamical behavior of the Greenland ice sheet (GrIS). We evaluate the contribution of basal motion from ice deformation measurements in boreholes drilled to the bed at two sites in the western marginal zone of the GrIS. We find a sustained high amount of basal motion contribution to surface velocity of 44-73 percent in winter, and up to 90 percent in summer. Measured ice deformation rates show an unexpected variation with depth that can be explained with the help of an ice-flow model as a consequence of stress transfer from slippery to sticky areas. This effect necessitates the use of high-order ice-flow models, not only in regions of fast-flowing ice streams but in all temperate-based areas of the GrIS. The agreement between modeled and measured deformation rates confirms that the recommended values of the temperature-dependent flow rate factor A are a good choice for ice-sheet models.

  1. Correspondence between North Pacific Ocean ventilation, Cordilleran Ice Sheet variations, and North Atlantic Heinrich Events

    NASA Astrophysics Data System (ADS)

    Walczak, M. H.; Mix, A.; Fallon, S.; Praetorius, S. K.; Cowan, E. A.; Du, J.; Hobern, T.; Padman, J.; Fifield, L. K.; Stoner, J. S.; Haley, B. A.

    2017-12-01

    Much remains unresolved concerning the origin and global implications of the episodes of rapid glacial failure in the North Atlantic known as Heinrich Events. Thought to occur during or at the termination of the coldest of the abrupt stadial climate events known as Dansgaard-Oschger cycles, various trigger mechanisms have been theorized, including external forcing in the form of oceanic or atmospheric warming, internal dynamics of the large Laurentide ice sheet, or the episodic failure of another (presumably European) ice sheet. Heinrich events may also be associated with a decrease in North Atlantic deep-water formation. New results from Gulf of Alaska IODP Expedition 341 reveal events of Cordilleran Ice Sheet retreat (based on ice-rafted detritus and sedimentation rates) synchronous with reorganization of ocean circulation (based on benthic-planktic 14C pairs) spanning the past 45,000 years on an independent high-resolution radiocarbon-based chronology. We document the relationship between these Pacific records and the North Atlantic Heinrich events, and find the data show an early Pacific expression of ice sheet instability in the form of pulses of Cordilleran glacial discharge. The benthic radiocarbon anomalies in the Northeast Pacific contemporaneous with Cordilleran discharge events indicate a close coupling of ice-ocean dynamics throughout Marine Isotope Stage 2. These data are hard to reconcile with triggering in the North Atlantic or internal to the Laurentide ice sheet, requiring us to re-think both the mechanisms that generate Heinrich events and their far-field impacts.

  2. A Climate-Data Record (CDR) of the "Clear-Sky" Surface Temperature of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Hall, Dorothy K.; Comiso, Josefino C.; DiGirolamo, Nocolo E.; Shuman, Christopher A.

    2011-01-01

    We have developed a climate-data record (CDR) of "clear-sky" ice-surface temperature (IST) of the Greenland Ice Sheet using Moderate-Resolution Imaging Spectroradiometer (MODIS) data. The CDR provides daily and monthly-mean IST from March 2000 through December 2010 on a polar stereographic projection at a resolution of 6.25 km. The CDR is amenable to extension into the future using Visible/Infrared Imager Radiometer Suite (VIIRS) data. Regional "clear-sky" surface temperature increases since the early 1980s in the Arctic, measured using Advanced Very High Resolution Radiometer (AVHRR) infrared data, range from 0.57 +/- 0.02 to 0.72 +/- 0.1 c per decade. Arctic warming has important implications for ice-sheet mass balance because much of the periphery of the Greenland Ice Sheet is already near O C during the melt season, and is thus vulnerable to rapid melting if temperatures continue to increase. An increase in melting of the ice sheet would accelerate sea-level rise, an issue affecting potentially billions of people worldwide. The IST CDR will provide a convenient data set for modelers and for climatologists to track changes of the surface temperature of the ice sheet as a whole and of the individual drainage basins on the ice sheet. The daily and monthly maps will provide information on surface melt as well as "clear-sky" temperature. The CDR will be further validated by comparing results with automatic-weather station data and with satellite-derived surface-temperature products.

  3. Glacial removal of late Cenozoic subglacially emplaced volcanic edifices by the West Antarctic ice sheet

    USGS Publications Warehouse

    Behrendt, John C.; Blankenship, D.D.; Damaske, D.; Cooper, A. K.

    1995-01-01

    Local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the Ross Sea, northwestern Ross Ice Shelf, and the West Antarctic ice sheet, reveal a linear magnetic rift fabric and numerous subcircular, high-amplitude anomalies. Geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the West Antarctic rift system. In contrast, we suggest glacial removal of edifices of volcanic sources of many more anomalies. Magnetic models, controlled by marine seismic reflection and radar ice-sounding data, allow us to infer that glacial removal of the associated late Cenozoic volcanic edifices (probably debris, comprising pillow breccias, and hyaloclastites) has occurred essentially concomitantly with their subglacial eruption. "Removal' of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than "erosion', given its fragmented, ice-contact origin. The exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet. -from Authors

  4. Marine ice sheet model performance depends on basal sliding physics and sub-shelf melting

    NASA Astrophysics Data System (ADS)

    Gladstone, Rupert Michael; Warner, Roland Charles; Galton-Fenzi, Benjamin Keith; Gagliardini, Olivier; Zwinger, Thomas; Greve, Ralf

    2017-01-01

    Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flow-line model accounting for all stress components and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting.Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction in resolution dependence.A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line.Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), impact on capability to simulate marine ice sheets. If there exists an abrupt change across the grounding line in either basal drag or basal melting, then high resolution will be required to solve the problem. However, the plausible combination of a physical dependency of basal drag on effective pressure, and the possibility of low ice shelf basal melt rates next to the grounding line, may mean that some marine ice sheet systems can be reliably simulated at

  5. Incorporation of ice sheet models into an Earth system model: Focus on methodology of coupling

    NASA Astrophysics Data System (ADS)

    Rybak, Oleg; Volodin, Evgeny; Morozova, Polina; Nevecherja, Artiom

    2018-03-01

    Elaboration of a modern Earth system model (ESM) requires incorporation of ice sheet dynamics. Coupling of an ice sheet model (ICM) to an AOGCM is complicated by essential differences in spatial and temporal scales of cryospheric, atmospheric and oceanic components. To overcome this difficulty, we apply two different approaches for the incorporation of ice sheets into an ESM. Coupling of the Antarctic ice sheet model (AISM) to the AOGCM is accomplished via using procedures of resampling, interpolation and assigning to the AISM grid points annually averaged meanings of air surface temperature and precipitation fields generated by the AOGCM. Surface melting, which takes place mainly on the margins of the Antarctic peninsula and on ice shelves fringing the continent, is currently ignored. AISM returns anomalies of surface topography back to the AOGCM. To couple the Greenland ice sheet model (GrISM) to the AOGCM, we use a simple buffer energy- and water-balance model (EWBM-G) to account for orographically-driven precipitation and other sub-grid AOGCM-generated quantities. The output of the EWBM-G consists of surface mass balance and air surface temperature to force the GrISM, and freshwater run-off to force thermohaline circulation in the oceanic block of the AOGCM. Because of a rather complex coupling procedure of GrIS compared to AIS, the paper mostly focuses on Greenland.

  6. The evolution of the Antarctic ice sheet at the Eocene-Oligocene Transition.

    NASA Astrophysics Data System (ADS)

    Ladant, Jean-Baptiste; Donnadieu, Yannick; Dumas, Christophe

    2017-04-01

    An increasing number of studies suggest that the Middle to Late Eocene has witnessed the waxing and waning of relatively small ephemeral ice sheets. These alternating episodes culminated in the Eocene-Oligocene transition (34 - 33.5 Ma) during which a sudden and massive glaciation occurred over Antarctica. Data studies have demonstrated that this glacial event is constituted of two 50 kyr-long steps, the first of modest (10 - 30 m of equivalent sea level) and the second of major (50 - 90 m esl) glacial amplitude, and separated by 200 kyrs. Since a decade, modeling studies have put forward the primary role of CO2 in the initiation of this glaciation, in doing so marginalizing the original "gateway hypothesis". Here, we investigate the impacts of CO2 and orbital parameters on the evolution of the ice sheet during the 500 kyrs of the EO transition using a tri-dimensional interpolation method. The latter allows precise orbital variations, CO2 evolution and ice sheet feedbacks (including the albedo) to be accounted for. Our results show that orbital variations are instrumental in initiating the first step of the EO glaciation but that the primary driver of the major second step is the atmospheric pCO2 crossing a modelled glacial threshold of 900 ppm. Although model-dependant, this higher glacial threshold makes a stronger case for ephemeral Middle-Late Eocene ice sheets. In addition, sensitivity tests demonstrate that the small first step only exists if the absolute pCO2 value remains within 100 ppm higher than the glacial threshold during the first 250 kyrs of the transition. Thereby, the pCO2 sufficiently counterbalances the strong insolation minima occurring at 33.9 and 33.8 Ma but is low enough to allow the ice sheet to nucleate. Nevertheless, questions remain as to what may cause this pCO2 drop.

  7. Radar Interferometry Studies of the Mass Balance of Polar Ice Sheets

    NASA Technical Reports Server (NTRS)

    Rignot, Eric (Editor)

    1999-01-01

    The objectives of this work are to determine the current state of mass balance of the Greenland and Antarctic Ice Sheets. Our approach combines different techniques, which include satellite synthetic-aperture radar interferometry (InSAR), radar and laser altimetry, radar ice sounding, and finite-element modeling. In Greenland, we found that 3.5 times more ice flows out of the northern part of the Greenland Ice Sheet than previously accounted for. The discrepancy between current and past estimates is explained by extensive basal melting of the glacier floating sections in the proximity of the grounding line where the glacier detaches from its bed and becomes afloat in the ocean. The inferred basal melt rates are very large, which means that the glaciers are very sensitive to changes in ocean conditions. Currently, it appears that the northern Greenland glaciers discharge more ice than is being accumulated in the deep interior, and hence are thinning. Studies of temporal changes in grounding line position using InSAR confirm the state of retreat of northern glaciers and suggest that thinning is concentrated at the lower elevations. Ongoing work along the coast of East Greenland reveals an even larger mass deficit for eastern Greenland glaciers, with thinning affecting the deep interior of the ice sheet. In Antarctica, we found that glaciers flowing into a large ice shelf system, such as the Ronne Ice Shelf in the Weddell Sea, exhibit an ice discharge in remarkable agreement with mass accumulation in the interior, and the glacier grounding line positions do not migrate with time. Glaciers flowing rapidly into the Amudsen Sea, unrestrained by a major ice shelf, are in contrast discharging more ice than required to maintain a state of mass balance and are thinning quite rapidly near the coast. The grounding line of Pine Island glacier (see diagram) retreated 5 km in 4 years, which corresponds to a glacier thinning rate of 3.5 m/yr. Mass imbalance is even more negative

  8. The extent and timing of the last British-Irish Ice Sheet offshore of west Ireland-preliminary findings

    NASA Astrophysics Data System (ADS)

    Peters, Jared; Benetti, Sara; Dunlop, Paul; Cofaigh, Colm Ó.

    2014-05-01

    Recently interpreted marine geophysical data from the western Irish shelf has provided the first direct evidence that the last British-Irish Ice Sheet (BIIS) extended westwards onto the Irish continental shelf as a grounded ice mass composed of several lobes with marine-terminating margins. Marine terminating ice margins are known to be sensitive to external forcing mechanisms and currently there is concern regarding the future stability of marine based ice sheets, such as the West Antarctic Ice Sheet, in a warming world. Given its position, the glaciated western Irish continental shelf is a prime location to investigate the processes of how marine-based ice sheets responded to past climatic and oceanic events, which may in turn help us better predict the future trajectory of the marine sectors of modern Ice Sheets. However, despite the potential importance of the former Irish ice margin to our understanding of ice sheet behaviour, the timing and nature of its advance and retreat is currently poorly understood. This study aims to describe the depositional history of the last BIIS on the continental shelf west of Ireland and age-constrain the rate of retreat of two ice lobes that extended from Galway Bay and Clew Bay. This is being accomplished through a multifaceted analysis of at least 29 sediment cores gathered across the continental shelf offshore of counties Galway and Mayo, Ireland. This poster shows results from initial sedimentological descriptions of cores from the mid to outer shelf, which support previous geomorphic interpretations of BIIS history. Preliminary palaeoenvironmental results from ongoing micropaleontological analyses are also discussed and provide new data that verifies sedimentary interpretations on ice proximity. Finally, results from several radiocarbon dates are discussed, which limit these deposits to the last glacial maximum and constrain the timings of ice advance and retreat on the continental shelf west of Ireland.

  9. High geothermal heat flux measured below the West Antarctic Ice Sheet

    PubMed Central

    Fisher, Andrew T.; Mankoff, Kenneth D.; Tulaczyk, Slawek M.; Tyler, Scott W.; Foley, Neil

    2015-01-01

    The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m2, significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m2. The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region. PMID:26601210

  10. High geothermal heat flux measured below the West Antarctic Ice Sheet.

    PubMed

    Fisher, Andrew T; Mankoff, Kenneth D; Tulaczyk, Slawek M; Tyler, Scott W; Foley, Neil

    2015-07-01

    The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m(2), significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m(2). The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.

  11. Full Stokes finite-element modeling of ice sheets using a graphics processing unit

    NASA Astrophysics Data System (ADS)

    Seddik, H.; Greve, R.

    2016-12-01

    Thermo-mechanical simulation of ice sheets is an important approach to understand and predict their evolution in a changing climate. For that purpose, higher order (e.g., ISSM, BISICLES) and full Stokes (e.g., Elmer/Ice, http://elmerice.elmerfem.org) models are increasingly used to more accurately model the flow of entire ice sheets. In parallel to this development, the rapidly improving performance and capabilities of Graphics Processing Units (GPUs) allows to efficiently offload more calculations of complex and computationally demanding problems on those devices. Thus, in order to continue the trend of using full Stokes models with greater resolutions, using GPUs should be considered for the implementation of ice sheet models. We developed the GPU-accelerated ice-sheet model Sainō. Sainō is an Elmer (http://www.csc.fi/english/pages/elmer) derivative implemented in Objective-C which solves the full Stokes equations with the finite element method. It uses the standard OpenCL language (http://www.khronos.org/opencl/) to offload the assembly of the finite element matrix on the GPU. A mesh-coloring scheme is used so that elements with the same color (non-sharing nodes) are assembled in parallel on the GPU without the need for synchronization primitives. The current implementation shows that, for the ISMIP-HOM experiment A, during the matrix assembly in double precision with 8000, 87,500 and 252,000 brick elements, Sainō is respectively 2x, 10x and 14x faster than Elmer/Ice (when both models are run on a single processing unit). In single precision, Sainō is even 3x, 20x and 25x faster than Elmer/Ice. A detailed description of the comparative results between Sainō and Elmer/Ice will be presented, and further perspectives in optimization and the limitations of the current implementation.

  12. Estimates of Ice Sheet Mass Balance from Satellite Altimetry: Past and Future

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Zukor, Dorothy J. (Technical Monitor)

    2001-01-01

    A major uncertainty in predicting sea level rise is the sensitivity of ice sheet mass balance to climate change, as well as the uncertainty in present mass balance. Since the annual water exchange is about 8 mm of global sea level equivalent, the 20% uncertainty in current mass balance corresponds to 1.6 mm/yr in sea level change. Furthermore, estimates of the sensitivity of the mass balance to temperature change range from perhaps as much as - 10% to + 10% per K. A principal purpose of obtaining ice sheet elevation changes from satellite altimetry has been estimation of the current ice sheet mass balance. Limited information on ice sheet elevation change and their implications about mass balance have been reported by several investigators from radar altimetry (Seasat, Geosat, ERS-1&2). Analysis of ERS-1&2 data over Greenland for 7 years from 1992 to 1999 shows mixed patterns of ice elevation increases and decreases that are significant in terms of regional-scale mass balances. Observed seasonal and interannual variations in ice surface elevation are larger than previously expected because of seasonal and interannUal variations in precipitation, melting, and firn compaction. In the accumulation zone, the variations in firn compaction are modeled as a function of temperature leaving variations in precipitation and the mass balance trend. Significant interannual variations in elevation in some locations, in particular the difference in trends from 1992 to 1995 compared to 1995 to 1999, can be explained by changes in precipitation over Greenland. Over the 7 years, trends in elevation are mostly positive at higher elevations and negative at lower elevations. In addition, trends for the winter seasons (from a trend analysis through the average winter elevations) are more positive than the corresponding trends for the summer. At lower elevations, the 7-year trends in some locations are strongly negative for summer and near zero or slightly positive for winter. These

  13. Ice-Sheet Dynamics and Millennial-Scale Climate Variability in the North Atlantic across the Middle Pleistocene Transition (Invited)

    NASA Astrophysics Data System (ADS)

    Hodell, D. A.; Nicholl, J.

    2013-12-01

    During the Middle Pleistocene Transition (MPT), the climate system evolved from a more linear response to insolation forcing in the '41-kyr world' to one that was decidedly non-linear in the '100-kyr world'. Smaller ice sheets in the early Pleistocene gave way to larger ice sheets in the late Pleistocene with an accompanying change in ice sheet dynamics. We studied Sites U1308 (49° 52.7'N, 24° 14.3'W; 3871 m) and U1304 (53° 3.4'N, 33° 31.8'W; 3024 m) in the North Atlantic to determine how ice sheet dynamics and millennial-scale climate variability evolved as glacial boundary conditions changed across the MPT. The frequency of ice-rafted detritus (IRD) in the North Atlantic was greater during glacial stages prior to 650 ka (MIS 16), reflecting more frequent crossing of an ice volume threshold when the climate system spent more time in the 'intermediate ice volume' window, resulting in persistent millennial scale variability. The rarity of Heinrich Events containing detrital carbonate and more frequent occurrence of IRD events prior to 650 ka may indicate the presence of 'low-slung, slippery ice sheets' that flowed more readily than their post-MPT counterparts (Bailey et al., 2010). Ice volume surpassed a critical threshold across the MPT that permitted ice sheets to survive boreal summer insolation maxima, thereby increasing ice volume and thickness, lengthening glacial cycles, and activating the dynamical processes responsible for Laurentide Ice Sheet instability in the region of Hudson Strait (i.e., Heinrich events). The excess ice volume during post-MPT glacial maxima provided a large, unstable reservoir of freshwater to be released to the North Atlantic during glacial terminations with the potential to perturb Atlantic Meridional Overtunring Circulation. We speculate that orbital- and millennial-scale variability co-evolved across the MPT and the interaction of processes on orbital and suborbital time scales gave rise to the changing patterns of glacial

  14. Ice Sheet History from Antarctic Continental Margin Sediments: The ANTOSTRAT Approach

    USGS Publications Warehouse

    Barker, P.F.; Barrett, P.J.; Camerlenghi, A.; Cooper, A. K.; Davey, F.J.; Domack, E.W.; Escutia, C.; Kristoffersen, Y.; O'Brien, P.E.

    1998-01-01

    The Antarctic Ice Sheet is today an important part of the global climate engine, and probably has been so for most of its long existence. However, the details of its history are poorly known, despite the measurement and use, over two decades, of low-latitude proxies of ice sheet volume. An additional way of determining ice sheet history is now available, based on understanding terrigenous sediment transport and deposition under a glacial regime. It requires direct sampling of the prograded wedge of glacial sediments deposited at the Antarctic continental margin (and of derived sediments on the continental rise) at a small number of key sites, and combines the resulting data using numerical models of ice sheet development. The new phase of sampling is embodied mainly in a suite of proposals to the Ocean Drilling Program, generated by separate regional proponent groups co-ordinated through ANTOSTRAT (the Antarctic Offshore Acoustic Stratigraphy initiative). The first set of margin sites has now been drilled as ODP Leg 178 to the Antarctic Peninsula margin, and a first, short season of inshore drilling at Cape Roberts, Ross Sea, has been completed. Leg 178 and Cape Roberts drilling results are described briefly here, together with an outline of key elements of the overall strategy for determining glacial history, and of the potential contributions of drilling other Antarctic margins investigated by ANTOSTRAT. ODP Leg 178 also recovered continuous ultra-high-resolution Holocene biogenic sections at two sites within a protected, glacially-overdeepened basin (Palmer Deep) on the inner continental shelf of the Antarctic Peninsula. These and similar sites from around the Antarctic margin are a valuable resource when linked with ice cores and equivalent sections at lower latitude sites for studies of decadal and millenial-scale climate variation.

  15. Ice-sheet sourced juxtaposed turbidite systems in Labrador Sea

    USGS Publications Warehouse

    Hesse, R.; Klaucke, I.; Ryan, William B. F.; Piper, D.J.W.

    1997-01-01

    Ice-sheet sourced Pleistocene turbidite systems of the Labrador Sea are different from non-glacially influenced systems in their facies distribution and depositional processes. Two large-scale sediment dispersal systems are juxtaposed, one mud-dominated and associated with the Northwest Atlantic Mid-Ocean Channel (NAMOC), the other sand-dominated and forming a huge submarine braided sandplain. Co-existence of the two systems reflects grain-size separation of the coarse and fine fractions on an enormous scale, caused by sediment winnowing at the entrance points of meltwater from the Laurentide Ice Sheet (LIS) to the sea (Hudson Strait, fiords) and involves a complex interplay of depositional and redepositional processes. The mud-rich NAMOC system is multisourced and represents a basinwide converging system of tributary canyons and channels. It focusses its sand load to the central trunk channel in basin centre, in the fashion of a "reverse" deep-sea fan. The sand plain received its sediment from the Hudson Strait by turbidity currents that were generated either by failure of glacial prodelta slopes at the ice margin, or by direct meltwater discharges with high bedload concentration. We speculate that the latter might have been related to subglacial-lake outburst flooding through the Hudson Strait, possibly associated with ice-rafting (Heinrich) events.

  16. The impact of dynamic topography on the bedrock elevation and volume of the Pliocene Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Austermann, Jacqueline; Pollard, David; Mitrovica, Jerry X.; Moucha, Robert; Forte, Alessandro M.; DeConto, Robert M.

    2015-04-01

    Reconstructions of the Antarctic ice sheet over long timescales (i.e. Myrs) require estimates of bedrock elevation through time. Ice sheet models have accounted, with varying levels of sophistication, for changes in the bedrock elevation due to glacial isostatic adjustment (GIA), but they have neglected other processes that may perturb topography. One notable example is dynamic topography, the deflection of the solid surface of the Earth due to convective flow within the mantle. Numerically predicted changes in dynamic topography have been used to correct paleo shorelines for this departure from eustasy, but the effect of such changes on ice sheet stability is unknown. In this study we use numerical predictions of time-varying dynamic topography to reconstruct bedrock elevation below the Antarctic ice sheet during the mid Pliocene warm period (~3 Ma). Moreover, we couple this reconstruction to a three-dimensional ice sheet model to explore the impact of dynamic topography on the evolution of the Antarctic ice sheet since the Pliocene. Our modeling indicates significant uplift in the area of the Transantarctic Mountains (TAM) and the adjacent Wilkes basin. This predicted uplift, which is at the lower end of geological inferences of uplift of the TAM, implies a lower elevation of the basin in the Pliocene. Relative to simulations that do not include dynamic topography, the lower elevation leads to a smaller Antarctic Ice Sheet volume and a more significant retreat of the grounding line in the Wilkes basin, both of which are consistent with offshore sediment core data. We conclude that reconstructions of the Antarctic Ice Sheet during the mid-Pliocene warm period should be based on bedrock elevation models that include the impact of both GIA and dynamic topography.

  17. Rapid Holocene thinning of an East Antarctic outlet glacier driven by marine ice sheet instability

    PubMed Central

    Jones, R. S.; Mackintosh, A. N.; Norton, K. P.; Golledge, N. R.; Fogwill, C. J.; Kubik, P. W.; Christl, M.; Greenwood, S. L.

    2015-01-01

    Outlet glaciers grounded on a bed that deepens inland and extends below sea level are potentially vulnerable to ‘marine ice sheet instability'. This instability, which may lead to runaway ice loss, has been simulated in models, but its consequences have not been directly observed in geological records. Here we provide new surface-exposure ages from an outlet of the East Antarctic Ice Sheet that reveal rapid glacier thinning occurred approximately 7,000 years ago, in the absence of large environmental changes. Glacier thinning persisted for more than two and a half centuries, resulting in hundreds of metres of ice loss. Numerical simulations indicate that ice surface drawdown accelerated when the otherwise steadily retreating glacier encountered a bedrock trough. Together, the geological reconstruction and numerical simulations suggest that centennial-scale glacier thinning arose from unstable grounding line retreat. Capturing these instability processes in ice sheet models is important for predicting Antarctica's future contribution to sea level change. PMID:26608558

  18. The influence of meltwater on the thermal structure and flow of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Poinar, Kristin

    As the climate has warmed over the past decades, the amount of melt on the Greenland Ice Sheet has increased, and areas higher on the ice sheet have begun to melt regularly. This increase in melt has been hypothesized to enhance ice flow in myriad ways, including through basal lubrication and englacial refreezing. By developing and interpreting thermal ice-sheet models and analyzing remote sensing data, I evaluate the effect of these processes on ice flow and sea-level rise from the Greenland Ice Sheet. I first develop a thermal ice sheet model that is applicable to western Greenland. Key components of this model are its treatment of multiple phases (solid ice and liquid water) and its viscosity-dependent velocity field. I apply the model to Jakobshavn Isbrae, a fast-flowing outlet glacier. This is an important benchmark for my model, which I next apply to the topics outlined above. I use the thermal model to calculate the effect of englacial latent-heat transfer (meltwater refreezing within englacial features such as firn and crevasses) on ice dynamics in western Greenland. I find that in slow-moving areas, this can significantly warm the ice, but that englacial latent heat transfer has only a minimal effect on ice motion (60%) of the ice flux into the ocean, evidence of deep englacial warming is virtually absent. Thus, the effects of englacial latent heat transfer on ice motion are likely limited to slow-moving regions, which limits its importance to ice-sheet mass balance. Next, I couple a model for ice fracture to a modified version of my thermal model to calculate the depth and shape evolution of water-filled crevasses that form in crevasse fields. At most elevations and for typical water input volumes, crevasses penetrate to the top ~200--300 meters depth, warm the ice there by ~10°C, and may persist englacially, in a liquid state, for multiple decades. The surface hydrological network limits the amount of water that can reach most crevasses. We find that

  19. Towards a morphogenetic classification of eskers: Implications for modelling ice sheet hydrology

    NASA Astrophysics Data System (ADS)

    Perkins, Andrew J.; Brennand, Tracy A.; Burke, Matthew J.

    2016-02-01

    Validations of paleo-ice sheet hydrological models have used esker spacing as a proxy for ice tunnel density. Changes in crest type (cross-sectional shape) along esker ridges have typically been attributed to the effect of changing subglacial topography on hydro- and ice-dynamics and hence subglacial ice-tunnel shape. These claims assume that all eskers formed in subglacial ice tunnels and that all major subglacial ice tunnels produced a remnant esker. We identify differences in geomorphic context, sinuosity, cross-sectional shape, and sedimentary architecture by analysing eskers formed at or near the margins of the last Cordilleran Ice Sheet on British Columbia's southern Fraser Plateau, and propose a morphogenetic esker classification. Three morphogenetic types and 2 subtypes of eskers are classified based on differences in geomorphic context, ridge length, sinuosity, cross-sectional shape and sedimentary architecture using geophysical techniques and sedimentary exposures; they largely record seasonal meltwater flows and glacial lake outburst floods (GLOFs) through sub-, en- and supraglacial meltwater channels and ice-walled canyons. General principles extracted from these interpretations are: 1) esker ridge crest type and sinuosity strongly reflect meltwater channel type. Eskers formed in subglacial conduits are likely to be round-crested with low sinuosity (except where controlled by ice structure or modified by surging) and contain faults associated with flank collapse. Eskers formed near or at the ice surface are more likely to be sharp-crested, highly sinuous, and contain numerous faults both under ridge crest-lines and in areas of flank collapse. 2) Esker ridges containing numerous flat-crested reaches formed directly on the land-surface in ice-walled canyons (unroofed ice tunnels) or in ice tunnels at atmospheric pressure, and therefore likely record thin or dead ice. 3) Eskers containing macroforms exhibiting headward and downflow growth likely record

  20. A first assessment of Sentinel-3 SAR altimetry over ice sheets

    NASA Astrophysics Data System (ADS)

    McMillan, M.; Muir, A. S.; Shepherd, A.

    2017-12-01

    The first Sentinel-3 satellite was launched in 2016 and carries onboard a Ku-band Synthetic Aperture Radar (SAR) altimeter. With coverage up to a latitude of 81.5 degrees and a repeat period of 27 days, it offers the opportunity to measure surface topography and elevation change across much of the Antarctic and Greenland Ice Sheets, therefore continuing the existing 25 year radar altimeter record. The global operation of Sentinel-3 in SAR mode differs from all past Ku-band instruments; for the first time SAR measurements are routinely acquired across the interiors of the ice sheets; however unlike CryoSat-2 it does not carry an interferometer to aid signal retrieval in regions of complex coastal terrain. In view of these differences and the novel characteristics of the Sentinel-3 system, assessments of the performance of the instrument are required, to evaluate the satellite's utility for monitoring Earth's Polar regions. Here, we analyse data acquired during the first year of routine operations, to assess the performance of the Sentinel-3 SAR altimeter to date. We focus both on inland ice sheet regions, where Sentinel-3 provides the first operational SAR altimeter measurements, and also on coastal areas with more complex topography. We investigate SAR waveforms and retrieved elevations in both regions, and through comparison to measurements from earlier missions examine the impact of the different modes of operation. We also conduct a high level evaluation of the data, by comparing it to reference airborne altimetry, to provide an assessment of Sentinel-3 performance to date over ice sheets.

  1. Hypsometric Amplification of Greenland Ice Sheet Meltwater Release

    NASA Astrophysics Data System (ADS)

    van As, D.; Hasholt, B.; Mikkelsen, A. B.; Holtegaard Nielsen, M.; Box, J.; Claesson Liljedahl, L.; Lindback, K.; Pitcher, L. H.

    2017-12-01

    Proglacial discharge monitoring provides valuable insights in Greenland ice sheet meltwater release. We use a 2006-2016 discharge time series from the Watson River draining 12000 km2 of the ice sheet in southwest Greenland to investigate the large variability in catchment-total meltwater production. An observationally-constrained reconstruction of past discharge shows that meltwater release has on average increased by a factor of 1.5 since 2003 compared to the 1949-2002 period, and that interannual variability has disproportionally increased by a factor of 2.1, suggesting that melt amplifiers are at play. We derive a hypsometric amplification factor of 1.6, which is the result of the exponential melt area increase with rising temperature. Peak meltwater discharge events such as during the July 2012 flooding are due to this and other melt amplifiers, but also require intense melting over a period exceeding the multi-day transit time for high-elevation meltwater to pass through the glacial drainage system.

  2. Dark ice dynamics of the south-west Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Tedstone, Andrew J.; Bamber, Jonathan L.; Cook, Joseph M.; Williamson, Christopher J.; Fettweis, Xavier; Hodson, Andrew J.; Tranter, Martyn

    2017-11-01

    Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to changes in atmospheric circulation and atmospheric warming. Albedo reductions resulting from these changes have amplified surface melting. Some of the largest declines in GrIS albedo have occurred in the ablation zone of the south-west sector and are associated with the development of dark ice surfaces. Field observations at local scales reveal that a variety of light-absorbing impurities (LAIs) can be present on the surface, ranging from inorganic particulates to cryoconite materials and ice algae. Meanwhile, satellite observations show that the areal extent of dark ice has varied significantly between recent successive melt seasons. However, the processes that drive such large interannual variability in dark ice extent remain essentially unconstrained. At present we are therefore unable to project how the albedo of bare ice sectors of the GrIS will evolve in the future, causing uncertainty in the projected sea level contribution from the GrIS over the coming decades. Here we use MODIS satellite imagery to examine dark ice dynamics on the south-west GrIS each year from 2000 to 2016. We quantify dark ice in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years but so too does its duration (from 0 to > 80 % of June-July-August, JJA), intensity and the timing of its first appearance. Comparison of dark ice dynamics with potential meteorological drivers from the regional climate model MAR reveals that the JJA sensible heat flux, the number of positive minimum-air-temperature days and the timing of bare ice appearance are significant interannual synoptic controls. We use these findings to identify the surface processes which are most likely to explain recent dark ice dynamics. We suggest that whilst the spatial distribution of dark ice is best explained by outcropping of particulates from

  3. A combined surface/volume scattering retracking algorithm for ice sheet satellite altimetry

    NASA Technical Reports Server (NTRS)

    Davis, Curt H.

    1992-01-01

    An algorithm that is based upon a combined surface-volume scattering model is developed. It can be used to retrack individual altimeter waveforms over ice sheets. An iterative least-squares procedure is used to fit the combined model to the return waveforms. The retracking algorithm comprises two distinct sections. The first generates initial model parameter estimates from a filtered altimeter waveform. The second uses the initial estimates, the theoretical model, and the waveform data to generate corrected parameter estimates. This retracking algorithm can be used to assess the accuracy of elevations produced from current retracking algorithms when subsurface volume scattering is present. This is extremely important so that repeated altimeter elevation measurements can be used to accurately detect changes in the mass balance of the ice sheets. By analyzing the distribution of the model parameters over large portions of the ice sheet, regional and seasonal variations in the near-surface properties of the snowpack can be quantified.

  4. Antarctic Circumpolar Current Dynamics and Their Relation to Antarctic Ice Sheet and Perennial Sea-Ice Variability in the Central Drake Passage During the Last Climate Cycle

    NASA Astrophysics Data System (ADS)

    Kuhn, G.; Wu, S.; Hass, H. C.; Klages, J. P.; Zheng, X.; Arz, H. W.; Esper, O.; Hillenbrand, C. D.; Lange, C.; Lamy, F.; Lohmann, G.; Müller, J.; McCave, I. N. N.; Nürnberg, D.; Roberts, J.; Tiedemann, R.; Timmermann, A.; Titschack, J.; Zhang, X.

    2017-12-01

    The evolution of the Antarctic Ice Sheet during the last climate cycle and the interrelation to global atmospheric and ocean circulation remains controversial and plays an important role for our understanding of ice sheet response to modern global warming. The timing and sequence of deglacial warming is relevant for understanding the variability and sensitivity of the Antarctic Ice Sheet to climatic changes, and the continuing rise of atmospheric greenhouse gas concentrations. The Antarctic Ice Sheet is a pivotal component of the global water budget. Freshwater fluxes from the ice sheet may affect the Antarctic Circumpolar Current (ACC), which is strongly impacted by the westerly wind belt in the Southern Hemisphere (SHWW) and constricted to its narrowest extent in the Drake Passage. The flow of ACC water masses through Drake Passage is, therefore, crucial for advancing our understanding of the Southern Ocean's role in global meridional overturning circulation and global climate change. In order to address orbital and millennial-scale variability of the Antarctic ice sheet and the ACC, we applied a multi-proxy approach on a sediment core from the central Drake Passage including grain size, iceberg-rafted debris, mineral dust, bulk chemical and mineralogical composition, and physical properties. In combination with already published and new sediment records from the Drake Passage and Scotia Sea, as well as high-resolution data from Antarctic ice cores (WDC, EDML), we now have evidence that during glacial times a more northerly extent of the perennial sea-ice zone decreased ACC current velocities in the central Drake Passage. During deglaciation the SHWW shifted southwards due to a decreasing temperature gradient between subtropical and polar latitudes caused by sea ice and ice sheet decline. This in turn caused Southern Hemisphere warming, a more vigorous ACC, stronger Southern Ocean ventilation, and warm Circumpolar Deep Water (CDW) upwelling on Antarctic shelves

  5. Abrupt Shift in the Observed Runoff from the Southwest Greenland Ice Sheet?

    NASA Astrophysics Data System (ADS)

    Ahlstrom, A.; Petersen, D.; Box, J.; Langen, P. P.; Citterio, M.

    2016-12-01

    Mass loss of the Greenland ice sheet has contributed significantly to sea level rise in recent years and is considered a crucial parameter when estimating the impact of future climate change. Few observational records of sufficient length exist to validate surface mass balance models, especially the estimated runoff. Here we present an observation time series from 1975-2014 of discharge from a large proglacial lake, Tasersiaq, in West Greenland (66.3°N, 50.4°W) with a mainly ice-covered catchment. We argue that the discharge time series is representative measure of ice sheet runoff, making it the only observational record of runoff to exceed the 30-year period needed to assess the climatological state of the ice sheet. We proceed to isolate the runoff part of the signal from precipitation and identified glacial lake outburst floods from a small sub-catchment. Similarly, the impact from major volcanic eruptions is clearly identified. We examine the trend and annual variability in the annual discharge, relating it to likely atmospheric forcing mechanisms and compare the observational time series with modelled runoff from the regional climate model HIRHAM.

  6. Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet

    PubMed Central

    Schroeder, Dustin M.; Blankenship, Donald D.; Young, Duncan A.; Quartini, Enrica

    2014-01-01

    Heterogeneous hydrologic, lithologic, and geologic basal boundary conditions can exert strong control on the evolution, stability, and sea level contribution of marine ice sheets. Geothermal flux is one of the most dynamically critical ice sheet boundary conditions but is extremely difficult to constrain at the scale required to understand and predict the behavior of rapidly changing glaciers. This lack of observational constraint on geothermal flux is particularly problematic for the glacier catchments of the West Antarctic Ice Sheet within the low topography of the West Antarctic Rift System where geothermal fluxes are expected to be high, heterogeneous, and possibly transient. We use airborne radar sounding data with a subglacial water routing model to estimate the distribution of basal melting and geothermal flux beneath Thwaites Glacier, West Antarctica. We show that the Thwaites Glacier catchment has a minimum average geothermal flux of ∼114 ± 10 mW/m2 with areas of high flux exceeding 200 mW/m2 consistent with hypothesized rift-associated magmatic migration and volcanism. These areas of highest geothermal flux include the westernmost tributary of Thwaites Glacier adjacent to the subaerial Mount Takahe volcano and the upper reaches of the central tributary near the West Antarctic Ice Sheet Divide ice core drilling site. PMID:24927578

  7. Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet.

    PubMed

    Andrews, Lauren C; Catania, Ginny A; Hoffman, Matthew J; Gulley, Jason D; Lüthi, Martin P; Ryser, Claudia; Hawley, Robert L; Neumann, Thomas A

    2014-10-02

    Seasonal acceleration of the Greenland Ice Sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed via crevasses and moulins (vertical conduits connecting supraglacial water to the bed of the ice sheet). As the melt season progresses, the subglacial hydrologic system drains supraglacial meltwater more efficiently, decreasing basal water pressure and moderating the ice velocity response to surface melting. However, limited direct observations of subglacial water pressure mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood. Here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby (0.3-2 kilometres away) boreholes, indicating that moulins connect to an efficient, channelized component of the subglacial hydrologic system, which exerts the primary control on diurnal and multi-day changes in ice velocity. Our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the Paakitsoq region of western Greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin-connected channels to convey supraglacial melt. Instead, these observations suggest that decreasing late-season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system. Understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt-season dynamics affect ice velocity beyond the conclusion of the melt season.

  8. Mass Gains of the Antarctic Ice Sheet Exceed Losses

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Li, Jun; Robbins, John; Saba, Jack L.; Yi, Donghui; Brenner, Anita; Bromwich, David

    2012-01-01

    During 2003 to 2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 Gt/yr (2.5% of input), as derived from ICESat laser measurements of elevation change. The net gain (86 Gt/yr) over the West Antarctic (WA) and East Antarctic ice sheets (WA and EA) is essentially unchanged from revised results for 1992 to 2001 from ERS radar altimetry. Imbalances in individual drainage systems (DS) are large (-68% to +103% of input), as are temporal changes (-39% to +44%). The recent 90 Gt/yr loss from three DS (Pine Island, Thwaites-Smith, and Marie-Bryd Coast) of WA exceeds the earlier 61 Gt/yr loss, consistent with reports of accelerating ice flow and dynamic thinning. Similarly, the recent 24 Gt/yr loss from three DS in the Antarctic Peninsula (AP) is consistent with glacier accelerations following breakup of the Larsen B and other ice shelves. In contrast, net increases in the five other DS of WA and AP and three of the 16 DS in East Antarctica (EA) exceed the increased losses. Alternate interpretations of the mass changes driven by accumulation variations are given using results from atmospheric-model re-analysis and a parameterization based on 5% change in accumulation per degree of observed surface temperature change. A slow increase in snowfall with climate waRMing, consistent with model predictions, may be offsetting increased dynamic losses.

  9. Exploring changes in vertical ice extent along the margin of the East Antarctic Ice Sheet in western Dronning Maud Land - initial results of the MAGIC-DML collaboration

    NASA Astrophysics Data System (ADS)

    Lifton, N. A.; Newall, J. C.; Fredin, O.; Glasser, N. F.; Fabel, D.; Rogozhina, I.; Bernales, J.; Prange, M.; Sams, S.; Eisen, O.; Hättestrand, C.; Harbor, J.; Stroeven, A. P.

    2017-12-01

    Numerical ice sheet models constrained by theory and refined by comparisons with observational data are a central component of work to address the interactions between the cryosphere and changing climate, at a wide range of scales. Such models are tested and refined by comparing model predictions of past ice geometries with field-based reconstructions from geological, geomorphological, and ice core data. However, on the East Antarctic Ice sheet, there are few empirical data with which to reconstruct changes in ice sheet geometry in the Dronning Maud Land (DML) region. In addition, there is poor control on the regional climate history of the ice sheet margin, because ice core locations, where detailed reconstructions of climate history exist, are located on high inland domes. This leaves numerical models of regional glaciation history in this near-coastal area largely unconstrained. MAGIC-DML is an ongoing Swedish-US-Norwegian-German-UK collaboration with a focus on improving ice sheet models by combining advances in numerical modeling with filling critical data gaps that exist in our knowledge of the timing and pattern of ice surface changes on the western Dronning Maud Land margin. A combination of geomorphological mapping using remote sensing data, field investigations, cosmogenic nuclide surface exposure dating, and numerical ice-sheet modeling are being used in an iterative manner to produce a comprehensive reconstruction of the glacial history of western Dronning Maud Land. We will present an overview of the project, as well as field observations and preliminary in situ cosmogenic nuclide measurements from the 2016/17 expedition.

  10. Comparison of Retracking Algorithms Using Airborne Radar and Laser Altimeter Measurements of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Ferraro, Ellen J.; Swift, Calvin T.

    1995-01-01

    This paper compares four continental ice sheet radar altimeter retracking algorithms using airborne radar and laser altimeter data taken over the Greenland ice sheet in 1991. The refurbished Advanced Application Flight Experiment (AAFE) airborne radar altimeter has a large range window and stores the entire return waveform during flight. Once the return waveforms are retracked, or post-processed to obtain the most accurate altitude measurement possible, they are compared with the high-precision Airborne Oceanographic Lidar (AOL) altimeter measurements. The AAFE waveforms show evidence of varying degrees of both surface and volume scattering from different regions of the Greenland ice sheet. The AOL laser altimeter, however, obtains a return only from the surface of the ice sheet. Retracking altimeter waveforms with a surface scattering model results in a good correlation with the laser measurements in the wet and dry-snow zones, but in the percolation region of the ice sheet, the deviation between the two data sets is large due to the effects of subsurface and volume scattering. The Martin et al model results in a lower bias than the surface scattering model, but still shows an increase in the noise level in the percolation zone. Using an Offset Center of Gravity algorithm to retrack altimeter waveforms results in measurements that are only slightly affected by subsurface and volume scattering and, despite a higher bias, this algorithm works well in all regions of the ice sheet. A cubic spline provides retracked altitudes that agree with AOL measurements over all regions of Greenland. This method is not sensitive to changes in the scattering mechanisms of the ice sheet and it has the lowest noise level and bias of all the retracking methods presented.

  11. Ice-sheet-driven methane storage and release in the Arctic

    PubMed Central

    Portnov, Alexey; Vadakkepuliyambatta, Sunil; Mienert, Jürgen; Hubbard, Alun

    2016-01-01

    It is established that late-twentieth and twenty-first century ocean warming has forced dissociation of gas hydrates with concomitant seabed methane release. However, recent dating of methane expulsion sites suggests that gas release has been ongoing over many millennia. Here we synthesize observations of ∼1,900 fluid escape features—pockmarks and active gas flares—across a previously glaciated Arctic margin with ice-sheet thermomechanical and gas hydrate stability zone modelling. Our results indicate that even under conservative estimates of ice thickness with temperate subglacial conditions, a 500-m thick gas hydrate stability zone—which could serve as a methane sink—existed beneath the ice sheet. Moreover, we reveal that in water depths 150–520 m methane release also persisted through a 20-km-wide window between the subsea and subglacial gas hydrate stability zone. This window expanded in response to post-glacial climate warming and deglaciation thereby opening the Arctic shelf for methane release. PMID:26739497

  12. Landforms, sediments and dates to constrain rates and style of marine-influenced ice sheet decay; the BRITICE-CHRONO project.

    NASA Astrophysics Data System (ADS)

    Clark, Chris

    2014-05-01

    Uncertainty exists regarding the future mass of the Antarctic and Greenland ice sheets and how they will respond to forcings from sea level, and atmospheric and ocean temperatures. If we want to know more about the mechanisms and rate of change of shrinking ice sheets, then why not examine an ice sheet that has fully disappeared and track its retreat through time? If achieved in enough detail such information on ice retreat could be a data-rich playground for improving the next breed of numerical ice sheet models to be used in ice and sea level forecasting. We regard that the last British-Irish Ice Sheet is a good target for this work, on account of its small size, density of information and with its numerous researchers already investigating it. Geomorphological mapping across the British Isles and the surrounding continental shelf has revealed the nature and distribution of glacial landforms. Here we demonstrate how such data have been used to build a pattern of ice margin retreat. The BRITICE-CHRONO consortium of Quaternary scientists and glaciologists, are now working on a project running from 2012 - 2017 to produce an ice sheet wide database of geochronometric dates to constrain and then understand ice margin retreat. This is being achieved by focusing on 8 transects running from the continental shelf edge to a short distance (10s km) onshore and acquiring marine and terrestrial samples for geochronometric dating. The project includes funding for 587 radiocarbon, 140 OSL and 158 TCN samples for surface exposure dating; with sampling accomplished by two research cruises and 16 fieldwork campaigns. Results will reveal the timing and rate of change of ice margin recession for each transect, and combined with existing landform and dating databases, will be used to build an ice sheet-wide empirical reconstruction of retreat. Simulations using two numerical ice sheet models, fitted against the margin data, will help us understand the nature and significance of sea

  13. Land motion due to 20th century mass balance of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Kjeldsen, K. K.; Khan, S. A.

    2017-12-01

    Quantifying the contribution from ice sheets and glaciers to past sea level change is of great value for understanding sea level projections into the 21st century. However, quantifying and understanding past changes are equally important, in particular understanding the impact in the near-field where the signal is highest. We assess the impact of 20th century mass balance of the Greenland Ice Sheet on land motion using results from Kjeldsen et al, 2015. These results suggest that the ice sheet on average lost a minimum of 75 Gt/yr, but also show that the mass balance was highly spatial- and temporal variable, and moreover that on a centennial time scale changes were driven by a decreasing surface mass balance. Based on preliminary results we discuss land motion during the 20th century due to mass balance changes and the driving components surface mass balance and ice dynamics.

  14. Determination of Interannual to Decadal Changes in Ice Sheet Mass Balance from Satellite Altimetry

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Busalacchi, Antonioa J. (Technical Monitor)

    2001-01-01

    A major uncertainty in predicting sea level rise is the sensitivity of ice sheet mass balance to climate change, as well as the uncertainty in present mass balance. Since the annual water exchange is about 8 mm of global sea level equivalent, the +/- 25% uncertainty in current mass balance corresponds to +/- 2 mm/yr in sea level change. Furthermore, estimates of the sensitivity of the mass balance to temperature change range from perhaps as much as - 10% to + 10% per K. Although the overall ice mass balance and seasonal and inter-annual variations can be derived from time-series of ice surface elevations from satellite altimetry, satellite radar altimeters have been limited in spatial coverage and elevation accuracy. Nevertheless, new data analysis shows mixed patterns of ice elevation increases and decreases that are significant in terms of regional-scale mass balances. In addition, observed seasonal and interannual variations in elevation demonstrate the potential for relating the variability in mass balance to changes in precipitation, temperature, and melting. From 2001, NASA's ICESat laser altimeter mission will provide significantly better elevation accuracy and spatial coverage to 86 deg latitude and to the margins of the ice sheets. During 3 to 5 years of ICESat-1 operation, an estimate of the overall ice sheet mass balance and sea level contribution will be obtained. The importance of continued ice monitoring after the first ICESat is illustrated by the variability in the area of Greenland surface melt observed over 17-years and its correlation with temperature. In addition, measurement of ice sheet changes, along with measurements of sea level change by a series of ocean altimeters, should enable direct detection of ice level and global sea level correlations.

  15. Refreezing on the Greenland ice sheet: a model comparison

    NASA Astrophysics Data System (ADS)

    Steger, Christian; Reijmer, Carleen; van den Broeke, Michiel; Ligtenberg, Stefan; Kuipers Munneke, Peter; Noël, Brice

    2016-04-01

    Mass loss of the Greenland ice sheet (GrIS) is an important contributor to global sea level rise. Besides calving, surface melt is the dominant source of mass loss. However, only part of the surface melt leaves the ice sheet as runoff whereas the other part percolates into the snow cover and refreezes. Due to this process, part of the meltwater is (intermediately) stored. Refreezing thus impacts the surface mass balance of the ice sheet but it also affects the vertical structure of the snow cover due to transport of mass and energy. Due to the sparse availability of in situ data and the demand of future projections, it is inevitable to use numerical models to simulate refreezing and related processes. Currently, the magnitude of refrozen mass is neither well constrained nor well validated. In this study, we model the snow and firn layer, and compare refreezing on the GrIS as modelled with two different numerical models. Both models are forced with meteorological data from the regional climate model RACMO 2 that has been shown to simulate realistic conditions for Greenland. One model is the UU/IMAU firn densification model (FDM) that can be used both in an on- and offline mode with RACMO 2. The other model is SNOWPACK; a model originally designed to simulate seasonal snow cover in alpine conditions. In contrast to FDM, SNOWPACK accounts for snow metamorphism and microstructure and contains a more physically based snow densification scheme. A first comparison of the models indicates that both seem to be able to capture the general spatial and temporal pattern of refreezing. Spatially, refreezing occurs mostly in the ablation zone and decreases in the accumulation zone towards the interior of the ice sheet. Below the equilibrium line altitude (ELA) where refreezing occurs in seasonal snow cover on bare ice, the storage effect is only intermediate. Temporal patterns on a seasonal range indicate two peaks in refreezing; one at the beginning of the melt season where

  16. Data assimilation of surface altimetry on the North-Easter Ice Stream using the Ice Sheet System Model (ISSM)

    NASA Astrophysics Data System (ADS)

    Larour, Eric; Utke, Jean; Morlighem, Mathieu; Seroussi, Helene; Csatho, Beata; Schenk, Anton; Rignot, Eric; Khazendar, Ala

    2014-05-01

    Extensive surface altimetry data has been collected on polar ice sheets over the past decades, following missions such as Envisat and IceSat. This data record will further increase in size with the new CryoSat mission, the ongoing Operation IceBridge Mission and the soon to launch IceSat-2 mission. In order to make the best use of these dataset, ice flow models need to improve on the way they ingest surface altimetry to infer: 1) parameterizations of poorly known physical processes such as basal friction; 2) boundary conditions such as Surface Mass Balance (SMB). Ad-hoc sensitivity studies and adjoint-based inversions have so far been the way ice sheet models have attempted to resolve the impact of 1) on their results. As for boundary conditions or the lack thereof, most studies assume that they are a fixed quantity, which, though prone to large errors from the measurement itself, is not varied according to the simulated results. Here, we propose a method based on automatic differentiation to improve boundary conditions at the base and surface of the ice sheet during a short-term transient run for which surface altimetry observations are available. The method relies on minimizing a cost-function, the best fit between modeled surface evolution and surface altimetry observations, using gradients that are computed for each time step from automatic differentiation of the ISSM (Ice Sheet System Model) code. The approach relies on overloaded operators using the ADOLC (Automatic Differentiation by OverLoading in C++) package. It is applied to the 79 North Glacier, Greenland, for a short term transient spanning a couple of decades before the start of the retreat of the Zachariae Isstrom outlet glacier. Our results show adjustments required on the basal friction and the SMB of the whole basin to best fit surface altimetry observations, along with sensitivities each one of these parameters has on the overall cost function. Our approach presents a pathway towards assimilating

  17. Unveiling climate and ice-sheet history from drilling in high-latitude margins and future perspectives

    NASA Astrophysics Data System (ADS)

    Escutia Dotti, Carlota

    2010-05-01

    Polar ice is an important component of the climate system, affecting global sea level, ocean circulation and heat transport, marine productivity, and albedo. During the last decades drilling in the Arctic (IODP ACEX and Bering Expeditions) and in Antarctica (ODP Legs 178, 188, IODP Expedition 318 and ANDRILL) has revealed regional information about sea ice and ice sheets development and evolution. Integration of this data with numerical modeling provide an understanding of the early development of the ice sheets and their variability through the Cenozoic. Much of this work points to atmospheric CO2 and other greenhouse gases concentrations as important triggering mechanism driving the onset of glaciation and subsequent ice volume variability. With current increasing atmospheric greenhouse gases concentrations resulting in rapidly rising global temperatures, studies of polar climates become increasingly prominent on the research agenda. Despite of the relevance of the high-latitudes in the global climate systems, the short- and long-term history of the ice sheets and sea-ice and its relationships with paleoclimatic, paleoceanographic, and sea level changes is still poorly understood. A multinational, multiplatform scientific drilling strategy is being developed to recover key physical evidence from selected high-latitude areas. This strategy is aimed at addressing key knowledge gaps about the role of polar ice in climate change, targeting questions such as timing of events, rates of change, tipping points, regional variations, and northern vs. southern hemispheres (in phase or out-of-phase) variability. This data is critical to provide constrains to sea-ice and ice sheet models, which are the basis for forecasting the future of the cryosphere in a warming world.

  18. Did the Laurentide ice sheet survive through Marine Isotope Stage 9?

    NASA Astrophysics Data System (ADS)

    Carlson, A. E.; Tarasov, L.; Ullman, D. J.

    2016-12-01

    Looking at the global benthic oxygen isotope stack, only marine oxygen isotope stage (MIS) 7 stands out as an anomalous interglaciation with a higher oxygen isotope value than other interglaciations of the last half million years. However, benthic oxygen isotopes are an integrator of global ice volume plus temperature, and records of local ice-sheet change are needed to partition the sources of the global signal. Here we use the Laurentide ice-sheet (LIS) proximal record of IODP Site U1302/1303 off of Orphan Knoll to test LIS presence/absence on the eastern Canadian shield. Ice-sheet model calibrated Si relative to Sr is low in most interglaciations of the last 500 ka reflecting the removal of the LIS and its erosive power from the Canadian Shield. However, like MIS 7, MIS 9 has continued elevated inputs of Si. Furthermore, planktic oxygen isotopes do not decrease to full interglacial levels like in MIS 1, 5e and 11. MIS 9 had a similar orbital forcing as MIS 5e, but a much shorter period of elevated interglacial carbon dioxide concentration. Based on climate model simulations of LIS stability, we suggest that the reduced period of elevated atmospheric carbon dioxide allowed the LIS to survive through MIS 9 (like MIS 7), providing important constraints for the climatic thresholds necessary for a full interglaciation.

  19. Simulating Heinrich events in a coupled atmosphere-ocean-ice sheet model

    NASA Astrophysics Data System (ADS)

    Mikolajewicz, Uwe; Ziemen, Florian

    2016-04-01

    Heinrich events are among the most prominent events of long-term climate variability recorded in proxies across the northern hemisphere. They are the archetype of ice sheet - climate interactions on millennial time scales. Nevertheless, the exact mechanisms that cause Heinrich events are still under discussion, and their climatic consequences are far from being fully understood. We contribute to answering the open questions by studying Heinrich events in a coupled ice sheet model (ISM) atmosphere-ocean-vegetation general circulation model (AOVGCM) framework, where this variability occurs as part of the model generated internal variability without the need to prescribe external perturbations, as was the standard approach in almost all model studies so far. The setup consists of a northern hemisphere setup of the modified Parallel Ice Sheet Model (mPISM) coupled to the global coarse resolution AOVGCM ECHAM5/MPIOM/LPJ. The simulations used for this analysis were an ensemble covering substantial parts of the late Glacial forced with transient insolation and prescribed atmospheric greenhouse gas concentrations. The modeled Heinrich events show a marked influence of the ice discharge on the Atlantic circulation and heat transport, but none of the Heinrich events during the Glacial did show a complete collapse of the North Atlantic meridional overturning circulation. The simulated main consequences of the Heinrich events are a freshening and cooling over the North Atlantic and a drying over northern Europe.

  20. VISL: A Virtual Ice Sheet Laboratory For Outreach and K-12 Education

    NASA Astrophysics Data System (ADS)

    Halkides, D. J.; Larour, E. Y.; Cheng, D. L.; Perez, G.; Romero, V.; Saks, O.

    2014-12-01

    We present a prototype Virtual Ice Sheet Laboratory (VISL) geared to K-12 classrooms and the general public, with the goal of improving climate literacy, especially in regards to the crucial role of the polar ice sheets in Earth's climate and sea level. VISL will allow users to perform guided experiments using the Ice Sheet System Model (ISSM), a state-of-the-art ice flow model developed at NASA's Jet Propulsion Laboratory and UC Irvine that simulates the near-term evolution of the ice sheets on Greenland and Antarctica. VISL users will access ISSM via a graphical interface that can be launched from a web browser on a computer, tablet or smart phone. Users select climate conditions and run time by moving graphic sliders then watch how a given region evolves in time under those conditions. Lesson plans will include conceptual background, instructions for table top experiments related to the concepts addressed in a given lesson, and a guide for performing model experiments and interpreting their results. Activities with different degrees of complexity will aim for consistency with NGSS Physical Science criteria for different grade bands (K-2, 3-5, 6-8, and 9-12), although they will not be labeled as such to encourage a broad user base. Activities will emphasize the development of physical intuition and critical thinking skills, understanding conceptual and computational models, as well as observation recording, concept articulation, hypothesis formulation and testing, and mathematical analysis. At our present phase of development, we seek input from the greater science education and outreach communities regarding VISL's planned content, as well as additional features and topic areas that educators and students would find useful.

  1. Sensitivity of the Greenland Ice Sheet to Pliocene sea surface temperatures

    USGS Publications Warehouse

    Hill, Daniel J.; Dolan, Aisling M.; Haywood, Alan M.; Hunter, Stephen J.; Stoll, Danielle K.

    2010-01-01

    PRISM3).Use of these different SSTswithin theHadley CentreGCM(GeneralCirculationModel) and BASISM (BritishAntarctic Survey Ice Sheet Model), consistently show large reductions of Pliocene Greenland ice volumes compared to modern. The changes in climate introduced by the use of different SST reconstructions do change the predicted ice volumes, mainly through precipitation feedbacks. However, the models show a relatively low sensitivity of modelled Greenland ice volumes to different mid-Piacenzian SST reconstructions, with the largest SST induced changes being 20% of Pliocene ice volume or less than a metre of sea-level rise.

  2. Oceanic Low Blows Hitting Ice Sheets Where It Hurts

    NASA Technical Reports Server (NTRS)

    Bindschadler, Robert

    2006-01-01

    The recent acceleration, thinning and retreat of large outlet glaciers in both Antarctica and Greenland is altering the mass balance of these two large ice sheets and increasing their contribution to rising sea level. In this short Perspective solicited by Science for a special March 24th issue on sea level change, I argue that the cause of these bihemispheric changes is that warmer water has gained access to the undersides of these glaciers where they come afloat from the continent. This process is particularly effective at accelerating glaciers because the beds of the large outlet glaciers are well below sea level (1000 meters or more) but "guarded" downstream by a shallow moraine formed when the glacier was more advanced. Once warmer water can breach this moraine, it sinks in the colder, fresh water behind the moraine and reaches the submarine front of the glacier. The pressure melting effect lowers the melting point of this deep ice allowing the warmer water to melt ice at rates of many tens of meters per year. This melting reduces . the frictional hold of the bed on the ice, allowing the ice to accelerate in agreement with the observations, Hansen has discussed the likelihood that approximately half of the Earth's radiation imbalance is manifesting in warmer ocean waters and Levitus et al. have seen warming in ocean temperature measurements at mid and low latitudes. The behavior of these outlet glaciers indicates this ocean warmth is reaching polar waters. The prognosis is for a continuation of this process, more negative ice sheet mass balances and increased rates of sea level rise.

  3. Greenland Ice Sheet exports labile organic carbon to the Arctic oceans

    NASA Astrophysics Data System (ADS)

    Lawson, E. C.; Wadham, J. L.; Tranter, M.; Stibal, M.; Lis, G. P.; Butler, C. E. H.; Laybourn-Parry, J.; Nienow, P.; Chandler, D.; Dewsbury, P.

    2013-12-01

    Runoff from small glacier systems contains dissolved organic carbon (DOC), rich in protein-like, low molecular weight (LMW) compounds, designating glaciers as an important source of bioavailable carbon for downstream heterotrophic activity. Fluxes of DOC and particulate organic carbon (POC) exported from large Greenland catchments, however, remain unquantified, despite the Greenland Ice Sheet (GrIS) being the largest source of global glacial runoff (ca. 400 km3 yr-1). We report high and episodic fluxes of POC and DOC from a large (1200 km2) GrIS catchment during contrasting melt seasons. POC dominates organic carbon (OC) export (70-89% on average), is sourced from the ice sheet bed and contains a significant bioreactive component (9% carbohydrates). A major source for the "bioavailable" (free carbohydrates) LMW-DOC fraction is microbial activity on the ice sheet surface, with some further addition of LMW-DOC to meltwaters by biogeochemical processes at the ice sheet bed. The bioavailability of the exported DOC (30-58%) to downstream marine microorganisms is similar to that reported from other glacial watersheds. Annual fluxes of DOC and free carbohydrates during two melt seasons were similar, despite the ~ 2 fold difference in runoff fluxes, suggesting production-limited DOC sources. POC fluxes were also insensitive to an increase in seasonal runoff volumes, indicating supply-limitation of suspended sediment in runoff. Scaled to the GrIS, the combined DOC and POC fluxes (0.13-0.17 Tg C yr-1 DOC, 0.36-1.52 Tg C yr-1 mean POC) are of a similar order of magnitude to a large Arctic river system, and hence represent an important OC source to the North Atlantic, Greenland and Labrador Seas.

  4. A calving law for ice sheet models; Investigating the role of surface melt on dynamics of Greenland outlet glaciers

    NASA Astrophysics Data System (ADS)

    Nick, F. M.; van der Veen, C. J.; Vieli, A.

    2008-12-01

    alving of icebergs accounts for perhaps as much as half the ice transferred from the Greenland Ice Sheet into the surrounding ocean, and virtually all of the ice loss from the Antarctic Ice Sheet. We have formulated a calving model that can be readily incorporated into time-evolving numerical ice-flow models. Our model is based on downward penetration of water-filled surface crevasses and upward propagation of basal crevasses. A calving event occurs when the depth of the surface crevasse (which increases as melting progresses through the summer) reaches the height of the basal crevasse. Our numerical ice sheet model is able to reproduce observed seasonal changes of Greenland outlet glaciers, such as fluctuations in flow speed and terminus positions. We have applied the model to Helheim Glacier on the east coast, and Petermann Glacier in the northwest. Our model suggests that rapid retreat of the claving front is highly affected by the amplified calving rate due to increasing water level in surface crevasses during warmer summers. Our results show little response to seasonally enhanced basal lubrication from surface melt. This modeling study provides insights into the role of surface and basal hydrology to ice sheet dynamics and on how to incorporate calving in ice sheet models and therefore advances our ability to predict future ice sheet change.

  5. Evidence for a substantial West Antarctic ice sheet contribution to meltwater pulses and abrupt global sea level rise

    NASA Astrophysics Data System (ADS)

    Fogwill, C. J.; Turney, C. S.; Golledge, N. R.; Etheridge, D. M.; Rubino, M.; Thornton, D.; Woodward, J.; Winter, K.; van Ommen, T. D.; Moy, A. D.; Curran, M. A.; Rootes, C.; Rivera, A.; Millman, H.

    2015-12-01

    During the last deglaciation (21,000 to 7,000years ago) global sea level rise was punctuated by several abrupt meltwater spikes triggered by the retreat of ice sheets and glaciers world-wide. However, the debate regarding the relative timing, geographical source and the physical mechanisms driving these rapid increases in sea level has catalyzed debate critical to predicting future sea level rise and climate. Here we present a unique record of West Antarctic Ice Sheet elevation change derived from the Patriot Hills blue ice area, located close to the modern day grounding line of the Institute Ice Stream in the Weddell Sea Embayment. Combined isotopic signatures and gas volume analysis from the ice allows us to develop a record of local ice sheet palaeo-altitude that is assessed against independent regional high-resolution ice sheet modeling studies, allowing us to demonstrate that past ice sheet elevations across this sector of the WSE were considerably higher than those suggested by current terrestrial reconstructions. We argue that ice in the WSE had a significant influence on both pre and post LGM sea level rise including MWP-1A (~14.6 ka) and during MWP-1B (11.7-11.6 ka), reconciling past sea level rise and demonstrating for the first time that this sector of the WAIS made a significant and direct contribution to post LGM sea level rise.

  6. Warming Seas and Melting Ice Sheets

    NASA Image and Video Library

    2017-12-08

    Sea level rise is a natural consequence of the warming of our planet. We know this from basic physics. When water heats up, it expands. So when the ocean warms, sea level rises. When ice is exposed to heat, it melts. And when ice on land melts and water runs into the ocean, sea level rises. For thousands of years, sea level has remained relatively stable and human communities have settled along the planet’s coastlines. But now Earth’s seas are rising. Globally, sea level has risen about eight inches since the beginning of the 20th century and more than two inches in the last 20 years alone. All signs suggest that this rise is accelerating. Read more: go.nasa.gov/1heZn29 Caption: An iceberg floats in Disko Bay, near Ilulissat, Greenland, on July 24, 2015. The massive Greenland ice sheet is shedding about 300 gigatons of ice a year into the ocean, making it the single largest source of sea level rise from melting ice. Credits: NASA/Saskia Madlener NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  7. Antarctic ice sheet mass loss estimates using Modified Antarctic Mapping Mission surface flow observations

    NASA Astrophysics Data System (ADS)

    Ren, Diandong; Leslie, Lance M.; Lynch, Mervyn J.

    2013-03-01

    The long residence time of ice and the relatively gentle slopes of the Antarctica Ice Sheet make basal sliding a unique positive feedback mechanism in enhancing ice discharge along preferred routes. The highly organized ice stream channels extending to the interior from the lower reach of the outlets are a manifestation of the role of basal granular material in enhancing the ice flow. In this study, constraining the model-simulated year 2000 ice flow fields with surface velocities obtained from InSAR measurements permits retrieval of the basal sliding parameters. Forward integrations of the ice model driven by atmospheric and oceanic parameters from coupled general circulation models under different emission scenarios provide a range of estimates of total ice mass loss during the 21st century. The total mass loss rate has a small intermodel and interscenario spread, rising from approximately -160 km3/yr at present to approximately -220 km3/yr by 2100. The accelerated mass loss rate of the Antarctica Ice Sheet in a warming climate is due primarily to a dynamic response in the form of an increase in ice flow speed. Ice shelves contribute to this feedback through a reduced buttressing effect due to more frequent systematic, tabular calving events. For example, by 2100 the Ross Ice Shelf is projected to shed 40 km3 during each systematic tabular calving. After the frontal section's attrition, the remaining shelf will rebound. Consequently, the submerged cross-sectional area will reduce, as will the buttressing stress. Longitudinal differential warming of ocean temperature contributes to tabular calving. Because of the prevalence of fringe ice shelves, oceanic effects likely will play a very important role in the future mass balance of the Antarctica Ice Sheet, under a possible future warming climate.

  8. Understanding ice sheet evolution to avoid massive sea level rise instead of experiencing it (Louis Agassiz Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Rignot, Eric

    2017-04-01

    With unabated climate warming, massive sea level rise from the melting of ice sheets in Greenland and Antarctica looms at the horizon. This is unfortunately an experiment that we can afford to run only once. Satellite and airborne sensors have significantly helped reveal the magnitude of the mass balance of the ice sheets, where the changes take place, when they started, how they change with time and the nature of the physical processes controlling them. These observations have constrained the maturation of numerical modeling techniques for projecting changes in these ice sheets, including the coupling of ocean and ice sheet models, yet significant uncertainties remain to make these projections directly policy relevant and many challenges remain. I will review the state of balance of the ice sheets as we know it today and the fundamental processes that will drive fast ice sheet retreat and sea level change: ice-ocean interaction and iceberg calving. Ice-ocean interaction are dominated by the wind-forced intrusion of warm, salty, subsurface waters toward the ice sheet periphery to melt ice from below at rates orders of magnitude greater than at the surface. In Greenland, these rates are difficult to observe, but model simulations indicate rates of ice melt along vertical calving faces of meters per day, along with undercutting of the ice faces. Constraining the temperature of the ocean waters from high resolution models and observations, however, remains a significant challenge. I will describe the progress we have made in addressing one major issue which is the mapping of fjord bathymetry around Greenland to define the pathways for warm waters. In Antarctica, the rates of melt are measured from remote sensing data but averaged over long periods, so that we are dependent on in-situ observations to understand the interaction of ocean waters with ice within the sub-ice-shelf cavities. I will describe progress made in mapping the bathymetry of the ice shelves and how

  9. Microbial Degradation of 2,4-Dichlorophenoxyacetic Acid on the Greenland Ice Sheet

    PubMed Central

    Stibal, Marek; Bælum, Jacob; Holben, William E.; Sørensen, Sebastian R.; Jensen, Anders

    2012-01-01

    The Greenland ice sheet (GrIS) receives organic carbon (OC) of anthropogenic origin, including pesticides, from the atmosphere and/or local sources, and the fate of these compounds in the ice is currently unknown. The ability of supraglacial heterotrophic microbes to mineralize different types of OC is likely a significant factor determining the fate of anthropogenic OC on the ice sheet. Here we determine the potential of the microbial community from the surface of the GrIS to mineralize the widely used herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Surface ice cores were collected and incubated for up to 529 days in microcosms simulating in situ conditions. Mineralization of side chain- and ring-labeled [14C]2,4-D was measured in the samples, and quantitative PCR targeting the tfdA genes in total DNA extracted from the ice after the experiment was performed. We show that the supraglacial microbial community on the GrIS contains microbes that are capable of degrading 2,4-D and that they are likely present in very low numbers. They can mineralize 2,4-D at a rate of up to 1 nmol per m2 per day, equivalent to ∼26 ng C m−2 day−1. Thus, the GrIS should not be considered a mere reservoir of all atmospheric contaminants, as it is likely that some deposited compounds will be removed from the system via biodegradation processes before their potential release due to the accelerated melting of the ice sheet. PMID:22582066

  10. Dynamic influence of pinning points on marine ice-sheet stability: a numerical study in Dronning Maud Land, East Antarctica

    DOE PAGES

    Favier, Lionel; Pattyn, Frank; Berger, Sophie; ...

    2016-11-09

    The East Antarctic ice sheet is likely more stable than its West Antarctic counterpart because its bed is largely lying above sea level. However, the ice sheet in Dronning Maud Land, East Antarctica, contains marine sectors that are in contact with the ocean through overdeepened marine basins interspersed by grounded ice promontories and ice rises, pinning and stabilising the ice shelves. In this paper, we use the ice-sheet model BISICLES to investigate the effect of sub-ice-shelf melting, using a series of scenarios compliant with current values, on the ice-dynamic stability of the outlet glaciers between the Lazarev and Roi Baudouinmore » ice shelves over the next millennium. Overall, the sub-ice-shelf melting substantially impacts the sea-level contribution. Locally, we predict a short-term rapid grounding-line retreat of the overdeepened outlet glacier Hansenbreen, which further induces the transition of the bordering ice promontories into ice rises. Furthermore, our analysis demonstrated that the onset of the marine ice-sheet retreat and subsequent promontory transition into ice rise is controlled by small pinning points, mostly uncharted in pan-Antarctic datasets. Pinning points have a twofold impact on marine ice sheets. They decrease the ice discharge by buttressing effect, and they play a crucial role in initialising marine ice sheets through data assimilation, leading to errors in ice-shelf rheology when omitted. Our results show that unpinning increases the sea-level rise by 10%, while omitting the same pinning point in data assimilation decreases it by 10%, but the more striking effect is in the promontory transition time, advanced by two centuries for unpinning and delayed by almost half a millennium when the pinning point is missing in data assimilation. As a result, pinning points exert a subtle influence on ice dynamics at the kilometre scale, which calls for a better knowledge of the Antarctic margins.« less

  11. Dynamic influence of pinning points on marine ice-sheet stability: a numerical study in Dronning Maud Land, East Antarctica

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Favier, Lionel; Pattyn, Frank; Berger, Sophie

    The East Antarctic ice sheet is likely more stable than its West Antarctic counterpart because its bed is largely lying above sea level. However, the ice sheet in Dronning Maud Land, East Antarctica, contains marine sectors that are in contact with the ocean through overdeepened marine basins interspersed by grounded ice promontories and ice rises, pinning and stabilising the ice shelves. In this paper, we use the ice-sheet model BISICLES to investigate the effect of sub-ice-shelf melting, using a series of scenarios compliant with current values, on the ice-dynamic stability of the outlet glaciers between the Lazarev and Roi Baudouinmore » ice shelves over the next millennium. Overall, the sub-ice-shelf melting substantially impacts the sea-level contribution. Locally, we predict a short-term rapid grounding-line retreat of the overdeepened outlet glacier Hansenbreen, which further induces the transition of the bordering ice promontories into ice rises. Furthermore, our analysis demonstrated that the onset of the marine ice-sheet retreat and subsequent promontory transition into ice rise is controlled by small pinning points, mostly uncharted in pan-Antarctic datasets. Pinning points have a twofold impact on marine ice sheets. They decrease the ice discharge by buttressing effect, and they play a crucial role in initialising marine ice sheets through data assimilation, leading to errors in ice-shelf rheology when omitted. Our results show that unpinning increases the sea-level rise by 10%, while omitting the same pinning point in data assimilation decreases it by 10%, but the more striking effect is in the promontory transition time, advanced by two centuries for unpinning and delayed by almost half a millennium when the pinning point is missing in data assimilation. As a result, pinning points exert a subtle influence on ice dynamics at the kilometre scale, which calls for a better knowledge of the Antarctic margins.« less

  12. How much can Greenland melt? An upper bound on mass loss from the Greenland Ice Sheet through surface melting

    NASA Astrophysics Data System (ADS)

    Liu, X.; Bassis, J. N.

    2015-12-01

    With observations showing accelerated mass loss from the Greenland Ice Sheet due to surface melt, the Greenland Ice Sheet is becoming one of the most significant contributors to sea level rise. The contribution of the Greenland Ice Sheet o sea level rise is likely to accelerate in the coming decade and centuries as atmospheric temperatures continue to rise, potentially triggering ever larger surface melt rates. However, at present considerable uncertainty remains in projecting the contribution to sea level of the Greenland Ice Sheet both due to uncertainty in atmospheric forcing and the ice sheet response to climate forcing. Here we seek an upper bound on the contribution of surface melt from the Greenland to sea level rise in the coming century using a surface energy balance model coupled to an englacial model. We use IPCC Representative Concentration Pathways (RCP8.5, RCP6, RCP4.5, RCP2.6) climate scenarios from an ensemble of global climate models in our simulations to project the maximum rate of ice volume loss and related sea-level rise associated with surface melting. To estimate the upper bound, we assume the Greenland Ice Sheet is perpetually covered in thick clouds, which maximize longwave radiation to the ice sheet. We further assume that deposition of black carbon darkens the ice substantially turning it nearly black, substantially reducing its albedo. Although assuming that all melt water not stored in the snow/firn is instantaneously transported off the ice sheet increases mass loss in the short term, refreezing of retained water warms the ice and may lead to more melt in the long term. Hence we examine both assumptions and use the scenario that leads to the most surface melt by 2100. Preliminary models results suggest that under the most aggressive climate forcing, surface melt from the Greenland Ice Sheet contributes ~1 m to sea level by the year 2100. This is a significant contribution and ignores dynamic effects. We also examined a lower bound

  13. Laurentide ice sheet meltwater routing along the Iro-Mohawk River, eastern New York, USA

    NASA Astrophysics Data System (ADS)

    Porreca, Charles; Briner, Jason P.; Kozlowski, Andrew

    2018-02-01

    The rerouting of meltwater as the configuration of ice sheets evolved during the last deglaciation is thought to have led to some of the most significant perturbations to the climate system in the late Quaternary. However, the complex pattern of ice sheet meltwater drainage off the continents, and the timing of rerouting events, remains to be fully resolved. As the Laurentide Ice Sheet (LIS) retreated north of the Adirondack Uplands of northeastern New York State during the last deglaciation, a large proglacial lake, Lake Iroquois, found a lower outlet that resulted in a significant flood event. This meltwater rerouting event, from outflow via the Iro-Mohawk River valley (southern Adirondack Mountains) to the spillway at Covey Hill (northeastern Adirondack Mountains), is hypothesized to have taken place 13.2 ka and disturbed meridional circulation in the North Atlantic Ocean. However, the timing of the rerouting event is not certain because the event has not been directly dated. With improving the history of Lake Iroquois drainage in mind, we obtained cosmogenic 10Be exposure ages on a strath terrace on Moss Island, along the Iro-Mohawk River spillway. We hypothesize that Moss Island's strath terrace became abandoned during the rerouting event. Six 10Be ages from the strath surface average 14.8 ± 1.3 ka, which predates the previously published bracketing radiocarbon ages of 13.2 ka. Several possibilities for the discrepancy exist: (1) the 10Be age accurately represents the timing of a decrease in discharge through the Iro-Mohawk River spillway; (2) the age is influenced by inheritance. The 10Be ages from glacially sculpted surfaces on Moss Island above the strath terrace predate the deglaciation of the site by 5 to 35 ky; and (3) the abandonment of the Moss Island strath terrace relates to knickpoint migration and not the final abandonment of the Iro-Mohawk River as the Lake Iroquois spillway. Further study and application of cosmogenic 10Be exposure dating in the

  14. Modelling West Antarctic ice sheet growth and collapse through the past five million years.

    PubMed

    Pollard, David; DeConto, Robert M

    2009-03-19

    The West Antarctic ice sheet (WAIS), with ice volume equivalent to approximately 5 m of sea level, has long been considered capable of past and future catastrophic collapse. Today, the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams. Grounding lines are several hundred metres below sea level and the bed deepens upstream, raising the prospect of runaway retreat. Projections of future WAIS behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms. Its variation since the Last Glacial Maximum is best known, with grounding lines advancing to the continental-shelf edges around approximately 15 kyr ago before retreating to near-modern locations by approximately 3 kyr ago. Prior collapses during the warmth of the early Pliocene epoch and some Pleistocene interglacials have been suggested indirectly from records of sea level and deep-sea-core isotopes, and by the discovery of open-ocean diatoms in subglacial sediments. Until now, however, little direct evidence of such behaviour has been available. Here we use a combined ice sheet/ice shelf model capable of high-resolution nesting with a new treatment of grounding-line dynamics and ice-shelf buttressing to simulate Antarctic ice sheet variations over the past five million years. Modelled WAIS variations range from full glacial extents with grounding lines near the continental shelf break, intermediate states similar to modern, and brief but dramatic retreats, leaving only small, isolated ice caps on West Antarctic islands. Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years. Our simulation is in good agreement with a new sediment record (ANDRILL AND-1B) recovered from the western Ross Sea, indicating a long-term trend from more frequently collapsed to more glaciated states, dominant 40-kyr cyclicity in the Pliocene, and major retreats at

  15. Reconstruction of the Greenland ice sheet dynamics in a fully coupled Earth System Model

    NASA Astrophysics Data System (ADS)

    Rybak, Oleg; Volodin, Evgeny; Huybrechts, Philippe

    2016-04-01

    Earth system models (ESMs) are undoubtedly effective tools for studying climate dynamics. Incorporation of evolving ice sheets to ESMs is a challenging task because response times of the climate system and of ice sheets differ by several orders of magnitude. Besides, AO GCMs operate on spatial and temporal resolutions substantially differing from those of ice sheet models (ICMs). Therefore elaboration of an effective coupling methodology of an AO GCM and an ICM is the key problem of an ESM construction and utilization. Several downscaling strategies of varying complexity exist now of data exchange between modeled climate system and ice sheets. Application of a particular strategy depends on the research objectives. In our view, the optimum approach for model studying of significant environmental changes (e.g. glacial/interglacial transitions) when ice sheets undergo substantial evolution of geometry and volume would be an asynchronous coupling. The latter allows simulation in the interactive way of growth and decay of ice sheets in the changing climatic conditions. In the focus of the presentation, is the overview of coupling aspects of an AO GCM INMCM32 elaborated in the Institute of Numerical Mathematics (Moscow, Russia) to the Greenland ice sheet model (GrISM, Vrije Uninersiteit Brussel, Belgium). To provide interactive coupling of INMCM32 (spatial resolution 5°×4°, 21 vertical layers and temporal resolution 6 min. in the atmospheric block) and GrISM (spatial resolution 20×20 km, 51 vertical layers and 1 yr temporal resolution), we employ a special energy- and water balance model (EWBM-G), which serves as a buffer providing effective data exchange between INMCM32 and GrISM. EWBM-G operates in a rectangle domain including Greenland. Transfer of daily meanings of simulated climatic variables (air surface temperature and specific humidity) is provided on the lateral boundarias of the domain and inside the domain (sea level air pressure, wind speed and total

  16. Climate change and forest fires synergistically drive widespread melt events of the Greenland Ice Sheet.

    PubMed

    Keegan, Kaitlin M; Albert, Mary R; McConnell, Joseph R; Baker, Ian

    2014-06-03

    In July 2012, over 97% of the Greenland Ice Sheet experienced surface melt, the first widespread melt during the era of satellite remote sensing. Analysis of six Greenland shallow firn cores from the dry snow region confirms that the most recent prior widespread melt occurred in 1889. A firn core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from Northern Hemisphere forest fires reduced albedo below a critical threshold in the dry snow region, and caused the melting events in both 1889 and 2012. We use these data to project the frequency of widespread melt into the year 2100. Since Arctic temperatures and the frequency of forest fires are both expected to rise with climate change, our results suggest that widespread melt events on the Greenland Ice Sheet may begin to occur almost annually by the end of century. These events are likely to alter the surface mass balance of the ice sheet, leaving the surface susceptible to further melting.

  17. Land, Ocean and Ice sheet surface elevation retrieval from CALIPSO lidar measurements

    NASA Astrophysics Data System (ADS)

    Lu, X.; Hu, Y.

    2013-12-01

    Since launching in April 2006 the main objective of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission has been studying the climate impact of clouds and aerosols in the atmosphere. However, CALIPSO also collects information about other components of the Earth's ecosystem, such as lands, oceans and polar ice sheets. The objective of this study is to propose a Super-Resolution Altimetry (SRA) technique to provide high resolution of land, ocean and polar ice sheet surface elevation from CALIPSO single shot lidar measurements (70 m spot size). The land surface results by the new technique agree with the United States Geological Survey (USGS) National Elevation Database (NED) high-resolution elevation maps, and the ice sheet surface results in the region of Greenland and Antarctic compare very well with the Ice, Cloud and land Elevation Satellite (ICESat) laser altimetry measurements. The comparisons suggest that the obtained CALIPSO surface elevation information by the new technique is accurate to within 1 m. The effects of error sources on the retrieved surface elevation are discussed. Based on the new technique, the preliminary data products of along-track topography retrieved from the CALIPSO lidar measurements is available to the altimetry community for evaluation.

  18. Inland thinning on the Greenland ice sheet controlled by outlet glacier geometry

    NASA Astrophysics Data System (ADS)

    Felikson, Denis; Bartholomaus, Timothy C.; Catania, Ginny A.; Korsgaard, Niels J.; Kjær, Kurt H.; Morlighem, Mathieu; Noël, Brice; van den Broeke, Michiel; Stearns, Leigh A.; Shroyer, Emily L.; Sutherland, David A.; Nash, Jonathan D.

    2017-04-01

    Greenland’s contribution to future sea-level rise remains uncertain and a wide range of upper and lower bounds has been proposed. These predictions depend strongly on how mass loss--which is focused at the termini of marine-terminating outlet glaciers--can penetrate inland to the ice-sheet interior. Previous studies have shown that, at regional scales, Greenland ice sheet mass loss is correlated with atmospheric and oceanic warming. However, mass loss within individual outlet glacier catchments exhibits unexplained heterogeneity, hindering our ability to project ice-sheet response to future environmental forcing. Using digital elevation model differencing, we spatially resolve the dynamic portion of surface elevation change from 1985 to present within 16 outlet glacier catchments in West Greenland, where significant heterogeneity in ice loss exists. We show that the up-glacier extent of thinning and, thus, mass loss, is limited by glacier geometry. We find that 94% of the total dynamic loss occurs between the terminus and the location where the down-glacier advective speed of a kinematic wave of thinning is at least three times larger than its diffusive speed. This empirical threshold enables the identification of glaciers that are not currently thinning but are most susceptible to future thinning in the coming decades.

  19. Switch of flow direction in an Antarctic ice stream.

    PubMed

    Conway, H; Catania, G; Raymond, C F; Gades, A M; Scambos, T A; Engelhardt, H

    2002-10-03

    Fast-flowing ice streams transport ice from the interior of West Antarctica to the ocean, and fluctuations in their activity control the mass balance of the ice sheet. The mass balance of the Ross Sea sector of the West Antarctic ice sheet is now positive--that is, it is growing--mainly because one of the ice streams (ice stream C) slowed down about 150 years ago. Here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the Ross drainage system. We show that the flow in an area that once discharged into ice stream C has changed direction, now draining into the Whillans ice stream (formerly ice stream B). This switch in flow direction is a result of continuing thinning of the Whillans ice stream and recent thickening of ice stream C. Further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long-term climate change. We suggest that caution is needed when using observations of short-term mass changes to draw conclusions about the large-scale mass balance of the ice sheet.

  20. Inferring Firn Permeability from Pneumatic Testing: A Case Study on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Sommers, Aleah N.; Rajaram, Harihar; Weber, Eliezer P.; MacFerrin, Michael J.; Colgan, William T.; Stevens, C. Max

    2017-03-01

    Across the accumulation zone of the Greenland ice sheet, summer temperatures can be sufficiently warm to cause widespread melting, as was the case in July 2012 when the entire ice sheet experienced a brief episode of enhanced surface ablation. The resulting meltwater percolates into the firn and refreezes, to create ice lenses and layers within the firn column. This is an important process to consider when estimating the surface mass balance of the ice sheet. The rate of meltwater percolation depends on the permeability of the firn, a property that is not well constrained in the presence of refrozen ice layers and lenses. We present a novel, inexpensive method for measuring in-situ firn permeability using pneumatic testing, a well-established technique used in environmental engineering and hydrology. To illustrate the capabilities of this method, we estimate both horizontal and vertical permeability from pilot tests at six sites on the Greenland ice sheet: KAN-U, DYE-2, EKT, NASA-SE, Saddle, and EastGRIP. These sites cover a range of conditions from mostly dry firn (EastGRIP), to firn with several ice layers and lenses from refrozen meltwater (Saddle, NASA-SE, EKT), to firn with extensive ice layers (DYE-2 and KAN-U). The estimated permeability in firn without refrozen ice layers at EastGRIP agrees well with the range previously reported using an air permeameter to measure permeability through firn core samples at Summit, Greenland. At sites with ice lenses or layers, we find high degrees of anisotropy, with vertical permeability much lower than horizontal permeability. Pneumatic testing is a promising and low-cost technique for measuring firn permeability, particularly as meltwater production increases in the accumulation zone and ice layers and lenses from refrozen melt layers become more prevalent. In these initial proof-of-concept tests, the estimated permeabilities represent effective permeability at the meter scale. With appropriately higher vacuum pressures

  1. VISL: A Virtual Ice Sheet Laboratory For Outreach and K-12 Education

    NASA Astrophysics Data System (ADS)

    Cheng, D. L. C.; Halkides, D. J.; Larour, E. Y.; Moore, J.; Dunn, S.; Perez, G.

    2015-12-01

    We present an update on our developing Virtual Ice Sheet Laboratory (VISL). Geared to K-12 classrooms and the general public, VISL's main goal is to improve climate literacy, especially in regards to the crucial role of the polar ice sheets in Earth's climate and sea level. VISL will allow users to perform guided experiments using the Ice Sheet System Model (ISSM), a state-of-the-art ice flow model developed at NASA's Jet Propulsion Laboratory and UC Irvine that simulates the near-term evolution of the ice sheets on Greenland and Antarctica. VISL users will access ISSM via a graphical interface that can be launched from a web browser on a computer, tablet or smart phone. Users select climate conditions and run time by moving graphic sliders then watch how a given region evolves in time under those conditions. Lesson plans will include conceptual background, instructions for table top experiments related to the concepts addressed in a given lesson, and a guide for performing model experiments and interpreting their results. Activities with different degrees of complexity will aim for consistency with NGSS Physical Science criteria for different grade bands (K-2, 3-5, 6-8, and 9-12), although they will not be labeled as such to encourage a broad user base. Activities will emphasize the development of physical intuition and critical thinking skills, understanding conceptual and computational models, as well as observation recording, concept articulation, hypothesis formulation and testing, and mathematical analysis. At our present phase of development, we seek input from the greater science education and outreach communities regarding VISL's planned content, as well as additional features and topic areas that educators and students would find useful.

  2. Antarctic Glacial Isostatic Adjustment and Ice Sheet Mass Balance using GRACE: A Report from the Ice-sheet Mass Balance Exercise (IMBIE)

    NASA Astrophysics Data System (ADS)

    Ivins, E. R.; Wahr, J. M.; Schrama, E. J.; Milne, G. A.; Barletta, V.; Horwath, M.; Whitehouse, P.

    2012-12-01

    In preparation for the Inter-govermental Panel on Climate Change: Assessment Report 5 (IPCC AR5), ESA and NASA have formed a committee of experts to perform a formal set of comparative experiments concerning space observations of ice sheet mass balance. This project began in August of 2011 and has now concluded with a report submitted for Science (Shepherd et al., 2012). The focus of the work conducted is to re-evaluate scientific reports on the mass balance of Greenland ice sheet (GIS) and Antarctic ice sheet (AIS). The most serious discrepancies have been reported for the AIS, amounting to as much as 0.9 mm/yr in discrepant sea level contribution. A direct method of determining the AIS is by space gravimetry. However, for this method to contribute to our understanding of sea level change, we require knowledge of present-day non-elastic vertical movements of bedrock in Antarctica. Quantifying the uncertainty and bias caused by lack of observational control on models of regional glacial isostatic adjustment (GIA), was a major focus for our experiments. This regional process is the most problematic error source for GRACE-determinations of ice mass balance in Antarctica. While GIA likely dominates some large vertical motions in Antarctica that are now observed with GPS (Thomas et al., 2011, GRL), interpretations still require models. The reported uncertainty for space gravimetric (GRACE) based sea level sourcing is roughly 0.20 to 0.35 mm/yr. The uncertainty is also part of the error budget for mass balances derived from altimetry measurements, though at a much lower level. Analysis of the GRACE time series using CSR RL04 (2003.0-2010.10) for AIS mass balance reveals a small trend of order +1 to -24 Gt/yr without a GIA correction. Three periods were selected over which to perform inter-comparisons (see Table). One class of GIA models, that relies primarily on far field sea level reconstructions (e.g. ICE-5G), provide a GIA correction that places AIS mass imbalance (

  3. Radiocarbon chronology of the last deglaciation in the Baffin Bay reveals asynchronous melting of Arctic and Laurentide ice sheets

    NASA Astrophysics Data System (ADS)

    Jackson, Rebecca; Kucera, Michal; Vogt, Christoph; Wacker, Lukas

    2016-04-01

    The transition from the last ice age into the Holocene interglacial was characterised by rapid retreat of North American ice sheets, discharging large quantities of meltwater into the Labrador Sea. Whereas the meltwater chronology of the Laurentide Ice Sheet is well documented, the deglacial history of the American Arctic ice sheets (Inuit Ice sheet and northern Greenland Ice Sheet) draining into the Labrador Sea via the Baffin Bay is less well constrained. Here we present the first high-resolution radiocarbon-dated deglacial records from the Canadian and Greenland margins of the central Baffin Bay. Sedimentological and geochemical data confirm the presence during Termination I of two events of enhanced delivery of detrital carbonate (Baffin Bay Detrital Carbonate Events) dated to 14.2-13.7 ka BP and 12.7-11 ka BP. The events are synchronous across the Baffin Bay and their mineralogical signature indicates a common source of detrital carbonate from the Canadian Arctic, with a synchronous clastic source proximal to Greenland. The events postdate Heinrich layers and their onset is not linked to Greenland temperature change. This indicates that the deglaciation of American Arctic ice sheets and associated meltwater discharge were decoupled from the dominant North Atlantic climate mode.

  4. Storage and release of organic carbon from glaciers and ice sheets

    NASA Astrophysics Data System (ADS)

    Hood, Eran; Battin, Tom J.; Fellman, Jason; O'Neel, Shad; Spencer, Robert G. M.

    2015-02-01

    Polar ice sheets and mountain glaciers, which cover roughly 11% of the Earth's land surface, store organic carbon from local and distant sources and then release it to downstream environments. Climate-driven changes to glacier runoff are expected to be larger than climate impacts on other components of the hydrological cycle, and may represent an important flux of organic carbon. A compilation of published data on dissolved organic carbon from glaciers across five continents reveals that mountain and polar glaciers represent a quantitatively important store of organic carbon. The Antarctic Ice Sheet is the repository of most of the roughly 6 petagrams (Pg) of organic carbon stored in glacier ice, but the annual release of glacier organic carbon is dominated by mountain glaciers in the case of dissolved organic carbon and the Greenland Ice Sheet in the case of particulate organic carbon. Climate change contributes to these fluxes: approximately 13% of the annual flux of glacier dissolved organic carbon is a result of glacier mass loss. These losses are expected to accelerate, leading to a cumulative loss of roughly 15 teragrams (Tg) of glacial dissolved organic carbon by 2050 due to climate change -- equivalent to about half of the annual flux of dissolved organic carbon from the Amazon River. Thus, glaciers constitute a key link between terrestrial and aquatic carbon fluxes, and will be of increasing importance in land-to-ocean fluxes of organic carbon in glacierized regions.

  5. Storage and release of organic carbon from glaciers and ice sheets

    USGS Publications Warehouse

    Hood, Eran; Battin, Tom J.; Fellman, Jason; O'Neel, Shad; Spencer, Robert G. M.

    2015-01-01

    Polar ice sheets and mountain glaciers, which cover roughly 11% of the Earth's land surface, store organic carbon from local and distant sources and then release it to downstream environments. Climate-driven changes to glacier runoff are expected to be larger than climate impacts on other components of the hydrological cycle, and may represent an important flux of organic carbon. A compilation of published data on dissolved organic carbon from glaciers across five continents reveals that mountain and polar glaciers represent a quantitatively important store of organic carbon. The Antarctic Ice Sheet is the repository of most of the roughly 6 petagrams (Pg) of organic carbon stored in glacier ice, but the annual release of glacier organic carbon is dominated by mountain glaciers in the case of dissolved organic carbon and the Greenland Ice Sheet in the case of particulate organic carbon. Climate change contributes to these fluxes: approximately 13% of the annual flux of glacier dissolved organic carbon is a result of glacier mass loss. These losses are expected to accelerate, leading to a cumulative loss of roughly 15 teragrams (Tg) of glacial dissolved organic carbon by 2050 due to climate change — equivalent to about half of the annual flux of dissolved organic carbon from the Amazon River. Thus, glaciers constitute a key link between terrestrial and aquatic carbon fluxes, and will be of increasing importance in land-to-ocean fluxes of organic carbon in glacierized regions.

  6. Molecular characterization of dissolved organic matter associated with the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Bhatia, Maya P.; Das, Sarah B.; Longnecker, Krista; Charette, Matthew A.; Kujawinski, Elizabeth B.

    2010-07-01

    Subsurface microbial oxidation of overridden soils and vegetation beneath glaciers and ice sheets may affect global carbon budgets on glacial-interglacial timescales. The likelihood and magnitude of this process depends on the chemical nature and reactivity of the subglacial organic carbon stores. We examined the composition of carbon pools associated with different regions of the Greenland ice sheet (subglacial, supraglacial, proglacial) in order to elucidate the type of dissolved organic matter (DOM) present in the subglacial discharge over a melt season. Electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry coupled to multivariate statistics permitted unprecedented molecular level characterization of this material and revealed that carbon pools associated with discrete glacial regions are comprised of different compound classes. Specifically, a larger proportion of protein-like compounds were observed in the supraglacial samples and in the early melt season (spring) subglacial discharge. In contrast, the late melt season (summer) subglacial discharge contained a greater fraction of lignin-like and other material presumably derived from underlying vegetation and soil. These results suggest (1) that the majority of supraglacial DOM originates from autochthonous microbial processes on the ice sheet surface, (2) that the subglacial DOM contains allochthonous carbon derived from overridden soils and vegetation as well as autochthonous carbon derived from in situ microbial metabolism, and (3) that the relative contribution of allochthonous and autochthonous material in subglacial discharge varies during the melt season. These conclusions are consistent with the hypothesis that, given sufficient time (e.g., overwinter storage), resident subglacial microbial communities may oxidize terrestrial material beneath the Greenland ice sheet.

  7. Assessing the Extent of Influence Subglacial Hydrology Has on Dynamic Ice Sheet Behavior

    NASA Astrophysics Data System (ADS)

    Babonis, G. S.; Csatho, B. M.

    2012-12-01

    Numerous recent studies have done an excellent job capturing and quantifying the complex pattern of dynamic changes of the Greenland Ice Sheet (GrIS) over the past several decades. The timing of changes in ice velocities and mass balance indicate that the mechanisms controlling these behaviors, both external and internal, act over variable spatial and temporal regimes, can change in rapid and complex fashion, and have significant effect on ice sheet behavior as well as sea level rise. With roughly half of the estimated ice loss from the GrIS attributed to dynamic processes, these changes account for about 250 Gt/yr (2003-2008), equivalence to 0.6 mm/yr sea level rise. One of the primary influences of dynamic ice behavior is ice sheet hydrology, including the storage and transport of water from the supraglacial to subglacial environment, and the subsequent development of water transport pathways, thus demonstrating the need for further characterization of the subglacial environment. Enhanced dynamic flow of ice due to the influence of meltwater distribution on the subglacial environment has been reported, including In-SAR observations of large velocity increases over short periods of time, suggesting regions where dynamic changes are likely being caused by changes in hydrology. Additionally, building upon the 1993-2011 laser altimetry record, analyzed by our Surface Elevation Reconstruction And Change detection (SERAC) procedure, we have detected complex patterns of rapid thickening and thinning patterns over several outlet glaciers. This study presents a comprehensive investigation of hydrologic control on dynamic glacier behavior for several key sites in Greenland. We combine a high resolution surface digital elevation model (DEM) derived by fusing space- and airborne laser altimetry observations and SPIRIT SPOT DEMs, with a high resolution, hydrologically-corrected bedrock DEM derived from a combination of CResIS and Operation Icebridge ice penetrating radar data

  8. Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise.

    PubMed

    Shannon, Sarah R; Payne, Antony J; Bartholomew, Ian D; van den Broeke, Michiel R; Edwards, Tamsin L; Fettweis, Xavier; Gagliardini, Olivier; Gillet-Chaulet, Fabien; Goelzer, Heiko; Hoffman, Matthew J; Huybrechts, Philippe; Mair, Douglas W F; Nienow, Peter W; Perego, Mauro; Price, Stephen F; Smeets, C J P Paul; Sole, Andrew J; van de Wal, Roderik S W; Zwinger, Thomas

    2013-08-27

    We assess the effect of enhanced basal sliding on the flow and mass budget of the Greenland ice sheet, using a newly developed parameterization of the relation between meltwater runoff and ice flow. A wide range of observations suggest that water generated by melt at the surface of the ice sheet reaches its bed by both fracture and drainage through moulins. Once at the bed, this water is likely to affect lubrication, although current observations are insufficient to determine whether changes in subglacial hydraulics will limit the potential for the speedup of flow. An uncertainty analysis based on our best-fit parameterization admits both possibilities: continuously increasing or bounded lubrication. We apply the parameterization to four higher-order ice-sheet models in a series of experiments forced by changes in both lubrication and surface mass budget and determine the additional mass loss brought about by lubrication in comparison with experiments forced only by changes in surface mass balance. We use forcing from a regional climate model, itself forced by output from the European Centre Hamburg Model (ECHAM5) global climate model run under scenario A1B. Although changes in lubrication generate widespread effects on the flow and form of the ice sheet, they do not affect substantial net mass loss; increase in the ice sheet's contribution to sea-level rise from basal lubrication is projected by all models to be no more than 5% of the contribution from surface mass budget forcing alone.

  9. Determination of Local Slope on the Greenland Ice Sheet Using a Multibeam Photon-Counting Lidar in Preparation for the ICESat-2 Mission

    NASA Technical Reports Server (NTRS)

    Brunt, Kelly M.; Neumann, Thomas Allen; Walsh, Kaitlin M.; Markus, Thorsten

    2013-01-01

    The greatest changes in elevation in Greenland and Antarctica are happening along the margins of the ice sheets where the surface frequently has significant slopes. For this reason, the upcoming Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission utilizes pairs of laser altimeter beams that are perpendicular to the flight direction in order to extract slope information in addition to elevation. The Multiple Altimeter Beam Experimental Lidar (MABEL) is a high-altitude airborne laser altimeter designed as a simulator for ICESat-2. The MABEL design uses multiple beams at fixed angles and allows for local slope determination. Here, we present local slopes as determined by MABEL and compare them to those determined by the Airborne Topographic Mapper (ATM) over the same flight lines in Greenland. We make these comparisons with consideration for the planned ICESat-2 beam geometry. Results indicate that the mean slope residuals between MABEL and ATM remain small (< 0.05 degrees) through a wide range of localized slopes using ICESat-2 beam geometry. Furthermore, when MABEL data are subsampled by a factor of 4 to mimic the planned ICESat-2 transmit-energy configuration, the results are indistinguishable from the full-data-rate analysis. Results from MABEL suggest that ICESat-2 beam geometry and transmit-energy configuration are appropriate for the determination of slope on approx. 90-m spatial scales, a measurement that will be fundamental to deconvolving the effects of surface slope from the ice-sheet surface change derived from ICESat-2.

  10. Determination of Local Slope on the Greenland Ice Sheet Using a Multibeam Photon-Counting Lidar in Preparation for the ICESat-2 Mission

    NASA Technical Reports Server (NTRS)

    Brunt, Kelly M.; Neumann, Thomas A.; Walsh, Kaitlin M.; Markus, Thorsten

    2014-01-01

    The greatest changes in elevation in Greenland and Antarctica are happening along the margins of the ice sheets where the surface frequently has significant slopes. For this reason, the upcoming Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission utilizes pairs of laser altimeter beams that are perpendicular to the flight direction in order to extract slope information in addition to elevation. The Multiple Altimeter Beam Experimental Lidar (MABEL) is a high-altitude airborne laser altimeter designed as a simulator for ICESat-2. The MABEL design uses multiple beams at fixed angles and allows for local slope determination. Here, we present local slopes as determined by MABEL and compare them to those determined by the Airborne Topographic Mapper (ATM) over the same flight lines in Greenland. We make these comparisons with consideration for the planned ICESat-2 beam geometry. Results indicate that the mean slope residuals between MABEL and ATM remain small (< 0.05?) through a wide range of localized slopes using ICESat-2 beam geometry. Furthermore, when MABEL data are subsampled by a factor of 4 to mimic the planned ICESat-2 transmit-energy configuration, the results are indistinguishable from the full-data-rate analysis. Results from MABEL suggest that ICESat-2 beam geometry and transmit-energy configuration are appropriate for the determination of slope on 90-m spatial scales, a measurement that will be fundamental to deconvolving the effects of surface slope from the ice-sheet surface change derived from ICESat-2.

  11. Snow Grain Size Retrieval over the Polar Ice Sheets with the Ice, Cloud, and land Elevation Satellite (ICESat) Observations

    PubMed Central

    Yang, Yuekui; Marshak, Alexander; Han, Mei; Palm, Stephen P.; Harding, David J.

    2018-01-01

    Snow grain size is an important parameter for cryosphere studies. As a proof of concept, this paper presents an approach to retrieve this parameter over Greenland, East and West Antarctica ice sheets from surface reflectances observed with the Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud, and land Elevation Satellite (ICESat) at 1064 nm. Spaceborne lidar observations overcome many of the disadvantages in passive remote sensing, including difficulties in cloud screening and low sun angle limitations; hence tend to provide more accurate and stable retrievals. Results from the GLAS L2A campaign, which began on 25 September and lasted until 19 November, 2003, show that the mode of the grain size distribution over Greenland is the largest (~300 μm) among the three, West Antarctica is the second (~220 μm) and East Antarctica is the smallest (~190 μm). Snow grain sizes are larger over the coastal regions compared to inland the ice sheets. These results are consistent with previous studies. Applying the broadband snow surface albedo parameterization scheme developed by Garder and Sharp (2010) to the retrieved snow grain size, ice sheet surface albedo is also derived. In the future, more accurate retrievals can be achieved with multiple wavelengths lidar observations. PMID:29636591

  12. Snow Grain Size Retrieval over the Polar Ice Sheets with the Ice, Cloud and Land Elevation Satellite (ICESat) Observations

    NASA Technical Reports Server (NTRS)

    Yang, Yuekui; Marshak, Alexander; Han, Mei; Palm, Stephen P.; Harding, David J.

    2016-01-01

    Snow grain size is an important parameter for cryosphere studies. As a proof of concept, this paper presents an approach to retrieve this parameter over Greenland, East and West Antarctica ice sheets from surface reflectances observed with the Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud, and land Elevation Satellite (ICESat) at 1064 nanometers. Spaceborne lidar observations overcome many of the disadvantages in passive remote sensing, including difficulties in cloud screening and low sun angle limitations; hence tend to provide more accurate and stable retrievals. Results from the GLAS L2A campaign, which began on 25 September and lasted until 19 November, 2003, show that the mode of the grain size distribution over Greenland is the largest (approximately 300 microns) among the three, West Antarctica is the second (220 microns) and East Antarctica is the smallest (190 microns). Snow grain sizes are larger over the coastal regions compared to inland the ice sheets. These results are consistent with previous studies. Applying the broadband snow surface albedo parameterization scheme developed by Garder and Sharp (2010) to the retrieved snow grain size, ice sheet surface albedo is also derived. In the future, more accurate retrievals can be achieved with multiple wavelengths lidar observations.

  13. Greenland Ice Sheet exports labile organic carbon to the Arctic oceans

    NASA Astrophysics Data System (ADS)

    Lawson, E. C.; Wadham, J. L.; Tranter, M.; Stibal, M.; Lis, G. P.; Butler, C. E. H.; Laybourn-Parry, J.; Nienow, P.; Chandler, D.; Dewsbury, P.

    2014-07-01

    Runoff from small glacier systems contains dissolved organic carbon (DOC) rich in protein-like, low molecular weight (LMW) compounds, designating glaciers as an important source of bioavailable carbon for downstream heterotrophic activity. Fluxes of DOC and particulate organic carbon (POC) exported from large Greenland catchments, however, remain unquantified, despite the Greenland Ice Sheet (GrIS) being the largest source of global glacial runoff (ca. 400 km3 yr-1). We report high and episodic fluxes of POC and DOC from a large (>600 km2) GrIS catchment during contrasting melt seasons. POC dominates organic carbon (OC) export (70-89% on average), is sourced from the ice sheet bed, and contains a significant bioreactive component (9% carbohydrates). A major source of the "bioavailable" (free carbohydrate) LMW-DOC fraction is microbial activity on the ice sheet surface, with some further addition of LMW-DOC to meltwaters by biogeochemical processes at the ice sheet bed. The bioavailability of the exported DOC (26-53%) to downstream marine microorganisms is similar to that reported from other glacial watersheds. Annual fluxes of DOC and free carbohydrates during two melt seasons were similar, despite the approximately two-fold difference in runoff fluxes, suggesting production-limited DOC sources. POC fluxes were also insensitive to an increase in seasonal runoff volumes, indicating a supply limitation in suspended sediment in runoff. Scaled to the GrIS, the combined DOC (0.13-0.17 Tg C yr-1 (±13%)) and POC fluxes (mean = 0.36-1.52 Tg C yr-1 (±14%)) are of a similar order of magnitude to a large Arctic river system, and hence may represent an important OC source to the near-coastal North Atlantic, Greenland and Labrador seas.

  14. Leveraging Cloud Technology to Provide a Responsive, Reliable and Scalable Backend for the Virtual Ice Sheet Laboratory Using the Ice Sheet System Model and Amazon's Elastic Compute Cloud

    NASA Astrophysics Data System (ADS)

    Perez, G. L.; Larour, E. Y.; Halkides, D. J.; Cheng, D. L. C.

    2015-12-01

    The Virtual Ice Sheet Laboratory(VISL) is a Cryosphere outreach effort byscientists at the Jet Propulsion Laboratory(JPL) in Pasadena, CA, Earth and SpaceResearch(ESR) in Seattle, WA, and the University of California at Irvine (UCI), with the goal of providing interactive lessons for K-12 and college level students,while conforming to STEM guidelines. At the core of VISL is the Ice Sheet System Model(ISSM), an open-source project developed jointlyat JPL and UCI whose main purpose is to model the evolution of the polar ice caps in Greenland and Antarctica. By using ISSM, VISL students have access tostate-of-the-art modeling software that is being used to conduct scientificresearch by users all over the world. However, providing this functionality isby no means simple. The modeling of ice sheets in response to sea and atmospheric temperatures, among many other possible parameters, requiressignificant computational resources. Furthermore, this service needs to beresponsive and capable of handling burst requests produced by classrooms ofstudents. Cloud computing providers represent a burgeoning industry. With majorinvestments by tech giants like Amazon, Google and Microsoft, it has never beeneasier or more affordable to deploy computational elements on-demand. This isexactly what VISL needs and ISSM is capable of. Moreover, this is a promisingalternative to investing in expensive and rapidly devaluing hardware.

  15. Simulations of Antarctic ice shelves and the Southern Ocean in the POP2x ocean model coupled with the BISICLES ice-sheet model

    NASA Astrophysics Data System (ADS)

    Asay-Davis, Xylar; Martin, Daniel; Price, Stephen; Maltrud, Mathew

    2014-05-01

    We present initial results from Antarctic, ice-ocean coupled simulations using large-scale ocean circulation and ice-sheet evolution models. This presentation focuses on the ocean model, POP2x, which is a modified version of POP, a fully eddying, global-scale ocean model (Smith and Gent, 2002). POP2x allows for circulation beneath ice shelf cavities using the method of partial top cells (Losch, 2008). Boundary layer physics, which control fresh water and salt exchange at the ice-ocean interface, are implemented following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013) and with results from other idealized ice-ocean coupling test cases (e.g., Goldberg et al., 2012). A companion presentation, 'Fully resolved whole-continent Antarctica simulations using the BISICLES AMR ice sheet model coupled with the POP2x Ocean Model', concentrates more on the ice-sheet model, BISICLES (Cornford et al., 2012), which includes a 1st-order accurate momentum balance (L1L2) and uses block structured, adaptive-mesh refinement to more accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. For idealized test cases focused on marine-ice sheet dynamics, BISICLES output compares very favorably relative to simulations based on the full, nonlinear Stokes momentum balance (MISMIP-3d; Pattyn et al., 2013). Here, we present large-scale (Southern Ocean) simulations using POP2x at 0.1 degree resolution with fixed ice shelf geometries, which are used to obtain and validate modeled submarine melt rates against observations. These melt rates are, in turn, used to force evolution of the BISICLES model. An offline-coupling scheme, which we compare with the ice-ocean coupling work of Goldberg et al. (2012), is then used to

  16. Translating hydrologically-relevant variables from the ice sheet model SICOPOLIS to the Greenland Analog Project hydrologic modeling domain

    NASA Astrophysics Data System (ADS)

    Vallot, Dorothée; Applegate, Patrick; Pettersson, Rickard

    2013-04-01

    Projecting future climate and ice sheet development requires sophisticated models and extensive field observations. Given the present state of our knowledge, it is very difficult to say what will happen with certainty. Despite the ongoing increase in atmospheric greenhouse gas concentrations, the possibility that a new ice sheet might form over Scandinavia in the far distant future cannot be excluded. The growth of a new Scandinavian Ice Sheet would have important consequences for buried nuclear waste repositories. The Greenland Analogue Project, initiated by the Swedish Nuclear Fuel and Waste Management Company (SKB), is working to assess the effects of a possible future ice sheet on groundwater flow by studying a constrained domain in Western Greenland by field measurements (including deep bedrock drilling in front of the ice sheet) combined with numerical modeling. To address the needs of the GAP project, we interpolated results from an ensemble of ice sheet model runs to the smaller and more finely resolved modeling domain used in the GAP project's hydrologic modeling. Three runs have been chosen with three fairly different positive degree-day factors among those that reproduced the modern ice margin at the borehole position. The interpolated results describe changes in hydrologically-relevant variables over two time periods, 115 ka to 80 ka, and 20 ka to 1 ka. In the first of these time periods, the ice margin advances over the model domain; in the second time period, the ice margin retreats over the model domain. The spatially-and temporally dependent variables that we treated include the ice thickness, basal melting rate, surface mass balance, basal temperature, basal thermal regime (frozen or thawed), surface temperature, and basal water pressure. The melt flux is also calculated.

  17. The geomorphic signature of present ice-sheet flow in the radar-sounded subglacial record: Pine Island Glacier, West Antarctica

    NASA Astrophysics Data System (ADS)

    Bingham, R. G.; Davies, D.; King, E. C.; Vaughan, D. G.; Cornford, S. L.; Brisbourne, A.; Smith, A.; De Rydt, J.; Graham, A. G. C.; Spagnolo, M.

    2016-12-01

    Deglaciated landscapes and landforms are much used in the quest to reconstruct past ice-sheet behaviour, on the principle that aspects of landform shapes, sizes and relative associations "fossilise" palaeo-ice-sheet processes. Such techniques have been widely used around the margin of the marine West Antarctic Ice Sheet, taking advantage of bathymetric surveying techniques which have exposed a rich suite of landform assemblages across West Antarctica's continental shelf. Though these geomorphological interpretations are solidly grounded in glacial geological theory, there has, until now, been little ability to compare these deglaciated, and potentially postglacially-modified, landforms offshore with landforms currently situated (and potentially still evolving) beneath the contemporary ice sheet. This paper presents a widespread view of glacial landforms presently situated beneath 1-2 km of ice in multi-square-km "windows to the bed" distributed throughout the catchment of Pine Island Glacier, West Antarctica. Imaged over three field seasons between 2007 and 2013 by dedicated radar surveys designed specifically to capture landforms analogous to those surveyed offshore by bathymetric surveying, the results provide significant insights for the interpretation of palaeo-ice-stream landforms, and their use in modelling ice-ocean interactions around the fringes of marine ice sheets. We show that landform sizes, shapes and associations vary significantly around Pine Island Glacier's catchment. The key controls appear to be substrate composition, pre-existing tectonic structure, and longstanding spatial stability of the ice-stream's flow distribution. The findings offer crucial information for modelling ice coupling to the bed, which should feed through to wider efforts to reconstruct the past behaviour of this significant marine ice sheet using its palaeoglacial landforms.

  18. Century/millennium internal climate oscillations in an ocean-atmosphere-continental ice sheet model

    NASA Technical Reports Server (NTRS)

    Birchfield, Edward G.; Wang, Huaxiao; Rich, Jonathan J.

    1994-01-01

    We demonstrate in a simple climate model that there exist nonlinear feedbacks between the atmosphere, ocean, and ice sheets capable of producing century/millennium timescale internal oscillations resembling those seen in the paleoclimate record. Feedbacks involve meridional heat and salt transports in the North Atlantic, surface ocean freshwater fluxes associated with melting and growing continental ice sheets in the northen hemisphere and with Atlantic to Pacific water vapor transport. The positive feedback between the production of North Atlantic Deep Water (NADW) and the meridional salt transport by the Atlantic thermohaline circulation tends to destabilize the climate system, while the negative feedback between the freshwater flux, either to or from the continental ice sheets, and meridional heat flux to the high-latitude North Atlantic, accomplished by the thermohaline circulation, stabilizes the system. The thermohaline circulation plays a central role in both positive and negative feedbacks because of its transport of both heat and salt. Because of asymmetries between the growth and melt phases the oscillations are, in general, accompanied by a growing or decreasing ice volume over each cycle, which in the model is reflected by increasing or decreasing mean salinity.

  19. Physical basis for a thick ice shelf in the Arctic Basin during the penultimate glacial maximum

    NASA Astrophysics Data System (ADS)

    Gasson, E.; DeConto, R.; Pollard, D.; Clark, C.

    2017-12-01

    A thick ice shelf covering the Arctic Ocean during glacial stages was discussed in a number of publications in the 1970s. Although this hypothesis has received intermittent attention, the emergence of new geophysical evidence for ice grounding in water depths of up to 1 km in the central Arctic Basin has renewed interest into the physical plausibility and significance of an Arctic ice shelf. Various ice shelf configurations have been proposed, from an ice shelf restricted to the Amerasian Basin (the `minimum model') to a complete ice shelf cover in the Arctic. Attempts to simulate an Arctic ice shelf have been limited. Here we use a hybrid ice sheet / shelf model that has been widely applied to the Antarctic ice sheet to explore the potential for thick ice shelves forming in the Arctic Basin. We use a climate forcing appropriate for MIS6, the penultimate glacial maximum. We perform a number of experiments testing different ice sheet / shelf configurations and compare the model results with ice grounding locations and inferred flow directions. Finally, we comment on the potential significance of an Arctic ice shelf to the global glacial climate system.

  20. Bayesian prediction of future ice sheet volume using local approximation Markov chain Monte Carlo methods

    NASA Astrophysics Data System (ADS)

    Davis, A. D.; Heimbach, P.; Marzouk, Y.

    2017-12-01

    We develop a Bayesian inverse modeling framework for predicting future ice sheet volume with associated formal uncertainty estimates. Marine ice sheets are drained by fast-flowing ice streams, which we simulate using a flowline model. Flowline models depend on geometric parameters (e.g., basal topography), parameterized physical processes (e.g., calving laws and basal sliding), and climate parameters (e.g., surface mass balance), most of which are unknown or uncertain. Given observations of ice surface velocity and thickness, we define a Bayesian posterior distribution over static parameters, such as basal topography. We also define a parameterized distribution over variable parameters, such as future surface mass balance, which we assume are not informed by the data. Hyperparameters are used to represent climate change scenarios, and sampling their distributions mimics internal variation. For example, a warming climate corresponds to increasing mean surface mass balance but an individual sample may have periods of increasing or decreasing surface mass balance. We characterize the predictive distribution of ice volume by evaluating the flowline model given samples from the posterior distribution and the distribution over variable parameters. Finally, we determine the effect of climate change on future ice sheet volume by investigating how changing the hyperparameters affects the predictive distribution. We use state-of-the-art Bayesian computation to address computational feasibility. Characterizing the posterior distribution (using Markov chain Monte Carlo), sampling the full range of variable parameters and evaluating the predictive model is prohibitively expensive. Furthermore, the required resolution of the inferred basal topography may be very high, which is often challenging for sampling methods. Instead, we leverage regularity in the predictive distribution to build a computationally cheaper surrogate over the low dimensional quantity of interest (future ice