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Sample records for future ice sheet

  1. Commitments to future retreat of Antarctic and Greenland ice sheets

    NASA Astrophysics Data System (ADS)

    DeConto, Robert; Pollard, David

    2016-04-01

    The agreement reached at the COP21 United Nations Conference on Climate Change is aimed at limiting future increases in global mean temperature below 2°C. Here, we use a continental ice sheet/shelf model with new treatments of meltwater-enhanced calving (hydrofracturing) and marine terminating ice-cliffs, to explore future commitments to sea-level rise given limits of global mean warming between 1 and 3°C. In this case, ice-sheet model physics are calibrated against past ice-sheet response to temperatures warmer than today. The ice-sheet model is coupled to highly resolved atmosphere and ocean-model components, with imposed limits on future warming designed to mimic the idealized limits discussed at COP21. Both the short and long-term potential rise in global mean sea level are discussed in light of the range of allowances agreed in Paris. We also explore the sensitivity of Greenland and Antarctic ice sheets to plausible ranges of atmospheric versus ocean warming consistent with global mean temperatures between 1 and 3°C; and the resulting long-term commitments to sea-level rise over the coming centuries and millennia.

  2. Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise

    USGS Publications Warehouse

    Overpeck, J.T.; Otto-Bliesner, B. L.; Miller, G.H.; Muhs, D.R.; Alley, R.B.; Kiehl, J.T.

    2006-01-01

    Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.

  3. Learning from the past: Antarctic Eemian ice sheet dynamics as an analogy for future warming.

    NASA Astrophysics Data System (ADS)

    Sutter, Johannes; Thoma, Malte; Grosfeld, Klaus; Gierz, Paul; Lohmann, Gerrit

    2015-04-01

    Facing considerable warming during this century the stability of the West Antarctic Ice Sheet is under increasing scrutiny. Recent observations suggest that the marine ice sheet instability of the WAIS has already started . We investigate the dynamic evolution of the Antarctic Ice Sheet during the last interglacial, forcing a state of the art 3D ice sheet model with Eemian boundary conditions. We elucidate the role of ocean warming and surface mass balance on the coupled ice sheet/shelf and grounding line dynamics. Special focus lies on an ice sheet modeling assessment of Antarctica's potential contribution to global sea level rise during the Eemian. The transient model runs are forced by time slice experiments of a fully coupled atmosphere-ocean global circulation model, as well as different sets of sea level and bedrock reconstructions. The model result show strong evidences for a severe ice-sheet retreat in West Antartica, leading to substantical contribution to global sea level from the Southern Hemisphere. Additionally we compare future warming scenarios of West Antarctic Ice Sheet dynamics to our paleo ice sheet modeling studies.

  4. The Importance of History for Predicting the Future of the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Bindschadler, R.

    2008-12-01

    The West Antarctic Ice Sheet (WAIS) initiative began in 1990, following on earlier studies of the 'Siple Coast' ice streams and the Ross Ice Shelf. The past nearly two decades of field and satellite research of the West Antarctic ice sheet have produced an astounding number of discoveries, not the least of which is the variability of the West Antarctic ice sheet on time scales from seconds (yes, seconds!) to many millennia. The shorter-time-scale variations, such as the recent acceleration and thinning of glaciers draining into the Amundsen Sea, have illustrated serious weaknesses in what were once regarded as excellent models of ice sheet dynamics. Repairing this modeling capability requires understanding and incorporating external and internal processes previously regarded as less important. Ice-sheet history remains the best means to test, tune and validate numerical models of ice sheets. Cenozoic-age behavior may seem too ancient to matter to a centennial-time-scale focus on the future, but it is precisely through a long history, that the variety of more extreme ice sheet configurations can be extracted. Such upper or lower bound estimates have served the WAIS community well over the years to help justify research needed to assess the probability of dramatic behavior. Now, with the necessity of model revisions central to the WAIS effort, time histories of ice sheet behavior over both short and long time scales will return to a position of extreme importance.

  5. 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.

  6. Long-term future contribution of the Greenland ice sheet to sea level rise

    NASA Astrophysics Data System (ADS)

    Calov, Reinhard; Robinson, Alex; Ganopolski, Andrey

    2015-04-01

    We investigate the impact of future cumulative anthropogenic emissions on the fate of the Greenland ice sheet. For this study, we use the polythermal ice sheet model SICOPOLIS, which is bi-directionally coupled with the regional climate model of intermediate complexity REMBO. We constrain our model parameters with simulations over two glacial cycles employing anomalies from the global CLIMBER-2 model. CLIMBER-2 treats the major components or the Earth system, including atmosphere, ocean, terrestrial vegetation and carbon cycle. As constraints we include the cumulative error in ice thickness, the surface mass balance partition (ratio between precipitation and ice discharge) and the ice elevation drop between Eemian and present-day at the NEEM ice core location. Our model includes a new ice discharge parameterization, which describes the ice loss via small-scale outlet glaciers in a heuristic statistical approach. Using the large-ensemble of model versions consistent with our constraints, we estimate the range of the long-term future contribution of the Greenland ice sheet to sea-level rise under global warming. On the 100,000-year time scale, there is a visible modulation over the CO2 signal in the simulated Greenland ice volume caused by the 20,000 years precessional cycle of insolation. In nearly all of our scenarios (500 to 5000 Gt carbon cumulative emissions), the Greenland is sheet fully decays in the future after at least 40,000 years. For the extreme scenario (5000 Gt), the Greenland ice sheet decays much faster - after about 5000 years, while there is still 80% of the ice sheet left after 40,000 years only for the model versions with a low temperature sensitivity and the low cumulative carbon emission scenario (500 Gt). Our results underline that without future negative CO2 emissions, irreversible loss of Greenland ice sheet is essentially unavoidable.

  7. 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.

  8. A smaller Antarctic Ice Sheet in the Pliocene and in the Future

    NASA Astrophysics Data System (ADS)

    DeConto, Robert; Pollard, David

    2013-04-01

    The middle Pliocene epoch (around 3.3 to 3) million years ago is often considered an analogue for future global climatic conditions, because mixing ratios of atmospheric CO2 were similar to today and global mean temperature was about 3°C warmer, comparable to projections of future climate at the end of this century (IPCC 2007). Importantly, some estimates of mid-Pliocene sea level are >20 m higher than today, implying the potential for significant retreat of the East Antarctic Ice Sheet (EAIS), in addition to complete loss of the Greenland and West Antarctic Ice Sheets (WAIS). Until now, most climate-ice sheet modeling studies have failed to simulate substantial Pliocene retreat of the East Antarctic ice margin, because at 400 ppmv CO2, atmospheric conditions on the steep flanks of the ice sheet remain relatively cold, even during the warmest austral summer orbits. Here, we use a hybrid ice sheet-shelf model coupled to a high-resolution regional climate model, to test the potential for both West and East Antarctic Ice Sheet retreat during the warm Pliocene and in the long-term future. In these simulations we apply new treatments of ice shelf calving and basal sliding (assuming a relationship between basal sliding coefficients and the rate of liquid water supply at the bed), and improved sub-glacial bathymetry using BEDMAP2. A range of plausible ocean warming scenarios (based on offline ocean modeling) are combined with the high-resolution regional climate model simulations to simulate the ice sheet's response to both Pliocene and long term future scenarios with elevated CO2. Unlike our previous studies, the combination of improved bathymetric detail and more physically based model treatments of calving and basal sliding result in substantial grounding line retreat into the Wilkes sub-glacial basin of East Antarctica during the Pliocene, adding several meters of equivalent sea level in addition to the contribution from a retreated WAIS. In long-term (10^3-yr

  9. Current and future darkening of the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Tedesco, Marco; Stroeve, Julienne; Fettweis, Xavier; Warren, Stephen; Doherty, Sarah; Noble, Erik; Alexander, Patrick

    2015-04-01

    Surface melting over the Greenland ice sheet (GIS) promotes snow grains growth, reducing albedo and further enhancing melting through the increased amount of absorbed solar radiation. Using a combination of remote sensing data and outputs of a regional climate model, we show that albedo over the GIS decreased significantly from 1996 to 2012. Further, we show that most of this darkening can be accounted for by enhanced snow grain growth and the expansion of areas where bare ice is exposed, both of which are driven by increases in snow warming. An analysis of the impact of light-absorbing impurities on albedo trends detected from spaceborne measurements was inconclusive because the estimated impact for concentrations of impurities of order of magnitude found in Greenland is within the albedo uncertainty retrievable from space-based instruments. However, neither models nor observations show an increase in pollutants (black carbon and associated organics) in the atmosphere over the GIS in this time period. Additionally, we could not identify trends in the number of fires over North America and Russia, assumed to be among the sources of soot for Greenland. We did find that a 'dark band' of tilted ice plays a crucial role in decreasing albedo along the west margin, and there is some indication that dust deposition to the GIS may be decreasing albedo in this region but this is not conclusive. In addition to looking at the direct impact of impurities on albedo, we estimated the impact of impurities on albedo via their influence on grain growth and found it is relatively small (~ 1- 2 %), though more sophisticated analysis needs to be carried out. Projections obtained under different warming scenarios consistently point to a continued darkening, with anomalies in albedo driven solely by the effects of climate warming of as much as -0.12 along the west margin of the GIS by the end of this century (with respect to year 2000). Projected darkening is likely underestimated

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

    PubMed

    Gomez, Natalya; Pollard, David; Holland, David

    2015-11-10

    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.

  11. Global ice sheet modeling

    SciTech Connect

    Hughes, T.J.; Fastook, J.L.

    1994-05-01

    The University of Maine conducted this study for Pacific Northwest Laboratory (PNL) as part of a global climate modeling task for site characterization of the potential nuclear waste respository site at Yucca Mountain, NV. The purpose of the study was to develop a global ice sheet dynamics model that will forecast the three-dimensional configuration of global ice sheets for specific climate change scenarios. The objective of the third (final) year of the work was to produce ice sheet data for glaciation scenarios covering the next 100,000 years. This was accomplished using both the map-plane and flowband solutions of our time-dependent, finite-element gridpoint model. The theory and equations used to develop the ice sheet models are presented. Three future scenarios were simulated by the model and results are discussed.

  12. AMOCMIP: Probabilistic projections of future AMOC evolution driven by global warming and Greenland Ice Sheet melt.

    NASA Astrophysics Data System (ADS)

    Bakker, P.; Ohgaito, R.; Abe-Ouchi, A.; Swingedouw, D.; Saenko, O.; Marsland, S. J.; Bi, D.; Schmittner, A.; Hu, A.; Mernild, S. H.; Yin, J.; Beadling, R. L.; Lenaerts, J.; van den Broeke, M.

    2015-12-01

    We present results of a community effort to use state-of-the-science climate models to simulate the impact of the partial melt of the Greenland Ice Sheet on the Atlantic Meridional Overturning Circulation (AMOC) under future global warming: the AMOC Model Intercomparison Project (AMOCMIP). The evolution of the AMOC is one of the key uncertainties of future climate projections. Climate models taking part in CMIP5 showed that over the 21st century the AMOC might reduce by 20-30% under the intermediate Representative Concentration Pathways (RCP) 4.5 scenario and by 36-44% under the high-end RCP8.5 scenario relative to preindustrial values. However, these projections didn't include the predicted partial melting of the Greenland Ice Sheet. Using a combination of observations and regional climate model experiments, realistic scenarios of the future mass loss of the Greenland Ice Sheet are constructed for different RCPs that span the next three centuries. These melt water scenarios have been included in various climate models to investigate its role in the simulated future evolution of the climate in general and more specifically the AMOC. To rigorously quantify the uncertainties of the AMOC projections and the probability of a future AMOC collapse we have used an AMOC emulator. This physics-based approach allows us to include uncertainties in the AMOC's sensitivity to temperature and salinity changes, as well as uncertainties of future global warming, polar amplification and melt rates of the Greenland Ice Sheet. Based on the first AMOCMIP simulations of future AMOC evolution forced by changes in greenhouse-gas concentrations and Greenland Ice Sheet melt, we will present probabilistic projections of future AMOC changes and the probability of an AMOC collapse.

  13. 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

  14. 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.; Peteet, Dorothy M.

    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

  15. Predicting uncertainty in future marine ice sheet volume using Bayesian statistical methods

    NASA Astrophysics Data System (ADS)

    Davis, A. D.

    2015-12-01

    The marine ice instability can trigger rapid retreat of marine ice streams. Recent observations suggest that marine ice systems in West Antarctica have begun retreating. However, unknown ice dynamics, computationally intensive mathematical models, and uncertain parameters in these models make predicting retreat rate and ice volume difficult. In this work, we fuse current observational data with ice stream/shelf models to develop probabilistic predictions of future grounded ice sheet volume. Given observational data (e.g., thickness, surface elevation, and velocity) and a forward model that relates uncertain parameters (e.g., basal friction and basal topography) to these observations, we use a Bayesian framework to define a posterior distribution over the parameters. A stochastic predictive model then propagates uncertainties in these parameters to uncertainty in a particular quantity of interest (QoI)---here, the volume of grounded ice at a specified future time. While the Bayesian approach can in principle characterize the posterior predictive distribution of the QoI, the computational cost of both the forward and predictive models makes this effort prohibitively expensive. To tackle this challenge, we introduce a new Markov chain Monte Carlo method that constructs convergent approximations of the QoI target density in an online fashion, yielding accurate characterizations of future ice sheet volume at significantly reduced computational cost.Our second goal is to attribute uncertainty in these Bayesian predictions to uncertainties in particular parameters. Doing so can help target data collection, for the purpose of constraining the parameters that contribute most strongly to uncertainty in the future volume of grounded ice. For instance, smaller uncertainties in parameters to which the QoI is highly sensitive may account for more variability in the prediction than larger uncertainties in parameters to which the QoI is less sensitive. We use global sensitivity

  16. Will ice flow in land-terminating regions of the Greenland ice sheet accelerate under future climate warming?

    NASA Astrophysics Data System (ADS)

    Doyle, S. H.; Hubbard, A.

    2015-12-01

    Recent observations and modelling studies investigating the dynamic response of land-terminating regions of the Greenland ice sheet to a warmer climate remain at best unreconciled and at worst equivocal and contradictory. Some studies suggest that ice flow will be regulated over annual time scales by the development of efficient subglacial drainage. Others suggest that such self-regulation processes may not be effective at higher elevations and that the recent and projected expansion of supraglacial lakes further into the ice sheet interior has lead to increased ice flow at high elevations. On the other hand, the observation that rapid in situ supraglacial lake drainage events may be triggered by precursory basal motion have led to the argument that, by inference, such lake drainage in the interior may be impossible, or at least hindered, by reduced strain rates and lack of surface crevasses in these regions. The response of the Greenland ice sheet to a warmer, wetter climate, in which late summer and autumnal cyclonic weather events drive widespread melt, rainfall and transient accelerations may also need to be accounted for in assessments of future Greenland ice mass loss if predicted changes in Greenland's climate are realised. This talk will critically assess recent insights gained into this topic, attempt to resolve some of them, and suggest directions for future research.

  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

    ) provides a good analogue for this purpose due to similarities in geology (in the selected study area), and the climate conditions and ice sheet size in Kangerlussuaq resemble the expected conditions in Fennoscandia during future glaciations. In 2005 and 2008 reconnaissance field trips were made to Kangerlussuaq, which confirmed the suitability of the area for the planned studies. According to the present Work Programme the investigations will be carried out in 2009-2012. The project is divided into four subprojects (SPA, SPB, SPC and SPD) addressing specific and different topics at or in relation to the ice margin: SPA (ice sheet hydrology and glacial groundwater formation); SPB (subglacial ice sheet hydrology), SPC (hydrogeochemistry and hydrogeology) and SPD (periglacial environment: biosphere and permafrost). The main objectives of SPA and SPB are to gain a better process understanding of supra- and subglacial hydrology. Qualitative and quantitative knowledge of the mechanisms, rates and distribution of the melt water recharge through the ice down to the bed, location and extension of warm-based areas and hydraulic pressure conditions at the base are the key issues to be studied. This will be made by meteorological observations, GPS measurements, radar surveys, drilling through the ice sheet and by ice sheet modelling. SPC will further study the fate of melt water by extending the investigations into the bedrock. It is assumed that the high hydraulic pressures at the ice sheet bed force water into the fracture network prevailing in the bedrock. However, it is not known how the fracture network behaves under loading, what is the proportion of recharging water compared to the drainage through the bed sediments, what is the intrusion depth, how long the meltwater can sustain its oxic nature and what chemical composition the recharging water has when and if it reaches repository depth (400-700 m). SPC seeks to answer these questions by drilling and instrumenting boreholes

  18. Large-Ensemble modeling of last deglacial and future variations of the Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Pollard, David; DeConto, Robert; Chang, Won; Applegate, Patrick; Haran, Murali

    2015-04-01

    Recent observations of thinning and retreat of the Pine Island and Thwaites Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as particularly vulnerable to future climate change. To date, most future modeling of these glaciers has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data from ~20,000 years BP to present, focusing on the ASE but including other sectors of Antarctica. Following several recent ice-sheet studies, we use Large-Ensemble statistical techniques, performing sets of ~500 to 1000 runs with varying model parameters. The model is run for the last 40 kyrs on 10 to 20-km grids, both on continental domains and also on nested domains over West Antarctica. Various types of objective scores for each run are calculated using reconstructed past grounding lines, relative sea level records, measured uplift rates, and cosmogenic elevation-age data. Runs are extended into the future few millennia using RCP scenarios. The goal is to produce calibrated probabilistic ranges of model parameter values and quantified envelopes of future ice retreat. Preliminary results are presented for Large Ensembles with (i) Latin HyperCube sampling in high-dimensional parameter space, using statistical emulators and Markov Chain Monte Carlo techniques, and (ii) dense "factorial" sampling with a smaller number of parameters. Different ways of combining the types of scores listed above are explored. One robust conclusion is that for the warmer future RCP scenarios, most reasonable parameter combinations produce retreat deep into the West Antarctic interior. Recently proposed mechanisms of hydrofracturing and ice-cliff failure accelerate future West Antarctic retreat, and later produce retreat into East Antarctic basins.

  19. Modeling Greenland ice sheet present-day and near-future runoff contribution.

    NASA Astrophysics Data System (ADS)

    Peano, Daniele; Colleoni, Florence; Masina, Simona

    2014-05-01

    The last IPCC report [AR5, IPCC] has shown an increasing contribution from Greenland melting to global sea-level over the last decade, increasing from 0.09 mm/year (period 1992-2001) to 0.59 mm/year (period 2002-2011). Given its strategic location, i.e. close to the main North Atlantic ocean convection sites, it is therefore of importance to better assess ice sheet melting and its impact on regional ocean processes. So far, runoff estimate from ice sheet has been poorly constrained (e.g. [Hanna et al., 2005], [Hanna et al., 2008]) and most of the time the few estimates comes from regional atmospheric models or general circulation models (e.g. [Edwards et al., 2013], [Fettweis et al., 2013]). Here, we present the results from the implementation of a routing scheme into the thermo-mechanical ice sheet-ice shelves model GRISLI [Ritz et al, 2001], applied to the Greenland ice sheet mass evolution over the 20th and 21st centuries. The routing scheme is based on the "multiple flow direction" developed by [Quinn et al., 1991]. We further improved this scheme by considering topographic depressions as possible "lakes" to be filled by meltwater. In this way, when a depression is filled, only the extra water is routed towards the Greenland coasts. This allow us to obtain an estimate of the total amount of freshwater reaching the ocean at each time step of the model integration, as well as a time-varying spatial distribution of the runoff along the coasts of Greenland. This routing scheme is applied in routing both surface and basal meltwater. Surface meltwater is computed by means of a PDD method [Fausto et al., 2007] on which only a fraction is considered for routing while the basal melting rate is part of the heat balance at the ice-bed interface. Runoff is simulated on a 5km x 5km horizontal grid and validation is performed over the 20th century using mean annual total precipitation and air temperature at 2 meters from Era-Interim reanalysis [Dee et al., 2011]. Near future

  20. 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.

  1. Modeling of past and future variations of the Antarctic Ice Sheet with Large Ensembles.

    NASA Astrophysics Data System (ADS)

    Pollard, D.; Deconto, R. M.; Chang, W.; Applegate, P. J.; Haran, M.

    2015-12-01

    Recent observations of thinning and retreat of the Pine Island and Thwaites Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as particularly vulnerable to future climate change. To date, most future modelingof these glaciers has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data from ~20,000 years BP to present, focusing on the ASE but including other sectors of Antarctica. Following several recent ice-sheet studies, we use Large Ensemble statistical methods, performing sets of ~600 runs over the last 30,000 years with systematically varying model parameters. Objective scores for each run are calculated using modern data and past reconstructed grounding lines, relative sea level records, cosmogenic elevation-age data and uplift rates.Two types of statistical methods are used to analyze the Large-Ensembleresults: simple averaging weighted by the aggregate score, and more advancedBayesian emulation and calibration methods that rigorously account for some of the uncertainties in the model and observations. Results for best-fit parameter ranges and envelopes of equivalent sea-level rise with the simple averaging method agree quite well with the more advanced techniques, but only for a Large Ensemble with dense (Full Factorial) parameter sampling. Runs are extended into the future using RCP scenarios, with drastic retreat mechanisms of hydrofracturing and structural ice-cliff failure. In most runs this produces grounding-line retreat into the West Antarctic interior, and into East Antarctic basins for RCP8.5, and the Large Ensemble analysis provides sea-level-rise envelopes with well defined parametric uncertainty bounds.

  2. Subglacial floods beneath ice sheets.

    PubMed

    Evatt, G W; Fowler, A C; Clark, C D; Hulton, N R J

    2006-07-15

    Subglacial floods (jökulhlaups) are well documented as occurring beneath present day glaciers and ice caps. In addition, it is known that massive floods have occurred from ice-dammed lakes proximal to the Laurentide ice sheet during the last ice age, and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago. We propose that drainage of lakes from beneath ice sheets will generally occur in a time-periodic fashion, and that such floods can be of severe magnitude. Such hydraulic eruptions are likely to have caused severe climatic disturbances in the past, and may well do so in the future.

  3. Present-day and future Antarctic ice sheet climate and surface mass balance in the Community Earth System Model

    NASA Astrophysics Data System (ADS)

    Lenaerts, Jan T. M.; Vizcaino, Miren; Fyke, Jeremy; van Kampenhout, Leo; van den Broeke, Michiel R.

    2016-09-01

    We present climate and surface mass balance (SMB) of the Antarctic ice sheet (AIS) as simulated by the global, coupled ocean-atmosphere-land Community Earth System Model (CESM) with a horizontal resolution of {˜ }1° in the past, present and future (1850-2100). CESM correctly simulates present-day Antarctic sea ice extent, large-scale atmospheric circulation and near-surface climate, but fails to simulate the recent expansion of Antarctic sea ice. The present-day Antarctic ice sheet SMB equals 2280 ± 131 {Gt year^{-1}}, which concurs with existing independent estimates of AIS SMB. When forced by two CMIP5 climate change scenarios (high mitigation scenario RCP2.6 and high-emission scenario RCP8.5), CESM projects an increase of Antarctic ice sheet SMB of about 70 {Gt year^{-1}} per degree warming. This increase is driven by enhanced snowfall, which is partially counteracted by more surface melt and runoff along the ice sheet's edges. This intensifying hydrological cycle is predominantly driven by atmospheric warming, which increases (1) the moisture-carrying capacity of the atmosphere, (2) oceanic source region evaporation, and (3) summer AIS cloud liquid water content.

  4. Large-Ensemble modeling of past and future variations of the Antarctic Ice Sheet with a coupled ice-Earth-sea level model

    NASA Astrophysics Data System (ADS)

    Pollard, David; DeConto, Robert; Gomez, Natalya

    2016-04-01

    To date, most modeling of the Antarctic Ice Sheet's response to future warming has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data of the last ~20,000 years to test the model against the large-scale variations during this period. The ice model is coupled to a global Earth-sea level model to improve modeling of the bedrock response and to capture ocean-ice gravitational interactions. Following several recent ice-sheet studies, we use Large Ensemble (LE) statistical methods, performing sets of 625 runs from 30,000 years to present with systematically varying model parameters. Objective scores for each run are calculated using modern data and past reconstructed grounding lines, relative sea level records, cosmogenic elevation-age data and uplift rates. The LE results are analyzed to calibrate 4 particularly uncertain model parameters that concern marginal ice processes and interaction with the ocean. LE's are extended into the future with climates following RCP scenarios. An additional scoring criterion tests the model's ability to reproduce estimated sea-level high stands in the warm mid-Pliocene, for which drastic retreat mechanisms of hydrofracturing and ice-cliff failure are needed in the model. The LE analysis provides future sea-level-rise envelopes with well-defined parametric uncertainty bounds. Sensitivities of future LE results to Pliocene sea-level estimates, coupling to the Earth-sea level model, and vertical profiles of Earth properties, will be presented.

  5. 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

  6. 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.

  7. 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…

  8. Polar predictability: exploring the influence of GCM and regional model uncertainty on future ice sheet climates

    NASA Astrophysics Data System (ADS)

    Reusch, D. B.

    2015-12-01

    Evaluating uncertainty in GCMs and regional-scale forecast models is an essential step in the development of climate change predictions. Polar-region skill is particularly important due to the potential for changes affecting both local (ice sheet) and global (sea level) environments through more frequent/intense surface melting and changes in precipitation type/amount. High-resolution, regional-scale models also use GCMs as a source of boundary/initial conditions in future scenarios, thus inheriting a measure of GCM-derived externally-driven uncertainty. We examine inter- and intramodel uncertainty through statistics from decadal climatologies and analyses of variability based on self-organizing maps (SOMs), a nonlinear data analysis tool. We evaluate a 19-member CMIP5 subset and the 30-member CESM1.0-CAM5-BGC Large Ensemble (CESMLE) during polar melt seasons (boreal/austral summer) for recent (1981-2000) and future (2081-2100, RCP 8.5) decades. Regional-model uncertainty is examined with a subset of these GCMs driving Polar WRF simulations. Decadal climatologies relative to a reference (recent: the ERA-Interim reanalysis; future: a skillful modern GCM) identify model uncertainty in bulk, e.g., BNU-ESM is too warm, CMCC-CM too cold. While quite useful for model screening, diagnostic benefit is often indirect. SOMs extend our diagnostics by providing a concise, objective summary of model variability as a set of generalized patterns. Joint analysis of reference and test models summarizes the variability of multiple realizations of climate (all the models), benchmarks each model versus the reference (frequency analysis helps identify the patterns behind GCM bias), and places each GCM in a common context. Joint SOM analysis of CESMLE members shows how initial conditions contribute to differences in modeled climates, providing useful information about internal variability, such as contributions from each member to overall uncertainty using pattern frequencies. In the

  9. Assessing the impact of ocean temperature on the contribution of the Greenland ice sheet to future sea-level rise with a heuristic statistical approach

    NASA Astrophysics Data System (ADS)

    Calov, R.; Ganopolski, A.; Robinson, A.; Beckmann, J.; Alexander, D.; Perrette, M.

    2015-12-01

    We present large ensembles of simulations of the contribution of the Greenland ice sheet to sea-level rise under RCP2.6, 4.5 and 8.5 future scenarios over 300 years, including several extended simulations over millennia. To this end, we utilise the ice sheet model SICOPOLIS coupled with the regional climate system model of intermediate complexity REMBO. The loss of ice into fjords via outlet glaciers is resolved in a heuristic statistical approach. In this framework, we recently included into our model an ocean temperature parameterization to assess the impact of ice loss of the Greenland ice sheet. This work precedes our planned more comprehensive approach, which will resolve all important ice loss processes in a model of intermediate complexity of the Greenland glacial system. Our large ensembles of simulations under future scenarios serve to estimate the uncertainty of the impact of future ocean temperatures on the state of the Greenland ice sheet. A suite of initial configurations of the ice sheet is generated via paleo simulations over two glacial cycles by introducing present-day and Eemian constraints, such as a range in the mass balance partition, the shape of the ice sheet, the Eemian ice surface elevation change at an upstream position of the NEEM and further local constraints. The paleo simulations support the choice of the range of parameters of our ocean temperature parameterization. The large ensemble simulations together with the paleo constraints enable us to improve estimates onthe range of the possible impact of future changes in ocean temperature around Greenland on the ice loss of the Greenland ice sheet.

  10. The Elementary Marine Ice Sheet Model (EMISM)

    NASA Astrophysics Data System (ADS)

    Pattyn, Frank

    2015-04-01

    behaviour is in line with recent model simulations of Pine Island and Thwaites Glacier systems. We perform a series of sensitivity experiments with EMISM and compare results to recent model intercomparisons of the Antarctic ice sheet (e.g., SeaRISE, Favier et al. (2013)). Future developments include the implementation of a variant of the coupled SSA/SIA to account for ice stream flow, upstream of grounding lines.

  11. Simulations of the Greenland ice sheet 100 years into the future with the full Stokes model Elmer/Ice

    NASA Astrophysics Data System (ADS)

    Seddik, H.; Greve, R.; Zwinger, T.; Gillet-Chaulet, F.; Gagliardini, O.

    2011-12-01

    The full Stokes thermo-mechanically coupled model Elmer/Ice is applied to the Greenland ice sheet. Elmer/Ice employs the finite element method to solve the full Stokes equations, the temperature evolution equation and the evolution equation of the free surface. The general framework of this modeling effort is a contribution to the Sea-level Response to Ice Sheet Evolution (SeaRISE) assessment project, a community-organized effort to estimate the likely range of ice sheet contributions to sea level rise over the next few hundred years (http://tinyurl.com/srise-lanl, http://tinyurl.com/srise-umt). The present geometry (surface and basal topographies) is derived from data where the basal topography was created with the preservation of the troughs at the Jakobshavn Ice Stream, Helheim, Kangerdlussuaq and Petermann glaciers. A mesh of the computational domain is created using an initial footprint which contains elements of 5 km horizontal resolution and to limit the number elements on the footprint while maximizing the spatial resolution, an anisotropic mesh adaptation scheme is employed based on the Hessian matrix of the observed surface velocities. The adaptation is carried out with the tool YAMS and the final footprint is vertically extruded to form a 3D mesh of 320880 elements with 17 equidistant, terrain-following layers. The numerical solution of the Stokes and the heat transfer equations employs direct solvers with stabilization procedures. The boundary conditions are such that the temperature at the surface uses the present-day mean annual air temperature given by a parameterization or directly from the available data, the geothermal heat flux at the bedrock is given by data and the lateral sides are open boundaries. A non-linear Weertman law is used for the basal sliding. Results for the SeaRISE 2011 sensitivity experiments are presented so that six different experiments have been conducted, grouped in two sets. The Set C (three experiments) applies a change to

  12. Palaeoclimate science: Pulsating ice sheet

    NASA Astrophysics Data System (ADS)

    Vieli, Andreas

    2017-02-01

    During the last ice age, huge numbers of icebergs were episodically discharged from an ice sheet that covered North America. Numerical modelling suggests that these events resulted from a conceptually simple feedback cycle. See Letter p.332

  13. Impact of realistic future ice sheet discharge on the Atlantic ocean

    NASA Astrophysics Data System (ADS)

    van den Berk, Jelle

    2015-04-01

    Royal Netherlands Meteorological Institute, De Bilt, The Netherlands A high-end scenario of polar ice loss from the Greenland and Antarctic ice sheet is presented with separate projections for different mass-loss sites up to the year 2100. The resultant freshwater forcing is applied to a global climate model and the effects on sea-level rise are discussed. The simulations show strong sea level rise on the Antarctic continental shelves. To separate the effects of atmospheric warming and melt water we then ran four simulations. One without either forcing, one with both and two with one of each separately. Melt water leads to a slight additional depression of the Atlantic overturning circulation, but a strong decrease remains absent. The bulk of the strength reduction is due to higher atmospheric temperatures which inhibits deep water formation in the North Atlantic. The melt water freshens the upper layers of the ocean, but does not strongly impact buoyancy. The balance between North Atlantic Deep Water and Antarctic Bottom Water must then remain relatively unaffected. Only applying the melt water forcing to the Northern Hemisphere does not lead to a stronger effect. We conclude that the meltwater scenario only impacts the overturning circulation superficially because the deeper ocean is not affected. Transport through Bering Strait and across the zonal section at the latitude of Cape Agulhas is increased by increased atmospheric temperatures and adds some inertia to these transports. Reversing the atmospheric forcing bears this out when the transport then further increases. The freshwater, however, mitigates this inertia somewhat.

  14. 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

  15. 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.

  16. LAKES BENEATH THE ICE SHEET: The Occurrence, Analysis, and Future Exploration of Lake Vostok and Other Antarctic Subglacial Lakes

    NASA Astrophysics Data System (ADS)

    Siegert, Martin J.

    2005-01-01

    Airborne geophysics has been used to identify more than 100 lakes beneath the ice sheets of Antarctica. The largest, Lake Vostok, is more than 250 km in length and 1 km deep. Subglacial lakes occur because the ice base is kept warm by geothermal heating, and generated meltwater collects in topographic hollows. For lake water to be in equilibrium with the ice sheet, its roof must slope ten times more than the ice sheet surface. This slope causes differential temperatures and melting/freezing rates across the lake ceiling, which excites water circulation. The exploration of subglacial lakes has two goals: to find and understand the life that may inhabit these unique environments and to measure the climate records that occur in sediments on lake floors. The technological developments required for in situ measurements mean, however, that direct studies of subglacial lakes may take several years to happen.

  17. 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.

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

    PubMed Central

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

    2015-01-01

    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. PMID:25908601

  19. Irregular oscillations of the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    Macayeal, Douglas R.

    1993-01-01

    Model simulations of the West Antarctic ice sheet suggest that sporadic, perhaps chaotic, collapse (complete mobilization) of the ice sheet occurred throughout the past one million years. The irregular behavior is due to the slow equilibration time of the distribution of basal till, which lubricates ice-sheet motion. This nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history, and in particular, whether the present basal till distribution predisposes the ice sheet towards rapid change.

  20. Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise

    PubMed Central

    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

    2013-01-01

    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. PMID:23940337

  1. 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.

  2. 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

  3. Clouds enhance Greenland ice sheet meltwater runoff

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    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.

  4. Paleoclimate perspectives on Antarctic ice sheet sensitivity

    NASA Astrophysics Data System (ADS)

    Naish, Timothy

    2015-04-01

    Near- and long-term future projections of global mean sea level rise (SLR) are hampered by a lack of understanding of the potential dynamic contribution of the polar ice sheets, and in particular the Antarctic ice sheets. With the completion of the Intergovernmental Panel on Climate Change's Assessment Report a major challenge continues to be placing an upper bound in sea-level projections for 2100 and beyond. The so-called "deterministic" approach which sums observed- and model-projected trends in the known contributions (e.g. ice sheet and glacier surface mass balance, ocean thermal expansion and ground water storage changes) implies a "likely" upper bound of +100cm by 2080-2100. The "semi-empirical" approach which scales past observed sea-level change to mean surface temperature, and uses this relationship to scale future temperature scenarios, predicts a significantly higher upper bound of up to ~2m by 2100. The discrepancy between the two approaches may in part reflect the poorly understood contribution of ice dynamics - that is the rate of flow of ice sheets into the ocean. An ensemble of Antarctic ice sheet models produces highly divergent results for future sea-level projections, primarily because of uncertainties around the mass changes in the East Antarctic Ice Sheet with some models showing increased precipitation driving a positive mass balance overall, even with loss of the marine-based West Antarctic Ice Sheet (WAIS). Current best estimates suggest a 10-20cm dynamic ice sheet contribution by 2100 to global SLR. Of concern is that marine based ice sheets are highly sensitive to increases in ocean temperature at their margins and rapid disintegration may ensue if the ice sheets grounding lines retreat into deep sub-glacial basins. Recent studies show the highest rates of ice sheet thinning and retreat are occurring at locations around the WAIS where the surface ocean has warmed, and that some WAIS loss may now be irreversible. Geological records allow

  5. Cryosphere: Warming ocean erodes ice sheets

    NASA Astrophysics Data System (ADS)

    Kusahara, Kazuya

    2016-01-01

    Antarctic ice sheets are a key player in sea-level rise in a warming climate. Now an ice-sheet modelling study clearly demonstrates that an Antarctic ice sheet/shelf system in the Atlantic Ocean will be regulated by the warming of the surrounding Southern Ocean, not by marine-ice-sheet instability.

  6. Ice flow of the Antarctica Ice Sheet

    NASA Astrophysics Data System (ADS)

    Mouginot, J.; Scheuchl, B.; Rignot, E. J.

    2011-12-01

    Ice velocity is fundamental characteristic of the dynamics of ice sheets and is essential to know for calculating the mass budget of ice sheet and for controlling ice sheet numerical models with realistic boundary conditions. Until recently, data were mostly available on a discrete basis over small areas with variable precision. Here, we report on our results of processing ice velocity from the interferometric synthetic-aperture radar data acquired by ALOS PALSAR in 2006 to 2010 by the Japan Aerospace Exploration Agency (JAXA), by ENVISAT ASAR in 2007 to 2009, by RADARSAT-2 in 2009 and 2011, by RADARSAT-1 in 1997 and 2000 and by ERS-1 & -2 in 1996 in the framework of the International Polar Year 2007-2009. The result is the most comprehensive and precise high-resolution digital map of ice motion ever produced on the Antarctic continent. While important surprises are found along the coastline, it is in the interior that this map is revealing the most interesting features. The data reveal widespread, patterned, enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland, over the entire continent. We show that the ice motion along these flow features has a strong basal slip component. This has far reaching implications for the modeling of ice sheet flow and evolution. In addition, our multi-year coverage of the coastal sectors reveal the beginning of an acceleration on Thwaites glacier and a wave of accelerated flow propagating inland rapidly on Pine Island Glacier between 2006 and 2010. This work was conducted at the Department of Earth System Science, University of California Irvine under a contract with the National Aeronautics and Space Administration's MEaSUREs program.

  7. Isochronal ice sheet model: Simulate englacial tracer transport to reconstruct past climates and ice sheet volumes

    NASA Astrophysics Data System (ADS)

    Born, Andreas

    2015-04-01

    The full history of ice sheet and climate interactions is recorded in the vertical profiles of isotopic and other geochemical tracers in polar ice sheets. In addition, recent advances in radiostratigraphy uncover the englacial layering that contains information of past surface topographies and thus ice sheet volumes and sea level. Numerical simulations of these archives could afford great advances both in the interpretation of paleoclimatic tracers as well as to help improve ice sheet models themselves and future projections. However, fundamental mathematical shortcomings in existing ice sheet models subject tracers to spurious diffusion that renders such attempts unfeasible. 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. Simulations of the last glacial cycle are presented that show good skill in reproducing the reconstructed profile of the oxygen isotopic ratio (δ18O) and the age scale (http://www.climate.unibe.ch/~born/ice_model.html).

  8. 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.

  9. West antarctic ice sheet collapse: Chimera or clear danger

    SciTech Connect

    Alley, R.B. ); MacAyeal, D.R. )

    1993-01-01

    The specter of a west antarctic collapse has been with us for 25 years. Recently, certain official assessments concerned primarily with the future response to projected global warming have concluded that Antarctica will not cause much sea-level rise within the planning horizon of a century or so. At the same time startling new results on ice sheet (in)stability have been emerging, pointing to less stability then previously believed. Some recent results are reviewed briefly: Heinrich layers in the North Atlantic show basally lubricated surges of the Laurentide ice sheet; the west antarctic ice sheet collapsed recently; the modern west antarctic ice sheet is changing rapidly locally; the bed of ice stream B is exceptionally well lubricated by water and water-saturated soft sediments; the modern ice sheet is thinning slowly on average; a model west antarctic ice sheet undergoes rapid collapses long after forcing and probably related to penetration of warmth to the bed. 23 refs.

  10. Monitoring ice sheet behavior from space

    NASA Astrophysics Data System (ADS)

    Bindschadler, Robert

    1998-02-01

    Satellite remote sensing has revolutionized ice sheet research. A variety of instruments sensitive to different parts of the electromagnetic spectrum take what the human eye detects as a flat, white desert and provide data sets rich in scientific information. Image-based maps of ice sheets are becoming commonplace and have become an integral component of field work. More than a pretty picture, the digital character of the satellite data from these instruments has become fundamental to the production of elevation, motion, accumulation, and reflectance data sets. Visible imagery shows the scientist a wealth of features that offer clues to the history and current behavior of the ice sheet. Radar and microwave imagery provide information from beneath the surface and have been used to estimate snow accumulation rates. Interferometry principles have recently been applied to measure surface topography and ice motion with unparalleled precision. Nonimaging instruments also keep a watchful eye, monitoring the ice sheet for indications of growth or shrinkage. Further expansion of the uses of satellite data is anticipated in the future.

  11. 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.

  12. Greenland Ice Sheet glacier motion and ice loss: New understanding of ice sheet behavior through remote sensing

    NASA Astrophysics Data System (ADS)

    Moon, T. A.; Fahnestock, M. A.; Scambos, T.; Joughin, I.

    2015-12-01

    Ice loss from the Greenland Ice Sheet makes up roughly a third of current sea level rise, also generating substantial local and regional freshwater fluxes. Containing more than 6 meters of sea level rise equivalent in ice, Greenland has the potential to contribute much more to rising ocean levels and freshening water in the future. Understanding the dynamics of the ice sheet, particularly the behavior of fast flowing coastal outlet glaciers, is critical to improving predictions of future ice sheet change and associated impacts. Combining velocity, glacier ice front, sea ice, and ice sheet surface melt data, we made several important advances in characterizing and understanding seasonal glacier behavior and the processes driving change: 1) seasonal velocity patterns fall into at least 3 distinct patterns, 2) these seasonal velocity patterns likely indicate differences in glacier responsiveness to ocean versus subglacial hydrologic processes, and 3) in some regions seasonal versus multi-year velocity changes appear most strongly influenced by different environmental factors. Further progress was previously hampered by limits in measurement resolution across space and time. To address this challenge, we are creating a new - and continuously growing - ice velocity dataset from Landsat 8 imagery. This data stream supports comprehensive global measurements of ice flow, providing a leap in our understanding of ice sheet motion across space and time. We offer a high-level discussion of our research findings and an introduction to the new Landsat 8-enabled data stream. Our results and measurement capabilities deliver critical new knowledge about ice sheet behavior and interaction with ocean and climate factors. These advances, in turn, have important implications for other elements of Earth system research, including climate, oceanography, and biology.

  13. Equilibrium ice sheet scaling in climate modeling

    SciTech Connect

    Verbitsky, M.Y.

    1992-03-01

    A set of simple scaling formulas related to ice sheet evolution is derived from the dynamic and thermodynamic equations for ice and is used to consider two common situations: (1) when we wish to estimate potential ice sheet characteristics given the prescribed net snow accumulation over an area; and (b) when we wish to reconstruct net snow accumulation and vertical temperature difference within the ice sheet area and volume. The scaling formulas are applied to the present day Antarctic and Greenland ice sheets, as well as to some ancient ice sheets, and are used to estimate the potential global sea level change due to greenhouse warming. 23 refs.

  14. The contribution to future flood risk in the Severn Estuary from extreme sea level rise due to ice sheet mass loss

    NASA Astrophysics Data System (ADS)

    Quinn, N.; Bates, P. D.; Siddall, M.

    2013-11-01

    In this paper, we assess the risk of future coastal flooding in the Severn Estuary, examining the contribution from low probability extreme sea level rise scenarios resulting from the possibility of increased rates of ice sheet mass loss in the coming century. A simple asymmetric probability distribution is constructed to include sea level rise scenarios of up to 1.9 m by 2100, based on recent assessments of future sea level rise in the UK. A regular sampling procedure, sampling every 1 mm, is used to increase the boundary water levels associated with a current 1:200 year event to force a two-dimensional hydrodynamic model of coastal inundation to examine the influence of sea level rise on inundation of the Somerset Levels region. From the resulting ensemble of predictions an estimation of risk (conditioned upon the hazard and the probability of occurrence) by 2100 is established. The results indicate that although the likelihood of extreme sea level rise due to rapid ice sheet mass loss is low, the resulting hazard can be large, resulting in a significant (29.7%) increase to the projected risk. These findings clearly demonstrate that uncertainty in future sea level rise, mostly associated with the rate of ice sheet mass loss, is a vital component of coastal flood risk, and therefore, needs to be accounted for by decision makers when considering mitigation policies related to coastal flooding.

  15. Ice Sheet Retention Structures,

    DTIC Science & Technology

    1983-12-01

    Protection Manual (U.S. Army water velocities that are too high to allow an ice cover Corps of Engineers 1973). A paper by Czerniak et al. to form...phenomena. Physical hydraulic model stu- dian Journal of Civil Engineering, 4: 380. dies should precede the design and installation of most Czerniak

  16. 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).

  17. 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

  18. Very high resolution modelling of the Surface Mass Balance of the Greenland Ice Sheet: Present day conditions and future prospects.

    NASA Astrophysics Data System (ADS)

    Mottram, Ruth; Aðalgeirsdóttir, Guðfinna; Boberg, Fredrik; Hesselbjerg Christensen, Jens; Bøssing Christensen, Ole; Langen, Peter; Rodehacke, Christian; Stendel, Martin; Yang, Shuting

    2014-05-01

    Recent experiments with the Regional Climate Model (RCM) HIRHAM5 have produced new surface mass balance (SMB) estimates at the unprecedented high horizontal resolution of 0.05 degrees (~5.5km). These simulations indicate a present day SMB of 347 ± 98 Gt/year over the whole ice sheet averaged over the period 1989 - 2012 driven by the ERA-Interim reanalysis dataset. We validate accumulation rates over the ice sheet using estimates from shallow firn cores to confirm the importance of resolution to accurate estimates of accumulation. Comparison with PROMICE and GC-Net automatic weather station observations shows the model represents present day climate and climate variability well when driven by the ERA-Interim reanalysis dataset. Comparison with a simulation at 0.25 degrees (~27km) resolution from the same model shows a significantly different calculated SMB over the whole ice sheet, largely due to changes in precipitation distribution over Greenland. The very high resolution requires a more sophisticated treatment of sub-grid scale processes in the snow pack including meltwater retention and refreezing and an enhanced albedo scheme. Our results indicate retention processes account for a significant proportion of the total surface budget based on a new parameterization scheme in the model. SMB projections, driven by the EC-Earth Global Climate Model (GCM) at the boundaries for the RCP 4.5 scenario indicate a declining surface mass balance over the 21st century with some compensation for warmer summer temperatures and enhanced melt in the form of increased precipitation. A cold bias in the driving GCM for present day conditions suggests that this simulation likely underestimates the change in SMB. However, the downscaled precipitation fields compare well with those in the reanalysis driven simulations. A soon-to-be complete simulation uses driving fields from the GCM running the RCP8.5 scenario.

  19. Hysteresis and feedback of ice sheet response

    NASA Astrophysics Data System (ADS)

    Abe-Ouchi, A.; Saito, F.; Takahashi, K.

    2014-12-01

    Investigating the response of ice sheets to climatic forcings in the past by climate-ice sheet modelling is important for understanding the ice sheets' change. The 100-kyr cycle of the large Northern Hemisphere ice sheets and fast termination of the glacial cycle are the prominent pattern known from paleoclimate records. We simulate the past glacial cycles with an ice sheet model, IcIES in combination with a general circulation model, MIROC, using the time series of insolation and atmospheric CO2. Feedback processes between ice sheet and atmosphere such as the ice albedo feedback, the elevation-mass balance feedback, desertification effect and stationary wave feedback are analyzed. We show that the threshold of termination of the glacial cycles can be explained by the pattern of the hysteresis of ice sheet change, i.e. multiple steady states of the ice sheets under climatic forcings. We find that slope of the upper branch of the multiple equilibria curve for Laurentide ice volumes is fundamental for the observed glacial patterns. Finally, we discuss the similarity and difference between the hysteresis structure of ice sheets variation for Northern Hemisphere ice sheets, Antarctica and Greenland.

  20. 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.

  1. Mass balance of polar ice sheets.

    PubMed

    Rignot, Eric; Thomas, Robert H

    2002-08-30

    Recent advances in the determination of the mass balance of polar ice sheets show that the Greenland Ice Sheet is losing mass by near-coastal thinning, and that the West Antarctic Ice Sheet, with thickening in the west and thinning in the north, is probably thinning overall. The mass imbalance of the East Antarctic Ice Sheet is likely to be small, but even its sign cannot yet be determined. Large sectors of ice in southeast Greenland, the Amundsen Sea Embayment of West Antarctica, and the Antarctic Peninsula are changing quite rapidly as a result of processes not yet understood.

  2. 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.

  3. 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.

  4. Simulating the Antarctic ice sheet in the late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, B.; Dolan, A. M.; Bernales, J.; Gasson, E.; Goelzer, H.; Golledge, N. R.; Sutter, J.; Huybrechts, P.; Lohmann, G.; Rogozhina, I.; Abe-Ouchi, A.; Saito, F.; van de Wal, R. S. W.

    2015-05-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The late Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of five sensitivity experiments. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, considering the models are set up with their own parameter settings. For the Pliocene, the results demonstrate the difficulty of all six models used here to simulate a significant retreat or re-advance of the East Antarctic ice grounding line, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. The specific sea-level contribution of the Antarctic ice sheet at this point cannot be conclusively determined, whereas improved grounding line physics could be essential for a correct representation of the migration of the grounding-line of the Antarctic ice sheet during the Pliocene.

  5. Impact of bedrock description on modeling ice sheet dynamics

    NASA Astrophysics Data System (ADS)

    Durand, G.; Gagliardini, O.; Favier, L.; Zwinger, T.; le Meur, E.

    2011-10-01

    Recent glaciological surveys have revealed a significant increase of ice discharge from polar ice caps into the ocean. In parallel, ice flow models have been greatly improved to better reproduce current changes and forecast the future behavior of ice sheets. For these models, surface topography and bedrock elevation are crucial input parameters that largely control the dynamics and the ensuing overall mass balance of the ice sheet. For obvious reasons of inaccessibility, only sparse and uneven bedrock elevation data is available. This raw data is processed to produce Digital Elevation Models (DEMs) on a regular 5 km grid. These DEMs are used to constrain the basal boundary conditions of all ice sheet models. Here, by using a full-Stokes finite element code, we examine the sensitivity of an ice flow model to the accuracy of the bedrock description. In the context of short-term ice sheet forecast, we show that in coastal regions, the bedrock elevation should be known at a resolution of the order of one kilometer. Conversely, a crude description of the bedrock in the interior of the continent does not affect modeling of the ice outflow into the ocean. These findings clearly indicate that coastal regions should be prioritized during future geophysical surveys. They also indicate that a paradigm shift is required to change the current design of DEMs describing the bedrock below the ice sheets: they must give users the opportunity to incorporate high-resolution bedrock elevation data in regions of interest.

  6. 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.

  7. A Prediction of Increase in Subglacial Volcanism Beneath the West Antarctic Ice Sheet (WAIS) as Future Deglaciation Caused by Ocean Circulation Proceeds

    NASA Astrophysics Data System (ADS)

    Behrendt, J. C.; LeMasurier, W. E.

    2015-12-01

    Many decades of aeromagnetic surveying (e.g. Behrendt, 1964; 2013; and others) over the West Antarctic Ice sheet (WAIS) have shown >1000 high amplitude, shallow source magnetic anomalies interpreted as as indicating subglacial volcanic centers of late Cenozoic age to presently active. Similar anomalies exist over exposed volcanic rocks bordering the WAIS in places.Recent papers (e.g. Wouters et al., 2015; Paolo, et al.; 2015 and others) based on satellite altimetry have shown dramatic thinning and retreat of ice shelves, particularly those bordering the Amundsen and Bellingshausen Seas, caused by melting from circulation of warming sea water. Previous workers have shown that when ice shelves collapse, the ice streams previously dammed by them accelerate an order of magnitude higher velocity, and surface elevation decreases. GRACE satellite interpretations (e.g. Velicogna et al., and others) indicate mass loss of WAIS in recent years.The bed elevation beneath the WAIS deepens inland from the Amundsen and Bellingshausen coasts, although high relief volcanic topography is present in a number of areas beneath the ice.Crowley et a. (2015) have shown that glacial cycles may drive production of oceanic crust by lowering pressure in the mantle resulting in increased melting and magma production. Increased volcanism due to deglaciation in Iceland has apparently produced increased in volcanic activity there. Deglaciation of the Norwegian continental shelf has resulted in faulting of the sea floor and similar faulting has been reported of the Ross Sea shelf following deglaciation there.I suggest here that as the WAIS collapses in the future resulting from climate change, an increase in volcanic activity beneath the ice might be expected. This may provide a feedback mechanism for increase in ice melting.

  8. 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

    decades. An extensible design will allow for continued use of the CMCT as future altimetry, gravimetry, and other remotely sensed data become available for use in ice sheet model validation.« less

  9. 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

    Greenland over the past few decades. An extensible design will allow for continued use of the CmCt as future altimetry, gravimetry, and other remotely sensed data become available for use in ice sheet model validation.

  10. Considering thermal-viscous collapse of the Greenland ice sheet.

    PubMed

    Colgan, William; Sommers, Aleah; Rajaram, Harihar; Abdalati, Waleed; Frahm, Joel

    2015-07-01

    We explore potential changes in Greenland ice sheet form and flow associated with increasing ice temperatures and relaxing effective ice viscosities. We define "thermal-viscous collapse" as a transition from the polythermal ice sheet temperature distribution characteristic of the Holocene to temperate ice at the pressure melting point and associated lower viscosities. The conceptual model of thermal-viscous collapse we present is dependent on: (1) sufficient energy available in future meltwater runoff, (2) routing of meltwater to the bed of the ice sheet interior, and (3) efficient energy transfer from meltwater to the ice. Although we do not attempt to constrain the probability of thermal-viscous collapse, it appears thermodynamically plausible to warm the deepest 15% of the ice sheet, where the majority of deformational shear occurs, to the pressure melting point within four centuries. First-order numerical modeling of an end-member scenario, in which prescribed ice temperatures are warmed at an imposed rate of 0.05 K/a, infers a decrease in ice sheet volume of 5 ± 2% within five centuries of initiating collapse. This is equivalent to a cumulative sea-level rise contribution of 33 ± 18 cm. The vast majority of the sea-level rise contribution associated with thermal-viscous collapse, however, would likely be realized over subsequent millennia.

  11. 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.

  12. Seasonal Greenland Ice Sheet ice flow variations in regions of differing bed and surface topography

    NASA Astrophysics Data System (ADS)

    Sole, A. J.; Livingstone, S. J.; Rippin, D. M.; Hill, J.; McMillan, M.; Quincey, D. J.

    2015-12-01

    The contribution of the Greenland Ice Sheet (GrIS) to future sea-level rise is uncertain. Observations reveal the important role of basal water in controlling ice-flow to the ice sheet margin. In Greenland, drainage of large volumes of surface meltwater to the ice sheet bed through moulins and hydrofracture beneath surface lakes dominates the subglacial hydrological system and provides an efficient means of moving mass and heat through the ice sheet. Ice surface and bed topography influence where meltwater can access the bed, and the nature of its subsequent flow beneath the ice. However, no systematic investigation into the influence of topographic variability on Greenland hydrology and dynamics exists. Thus, physical processes controlling storage and drainage of surface and basal meltwater, and the way these affect ice flow are not comprehensively understood. This presents a critical obstacle in efforts to predict the future evolution of the GrIS. Here we present high-resolution satellite mapping of the ice-surface drainage network (e.g. lakes, channels and moulins) and measurements of seasonal variations in ice flow in south west Greenland. The region is comprised of three distinct subglacial terrains which vary in terms of the amplitude and wavelength and thus the degree to which basal topography is reflected in the ice sheet surface. We find that the distribution of surface hydrological features is related to the transfer of bed topography to the ice sheet surface. For example, in areas of thinner ice and high bed relief, moulins occur more frequently and are more uniformly dispersed, indicating a more distributed influx of surface-derived meltwater to the ice sheet bed. We investigate the implications of such spatial variations in surface hydrology on seasonal ice flow rates.

  13. Swath Measurements of Ice Sheet Bottom Topography and Radar Reflectivity

    NASA Astrophysics Data System (ADS)

    Freeman, A.; Gogineni, P. S.; Jezek, K. C.; Rodriguez, E.; Wu, X.

    2009-12-01

    Ice sheet thickness is a fundamental measurement for understanding the dynamics of large ice sheets (terrestrial or extraterrestrial). Radar is the primary tool used to measure ice thickness but a major challenge is accurately measuring the arrival time of the basal echo in the presence of surface clutter, which may arise from processes such as wind driven deposition and erosion or crevassing. Essentially, the basal echo strength, which is weak because of attenuation through the ice, becomes comparable to the surface scattering signal even though the coincident surface return comes from a large, off-nadir angle. During the past 4 years, we explored three surface clutter rejection techniques and applied them to data collected with 150/450 MHz radars operated from aircraft over the Greenland Ice Sheet. We also investigated how the techniques could be used to go beyond nadir sounding of ice sheets and, when operated used with broad-beam antennas, could successfully acquire 3-dimensional intensity images of the ice sheet base. In this paper, we describe experiments to image the ice sheet base using: synthetic aperture radar (SAR) interferogram filtering; SAR tomography; and beam steering. For the case of a broad beam antenna array, we show that interferograms filtering provides the highest quality topographic data from both the left and right sides of the aircraft but only under optimal conditions. We show that a beam-steering/radar tomography hybrid algorithm provides the most robust topography and also yields an intensity map. We provide example topographies for the base of the Greenland Ice Sheet and suggest how the approach could be used for future sounding of extraterrestrial ice. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a grant from the National Aeronautics and Space Administration. 3-d radar image of the base of the ice sheet. Scene is an orthorectified mosaic located just

  14. 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.

  15. 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 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 (???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, ???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 today and atmospheric CO 2 concentration was as high as ???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 CO2. ??2009 Macmillan Publishers Limited. All rights reserved.

  16. 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).

  17. 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.

  18. Ice sheets. Volume loss from Antarctic ice shelves is accelerating.

    PubMed

    Paolo, Fernando S; Fricker, Helen A; Padman, Laurie

    2015-04-17

    The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using 18 years of continuous satellite radar altimeter observations, we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 cubic kilometers per year for 1994-2003 to rapid loss of 310 ± 74 cubic kilometers per year for 2003-2012. West Antarctic losses increased by ~70% in the past decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades.

  19. Subglacial lake drainage detected beneath the Greenland ice sheet

    PubMed Central

    Palmer, Steven; McMillan, Malcolm; Morlighem, Mathieu

    2015-01-01

    The contribution of the Greenland ice sheet to sea-level rise has accelerated in recent decades. Subglacial lake drainage events can induce an ice sheet dynamic response—a process that has been observed in Antarctica, but not yet in Greenland, where the presence of subglacial lakes has only recently been discovered. Here we investigate the water flow paths from a subglacial lake, which drained beneath the Greenland ice sheet in 2011. Our observations suggest that the lake was fed by surface meltwater flowing down a nearby moulin, and that the draining water reached the ice margin via a subglacial tunnel. Interferometric synthetic aperture radar-derived measurements of ice surface motion acquired in 1995 suggest that a similar event may have occurred 16 years earlier, and we propose that, as the climate warms, increasing volumes of surface meltwater routed to the bed will cause such events to become more common in the future. PMID:26450175

  20. Subglacial lake drainage detected beneath the Greenland ice sheet.

    PubMed

    Palmer, Steven; McMillan, Malcolm; Morlighem, Mathieu

    2015-10-09

    The contribution of the Greenland ice sheet to sea-level rise has accelerated in recent decades. Subglacial lake drainage events can induce an ice sheet dynamic response--a process that has been observed in Antarctica, but not yet in Greenland, where the presence of subglacial lakes has only recently been discovered. Here we investigate the water flow paths from a subglacial lake, which drained beneath the Greenland ice sheet in 2011. Our observations suggest that the lake was fed by surface meltwater flowing down a nearby moulin, and that the draining water reached the ice margin via a subglacial tunnel. Interferometric synthetic aperture radar-derived measurements of ice surface motion acquired in 1995 suggest that a similar event may have occurred 16 years earlier, and we propose that, as the climate warms, increasing volumes of surface meltwater routed to the bed will cause such events to become more common in the future.

  1. Ice sheets viewed from the ocean: the contribution of marine science to understanding modern and past ice sheets.

    PubMed

    Ó Cofaigh, Colm

    2012-12-13

    Over the last two decades, marine science, aided by technological advances in sediment coring, geophysical imaging and remotely operated submersibles, has played a major role in the investigation of contemporary and former ice sheets. Notable advances have been achieved with respect to reconstructing the extent and flow dynamics of the large polar ice sheets and their mid-latitude counterparts during the Quaternary from marine geophysical and geological records of landforms and sediments on glacier-influenced continental margins. Investigations of the deep-sea ice-rafted debris record have demonstrated that catastrophic collapse of large (10(5)-10(6) km(2)) ice-sheet drainage basins occurred on millennial and shorter time scales and had a major influence on oceanography. In the last few years, increasing emphasis has been placed on understanding physical processes at the ice-ocean interface, particularly at the grounding line, and on determining how these processes affect ice-sheet stability. This remains a major challenge, however, owing to the logistical constraints imposed by working in ice-infested polar waters and ice-shelf cavities. Furthermore, despite advances in reconstructing the Quaternary history of mid- and high-latitude ice sheets, major unanswered questions remain regarding West Antarctic ice-sheet stability, and the long-term offshore history of the East Antarctic and Greenland ice sheets remains poorly constrained. While these are major research frontiers in glaciology, and ones in which marine science has a pivotal role to play, realizing such future advances will require an integrated collaborative approach between oceanographers, glaciologists, marine geologists and numerical modellers.

  2. Can ice sheets trigger abrupt climatic change?

    SciTech Connect

    Hughes, T.

    1996-11-01

    The discovery in recent years of abrupt climatic changes in climate proxy records from Greenland ice cores and North Atlantic sediment cores, and from other sites around the world, has diverted attention from gradual insolation changes caused by Earth`s orbital variations to more rapid processes on Earth`s surface as forcing Quaternary climatic change. In particular, forcing by ice sheets has been quantified for a major ice stream that drained the Laurentide Ice Sheet along Hudson Strait. The history of these recent discoveries leading to an interest in ice sheets is reviewed, and a case is made that ice sheets may drive abrupt climatic change that is virtually synchronous worldwide. Attention is focused on abrupt inception and termination of a Quaternary glaciation cycle, abrupt changes recorded as stadials and interstadials within the cycle, abrupt changes in ice streams that trigger stadials and interstadials, and abrupt changes in the Laurentide Ice Sheet linked to effectively simultaneous abrupt changes in its ice streams. Remaining work needed to quantify further these changes is discussed. 90 refs., 14 figs.

  3. Application of GRACE to the Evaluation of an Ice Flow Model of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Schlegel, N.; Wiese, D. N.; Watkins, M. M.; Larour, E. Y.; Box, J. E.; Fettweis, X.; van den Broeke, M. R.; Morlighem, M.; Boening, C.; Seroussi, H. L.

    2014-12-01

    Quantifying Greenland's future contribution to sea level rise is a challenging task and requires accurate estimates of ice flow sensitivity to climate change. Transient ice flow models are promising tools for estimating future ice sheet behavior. However, confidence in these types of future projections is low, especially because evaluation of model historical runs is so challenging due to the scarcity of continental-wide data for validation. For more than a decade, NASA's GRACE has continuously acquired time-variable measurements of the Earth's gravity field and has provided unprecedented surveillance of mass balance of the ice sheets, offering an opportunity for ice sheet model evaluation. Here, we take advantage of a new high-resolution (~300 km) monthly mascon solution for the purpose of mass balance comparison with an independent, historical ice flow model simulation using the Ice Sheet System Model (ISSM). The comparison highlights which regions of the ice sheet differ most from GRACE. Investigation of regional differences in trends and seasonal amplitudes between simulations forced with three different Regional Climate Model (RCM)-based estimates of surface mass balance (SMB) allows us to make conclusions about the relative contributions of various error sources in the model hindcast. This study constitutes the first regional comparison of GRACE data and an ice sheet model. Conclusions will aid in the improvement of RCM SMB estimates as well as ice sheet simulation estimates of present and future rates of sea level rise. 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 Program and President's and Director's Fund Program.

  4. Iceberg Armadas and Pliocene Antarctic Ice Sheet Retreat

    NASA Astrophysics Data System (ADS)

    Dolan, A. M.; Hill, D. J.; van de Flierdt, T.; Cook, C.; Haywood, A. M.

    2011-12-01

    Pliocene East Antarctic Ice Sheet. IRD data and iceberg modelling provides us with a unique window into the changes and oscillations that must have occurred during the Pliocene. While this approach does not give a complete picture of ice sheets during the Pliocene it can be used to evaluate existing reconstructions and provide important constraints on any future reconstructions.

  5. Greenland Ice Sheet retreat during the Eemian

    NASA Astrophysics Data System (ADS)

    van de Berg, W. J.; Helsen, M. M.; van de Wal, R. S. W.; van den Broeke, M. R.; Oerlemans, J.

    2012-04-01

    We present a new estimate of the evolution of the Greenland Ice Sheet through the Eemian (130 till 115 ky BP). This estimate is determined using the 3D 'shallow' ice sheet model ANICE and the regional climate model RACMO2/GR. The two models are time-slice coupled with an interval of 1500 years. 3D interpolated surface mass balance fields from RACMO2/GR force ANICE. Eemian and post-Eemian climate from the GCM ECHO-G drives RACMO2/GR on its lateral boundaries. These boundaries are gradually adjusted from maximum Eemian conditions to post-Eemian inception conditions, following the orbital parameters and Greenhouse gas concentrations derived from ice cores. The simulation shows a steady mass loss till the insolation conditions decline and the summer climate cools, with a typical rate of mass loss equivalent to 5 cm sea level rise per century for most of the time. Once summer start to cool the Greenland ice sheet recovers fast. The maximum ice loss is about 2 m eustatic sea level compared to present day volume and originates predominantly from southwest Greenland. Our results align with paleo-observations of Eemian ice sheet existence in South Greenland. Strong summer radiation also induces ice retreat in northern Greenland. Moreover, it agrees with preceding studies that the Greenland ice sheet had only a limited contribution to the Eemian sea level high stand. A finding of this novel approach is the impact of topographic pinpoints on the ice sheet evolution. Subglacial topography, like at 52° W 72° N (near Uummannaq), cause promontories in the ice sheet that enhances snowfall. Locations with high snowfall react less on warming than dry locations, because more melt is needed before all snow is removed, and the more efficient ice melt starts. The reduced ice depth also buttresses inland ice, limiting the ice sheet response to enhanced ablation. As a result, this topographical feature becomes the northern limit of significant ice sheet retreat, and shields the north

  6. Solar radiation management geoengineering and the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Applegate, P. J.; Keller, K.

    2014-12-01

    Several authors have suggested that technologies for modifying the Earth's climate should be developed so that they can be deployed if a climate emergency seems imminent. These technologies are generally called geoengineering, or climate engineering. Solar radiation management is perhaps the most commonly discussed geoengineering technique. It involves lofting reflective particles into the upper atmosphere, in imitation of explosive volcanic eruptions that produce measurable cooling at the Earth's surface. Given that geoengineering is intended to reduce surface air temperatures, some authors suggest that it could be used to prevent sea level rise from ice sheet mass loss. The Greenland Ice Sheet is an obvious target for geoengineering-based efforts to avoid sea level rise, because it is large and vulnerable to surface air temperature increases. To evaluate this possibility, we use a three-dimensional, shallow-ice sheet model (SICOPOLIS; sicopolis.greveweb.net) to examine the ability of geoengineering to reduce sea level contributions from the Greenland Ice Sheet. Although this model is highly simplified, its speed of execution allows us to investigate many different potential geoengineering scenarios, covering tens of thousands of model years (excluding spinup). We examine stylized geoengineering scenarios, including both aggressive, sustained geoengineering and more moderate efforts. We comment on the implications of our model results for the ability of geoengineering to reduce future ice sheet-driven sea level change, and how our results might change if our experiments were repeated with a more sophisticated ice sheet model.

  7. Climatic Drivers of Past Antarctic Ice Sheet Evolution Add Nonlinearly

    NASA Astrophysics Data System (ADS)

    Tigchelaar, M.; Timmermann, A.; Pollard, D.; Friedrich, T.; Heinemann, M.

    2015-12-01

    The Antarctic ice sheet has varied substantially in shape and volume in the past, with evidence for strong regional differences in evolution history. Recent observations of change in the Antarctic environment indicate that different regions respond differently to ongoing changes in global climate -- over the West Antarctic Ice Sheet strong increases in sub-shelf melt rates indicate a sensitivity to changes in ocean temperature and circulation, while in East Antarctica the mass balance is increasingly positive due to an increase in accumulation in response to rising temperatures. Modeling the long term evolution of the Antarctic ice sheet can help address questions about its regional sensitivity to external forcing. We have conducted experiments with an established ice sheet model over the last eight glacial cycles using spatially and temporally varying climate forcing from an EMIC. These simulations indicate a glacial-interglacial amplitude of ~11m SLE. Using a series of sensitivity experiments we address the dominant climatic forcing of this evolution. While sea level changes are the main driver of grounding line movement, they alone are not sufficient to explain the full glacial amplitude. Local insolation changes contribute to the initiation of terminations, while accumulation and sub-shelf melt changes feed back positively and negatively respectively onto the ice sheet evolution. This implies that climatic drivers add nonlinearly and the full spectrum of climate forcing needs to be considered when evaluating the sensitivity of the Antarctic ice sheet to past and future climate change.

  8. Microbial abundance in surface ice on the Greenland Ice Sheet

    PubMed Central

    Stibal, Marek; Gözdereliler, Erkin; Cameron, Karen A.; Box, Jason E.; Stevens, Ian T.; Gokul, Jarishma K.; Schostag, Morten; Zarsky, Jakub D.; Edwards, Arwyn; Irvine-Fynn, Tristram D. L.; Jacobsen, Carsten S.

    2015-01-01

    Measuring microbial abundance in glacier ice and identifying its controls is essential for a better understanding and quantification of biogeochemical processes in glacial ecosystems. However, cell enumeration of glacier ice samples is challenging due to typically low cell numbers and the presence of interfering mineral particles. We quantified for the first time the abundance of microbial cells in surface ice from geographically distinct sites on the Greenland Ice Sheet (GrIS), using three enumeration methods: epifluorescence microscopy (EFM), flow cytometry (FCM), and quantitative polymerase chain reaction (qPCR). In addition, we reviewed published data on microbial abundance in glacier ice and tested the three methods on artificial ice samples of realistic cell (102–107 cells ml−1) and mineral particle (0.1–100 mg ml−1) concentrations, simulating a range of glacial ice types, from clean subsurface ice to surface ice to sediment-laden basal ice. We then used multivariate statistical analysis to identify factors responsible for the variation in microbial abundance on the ice sheet. EFM gave the most accurate and reproducible results of the tested methodologies, and was therefore selected as the most suitable technique for cell enumeration of ice containing dust. Cell numbers in surface ice samples, determined by EFM, ranged from ~ 2 × 103 to ~ 2 × 106 cells ml−1 while dust concentrations ranged from 0.01 to 2 mg ml−1. The lowest abundances were found in ice sampled from the accumulation area of the ice sheet and in samples affected by fresh snow; these samples may be considered as a reference point of the cell abundance of precipitants that are deposited on the ice sheet surface. Dust content was the most significant variable to explain the variation in the abundance data, which suggests a direct association between deposited dust particles and cells and/or by their provision of limited nutrients to microbial communities on the GrIS. PMID:25852678

  9. Holocene deceleration of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    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.

  10. Models for polythermal ice sheets and glaciers

    NASA Astrophysics Data System (ADS)

    Hewitt, Ian J.; Schoof, Christian

    2017-02-01

    Polythermal ice sheets and glaciers contain both cold ice and temperate ice. We present two new models to describe the temperature and water content of such ice masses, accounting for the possibility of gravity- and pressure-driven water drainage according to Darcy's law. Both models are based on the principle of energy conservation; one additionally invokes the theory of viscous compaction to calculate pore water pressure, and the other involves a modification of existing enthalpy gradient methods to include gravity-driven drainage. The models self-consistently predict the evolution of temperature in cold ice and of water content in temperate ice. Numerical solutions are described, and a number of illustrative test problems are presented, allowing comparison with existing methods. The suggested models are simple enough to be incorporated in existing ice-sheet models with little modification.

  11. Controls on interior West Antarctic Ice Sheet Elevations: inferences from geologic constraints and ice sheet modeling

    NASA Astrophysics Data System (ADS)

    Ackert, Robert P.; Putnam, Aaron E.; Mukhopadhyay, Sujoy; Pollard, David; DeConto, Robert M.; Kurz, Mark D.; Borns, Harold W.

    2013-04-01

    Knowledge of the West Antarctic Ice Sheet (WAIS) response to past sea level and climate forcing is necessary to predict its response to warmer temperatures in the future. The timing and extent of past interior WAIS elevation changes provides insight to WAIS behavior and constraints for ice sheet models. Constraints prior to the Last Glacial Maximum (LGM) however, are rare. Surface exposure ages of glacial erratics near the WAIS divide at Mt. Waesche in Marie Byrd Land, and at the Ohio Range in the Transantarctic Mountains, range from ∼10 ka to >500 ka without a dependence on elevation. The probability distribution functions (PDF) of the exposure ages at both locations, are remarkably similar. During the last glaciation, maximum interior ice elevations as recorded by moraines and erratics were reached between 10 ka and 12 ka. However, most exposure ages are older than the LGM and cluster around ∼40 ka and ∼80 ka. The peak in the exposure age distributions at ∼40 ka includes ages of alpine moraine boulders at Mercer Ridge in the Ohio Range. Comparison of the PDF of exposures ages from the Ohio Range and Mt. Waesche with the temperature record from the Fuji Dome ice core indicates that the youngest peak in the exposure age distributions corresponds to the abrupt warming during the Last Glacial termination. A prominent peak in the Ohio Range PDF corresponds to the penultimate termination (stage 5e). During the intervening glacial period, there is not a consistent relationship between the peaks in the PDF at each location and temperature. A combined ice sheet/ice shelf model with forcing scaled to marine δ18O predicts that interior WAIS elevations near the ice divide have varied ∼300 m over the Last Glacial cycle. Peaks in the PDF correspond to model highstands over the last 200 ka. In the simulated elevation history, maximum ice elevations at Ohio Range (+100 m) and Mt. Waesche (+60 m) occur at ∼10 ka, in agreement with observations from these sites

  12. Modelling water isotopes in polar ice sheets

    NASA Astrophysics Data System (ADS)

    Lhomme, Nicolas

    2005-07-01

    Concentrations of water isotopes in marine sediments and ice cores are a key indicator for estimating global and regional fluctuations of past temperatures. Interpreting these concentrations requires an understanding of the storage capacity and exchanges among the ocean, atmosphere and cryosphere as well as an understanding of the dynamical behaviour of these reservoirs. The contribution of the latter remains poorly established because of the paucity of deep ice cores in Greenland and Antarctica and the difficulty of interpreting these cores. To obtain the water isotope composition of polar ice sheets and gain an understanding of their stratigraphy, I develop a tracer transport method first proposed by Clarke and Marshall (2002) and significantly improve it by introducing an interpolation technique that accounts for the particular age-depth relationship of ice sheets. I combine the tracers with numerical models of ice dynamics to predict the fine layering of polar ice masses such that it is locally validated at ice core sites, hence setting a new method to constrain reconstructions of ice sheets' climatic and dynamic histories. This framework is first applied and tested with the UBC Ice Sheet Model of Marshall and Clarke (1997). I predict the three-dimensional time-evolving stratigraphy of the Greenland Ice Sheet and use the ice core records predicted at GRIP, Dye 3 and Camp Century to better determine the minimal ice extent during the Eemian, 127 kyr ago, when the Earth's climate was somewhat similar to the present. I suggest that 3.5--4.5 m of sea level rise could be attributed to melting in Greenland. Tracers are also applied to Antarctica with the LGGE Ice Sheet Model of Ritz et al. (2001). The three-dimensional model is compared to simple flow models at the deep ice core sites of Dome C, Vostok and Dome Fuji to test the hypotheses on depositional and dynamical conditions used for interpreting ice cores. These studies lead to a well-constrained stratigraphic

  13. Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, Bas; Dolan, Aisling; Bernales, Jorge; Gasson, Edward; Goelzer, Heiko; Golledge, Nick; Sutter, Johannes; Huybrechts, Phillipe; Lohmann, Gerrit; Rogozhina, Irina; Abe-Ouchi, Ayako; Saito, Fuyuki; van de Wal, Roderik

    2015-04-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of four sensitivity experiments. Ice-sheet model forcing fields are taken from the HadCM3 atmosphere-ocean climate model runs for the pre-industrial and the Pliocene. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, although the models are setup with their own parameter settings. For the Pliocene simulations using the Bedmap1 bedrock topography, some models show a small retreat of the East Antarctic ice sheet, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. This can be ascribed to either the surface mass balance, as the HadCM3 Pliocene climate shows a significant increase over the Wilkes and Aurora basin, or the initial bedrock topography. For the latter, our simulations with the recently

  14. Experimental design for three interrelated Marine Ice-Sheet and Ocean Model Intercomparison Projects

    NASA Astrophysics Data System (ADS)

    Asay-Davis, X. S.; Cornford, S. L.; Durand, G.; Galton-Fenzi, B. K.; Gladstone, R. M.; Gudmundsson, G. H.; Hattermann, T.; Holland, D. M.; Holland, D.; Holland, P. R.; Martin, D. F.; Mathiot, P.; Pattyn, F.; Seroussi, H.

    2015-11-01

    Coupled ice sheet-ocean models capable of simulating moving grounding lines are just becoming available. Such models have a broad range of potential applications in studying the dynamics of marine ice sheets and tidewater glaciers, from process studies to future projections of ice mass loss and sea level rise. The Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP) is a community effort aimed at designing and coordinating a series of model intercomparison projects (MIPs) for model evaluation in idealized setups, model verification based on observations, and future projections for key regions in the West Antarctic Ice Sheet (WAIS). Here we describe computational experiments constituting three interrelated MIPs for marine ice sheet models and regional ocean circulation models incorporating ice shelf cavities. These consist of ice sheet experiments under the Marine Ice Sheet MIP third phase (MISMIP+), ocean experiments under the ice shelf-ocean MIP second phase (ISOMIP+) and coupled ice sheet-ocean experiments under the MISOMIP first phase (MISOMIP1). All three MIPs use a shared domain with idealized bedrock topography and forcing, allowing the coupled simulations (MISOMIP1) to be compared directly to the individual component simulations (MISMIP+ and ISOMIP+). The experiments, which have qualitative similarities to Pine Island Glacier Ice Shelf and the adjacent region of the Amundsen Sea, are designed to explore the effects of changes in ocean conditions, specifically the temperature at depth, on basal melting and ice dynamics. In future work, differences between model results will form the basis for evaluation of the participating models.

  15. 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

  16. Pliocene retreat of Greenland and Antarctic Ice Sheet margins (Invited)

    NASA Astrophysics Data System (ADS)

    Deconto, R. M.; Pollard, D.

    2013-12-01

    The middle Pliocene epoch (~3 million years ago) is often considered an analogue for future global climatic conditions, because global mean temperatures were comparable to projections of future climate at the end of this century. Importantly, some estimates of mid-Pliocene sea level are >20 m higher than today, implying the potential for significant retreat of the East Antarctic Ice Sheet (EAIS), in addition to the loss of the Greenland and West Antarctic Ice Sheets (WAIS). Here, we use a hybrid ice sheet-shelf model with freely migrating grounding lines coupled to a high-resolution regional climate model to test the potential for both West and East Antarctic Ice Sheet retreat during the warm Pliocene and in long-term future scenarios with elevated CO2. In these simulations we apply new treatments of i) ice shelf calving (accounting for the effects of divergent ice flow and surface melt water on crevassing), ii) ice-cliff mechanics at the grounding line, iii) improved sub-glacial bathymetry using BEDMAP2, and iv) a range of plausible ocean warming scenarios based on offline ocean modeling. In warm Pliocene simulations, the combination of improved bathymetric detail and more physically based model treatments of floating and grounded calving fronts substantially increases the rates and magnitudes of ice sheet retreat into over-deepened subglacial basins in both in West and East Antarctica. These new results imply the EAIS margin did indeed contribute to elevated (and orbitally paced) Pliocene sea levels, with Antarctica contributing up to ~20m equivalent sea level during the warmest intervals. In long-term (10^3-4-yr) future simulations using the same model physics, we find these new mechanisms produce a much more sensitive and vulnerable ice sheet than previously considered, with the potential for substantial future retreat of both WAIS and parts of the East Antarctic margin in response to the combined effects of increased surface melt on ice shelf surfaces and

  17. 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.

  18. Conditions for bubble elongation in cold ice-sheet ice

    USGS Publications Warehouse

    Alley, R.B.; Fitzpatrick, J.J.

    1999-01-01

    Highly elongated bubbles are sometimes observed in ice-sheet ice. Elongation is favored by rapid ice deformation, and opposed by diffusive processes. We use simple models to show that vapor transport dominates diffusion except possibly very close to the melting point, and that latent-heat effects are insignificant. Elongation is favored by larger bubbles at pore close-off, but is nearly independent of bubble compression below close-off. The simple presence of highly elongated bubbles indicates only that a critical ice-strain rate has been exceeded for significant time, and provides no information on possible disruption of stratigraphic continuity by ice deformation.

  19. The contribution to future flood risk in the Severn Estuary from extreme sea level rise due to ice sheet mass loss

    NASA Astrophysics Data System (ADS)

    Quinn, N.; Bates, P. D.; Siddall, M.

    2013-12-01

    The rate at which sea levels will rise in the coming century is of great interest to decision makers tasked with developing mitigation policies to cope with the risk of coastal inundation. Accurate estimates of future sea levels are vital in the provision of effective policy. Recent reports from UK Climate Impacts Programme (UKCIP) suggest that mean sea levels in the UK may rise by as much as 80 cm by 2100; however, a great deal of uncertainty surrounds model predictions, particularly the contribution from ice sheets responding to climatic warming. For this reason, the application of semi-empirical modelling approaches for sea level rise predictions has increased of late, the results from which suggest that the rate of sea level rise may be greater than previously thought, exceeding 1 m by 2100. Furthermore, studies in the Red Sea indicate that rapid sea level rise beyond 1m per century has occurred in the past. In light of such research, the latest UKCIP assessment has included a H++ scenario for sea level rise in the UK of up to 1.9 m which is defined as improbable but, crucially, physically plausible. The significance of such low-probability sea level rise scenarios upon the estimation of future flood risk is assessed using the Somerset levels (UK) as a case study. A simple asymmetric probability distribution is constructed to include sea level rise scenarios of up to 1.9 m by 2100 which are added to a current 1:200 year event water level to force a two-dimensional hydrodynamic model of coastal inundation. From the resulting ensemble predictions an estimation of risk by 2100 is established. The results indicate that although the likelihood of extreme sea level rise due to rapid ice sheet mass loss is low, the resulting hazard can be large, resulting in a significant (27%) increase to the projected annual risk. Furthermore, current defence construction guidelines for the coming century in the UK are expected to account for 95% of the sea level rise distribution

  20. Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, B.; Dolan, A. M.; Bernales, J.; Gasson, E.; Goelzer, H.; Golledge, N. R.; Sutter, J.; Huybrechts, P.; Lohmann, G.; Rogozhina, I.; Abe-Ouchi, A.; Saito, F.; van de Wal, R. S. W.

    2014-11-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of four sensitivity experiments. Ice-sheet model forcing fields are taken from the HadCM3 atmosphere-ocean climate model runs for the pre-industrial and the Pliocene. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, although the models are setup with their own parameter settings. For the Pliocene simulations using the Bedmap1 bedrock topography, some models show a small retreat of the East Antarctic ice sheet, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. This can be ascribed to either the surface mass balance, as the HadCM3 Pliocene climate shows a significant increase over the Wilkes and Aurora basin, or the initial bedrock topography. For the latter, our simulations with the recently

  1. Greenland Ice Sheet Today: A daily look at surface melt of the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Leslie, S. R.; Gergely, K. L.; Beitler, J.; Scambos, T. A.; Stroeve, J. C.

    2013-12-01

    An increase in the surface melt of the Greenland ice sheet in recent decades signals the waning of the ice sheet in a changing climate. The unprecedented intense surface melt of the ice sheet in 2012 prompted NASA and NSIDC to launch Greenland Ice Sheet Today, a Web site that offers daily updated satellite data and periodic scientific analysis on surface melting of the Greenland ice sheet. Near-real-time melt data are derived from an algorithm that estimates melt and is applied to DMSP SSMIS brightness temperatures gridded to a 25km EASE-Grid. These data are then used to generate a daily melt image, a cumulative melt days image, and a daily melt graph. Contextual background information on ice sheets as well as scientific discussions about the status of the Greenland ice sheet are posted periodically. Greenland Ice Sheet Today serves to keep a wide range of user communities informed about a crucial part of the Earth's cryosphere and here we examine the development of and reactions to the Web site.

  2. Ice Velocity Measurements From The First Sentinel-1a Full Antarctic Ice Sheet Campaign

    NASA Astrophysics Data System (ADS)

    Hogg, A. E.; Shepherd, A.; Gourmelen, N.; Nagler, T.

    2015-12-01

    Island Glacier in West Antarctica InSAR is less successful. We conclude that the short 12-day repeat period and large scale coverage of Sentinel-1a is an invaluable new resource, which in the future will be a critical tool for monitoring ice velocity change and dynamic instability of the Antarctic ice sheet.

  3. 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.

  4. Assessing the predictability of a coupled climate-ice sheet model system for the response of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Adalgeirsdottir, G.; Stendel, M.; Bueler, E.; Christensen, J. H.; Drews, M.; Mottram, R.

    2009-04-01

    The wild card for reliable sea level rise prediction is the contribution of the Greenland Ice Sheet. There is an urgent need to determine the predictability of models that simulate the response of Greenland Ice Sheet to rising temperatures and the amount of freshwater flux that can be expected into the ocean. Modelling efforts have been limited by poorly known boundary and initial conditions, low resolution and lack of presentation of fast flowing ice streams. We address these limitations by building a model system consisting of a high resolution regional climate model (HIRHAM4), that has been run for the period 1950-2080 at 25 km, and Parallel Ice Sheet Model (PISM), which simulates spatially and temporally varying ice streams by combining the solutions of the Shallow Shelf and Shallow Ice Approximations. The surface mass balance is simulated with a positive-degree-day method. The important and poorly constrained model component is the past climate forcing, which serves the purpose of initializing the model by simulating the present ice sheet and observed rate of mass changes. Simulated gradients of mass loss due to warming trends of past decade and prediction for the future are presented as well as estimated sensitivities due to the various model component uncertainties.

  5. 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.

  6. 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.

  7. Ice Thicknesses and Driving Stresses of Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Zwally, H. J.; Saba, J.; Giovinetto, M.

    1999-01-01

    Surface elevations from satellite radar altimetry (Geosat, Seasat, and ERS-1) and bedrock topography from airborne radar sounding (Simon Ekholm's Danish compilation) are combined to derive maps of the driving stresses in the Greenland ice sheet. The stress vector, tau = rho g h sin(alpha), is calculated using surface slope vectors, alpha, from surface elevations and ice thicknesses, h, from the difference between surface and basal elevations. Since the 5-km scale of the surface slope is only about 2 times the ice thickness, the stress maps show spatial variations indicative of longitudinal stress gradients associated with topographic undulations. Values of alpha generally vary from near zero at the ice divides to maxima values around 120 kpa, returning to near zero in a narrow band at the edges. The distribution of alpha's peaks at 60 kpa with an approximate sigma of +/- 20 kpa. Areas of very low alpha near the origin of the northeast ice stream may indicate small sub-glacial lakes. The profile of alpha, down the ice stream from near the ice divide, increases to a maximum of about 120 kpa near the margin, which is characteristic of East Antarctic outlet glaciers and in contrast to West Antarctic ice streams where alpha has maximum values 400 to 500 km inland from the grounding lines. Overall distributions of alpha values are compared with those for the Antarctic ice sheet and the Mars Northern ice cap.

  8. 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

  9. Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, Bas; Dolan, Aisling M.; Hill, Daniel J.; van de Wal, Roderik S. W.

    2014-05-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene Warm Period (also known as the PRISM interval: 3.29 to 2.97 million years before present) can serve as a potential analogue for projected future climates, with a global annual mean surface-air temperature warming of 1.76 °C. Although Pliocene ice locations and surface extents are still poorly constrained, a significant contribution to sea-level rise should be expected from Greenland and West and, possibly, East Antarctica based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of the ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of sensitivity experiments. Ice-sheet model forcing fields are taken from the PlioMIP results incorporating multiple coupled atmosphere-ocean general circulation models (GCM). We show that ice-sheet models simulate a present-day ice sheet which is comparable to the observations, and find no systematic biases introduced when using different GCM forcing relative to observational climate forcing. This project includes multiple ice-sheet models forced with multiple climate model output, from which a comprehensive assessment can be made as to the uncertainties of ice-sheet extent on Antarctica. These results may eventually serve as a new constraint on the extent of the Antarctic ice sheet during the Late-Pliocene Warm Period

  10. 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

  11. Using the level set method to track ice sheet boundaries

    NASA Astrophysics Data System (ADS)

    Lindsey, D. S.; Dupont, T. K.

    2009-12-01

    Simulating ice-sheet volume changes requires tracking the interface of ice and its surrounding media, e.g. water, air, and sediment or rock. This can be challenging when using a fixed, or Eulerian, grid and allowing the interface to move via kinematic boundary conditions. For example, the interface may fall between grid points at a given point in time, making the application of boundary conditions less than straightforward. The level set method of Osher and Sethian (1988) offers an alternative approach, wherein a continuous level set function evolves within the domain via the combined kinematics of ice and its encompassing materials. The methods true strength lies in tracking the interface of two materials through time. Pralong and Funk (2004) applied this method to the movement of a glacier’s ice/air interface, offering a glimpse of the potential of this method for glaciology. Here we perform a simple preliminary test of the method for a two-dimensional (flow-line) model of an ice shelf, comparing the results to analytic approximations of the movement of both the ice/air interface and the ice front. Future experiments will incorporate grounded ice and include basal and lateral-shear stresses. The ultimate goal of this work is provide a practical approach for two and three-dimensional ice-sheet models to naturally track their moving boundaries.

  12. Measuring Ice Sheet Height with ICESat-2

    NASA Astrophysics Data System (ADS)

    Walsh, K.; Smith, B.; Neumann, T.; Hancock, D.

    2015-12-01

    ICESat-2 is NASA's next-generation laser altimeter, designed to measure changes in ice sheet height and sea ice freeboard. Over the ice sheets, it will use a continuous repeat-track pointing strategy to ensure that it accurately measures elevation changes along a set of reference tracks. Over most of the area of Earth's ice sheets, ICESat-2 will provide coverage with a track-to-track spacing better than ~3 km. The onboard ATLAS instrument will use a photon-counting approach to provide a global geolocated photon point cloud, which is then converted into surface-specific elevation data sets. In this presentation, we will outline our strategy for taking the low-level photon point cloud and turning it into measurements posted at 20 m along-track for a set of pre-defined reference points by (1) selecting groups of photon events (PEs) around each along-track point, (2) refining the initial PE selection by fitting selected PEs with an along-track segment model and eliminating outliers to the model, (3) applying histogram-based corrections to the surface height based on the residuals to the along-track segment model, (4) calculate error estimates based on estimates of relative contributions of signal and noise PEs to the observed PE count, and (5) determining the final location and surface height of the along-track segment. These measurements are then corrected for short-scale (100-200 m) across-track surface topography around the reference points to develop a time series of land ice heights. The resulting data products will allow us to measure ice sheet elevation change with a point-for-point accuracy of a few centimeters over Earth's ice sheets.

  13. 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.

  14. Climate Model Dependency and Understanding the Antarctic Ice Sheet during the Warm Late Pliocene

    NASA Astrophysics Data System (ADS)

    Dolan, Aisling; de Boer, Bas; Bernales, Jorge; Hunter, Stephen; Haywood, Alan

    2016-04-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals of Earth history is fundamentally important. A warm period in the Late Pliocene (3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions and geological evidence. Following a five year international project PLISMIP (Pliocene Ice Sheet Modeling Intercomparison Project) we present the final set of results which quantify uncertainty in climate model-based predictions of the Antarctic ice sheet. In this study we use an ensemble of climate model forcings within a multi-ice sheet model framework to assess the climate (model) dependency of large scale features of the Antarctic ice sheet. Seven coupled atmosphere-ocean climate models are used to derive surface temperature, precipitation and oceanic forcing that drive three ice sheet models (over the grounded and floating domain). Similar to results presented over Greenland, we show that the reconstruction of the Antarctic ice sheet is sensitive to which climate model is used to provide the forcing field. Key areas of uncertainty include West Antarctica, the large subglacial basins of East Antarctica and the overall thickness of the continental interior of East Antarctica. We relate the results back to geological proxy data, such as those relating to exposure rates which provide information on potential ice sheet thickness. Finally we discuss as to whether the choice of modelling framework (i.e. climate model and ice sheet model used) or the choice of boundary conditions causes the greatest uncertainty in ice sheet reconstructions of the warm Pliocene.

  15. Bedforms of the Keewatin Ice Sheet, Canada

    NASA Astrophysics Data System (ADS)

    Aylsworth, J. M.; Shilts, W. W.

    1989-05-01

    By compiling glacial bedforms on a map that covers most of one sector of the Laurentide Ice Sheet, it is possible to make some suggestions about their genesis based largely on spatial relationships. It can be concluded that drumlins and ribbed moraine form at the base of actively flowing ice under similar dynamic conditions. For either landform to exist, however, there must have been enough sediment available in the base of the glacier to leave or form a feature large enough to be recognizable. The presence or absence of sufficient load is related to the geology of the glacier bed and has little to do with regionally changing dynamics of the ice-water system. Likewise, given sufficient load, it is evident that whether drumlins formed or whether ribbed moraine formed in a certain area is a function of the physical nature of the load which is, again, related to geology of the source outcrops. Whether the physical characteristics come into play after the sediment has been released from the ice and is being reshaped by basal drag, streamlining, etc., or whether the nature of the load while entrained changes the behaviour of the basal part of the ice is unclear. Physical characteristics of the basal sediment load have apparently promoted internal thrusting of coherent slabs of entrained debris and ice to form ribbed moraine on melting, whereas drumlins may reflect moulding of plastic subglacial debris or erosional streamlining of both the unconsolidated glacial substrate and bedrock. The observation that many eskers cross drumlin fields at nearly right angles to their orientation suggests that conditions producing streamlining and those pertaining to subglacial drainage are separated in time and circumstance. The general occurrence of drumlins and eskers throughout the sediment-rich portions of the Keewatin Ice Sheet, from Zone 1 to its edge, is difficult to reconcile with the restriction and intimate association of these forms with ribbed moraine almost exclusively in

  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. Preservation of landforms under ice sheets and ice caps

    NASA Astrophysics Data System (ADS)

    Kleman, Johan

    1994-02-01

    This article addresses the question of whether or not distinct glacial and non-glacial landforms can survive beneath ice sheets and ice caps with little or no morphological alteration. A review of recent work documents the existence of pre-last stadial landforms and landscapes in areas covered by the Fennoscandian and Laurentide ice sheets. A substantial number of independent works indicate that landforms such as eskers, drainage channels and boulder fields have escaped destruction despite complete ice overriding during several tens of millenia. Full preservation of former ground surfaces or delicate landforms probably is linked to areas where the ice-sheet base was continuously frozen to its bed. Larger "robust" landforms, such as large drumlins, appear to have been preserved even under wet-based conditions. In glaciated areas, patches preserved under dry (cold)-based conditions provide important windows towards the past, showing landscapes that were destroyed in surrounding areas affected by wet-based and eroding ice. Some consequences for the research fields of non-glacial geomorphology, archaeology and botany include the possibility of subglacial museums and refugia. A time/ space model describes geomorphological access to information from older events in glaciated areas.

  18. Satellite remote sensing for ice sheet research

    NASA Technical Reports Server (NTRS)

    Thomas, R. H.; omplexity of the land cover and land use p; omplexity of the land cover and land use p

    1985-01-01

    Potential research applications of satellite data over the terrestrial ice sheets of Greenland and Antarctica are assessed and actions required to ensure acquisition of relevant data and appropriate processing to a form suitable for research purposes are recommended. Relevant data include high-resolution visible and SAR imagery, infrared, passive-microwave and scatterometer measurements, and surface topography information from laser and radar altimeters.

  19. Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development

    NASA Astrophysics Data System (ADS)

    Kirchner, N.; Ahlkrona, J.; Gowan, E. J.; Lötstedt, P.; Lea, J. M.; Noormets, R.; von Sydow, L.; Dowdeswell, J. A.; Benham, T.

    2016-09-01

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice

  20. 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.

  1. Moulin distribution and formation on the southwest Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Chu, V. W.; Smith, L. C.; Gleason, C. J.; Yang, K.; Poinar, K.; Joughin, I.; Pitcher, L. H.

    2015-12-01

    River moulins represent a significant connection between surface meltwater generated on the Greenland ice sheet and subglacial drainage networks, where increased meltwater can enhance ice sliding dynamics. In this study, a new high-resolution moulin map is created from WorldView-1/2 imagery acquired during the 2012 record melt year for a 12,500 km2 area near Russell Glacier in southwest Greenland. A total of 1,236 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. We find the presence of moulins up to 1787 m elevation, with 11% of moulins found above 1600 m elevation: higher than previously mapped moulins and where glaciological theory suggests few moulins should form. Our study observes moulins in both extensional and compressional ice flow regimes (28% of moulins are found in areas of high extensional strain rate >0.005 yr-1), suggesting that strain rates are not a strong indicator of the likelihood for moulin formation. Overall, moulin density tends to increase with higher bed elevation, thinner ice, lower surface slope, higher velocity, and higher strain rate. In sum, moulins are most common in crevassed, thinner ice near the ice sheet edge, but significant quantities also develop at high elevations. This indicates that future inland expansion of melting may create hydrologic connections between the surface and the bed at higher elevations than previously thought.

  2. 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.; Price, S. F.; Seroussi, H.

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-12-01

    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-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.

  4. Ice sheets promote speciation in boreal birds.

    PubMed

    Weir, Jason T; Schluter, Dolph

    2004-09-22

    The premise that Pleistocene ice ages played an important role in generating present-day species diversity has been challenged by genetic data indicating that most of the youngest terrestrial species on Earth coalesced long before major glacial advances. However, study has been biased towards faunas distributed at low latitudes that were not directly fragmented by advancing ice sheets. Using mitochondrial sequence divergence and a molecular clock, we compared the coalescence times of pairs of avian species belonging to superspecies complexes from the high-latitude boreal forest with those of sub-boreal and tropical avifaunas of the New World. Remarkably, all coalescence events in boreal superspecies date to the Pleistocene, providing direct evidence that speciation was commonly initiated during recent glacial periods. A pattern of endemism in boreal superspecies plausibly links the timing of divergence to the fragmentation of the boreal forest by ice sheets during the Mid- and Late Pleistocene. In contrast to the boreal superspecies, only 56% of sub-boreal and 46% of tropical superspecies members coalesced during the Pleistocene, suggesting that avifaunas directly fragmented by ice sheets experienced rapid rates of diversification, whereas those distributed farther south were affected to a lesser extent. One explanation for the absence of pre-Pleistocene superspecies in boreal avifaunas is that strong selection pressures operated in boreal refugia, causing superspecies members to achieve ecological differentiation at an accelerated rate.

  5. Ice sheets promote speciation in boreal birds.

    PubMed Central

    Weir, Jason T.; Schluter, Dolph

    2004-01-01

    The premise that Pleistocene ice ages played an important role in generating present-day species diversity has been challenged by genetic data indicating that most of the youngest terrestrial species on Earth coalesced long before major glacial advances. However, study has been biased towards faunas distributed at low latitudes that were not directly fragmented by advancing ice sheets. Using mitochondrial sequence divergence and a molecular clock, we compared the coalescence times of pairs of avian species belonging to superspecies complexes from the high-latitude boreal forest with those of sub-boreal and tropical avifaunas of the New World. Remarkably, all coalescence events in boreal superspecies date to the Pleistocene, providing direct evidence that speciation was commonly initiated during recent glacial periods. A pattern of endemism in boreal superspecies plausibly links the timing of divergence to the fragmentation of the boreal forest by ice sheets during the Mid- and Late Pleistocene. In contrast to the boreal superspecies, only 56% of sub-boreal and 46% of tropical superspecies members coalesced during the Pleistocene, suggesting that avifaunas directly fragmented by ice sheets experienced rapid rates of diversification, whereas those distributed farther south were affected to a lesser extent. One explanation for the absence of pre-Pleistocene superspecies in boreal avifaunas is that strong selection pressures operated in boreal refugia, causing superspecies members to achieve ecological differentiation at an accelerated rate. PMID:15347509

  6. Ice Sheet Monitoring Using Latest Generation SAR Satellites

    NASA Astrophysics Data System (ADS)

    Scheuchl, B.; Mouginot, J.; Rignot, E. J.; Li, X.

    2015-12-01

    Remote sensing is a crucial component to gain insight in the worlds ice sheets and glaciers. Spaceborne Synthetic Aperture Radar data have proven to be a key resource to monitor the great ice sheets in Antarctica and Greenland. International efforts undertaken during the last International Polar Year resulted in the collection of vast amounts of data to generate the first continent-wide ice velocity map of Antarctica, a series of full velocity maps of Greenland, and time series data in key regions. The Antarctic grounding line was also mapped at unprecedented accuracy using InSAR. The end of several SAR missions since 2010 has posed a significant challenge in the effort to provide ongoing data acquisitions. New generation missions show potential to not only fill the data gap, but to make the collection of ice sheet data part of the ongoing acquisition scenarios, therefore ensuring data continuity. New modes, like the TOPSAR mode used for Sentinel-1A, provide new opportunities but also pose processing challenges, particularly if the entire area monitored is in motion. Several future missions are in various stages of development, thus further adding to the suite of sensors potentially available to collect data in Polar Regions going forward. The NASA-ISRO L-and S-band mission, planned for launch in 2020, will be a pure science mission with an open data policy, thus again changing the data availability and data access situation for the better. In international collaboration through the Polar Space Task Group, space agencies coordinate their science acquisitions in Polar Regions. With broad input from the larger ice sheet science community, we have worked closely with space agencies to define science requirements and to develop acquisition scenarios that maximize science value for ice sheets. Here we highlight the collaboration effort, summarize the input of the ice sheet science community to the Polar Space Task Group, and present the acquisition plans that resulted

  7. Response of a marine ice sheet to changes at the grounding line

    SciTech Connect

    Van der Veen, C.J.

    1985-01-01

    A numerical model was designed to study the stability of a marine ice sheet, and used to do some basic experiments. The ice-shelf/ice-sheet interaction enters through the flow law in which the longitudinal stress is also taken into account. Instead of applying the model to some (measured) profile and showing that this is unstable (as is common practice in other studies), an attempt is made to simulate a whole cycle of growth and retreat of a marine ice sheet, although none of the model sheets is particularly sensitive to changes in environmental conditions. The question as to what might happen to the West Antarctic Ice Sheet in the near future when a climatic warming can be expected as a result of the CO/sub 2/ effect, seems to be open for discussion again. From the results presented in this paper one can infer that a collapse, caused by increased melting on the ice shelves, is not very likely.

  8. 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.

  9. Geophysical constraints on the dynamics and retreat of the Barents Sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation

    NASA Astrophysics Data System (ADS)

    Patton, Henry; Andreassen, Karin; Bjarnadóttir, Lilja R.; Dowdeswell, Julian A.; Winsborrow, Monica C. M.; Noormets, Riko; Polyak, Leonid; Auriac, Amandine; Hubbard, Alun

    2015-12-01

    Our understanding of processes relating to the retreat of marine-based ice sheets, such as the West Antarctic Ice Sheet and tidewater-terminating glaciers in Greenland today, is still limited. In particular, the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales. Over numerous glaciations during the Quaternary, a marine-based ice sheet has waxed and waned over the Barents Sea continental shelf, characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming. Increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this Barents Sea ice sheet (BSIS), particularly so from its Late Weichselian maximum extent. We present a review of existing geophysical constraints that detail the dynamic evolution of the BSIS through the last glacial cycle, providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior. Although constraining data are generally spatially sporadic across the Barents and Kara Seas, behaviors such as ice sheet thinning, major ice divide migration, asynchronous and rapid flow switching, and ice stream collapses are all evident. Further investigation into the drivers and mechanisms of such dynamics within this unique paleo-analogue is seen as a key priority for advancing our understanding of marine-based ice sheet deglaciations, both in the deep past and in the short-term future.

  10. 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.

  11. Ice stream reorganization and ice sheet mass balance following the reactivation of Kamb Ice Stream, West Antarctica

    NASA Astrophysics Data System (ADS)

    Bougamont, Marion; Christoffersen, Poul; Price, Stephen; Carter, Sasha

    2015-04-01

    Ice streams in Antarctica account for most of the ice volume discharged to the ocean, and their flow variability greatly influences the mass balance of the ice sheet. Today, the Siple Coast region of West Antarctica is the only one to experience a positive mass balance (~36Gt/yr), as a consequence of the stagnation of Kamb Ice Stream about 170 years ago and the ongoing slowdown of Whillans Ice Stream. However, this positive trend could be temporary; past studies have shown that both ice streams experienced significant flow variability over the past millennia, with stagnation typically followed by reactivation on centennial timescales, occurring in response to internal processes. The impact this variability may have on the future mass balance of the WAIS remains unknown. Here, we explore the future flow variability of the Siple Coast ice streams by using a three-dimensional higher-order ice sheet model (CISM), coupled to a physically-based basal processes model and a model of regional hydrology. To obtain realistic initial flow conditions, we assimilate available velocity data for this region from 1997. We perform forward simulations over a 200 year period, during which the basal properties evolve according to the distribution of meltwater beneath the ice and its drainage/flow through a subglacial till layer. First, we assume that the bed evolves according to ice-till interactions with only local exchange of water between the ice and till. Next, we include a model of the regional basal water system capable of transporting water over long distances, so that meltwater is routed laterally along the bed before interacting with the till layer. We also explore the effect of geothermal heat flux uncertainties. We find that ice discharge to the grounding line is larger and more sustained in time when the regional water system is included in the simulations. Still, in all experiments, the main future perturbation to the current state of flow follows from the reactivation of

  12. Uncertainty in Ice Crystal Orientation Distributions in Ice Sheets

    NASA Astrophysics Data System (ADS)

    Hay, Michael; Waddington, Edwin

    2016-04-01

    Crystal-orientation fabrics in polar ice sheets have a strong influence on ice flow due to the plastic anisotropy of ice. Crystal orientations evolve primarily in response to applied strain, but are also affected by temperature, impurities, interactions with neighbors, and other factors. While the evolution of each ice crystal is physically deterministic, in limited samples, such as those from ice-core thin sections, measured samples are stochastic due to sampling error. Even in continuum representations from models, crystal orientation distribution functions (ODFs) can be treated as stochastic due to uncertainties in how they developed. Here, we present results on the statistics of crystal orientation fabrics. We show a first-order estimate of the sampling distribution of fabric eigenvalues and fabric eigenvectors from ice-core thin sections. We also analyze uncertainty in electron backscatter diffraction measurements. In addition to sampling error, the strain histories of fabrics are generally poorly constrained, and may have varied in unknown ways through time. Nearby layers in ice sheets can also experience different strain histories due to inherent variabilities such as transient flow, or differences in impurities. This means that the continuum ODF itself can be treated as stochastic, because it depends on an effectively-stochastic unknown strain-history. To explore this, we analyze the effects of strain and vorticity variability on the evolution of the continuum ice-crystal ODF. We recast Jeffery's equation for the evolution of the ODF as a stochastic differential equation, with vorticity and strain perturbed by Gaussian processes. From this, we run a Monte-Carlo ensemble to determine likely bounds of true continuum ODF variability in response to random perturbations of strain and vorticity.

  13. Statistical Variability of Ice Crystal Orientations in Ice Sheets

    NASA Astrophysics Data System (ADS)

    Hay, M.; Waddington, E. D.

    2015-12-01

    Crystal-orientation fabrics in polar ice sheets have a strong influence on ice flow due to the plastic anisotropy of ice. Crystal orientations evolve primarily in response to applied strain, but are also affected by temperature, impurities, interactions with neighbors, and other factors. While the evolution of each ice crystal is physically deterministic, in limited samples, such as those from ice-core thin sections, measured samples are stochastic due to sampling error. Even in continuum representations from models, crystal ODFs can be treated as stochastic due to uncertainties in how they developed. Here, we present results on the statistics of crystal orientation fabrics. We show a first-order estimate of the sampling distribution of fabric eigenvalues and fabric eigenvectors from ice-core thin sections. In addition, the strain histories of fabrics are generally poorly constrained, and may have varied in unknown ways through time. Nearby layers in ice sheets can also experience different strain histories due to inherent variabilities such as transient flow, or differences in impurities. This means that the continuum ODF itself can be treated as stochastic, because it depends on an effectively-stochastic unknown strain-history. To explore this, we analyze the effects of strain and vorticity variability on the evolution of the continuum ice-crystal ODF. We recast Jeffery's equation for the evolution of the ODF as a stochastic differential equation, with vorticity and strain perturbed by Gaussian processes. From this, we run a Monte-Carlo ensemble to determine likely bounds of true continuum ODF variability in response to random perturbations of strain and vorticity.

  14. The First Annual West Antarctic Ice Sheet (WAIS) Science Workshop

    NASA Technical Reports Server (NTRS)

    Bindschadler, Robert A. (Editor)

    1993-01-01

    A compilation of abstracts presented at the workshop are presented. The goal was to answer the question, what is the future behavior and potential for rapid collapse of the West Antarctic Ice Sheet (WAIS)? The workshop was organized into four sessions corresponding to the four objectives identified as necessary to reach the WAIS workshop goal: history, current behavior, internal dynamics, and environmental interactions. Presentations were organized by their relevance to each objective, rather than by discipline.

  15. 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.

    2016-12-01

    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.

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

    PubMed

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

    2017-01-05

    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.

  17. 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.

  18. Flow-pattern evolution of the last British Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hughes, Anna L. C.; Clark, Chris D.; Jordan, Colm J.

    2014-04-01

    We present a 10-stage reconstruction of the evolution in ice-flow patterns of the last British Ice Sheet from build-up to demise derived from geomorphological evidence. 100 flowsets identified in the subglacial bedform record (drumlins, mega-scale glacial lineations, and ribbed moraine) are combined with ancillary evidence (erratic-transport paths, absolute dates and a semi-independently reconstructed retreat pattern) to define flow patterns, ice divides and ice-sheet margins during build-up, maximum glaciation and retreat. Overprinting and cross-cutting of landform assemblages are used to define the relative chronology of flow patterns and a tentative absolute chronology is presented based on a collation of available dates for ice advance and retreat. The ice-flow configuration of the last British Ice Sheet was not static. Some ice divides were remarkably stable, persisting through multiple stages of the ice-sheet evolution, whereas others were transient features existing for a short time and/or shifting in position 10s km. The 10 reconstructed stages of ice-sheet geometry capture two main modes of operation; first as an integrated ice sheet with a broadly N-S orientated ice divide, and second as a multi-domed ice sheet orientated parallel with the shelf edge. A thick integrated ice sheet developed as ice expanded out of source areas in Scotland to envelop southerly ice caps in northern England and Wales, and connect with the Irish Ice Sheet to the west and the Scandinavian Ice Sheet across the North Sea. Following break-up of ice over the North Sea, ice streaming probably drove mass loss and ice-sheet thinning to create a more complex divide structure, where ice-flow patterns were largely controlled by the form of the underlying topography. Ice surface lowering occurred before separation of, and retreat to, multiple ice centres centred over high ground. We consider this 10-stage reconstruction of the evolution in ice-sheet configuration to be the simplest palaeo

  19. Effect of microorganism on Greenland ice sheet surface temperature change

    NASA Astrophysics Data System (ADS)

    Shimada, R.; Takeuchi, N.; Aoki, T.

    2012-12-01

    Greenland ice sheet holds approximately 10% of the fresh water on earth. If it melts all, sea level rises about 7.2meter. It is reported that mass of Greenland ice sheet is decreasing with temperature rising of climate change. Melting of the coastal area is particularly noticeable. It is established that 4 to 23% of the sea level rising from 1993 to 2005 is caused by the melting of Greenland ice sheet. In 2010, amount of melting per year became the largest than the past. However many climate models aren't able to simulate the recent melting of snow and ice in the Arctic including Greenland. One of the possible causes is albedo reduction of snow and ice surface by light absorbing snow impurities such as black carbon and dust and by glacial microorganisms. But there are few researches for effect of glacial microorganism in wide area. So it is important to clarify the impact of glacial microorganisms in wide area. The purpose of this study is to clarify the effect of microorganism on Greenland ice sheet surface temperature change using satellite images of visible, near infrared and thermal infrared wavelength range and observation carried out in northwestern Greenland. We use MODIS Land Surface Temperature Product as ice sheet surface temperature. It estimates land surface temperature based on split window method using thermal infrared bands. MODIS data is bound to cover the whole of Greenland, and calculated the ratio of the temperature change per year. Analysis period is from December 2002 to November 2010. Results of calculating Greenland ice sheet surface temperature change using the MODIS data, our analysis shows that it is upward trend in the whole region. We find a striking upward trend in northern and western part of Greenland. The rate is 0.33±0.03 degree Celsius per a year from 47.5°W to 49°W. While in the coastal area from 49°W to 50.7°W, the rate is 0.26±0.06 degree Celsius per a year. This large upward trend area is the same area as dark region

  20. Historic and Future Ice Storms

    NASA Astrophysics Data System (ADS)

    Klima, K.; Morgan, M. G.

    2014-12-01

    Ice storm losses from business interruption as well as transportation and health damages can range into billions of dollars. For instance, the December 2008 New England and Upstate New York ice storm caused four deaths and monetary damages between 2.5 and 3.7 billion, and the 2008 Chinese winter storms resulted in over 130 deaths and over 20 billion in damages. Informal discussions with ice storm experts indicate that due to competing temperature and precipitation effects as well as local topographic effects, it is unclear how exactly climate change will affect ice storms. Here we ask how incident frequencies might change in a future climate at four weather stations prone to ice storms. Using historical atmospheric soundings, we conduct a thought experiment where we perturb the temperatures as might be expected in a future climate. We then discuss changes in monthly frequency of ice storms.

  1. Moulin density controls drainage development beneath the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Banwell, Alison; Hewitt, Ian; Willis, Ian; Arnold, Neil

    2016-12-01

    Uncertainty remains about how the surface hydrology of the Greenland ice sheet influences its subglacial drainage system, affecting basal water pressures and ice velocities, particularly over intraseasonal and interseasonal timescales. Here we apply a high spatial (200 m) and temporal (1 h) resolution subglacial hydrological model to a marginal (extending 25 km inland), land-terminating, 200 km2 domain in the Paakitsoq region, West Greenland. The model is based on that by Hewitt (2013) but adapted for use with both real topographic boundary conditions and calibrated modeled water inputs. The inputs consist of moulin hydrographs, calculated by a surface routing and lake-filling/draining model, which is forced with distributed runoff from a surface energy-balance model. Results suggest that the areal density of lake-bottom moulins and their timing of opening during the melt season strongly affects subglacial drainage system development. A higher moulin density causes an earlier onset of subglacial channelization (i.e., water transport through channels rather than the distributed sheet), which becomes relatively widespread across the bed, whereas a lower moulin density results in a later onset of channelization that becomes less widespread across the bed. In turn, moulin density has a strong control on spatial and temporal variations in subglacial water pressures, which will influence basal sliding rates, and thus ice motion. The density of active surface-to-bed connections should be considered alongside surface melt intensity and extent in future predictions of the ice sheet's dynamics.

  2. 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

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

    NASA Astrophysics Data System (ADS)

    Abdalati, W.

    2004-12-01

    The mass balance of high-latitude glaciers and ice sheets is highly variable on a wide range of spatial and temporal scales, but through a combination of remote sensing and in situ measurements, some significant changes have been observed in recent years. 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 between the ice sheet and a warming climate. Throughout much of the rest of the Arctic, glaciers and ice caps have been shrinking in recent decades, with increased mass losses during the 1990s in parts of Canada and Alaska. The picture is more complicated in the southern hemisphere, where Antarctic ice is growing in some areas, shrinking dramatically in others, and is essentially in balance elsewhere. The West Antarctic Ice Sheet (WAIS) shows thinning along its northern margin, particularly in the glaciers that flow into the Amundsen Sea. The western portions of the WAIS, however, show thickening, but in the aggregate the mass loss is believed to exceed the gain. Changes in the East Antarctic Ice Sheet are small, but we don't know at this point whether it is growing or shrinking. On the Antarctic Peninsula, the rapid disintegration of the Larsen B ice shelf has resulted in acceleration and thinning of a small number of glaciers that once fed the ice shelf. This behavior raises questions about relatively near-term consequences of climate change and the Antarctic Ice Sheet's contribution to sea level rise. These recent observations offer only a snapshot in time of their long-term behavior, but they are providing crucial information about the current state of ice mass balance and the mechanisms that control it. As we continue to learn more through a combination of remote sensing

  4. 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.

  5. 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.

  6. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat

    PubMed Central

    Scherer, Reed P.; DeConto, Robert M.; Pollard, David; Alley, Richard B.

    2016-01-01

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise. PMID:27649516

  7. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat

    NASA Astrophysics Data System (ADS)

    Scherer, Reed P.; Deconto, Robert M.; Pollard, David; Alley, Richard B.

    2016-09-01

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.

  8. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat.

    PubMed

    Scherer, Reed P; DeConto, Robert M; Pollard, David; Alley, Richard B

    2016-09-20

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.

  9. Tectonics and ice sheet dynamics of West Antarctic margins

    NASA Astrophysics Data System (ADS)

    Gohl, Karsten

    2010-05-01

    An understanding of the glacial history of the Amundsen Sea Embayment (ASE) and Pine Island Bay (PIB) is essential for proposing models on the future development of the West Antarctic Ice Sheet. This requires an understanding of the tectonic history and knowledge of tectonic features such as lineaments, ridges, sills and basins, because basement morphology and inherited erosional features control the flow direction of ice-sheets and the influx of Circum-Polar Deep Water (CDW). This is an attempt to reconstruct the tectonic history with the aim to search for basement features and crustal boundaries which may be correlated to the flow and dynamics of the ice-sheet. The Amundsen Sea Embayment of West Antarctica is in a prominent location for a series of tectonic and magmatic events from Paleozoic to Cenozoic times. Seismic, magnetic and gravity data from the embayment and PIB reveal the crustal thickness and significant tectonic features. NE-SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone which was active during or in the run-up to the breakup process between Chatham Rise and West Antarctica before or at 90 Ma. NW-SE trending gravity and magnetic anomalies, following a prolongation of Peacock Sound, indicate the extensional southern boundary to the Bellingshausen Plate which was active between 79 and 61 Ma. It is likely that the prominent Pine Island Trough follows a structural boundary between the crustal blocks of Ellsworth Land and Marie Byrd Land. Data are shown from the ASE and PIB which can be interpreted in context with the reconstruction of the ice advance and retreat history in this area. Differences in the behaviour of the ice-sheet are shown to exist for the western and eastern parts of PIB due to basement structures affecting the inflow of CDW.

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

    USGS Publications Warehouse

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

    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.

  11. 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.

  12. 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

  13. Reconstructing the last Newfoundland Ice Sheet,Canada.

    NASA Astrophysics Data System (ADS)

    McHenry, Maureen; Dunlop, Paul

    2015-04-01

    The Newfoundland Ice Sheet which formed part of the North American Ice Sheet Complex was situated on the margins of the northwest Atlantic Ocean during the Wisconsinan glaciation (~80ka BP to 10ka BP). This complex consisted of the Laurentide, the Cordilleran and Innuitian Ice Sheets, the Canadian Maritime Provinces Ice Cover and the Newfoundland Ice Sheet (NIS). Although all were confluent at the last glacial maximum, the NIS is known to have supported independent ice centres with advances from the Laurentide Ice Sheet being restricted to Newfoundland's northern and western margins. Given its distinctive position, it is likely the evolution of the NIS through the last glacial cycle was influenced by a number of external and internal drivers including configuration changes in the Laurentide Ice Sheet, ice stream initiation and shutdown, changes in oceanic circulation and fluctuating sea levels and climate signals from the wider Amphi-North Atlantic. As such Newfoundland is a key location for investigating ice sheet response to a number of internal and external forcing mechanisms during glacial cycles. An established technique for reconstructing former ice sheet behaviour is the mapping and spatial analysis of glacial landforms. This provides a valuable record of former ice sheet extent and behaviour through time as well as ice sheet retreat during deglaciation. Here we present new mapping based on our interpretation of SPOT satellite imagery and Digital Elevation Models of the entire Island of Newfoundland as well as swath bathymetric imagery from several locations offshore. Our new database consisting of ~150,000 individually mapped subglacial bedforms that includes drumlins, crag and tails, glacially moulded bedrock lineations and ribbed moraines significantly increases the known landform record in this region. The new database shows Newfoundland has a complex palimpsest landscape that records multiple ice sheet events across the island. Here we report our

  14. 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.

  15. Greenland ice sheet motion insensitive to exceptional meltwater forcing

    PubMed Central

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

    2013-01-01

    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 ∼3.9σ 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. PMID:24248343

  16. Growth of Greenland ice sheet - Measurement

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Bindschadler, Robert A.; Marsh, James G.; Brenner, Anita C.; Major, Judy A.

    1989-01-01

    Measurements of ice-sheet elevation change by satellite altimetry show that the Greenland surface elevation south of 72 deg north latitude is increasing. The vertical velocity of the surface is 0.20 + or - 0.06 meters/year from measured changes in surface elevations at 5906 intersections between Geosat paths in 1985 and Seasat in 1978, and 0.28 + or - 0.02 meters/year from 256,694 intersections of Geosat paths during a 548-day period of 1985 to 1986.

  17. 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.

  18. 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.

  19. 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

  20. Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin.

    PubMed

    Feldmann, Johannes; Levermann, Anders

    2015-11-17

    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.

  1. 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).

  2. Improving Constraints on Paleo Ice Sheets in the Amundsen Sea Embayment

    NASA Astrophysics Data System (ADS)

    Larter, Robert D.; Gohl, Karsten; Bentley, Michael J.

    2010-01-01

    Amundsen Sea Embayment: Tectonic and Climatic Evolution; Granada, Spain, 9 September 2009; Geoscientists working on the Amundsen Sea Embayment (ASE) of West Antarctica met at a workshop during the First Antarctic Climate Evolution Symposium to discuss recent advances from, and future priorities for, work in this region. The ASE is the most rapidly changing sector of the West Antarctic Ice Sheet (WAIS) and contains enough ice to raise sea level by 1.2 meters. Ice sheet modeling studies suggest that this sector of the WAIS is potentially unstable. Considerable efforts have been made through several national Antarctic programs to acquire new data on the geological structure, subglacial topography, bathymetry, and glacial history of this remote region. These data are important for establishing boundary conditions for ice sheet modeling, for providing constraints on past ice sheet changes that can be used to test models, and for putting recent changes into a longer-term context.

  3. 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.

  4. 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

  5. Rapid Holocene thinning of an East Antarctic outlet glacier driven by marine ice sheet instability.

    PubMed

    Jones, R S; Mackintosh, A N; Norton, K P; Golledge, N R; Fogwill, C J; Kubik, P W; Christl, M; Greenwood, S L

    2015-11-26

    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.

  6. Boundary layer stability acts to ballast the mass of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Berkelhammer, M. B.; Noone, D. C.; Steen-Larsen, H. C.; O'Neill, M.; Raudzens Bailey, A.; Cox, C.; Schneider, D. P.

    2014-12-01

    The mass of the Greenland Ice Sheet has been reduced over recent decades as a consequence of warming, the impact of which is already detectable on global sea level. However, temperature projections suggest that at interior high-altitude sites on the ice it could be decades or more before warming forces these regions to transition from a dry to wet snow facies. Shifts in boundary layer dynamics, including atmosphere-ice sheet hydrological exchange and cloud radiative forcing could expedite or delay this transition. These processes are important with respect to future ice sheet stability, yet they remain difficult to constrain. Using continuous in situ measurements of vertical profiles of the isotopic composition of water vapor at Summit Camp, the highest observatory on the ice sheet, we document the presence of a hydrologic balance between surface sublimation and condensation fluxes. This exists because of a nearly persistent temperature inversion, which hinders the efficiency with which surface water vapor mixes into the free atmosphere. In the presence of a strong temperature inversion, fog and ice particles form near the ice-atmosphere interface from surface moisture fluxes. When this condensate precipitates on or settles to the surface, it ballasts the ice sheet's mass. A decade-long trend towards lower annual accumulation at Summit may therefore reflect continuous replacement of the near surface atmosphere due to reduced atmospheric stability. If this tendency toward destabilization continues, it could accelerate mass loss at interior sites on the ice sheet. The role of boundary layer stability in ice sheet hydrological budgets discussed here is applicable beyond the accumulation zone of the Greenland Ice Sheet.

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

    SciTech Connect

    Waisman, Haim; Bell, Robin; Keyes, David; Boman, Erik Gunnar; Tuminaro, Raymond Stephen

    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 in 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.

  8. 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

  9. 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.

  10. Ice-age Ice-sheet Rheology: Constraints from the Last Glacial Maximum Form of the Laurentide Ice Sheet

    NASA Technical Reports Server (NTRS)

    Peltier, W. Richard; Goldsby, David L.; Kohlstedt, David L.; Tarasov, Lev

    2000-01-01

    State-ot-the-art thermomechanical models of the modern Greenland ice and the ancient Laurenticle ice sheet that covered Canada at the Last Glacial Maximum (LGM) are not able to explain simultaneously the observed forms of these cryospheric structures when the same, anisotropy-enhanced, version of the conventional Glen flow law is employed to describe their rheology. The LGM Laurenticle ice sheet. predicted to develop in response to orbital climate forcing, is such that the ratio of its thickness to its horizontal extent is extremely large compared to the aspect ratio inferred on the basis of surface-geomorphological and solid-earth-geophysical constraints. We show that if the Glen flow law representation of the rheology is replaced with a new rheology based upon very high quality laboratory measurements of the stress-strain-rate relation, then the aspect ratios of both the modern Greenland ice sheet and the Laurenticle ice sheet, that existed at the LGM, are simultaneously explained with little or no retuning of the flow law.

  11. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years.

    PubMed

    Bierman, Paul R; Shakun, Jeremy D; Corbett, Lee B; Zimmerman, Susan R; Rood, Dylan H

    2016-12-07

    Climate models show that ice-sheet melt will dominate sea-level rise over the coming centuries, but our understanding of ice-sheet variations before the last interglacial 125,000 years ago remains fragmentary. This is because terrestrial deposits of ancient glacial and interglacial periods are overrun and eroded by more recent glacial advances, and are therefore usually rare, isolated and poorly dated. In contrast, material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time-varying state of major ice sheets. Here we show that the East Greenland Ice Sheet existed over the past 7.5 million years, as indicated by beryllium and aluminium isotopes ((10)Be and (26)Al) in quartz sand removed by deep, ongoing glacial erosion on land and deposited offshore in the marine sedimentary record. During the early Pleistocene epoch, ice cover in East Greenland was dynamic; in contrast, East Greenland was mostly ice-covered during the mid-to-late Pleistocene. The isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions. Although our data are continuous, they are from low-deposition-rate sites and sourced only from East Greenland. Consequently, the signal of extensive deglaciation during short, intense interglacials could be missed or blurred, and we cannot distinguish between a remnant ice sheet in the East Greenland highlands and a diminished continent-wide ice sheet. A clearer constraint on the behaviour of the ice sheet during past and, ultimately, future interglacial warmth could be produced by (10)Be and (26)Al records from a coring site with a higher deposition rate. Nonetheless, our analysis challenges the possibility of complete and extended deglaciation over the past several million years.

  12. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years

    NASA Astrophysics Data System (ADS)

    Bierman, Paul R.; Shakun, Jeremy D.; Corbett, Lee B.; Zimmerman, Susan R.; Rood, Dylan H.

    2016-12-01

    Climate models show that ice-sheet melt will dominate sea-level rise over the coming centuries, but our understanding of ice-sheet variations before the last interglacial 125,000 years ago remains fragmentary. This is because terrestrial deposits of ancient glacial and interglacial periods are overrun and eroded by more recent glacial advances, and are therefore usually rare, isolated and poorly dated. In contrast, material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time-varying state of major ice sheets. Here we show that the East Greenland Ice Sheet existed over the past 7.5 million years, as indicated by beryllium and aluminium isotopes (10Be and 26Al) in quartz sand removed by deep, ongoing glacial erosion on land and deposited offshore in the marine sedimentary record. During the early Pleistocene epoch, ice cover in East Greenland was dynamic; in contrast, East Greenland was mostly ice-covered during the mid-to-late Pleistocene. The isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions. Although our data are continuous, they are from low-deposition-rate sites and sourced only from East Greenland. Consequently, the signal of extensive deglaciation during short, intense interglacials could be missed or blurred, and we cannot distinguish between a remnant ice sheet in the East Greenland highlands and a diminished continent-wide ice sheet. A clearer constraint on the behaviour of the ice sheet during past and, ultimately, future interglacial warmth could be produced by 10Be and 26Al records from a coring site with a higher deposition rate. Nonetheless, our analysis challenges the possibility of complete and extended deglaciation over the past several million years.

  13. 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

  14. 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.

  15. 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.

  16. The Eurasian Ice Sheet and the deglaciation of western Norway

    NASA Astrophysics Data System (ADS)

    Svendsen, John Inge; Mangerud, Jan; Briner, Jason; Hughes, Anna L. C.; Lohne, Øystein S.; Goehring, Brent M.; Gyllencreutz, Richard

    2013-04-01

    New time-slice reconstructions of the Eurasian Ice Sheet limits reveal that the timing of both the maximum ice sheet extent and the subsequent retreat were spatially variable. This variability most likely reflects regional contrasts in geographic setting, internal ice sheet dynamics and the forcing mechanisms. Here we report fresh results from an ongoing field campaign in southern Norway. The inferred ice sheet history is based on a number of radiocarbon dates from various geological contexts as well cosmogenic nuclide (CN) dating of glacially transported boulders. We discuss the ice-recession along the western flank of the Scandinavian Ice Sheet since the Last Glacial Maximum (LGM) and until the final deglaciation of the fjords. One important conclusion is that the ice stream that occupied the Norwegian Trough, and that was active during the LGM, broke up extremely rapid at around 20 ka leaving the islands Utsira and Karmøy permanently ice free. The adjacent areas of the Norwegian mainland remained ice covered for another 4 thousand years until about 16 ka when the ice margin along the coast gradually started to retreat eastwards. However, this second stage of ice sheet retreat was interrupted by several re-advances. The largest advance probably peaked at the very end of the Younger Dryas stadial (11.6 ka) forming a system of prominent end moraines along the coast. The outlet glaciers that filled the main fjord troughs during this event started to break up very rapidly by means of calving at the onset of the Holocene warming, evidently a climatic response. The main fjords became totally ice free not later than 500 years into the Holocene, and a thousand years later the remaining part of the ice sheet was gone. It is calculated that the melting during the early Holocene (11.6-10.0 ka) was more than ten times faster than at Greenland today.

  17. Inland thinning of West Antarctic Ice Sheet steered along subglacial rifts.

    PubMed

    Bingham, Robert G; Ferraccioli, Fausto; King, Edward C; Larter, Robert D; Pritchard, Hamish D; Smith, Andrew M; Vaughan, David G

    2012-07-25

    Current ice loss from the West Antarctic Ice Sheet (WAIS) accounts for about ten per cent of observed global sea-level rise. Losses are dominated by dynamic thinning, in which forcings by oceanic or atmospheric perturbations to the ice margin lead to an accelerated thinning of ice along the coastline. Although central to improving projections of future ice-sheet contributions to global sea-level rise, the incorporation of dynamic thinning into models has been restricted by lack of knowledge of basal topography and subglacial geology so that the rate and ultimate extent of potential WAIS retreat remains difficult to quantify. Here we report the discovery of a subglacial basin under Ferrigno Ice Stream up to 1.5 kilometres deep that connects the ice-sheet interior to the Bellingshausen Sea margin, and whose existence profoundly affects ice loss. We use a suite of ice-penetrating radar, magnetic and gravity measurements to propose a rift origin for the basin in association with the wider development of the West Antarctic rift system. The Ferrigno rift, overdeepened by glacial erosion, is a conduit which fed a major palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open-ocean water back to the ice front to reinforce dynamic thinning. We show that dynamic thinning from both the Bellingshausen and Amundsen Sea region is being steered back to the ice-sheet interior along rift basins. We conclude that rift basins that cut across the WAIS margin can rapidly transmit coastally perturbed change inland, thereby promoting ice-sheet instability.

  18. 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.

  19. Self-inhibiting growth of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Langen, P. L.; Solgaard, A. M.; Hvidberg, C. S.

    2012-04-01

    The build-up of the Greenland Ice Sheet from ice free conditions is studied in an ice sheet model (ISM) driven by fields from an atmospheric general circulation model (GCM). Experiments where the two are coupled offline are performed and augmented by one where an intermediate ice sheet configuration, taken as a snap shot during the regrowth in the ISM, is coupled back to the GCM. It is found that several open questions regarding reversibility or irreversibility of a disintegration of the Greenland Ice Sheet may be reconciled with these experiments. Running the ISM with GCM fields corresponding to a present day ice sheet configuration leads to regrowth, while considerations of the GCM's snow accumulation in an ice free run point to irreversibility. Forcing the ISM with the GCM fields corresponding to the ice free state leads to extensive regrowth which, however, is halted when an intermediate recoupling step is included. This inhibition of further growth is believed to be due to a Föhn effect of moist air parcels being lifted over the intermediate ice sheet and arriving in the Greenland interior with high temperatures.

  20. 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.

  1. 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

  2. Measurements of Past Ice Sheet Elevations in Interior West Antarctica.

    PubMed

    Ackert; Barclay; Borns; Calkin; Kurz; Fastook; Steig

    1999-10-08

    A lateral moraine band on Mount Waesche, a volcanic nunatak in Marie Byrd Land, provides estimates of past ice sheet surface elevations in West Antarctica. Helium-3 and chlorine-36 surface exposure ages indicate that the proximal part of the moraine, up to 45 meters above the present ice surface, was deposited about 10,000 years ago, substantially later than the maximum ice extent in the Ross Embayment. The upper distal part of the moraine may record multiple earlier ice sheet high stands. A nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at Mount Waesche relative to the maximum ice extent in the Ross Sea. The glacial geologic evidence, coupled with the ice sheet model, indicates that the contribution of the Ross Sea sector of the West Antarctic Ice Sheet to Holocene sea level rise was only about 3 meters. These results eliminate West Antarctic ice as the principle source of the large meltwater pulse during the early Holocene.

  3. 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.

  4. 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

  5. Modeling the Fracture of Ice Sheets on Parallel Computers

    SciTech Connect

    Waisman, Haim; Tuminaro, Ray

    2013-10-10

    The objective of this project was to investigate the complex fracture of ice and understand its role within larger ice sheet simulations and global climate change. This objective was achieved by developing novel physics based models for ice, novel numerical tools to enable the modeling of the physics and by collaboration with the ice community experts. 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. To this end, our research findings through this project offers significant advancement to the field and closes a large gap of knowledge in understanding and modeling the fracture of ice sheets in the polar regions. Thus, we believe that our objective has been achieved and our research accomplishments are significant. This is corroborated through a set of published papers, posters and presentations at technical conferences in the field. In particular significant progress has been made in the mechanics of ice, fracture of ice sheets and ice shelves in polar regions and sophisticated numerical methods that enable the solution of the physics in an efficient way.

  6. Antarctic marine ice sheet retreat in the Ross Sea during the early Holocene

    NASA Astrophysics Data System (ADS)

    Mckay, R. M.; Golledge, N.; Naish, T.; Maas, S.; Levy, R. H.; Kuhn, G.; Lee, J. I.; Dunbar, G. B.

    2015-12-01

    Geological constraints on the timing of the retreat of the Last Glacial Maximum (LGM) Antarctic Ice Sheets provide critical insights into the processes controlling marine-based ice sheet stability. The over-deepened, seaward shallowing bathymetry of Antarctica's continental shelves is ideally configured to promote past, and potentially future, marine ice-sheet instability. The retreat history of the LGM ice sheet in the Ross Sea region is primarily constrained by C-14 ages on coastal beach ridges and relict penguin colonies along the Transantarctic Mountain front in the Western Ross Sea. Although these terrestrial sites offer more reliable dates than imprecise C-14 chronologies derived from bulk marine sediments, they may reflect retreat of local piedmont glaciers derived from East Antarctic outlet glaciers rather than representing the timing of retreat of the ice sheet in the central Ross Embayment. We present a sedimentary facies succession and foraminifera-based C-14 chronology from a core collected beneath the Ross Ice Shelf via a hot water drill access hole used for the ANDRILL Coulman High site survey. The site is to the east of Ross Island and distal from the coast, and yields a minimum age for glacial retreat that is approximately 1000 yrs earlier than suggested by coastal records along the nearby Victoria Land coast. We examine the implications of this constraint on the timing of ice sheet retreat in the context of model simulations and new multi-beam bathymetry data acquired in the Western Ross Sea. On the basis of these data we hypothesize that marine-based ice sheet retreat was triggered by oceanic forcings along most of the Pacific Ocean coastline of Antarctica simultaneously, but continued retreat in the Ross Sea occurred primarily as a consequence of marine ice sheet instability.

  7. 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.

  8. Recent ice sheet mass change observations from GRACE mascon solutions

    NASA Astrophysics Data System (ADS)

    Luthcke, S. B.; Zwally, H. J.; Rowlands, D. D.; Abdalati, W.; Nerem, R. S.; Ray, R. D.; Lemoine, F.; Chinn, D.

    2006-12-01

    On multi-decadal time scales or longer, the most important processes affecting sea level are those associated with the mass balance over the Earth's ice sheets. The vulnerability of the cryosphere to climate change along with the difficulty in acquiring uniform in situ observations in these inhospitable regions, makes the problem of understanding ice sheet mass trends a high Earth science research priority at NASA. The Gravity Recovery and Climate Experiment (GRACE) mission has acquired ultra-precise inter-satellite ranging data since 2002. These data provide new opportunities to observe and understand ice mass changes at unprecedented temporal and spatial resolution. In order to improve upon the ice mass trend observations obtained from GRACE, we have employed unique data analysis approaches to obtain high resolution local mass change (mascon solutions) from GRACE inter-satellite observations alone. We have applied our mascon solution technique to the Greenland and Antarctic ice sheets estimating surface mass change for irregularly shaped regions defined by the ice sheet drainage basins and further sub-divided by elevation. We estimate the surface mass change of each ice sheet drainage basin sub-divided by elevation at 10-day resolution. We have computed multi-year time series of surface mass change for each ice sheet drainage basin. These mascon solutions provide unprecedented observations of the seasonal and inter-annual evolution of ice-sheet mass flux. In this presentation we discuss our analysis techniques and the details of our ice sheet mascon solutions, as well as compare these results with mass change observations derived from NASA's ICESat mission.

  9. Evidence for a former large ice sheet in the Orville Coast- Ronne Ice Shelf area, Antarctica.

    USGS Publications Warehouse

    Carrara, P.

    1981-01-01

    The Orville Coast area of the Antarctic Peninsula was extensively glacierized in the past. Striations, polished rock surfaces, and erratics on nunatak summits indicate that this area was covered by a broad regional ice sheet whose grounded ice margin was on the continental shelf, in the present-day Ronne Ice Shelf area. If the glacial history of Antarctica has been controlled by eustatic sea-level changes, the destruction of this ice sheet would have been contemporaneous with that of the Ross Sea ice sheet due to the world-wide rise of eustatic sea-level at the end of the Wisconsin glaciation. -Author

  10. 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.

  11. Permafrost-ice-sheet interactions during the Quaternary

    NASA Astrophysics Data System (ADS)

    Willeit, Matteo; Ganopolski, Andrey

    2016-04-01

    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. We recently included a permafrost module in the Earth system model CLIMBER-2 to explore the coupled Northern Hemisphere (NH) permafrost-ice-sheet evolution during the Quaternary. The model has been shown to perform generally well at reproducing present-day permafrost extent and thickness. Modelled permafrost extent at the Last Glacial Maximum (LGM) agrees well with reconstructions and previous modeling estimates. In a previous study we showed that 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. In transient model simulations of the "40 kyr world" of the early Pleistocene we show that when all continents are covered by a thick sediment layer the response of ice volume to the obliquity component of orbital forcing is enhanced while the response to precession is dampened. We therefore argue that permafrost could have played a role for ice sheet evolution when all continents were covered by a thick sediment layer, as was likely the case in the early Pleistocene before the sediment layer was gradually eroded by expanding ice sheets over parts of northern Canada and Scandinavia.

  12. The environment and evolution of the West Antarctic ice sheet: setting the stage.

    PubMed

    Bindschadler, Robert

    2006-07-15

    The West Antarctic ice sheet is the last ice sheet of the type cradled in a warm, marine geologic basin. Its perimeter stretches into the surrounding seas allowing warmer ocean waters to reach the undersides of its floating ice shelves and its relatively low surface elevation permits snow-carrying storms to extend well into its interior. This special environment has given rise to theories of impending collapse and for the past quarter-century has challenged researchers who seek a quantitative prediction of its future behaviour and the corresponding effect on sea level. Observations confirm changes on a variety of time scales from the quaternary to less than a minute. The dynamics of the ice sheet involve the complex interaction of ice that is warm at its base and cold along the margins of ice streams; subglacial till that is composed of a combination of marine sediment and eroded sedimentary rocks; and water that moves primarily between the ice and bed, but whose flow direction can differ from the direction of ice motion. The pressure of the water system is often sufficient to float the ice sheet locally and small changes in the amount of water in the till can cause it to rapidly switch from very weak to very stiff.

  13. 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

  14. Sensitivity of the Ice Sheet System Model to direct surface mass balance forcing over the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Schlegel, N. J.; Seroussi, H. L.; Morlighem, M.; Larour, E. Y.; Box, J. E.

    2011-12-01

    The Greenland Ice Sheet, which extends south of the Arctic Circle, is vulnerable to temperature perturbations in the Northern Hemisphere, and its complete retreat would raise global sea level by about 7 meters. Models of the ice sheet's past behavior suggest that Greenland's severe retreat was largely responsible for sea-level rise during the last interglacial period. A clear understanding of exactly how the ice sheet responded to past climate change requires a high-degree of spatial resolution, especially within the ice sheet's large drainage basins, as they contain outlets capable of high-velocity flow. The newly developed Ice Sheet System Model (ISSM) is a finite-element model capable of simulating transient ice flow on an anisotropic mesh. The adaptable mesh can be refined to higher resolutions in the areas of enhanced ice flow. These features offer a distinct advantage over previous models of the Greenland Ice Sheet, specifically in terms of modeling fast-flowing outlet glaciers. With use of established ISSM capabilities, we examined the sensitivity of Greenland's outlet glaciers to the new Arctic System Reanalysis (ASR) reconstruction of yearly surface mass balance forcing of the last 150 years. 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.

  15. Basal Dynamics and Internal Structure of Ice Sheets

    NASA Astrophysics Data System (ADS)

    Wolovick, Michael J.

    The internal structure of ice sheets reflects the history of flow and deformation experienced by the ice mass. Flow and deformation are controlled by processes occurring within the ice mass and at its boundaries, including surface accumulation or ablation, ice rheology, basal topography, basal sliding, and basal melting or freezing. The internal structure and basal environment of ice sheets is studied with ice-penetrating radar. Recently, radar observations in Greenland and Antarctica have imaged large englacial structures rising from near the bed that deform the overlying stratigraphy into anticlines, synclines, and overturned folds. The mechanisms that may produce these structures include basal freeze-on, travelling slippery patches at the ice base, and rheological contrasts within the ice column. In this thesis, I explore the setting and mechanisms that produce large basal stratigraphic structures inside ice sheets. First, I use radar data to map subglacial hydrologic networks that deliver meltwater uphill towards freeze-on structures in East Antarctica. Next, I use a thermomechanical flowline model to demonstrate that trains of alternating slippery and sticky patches can form underneath ice sheets and travel downstream over time. The disturbances to the ice flow field produced by these travelling patches produce stratigraphic folds resembling the observations. I then examine the overturned folds produced by a single travelling sticky patch using a kinematic flowline model. This model is used to interpret stratigraphic measurements in terms of the dynamic properties of basal slip. Finally, I use a simple local one-dimensional model to estimate the thickness of basal freeze-on that can be produced based on the supply of available meltwater, the thermal boundary conditions, ice sheet geometry, and the ice flow regime.

  16. The sea level response to ice sheet freshwater forcing in the Community Earth System Model

    NASA Astrophysics Data System (ADS)

    Slangen, Aimée B. A.; Lenaerts, Jan T. M.

    2016-10-01

    We study the effect of a realistic ice sheet freshwater forcing on sea-level change in the fully coupled Community Earth System Model (CESM) showing not only the effect on the ocean density and dynamics, but also the gravitational response to mass redistribution between ice sheets and the ocean. We compare the ‘standard’ model simulation (NO-FW) to a simulation with a more realistic ice sheet freshwater forcing (FW) for two different forcing scenario’s (RCP2.6 and RCP8.5) for 1850-2100. The effect on the global mean thermosteric sea-level change is small compared to the total thermosteric change, but on a regional scale the ocean steric/dynamic change shows larger differences in the Southern Ocean, the North Atlantic and the Arctic Ocean (locally over 0.1 m). The gravitational fingerprints of the net sea-level contributions of the ice sheets are computed separately, showing a regional pattern with a magnitude that is similar to the difference between the NO-FW and FW simulations of the ocean steric/dynamic pattern. Our results demonstrate the importance of ice sheet mass loss for regional sea-level projections in light of the projected increasing contribution of ice sheets to future sea-level rise.

  17. 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

  18. 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.

  19. 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.

  20. Ice sheet topography from retracked ERS-1 altimetry

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Brenner, Anita C.; Dimarzio, John; Seiss, Timothy

    1994-01-01

    An objective of the ERS-1 radar altimeter is to measure the surface topography of the polar ice sheets to a precision on the order of a meter. ERS-1 Waveform Altimeter Product (WAP) data was corrected for several processing errors. A range correction from the WAP waveforms, using the multiparameter retracking algorithm to account for range tracking limitations inherent to radar altimetry, was derived. From crossover analysis, the resulting precision is shown to be about 2.1 m in ocean mode and 2.2 m in ice mode. A topography map, produced with 23 days of corrected data, shows details of the western part of west Antarctic ice sheet and part of the Ross ice shelf including ice divides, ice stream boundaries, and ice shelf grounding lines.

  1. 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.

  2. Radiostratigraphy and age structure of the Greenland Ice Sheet.

    PubMed

    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

    2015-02-01

    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.

  3. Evidence for an extensive Antarctic Ice Sheet by 37 Ma

    NASA Astrophysics Data System (ADS)

    Carter, Andrew; Riley, Teal; Hillenbrand, Claus-Dieter; Rittner, Martin

    2016-04-01

    We present observational evidence that both the East and West Antarctic ice sheets had expanded to the coast by 37 Ma, predating, by at least 3 Myr, a major drop in atmospheric CO2 at the Eocene-Oligocene boundary widely considered responsible for Antarctic Ice Sheet expansion. Our evidence comes from the provenance (geochronology, thermochronometry, mineralogy) of iceberg-rafted debris identified in Late Eocene marine sediments from (ODP) Leg 113 Site 696 in the NW Weddell Sea. The existence of an significant Antarctic Ice Sheet in a Late Eocene high pCO2 world calls into question the role of atmospheric CO2 concentrations as the dominant mechanism for ice sheet expansion and whether topography and ocean circulation only play a secondary role.

  4. Sturgis and Tekonsha ice advances: Evidence for thin ice sheets in southern Michigan

    SciTech Connect

    Straw, W.T.; Kehew, A.E.; Passero, R.N. . Dept. of Geology)

    1993-03-01

    Advance of an ice sheet through the Saginaw Lowland to the position of Sturgis, in St. Joseph County, Michigan initially produced the Sturgis Moraine and adjacent outwash apron. As this ice sheet down wasted, meltwater impounded by it overtopped ice divides to form meltwater streams that cut canyons in the ice and underlying glacial sediments. Continued wasting was attended by formation of supraglacial streams that developed waterfalls as they eroded headward. Complete melting of the ice revealed a subglacial topography marked by drumlins, many of which are irregular, attesting to formation near the attenuated margin of the ice sheet. Prior to complete wasting of this stagnant ice mass, ice of the Lake Michigan Lobe advanced from the west-northwest to form the Tekonsha Moraine. Exposed only where this relatively thin ice sheet advanced onto a highland in east-central Kalamazoo County, the southwestern extension of this moraine was buried by outwash from ice that formed the very prominent Kalamazoo Moraine. That the Tekonsha ice sheet stagnated and wasted in place is revealed by two prominent elongate kames, several escarpments, lake and wetland basins produced by melting of ice masses buried by glacial outwash, and sequential filling of the lowland produced by melting of this ice sheet by a series of alluvial fans. The sequence of fan emplacement is indicated by marginal relationships of the fans and the number and size of depressions formed by melting of relic blocks of ice. The first-formed Dry Prairie fan is marked by numerous large depressions while the Prairie Ronde, the last formed fan exhibits only small widely spaced depressions formed by melting of the last vestiges of this ice sheet.

  5. Uncertainty Quantification for Large-Scale Ice Sheet Modeling

    SciTech Connect

    Ghattas, Omar

    2016-02-05

    This report summarizes our work to develop advanced forward and inverse solvers and uncertainty quantification capabilities for a nonlinear 3D full Stokes continental-scale ice sheet flow model. The components include: (1) forward solver: a new state-of-the-art parallel adaptive scalable high-order-accurate mass-conservative Newton-based 3D nonlinear full Stokes ice sheet flow simulator; (2) inverse solver: a new adjoint-based inexact Newton method for solution of deterministic inverse problems governed by the above 3D nonlinear full Stokes ice flow model; and (3) uncertainty quantification: a novel Hessian-based Bayesian method for quantifying uncertainties in the inverse ice sheet flow solution and propagating them forward into predictions of quantities of interest such as ice mass flux to the ocean.

  6. Increased future ice discharge from Antarctica owing to higher snowfall.

    PubMed

    Winkelmann, R; Levermann, A; Martin, M A; Frieler, K

    2012-12-13

    Anthropogenic climate change is likely to cause continuing global sea level rise, but some processes within the Earth system may mitigate the magnitude of the projected effect. Regional and global climate models simulate enhanced snowfall over Antarctica, which would provide a direct offset of the future contribution to global sea level rise from cryospheric mass loss and ocean expansion. Uncertainties exist in modelled snowfall, but even larger uncertainties exist in the potential changes of dynamic ice discharge from Antarctica and thus in the ultimate fate of the precipitation-deposited ice mass. Here we show that snowfall and discharge are not independent, but that future ice discharge will increase by up to three times as a result of additional snowfall under global warming. Our results, based on an ice-sheet model forced by climate simulations through to the end of 2500 (ref. 8), show that the enhanced discharge effect exceeds the effect of surface warming as well as that of basal ice-shelf melting, and is due to the difference in surface elevation change caused by snowfall on grounded versus floating ice. Although different underlying forcings drive ice loss from basal melting versus increased snowfall, similar ice dynamical processes are nonetheless at work in both; therefore results are relatively independent of the specific representation of the transition zone. In an ensemble of simulations designed to capture ice-physics uncertainty, the additional dynamic ice loss along the coastline compensates between 30 and 65 per cent of the ice gain due to enhanced snowfall over the entire continent. This results in a dynamic ice loss of up to 1.25 metres in the year 2500 for the strongest warming scenario. The reported effect thus strongly counters a potential negative contribution to global sea level by the Antarctic Ice Sheet.

  7. Sensitivity of the Southern Ocean to enhanced regional Antarctic ice sheet meltwater input

    NASA Astrophysics Data System (ADS)

    Fogwill, C. J.; Phipps, S. J.; Turney, C. S. M.; Golledge, N. R.

    2015-10-01

    Despite advances in our understanding of the processes driving contemporary sea level rise, the stability of the Antarctic ice sheets and their contribution to sea level under projected future warming remains uncertain due to the influence of strong ice-climate feedbacks. Disentangling these feedbacks is key to reducing uncertainty. Here we present a series of climate system model simulations that explore the potential effects of increased West Antarctic Ice Sheet (WAIS) meltwater flux on Southern Ocean dynamics. We project future changes driven by sectors of the WAIS, delivering spatially and temporally variable meltwater flux into the Amundsen, Ross, and Weddell embayments over future centuries. Focusing on the Amundsen Sea sector of the WAIS over the next 200 years, we demonstrate that the enhanced meltwater flux rapidly stratifies surface waters, resulting in a significant decrease in the rate of Antarctic Bottom Water (AABW) formation. This triggers rapid pervasive ocean warming (>1°C) at depth due to advection from the original site(s) of meltwater input. The greatest warming is predicted along sectors of the ice sheet that are highly sensitized to ocean forcing, creating a feedback loop that could enhance basal ice shelf melting and grounding line retreat. Given that we do not include the effects of rising CO2—predicted to further reduce AABW formation—our experiments highlight the urgent need to develop a new generation of fully coupled ice sheet climate models, which include feedback mechanisms such as this, to reduce uncertainty in climate and sea level projections.

  8. Experimental design for three interrelated marine ice sheet and ocean model intercomparison projects: MISMIP v. 3 (MISMIP +), ISOMIP v. 2 (ISOMIP +) and MISOMIP v. 1 (MISOMIP1)

    NASA Astrophysics Data System (ADS)

    Asay-Davis, Xylar S.; Cornford, Stephen L.; Durand, Gaël; Galton-Fenzi, Benjamin K.; Gladstone, Rupert M.; Hilmar Gudmundsson, G.; Hattermann, Tore; Holland, David M.; Holland, Denise; Holland, Paul R.; Martin, Daniel F.; Mathiot, Pierre; Pattyn, Frank; Seroussi, Hélène

    2016-07-01

    Coupled ice sheet-ocean models capable of simulating moving grounding lines are just becoming available. Such models have a broad range of potential applications in studying the dynamics of marine ice sheets and tidewater glaciers, from process studies to future projections of ice mass loss and sea level rise. The Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP) is a community effort aimed at designing and coordinating a series of model intercomparison projects (MIPs) for model evaluation in idealized setups, model verification based on observations, and future projections for key regions of the West Antarctic Ice Sheet (WAIS). Here we describe computational experiments constituting three interrelated MIPs for marine ice sheet models and regional ocean circulation models incorporating ice shelf cavities. These consist of ice sheet experiments under the Marine Ice Sheet MIP third phase (MISMIP+), ocean experiments under the Ice Shelf-Ocean MIP second phase (ISOMIP+) and coupled ice sheet-ocean experiments under the MISOMIP first phase (MISOMIP1). All three MIPs use a shared domain with idealized bedrock topography and forcing, allowing the coupled simulations (MISOMIP1) to be compared directly to the individual component simulations (MISMIP+ and ISOMIP+). The experiments, which have qualitative similarities to Pine Island Glacier Ice Shelf and the adjacent region of the Amundsen Sea, are designed to explore the effects of changes in ocean conditions, specifically the temperature at depth, on basal melting and ice dynamics. In future work, differences between model results will form the basis for the evaluation of the participating models.

  9. Holocene accumulation and ice flow near the West Antarctic Ice Sheet Divide ice core site

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    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 kyr 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 km 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% lower than modern at 9.2 kyr before present (B.P.), increased by 40% from 9.2 to 2.3 kyr B.P., and decreased by at least 10% over the past 2 kyr 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 km of its modern position. Continent-scale ice-sheet models used for reconstructions of West Antarctic ice volume should incorporate this accumulation history.

  10. Self-inhibiting growth of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Langen, P. L.; Solgaard, A. M.; Hvidberg, C. S.

    2012-06-01

    The build-up of the Greenland Ice Sheet (GrIS) from ice-free conditions is studied in an ice sheet model (ISM) driven by fields from an atmospheric general circulation model (GCM) to demonstrate the importance of coupling between the two components. Experiments where the two are coupled off-line are augmented by one where an intermediate ice sheet configuration is coupled back to the GCM. Forcing the ISM with GCM fields corresponding to the ice-free state leads to extensive regrowth which, however, is halted when the intermediate recoupling step is included. This inhibition of further growth is due to a Föhn effect of moist air parcels being lifted over the intermediate ice sheet and arriving in the low-lying Greenland interior with high temperatures. This demonstrates that two-way coupling between the atmosphere and the ice sheet is essential for understanding the dynamics and that large scale conditions cooler than those of today may be necessary for the GrIS to regrow to the present volume.

  11. Sources, cycling and export of nitrogen on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Wadham, Jemma Louise; Hawkings, Jonathan; Telling, Jon; Chandler, Dave; Alcock, Jon; O'Donnell, Emily; Kaur, Preeti; Bagshaw, Elizabeth; Tranter, Martyn; Tedstone, Andre; Nienow, Peter

    2016-11-01

    Fjord and continental shelf environments in the polar regions are host to some of the planet's most productive ecosystems and support economically important fisheries. Their productivity, however, is often critically dependent upon nutrient supply from upstream terrestrial environments delivered via river systems. In glacially fed coastal ecosystems, riverine nutrients are largely sourced from melting snow and ice. The largest and most extensive glacially fed coastal ecosystem in the Arctic is that bordering the Greenland Ice Sheet. The future primary productivity of this ecosystem, however, is uncertain. A potential increase in primary productivity driven by reduced sea ice extent and associated increased light levels may be curtailed by insufficient nutrient supply, and specifically nitrogen. Research on small valley glaciers indicates that glaciers are important sources of nitrogen to downstream environments. However, no data exist from ice sheet systems such as Greenland. Time series of nitrogen concentrations in runoff are documented from a large Greenland glacier, demonstrating seasonally elevated fluxes to the ocean. Fluxes are highest in mid-summer, when nitrogen limitation is commonly reported in coastal waters. It is estimated that approximately half of the glacially exported nitrogen is sourced from microbial activity within glacial sediments at the surface and bed of the ice sheet, doubling nitrogen fluxes in runoff. Summer dissolved inorganic nitrogen fluxes from the Greenland Ice Sheet (30-40 Gg) are a similar order of magnitude to those from a large Arctic river (Holmes et al., 2012). Nitrogen yields from the ice sheet (236 kg TDN km-2 a-1), however, are approximately double those from Arctic riverine catchments. We assert that this ice sheet nitrogen subsidy to Arctic coastal ecosystems may be important for understanding coastal biodiversity, productivity and fisheries and should be considered in future biogeochemical modelling studies of coastal

  12. 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.

  13. 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.

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

    NASA Astrophysics Data System (ADS)

    McMillan, Malcolm; Shepherd, Andrew; Roca, Monica; Escorihuela, Maria Jose; Thibaut, Pierre; Remy, Frederique; Escola, Roger; Benveniste, Jerome; Ambrozio, Americo; Restano, Marco

    2016-04-01

    Since the launch of ERS-1 in 1991, polar-orbiting satellite radar altimeters have provided a near continuous record of ice sheet elevation change, yielding estimates of ice sheet mass imbalance at the scale of individual ice sheet basins. One of the principle challenges associated with radar altimetry comes from the relatively large ground footprint of conventional pulse-limited radars, which limits their capacity to make reliable 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 heralded the start of a new era of SAR altimetry, although full SAR coverage of the polar ice sheets will only be achieved with the launch of the first Sentinel-3 satellite in January 2016. Because of the heritage of SAR altimetry provided by CryoSat, current SAR altimeter processing techniques have to some extent been optimized and evaluated for water and sea ice surfaces. This leaves several outstanding issues related to the development and evaluation of SAR altimetry for ice sheets, including improvements to SAR processing algorithms and SAR altimetry waveform retracking procedures. Here we will outline SPICE (Sentinel-3 Performance Improvement for Ice Sheets), a 2 year project which began in September 2015 and is funded by ESA's SEOM (Scientific Exploitation of Operational Missions) programme. This project aims to contribute to the development and understanding of ice sheet SAR altimetry through the emulation of Sentinel-3 data from dedicated CryoSat SAR acquisitions made at several sites in Antarctica. More specifically, the project aims to (1) evaluate and improve the current Delay-Doppler processing and SAR waveform retracking algorithms, (2) evaluate higher level SAR altimeter data, and (3) investigate radar wave interaction with the snowpack. We will provide a broad overview of

  15. Retreat history of the East Antarctic Ice Sheet since the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Mackintosh, Andrew N.; Verleyen, Elie; O'Brien, Philip E.; White, Duanne A.; Jones, R. Selwyn; McKay, Robert; Dunbar, Robert; Gore, Damian B.; Fink, David; Post, Alexandra L.; Miura, Hideki; Leventer, Amy; Goodwin, Ian; Hodgson, Dominic A.; Lilly, Katherine; Crosta, Xavier; Golledge, Nicholas R.; Wagner, Bernd; Berg, Sonja; van Ommen, Tas; Zwartz, Dan; Roberts, Stephen J.; Vyverman, Wim; Masse, Guillaume

    2014-09-01

    The East Antarctic Ice Sheet (EAIS) is the largest continental ice mass on Earth, and documenting its evolution since the Last Glacial Maximum (LGM) is important for understanding its present-day and future behaviour. As part of a community effort, we review geological evidence from East Antarctica that constrains the ice sheet history throughout this period (˜30,000 years ago to present). This includes terrestrial cosmogenic nuclide dates from previously glaciated regions, 14C chronologies from glacial and post-glacial deposits onshore and on the continental shelf, and ice sheet thickness changes inferred from ice cores and continental-scale ice sheet models. We also include new 14C dates from the George V Land - Terre Adélie Coast shelf. We show that the EAIS advanced to the continental shelf margin in some parts of East Antarctica, and that the ice sheet characteristically thickened by 300-400 m near the present-day coastline at these sites. This advance was associated with the formation of low-gradient ice streams that grounded at depths of >1 km below sea level on the inner continental shelf. The Lambert/Amery system thickened by a greater amount (800 m) near its present-day grounding zone, but did not advance beyond the inner continental shelf. At other sites in coastal East Antarctica (e.g. Bunger Hills, Larsemann Hills), very little change in the ice sheet margin occurred at the LGM, perhaps because ice streams accommodated any excess ice build up, leaving adjacent, ice-free areas relatively unaffected. Evidence from nunataks indicates that the amount of ice sheet thickening diminished inland at the LGM, an observation supported by ice cores, which suggest that interior ice sheet domes were ˜100 m lower than present at this time. Ice sheet recession may have started ˜18,000 years ago in the Lambert/Amery glacial system, and by ˜14,000 years ago in Mac.Robertson Land. These early pulses of deglaciation may have been responses to abrupt sea-level rise

  16. Ocean temperature thresholds for Last Interglacial West Antarctic Ice Sheet collapse

    NASA Astrophysics Data System (ADS)

    Sutter, Johannes; Gierz, Paul; Grosfeld, Klaus; Thoma, Malte; Lohmann, Gerrit

    2016-03-01

    The West Antarctic Ice Sheet (WAIS) is considered the major contributor to global sea level rise in the Last Interglacial (LIG) and potentially in the future. Exposed fossil reef terraces suggest sea levels in excess of 7 m in the last warm era, of which probably not much more than 2 m are considered to originate from melting of the Greenland Ice Sheet. We simulate the evolution of the Antarctic Ice Sheet during the LIG with a 3-D thermomechanical ice sheet model forced by an atmosphere-ocean general circulation model (AOGCM). Our results show that high LIG sea levels cannot be reproduced with the atmosphere-ocean forcing delivered by current AOGCMs. However, when taking reconstructed Southern Ocean temperature anomalies of several degrees, sensitivity studies indicate a Southern Ocean temperature anomaly threshold for total WAIS collapse of 2-3°C, accounting for a sea level rise of 3-4 m during the LIG. Potential future Antarctic Ice Sheet dynamics range from a moderate retreat to a complete collapse, depending on rate and amplitude of warming.

  17. Reconstruction of changes in the Weddell Sea sector of the Antarctic Ice Sheet since the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Hillenbrand, Claus-Dieter; Bentley, Michael J.; Stolldorf, Travis D.; Hein, Andrew S.; Kuhn, Gerhard; Graham, Alastair G. C.; Fogwill, Christopher J.; Kristoffersen, Yngve; Smith, James. A.; Anderson, John B.; Larter, Robert D.; Melles, Martin; Hodgson, Dominic A.; Mulvaney, Robert; Sugden, David E.

    2014-09-01

    The Weddell Sea sector is one of the main formation sites for Antarctic Bottom Water and an outlet for about one fifth of Antarctica's continental ice volume. Over the last few decades, studies on glacial-geological records in this sector have provided conflicting reconstructions of changes in ice-sheet extent and ice-sheet thickness since the Last Glacial Maximum (LGM at ca 23-19 calibrated kiloyears before present, cal ka BP). Terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the Weddell Sea, ice-sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding-line advance at the LGM. Other marine geological and geophysical studies concluded that subglacial bedforms mapped on the Weddell Sea continental shelf, subglacial deposits and sediments over-compacted by overriding ice recovered in cores, and the few available radiocarbon ages from marine sediments are consistent with major ice-sheet advance at the LGM. Reflecting the geological interpretations, different ice-sheet models have reconstructed conflicting LGM ice-sheet configurations for the Weddell Sea sector. Consequently, the estimated contributions of ice-sheet build-up in the Weddell Sea sector to the LGM sea-level low-stand of ˜130 m vary considerably. In this paper, we summarise and review the geological records of past ice-sheet margins and past ice-sheet elevations in the Weddell Sea sector. We compile marine and terrestrial chronological data constraining former ice-sheet size, thereby highlighting different levels of certainty, and present two alternative scenarios of the LGM ice-sheet configuration, including time-slice reconstructions for post-LGM grounding-line retreat. Moreover, we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research. The aim

  18. 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

  19. Global ice-sheet system interlocked by sea level

    NASA Astrophysics Data System (ADS)

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

    1986-07-01

    Denton and Hughes (1983, Quaternary Research20, 125-144) postulated that sea level linked a global ice-sheet system with both terrestrial and grounded marine components during late Quaternary ice ages. Summer temperature changes near Northern Hemisphere melting margins initiated sea-level fluctuations that controlled marine components in both polar hemispheres. It was further proposed that variations of this ice-sheet system amplified and transmitted Milankovitch summer half-year insolation changes between 45 and 75°N into global climatic changes. New tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the Antarctic Ice Sheet, thus fitting the concept of a globally interlocked ice-sheet system. But recent atmospheric modeling results ( Manabe and Broccoli, 1985, Journal of Geophysical Research90, 2167-2190) suggest that factors other than areal changes of the grounded Antarctic Ice Sheet strongly influenced Southern Hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres. Atmospheric carbon dioxide linked to high-latitude oceans is the most likely candidate ( Shackleton and Pisias, 1985, Atmospheric carbon dioxide, orbital forcing, and climate. In "The Carbon Cycle and Atmospheric CO 2: Natural Variations Archean to Present" (E. T. Sundquest and W. S. Broecker, Eds.), pp. 303-318. Geophysical Monograph 32, American Geophysical Union, Washington, D.C.), but another potential influence was high-frequency climatic oscillations (2500 yr). It is postulated that variations in atmospheric carbon dioxide acted through an Antarctic ice shelf linked to the grounded ice sheet to produce and terminate Southern Hemisphere ice-age climate. It is further postulated that Milankovitch summer insolation combined with a warm high-frequency oscillation caused marked recession of Northern Hemisphere ice-sheet melting margins and the North Atlantic polar front about 14,000 14C yr B.P. This

  20. Basal Melt Under the Interior of the Greenland Ice Sheet: Comparison of Models, Deep Ice Cores, and Radar Observations

    NASA Astrophysics Data System (ADS)

    Rezvanbehbahani, S.; Stearns, L. A.; van der Veen, C. J.

    2014-12-01

    Basal ice temperature is a critical boundary condition for ice sheet models. It modulates the basal melt rate and sliding conditions, and also affects the ice hardness which alters the deformational velocity. Therefore, in order to obtain reliable estimates on the future mass loss of the ice sheets using numerical models, basal ice temperature is of paramount importance. In this study, the basal temperature and basal melt rate under the Greenland Ice Sheet are estimated using the Robin temperature solution. The analytical Robin solution is obtained by solving the heat conservation equation for steady state conditions, assuming that advection and diffusion are significant only in the vertical direction. In this study, the sensitivity of the basal temperature obtained from the Robin solution to changes in input parameters, including changes in atmospheric conditions, ice thickness, and geothermal heat flux is tested. Although the Robin solution is frequently used in glaciology, there has been no quantitative study to estimate the effect of neglecting the horizontal advection on basal temperatures in regions of higher velocity. Here, a two-dimensional model is applied to quantify the effect of horizontal heat advection on basal temperatures. Overall, horizontal heat advection lowers the basal temperature except in regions where surface mass balance gradients are negative along the flow. Comparing the results from the 2D temperature model to the Robin solution along multiple flowlines of the Greenland Ice Sheet suggest that the horizontal heat advection alters the basal temperatures by less than 3°C up to 30-45% of the flow distance away from the ice divide; at greater distances this difference increases rapidly. All simulations using the Robin solution predict substantial basal melting under the northeast drainage basin of the ice sheet. Our 2D model results also show that because of the negative surface mass balance gradient, horizontal heat advection increases the

  1. 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.

  2. 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

  3. Drainage beneath ice sheets: groundwater-channel coupling, and the origin of esker systems from former ice sheets

    NASA Astrophysics Data System (ADS)

    Boulton, G. S.; Hagdorn, M.; Maillot, P. B.; Zatsepin, S.

    2009-04-01

    The nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic. An experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in Europe, which we regard as a fossil imprint of major longitudinal drainage channels. We conclude that the large-scale distribution and spacing of major eskers is consistent with the theory of groundwater control, in which esker spacing is partly controlled by the transmissivity of the bed. It is concluded that esker patterns reflect the large-scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet's dynamic regime. The theory is then used to deduce: basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat; patterns of palaeo-groundwater flow and head distribution; and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin. Major channel/esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet, although major dynamic events are demonstrably associated with major shifts in the hydraulic regime. Modelling suggests: that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations, with groundwater flow predominantly transverse to ice flow; that the circulation pattern has the potential to create large-scale anomalies in groundwater chemistry; and that the spacing of channels will change through the glacial cycle, influencing water pressures in stream tunnels, subglacial hydraulic gradients and effective pressure. If the latter is reduced sufficiently, it could trigger enhanced bed deformation, thus coupling drainage to ice sheet movement. It suggests the

  4. Global Geodetic Signatures of the Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    James, Thomas S.; Ivins, Erik R.

    1997-01-01

    Four scenarios of present day Antarctic ice sheet mass change are developed from comprehensive reviews of the available glaciological and oceanographic evidence. The gridded scenarios predict widely varying contributions to secular sea level change xi ranging from -1.1 to 0.45 mm/yr, and predict polar motion m and time-varying low-degree gravitational coefficients J(sub l) that differ significantly from earlier estimates. A reasonably linear relationship between the rate of sea level change from Antarctica xi(sub A) and the predicted Antarctic J(sub l) is found for the four scenarios. This linearity permits a series of forward models to be constructed that incorporate the effects of ice mass changes in Antarctica, Greenland, and distributed smaller glaciers, as well as postglacial rebound (assuming the ICE-3G deglaciation history), with the goal of obtaining optimum reconciliation between observed constraints on J(sub l) and sea level rise xi. Numerous viable combinations of lower mantle viscosity and hydrologic sources are found that safely "observed" in the range of 1 to 2-2.5 mm/yr and observed J(sub l) for degrees 2, 3, and 4. In contrast, rates of global sea level rise above 2.5 mm/yr are inconsistent with available J(sub l) observations. The successful composite models feature a pair of lower mantle viscosity solutions arising from the sensitivity of J(sub l) to glacial rebound. The paired values are well separated at xi = 1 mm/yr, but move closer together as xi is increased, and, in fact, merge around xi = 2 - 2.5 mm/yr, revealing an intimate relation between xi and preferred lower mantle viscosity. This general pattern is quite robust and persists for different J(sub l) solutions, for variations in source assumptions, and for different styles of lower mantle viscosity stratification. Tighter J(sub l) constraints for l greater than 2 may allow some viscosity stratification schemes and source assumptions to be excluded in the future. For a given total

  5. 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.

  6. 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.

  7. 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.

  8. Testing of SIR (a transformable robotic submarine) in Lake Tahoe for future deployment at West Antarctic Ice Sheet grounding lines of Siple Coast

    NASA Astrophysics Data System (ADS)

    Powell, R. D.; Scherer, R. P.; Griffiths, I.; Taylor, L.; Winans, J.; Mankoff, K. D.

    2011-12-01

    A remotely operated vehicle (ROV) has been custom-designed and built by DOER Marine to meet scientific requirements for exploring subglacial water cavities. This sub-ice rover (SIR) will explore and quantitatively document the grounding zone areas of the Ross Ice Shelf cavity using a 3km-long umbilical tether by deployment through an 800m-long ice borehole in a torpedo shape, which is also its default mode if operational failure occurs. Once in the ocean cavity it transforms via a diamond-shaped geometry into a rectangular form when all of its instruments come alive in its flight mode. Instrumentation includes 4 cameras (one forward-looking HD), a vertical scanning sonar (long-range imaging for spatial orientation and navigation), Doppler current meter (determine water current velocities), multi-beam sonar (image and swath map bottom topography), sub-bottom profiler (profile sub-sea-floor sediment for geological history), CTD (determine salinity, temperature and depth), DO meter (determine dissolved oxygen content in water), transmissometer (determine suspended particulate concentrations in water), laser particle-size analyzer (determine sizes of particles in water), triple laser-beams (determine size and volume of objects), thermistor probe (measure in situ temperatures of ice and sediment), shear vane probe (determine in situ strength of sediment), manipulator arm (deploy instrumentation packages, collect samples), shallow ice corer (collect ice samples and glacial debris), water sampler (determine sea water/freshwater composition, calibrate real-time sensors, sample microbes), shallow sediment corer (sample sea floor, in-ice and subglacial sediment for stratigraphy, facies, particle size, composition, structure, fabric, microbes). A sophisticated array of data handling, storing and displaying will allow real-time observations and environmental assessments to be made. This robotic submarine and other instruments will be tested in Lake Tahoe in September, 2011 and

  9. Ice Sheet Oscillations During the Last Deglaciation in Western Norway

    NASA Astrophysics Data System (ADS)

    Lohne, O. S.; Mangerud, J.; Svendsen, J.; Gyllencreutz, R.

    2009-12-01

    About 25-26ka cal BP, the Scandinavian Ice Sheet merged with the British Ice Sheet on the continental shelf in the northern part of the North Sea (Sejrup et al., 2009). The Norwegian Channel, that hosted a major ice stream, became finally deglaciated at around 18.5ka cal BP. The deglaciation of western Norway started first in the south and not later than 17-16 ka cal BP, the Jæren region was partly ice-free (Knudsen, 2006). The early deglaciation history in the Hardangerfjord-Bergen area was characterized by some rapid ice-margin oscillations. The extent and timing of the ice front variations are not well constrained, but probably they occurred sometime between 16-14.5ka cal BP. A main difficulty with the exact timing is the radiocarbon plateau around 12.3 ka 14C BP. Subsequently, during the Allerød, the ice sheet withdrew more than 50 km from the coastline, until it again started to re-advance. The recorded sea-level response of this re-advance indicates that the re-growth of the ice sheet started in the mid-Allerød, approximately at 13.6ka cal BP (Lohne et al., 2007), and it seems clear that the resulting ice sheet advance continued until the very end of the Younger Dryas when the prominent Herdla-Halsnøy Moraine was formed. In the 500-800 m deep Hardangerfjord the re-advance stopped at a bedrock threshold where the Halsnøy Moraine subsequently formed. As appear from the sediment stratigraphy in a lake basin on Halsnøy the advancing ice-margin reached the island slightly before the Vedde Ash fall (c. 12.1ka cal BP). The ice-front then halted on the bedrock sill, but stratigraphic evidence indicates that the fjord-glacier continued to grow in thickness. Marginal moraines from Halsnøy can be traced up to a level of about 1000 m a.s.l., 40 km further inland, reflecting a massive buildup of the ice sheet. The advancing ice-sheet reached its maximum position, both in the Bergen and Hardangerfjorden area, late in the YD. A more precise dating by radiocarbon is

  10. 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

  11. 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.

  12. A continuum mixture model of ice stream thermomechanics in the Laurentide Ice Sheet 1. Theory

    NASA Astrophysics Data System (ADS)

    Marshall, Shawn J.; Clarke, Garry K. C.

    1997-01-01

    We employ a continuum mixture framework to incorporate ice streams in a three-dimensional thermomechanical model of the Laurentide Ice Sheet. The ice mass is composed of a binary mixture of sheet ice, which deforms by viscous creep, and stream ice, which flows by sliding and/or sediment deformation at the bed. Dynamic and thermal evolutions are solved for each component in the mixture, with coupling rules to govern transfer between flow regimes. We describe two different transfer mechanisms: (1) creep exchange, the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet, and (2) bed exchange, the activation, growth, and deactivation of ice streams, perpetrated by transfers of bed area between flow constituents. This paper develops the underlying mixture theory. We express the governing equations for mass, momentum, and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics. A companion paper in this issue explores mixture and ice stream behavior in applications with the Laurentide Ice Sheet.

  13. Arctic greenhouse-gas storage and release modulated by late-glacial ice sheet fluctuations

    NASA Astrophysics Data System (ADS)

    Portnov, Alexey; Mienert, Jurgen; Vadakkepuliyambatta, Sunil; Patton, Henry; Andreassen, Karin; Winsborrow, Monica; Knies, Jochen; Hubbard, Alun I.

    2016-04-01

    The subglacial footprint of the Barents Sea Ice sheet which advanced across northern Eurasia from 26 to 22 ka BP had a major impact on the underlying gas hydrate stability zone (GHSZ) leading to storage of methane and other hydrocarbons. With the onset of deglaciation, these hydrocarbon rich hydrates dissociated, releasing potent greenhouse gas into the ocean and possibly atmosphere over a period of thousands of years. We present a wide-range of observational data acquired from offshore western Svalbard and the Barents Sea to robustly constrain a coupled model of the subglacial evolution of gas hydrate reservoirs during and after the Last Glacial Maximum (LGM). Our results indicate that even under minimum ice thickness reconstructions, an extensive, ~500-meter thick GHSZ existed beneath the ice sheet in our study area offshore of western Svalbard (Portnov et al., 2016). An offshore corridor of methane release did though also persist throughout maximum ice conditions on the upper continental margin. Throughout the LGM a marine ice sheet directly comparable to those of Greenland and Antarctica today inundated the continental margin offshore of western Svalbard and the vast shelf areas of the Barents Sea. However, with climatic amelioration the Barents Sea ice sheet experienced a 4ka period of dynamic retreat with concurrent flooding of the shelf by rising sea levels, which provided a high magnitude perturbation to the substrate pressure and temperature domains. By analogy, the future response of Polar ice sheets is an emerging concern as their ongoing thinning and retreat will likewise perturb the present day subglacial GHSZ leading to potential widespread gas hydrate destabilisation and release. Portnov, Alexey, et al. "Ice-sheet-driven methane storage and release in the Arctic", Nature Comm. DOI: 10.1038/ncomms10314. (2016).

  14. 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

  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. 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.

  17. Global warming and the growth of ice sheets

    SciTech Connect

    Ledley, T.S.; Chu, S.

    1994-01-01

    Recent research has suggested that warmer conditions, that may result from increased levels of CO{sub 2} in the atmosphere, may induce the growth of the Northern Hemisphere ice sheets through the impact of warmer temperature on the water carrying capacity of the atmosphere and thus on precipitation. In this study we examine this possibility using a coupled energy balance climate-thermodynamic sea ice model. Results indicate that if summer ice albedo is high enough, and there is some mechanism for initially maintaining ice through the summer season, then it may be possible to have ice sheet growth under the conditions of CO{sub 2} induced warming. 30 refs., 5 figs., 2 tabs.

  18. 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.

  19. Sensitivity and initialisation of a moving mesh ice sheet model

    NASA Astrophysics Data System (ADS)

    Bonan, Bertrand; Baines, Michael J.; Nichols, Nancy K.; Partridge, Dale

    2014-05-01

    One of the most important issues in marine ice sheet modelling is tracking accurately the evolution of the snout and the grounding line. Here we present a moving mesh method that is well-suited to tracking moving phenomena accurately. We study the behaviour of the method for a flow line version using the Shallow Ice Approximation. The solution procedure uses the conservation of mass fractions to define a deformation velocity that generates movement of the support of the ice. The ice thickness is recovered using the conservation principle. In order to initialise the moving mesh model, we apply advanced inverse data assimilation techniques to the system. We develop particularly an Ensemble Kalman Filter (EnKF) approach in this context. EnKF is an efficient Monte-Carlo method based on Gaussian assumptions. Contrary to variational methods, it does not require the development of the adjoint of the model. The data assimilation procedure treats both the mesh point positions and the ice sheet thickness as unknown state variables and updates both of these at each assimilation step. The advantage of the ensemble approach is that it enables the sensitivity of the system to be understood and, more importantly, provides information on the correlations between the variables, in particular between the grid and the ice thickness. We demonstrate the success of the technique for noisy, infrequent, partial measurements of ice thickness, both with and without noisy measurements of the terminus position. The moving mesh and EnKF methods can be extended to coupled models of grounded ice sheets and floating ice shelves. The covariances between the states of the system at the grounding line derived by the method will provide valuable information on the coupled system. The techniques presented here can also be extended to include the estimation of parameters, such as the basal sliding coefficient and the bedrock topography, and to two dimensional ice sheets.

  20. A simple equation for the melt elevation feedback of ice sheets

    NASA Astrophysics Data System (ADS)

    Levermann, Anders; Winkelmann, Ricarda

    2016-08-01

    In recent decades, the Greenland Ice Sheet has been losing mass and has thereby contributed to global sea-level rise. The rate of ice loss is highly relevant for coastal protection worldwide. The ice loss is likely to increase under future warming. Beyond a critical temperature threshold, a meltdown of the Greenland Ice Sheet is induced by the self-enforcing feedback between its lowering surface elevation and its increasing surface mass loss: the more ice that is lost, the lower the ice surface and the warmer the surface air temperature, which fosters further melting and ice loss. The computation of this rate so far relies on complex numerical models which are the appropriate tools for capturing the complexity of the problem. By contrast we aim here at gaining a conceptual understanding by deriving a purposefully simple equation for the self-enforcing feedback which is then used to estimate the melt time for different levels of warming using three observable characteristics of the ice sheet itself and its surroundings. The analysis is purely conceptual in nature. It is missing important processes like ice dynamics for it to be useful for applications to sea-level rise on centennial timescales, but if the volume loss is dominated by the feedback, the resulting logarithmic equation unifies existing numerical simulations and shows that the melt time depends strongly on the level of warming with a critical slowdown near the threshold: the median time to lose 10 % of the present-day ice volume varies between about 3500 years for a temperature level of 0.5 °C above the threshold and 500 years for 5 °C. Unless future observations show a significantly higher melting sensitivity than currently observed, a complete meltdown is unlikely within the next 2000 years without significant ice-dynamical contributions.

  1. 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

  2. Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration?

    NASA Astrophysics Data System (ADS)

    Glasser, N. F.; Jennings, S. J. A.; Hambrey, M. J.; Hubbard, B.

    2014-07-01

    Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr-1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

  3. The Greenland Ice Sheet as a hot spot of phosphorus weathering and export in the Arctic

    NASA Astrophysics Data System (ADS)

    Hawkings, Jon; Wadham, Jemma; Tranter, Martyn; Telling, Jon; Bagshaw, Elizabeth; Beaton, Alexander; Simmons, Sarah-Louise; Chandler, David; Tedstone, Andrew; Nienow, Peter

    2016-02-01

    The contribution of ice sheets to the global biogeochemical cycle of phosphorus is largely unknown, due to the lack of field data. Here we present the first comprehensive study of phosphorus export from two Greenland Ice Sheet glaciers. Our results indicate that the ice sheet is a hot spot of phosphorus export in the Arctic. Soluble reactive phosphorus (SRP) concentrations, up to 0.35 µM, are similar to those observed in Arctic rivers. Yields of SRP are among the highest in the literature, with denudation rates of 17-27 kg P km-2 yr-1. Particulate phases, as with nonglaciated catchments, dominate phosphorus export (>97% of total phosphorus flux). The labile particulate fraction differs between the two glaciers studied, with significantly higher yields found at the larger glacier (57.3 versus 8.3 kg P km-2 yr-1). Total phosphorus yields are an order of magnitude higher than riverine values reported in the literature. We estimate that the ice sheet contributes ~15% of total bioavailable phosphorus input to the Arctic oceans (~11 Gg yr-1) and dominates total phosphorus input (408 Gg yr-1), which is more than 3 times that estimated from Arctic rivers (126 Gg yr-1). We predict that these fluxes will rise with increasing ice sheet freshwater discharge in the future.

  4. The Influence of Earth structure on a coupled Antarctic ice sheet - sea level model

    NASA Astrophysics Data System (ADS)

    Gomez, N. A.; Pollard, D.; Holland, D. M.; Latychev, K.

    2014-12-01

    Earth structure beneath the Antarctic Ice Sheet is characterized by significant lateral variability. A stable, thick craton exists in the east, while the west is underlain by a large continental rift system and a relatively thin lithosphere. Moreover, high resolution seismic tomography indicates slow wave speeds in the shallow mantle below WAIS, suggesting a hot, low viscosity asthenosphere. Variations in viscoelastic Earth structure can impact predictions of relative sea-level change and present-day crustal deformation rates by: 1) altering the timing and geometry of load-induced Earth deformation; and 2) perturbing, via a sea-level feedback (Gomez et al., EPSL, 2013), the timing and extent of the ice-sheet retreat. In this talk we use a coupled ice sheet - sea level model to explore the sensitivity of predictions of glacial isostatic adjustment and ice-sheet evolution in the Antarctic region to variations in Earth model parameters. We begin with a large suite of simulations in which 1-D (depth dependent) viscosity structure is varied over ranges that capture depth profiles inferred beneath the West and East Antarctic. We also present a simulation that incorporate 3-D variations in lithospheric thickness and mantle viscosity. The calculations will focus both on the evolution of the region since the Last Glacial Maximum and on projections of future, climate change driven ice-sheet retreat.

  5. Greenland ice sheet melting during the last interglacial

    NASA Astrophysics Data System (ADS)

    Langebroek, Petra M.; Nisancioglu, Kerim H.

    2016-04-01

    During the last interglacial period (LIG) peak temperatures over Greenland were several degrees warmer than today. The Greenland ice sheet (GIS) retreated causing a global sea-level rise in the order of several meters. Large uncertainties still exist in the exact amount of melt and on the source location of this melt. Here we examine the GIS response to LIG temperature and precipitation patterns using the SICOPOLIS ice sheet model. The LIG climate was simulated by forcing the Norwegian Earth System Model (NorESM) with the appropriate greenhouse gases and orbital settings. The resulting LIG ice volume evolution strongly depends on the chosen value of uncertain model parameters for the ice sheet (e.g. basal sliding parameter, PDD factors, and atmospheric temperature lapse rate). We reduce the uncertainty by evaluating an ensemble of model results against present-day observations of ice sheet size, elevation and stability, together with paleo information from deep ice cores. We find a maximum GIS reduction equivalent to 0.8 to 2.2m of global sea-level rise. In this model set-up most of the melting occurs in southwestern Greenland.

  6. Assessing the links between Greenland Ice Sheet Surface Mass Balance and Arctic climate using Climate Models and Observations

    NASA Astrophysics Data System (ADS)

    Mottram, Ruth; Rodehacke, Christian; Boberg, Fredrik; Langen, Peter; Sloth Madsen, Marianne; Høyer Svendsen, Synne; Yang, Shuting; Hesselbjerg Christensen, Jens; Olesen, Martin

    2016-04-01

    Changes in different parts of the Arctic cryosphere may have knock-on effects on other parts of the system. The fully coupled climate model EC-Earth, which includes the ice sheet model PISM, is a useful tool to examine interactions between sea ice, ice sheet, ocean and atmosphere. Here we present results from EC-Earth experimental simulations that show including an interactive ice sheet model changes ocean circulation, sea ice extent and regional climate with, for example, a dampening of the expected increase in Arctic temperatures under the RCP scenarios when compared with uncoupled experiments. However, the relatively coarse resolution of the climate model likely influences the calculated surface mass balance forcing applied to the ice sheet model and it is important therefore to evaluate the model performance over the ice sheet. Here, we assess the quality of the climate forcing from the GCM to the ice sheet model by comparing the energy balance and surface mass balance (SMB) output from EC-Earth with that from a regional climate model (RCM) run at very high resolution (0.05 degrees) over Greenland. The RCM, HIRHAM5, has been evaluated over a wide range of climate parameters for Greenland which allows us to be confident it gives a representative climate forcing for the Greenland ice sheet. To evaluate the internal variability in the climate forcing, we compare simulations from HIRHAM5 forced with both the EC-Earth historical emissions and the ERA-Interim reanalysis on the boundaries. The EC-Earth-PISM RCP8.5 scenario is also compared with an EC-Earth run without an ice sheet to assess the impact of an interactive ice sheet on likely future changes. To account for the resolution difference between the models we downscale both EC-Earth and HIRHAM5 simulations with a simple offline energy balance model (EBM).

  7. CLIVAR Exchanges No. 62: Sea Level Rise, Ocean/Ice Shelf Interactions and Ice Sheets

    SciTech Connect

    Pirani, Anna; Danabasoglu, Gokhan; Griffies, Stephen; Marsland, Simon

    2013-08-01

    This special issue of CLIVAR Exchanges is devoted to presenting a selection of the science contributed by both speakers and poster presenters at the CLIVAR Workshop on Sea Level Rise, Ocean/Ice Shelf Interactions and Ice Sheets at CSIRO Marine and Atmospheric Research in Hobart, Australia, on 18-20 February 2013. The workshop brought together leading international scientists and early-career researchers from the ocean, ice-sheet, ice-shelf, and sea-level rise modelling and observational communities to explore the state-of-science and emerging pathways for development of the next generation of coupled climate models.

  8. 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.

  9. Simulating the response of glacial ice-sheets to past abrupt climate changes

    NASA Astrophysics Data System (ADS)

    Banderas, Rubén; Álvarez-Solas, Jorge; Robinson, Alexander; Montoya, Marisa

    2016-04-01

    Dansgaard-Oeschger (D/O) events were recurrent glacial abrupt climatic transitions between cold and warm conditions over Greenland with an approximate characteristic time of a thousand years. The uncertainties among the available sea level reconstructions hinder our understanding of the interactions between climate and global ice volume. In addition, only limited highly-resolved and continuous sea level records exist. Thus, the millennial time-scale response of glacial ice-sheets to past abrupt climate changes is not well known. Here, we use a hybrid ice sheet-ice shelf model in order to investigate the response of glacial ice-sheets to the influence of millennial-scale climate variability. An ensemble of simulations is performed by forcing the model with a wide range of time-varying climatologies derived from proxy data and from some of the currently available climate model simulations. The assessment of the resulting suite of transient simulations will contribute to constrain the inadequacies of sea level reconstructions in terms of amplitude and timing and will help to understand the implications of glacial abrupt climate changes in past sea level variability. Furthermore, our experiments could be useful to elucidate the mechanisms that involve the interactions between climate and ice sheets on millennial time scales, including future climate change.

  10. 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.

  11. An ice sheet model of reduced complexity for paleoclimate studies

    NASA Astrophysics Data System (ADS)

    Neff, Basil; Born, Andreas; Stocker, Thomas F.

    2016-04-01

    IceBern2D is a vertically integrated ice sheet model to investigate the ice distribution on long timescales under different climatic conditions. It is forced by simulated fields of surface temperature and precipitation of the Last Glacial Maximum and present-day climate from a comprehensive climate model. This constant forcing is adjusted to changes in ice elevation. Due to its reduced complexity and computational efficiency, the model is well suited for extensive sensitivity studies and ensemble simulations on extensive temporal and spatial scales. It shows good quantitative agreement with standardized benchmarks on an artificial domain (EISMINT). Present-day and Last Glacial Maximum ice distributions in the Northern Hemisphere are also simulated with good agreement. Glacial ice volume in Eurasia is underestimated due to the lack of ice shelves in our model. The efficiency of the model is utilized by running an ensemble of 400 simulations with perturbed model parameters and two different estimates of the climate at the Last Glacial Maximum. The sensitivity to the imposed climate boundary conditions and the positive degree-day factor β, i.e., the surface mass balance, outweighs the influence of parameters that disturb the flow of ice. This justifies the use of simplified dynamics as a means to achieve computational efficiency for simulations that cover several glacial cycles. Hysteresis simulations over 5 million years illustrate the stability of the simulated ice sheets to variations in surface air temperature.

  12. Greenland Ice sheet mass balance from satellite and airborne altimetry

    NASA Astrophysics Data System (ADS)

    Khan, S. A.; Bevis, M. G.; Wahr, J. M.; Wouters, B.; Sasgen, I.; van Dam, T. M.; van den Broeke, M. R.; Hanna, E.; Huybrechts, P.; Kjaer, K.; Korsgaard, N. J.; Bjork, A. A.; Kjeldsen, K. K.

    2013-12-01

    Ice loss from the Greenland Ice Sheet (GrIS) is dominated by loss in the marginal areas. Dynamic induced ice loss and its associated ice surface lowering is often largest close to the glacier calving front and may vary from rates of tens of meters per years to a few meters per year over relatively short distances. Hence, high spatial resolution data are required to accurately estimate volume changes. Here, we estimate ice volume change rate of the Greenland ice sheet using data from Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter during 2003-2009 and CryoSat-2 data during 2010-2012. To improve the volume change estimate we supplement the ICESat and CryoSat data with altimeter surveys from NASA's Airborne Topographic Mapper (ATM) during 2003-2012 and NASA's Land, Vegetation and Ice Sensor (LVIS) during 2007-2012. The Airborne data are mainly concentrated along the ice margin and therefore significantly improve the estimate of the total volume change. Furthermore, we divide the GrIS into six major drainage basins and provide volume loss estimates during 2003-2006, 2006-2009 and 2009-2012 for each basin and separate between melt induced and dynamic ice loss. In order to separate dynamic ice loss from melt processes, we use SMB values from the Regional Atmospheric Climate Model (RACMO2) and SMB values from a positive degree day runoff retention model (Janssens & Huybrechts 2000, Hanna et al. 2011 JGR, updated for this study). Our results show increasing SMB ice loss over the last decade, while dynamic ice loss increased during 2003-2009, but has since been decreasing. Finally, we assess the estimated mass loss using GPS observations from stations located along the edge of the GrIS and measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite gravity mission. Hanna, E., et al. (2011), Greenland Ice Sheet surface mass balance 1870 to 2010 based on Twentieth Century Reanalysis, and links with global climate forcing, J. Geophys. Res

  13. Modelling the Laurentide Ice Sheet using improved ice margin chronologies and glacio-isostatic observations

    NASA Astrophysics Data System (ADS)

    Gowan, Evan; Tregoning, Paul; Purcell, Anthony; Lambeck, Kurt

    2013-04-01

    Creating models of the Laurentide ice sheet is challenging, due to the deficiency of chronological constraints and the uneven spatial resolution of data to determine the evolution of the glacio-isostatic response after deglaciation. Previous models relied on uncalibrated radiocarbon constrained margins that proved to have deficiencies in recent studies. Additionally, many recent Laurentide ice sheet models have been developed by incorporating climatic parameters that are poorly resolved for the late glacial period. We present a new ice sheet model by an iterative process of changing basal shear stress values and ice sheet margin location. A particular focus of this study is to determine the thickness and extent of the western Laurentide ice sheet, where there were few well dated observations of glacio-isostatic motion until recently. The volume of an ice sheet during long periods depends mostly on basal shear stress and margin position, which are the main parameters that we vary to fit our model to glacio-isostatic observations. We build our ice model using the assumption of perfectly plastic, steady-state conditions, with variable basal shear stress. Basal shear stress values depend on the surficial geology underlying the ice, and are at a minimum in offshore regions that have soft, deformable sediments, and at a maximum in areas with exposed crystalline bedrock. This approach may not capture dynamic and short lived features of the ice sheet, such as ice streams and stagnant ice, but gives an approximation of average conditions to produce ice volumes that fit geophysical observations. We adjust the margin location when the shear stress conditions alone cannot account for the observed glacio-isostatic response. The constraints on the response include relative sea level benchmarks, sea level highstand positions and proglacial lakes. We repeat the analysis using different rheological profiles to determine the dependence the Earth model has on the estimation of ice

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

    NASA Astrophysics Data System (ADS)

    Benveniste, Jérôme; Escolà, Roger; Roca, Mònica; Ambrózio, Américo; Restano, Marco; McMillan, Malcolm; Escorihuela, Maria Jose; Shepherd, Andrew; Thibaut, Pierre; Remy, Frederique

    2016-07-01

    Since the launch of ERS-1 in 1991, polar-orbiting satellite radar altimeters have provided a near continuous record of ice sheet elevation change, yielding estimates of ice sheet mass imbalance at the scale of individual ice sheet basins. One of the principle challenges associated with radar altimetry comes from the relatively large ground footprint of conventional pulse-limited radars, which limits their capacity to make reliable 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 by the European Space Agency heralded the start of a new era of SAR altimetry, although full SAR coverage of the polar ice sheets will only be achieved with the launch of the first Sentinel-3 satellite in February 2016. Because of the heritage of SAR altimetry provided by CryoSat-2, current SAR altimeter processing techniques have been optimized and evaluated for water and sea ice surfaces. This leaves several outstanding issues related to the development and evaluation of SAR altimetry for ice sheets, including improvements to SAR processing algorithms and SAR altimetry waveform retracking procedures. Here we will present interim results from SPICE (Sentinel-3 Performance Improvement for Ice Sheets), a 2 year project that focuses on the expected performance of Sentinel-3 SAR altimetry over the Polar ice sheets. The project, which began in September 2015 and is funded by ESA's SEOM (Scientific Exploitation of Operational Missions) programme, aims to contribute to the development and understanding of ice sheet SAR altimetry through the emulation of Sentinel-3 data from dedicated CryoSat SAR acquisitions made at several sites in Antarctica and Greenland. More specifically, the project aims to (1) evaluate and improve the current Delay-Doppler processing and SAR waveform retracking

  15. The response of the West Antarctic Ice Sheet to ocean warming beneath the Filchner Ronne Ice Shelf

    NASA Astrophysics Data System (ADS)

    Goeller, Sebastian; Timmermann, Ralph; Thoma, Malte

    2015-04-01

    The ice flow at the margins of the West Antarctic Ice Sheet (WAIS) is moderated by large ice shelves. Their buttressing effect substantially controls the mass balance of the WAIS and thus its contribution to sea level rise. The stability of these ice shelves results from the balance of mass gain by accumulation and ice flow from the adjacent ice sheet and mass loss by calving and basal melting due to the ocean heat flux. Recent results of ocean circulation models indicate that warm circumpolar water of the Southern Ocean may override the submarine slope front of the Antarctic Continent and boost basal ice shelf melting. In particular, ocean simulations for several of the IPCC's future climate scenarios demonstrate the redirection of a warm coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf (FRIS) within the next decades. In this study, we couple the finite elements ocean circulation model FESOM and the three-dimensional thermomechanical ice flow model RIMBAY to investigate the sensitivity of the ice dynamics within the entire FRIS catchment to simulated future basal shelf melt rates. Our simulations indicate a high sensitivity of the ice dynamics for the Möller and the Institute Ice Stream but only very little response of other ice streams like Rutford, Foundation and Recovery Ice Stream to enhanced basal shelf melting. The grounding line between the Möller and Institute Ice Streams is located on a submarine ridge in front of a deep trough further inland. In this area, basal shelf melting causes a local thinning of the FRIS. The consequent initial retreat of the grounding line continues once it reaches the adjacent reverse-sloped bedrock. We state, that a possible 'point of no return' for a vast grounding line retreat along this steep reverse bedrock slope might have been crossed already even for simulated present-day melt rates, indicating that the WAIS is currently not in equlibrium. Furthermore, our simulations show an

  16. 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.

  17. Younger Dryas interval and outflow from the Laurentide ice sheet

    USGS Publications Warehouse

    Moore, T.C.; Walker, J.C.G.; Rea, David K.; Lewis, C.F.M.; Shane, L.C.K.; Smith, A.J.

    2000-01-01

    A boxmodel of the Great Lakes is used to estimate meltwater flow into the North Atlantic between 8000 and 14,000 calendar years B.P. Controls on the model include the oxygen isotopic composition of meltwaters and lake waters as measured in the shells of ostracodes. Outflow rates are highest when oxygen isotopic values of the lake waters are most negative, denoting a maximum glacial meltwater component. Flow rates reach maximum values before the onset of the Younger Dryas and after it ends. These maxima appear to be correlative with the major meltwater pulses MWP 1A and 1B. Although the resumption of North Atlantic Deep Water formation may be tied to the reduction in ice sheet melting, neither the onset nor the end of the Younger Dryas, as recorded in the Greenland Ice Sheet Project (GISP2) records, appear tied to maxima in meltwater outflow from the Laurentide ice sheet. Copyright 2000 by the American Geophysical Union.

  18. 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.

  19. 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; 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; Sorensen, Louise Sandberg; Scambos, Ted A.; Yi, Dohngui; Zwally, H. Jay

    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.

  20. Miocene to recent ice elevation variations from the interior of the West Antarctic ice sheet: Constraints from geologic observations, cosmogenic nuclides and ice sheet modeling

    NASA Astrophysics Data System (ADS)

    Mukhopadhyay, Sujoy; Ackert, Robert P.; Pope, Allen E.; Pollard, David; DeConto, Robert M.

    2012-07-01

    Observations of long-term West Antarctic Ice Sheet (WAIS) behavior can be used to test and constrain dynamic ice sheet models. Long-term observational constraints are however, rare. Here we present the first constraints on long-term (Miocene-Holocene) WAIS elevation from the interior of the ice sheet near the WAIS divide. We use geologic observations and measurements of cosmogenic 21Ne and 10Be in bedrock surfaces to constrain WAIS elevation variations to <160 m above the present-day ice levels since 7 Ma, and <110 m above present-day ice levels since 5.4 Ma. The cosmogenic nuclide data indicate that bedrock surfaces 35 m above the present-day ice levels had near continuous exposure over the past 3.5 Ma, requiring average interior WAIS elevations to have been similar to, or lower than present, since the beginning of the Pliocene warm period. We use a continental ice sheet model to simulate the history of ice cover at our sampling sites and thereby compute the expected concentration of the cosmogenic nuclides. The ice sheet model indicates that during the past 5 Ma interior WAIS elevations of >65 m above present-day ice levels at the Ohio Range occur only rarely during brief ice sheet highstands, consistent with the observed cosmogenic nuclide data. Furthermore, the model's prediction that highstand elevations have increased on average since the Pliocene is in good agreement with the cosmogenic nuclide data that indicate the highest ice elevation over the past 5 Ma was reached during the highstand at 11 ka. Since the simulated cosmogenic nuclide concentrations derived from the model's ice elevation history are in good agreement with our measurements, we suggest that the model's prediction of more frequent collapsed-WAIS states and smaller WAIS volumes during the Pliocene are also correct.

  1. 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.

  2. Pan–ice-sheet glacier terminus change in East Antarctica reveals sensitivity of Wilkes Land to sea-ice changes

    PubMed Central

    Miles, Bertie W. J.; Stokes, Chris R.; Jamieson, Stewart S. R.

    2016-01-01

    The dynamics of ocean-terminating outlet glaciers are an important component of ice-sheet mass balance. Using satellite imagery for the past 40 years, we compile an approximately decadal record of outlet-glacier terminus position change around the entire East Antarctic Ice Sheet (EAIS) marine margin. We find that most outlet glaciers retreated during the period 1974–1990, before switching to advance in every drainage basin during the two most recent periods, 1990–2000 and 2000–2012. The only exception to this trend was in Wilkes Land, where the majority of glaciers (74%) retreated between 2000 and 2012. We hypothesize that this anomalous retreat is linked to a reduction in sea ice and associated impacts on ocean stratification, which increases the incursion of warm deep water toward glacier termini. Because Wilkes Land overlies a large marine basin, it raises the possibility of a future sea level contribution from this sector of East Antarctica. PMID:27386519

  3. Pan-ice-sheet glacier terminus change in East Antarctica reveals sensitivity of Wilkes Land to sea-ice changes.

    PubMed

    Miles, Bertie W J; Stokes, Chris R; Jamieson, Stewart S R

    2016-05-01

    The dynamics of ocean-terminating outlet glaciers are an important component of ice-sheet mass balance. Using satellite imagery for the past 40 years, we compile an approximately decadal record of outlet-glacier terminus position change around the entire East Antarctic Ice Sheet (EAIS) marine margin. We find that most outlet glaciers retreated during the period 1974-1990, before switching to advance in every drainage basin during the two most recent periods, 1990-2000 and 2000-2012. The only exception to this trend was in Wilkes Land, where the majority of glaciers (74%) retreated between 2000 and 2012. We hypothesize that this anomalous retreat is linked to a reduction in sea ice and associated impacts on ocean stratification, which increases the incursion of warm deep water toward glacier termini. Because Wilkes Land overlies a large marine basin, it raises the possibility of a future sea level contribution from this sector of East Antarctica.

  4. 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.

  5. Oxygen Isotope Mass-Balance Constraints on Pliocene Sea Level and East Antarctic Ice Sheet Stability

    NASA Astrophysics Data System (ADS)

    Winnick, M. J.; Caves, J. K.

    2015-12-01

    The mid-Pliocene Warm Period (MPWP, 3.3-2.9 Ma), with reconstructed atmospheric pCO2 of 350-450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 ± 10 m above modern, requiring total loss of the Greenland (GIS) and marine West Antarctic Ice Sheets (WAIS) and a substantial loss of the East Antarctic Ice Sheet (EAIS), with only a concurrent 2-3 ºC rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (δ18Ob) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the δ18O of Antarctic ice (δ18Oi) was the same in the Pliocene as it is today, and while the sensitivity of δ18Ob to changing meltwater δ18O has been previously considered, these analyses neglect conservation of 18O/16O in the ocean-ice system. Using well-calibrated δ18O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the δ18Oi of the Pliocene Antarctic ice sheet was at minimum 1‰-4‰ higher than present. Assuming conservation of 18O/16O in the ocean-ice system, this requires lower Pliocene seawater δ18O (δ18Osw) without a corresponding change in ice sheet mass. This effect alone accounts for 5%-20% of the δ18Ob difference between the MPWP interglacials and the modern. With this amended isotope budget, we suggest that Pliocene GMSL was likely 9-13.5 m and very likely 5-17 m above modern, which suggests the EAIS is less sensitive to radiative forcing than previously inferred from the geologic record.

  6. Modeling Abrupt Change in Global Sea Level Arising from Ocean - Ice-Sheet Interaction

    SciTech Connect

    Holland, David M

    2011-09-24

    It is proposed to develop, validate, and apply a coupled ocean ice-sheet model to simulate possible, abrupt future change in global sea level. This research is to be carried out collaboratively between an academic institute and a Department of Energy Laboratory (DOE), namely, the PI and a graduate student at New York University (NYU) and climate model researchers at the Los Alamos National Laboratory (LANL). The NYU contribution is mainly in the area of incorporating new physical processes into the model, while the LANL efforts are focused on improved numerics and overall model development. NYU and LANL will work together on applying the model to a variety of modeling scenarios of recent past and possible near-future abrupt change to the configuration of the periphery of the major ice sheets. The project's ultimate goal is to provide a robust, accurate prediction of future global sea level change, a feat that no fully-coupled climate model is currently capable of producing. This proposal seeks to advance that ultimate goal by developing, validating, and applying a regional model that can simulate the detailed processes involved in sea-level change due to ocean ice-sheet interaction. Directly modeling ocean ice-sheet processes in a fully-coupled global climate model is not a feasible activity at present given the near-complete absence of development of any such causal mechanism in these models to date.

  7. AMOC projections driven by global warming and Greenland Ice Sheet melt

    NASA Astrophysics Data System (ADS)

    Bakker, Pepijn; Schmittner, Andreas; Lenaerts, Jan; Abe-Ouchi, Ayako; Bi, Dave; van den Broeke, Michiel; Hu, Aixue; Beadling, Rebecca Lynn; Marsland, Simon; Mernhild, Sebastian H.; Ohgaito, Rumi; Rodehacke, Christian; Saenko, Oleg; Swingedouw, Didier; Yang, Shuting; Yin, Jianjun

    2016-04-01

    The evolution of the Atlantic meridional overturning circulation (AMOC) is one of the key uncertainties of future climate projections. State-of-art climate models that took part in the CMIP5 project show that over the 21st century the AMOC might reduce by 20-30% under the intermediate RCP4.5 scenario and by 36-44% under the high end RCP8.5 scenario relative to preindustrial values. However, these projections neglect enhanced meltwater input from the Greenland Ice Sheet and lack a thorough uncertainty assessment. We present results of a community effort to use state-of-the-science climate models to simulate the impact of the partial melt of the Greenland Ice Sheet on the AMOC under future global warming up to the year 2300 (AMOCMIP). A probabilistic uncertainty assessment is presented based on a physics-based AMOC emulator and includes uncertainties in the AMOC's sensitivity to temperature and salinity changes, as well as uncertainties of future global warming, regional temperature amplification and melt rates of the Greenland Ice Sheet. We find that the impact of increased Greenland Ice Sheet melt on the AMOC strength is non-negligible, albeit strongly model dependent. The uncertainty analysis shows that the chance of an collapse of the AMOC is negligible if global temperature change remains below 2°C, but becomes more probable for larger warming.

  8. A Smoothed Particle Hydrodynamics Model for Ice Sheet and Ice Shelf Dynamics

    SciTech Connect

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

    2012-02-08

    Mathematical modeling of ice sheets is complicated by the non-linearity of the governing equations and boundary conditions. Standard grid-based methods require complex front tracking techniques and have limited capability to handle large material deformations and abrupt changes in bottom topography. As a consequence, numerical methods are usually restricted to shallow ice sheet and ice shelf approximations. We propose a new smoothed particle hydrodynamics (SPH) model for coupled ice sheet and ice shelf dynamics. SPH is a fully Lagrangian particle method. It is highly scalable and its Lagrangian nature and meshless discretization are well suited to the simulation of free surface flows, large material deformation, and material fragmentation. In this paper SPH is used to study ice sheet/ice shelf behavior, and the dynamics of the grounding line. The steady state position of the grounding line obtained from the SPH simulations is in good agreement with laboratory observations for a wide range of simulated bedrock slopes, and density ratios similar to those of ice and sea water. The numerical accuracy of the SPH algorithm is further verified by simulating the plane shear flow of two immiscible fluids and the propagation of a highly viscous blob of fluid along a horizontal surface. In the experiment, the ice was represented with a viscous newtonian fluid. For consistency, in the described SPH model the ice is also modeled as a viscous newtonian fluid. Typically, ice sheets are modeled as a non-Newtonian fluid, accounting for the changes in the mechanical properties of ice. Implementation of a non-Newtonian rheology in the SPH model is the subject of our ongoing research.

  9. 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

  10. 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.

  11. Discussing Progress in Understanding Ice Sheet-Ocean Interactions

    NASA Astrophysics Data System (ADS)

    Herraiz Borreguero, Laura; Mottram, Ruth; Cvijanovic, Ivana

    2010-11-01

    Advanced Climate Dynamics Course 2010: Ice Sheet-Ocean Interactions; Lyngen, Norway, 8-19 June 2010; Sea level rise is one of many expected consequences of climate change, with accompanying complex social and economic challenges. Major uncertainties in sea level rise projections relate to the response of ice sheets to sea level rise and the key role that interactions with the ocean may play. Recognizing that probably no comprehensive curriculum currently exists at any single university that covers this novel and interdisciplinary subject, the Advanced Climate Dynamics Courses (ACDC) team brought together a group of 40 international students, postdocs, and lecturers from diverse backgrounds to provide an overview and discussion of state-of-the-art research into ocean-ice sheet interactions and to propose research priorities for the next decade. Among the key issues addressed were small-scale processes near the Antarctic ice shelves and Greenland outlet glaciers. These are fast changing components in the climate system, often related to large-scale forcings (atmospheric teleconnections and oceanic circulation). Progress in understanding and modeling is hampered by the range of scales involved, the lack of observations, and the difficulties in constraining, initializing, and providing adequate boundary conditions for ice sheet and ocean models.

  12. Changes in the velocity structure of the Greenland Ice Sheet.

    PubMed

    Rignot, Eric; Kanagaratnam, Pannir

    2006-02-17

    Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66 degrees north between 1996 and 2000, which rapidly expanded to 70 degrees north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase.

  13. 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

  14. Analysis and retracking of continental ice sheet radar altimeter waveforms

    NASA Technical Reports Server (NTRS)

    Martin, T. V.; Brenner, A. C.; Zwally, H. J.; Bindschadler, R. A.

    1983-01-01

    The Seasat-1 radar altimeter data set acquired over both the Antarctic and Greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface. The radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans. Causal factors identified in this complicated response include sloping surfaces, undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam-limited footprint, off-track reflections, and dynamic lag of the altimeter tracking circuit. Retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters. The entire set of Seasat-1 altimetry over the continental ice sheets is being retracked by fitting a multi-parameter function to each waveform. Many waveforms have double ramps indicating near-normal reflections from two distinct portions of the ice surface within the altimeter beam. Two independent range measurements differing by less than 25 m are obtained from retracking the double-ramp waveforms.

  15. Seasonal ice flow patterns as indicators of subglacial hydrology on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Moon, T. A.; Fahnestock, M. A.; Scambos, T.; Joughin, I.; van den Broeke, M.; Klinger, M. J.

    2015-12-01

    Improvements in satellite coverage of the Greenland Ice Sheet have supported a substantial increase in the spatial and temporal resolution of surface velocity measurements. Previously, with seasonal TerraSAR-X satellite measurements of marine-terminating glaciers across the western and southeastern coasts, we identified three dominant and distinct seasonal velocity patterns. Two patterns likely indicate differences in the development of the subglacial hydrologic system, one suggesting development of efficient subglacial drainage during the summer melt season and the other without. Using this framework, we are now taking advantage of a new velocity record we created from Landsat 8 OLI imagery, which allows for better sampling across space and time, to examine local and regional variations in ice sheet surface velocity. Readily measurable, ice velocity holds strong potential as a proxy for understanding changes in subglacial hydrology, which is much more difficult to observe. We investigate seasonal velocity behavior from glacier termini toward the ice sheet interior and among separate glacier systems across the Greenland Ice Sheet as a way to understand changes in ice motion and ice sheet hydrology.

  16. 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

  17. Susceptibility of the Antarctic Ice Sheet to Changes in Ice Shelf Buttressing

    NASA Astrophysics Data System (ADS)

    Fürst, J. J.; Durand, G.; Gillet-chaulet, F.; Tavard, L.; Gagliardini, O.

    2014-12-01

    Higher surface air temperatures over the Antarctic Peninsula are hypothesised to have caused melt-pond formation, destabilisation and sudden disintegration of the Larsen B ice shelf in 2002. The almost total removal of the shelf resulted in an acceleration of the extant glacier fronts, upstream thinning and unabated ice loss up to this day. Similar thinning is observed for Thwaites and Pine Island Glaciers in the Amundsen Sea sector, but here, ocean warming is suspected for enhancing the shelf melting. In both cases, shelf geometries were altered in a way that upstream buttressing was reduced, an explanation for the observed accelerations. Since more than half of all Antarctic glaciers extend into floating shelves and since most of them showed no significant accelerations in the recent past, it remains unclear how susceptible the upstream ice sheet is to geometric changes of the corresponding shelves under further warming in the future. In this context, we aim at quantifying the dynamic susceptibility using ice geometry and surface velocities inferred from observations. To obtain the stress distribution near the grounding line, the shelf viscosity field is determined using a variational inverse method that optimises the mismatch between observed and modelled surface velocities. This allows us to compute a buttressing factor along the grounding line. Using this factor as one criterion, we succeed to a priori discern the segments of the grounding line in the Amundsen Sea sector that, in fact, retreated by now. An abrupt drop-off in buttressing across the main trunk of Thwaites Glacier can explain its asymmetric retreat pattern. Moreover, other regions in this sector are recognised as susceptible to further loss of shelf buttressing, where, for now, perturbations remain too weak for a distinct migration of the grounding line. With the chosen criteria, we are able to localise the regions that are prone to changes in the downstream shelves. This identification enables

  18. Susceptibility of the Antarctic ice sheet to changes in ice shelf buttressing

    NASA Astrophysics Data System (ADS)

    Fürst, Johannes J.; Durand, Gaël; Gillet-Chaulet, Fabien; Tavard, Laure; Gagliardini, Olivier

    2015-04-01

    Higher surface air temperatures over the Antarctic Peninsula are hypothesised to have caused melt-pond formation, destabilisation and sudden disintegration of the Larsen B ice shelf in 2002. The almost total removal of the shelf resulted in an acceleration of the extant glacier fronts, upstream thinning and unabated ice loss up to this day. Similar thinning is observed for Thwaites and Pine Island Glaciers in the Amundsen Sea sector, but here, ocean warming is suspected for enhancing the shelf melting. In both cases, shelf geometries were altered in a way that upstream buttressing was reduced, an explanation for the observed accelerations. Since more than half of all Antarctic glaciers extend into floating shelves and since most of them showed no significant accelerations in the recent past, it remains unclear how susceptible the upstream ice sheet is to geometric changes of the corresponding shelves under further warming in the future. In this context, we aim at quantifying the dynamic susceptibility using ice geometry and surface velocities inferred from observations. To obtain the stress distribution near the grounding line, the shelf viscosity field is determined using a variational inverse method that optimises the mismatch between observed and modelled surface velocities. This allows us to compute a buttressing factor along the grounding line. Using this factor as one criterion, we succeed to a priori discern the segments of the grounding line in the Amundsen Sea sector that, in fact, retreated by now. An abrupt drop-off in buttressing across the main trunk of Thwaites Glacier can explain its asymmetric retreat pattern. Moreover, other regions in this sector are recognised as susceptible to further loss of shelf buttressing, where, for now, perturbations remain too weak for a distinct migration of the grounding line. With the chosen criteria, we are able to localise the regions that are prone to changes in the downstream shelves. This identification enables

  19. Feedbacks between ice and ocean dynamics at the West Antarctic Filchner-Ronne Ice Shelf in future global warming scenarios

    NASA Astrophysics Data System (ADS)

    Goeller, Sebastian; Timmermann, Ralph

    2016-04-01

    The ice flow at the margins of the West Antarctic Ice Sheet is moderated by large ice shelves. Their buttressing effect substantially controls the mass balance of the WAIS and thus its contribution to sea level rise. The stability of these ice shelves results from the balance of mass gain by accumulation and ice flow from the adjacent ice sheet and mass loss by calving and basal melting due to the ocean heat flux. Recent results of ocean circulation models indicate that warm circumpolar water of the Southern Ocean may override the submarine slope front of the Antarctic Continent and boost basal ice shelf melting. In particular, ocean simulations for several of the IPCC's future climate scenarios demonstrate the redirection of a warm coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf within the next decades. In this study, we couple the finite elements ocean circulation model FESOM and the three-dimensional thermomechanical ice flow model RIMBAY to investigate the complex interactions between ocean and ice dynamics at the Filchner-Ronne Ice Shelf. We focus on the impact of a changing ice shelf cavity on ocean dynamics as well as the feedback of the resulting sub-shelf melting rates on the ice shelf geometry and implications for the dynamics of the adjacent marine-based Westantarctic Ice Sheet. Our simulations reveal the high sensitivity of grounding line migration to ice-ocean interactions within the Filchner-Ronne Ice Shelf and emphasize the importance of coupled model studies for realistic assessments of the Antarctic mass balance in future global warming scenarios.

  20. Ice-sheet influences on global Monsoon systems (Invited)

    NASA Astrophysics Data System (ADS)

    Timmermann, A.; Elison Timm, O.; Friedrich, T.; Abe-Ouchi, A.; Menviel, L.; Tigchelaar, M.

    2013-12-01

    The waxing and waning of the northern Hemisphere ice-sheets on orbital and millennial timescales and corresponding changes in atmospheric and oceanic circulation played an essential role in modulating monsoon systems globally. Here we review the mechanisms by which changes in ice-sheet orography, global sea-level and freshwater input into the North Atlantic can influence global wind patterns and tropical moisture convergence. Our analysis is based on a series of transient model simulations conducted with the newly developed 3-dimensional coupled ice-sheet-climate model iLOVE. Forced by orbital and greenhouse gas concentrations over the past 80 ka, this model realistically simulates the evolution of Northern Hemisphere ice volume. It is demonstrated that orbital-scale changes in ice-sheet orography influence the South American and African Monsoons, but leave Asian Monsoon systems relatively unaltered. On millennial timescales the situation is very different. Freshwater forcing from calving ice-sheets causes variations of the thermohaline circulation, North Atlantic sea surface temperatures and global wind patterns. Using an earth system model hindcast for the period 30-50 ka in combination with high-resolution hydroclimate proxies, we demonstrate that this mechanism can explain for the bulk of MIS3 global Monsoon variability on millennial-timescales. In addition to these remote influences, rainfall intensity in the dominant Monsoon regions is also modulated by precessional forcing and corresponding shifts of the meridional surface temperature gradients. This presentation will conclude with a brief discussion of gaps in our understanding of how orbital forcing affected Monsoons and Intertropical Convergence Zones during the Pleistocene.

  1. 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

  2. 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; Gogineni, S. Prasad; Leuschen, Carl; McConnell, Joseph R.

    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.

  3. Surface Drifters Track the Fate of Greenland Ice Sheet Meltwater

    NASA Astrophysics Data System (ADS)

    Hauri, Claudine; Truffer, Martin; Winsor, Peter; Lennert, Kunuk

    2014-07-01

    Understanding the fate and influence of glacial meltwater in heavily ice-covered fjord systems has proven difficult because previous measurement platforms were con­strained to deeper water to keep instrumentation safe from drifting icebergs. Now, using novel, satellite-tracked devices that can with­ stand multiple collisions with ice blocks (see Figure 1) without incurring much damage, scientists have obtained new and detailed data about the role of Greenland Ice Sheet meltwater and its trajectories through God­thåbsfjord in western Greenland.

  4. 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.

  5. Glaciers and ice sheets as a biome.

    PubMed

    Anesio, Alexandre M; Laybourn-Parry, Johanna

    2012-04-01

    The tundra is the coldest biome described in typical geography and biology textbooks. Within the cryosphere, there are large expanses of ice in the Antarctic, Arctic and alpine regions that are not regarded as being part of any biome. During the summer, there is significant melt on the surface of glaciers, ice caps and ice shelves, at which point microbial communities become active and play an important role in the cycling of carbon and other elements within the cryosphere. In this review, we suggest that it is time to recognise the cryosphere as one of the biomes of Earth. The cryospheric biome encompasses extreme environments and is typified by truncated food webs dominated by viruses, bacteria, protozoa and algae with distinct biogeographical structures.

  6. A statistical-dynamical approach to represent Greenland ocean-ice sheet interactions

    NASA Astrophysics Data System (ADS)

    Perrette, Mahé; Calov, Reinhard; Ganopolski, Andrey; Robinson, Alex

    2013-04-01

    An understanding of the dynamics of the Greenland ice sheet is fundamental, because of its potential to contribute strongly to future sea level rise. In recent years there has been a discussion about the role of the ocean in the Greenland ice sheet's present and future mass balance. The ocean interacts with the ice sheet's outlet glaciers via the water circulation in the fjords and considerably affects melting at the termini of the outlet glaciers. Processes related to this interaction are difficult to represent in Greenland-wide ice-sheet models because grid resolution of such models is typically 10 km, whereas large fjords are more commonly only 1 to 5 km wide. Local refinement techniques (e.g. finite elements with adaptive mesh) can be a way of addressing that problem but are still computationally expensive to run. Here we propose a simpler, statistical-dynamical approach suited for large ensemble simulations over 100- to 1000-year integration times, in the EMIC spirit: the fjord-outlet glacier system is restricted to its most fundamental dynamics, controlled by a handful of parameters describing the major characteristics of the system. The model has a generic structure, i.e., it is designed such that it applies to every Greenland outlet glacier. Some of its parameters are fixed by using the (little) available observational data - e.g. for Helheim, Kangerdlugssuaq and Jakobshavn Isbrae - other parameters may vary depending on location. It is not our aim to simulate every single small outlet glacier in its full accuracy; but we aim to represent, on average, important characteristics like ice discharge and general advance/retreat rate on a regional scale over major catchment areas. Aspects of the coupling strategy with the 3D ice-sheet model (SICOPOLIS) are discussed, e.g., critical issues such as the treatment of mass balance. Preliminary design and results will be presented.

  7. Smoothed particle hydrodynamics non-Newtonian model for ice-sheet and ice-shelf dynamics

    SciTech Connect

    Pan, W.; Tartakovsky, A. M.; Monaghan, J. J.

    2013-06-01

    Mathematical modeling of ice sheets is complicated by the non-linearity of the governing equations and boundary conditions. Standard grid-based methods require complex front tracking techniques and have limited capability to handle large material deformations and abrupt changes in bottom topography. As a consequence, numerical methods are usually restricted to shallow ice sheet and ice shelf approximations. We propose a new smoothed particle hydrodynamics (SPH) non-Newtonian model for coupled ice sheet and ice shelf dynamics. SPH, a fully Lagrangian particle method, is highly scalable and its Lagrangian nature and meshless discretization are well suited to the simulation of free surface flows, large material deformation, and material fragmentation. In this paper, SPH is used to study 3D ice sheet/ice shelf behavior, and the dynamics of the grounding line. The steady state position of the grounding line obtained from SPH simulations is in good agreement with laboratory observations for a wide range of simulated bedrock slopes, and density ratios, similar to those of ice and sea water. The numerical accuracy of the SPH algorithm is verif;ed by simulating Poiseuille flow, plane shear flow with free surface and the propagation of a blob of ice along a horizontal surface. In the laboratory experiment, the ice was represented with a viscous Newtonian fluid. In the present work, however, the ice is modeled as both viscous Newtonian fluid and non-Newtonian fluid, such that the effect of non-Newtonian rheology on the dynamics of grounding line was examined. The non-Newtonian constitutive relation is prescribed to be Glen’s law for the creep of polycrystalline ice. A V-shaped bedrock ramp is further introduced to model the real geometry of bedrock slope.

  8. Antarctic Ice Sheet Sensitivity to Atmospheric CO2 Variations During the Early to Mid-Miocene

    NASA Astrophysics Data System (ADS)

    Levy, R. H.; Harwood, D. M.; Florindo, F.; Sangiorgi, F.; Eagle, R.; von Eynatten, H.; Gasson, E.; Kuhn, G.; Tripati, A.; Deconto, R. M.; Fielding, C. R.; Field, B.; Golledge, N. R.; Mckay, R. M.; Naish, T.; Olney, M.; Pollard, D.; Schouten, S.; Talarico, F. M.; Warny, S.; Willmott, V.

    2015-12-01

    The Early to mid-Miocene (23 to 14 million years ago) is a compelling interval to study Antarctic ice sheet sensitivity to changes in atmospheric CO2 as oceanic and atmospheric circulation patterns in the southern hemisphere were broadly similar to present and reconstructed atmospheric CO2 concentrations were analogous to those projected for the next several decades. This time interval includes the Miocene Climatic Optimum (MCO), a period of global warmth during which average surface temperatures were 3 to 4°C higher than today. Miocene sediments in the AND-2A drill core from the Western Ross Sea, Antarctica provide direct information regarding ice sheet variability through this key time interval and offer insight into the potential Antarctic contribution to future sea level rise. A multi-proxy dataset derived from AND-2A identifies four distinct environmental "motifs" based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of marine-based ice across the Ross Sea. They all correlate with major positive shifts in benthic oxygen isotope records and episodes of sea-level fall, and generally coincide with intervals when atmospheric CO2 concentrations were below current levels (~400 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for significant ice mass loss during episodes of high (>400 ppm) atmospheric CO2. These results suggest that polar climate and the Antarctic Ice Sheet (AIS) were highly sensitive to relatively small changes in CO2 during the early to mid-Miocene, which is supported by numerical ice sheet and climate modelling.

  9. Modelling the marine advance of the last Cordilleran ice sheet

    NASA Astrophysics Data System (ADS)

    Seguinot, Julien; Rogozhina, Irina

    2014-05-01

    Marine advance of the last Cordilleran ice sheet onto the north-eastern Pacific continental shelf may have caused rapid fluctuations of sea level and potentially impacted upon human migration into North America. However the position of the former ice front was critically controlled by a process that remains poorly understood: glacier calving. Geomorphological reconstructions show that part of the presently oceanic areas were ice-covered, allowing for downstream formation of the well-studied Puget and Juan de Fuca lobes. Here we use a numerical glacier model (PISM) to reconstruct the former marine front of the Cordilleran ice sheet and its impact on upstream ice dynamics. Our simulations show that the use of a thickness-based calving law leads to a strong deficit of marine ice cover in the areas where existing reconstructions suggest its advance. In contrast, a physically-based parametrization of glacier calving using the main components of the strain rate tensor (eigencalving; A. Levermann, T. Albrecht, R. Winkelmann, M. A. Martin, M. Haseloff, and I. Joughin, The Cryosphere, 6, 273-286, 2012) reproduces the geomorphologically inferred ice extent.

  10. Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks at the Last Glacial Termination.

    PubMed

    Fogwill, C J; Turney, C S M; Golledge, N R; Etheridge, D M; Rubino, M; Thornton, D P; Baker, A; Woodward, J; Winter, K; van Ommen, T D; Moy, A D; Curran, M A J; Davies, S M; Weber, M E; Bird, M I; Munksgaard, N C; Menviel, L; Rootes, C M; Ellis, B; Millman, H; Vohra, J; Rivera, A; Cooper, A

    2017-01-05

    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 disentangle ice-climate feedbacks that are key to improving future projections. Whilst the sequence of events during this period is reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records, making it difficult to assess relationships between Antarctic ice-sheet (AIS) dynamics, climate change and sea level. Here we present results from a highly-resolved 'horizontal ice core' from the Weddell Sea Embayment, which records millennial-scale AIS dynamics across this extensive region. Counterintuitively, we find AIS mass-loss across the full duration of the Antarctic Cold Reversal (ACR; 14,600-12,700 yrs ago), with stabilisation during the subsequent millennia of atmospheric warming. Earth-system and ice-sheet modelling suggests these contrasting trends were likely Antarctic-wide, sustained by feedbacks amplified by the delivery of Circumpolar Deep Water onto the continental shelf. Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections. With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could amplify AIS mass loss and accelerate global sea-level rise.

  11. Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks at the Last Glacial Termination

    PubMed Central

    Fogwill, C. J.; Turney, C. S. M.; Golledge, N. R.; Etheridge, D. M.; Rubino, M.; Thornton, D. P.; Baker, A.; Woodward, J.; Winter, K.; van Ommen, T. D.; Moy, A. D.; Curran, M. A. J.; Davies, S. M.; Weber, M. E.; Bird, M. I.; Munksgaard, N. C.; Menviel, L.; Rootes, C. M.; Ellis, B.; Millman, H.; Vohra, J.; Rivera, A.; Cooper, A.

    2017-01-01

    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 disentangle ice-climate feedbacks that are key to improving future projections. Whilst the sequence of events during this period is reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records, making it difficult to assess relationships between Antarctic ice-sheet (AIS) dynamics, climate change and sea level. Here we present results from a highly-resolved ‘horizontal ice core’ from the Weddell Sea Embayment, which records millennial-scale AIS dynamics across this extensive region. Counterintuitively, we find AIS mass-loss across the full duration of the Antarctic Cold Reversal (ACR; 14,600–12,700 yrs ago), with stabilisation during the subsequent millennia of atmospheric warming. Earth-system and ice-sheet modelling suggests these contrasting trends were likely Antarctic-wide, sustained by feedbacks amplified by the delivery of Circumpolar Deep Water onto the continental shelf. Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections. With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could amplify AIS mass loss and accelerate global sea-level rise. PMID:28054598

  12. Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks at the Last Glacial Termination

    NASA Astrophysics Data System (ADS)

    Fogwill, C. J.; Turney, C. S. M.; Golledge, N. R.; Etheridge, D. M.; Rubino, M.; Thornton, D. P.; Baker, A.; Woodward, J.; Winter, K.; van Ommen, T. D.; Moy, A. D.; Curran, M. A. J.; Davies, S. M.; Weber, M. E.; Bird, M. I.; Munksgaard, N. C.; Menviel, L.; Rootes, C. M.; Ellis, B.; Millman, H.; Vohra, J.; Rivera, A.; Cooper, A.

    2017-01-01

    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 disentangle ice-climate feedbacks that are key to improving future projections. Whilst the sequence of events during this period is reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records, making it difficult to assess relationships between Antarctic ice-sheet (AIS) dynamics, climate change and sea level. Here we present results from a highly-resolved ‘horizontal ice core’ from the Weddell Sea Embayment, which records millennial-scale AIS dynamics across this extensive region. Counterintuitively, we find AIS mass-loss across the full duration of the Antarctic Cold Reversal (ACR; 14,600–12,700 yrs ago), with stabilisation during the subsequent millennia of atmospheric warming. Earth-system and ice-sheet modelling suggests these contrasting trends were likely Antarctic-wide, sustained by feedbacks amplified by the delivery of Circumpolar Deep Water onto the continental shelf. Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections. With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could amplify AIS mass loss and accelerate global sea-level rise.

  13. 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

  14. 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.

  15. Ice Sheets from Space: The Big Picture View (Louis Agassiz Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Joughin, I. R.

    2012-04-01

    Instead of heroic efforts to acquire a few dozen field-based measurements of ice-flow velocity at some of the Earth's most remote locations, computer servers now crunch satellite data to spew out millions of measurements, providing a whole new perspective on how ice sheets function. Prior to such observations, outlet glaciers and ice streams were perceived to respond slowly to climate change, with little change at timescales of months to decades. While some glaciers flow at steady rates, a remarkable number have responded to recent warming with large variations in speed over periods as short as seconds. A wide variety of behavior is observed with some glaciers steadily gaining speed, others accelerating and then leveling off, and a handful varying substantially with no clear trend. While in the minority, a few glaciers have steadily decelerated over the last decade. As examples ofrecent change, two of the most rapidly changing glaciers are examined: Jakobshavn Isbrae in Greenland and Pine Island Glacier in West Antarctica. In addition to documenting the dramatic changes on these glaciers, satellite observations provide important new spatio-temporal data sets with which to understand such change, particularly when used in conjunction with ice sheet models. Although models constrained by these data reveal that ice-shelf buttressing is an important control on ice-sheet flow, the actual speedups are a complex response to the loss of buttressing, involving several other feedbacks that contribute to the acceleration.Although satellite observations have produced tremendous progress in understanding ice sheets, so far these data have raised more questions than they have answered. The contribution that ice dynamics will make to future sea level remains poorly understood and remains a grand challenge for glaciology. The ongoing record of spaceborne measurements is and will remain a key component in addressing this challenge.

  16. On the impact of buttressing on numerical ice sheet models.

    NASA Astrophysics Data System (ADS)

    Cornford, Stephen; Martin, Daniel; Lee, Victoria; Payne, Antony; Ng, Esmond

    2016-04-01

    Idealized problems with little or no lateral variation are frequently used to study numerical ice sheet models that are then applied to realistic problems that have substantial lateral stresses. Given that the strong lateral variation can, for example, result in a stable grounding line on retrograde slope - an impossible result otherwise - it seems unwise to assume that any conclusion drawn from an unbuttressed flow-line geometry can be extrapolated to the general case. We will present results from two problems which do involve significant lateral stresses, the idealized MISMIP+ tests and 1000-year simulations of the entire Antarctic ice sheet, both resolved to sub-kilometre spatial resolution with the BISICLES ice sheet model. We will consider some numerical issues - for example, whether sub-grid friction schemes are as useful as they appear to be in flow-line problems. We will also consider the impact of the Coulomb limited basal traction law proposed by Tsai (2015), which results in flow-line marine ice streams that are more sensitive to climate perturbations than with the usual Weertman law: to what extent does that hold true in buttressed ice streams?

  17. Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed.

    PubMed

    Bougamont, M; Christoffersen, P; Hubbard, A L; Fitzpatrick, A A; Doyle, S H; Carter, S P

    2014-09-29

    The dynamic response of the Greenland Ice Sheet (GrIS) depends on feedbacks between surface meltwater delivery to the subglacial environment and ice flow. Recent work has highlighted an important role of hydrological processes in regulating the ice flow, but models have so far overlooked the mechanical effect of soft basal sediment. Here we use a three-dimensional model to investigate hydrological controls on a GrIS soft-bedded region. Our results demonstrate that weakening and strengthening of subglacial sediment, associated with the seasonal delivery of surface meltwater to the bed, modulates ice flow consistent with observations. We propose that sedimentary control on ice flow is a viable alternative to existing models of evolving hydrological systems, and find a strong link between the annual flow stability, and the frequency of high meltwater discharge events. Consequently, the observed GrIS resilience to enhanced melt could be compromised if runoff variability increases further with future climate warming.

  18. 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.

  19. Holocene Antarctic's coastal environment, ice sheet, and sea levels explored

    NASA Astrophysics Data System (ADS)

    Goodwin, I.; Berkman, P.; Hjort, C.; Hirakawa, K.

    Efforts are in the works to resolve a several-decade-long debate over the size and extent of the Antarctic ice sheet and its role in sea levels during the last glacial cycle. Researchers also want to find out more about the nature of environmental changes around the Antarctic coast throughout the Holocene, the sensitivity of the ice sheet to warm periods, and the significance of pre-Holocene marine fossils there.Scientists concerned with these issues presented their research priorities last fall at an Antarctic ice margin evolution (ANTIME) workshop, “Circum-Antarctic Coastal Environmental Variability and Sea Level History During the Late Quaternary.” These workshop participants included coastal and glacial geomorphologists, geochemists, and paleoecologists.

  20. Mantle viscosity and ice-age ice sheet topography

    SciTech Connect

    Peltier, W.R.

    1996-09-06

    Ice-age paleotopography and mantle viscosity can both be inferred from observations of Earth`s response to the most recent deglaciation event of the current ice age. This procedure requires iterative application of a theoretical model of the global process of glacial isostatic adjustment. Results demonstrate that the iterative inversion procedure converges to a paleotopography that is extremely close to that from the ICE-4G model. The accompanying mantle viscosity profile is furthermore shown to reconcile the requirements of aspherical geoid anomalies related to the mantle convection process, thus resolving a fundamental issue concerning mantle rheology. The combined model also explains postglacial sea level histories for the east cost of the United States. 28 refs., 9 figs.

  1. Concentrated englacial shear over rigid basal ice, West Antarctica: implications for modelling and ice sheet flow

    NASA Astrophysics Data System (ADS)

    Ross, Neil; Siegert, Martin

    2014-05-01

    Basal freeze-on, deformation and ice crystal fabric re-organisation have been invoked to explain thick, massive englacial units observed in the lower ice column of both the Antarctic and Greenland ice sheets. Whilst recognised as having very different rheological properties to overlying meteoric ice, studies assessing the impact of these basal units on the large-scale flow of an ice sheet have so far been limited. We report the discovery of a previously unknown, extensive (100 km long, more than 25 km wide, and up to 1 km thick) englacial unit of near-basal ice beneath the onset zone of the Institute Ice Stream, West Antarctica. Using radio-echo sounding observations, we describe the form and physical characteristics of this englacial unit, and its impact on the stratigraphy and internal deformation of the overlying ice. The lower englacial unit, characterised by a highly-deformed to massive structure, is inferred to be rheologically distinct from the overlying ice column. The overlying ice contains a series of englacial 'whirlwind' features, which are traceable and exhibit longitudinal continuity between flow-orthogonal radar lines. In our data, these whirlwinds are the representation of englacial layer buckling, and therefore provide robust evidence for enhanced ice flow. The interface between the primary ice units is sharp and abrupt, and at a macro-scale is characterised by a series of high-amplitude long-wavelength undulations. Immediately above this interface, whirlwind features are deformed and display evidence for flow-orthogonal horizontal shear, consistent with the deformation of the overlying ice across the basal ice unit. This phenomenon is not a local process, it is observed above the entirety of the currently mapped extent of the basal ice, nor is it dependent on flight orientation, the direction of shear is consistent regardless of flight orientation. These findings have clear significance for our understanding and ability to realistically model ice

  2. Subglacial roughness of the former Barents Sea ice sheet

    NASA Astrophysics Data System (ADS)

    Gudlaugsson, E.; Humbert, A.; Winsborrow, M.; Andreassen, K.

    2013-12-01

    The roughness of a glacier bed has high importance for the estimation of the sliding velocity and can also provide valuable insights into the dynamics and history of ice sheets, depending on scale. Measurement of basal properties in present-day ice sheets is restricted to ground-penetrating radar and seismics, with surveys retrieving relatively coarse data sets. Deglaciated areas, like the Barents Sea, can be surveyed by shipborne 2-D and 3-D seismics and multibeam sonar and provide the possibility of studying the basal roughness of former ice sheets and ice streams with high resolution. Here, for the first time, we quantify the subglacial roughness of the former Barents Sea ice sheet by estimating the spectral roughness of the basal topography. We also make deductions about the past flow directions by investigating how the roughness varies along a 2-D line as the orientation of the line changes. Lastly, we investigate how the estimated basal roughness is affected by the resolution of the basal topography data set by comparing the spectral roughness along a cross section using various sampling intervals. We find that the roughness typically varies on a similar scale as for other previously marine-inundated areas in West Antarctica, with subglacial troughs having very low roughness, consistent with fast ice flow and high rates of basal erosion. The resolution of the data set seems to be of minor importance when comparing roughness indices calculated with a fixed profile length. A strong dependence on track orientation is shown for all wavelengths, with profiles having higher roughness across former flow directions than along them.

  3. How might the North American ice sheet influence the Northwestern Eurasian climate?

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    During the last glacial period (∼21 000 years ago), two continental-scale ice sheets covered the Canada and northern Europe. It is now widely acknowledged that these past ice sheets 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 through simulated changes in the past glacial atmospheric circulation. Using the LMDz5 atmospheric circulation model, we carried out twelve experiments run under constant Last Glacial Maximum (LGM) conditions for insolation, greenhouse gases and ocean. In the all experiments, the Eurasian ice sheet is removed. The twelve 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 to disentangle the albedo and the topographic impacts of the North American ice sheet onto the climate. The results are compared to our baseline experiment where both the North American and the Eurasian ice sheets have been removed. In summer, we show that the only albedo effect of the American ice sheet modifies the pattern of planetary waves with respect to the no-ice sheet case, causing a cooling of the Eurasian region. By contrast, the atmospheric circulation changes induced by the topography of the North American ice sheet imply summer warming in Northwestern Eurasia. In winter, the Scandinavian and the Barents-Kara regions respond differently to the albedo effect: in response to atmospheric circulation changes, Scandinavia is warmed up and precipitation is more abundant whereas Barents-Kara area is cooled down, decreasing convection process and thus leading to less precipitation. The

  4. Ice sheet topography by satellite altimetry

    NASA Technical Reports Server (NTRS)

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

    1978-01-01

    The measured time between the transmission and rzturn of 13.9 GHz radar pulses from the GEOS 3 satellite (at a mean altitude of 844.5 km and an inclination of 114 deg 52 min) is used to determine the thickness of the Greenland ice cap, with an accuracy in surface elevation on the order of 2 m. Attention is given to changes in ice thickness as an indicator of climatic change in general, and change in mean sea level in particular. Each elevation data point obtained by the satellite represents an average along 0.67 km of ground track, and three-dimensional maps are presented to illustrate the data.

  5. Estimation of ice sheet attenuation by using radar sounder and ice core data

    NASA Astrophysics Data System (ADS)

    Ilisei, Ana-Maria; Li, Jilu; Gogineni, Sivaprasad; Bruzzone, Lorenzo

    2016-10-01

    Due to their great impact on the environment and society, the study of the ice sheets has become a major concern of the scientific community. In particular, the estimation of the ice attenuation is crucial since it enables a more precise characterization of the ice and basal conditions. Although such problem has been often addressed in the literature, the assessment of the ice attenuation is subject to several hypotheses and uncertainties, resulting in a wide range of possible interpretations of the properties of the ice. In this paper, we propose a method for constraining the ice attenuation profiles in the vicinity of an ice core by jointly using coincident radar sounder (RS) data (radargrams) and dielectric profile (DEP) data. Radargrams contain measurements of radar reflected power from ice subsurface dielectric discontinuities (layers) on wide areas. DEP data contain ice dielectric permittivity measurements collected at an ice core. The method relies on the detection of ice layers in the radargrams, the estimation of their depth and reflectivity from the DEP data, and the use of the radar equation for the estimation of ice attenuation through the whole ice column and locally at each layer position. The method has been applied to RS and DEP data acquired at the NEEM core site in Greenland. Experimental results confirm the effectiveness of the proposed method.

  6. Geologic controls on the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    Anandakrishnan, Sridhar

    1993-01-01

    The stability of the West Antarctic Ice Sheet is intertwined with its geologic history. The sub-ice geology and the possibility of active rifting and associated elevated heat-flux and volcanism might be determining factors in ice-sheet behavior. Seismic monitoring of natural events at the base of Ice Stream C reveals the presence of a young sedimentary basin beneath the ice stream. The sediments are presumed to be of glacio-marine origin, similar to those beneath Ice Stream B and in the Ross Sea. The young sediments are approximately 1/2 km thick at UpC camp, but thin abruptly southward to 100 m or less. We hypothesize the presence of a fault with a throw of 400 m to account for this (though we have not directly detected the fault), rather than invoking unrealistic basement dips. To extend these studies to critical inland regions, we suggest an expanded explosive-source seismic survey of the Byrd Subglacial Basin to determine the extent and character of the hypothesized rift basin. High-resolution seismic monitoring will detect layering in the sedimentary column, as well as possibly imaging faults directly.

  7. Sub-Kilometer Scale Basal Roughness of the Siple Coast Ice Streams, West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Young, D. A.; Blankenship, D. D.; Peters, M. E.

    2006-12-01

    The anastomosing series of dynamic, basally lubricated ice streams found on the Siple Coast of West Antarctica play an important role in regulating the mass balance of the West Antarctic Ice Sheet (WAIS). Geological controls on lubrication, elucidated by gravity, magnetics and seismic data, have proven important in understanding the evolution of these features. An additional indicator of basal properties, the basal roughness of ice sheets, may be an indicator of crustal geology and glacial modification, as well as a controlling parameter on ice dynamics and subglacial hydrology. For the Siple Coast ice streams, Fourier analysis of > 5 kilometer morphology (Siegert et al. 2004) revealed a correlation between ice streams and low bed roughness. Coherent high resolution data allows analysis of along track roughness at tens of meters resolution (Peters et al. 2005), however these data are limited in coverage. We extend roughness estimates into to the hundreds-of-meters length scale, using both frequency domain and autocorrelation methods, using incoherent 60 MHz radio echo sounding data collected between 1991 and 1996 on a five kilometer grid. The data cover the Bentley Subglacial Trench, Bindschadler Ice Stream, Siple Dome and the onset region of Kamb Ice Stream. SAR-processed coherent sounding data collected in 2001 are used to confirm these methods. We test for confinement of ice stream rapid basal motion to distinct morphological provinces; assess the hypothesis that marine sediments blanket much of interior of the basal WAIS; and look for correlation between ice flow and textural anisotropy.

  8. 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.

  9. Snow grain size retrieval over the polar ice sheets with the Ice, Cloud, and land Elevation Satellite (ICESat) observations

    NASA Astrophysics Data System (ADS)

    Yang, Yuekui; Marshak, Alexander; Han, Mei; Palm, Stephen P.; Harding, David J.

    2017-02-01

    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 and 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.

  10. 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.

  11. ICESHEET 1.0: a program to produce paleo-ice sheet reconstructions with minimal assumptions

    NASA Astrophysics Data System (ADS)

    Gowan, Evan J.; Tregoning, Paul; Purcell, Anthony; Lea, James; Fransner, Oscar J.; Noormets, Riko; Dowdeswell, J. A.

    2016-05-01

    We describe a program that produces paleo-ice sheet reconstructions using an assumption of steady-state, perfectly plastic ice flow behaviour. It incorporates three input parameters: ice margin, basal shear stress and basal topography. Though it is unlikely that paleo-ice sheets were ever in complete steady-state conditions, this method can produce an ice sheet without relying on complicated and unconstrained parameters such as climate and ice dynamics. This makes it advantageous to use in glacial-isostatic adjustment ice sheet modelling, which are often used as input parameters in global climate modelling simulations. We test this program by applying it to the modern Greenland Ice Sheet and Last Glacial Maximum Barents Sea Ice Sheet and demonstrate the optimal parameters that balance computational time and accuracy.

  12. Sedimentary record of ice divide migration and ice streams in the Keewatin core region of the Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hodder, Tyler J.; Ross, Martin; Menzies, John

    2016-06-01

    The Aberdeen Lake region of central mainland Nunavut is a former core region of the Laurentide Ice Sheet that is characterized by streamlined glacial landforms classified into multiple crosscutting flow sets and near continuous till blanket. The presence of widespread till near the centre of the Keewatin Ice Dome raises questions about its origin. Detailed drillcore logging revealed a complex stratigraphy consisting of at least 6 till units, variably preserved across the study area. Till provenance analysis indicates deposition by near opposite-trending ice flow phases, interpreted as evidence of reconfiguration of the Keewatin Ice Divide. At the surface, large north-northwesterly aligned landforms are present across the study area. The till stratigraphy within these landforms indicates the same NNW ice flow phase is responsible for considerable till production. This ice flow phase is also correlated to a long regional dispersal train of erratics toward the Gulf of Boothia. The production of an extensive, thick (~ 12 m), till sheet during the NNW-trending ice flow phase occurred far from the ice margin at a time of extensive ice cover of mainland Nunavut, likely from an east-west oriented ice divide. A deglacial westerly trending ice flow phase formed small drumlins atop the larger NNW streamlined till ridges and deposited a surficial till unit that is too thin to mask the NNW flow set across the study area. It is proposed that the Boothia paleo-ice stream catchment area propagated deep into the Laurentide Ice Sheet and contributed to significant till production in this core region of the Keewatin Sector prior to the westerly ice flow shift. The apparent relationship between till thickness and the size of the associated or correlated drumlins, flow sets, and dispersal trains indicates complex erosion/deposition interplay is involved in the formation of streamlined subglacial landforms.

  13. Stable microbial community composition on the Greenland Ice Sheet

    PubMed Central

    Musilova, Michaela; Tranter, Martyn; Bennett, Sarah A.; Wadham, Jemma; Anesio, Alexandre M.

    2015-01-01

    The first molecular-based studies of microbes in snow and on glaciers have only recently been performed on the vast Greenland Ice Sheet (GrIS). Aeolian microbial seeding is hypothesized to impact on glacier surface community compositions. Localized melting of glacier debris (cryoconite) into the surface ice forms cryoconite holes, which are considered ‘hot spots’ for microbial activity on glaciers. To date, few studies have attempted to assess the origin and evolution of cryoconite and cryoconite hole communities throughout a melt season. In this study, a range of experimental approaches was used for the first time to study the inputs, temporal and structural transformations of GrIS microbial communities over the course of a whole ablation season. Small amounts of aeolian (wind and snow) microbes were potentially seeding the stable communities that were already present on the glacier (composed mainly of Proteobacteria, Cyanobacteria, and Actinobacteria). However, the dominant bacterial taxa in the aeolian samples (Firmicutes) did not establish themselves in local glacier surface communities. Cryoconite and cryoconite hole community composition remained stable throughout the ablation season following the fast community turnover, which accompanied the initial snow melt. The presence of stable communities in cryoconite and cryoconite holes on the GrIS will allow future studies to assess glacier surface microbial diversity at individual study sites from sampling intervals of short duration only. Aeolian inputs also had significantly different organic δ13C values (-28.0 to -27.0‰) from the glacier surface values (-25.7 to -23.6‰), indicating that in situ microbial processes are important in fixing new organic matter and transforming aeolian organic carbon. The continuous productivity of stable communities over one melt season makes them important contributors to biogeochemical nutrient cycling on glaciers. PMID:25852658

  14. Stable microbial community composition on the Greenland Ice Sheet.

    PubMed

    Musilova, Michaela; Tranter, Martyn; Bennett, Sarah A; Wadham, Jemma; Anesio, Alexandre M

    2015-01-01

    The first molecular-based studies of microbes in snow and on glaciers have only recently been performed on the vast Greenland Ice Sheet (GrIS). Aeolian microbial seeding is hypothesized to impact on glacier surface community compositions. Localized melting of glacier debris (cryoconite) into the surface ice forms cryoconite holes, which are considered 'hot spots' for microbial activity on glaciers. To date, few studies have attempted to assess the origin and evolution of cryoconite and cryoconite hole communities throughout a melt season. In this study, a range of experimental approaches was used for the first time to study the inputs, temporal and structural transformations of GrIS microbial communities over the course of a whole ablation season. Small amounts of aeolian (wind and snow) microbes were potentially seeding the stable communities that were already present on the glacier (composed mainly of Proteobacteria, Cyanobacteria, and Actinobacteria). However, the dominant bacterial taxa in the aeolian samples (Firmicutes) did not establish themselves in local glacier surface communities. Cryoconite and cryoconite hole community composition remained stable throughout the ablation season following the fast community turnover, which accompanied the initial snow melt. The presence of stable communities in cryoconite and cryoconite holes on the GrIS will allow future studies to assess glacier surface microbial diversity at individual study sites from sampling intervals of short duration only. Aeolian inputs also had significantly different organic δ(13)C values (-28.0 to -27.0‰) from the glacier surface values (-25.7 to -23.6‰), indicating that in situ microbial processes are important in fixing new organic matter and transforming aeolian organic carbon. The continuous productivity of stable communities over one melt season makes them important contributors to biogeochemical nutrient cycling on glaciers.

  15. Antarctic Ice Sheet dynamics in the Ross Sea during the Early to Middle Miocene

    NASA Astrophysics Data System (ADS)

    Levy, Richard; Harwood, David; Florindo, Fabio; DeConto, Robert; Golledge, Nicholas; Hoffmann, Stefan; Kuhn, Gerhard; McKay, Robert; Naish, Timothy; Pollard, David; Sangiorgi, Francesca; von Eynatten, Hilmar

    2015-04-01

    A 1138-meter sediment core (AND-2A) recovered from the Southern McMurdo Sound sector of the Ross Sea comprises a near-continuous record of Antarctic climate and ice sheet variability through the Early to early Middle Miocene (20.2 to 14.5 million years ago), including an interval of inferred sustained global warmth known as the Miocene Climatic Optimum (MCO). The record preserves 55 sedimentary sequences that reflect cycles of glacial advance and retreat. A new analysis of proxy environmental data from the AND-2A core, and synthesis with regional geological information, show that the early to middle Miocene Antarctic climate ranged from cold polar conditions, similar to Antarctica during the Holocene, to those that characterise modern sub-polar environments. Four disconformities that punctuate the sedimentary sequence coincide with regionally mapped seismic discontinuities and reflect transient expansion of marine-based ice across the Ross Sea. The timing of these major marine-based ice sheet advances correlates with shifts in highly-resolved deep sea isotope records and major drops in eustatic sea-level indicating the global nature of these events. In contrast, three distinct intervals in the core indicate that this high latitude site was periodically influenced by an ice sheet margin that had retreated beyond the coastline. These relatively large-scale changes in climate and ice sheet extent occurred under atmospheric carbon dioxide concentrations that generally varied between 300 to 500 ppm. Therefore, our reconstructions suggest that Antarctica's climate and ice sheets were sensitive to modest changes in greenhouse gas forcing and support previous studies, which indicate that marine-based portions of the WAIS and EAIS can retreat under climatic conditions that were similar to those projected for our future under current levels of atmospheric CO2.

  16. The influence of meltwater on the thermal structure and flow of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Poinar, Kristin

    the depth and longevity of such crevasses is relatively robust to realistic increases in melt volumes over the coming century, so that we should not expect large changes in the englacial hydrological system under near-future climate regimes. These inferences put important constraints on the timescales of the Greenland supraglacial-to-subglacial water cycle. Finally, I assess the likelihood that higher-elevation surface melt could deliver water to regions where the bed is currently frozen. This hypothetical process is important because it could potentially greatly accelerate the seaward motion of the ice sheet. By analyzing surface strain rates and comparing them to my modeled basal temperature field, I find that this scenario is unlikely to occur: the conditions necessary to form surface-to-bed conduits are rarely found at higher elevations (~1600 meters) that may overlie frozen beds.

  17. Development and Use of Overland Traversing Capabilities As a Science Platform on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Mercer, J. L.; Haggerty, P.; Crain, R.; Olsen, R.; Burnside, J.; Wisneski, S.; Joyner, J.; Phillips, G.; Lever, J. H.; Weale, J.

    2012-12-01

    The National Science Foundation's Arctic Sciences Research Support and Logistics office, along with their prime support contractor (CH2MHill Polar Services) and the US Army Cold Regions Research and Engineering Laboratory (CRREL), has been developing heavy-haul overland traversing capabilities on the Greenland Ice Sheet since 2007. In the past 5 years, logistical, technological and environmental challenges have been overcome to develop efficient, well-functioning and safe traverse capabilities aimed at delivering fuel and cargo resupply to inland research stations. The next phase of this effort entails the development and implementation of science support using the traverse as a platform. With rapid changes occurring on the ice sheet, an increase in science interests over long sections of Greenland is anticipated. The traverse provides a potential platform for onboard instrumentation and integrated data collection systems, installation and routine maintenance of monitoring stations on the ice sheet, sample and data collection over transects along the ice sheet, or movement of large/heavy science equipment. This presentation will provide an overview and discussion of current and future traverse science support capabilities, as well as potential options for implementing science support using the traverse as a platform.

  18. On Early Holocene Ice-Sheet/Sea-Level Interactions

    NASA Astrophysics Data System (ADS)

    Tornqvist, T. E.; Hijma, M.

    2011-12-01

    Early Holocene sea-level change constitutes an imperfect, yet potentially valuable analog for future sea-level rise, given the rapidly disintegrating land-based ice under climate conditions of high-latitude Northern Hemisphere warming. The associated rates of eustatic sea-level rise (cm/yr order of magnitude) fall within the range of predictions for the latter part of the next century. However, the early Holocene eustatic sea-level history is otherwise rather poorly understood. Recent impetus has been provided by new records of both relative sea-level (RSL) change and ice-sheet retreat that are sometimes difficult to reconcile in terms of timing and magnitude of change. We first summarize the state-of-the-art on early Holocene sea-level change and then identify key near-term research needs. Recent studies have identified a number of decimeter to meter-scale sea-level jumps, several of which have been linked to catastrophic drainage of proglacial Lake Agassiz and the 8.2 ka cooling event. It is increasingly clear that this occurred by means of two successive jumps, separated by up to a few centuries, and only the latter (and final) one coinciding with the 8.2 ka climate event proper. We show that a considerable research effort, including near-field, intermediate-field, and far-field localities across the globe is needed to fully understand the timing and magnitude of these sea-level jumps. Accomplishing this goal would in addition offer a unique opportunity for rigorous testing of gravitational theory and associated sea-level fingerprinting that plays a critical role in predicting future sea-level change. A more enigmatic sea-level jump that has been identified around 7.6 ka has received renewed interest both by means of new RSL data from Fennoscandia and reconstructions of Laurentide Ice Sheet retreat. However, the proposed ~5 m abrupt rise in eustatic sea level cannot be detected in relatively nearby, detailed RSL records from NW Europe, thus presenting a

  19. The influence of Antarctic ice sheet topography change on Antarctic climate -- a positive feedback.

    NASA Astrophysics Data System (ADS)

    Steig, E. J.

    2015-12-01

    The potential for climate change to result in changes to the Antarctic ice sheet -- including the possibility of a full collapse of the West Antarctic Ice Shet (WAIS) is well known. Less well known is that the collapse of the ice sheet, if it occured, would have significant impcts on regional climate. We use climate model simulations to quantify the impact of topographic changes on the surface climate of Antarctica. As a general rule, lowered topography produces anomalous cyclonic circulation owing to fundamental atmospheric dynamical constraints. In the case of WAIS collapse, this causes increased flow of warm, maritime air toward the South Pole and cold-air advection from the East Antarctic plateau toward the Ross Sea and Marie Byrd Land, West Antarctica. This resulting pattern The result is cooling in some areas and warming in others, a pattern that is similar to that observed from ice core paleotemperature data for the last interglacial period, suggesting that WAIS collapse occurred at that time. We find that magnitude of the response is roughly linear with the magnitude of the imposed elevation change. The regional response over West Antarctica is large enough that it probably needs to be taken into account in modeling future changes to the ice sheet. Of particular interest is that lowering of the WAIS topography results in anomalous westerlies along the Amundsen Sea coastline. Anomalous westerlies in this region today are in large part responsible for the for intrusion of circumpolar deepwater onto the continental shelf, and the observed rapid thinning of West Antarctic ice shelves. A positive feedback may thus exist in which lowering of the WAIS surface from climate forcing may enhance that forcing, leading to further elevation lowering and ice sheet mass loss.

  20. Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure

    NASA Astrophysics Data System (ADS)

    Pollard, David; DeConto, Robert M.; Alley, Richard B.

    2015-02-01

    Geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last ∼25 million years, at times reaching 20 m or more above modern. If correct, this implies substantial variations in the size of the East Antarctic Ice Sheet (EAIS). However, most climate and ice sheet models have not been able to simulate significant EAIS retreat from continental size, given that atmospheric CO2 levels were relatively low throughout this period. Here, we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model-data conflict. In response to atmospheric and ocean temperatures typical of past warm periods, floating ice shelves may be drastically reduced or removed completely by increased oceanic melting, and by hydrofracturing due to surface melt draining into crevasses. Ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing, and may fail structurally if stresses exceed the ice yield strength, producing rapid retreat. Incorporating these mechanisms in our ice-sheet model accelerates the expected collapse of the West Antarctic Ice Sheet to decadal time scales, and also causes retreat into major East Antarctic subglacial basins, producing ∼17 m global sea-level rise within a few thousand years. The mechanisms are highly parameterized and should be tested by further process studies. But if accurate, they offer one explanation for past sea-level high stands, and suggest that Antarctica may be more vulnerable to warm climates than in most previous studies.

  1. Supraglacial bacterial community structures vary across the Greenland ice sheet.

    PubMed

    Cameron, Karen A; Stibal, Marek; Zarsky, Jakub D; Gözdereliler, Erkin; Schostag, Morten; Jacobsen, Carsten S

    2016-02-01

    The composition and spatial variability of microbial communities that reside within the extensive (>200 000 km(2)) biologically active area encompassing the Greenland ice sheet (GrIS) is hypothesized to be variable. We examined bacterial communities from cryoconite debris and surface ice across the GrIS, using sequence analysis and quantitative PCR of 16S rRNA genes from co-extracted DNA and RNA. Communities were found to differ across the ice sheet, with 82.8% of the total calculated variation attributed to spatial distribution on a scale of tens of kilometers separation. Amplicons related to Sphingobacteriaceae, Pseudanabaenaceae and WPS-2 accounted for the greatest portion of calculated dissimilarities. The bacterial communities of ice and cryoconite were moderately similar (global R = 0.360, P = 0.002) and the sampled surface type (ice versus cryoconite) did not contribute heavily towards community dissimilarities (2.3% of total variability calculated). The majority of dissimilarities found between cryoconite 16S rRNA gene amplicons from DNA and RNA was calculated to be the result of changes in three taxa, Pseudanabaenaceae, Sphingobacteriaceae and WPS-2, which together contributed towards 80.8 ± 12.6% of dissimilarities between samples. Bacterial communities across the GrIS are spatially variable active communities that are likely influenced by localized biological inputs and physicochemical conditions.

  2. 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.

  3. 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.

  4. Ice-sheet acceleration driven by melt supply variability.

    PubMed

    Schoof, Christian

    2010-12-09

    Increased ice velocities in Greenland are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice-ocean interactions in water-terminating outlet glaciers and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt but an increase in water input variability that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure. Using a model that captures the dynamic switching between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles and an increase in rain and surface lake drainage events rather than an increase in mean melt supply.

  5. A robust threshold retracking algorithm for measuring ice-sheet surface elevation change from satellite radar altimeters

    SciTech Connect

    Davis, C.H.

    1997-07-01

    A threshold retracking algorithm for processing ice-sheet altimeter data is presented. The primary purpose for developing this algorithm is detection of ice-sheet elevation change, where it is critical that a retracking algorithm produce repeatable elevations. The more consistent an algorithm is in selecting the retracking point the less likely that errors and/or biases will be introduced by the retracking scheme in the elevation-change measurement. The authors performed extensive comparisons between the threshold algorithm and two other widely used ice-sheet retracking algorithms on Geosat datasets comprised of over 60,000 crossover points. The results show that the threshold retracking algorithm, with a 10% threshold level, produces ice-sheet surface elevations that are more repeatable than the elevations derived from the other retracking algorithms. For this reason, the threshold retracking algorithm has been adopted by NASA/GSFC as an alternative to their existing algorithm for production of ice-sheet altimeter datasets under the NASA Pathfinder program. The threshold algorithm will be used to re-process existing ice-sheet altimeter datasets and to process the datasets from future altimeter missions.

  6. 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.

  7. An ice-sheet-wide framework for englacial attenuation from ice-penetrating radar data

    NASA Astrophysics Data System (ADS)

    Jordan, T. M.; Bamber, J. L.; Williams, C. N.; Paden, J. D.; Siegert, M. J.; Huybrechts, P.; Gagliardini, O.; Gillet-Chaulet, F.

    2016-07-01

    Radar inference of the bulk properties of glacier beds, most notably identifying basal melting, is, in general, derived from the basal reflection coefficient. On the scale of an ice sheet, unambiguous determination of basal reflection is primarily limited by uncertainty in the englacial attenuation of the radio wave, which is an Arrhenius function of temperature. Existing bed-returned power algorithms for deriving attenuation assume that the attenuation rate is regionally constant, which is not feasible at an ice-sheet-wide scale. Here we introduce a new semi-empirical framework for deriving englacial attenuation, and, to demonstrate its efficacy, we apply it to the Greenland Ice Sheet. A central feature is the use of a prior Arrhenius temperature model to estimate the spatial variation in englacial attenuation as a first guess input for the radar algorithm. We demonstrate regions of solution convergence for two input temperature fields and for independently analysed field campaigns. The coverage achieved is a trade-off with uncertainty and we propose that the algorithm can be "tuned" for discrimination of basal melt (attenuation loss uncertainty ˜ 5 dB). This is supported by our physically realistic ( ˜ 20 dB) range for the basal reflection coefficient. Finally, we show that the attenuation solution can be used to predict the temperature bias of thermomechanical ice sheet models and is in agreement with known model temperature biases at the Dye 3 ice core.

  8. 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; Herzfeld, Ute; Jacskon, Charles; Johnson, Jesse; Khroulev, Constantine; Larour, Eric; Levermann, Anders; Lipscomb, William H.; Martin, Maria A.; Morlighem, Mathieu; Parizek, Byron R; Pollard, David; Price, Stephen F.; Seroussi, Helene; Walker, Ryan; Wang, Wei Li

    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.

  9. Greenland Ice Sheet seasonal and spatial mass variability from model simulations and GRACE (2003-2012)

    NASA Astrophysics Data System (ADS)

    Alexander, Patrick M.; Tedesco, Marco; Schlegel, Nicole-Jeanne; Luthcke, Scott B.; Fettweis, Xavier; Larour, Eric

    2016-06-01

    Improving the ability of regional climate models (RCMs) and ice sheet models (ISMs) to simulate spatiotemporal variations in the mass of the Greenland Ice Sheet (GrIS) is crucial for prediction of future sea level rise. While several studies have examined recent trends in GrIS mass loss, studies focusing on mass variations at sub-annual and sub-basin-wide scales are still lacking. At these scales, processes responsible for mass change are less well understood and modeled, and could potentially play an important role in future GrIS mass change. Here, we examine spatiotemporal variations in mass over the GrIS derived from the Gravity Recovery and Climate Experiment (GRACE) satellites for the January 2003-December 2012 period using a "mascon" approach, with a nominal spatial resolution of 100 km, and a temporal resolution of 10 days. We compare GRACE-estimated mass variations against those simulated by the Modèle Atmosphérique Régionale (MAR) RCM and the Ice Sheet System Model (ISSM). In order to properly compare spatial and temporal variations in GrIS mass from GRACE with model outputs, we find it necessary to spatially and temporally filter model results to reproduce leakage of mass inherent in the GRACE solution. Both modeled and satellite-derived results point to a decline (of -178.9 ± 4.4 and -239.4 ± 7.7 Gt yr-1 respectively) in GrIS mass over the period examined, but the models appear to underestimate the rate of mass loss, especially in areas below 2000 m in elevation, where the majority of recent GrIS mass loss is occurring. On an ice-sheet-wide scale, the timing of the modeled seasonal cycle of cumulative mass (driven by summer mass loss) agrees with the GRACE-derived seasonal cycle, within limits of uncertainty from the GRACE solution. However, on sub-ice-sheet-wide scales, some areas exhibit significant differences in the timing of peaks in the annual cycle of mass change. At these scales, model biases, or processes not accounted for by models related

  10. 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

  11. Supraglacial fluvial landscape evolution on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Karlstrom, L.; Yang, K.

    2015-12-01

    In the ablation zone of the Greenland Ice Sheet, melting during the summer drives drainage development in which flow is routed downslope through a network of supgraglacial streams and lakes until it is sequestered by the englacial system or flows off of the glacier. This supraglacial drainage network sets the efficacy by which melt water is transport into the glacier and thus has important implications for coupling between ice sheet sliding and surface melt. Thermal erosion in supraglacial streams is rapid compared to other fluvial environments, raising the possibility that supraglacial topographic evolution is to some extent set by local fluvial incision rather than by underlying bedrock or iceflow. We study a series of supraglacial drainage basins on top of the West Greenland Ice Sheet between 1000-1500 m elevation using a combination of high-resolution images, and concurrent (2 m resolution) DEMs constructed from World View Imagery. Although large-scale topography correlates well with underlying bedrock topography, spectral filtering of the surface also reveals broad, low relief valleys that suggest fluvial modification at all elevations. We extract several hundred supraglacial stream longitudinal profiles per drainage basin, finding many channel segments that are clearly out of equilibrium but also numerous concave up channel segments that are not well correlated with underlying bedrock. These concave up segments have a similar power law exponent, suggesting similarities to equilibrium bedrock and alluvial rivers (although the exponent is different in this setting). We develop a stream-power model to predict equilibrium longitudinal profiles where erosion is due to melting driving by viscous dissipation of heat within streams. We speculate that fluvial erosion driven by viscous dissipation is in part responsible for shaping the Greenland Ice Sheet ablation zone annually, superimposed on long wavelength bedrock control of surface topography and basins.

  12. 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.

  13. 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.

  14. 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

  15. 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.

  16. 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.

  17. 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

  18. 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.

  19. New eyes in the sky measure glaciers and ice sheets

    USGS Publications Warehouse

    Kieffer, Hugh; Kargel, Jeffrey S.; Barry, Roger G.; Bindschadler, Robert; Bishop, Michael P.; MacKinnon, David; Ohmura, Atsumu; Raup, Bruce; Antoninetti, Massimo; Bamber, Jonathan; Braun, Mattias; Brown, Ian; Cohen, Denis; Copland, Luke; DueHagen, Jon; Engeset, Rune V.; Fitzharris, Blair; Fujita, Koji; Haeberli, Wilfried; Hagen, Jon Oue; Hall, Dorothy; Hoelzle, Martin; Johansson, Maria; Kaab, Andi; Koenig, Max; Konovalov, Vladimir; Maisch, Max; Paul, Frank; Rau, Frank; Reeh, Niels; Rignot, Eric; Rivera, Andres; De Ruyter de Wildt, Martiyn; Scambos, Ted; Schaper, Jesko; Scharfen, Greg; Shroder, Jack; Solomina, Olga; Thompson, David; van der Veen, Kees; Wohlleben, Trudy; Young, Neal

    2000-01-01

    The mapping and measurement of glaciers and their changes are useful in predicting sea-level and regional water supply, studying hazards and climate change [Haeberli et al., 1998],and in the hydropower industry Existing inventories cover only about 67,000 of the world's estimated 160,000 glaciers and are based on data collected over 50 years or more [e.g.,Haeberli et al., 1998]. The data available have proven that small ice bodies are disappearing at an accelerating rate and that the Antarctic ice sheet and its fringing ice shelves are undergoing unexpected, rapid change. According to many glaciologists, much larger fluctuations in land ice—with vast implications for society—are possible in the coming decades and centuries due to natural and anthropogenic climate change [Oppenheimer, 1998].

  20. Radiostratigraphy of the Greenland ice sheet from the 2009-2012 Operation IceBridge surveys

    NASA Astrophysics Data System (ADS)

    MacGregor, J. A.; Fahnestock, M. A.; Catania, G. A.; Paden, J. D.; Wagman, B. M.; Rybarski, S. C.; Young, S. K.; Mabrey, A. N.

    2012-12-01

    We are developing a comprehensive radiostratigraphy of the Greenland ice sheet from the nearly two decades (1993-2012) of airborne radar-sounding surveys collected by the University of Kansas and other institutions. This radiostratigraphy will be used to investigate the ice sheet's basal condition in conjunction with 1-D vertical strain-rate modeling and other remote-sensing data. It will also be gridded at the same resolution as upcoming surface- and bed-elevation models, so that next-generation ice-sheet models can treat these isochrones as internal boundary conditions. To trace this stratigraphy efficiently, we are using several new and recently developed semi-automatic methods (phase tracking, layer-dip mapping, and amplitude flattening). Here we present initial results derived from Operation IceBridge surveys between 2009 and 2012. These radar surveys recorded the common depth pattern of the Greenland ice sheet's internal stratigraphy across thousands of line kilometers (strong reflections in the Holocene, a hiatus between ~15-30 ka, and then a distinct reflector triplet). This pattern is most easily observed in its northern sector. However, intermittent basal freeze-on within this sector can disrupt the overlying internal stratigraphy significantly, despite high-resolution along-track sampling (< 1 m). This disruption is likely due to the unusual depth profiles of vertical strain rate there. In heavily disrupted regions, fewer than half a dozen reflectors can typically be traced reliably, whereas in undisrupted regions more than two dozen reflectors can often be traced across distances of several hundred kilometers. This dataset includes multiple reflectors dated to the last glacial period and which are widely observable throughout the Greenland ice sheet. Thus, radiostratigraphy has the potential to strongly constrain models of its evolution since the last interglacial period.

  1. 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

  2. Investigating Antarctic ice sheet subglacial processes beneath the Whillans Ice Plain, West Antarctica, using satellite altimetry and GPS

    NASA Astrophysics Data System (ADS)

    Siegfried, Matthew Ross

    conclude by proposing future studies that would address knowledge gaps currently preventing the inclusion of an evolving basal water system into large-scale predictive models of ice-sheet flow.

  3. Contrasting ice sheet response to early and late summer rapid supraglacial lake drainage events on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Stevens, L. A.; Behn, M. D.; Das, S. B.; Joughin, I. R.; Herring, T.; King, M. A.; McGuire, J. J.

    2013-12-01

    Across much of the ablation region of the western Greenland Ice Sheet, hydro-fracture events related to supraglacial lake drainages rapidly deliver large volumes of meltwater to the bed and create conduits providing efficient surface-to-bed drainage networks for the remainder of the summer melt season. Using a network of 20 GPS stations installed in 2011, and supplemented with a smaller network operating back to 2006, we observe ice surface motion during a series of lake draining hydro-fracture events. These data are used to investigate (1) the location and propagation geometry of the fracture opening and (2) the acceleration of ice in response to the rapid input of surface meltwater to the bed. Observations at the same location show varying surface motion following early versus late summer rapid lake drainage events from multiple years. During a late-season (July 29) rapid drainage in 2006, results from a single GPS station show velocity in the direction of mean ice flow and surface uplift returned to pre-drainage values within ~24 hours (Das et al., 2008), indicating the large subglacial meltwater pulse was efficiently dissipated into the subglacial hydrologic network. In contrast, an early-season (June 11) rapid drainage at the same lake in 2011 induced uplift that persisted for much longer. Specifically, we find that elevations at stations nearest the moulin did not return to pre-drainage elevations for 4 to 8 days post-drainage, suggesting a more inefficient subglacial hydrologic system during the early summer season. These results indicate that the ice-sheet response is modulated, at least in part, by the seasonal evolution of the subglacial hydrological system. We also plan to investigate new GPS data from 2 rapid drainage events in the early portion of the melt season in 2012 and 2013. Findings from these events will ultimately improve our understanding of the mechanics of ice-sheet hydro-fracture and the influence of surface meltwater on ice-sheet flow.

  4. Preservation of a preglacial landscape under the center of the Greenland Ice Sheet.

    PubMed

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

    2014-04-25

    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 nonerosive 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.

  5. 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

  6. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse

    PubMed Central

    Holloway, Max D.; Sime, Louise C.; Singarayer, Joy S.; Tindall, Julia C.; Bunch, Pete; Valdes, Paul J.

    2016-01-01

    Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming. PMID:27526639

  7. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse.

    PubMed

    Holloway, Max D; Sime, Louise C; Singarayer, Joy S; Tindall, Julia C; Bunch, Pete; Valdes, Paul J

    2016-08-16

    Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming.

  8. What can adjoint modelling tell about the response of the Greenland Ice Sheet to changes in basal sliding?

    NASA Astrophysics Data System (ADS)

    McGovern, Jonathan; Rutt, Ian; Murray, Tavi; Utke, Jean

    2013-04-01

    Studying the future behaviour of the Greenland Ice Sheet is important considering the ice sheet has a sea-level equivalent of 7 metres and the rate of mass loss from it is increasing (Velicogna, 2009). Examining the modelled response of the Greenland Ice Sheet to changes in forcing parameters can give insight into how it will behave in the future. The response of the ice sheet to specific changes in forcing parameters is referred to as the sensitivity. Being able to obtain model sensitivities in as little computation time as possible would be useful for examining the future response of the Greenland Ice Sheet. Adjoint models allow sensitivities to be obtained more efficiently than the conventional way, when considering spatially varying parameters. Conventionally, such sensitivities are obtained by perturbing a parameter at every grid point in turn and calculating the sensitivity at every grid point. Adjoint sensitivities, though, are calculated in a single step. This reduces the computational cost when obtaining sensitivities over large model domains. The adjoint method also has the advantage that it gives the exact value of the model sensitivity, rather than a finite difference approximation to it. We present the adjoint of a finite difference, shallow ice, thermomechanical ice sheet model with basal sliding, applied to the Greenland Ice Sheet. This adjoint model is obtained using the OpenAD automatic differentiation tool (Utke, 2006), which is open source. The adjoint model is validated by comparing adjoint and forward model sensitivities over 100 years. This work builds on the work of Heimbach (2009). We use the adjoint model to examine the sensitivity of the model to changes in basal sliding. About half the mass loss from the Greenland Ice Sheet occurs from surface runoff and half from dynamic mass loss (Broeke, 2009). Melt-water from Greenland Ice Sheet supra-glacial lakes can percolate to the bed through moulins. The melt-water that reaches the bed can then

  9. A test bed for investigating the flow of outlet glaciers and ice streams embedded in the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Calov, Reinhard; Rückamp, Martin; Schlegel, Rebecca; Ganopolski, Andrey; Humbert, Angelika

    2016-04-01

    Here, we define a test bed for fast flow regions and its vicinity embedded in an ice sheet. This test bed is designed for outlet glaciers and ice streams of the Greenland ice sheet. It consists of a fine resolution part with a manufactured basal trough over which the professional software COMSOL (Multiphysics Modeling Software) operates as a full-Stokes model. Results by COMSOL are compared with coarse resolution simulations with the ice-sheet model SICOPOLIS operating in shallow-ice-approximation mode and using parameterizations of the fast flow effects. For simplification, in this preliminary approach, both models run in isothermal mode. Definition of surface mass balance follows the EISMINT intercomparison project with parameters adapted to the Greenland ice sheet. In particular, we inspect with this test bed upstream and lateral flow effects of ice streams and outlet glaciers. We present first simulations with this approach, although presentation of the test bed itself is the main emphasis of this presentation.

  10. Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate

    DOE PAGES

    Cornford, S. L.; Martin, D. F.; Payne, A. J.; ...

    2015-03-23

    We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet. Each of the simulations begins with a geometry and velocity close to present day observations, and evolves according to variation in meteoric ice accumulation, ice shelf melting, and mesh resolution. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rates anomalies that remove most of the ice shelves overmore » a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions, ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Sensitivity to mesh resolution is spurious, and we find that sub-kilometer resolution is needed along most regions of the grounding line to avoid systematic under-estimates of the retreat rate, although resolution requirements are more stringent in some regions – for example the Amundsen Sea Embayment – than others – such as the Möller and Institute ice streams.« less

  11. Thin Ice -- Bipolar Super Interglacials and our Possible Future

    NASA Astrophysics Data System (ADS)

    Brigham-Grette, J.

    2015-12-01

    Evidence of exceptionally warm Arctic interglacials from Lake El'gygytgyn NE Russia, combined with the periodic collapse of the West Antarctic Ice Sheet (ANDRILL) suggests the need to reassess climate sensitivity and cryospheric dynamics on many timescales. It may be easier to melt significant portions of large sheets and drive large ecosystem changes in the high latitudes than we thought! Elevated global temperatures of only a few degrees (or less) combined with polar amplification seems to have repeatedly caused large changes in the cryosphere and global sea level, still under investigation given the present knowledge and complexity of glacio-isostatic adjustments. But increasing evidence of a forested Arctic and smaller polar ice sheets in the Pliocene when CO2 was around 400 ppm (like today) and during several more recent super interglacials (under Milankovitch forcing) most likely presages a warming future, partially hidden in recent years by the lagged response of the oceans, atmosphere, and cryosphere to anthropogenic influences. The future will depend on the ability of societies to recognize and respond to the consequences of significant environmental change. The challenge to the science community is to communicate this data-driven reality effectively and without politics.

  12. SeaRISE experiments revisited: potential sources of spread in multi-model projections of the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Saito, F.; Abe-Ouchi, A.; Takahashi, K.; Blatter, H.

    2016-01-01

    The present paper revisits the future surface-climate experiments on the Greenland ice sheet proposed by the Sea-level Response to Ice Sheet Evolution (SeaRISE; Bindschadler et al., 2013) study. The projections of the different SeaRISE participants show dispersion, which has not been examined in detail to date. A series of sensitivity experiments are conducted and analyzed using the ice-sheet model for integrated Earth-system studies (IcIES) by replacing one or more formulations of the model parameters with those adopted in other model(s). The results show that large potential sources of the dispersion among the projections of the different SeaRISE participants are differences in the initialization methods and in the surface mass balance methods, and both aspects have almost equal impact on the results. The treatment of ice-sheet margins in the simulation has a secondary impact on the dispersion. We conclude that spinning up the model using fixed topography through the spin-up period while the temperature is allowed to evolve according to the surface temperature history is the preferred representation, at least for the experiment configuration examined in the present paper. A benchmark model experimental setup that most of the numerical models can perform is proposed for future intercomparison projects, in order to evaluate the uncertainties relating to pure ice-sheet model flow characteristics.

  13. Numerical and theoretical treatment of grounding line movement and ice shelf buttressing in marine ice sheets

    NASA Astrophysics Data System (ADS)

    Goldberg, Daniel N.

    Understanding the dynamics of marine ice sheets is integral to studying the evolution of the Antarctic Ice Sheet in both the short and long terms. An important component of the dynamics, grounding line migration, has proved difficult to represent in numerical models, with undesirable behavior such as sensitivity to grid resolution having been observed. Most successful attempts at representing grounding line migration have made use of techniques that are only readily applicable to flowline models, such as Arbitrary Lagrangian-Eulerian schemes. It remains unclear whether a purely Eulerian flowline model can reproduce the actual solution of the governing differential equations, as well as what the theoretical properties of that solution are. In addition, in order to capture the stress transmission involved in another important dynamic component, the buttressing of a marine ice sheet by its ice shelf, the transverse flow direction must also be resolved. Here a numerical model is developed that solves the time-dependent Shelfy-Stream equations [MacAyeal, 1989] and makes use of mesh adaption techniques to overcome the difficulties typically associated with the numerics of grounding line migration. In the special case of a flowline model, it is shown that the Shelfy-Stream equations have a unique solution provided constraints on the initial condition and the forcing are satisfied, and the convergence properties of the model are examined. Model output is also compared with a recent benchmark for flowline models. It is shown that our model yields an accurate solution while using far less resources than would be required without mesh adaption. It is also shown that the mesh adapting techniques extend to two horizontal dimensions. Experiments are carried out to determine how both ice shelf buttressing and ice rises affect the marine instability predicted for an ice sheet on a foredeepened bed. It is found that buttressing is not always sufficient to stabilize such a sheet but

  14. Modeling the West Antarctic and Greenland ice sheets: New dynamic, thermodynamic, and isostatic insights

    NASA Astrophysics Data System (ADS)

    Parizek, Byron R.

    Numerical simulations indicate that the apparent long-term persistence and short-term variability of the Ross ice streams in West Antarctica are tied to regional thermal conditions and local basal lubrication. Modelling results suggest that the flux of latent heat in a throughgoing hydrologic system fed by melt beneath thick inland ice maintains the lubrication of the ice streams despite their tendency to freeze to the bed, and would allow additional thinning and grounding-line retreat. However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist. Furthermore, simulations indicate that the ice streams have experienced only small deglacial thickness changes and are thinning more rapidly than their beds are rising isostatically. Thickness changes of O (100)m are modelled at the modern grounding line through the last glacial cycle. Coupled ice and bedrock models indicate isostatic rebound is raising the ice sheet at the modern grounding line faster than the rising sea level is submerging it. While, in and of itself, this could potentially lead to a grounding-line re-advance, ice flow is modelled to respond to recent changes in temperature, accumulation rate, and basal processes more rapidly than it does to bedrock-elevation and/or sea-level fluctuations. Future projections of the Greenland ice sheet indicate a faster contribution to sea-level rise in a warming world than previously believed, based on numerical modelling using a parameterization of recent results showing surface-meltwater lubrication of Greenland ice flow (Zwally et al., 2002). Numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on Zwally et al. data under different global warming scenarios. Comparisons to reconstructions generated with a traditional sliding

  15. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

    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.

  17. Accurate and stable time stepping in ice sheet modeling

    NASA Astrophysics Data System (ADS)

    Cheng, Gong; Lötstedt, Per; von Sydow, Lina

    2017-01-01

    In this paper we introduce adaptive time step control for simulation of the evolution of ice sheets. The discretization error in the approximations is estimated using "Milne's device" by comparing the result from two different methods in a predictor-corrector pair. Using a predictor-corrector pair the expensive part of the procedure, the solution of the velocity and pressure equations, is performed only once per time step and an estimate of the local error is easily obtained. The stability of the numerical solution is maintained and the accuracy is controlled by keeping the local error below a given threshold using PI-control. Depending on the threshold, the time step Δt is bound by stability requirements or accuracy requirements. Our method takes a shorter Δt than an implicit method but with less work in each time step and the solver is simpler. The method is analyzed theoretically with respect to stability and applied to the simulation of a 2D ice slab and a 3D circular ice sheet. The stability bounds in the experiments are explained by and agree well with the theoretical results.

  18. Surface elevation contours of Greenland and Antarctic ice sheets

    NASA Technical Reports Server (NTRS)

    Zwally, H. J.; Bindschadler, R. A.; Brenner, A. C.; Martin, T. V.; Thomas, R. H.

    1983-01-01

    Preliminary results from Seasat radar altimetry over Antarctica north of 72 deg S and Greenland south of 72 deg N are presented. Surface elevations of the ice sheets, obtained from computer retracking of the radar altimeter waveforms, are contoured at 50-m intervals for Greenland and at 100-m intervals for Antarctica. Elevation differences at orbital crossover points are analyzed to obtain a precision of 1.9 m; this figure is partly determined by radial errors of approximately 1.0 m in orbital determination and partly by noise due to ice surface irregularities. Adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small, relatively flat region reduces the rms difference to 0.25 m, which is indicative of the optimum precision obtainable over the ice sheets. However, the precision degrades as the slope of the surface or amplitude of the undulations increases, yielding an overall precision of + or - 1.6 m.

  19. 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.

  20. Simultaneous solution for mass trends on the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Schoen, N.; Zammit-Mangion, A.; Rougier, J. C.; Flament, T.; Rémy, F.; Luthcke, S.; Bamber, J. L.

    2015-04-01

    The Antarctic Ice Sheet is the largest potential source of future sea-level rise. Mass loss has been increasing over the last 2 decades for the West Antarctic Ice Sheet (WAIS) but with significant discrepancies between estimates, especially for the Antarctic Peninsula. Most of these estimates utilise geophysical models to explicitly correct the observations for (unobserved) processes. Systematic errors in these models introduce biases in the results which are difficult to quantify. In this study, we provide a statistically rigorous error-bounded trend estimate of ice mass loss over the WAIS from 2003 to 2009 which is almost entirely data driven. Using altimetry, gravimetry, and GPS data in a hierarchical Bayesian framework, we derive spatial fields for ice mass change, surface mass balance, and glacial isostatic adjustment (GIA) without relying explicitly on forward models. The approach we use separates mass and height change contributions from different processes, reproducing spatial features found in, for example, regional climate and GIA forward models, and provides an independent estimate which can be used to validate and test the models. In addition, spatial error estimates are derived for each field. The mass loss estimates we obtain are smaller than some recent results, with a time-averaged mean rate of -76 ± 15 Gt yr-1 for the WAIS and Antarctic Peninsula, including the major Antarctic islands. The GIA estimate compares well with results obtained from recent forward models (IJ05-R2) and inverse methods (AGE-1). The Bayesian framework is sufficiently flexible that it can, eventually, be used for the whole of Antarctica, be adapted for other ice sheets and utilise data from other sources such as ice cores, accumulation radar data, and other measurements that contain information about any of the processes that are solved for.

  1. Simultaneous solution for mass trends on the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Schön, N.; Zammit-Mangion, A.; Bamber, J. L.; Rougier, J.; Flament, T.; Rémy, F.; Luthcke, S. B.

    2014-06-01

    The Antarctic Ice Sheet is the largest potential source of future sea-level rise. Mass loss has been increasing over the last two decades in the West Antarctic Ice Sheet (WAIS), but with significant discrepancies between estimates, especially for the Antarctic Peninsula. Most of these estimates utilise geophysical models to explicitly correct the observations for (unobserved) processes. Systematic errors in these models introduce biases in the results which are difficult to quantify. In this study, we provide a statistically rigorous, error-bounded trend estimate of ice mass loss over the WAIS from 2003-2009 which is almost entirely data-driven. Using altimetry, gravimetry, and GPS data in a hierarchical Bayesian framework, we derive spatial fields for ice mass change, surface mass balance, and glacial isostatic adjustment (GIA) without relying explicitly on forward models. The approach we use separates mass and height change contributions from different processes, reproducing spatial features found in, for example, regional climate and GIA forward models, and provides an independent estimate, which can be used to validate and test the models. In addition, full spatial error estimates are derived for each field. The mass loss estimates we obtain are smaller than some recent results, with a time-averaged mean rate of -76 ± 15 GT yr-1 for the WAIS and Antarctic Peninsula (AP), including the major Antarctic Islands. The GIA estimate compares very well with results obtained from recent forward models (IJ05-R2) and inversion methods (AGE-1). Due to its computational efficiency, the method is sufficiently scalable to include the whole of Antarctica, can be adapted for other ice sheets and can easily be adapted to assimilate data from other sources such as ice cores, accumulation radar data and other measurements that contain information about any of the processes that are solved for.

  2. 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.

  3. Quantifying supraglacial lake volumes on the Greenland ice sheet from spaceborne optical sensors

    NASA Astrophysics Data System (ADS)

    Moussavi, Mahsa S.

    The acceleration of mass loss from the Greenland ice sheet (GrIS) over the last two decades is of great significance when considering the associated increasing rates of its contribution to sea level rise. With meltwater runoff accounting for more than half of Greenland's contribution to sea level, it is imperative to improve our understanding of the ice sheet hydrologic processes. This dissertation describes research to quantify surface meltwater volumes, specifically stored in supraglacial lakes, over large areas of the ablation region from spaceborne observations made by current and future optical sensors. Methods for remote-derivation and validation of supraglacial water depths from WorldView-2, Landsat 7's Enhanced Thematic Mapper (ETM+), and Landsat 8's Operational Land Imager (OLI) are discussed. While enabling large-scale assessments of lake volumes with unprecedented levels of accuracy and precision, these methods address the major limitation of spaceborne bathymetry, which is its dependence on the availability of costly in-situ measurements. This dissertation also investigates the potential capability of the future ICESat-2 laser altimetry mission in providing depth information over supraglacial lakes, which could be used in conjunction with passive optical data to assess lake volumes. A nominal (first-order) prediction of ICESat-2's performance suggests that it might be able to retrieve lake depths down to 6 m. Given such a premise, this dissertation takes one step further to develop an automatic depth-retrieval algorithm from simulated ICESat-2 datasets, provided by the mission's Project team at Goddard Space Flight Center (GSFC). By developing a surface detection algorithm for the most complex surfaces that ICESat-2 will be looking at, namely forested ecosystems, this dissertation provides a method applicable to land ice, sea ice, and water surfaces. Furthermore, as part of ICESat-2's science definition team efforts and in direct support of the mission

  4. Expanded Late Wisconsinan ice cap and ice sheet margins in the western Queen Elizabeth Islands, Arctic Canada

    NASA Astrophysics Data System (ADS)

    Nixon, F. Chantel; England, John H.

    2014-05-01

    Recent mapping of surficial geology and geomorphology in the western Canadian High Arctic (Melville and Eglinton islands), together with new radiocarbon dates acquired from ice-contact raised marine sediments, document expanded late Wisconsinan ice limits for the northwest Laurentide Ice Sheet and the western Innuitian Ice Sheet. An extension of the northwestern margin of the Laurentide Ice Sheet onto Eglinton Island is proposed based on evidence from till containing erratics derived from the Canadian Shield and a pattern of meltwater channels indicating ice retreat offshore into M'Clure Strait. Expansion of the western Melville Island Ice Cap (part of the western, lowland sector of the Innuitian Ice Sheet) to its offshore late Wisconsinan limit was facilitated by coalescence with the Laurentide Ice Sheet, whose buttressing allowed thickening to occur. Estimates of ice extent and thickness (>500 m) of the western Melville Island Ice Cap are in agreement with high marine limits (≤70 m asl). Lateral and proglacial meltwater channels, moraines and glaciomarine, glaciolacustrine and glaciofluvial deposits indicate radial retreat of the western Melville Island Ice Cap onto central highlands after ˜13.0 cal ka BP. Older marine limit shorelines on southern Eglinton Island (˜13.6 cal ka BP) are broadly synchronous with the early and rapid deglaciation of other areas formerly glaciated by the northwestern Laurentide Ice Sheet to the southeast and southwest (˜14.2-13.6 cal ka BP). The collapse of the northwest Laurentide Ice Sheet in M'Clure Strait beginning at ˜14.2 cal ka BP, in addition to prior inferred thinning, opens the possibility that it made a significant contribution to meltwater pulse 1A.

  5. 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

  6. Predicting Ice Sheet and Climate Evolution at Extreme Scales

    SciTech Connect

    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 development 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, or

  7. Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate

    NASA Astrophysics Data System (ADS)

    Cornford, S. L.; Martin, D. F.; Payne, A. J.; Ng, E. G.; Le Brocq, A. M.; Gladstone, R. M.; Edwards, T. L.; Shannon, S. R.; Agosta, C.; van den Broeke, M. R.; Hellmer, H. H.; Krinner, G.; Ligtenberg, S. R. M.; Timmermann, R.; Vaughan, D. G.

    2015-08-01

    We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet, deploying sub-kilometer resolution around the grounding line since coarser resolution results in substantial underestimation of the response. Each of the simulations begins with a geometry and velocity close to present-day observations, and evolves according to variation in meteoric ice accumulation rates and oceanic ice shelf melt rates. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rate anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions and ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Within the Amundsen Sea Embayment the largest single source of variability is the onset of sustained retreat in Thwaites Glacier, which can triple the rate of eustatic sea level rise.

  8. Traveling slippery patches produce thickness-scale folds in ice sheets

    NASA Astrophysics Data System (ADS)

    Wolovick, Michael J.; Creyts, Timothy T.; Buck, W. Roger; Bell, Robin E.

    2014-12-01

    Large, complex stratigraphic folds that rise as high as 60% of the local ice thickness have been observed in ice sheets on Antarctica and Greenland. Here we show that ice deformation caused by heterogeneous and time-variable basal sliding can produce the observed structures. We do this using a thermomechanical ice sheet model in which sliding occurs when the base approaches the melting point and slippery patches develop. These slippery patches emerge and travel downstream because of a feedback between ice deformation, vertical flow, and temperature. Our model produces the largest overturned structures, comparable to observations, when the patches move at about the ice column velocity. We conclude that the history of basal slip conditions is recorded in the ice sheet strata. These basal conditions appear to be dynamic and heterogeneous even in the slow-flowing interior regions of large ice sheets.

  9. Aerosol size distribution at Nansen Ice Sheet Antarctica

    NASA Astrophysics Data System (ADS)

    Belosi, F.; Contini, D.; Donateo, A.; Santachiara, G.; Prodi, F.

    2012-04-01

    During austral summer 2006, in the framework of the XXII Italian Antarctic expedition of PNRA (Italian National Program for Research in Antarctica), aerosol particle number size distribution measurements were performed in the 10-500 range nm over the Nansen Ice Sheet glacier (NIS, 74°30' S, 163°27' E; 85 m a.s.l), a permanently iced branch of the Ross Sea. Observed total particle number concentrations varied between 169 and 1385 cm- 3. A monomodal number size distribution, peaking at about 70 nm with no variation during the day, was observed for continental air mass, high wind speed and low relative humidity. Trimodal number size distributions were also observed, in agreement with measurements performed at Aboa station, which is located on the opposite side of the Antarctic continent to the NIS. In this case new particle formation, with subsequent particle growth up to about 30 nm, was observed even if not associated with maritime air masses.

  10. 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.

  11. Self-consistent ice-sheet properties: ice dynamics, temperature, accumulation, delta-age and chronologies for ice cores and radar isochrones

    NASA Astrophysics Data System (ADS)

    Lundin, J.; Waddington, E. D.; Conway, H.

    2011-12-01

    Ice sheet behavior has not previously been modeled to force self-consistency, to determine histories of accumulation, temperature, and ice dynamics that incorporate the ice-age/gas-age offset (delta-age) and sparse depth-age measurements from ice cores. An iterative scheme is used to combine several modular components into one self-consistent model. The goal is to determine a suite of histories constrained by the depth-age data from ice cores and ice radar that are part of a physically self-consistent ice sheet. The model is tested using a synthetic data set resembling WAIS divide. Using synthetic data provides proof of concept that histories of accumulation, temperature and ice dynamics can be recovered by the self-consistent model, and that the depth-age from ice cores and ice radar can be matched. Results from synthetic data show we can recover the ice-sheet properties used to generate the data and we can improve the depth-age chronologies by interpolating with an ice-flow model where data are sparse. When this self-consistent model can be applied to field data, results will (1) improve chronologies for ice cores and radar layers, (2) determine histories of accumulation for GCM modelling, and (3) improve estimates of past ice sheet configurations, incorporating data from ice cores and ice radar.

  12. Greenland Ice Sheet Program. 1979. Phase 1. Casing Operation.

    DTIC Science & Technology

    1980-06-01

    A-l0-1119 699 COLD. REGIONS RESEARCH AND ENGINEERING LAS HANOVER NH F/6 8/12 GREENLAND ICE SHEET PROGRAM. 1979. PHASE 1. CASING OPERATION. (U) JNAB J...Thermal drills Drills Glac iology Greenland 124 AnRACr (cimEaeu m reverse ssNnouiesaw ad Middfr by block nmber) )A modified CHREL thermal drill was used at...DYE-3 in Greenland to drill a 8.T5-in.-diamneter hole 251 ft deep for the installation of a steel casing. This activity was accomplished by a drill

  13. Collection and curation of IDPs in the stratosphere and below. Part 2: The Greenland and Antarctic ice sheets

    NASA Technical Reports Server (NTRS)

    Maurette, Michel; Hammer, C.; Harvey, R.; Immel, G.; Kurat, G.; Taylor, S.

    1994-01-01

    In a companion paper, Zolensky discusses interplanetary dust particles (IDP's) collected in the stratosphere. Here, we describe the recovery of much larger unmelted to partially melted IDP's from the Greenland and Antarctica ice sheet, and discuss problems arising in their collection and curation, as well as future prospects for tackling these problems.

  14. 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.

  15. Observations from the Programme for Monitoring of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Andersen, S. B.; Ahlstrom, A. P.; Andersen, M. L.; Box, J. E.; Citterio, M.; Colgan, W. T.; Fausto, R. S.; van As, D.; Forsberg, R.; Skourup, H.; Sandberg Sørensen, L.; Kristensen, S. S.; Dall, J.; Kusk, A.; Petersen, D.

    2014-12-01

    The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) is as an on-going effort initiated in 2007 to monitor changes in the mass budget of the Greenland Ice Sheet. The aim of the programme is to quantify the mass loss of the Greenland ice sheet and track changes in the extent of the glaciers, ice caps and ice sheet margin. Specifically, PROMICE aims to estimate the mass loss derived from three fundamentally different sources: Surface melt water runoff from the ice sheet margin Iceberg production Mass loss of individual glaciers and ice caps surrounding the ice sheet The first is observed by a network of automatic weather stations (AWS) on the ice sheet margin measuring ice ablation as well as meteorological parameters. The second is determined by establishing a so-called 'flux gate' along the entire ice sheet margin and keeping track of the ice passing through this gate. The flux gate is obtained from airborne surveys of ice sheet surface elevation and thickness. The volume of the ice passing through the gate is derived from maps of the surface velocity of the ice sheet, produced from satellite radar. The third is investigated through regular mapping of area and elevation of the approximately 20.000 individual glaciers and ice caps in Greenland. Mapping is carried out using recent satellite imagery as well as aerial ortho-photos. Within PROMICE data sets from these activities are collected. They include observations from the network of currently about 20 AWS on the margin of the Greenland ice sheet. Airborne surveys, yielding surface elevation and ice depth along the entire margin of the Greenland ice sheet carried out in 2007 and 2011. A map of all Greenland ice masses, based on the highest detail aero-photogrammetric maps produced from mid-80's aerial photographs. Real-time data from the PROMICE AWS network is shown at the web site www.promice.org and the data is freely available for download. Data from the airborne surveys and mapping activities are

  16. The darkening of the Greenland ice sheet: trends, drivers and projections (1981-2100)

    NASA Astrophysics Data System (ADS)

    Tedesco, M.; Doherty, S.; Fettweis, X.; Alexander, P.; Jeyaratnam, J.; Noble, E.; Stroeve, J.

    2015-10-01

    The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using spaceborne multispectral data collected during the three decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade-1 between 1996 and 2012. The negative trend is confined to the regions of the GrIS that undergo melting in summer with the dry-snow zone showing no trend. The period 1981-1996 showed no statistically significant trend. The analysis of the outputs of a regional climate model indicates that the drivers of the observed albedo decrease is imputable to a combination of increased near-surface temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, as well as by trends in light-absorbing impurities on the snow and ice surfaces. Neither aerosol models nor in situ observations indicate increasing trends in impurities in the atmosphere over Greenland, suggesting that their apparent increase in snow and ice might be related to the exposure of a "dark band" of dirty ice and to the consolidation of impurities at the surface with melt. Albedo projections through the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies in 2100 averaged over the whole ice sheet lower than in 2000 by 0.08, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in projected melting and because the model albedo scheme does not currently include light-absorbing impurities and the effect of biological activity, which themselves have a positive feedback, leading to increased melting, grain growth and darkening.

  17. 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

  18. IceBridge Radar as a Tool for Understanding Accumulation Variability throughout the Western Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Lewis, G.; Hawley, R. L.; Osterberg, E. C.; Marshall, H. P.

    2015-12-01

    Understanding the mass balance of the Greenland Ice Sheet (GIS) in a warming climate is of critical interest to scientists and the general public in the context of future sea-level rise. The mass balance of the GIS has been increasingly negative over recent decades, with a conservative estimate of ice sheet mass loss of 2000 Gt yr-1 over the 1988-2014 period. Snow accumulation, the critical input to surface mass balance, varies through time and regionally around Greenland and from the coast to interior, but is constrained by relatively few observations. An improved understanding of temporal and spatial snow accumulation variability will thus reduce uncertainties in GIS mass balance models and sea-level rise projections. Here we quantify spatial and temporal variations in snow accumulation in central and southwest Greenland using data from NASA's Operation IceBridge Accumulation Radar. We estimate depth-density and depth-age relationships using a Herron-Langway density model and Nye flow model, which are iteratively calculated at 50 km intervals along each radar line. We then trace isochrones to calculate spatial and temporal variability in average snow accumulation along several flight paths throughout central and southwestern Greenland. Depth-density, depth-age, and snow accumulation calculations are calibrated at Summit using data from the GISP2, Katie, and SM07C ice cores. Accumulation results will be verified by a series of snow pits, shallow firn cores, and ice-penetrating radar profiles collected on the Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) through southwest Greenland during spring 2016 and 2017.

  19. Tracking millennial-scale Holocene glacial advance and retreat using osmium isotopes: Insights from the Greenland ice sheet

    USGS Publications Warehouse

    Rooney, Alan D.; Selby, David; Llyod, Jeremy M.; Roberts, David H.; Luckge, Andreas; Sageman, Bradley B.; Prouty, Nancy G.

    2016-01-01

    High-resolution Os isotope stratigraphy can aid in reconstructing Pleistocene ice sheet fluctuation and elucidating the role of local and regional weathering fluxes on the marine Os residence time. This paper presents new Os isotope data from ocean cores adjacent to the West Greenland ice sheet that have excellent chronological controls. Cores MSM-520 and DA00-06 represent distal to proximal sites adjacent to two West Greenland ice streams. Core MSM-520 has a steadily decreasing Os signal over the last 10 kyr (187Os/188Os = 1.35–0.81). In contrast, Os isotopes from core DA00-06 (proximal to the calving front of Jakobshavn Isbræ) highlight four stages of ice stream retreat and advance over the past 10 kyr (187Os/188Os = 2.31; 1.68; 2.09; 1.47). Our high-resolution chemostratigraphic records provide vital benchmarks for ice-sheet modelers as we attempt to better constrain the future response of major ice sheets to climate change. Variations in Os isotope composition from sediment and macro-algae (seaweed) sourced from regional and global settings serve to emphasize the overwhelming effect weathering sources have on seawater Os isotope composition. Further, these findings demonstrate that the residence time of Os is shorter than previous estimates of ∼104 yr.

  20. Tracking millennial-scale Holocene glacial advance and retreat using osmium isotopes: Insights from the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Rooney, Alan D.; Selby, David; Lloyd, Jeremy M.; Roberts, David H.; Lückge, Andreas; Sageman, Bradley B.; Prouty, Nancy G.

    2016-04-01

    High-resolution Os isotope stratigraphy can aid in reconstructing Pleistocene ice sheet fluctuation and elucidating the role of local and regional weathering fluxes on the marine Os residence time. This paper presents new Os isotope data from ocean cores adjacent to the West Greenland ice sheet that have excellent chronological controls. Cores MSM-520 and DA00-06 represent distal to proximal sites adjacent to two West Greenland ice streams. Core MSM-520 has a steadily decreasing Os signal over the last 10 kyr (187Os/188Os = 1.35-0.81). In contrast, Os isotopes from core DA00-06 (proximal to the calving front of Jakobshavn Isbræ) highlight four stages of ice stream retreat and advance over the past 10 kyr (187Os/188Os = 2.31; 1.68; 2.09; 1.47). Our high-resolution chemostratigraphic records provide vital benchmarks for ice-sheet modelers as we attempt to better constrain the future response of major ice sheets to climate change. Variations in Os isotope composition from sediment and macro-algae (seaweed) sourced from regional and global settings serve to emphasize the overwhelming effect weathering sources have on seawater Os isotope composition. Further, these findings demonstrate that the residence time of Os is shorter than previous estimates of ∼104 yr.

  1. Increasing temperature forcing reduces the Greenland Ice Sheet's response time scale

    NASA Astrophysics Data System (ADS)

    Applegate, Patrick J.; Parizek, Byron R.; Nicholas, Robert E.; Alley, Richard B.; Keller, Klaus

    2015-10-01

    Damages from sea level rise, as well as strategies to manage the associated risk, hinge critically on the time scale and eventual magnitude of sea level rise. Satellite observations and paleo-data suggest that the Greenland Ice Sheet (GIS) loses mass in response to increased temperatures, and may thus contribute substantially to sea level rise as anthropogenic climate change progresses. The time scale of GIS mass loss and sea level rise are deeply uncertain, and are often assumed to be constant. However, previous ice sheet modeling studies have shown that the time scale of GIS response likely decreases strongly with increasing temperature anomaly. Here, we map the relationship between temperature anomaly and the time scale of GIS response, by perturbing a calibrated, three-dimensional model of GIS behavior. Additional simulations with a profile, higher-order, ice sheet model yield time scales that are broadly consistent with those obtained using the three-dimensional model, and shed light on the feedbacks in the ice sheet system that cause the time scale shortening. Semi-empirical modeling studies that assume a constant time scale of sea level adjustment, and are calibrated to small preanthropogenic temperature and sea level changes, may underestimate future sea level rise. Our analysis suggests that the benefits of reducing greenhouse gas emissions, in terms of avoided sea level rise from the GIS, may be greatest if emissions reductions begin before large temperature increases have been realized. Reducing anthropogenic climate change may also allow more time for design and deployment of risk management strategies by slowing sea level contributions from the GIS.

  2. How much did the North American ice sheet contribute to Meltwater Pulse 1a?

    NASA Astrophysics Data System (ADS)

    Gregoire, Lauren; Otto-Bliesner, Bette; Valdes, Paul

    2016-04-01

    Constraining the source of Meltwater Pulse 1a (MWP1a), a sea level rise of 12 to 22 m in less than 350 years, 14.6 ka (Deschamps et al., 2012) is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change. The North American ice sheet could have been a major contributor to this event due to the abrupt Northern Hemisphere Bølling warming at 14.7 ka and the collapse of the ice saddle between the Cordilleran and Laurentide ice sheets which caused accelerated melt (Gregoire et al., 2012). Here, we combine modelling of the North American ice sheet with observational constraints of ice extent evolution and sea level change to evaluate how and how much the North American ice sheet could have contributed to MWP1a. We drive an ice sheet model offline with transient climate experiments of the last deglaciation (21-7 ka) performed with two General Circulation Models (CCSM3 and FAMOUS) and run perturbed physics ensembles of ice sheet model experiments to take into account both climate and ice sheet model uncertainties. By ruling out experiments which do not match the evolution of ice extent and volume through the deglaciation (21-7 ka), we determine the range of plausible sea level rise associated with MWP1a. The North American ice sheet produces a sea level rise of 3-6 m in 350 years in response to the Bølling warming and 7-10 m over the same duration due to the ice saddle collapse during the separation of the Cordilleran and Laurentide ice sheets. Although not seen in our experiments, it is possible that the Bølling abrupt warming triggered the saddle collapse, in which case the meltwater flux would have been substantially amplified.

  3. Effect of subglacial volcanism on changes in the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    Behrendt, John C.

    1993-01-01

    Rapid changes in the West Antarctic Ice Sheet (WAIS) may affect future global sea-level changes. Alley and Whillans note that 'the water responsible for separating the glacier from its bed is produced by frictional dissipation and geothermal heat,' but assume that changes in geothermal flux would ordinarily be expected to have slower effects than glaciological parameters. I suggest that episodic subglacial volcanism and geothermal heating may have significantly greater effects on the WAIS than is generally appreciated. The WAIS flows through the active, largely asiesmic West Antarctic rift system (WS), which defines the sub-sea-level bed of the glacier. Various lines of evidence summarized in Behrendt et al. (1991) indicate high heat flow and shallow asthenosphere beneath the extended, weak lithosphere underlying the WS and the WAIS. Behrendt and Cooper suggest a possible synergistic relation between Cenozoic tectonism, episodic mountain uplift and volcanism in the West Antarctic rift system, and the waxing and waning of the Antarctic ice sheet beginning about earliest Oligocene time. A few active volcanoes and late-Cenozoic volcanic rocks are exposed throughout the WS along both flanks, and geophysical data suggest their presence beneath the WAIS. No part of the rift system can be considered inactive. I propose that subglacial volcanic eruptions and ice flow across areas of locally (episodically?) high heat flow--including volcanically active areas--should be considered possibly to have a forcing effect on the thermal regime resulting in increased melting at the base of the ice streams.

  4. Sensitivity of ocean circulation and sea-ice conditions to loss of West Antarctic ice shelves and ice sheet

    NASA Astrophysics Data System (ADS)

    Bougamont, Marion; Hunke, Elizabeth C.; Tulaczyk, Slawek

    We use a global coupled ocean-sea ice model to test the hypothesis that the disintegration of the West Antarctic ice sheet (WAIS), or just its ice shelves, may modify ocean circulation and sea-ice conditions in the Southern Ocean. We compare the results of three model runs: (1) a control run with a standard (modern) configuration of landmask in West Antarctica, (2) a no-shelves run with West Antarctic ice shelves removed and (3) a no-WAIS run. In the latter two runs, up to a few million square kilometres of new sea surface area opens to sea-ice formation, causing the volume and extent of Antarctic sea-ice cover to increase compared with the control run. In general, near-surface waters are cooler around Antarctica in the no-shelves and no-WAIS model runs than in the control run, while warm intermediate and deep waters penetrate further south, increasing poleward heat transport. Varying regional responses to the imposed changes in landmask configuration are determined by the fact that Antarctic polynyas and fast ice develop in different parts of the model domain in each run. Model results suggest that changes in the extent of WAIS may modify oceanographic conditions in the Southern Ocean.

  5. 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.

  6. Advances in Antarctic Mantle and Crustal Physics and Implications for Ice Sheet Models and Isostatic Adjustment Measurements

    NASA Astrophysics Data System (ADS)

    Ivins, Erik; Adhikari, Surendra; Seroussi, Helene; Larour, Eric; Wiens, Douglas; Scheinert, Mirko; Csatho, Beata; James, Thomas; Nyblade, Andrew

    2015-04-01

    The problem of improving both solid Earth structure models and assembling an appropriate tectonic framework for Antarctica is challenging for many reasons. The vast ice sheet cover is just one item in a long list of difficult observational challenges faced by solid Earth scientists. The ice sheet has a unique potential for causing relatively rapid global sea-level rise over the next couple of hundred years. This potential provides great impetus for employing extraordinary efforts to improve our knowledge of the thermo-mechanical properties and of the mass and energy transport systems operating in the underlying solid Earth. In this presentation we discuss the role of seismic mapping of the mantle and crust, heat flux inferences, models and measurements as they affect the state of the ice sheet and the predictions of present-day and future solid Earth glacial isostatic adjustment, global and regional sea-level variability. To illustrate the sensitivity to solid Earth parameters for deriving a model temperature at the base of the ice sheet, Tb, we have computed the differences between two models to produce maps of δTb, the differential temperature to the melting point at the base of the ice sheet using the ISSM 3-D Stokes flow model. A 'cold' case (with surface crustal heat flux qGHF = 40 mW/m^2) is compared to a 'hot' geothermal flux case (qGHF = 60 mW/m^2). Differences of δ Tb = 6 - 10 °C are predicted between the two heat flux assumptions, and these have associated differences in predicted ice velocities of a factor of 1.8-3.6. We also explore the hypothesis of a mantle plume, and its potential compatibility or incompatibility with basal ice sheet conditions in West Antarctica.

  7. Meltwater export of prokaryotic cells from the Greenland ice sheet.

    PubMed

    Cameron, Karen A; Stibal, Marek; Hawkings, Jon R; Mikkelsen, Andreas B; Telling, Jon; Kohler, Tyler J; Gözdereliler, Erkin; Zarsky, Jakub D; Wadham, Jemma L; Jacobsen, Carsten S

    2017-02-01

    Microorganisms are flushed from the Greenland Ice Sheet (GrIS) where they may contribute towards the nutrient cycling and community compositions of downstream ecosystems. We investigate meltwater microbial assemblages as they exit the GrIS from a large outlet glacier, and as they enter a downstream river delta during the record melt year of 2012. Prokaryotic abundance, flux and community composition was studied, and factors affecting community structures were statistically considered. The mean concentration of cells exiting the ice sheet was 8.30 × 10(4) cells mL(-1) and we estimate that ∼1.02 × 10(21) cells were transported to the downstream fjord in 2012, equivalent to 30.95 Mg of carbon. Prokaryotic microbial assemblages were dominated by Proteobacteria, Bacteroidetes, and Actinobacteria. Cell concentrations and community compositions were stable throughout the sample period, and were statistically similar at both sample sites. Based on our observations, we argue that the subglacial environment is the primary source of the river-transported microbiota, and that cell export from the GrIS is dependent on discharge. We hypothesise that the release of subglacial microbiota to downstream ecosystems will increase as freshwater flux from the GrIS rises in a warming world.

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

    PubMed

    Gasson, Edward; DeConto, Robert M; Pollard, David; Levy, Richard H

    2016-03-29

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

    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.

  10. Ice-age simulations with a caving ice-sheet model

    SciTech Connect

    Pollard, D.

    1982-01-01

    Further extensions and results of a simple northern hemispheric ice-sheet model are described for the Quaternary ice ages. The basic model predicts ice thickness and bedrock deformation in a north-south cross section, with a prescribed snow-budget distribution shifted uniformly in space to represent the orbital perturbations. An ice calving parameterization crudely representing proglacial lakes or marine incursions can attack the ice whenever the tip drops below sea level. As in Pollard (1982a) the basic model produces a large approx. 100,000-yr response and agrees fairly well with the delta/sup 18/O deep-sea core records. Three extensions of the model are described: an alternative treatment of bedrock deformation, a more realistic ice-shelf model of ice calving, and a generalized parameterization for such features as the North Atlantic deglacial meltwater layer. Much the same ice-age simulations and agreement with the delta/sup 18/O records as with the original model are still obtained. The observed phase-correlation between the 100,000-yr cycles and eccentricity is examined. First, the model is shown to give a approx. 100,000-yr response to nearly any kind of higher-frequency forcing. Although over the last two million years the model phase is mainly controlled by the precessional modulation due to eccentricity, over just the last 600,000-yr the observed phase can also be simulated with eccentricity held constant. A definitive conclusion on the phase-control of the Quaternary ice ages is prevented by uncertainty in the deep-sea core time scales before approx. 600,000-yr B.P. In an appendix the model is applied to West Antarctica where unforced internal oscillations occur with periods of about 50,000-yr.

  11. The effects of climate uncertainty on the stability of the Antarctic ice sheet during the mid-Pliocene warm period

    NASA Astrophysics Data System (ADS)

    Bernales, Jorge; Häfliger, Tonio; Rogozhina, Irina; Thomas, Maik

    2015-04-01

    The mid-Pliocene (3.15 to 2.85 million years before present) is the most recent period in Earth's history when temperatures and CO2 concentrations were sustainedly higher than pre-industrial values [1], representing an ideal interval for studying the climate system under conditions similar to those projected for the end of this century. In these projections, the response of the Antarctic ice sheet (AIS) remains uncertain, including areas generally considered stable under a warming climate. Therefore, a better understanding of AIS's behaviour during periods like the mid-Pliocene will provide valuable information on the potential vulnerability of the composite parts of the AIS in the future. For this purpose, we have designed numerical experiments of the AIS dynamics during the mid-Pliocene warm period using the continental-scale ice sheet-shelf model SICOPOLIS [2]. To account for the uncertainties in the configuration of the AIS and climate conditions prior to this period, we employ a wide range of initial ice sheet configurations and climatologies, including modern observations, the results from the Pliocene Model Intercomparison Project (PlioMIP) climate experiments [3], and perturbations to single climatic fields, allowing us to assess the vulnerability of different AIS sectors to specific forcing mechanisms. Our simulations show that the West Antarctic ice sheet remains largely ice-free under the chosen range of climate conditions, except for small portions grounded above sea level. On the contrary, the East Antarctic ice sheet (EAIS) shows no signs of potential collapse, with an ice loss over a few peripheral sectors largely compensated by an increase in ice volume over the interior due to increased precipitation rates and surface temperatures remaining well below the freezing point. Furthermore, our results contrast with existing hypotheses that cast doubt on the stability of the EAIS during the mid-Pliocene warm period. References [1] Cook, C. P., et al

  12. Probabilistic framework for assessing the ice sheet contribution to sea level change.

    PubMed

    Little, Christopher M; Urban, Nathan M; Oppenheimer, Michael

    2013-02-26

    Previous sea level rise (SLR) assessments have excluded the potential for dynamic ice loss over much of Greenland and Antarctica, and recently proposed "upper bounds" on Antarctica's 21st-century SLR contribution are derived principally from regions where present-day mass loss is concentrated (basin 15, or B15, drained largely by Pine Island, Thwaites, and Smith glaciers). Here, we present a probabilistic framework for assessing the ice sheet contribution to sea level change that explicitly accounts for mass balance uncertainty over an entire ice sheet. Applying this framework to Antarctica, we find that ongoing mass imbalances in non-B15 basins give an SLR contribution by 2100 that: (i) is comparable to projected changes in B15 discharge and Antarctica's surface mass balance, and (ii) varies widely depending on the subset of basins and observational dataset used in projections. Increases in discharge uncertainty, or decreases in the exceedance probability used to define an upper bound, increase the fractional contribution of non-B15 basins; even weak spatial correlations in future discharge growth rates markedly enhance this sensitivity. Although these projections rely on poorly constrained statistical parameters, they may be updated with observations and/or models at many spatial scales, facilitating a more comprehensive account of uncertainty that, if implemented, will improve future assessments.

  13. A parallel high-order accurate finite element nonlinear Stokes ice sheet model and benchmark experiments: A PARALLEL FEM STOKES ICE SHEET MODEL

    SciTech Connect

    Leng, Wei; Ju, Lili; Gunzburger, Max; Price, Stephen; Ringler, Todd

    2012-01-04

    The numerical modeling of glacier and ice sheet evolution is a subject of growing interest, in part because of the potential for models to inform estimates of global sea level change. This paper focuses on the development of a numerical model that determines the velocity and pressure fields within an ice sheet. Our numerical model features a high-fidelity mathematical model involving the nonlinear Stokes system and combinations of no-sliding and sliding basal boundary conditions, high-order accurate finite element discretizations based on variable resolution grids, and highly scalable parallel solution strategies, all of which contribute to a numerical model that can achieve accurate velocity and pressure approximations in a highly efficient manner. We demonstrate the accuracy and efficiency of our model by analytical solution tests, established ice sheet benchmark experiments, and comparisons with other well-established ice sheet models.

  14. 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

  15. 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.

  16. Laser altimetry reveals complex pattern of Greenland Ice Sheet dynamics.

    PubMed

    Csatho, Beata M; Schenk, Anton F; van der Veen, Cornelis J; Babonis, Gregory; Duncan, Kyle; Rezvanbehbahani, Soroush; van den Broeke, Michiel R; Simonsen, Sebastian B; Nagarajan, Sudhagar; van Angelen, Jan H

    2014-12-30

    We present a new record of ice thickness change, reconstructed at nearly 100,000 sites on the Greenland Ice Sheet (GrIS) from laser altimetry measurements spanning the period 1993-2012, partitioned into changes due to surface mass balance (SMB) and ice dynamics. We estimate a mean annual GrIS mass loss of 243 ± 18 Gt ⋅ y(-1), equivalent to 0.68 mm ⋅ y(-1) sea level rise (SLR) for 2003-2009. Dynamic thinning contributed 48%, with the largest rates occurring in 2004-2006, followed by a gradual decrease balanced by accelerating SMB loss. The spatial pattern of dynamic mass loss changed over this time as dynamic thinning rapidly decreased in southeast Greenland but slowly increased in the southwest, north, and northeast regions. Most outlet glaciers have been thinning during the last two decades, interrupted by episodes of decreasing thinning or even thickening. Dynamics of the major outlet glaciers dominated the mass loss from larger drainage basins, and simultaneous changes over distances up to 500 km are detected, indicating climate control. However, the intricate spatiotemporal pattern of dynamic thickness change suggests that, regardless of the forcing responsible for initial glacier acceleration and thinning, the response of individual glaciers is modulated by local conditions. Recent projections of dynamic contributions from the entire GrIS to SLR have been based on the extrapolation of four major outlet glaciers. Considering the observed complexity, we question how well these four glaciers represent all of Greenland's outlet glaciers.

  17. Glacial isostatic stress shadowing by the Antarctic ice sheet

    NASA Technical Reports Server (NTRS)

    Ivins, E. R.; James, T. S.; Klemann, V.

    2005-01-01

    Numerous examples of fault slip that offset late Quaternary glacial deposits and bedrock polish support the idea that the glacial loading cycle causes earthquakes in the upper crust. A semianalytical scheme is presented for quantifying glacial and postglacial lithospheric fault reactivation using contemporary rock fracture prediction methods. It extends previous studies by considering differential Mogi-von Mises stresses, in addition to those resulting from a Coulomb analysis. The approach utilizes gravitational viscoelastodynamic theory and explores the relationships between ice mass history and regional seismicity and faulting in a segment of East Antarctica containing the great Antarctic Plate (Balleny Island) earthquake of 25 March 1998 (Mw 8.1). Predictions of the failure stress fields within the seismogenic crust are generated for differing assumptions about background stress orientation, mantle viscosity, lithospheric thickness, and possible late Holocene deglaciation for the D91 Antarctic ice sheet history. Similar stress fracture fields are predicted by Mogi-von Mises and Coulomb theory, thus validating previous rebound Coulomb analysis. A thick lithosphere, of the order of 150-240 km, augments stress shadowing by a late melting (middle-late Holocene) coastal East Antarctic ice complex and could cause present-day earthquakes many hundreds of kilometers seaward of the former Last Glacial Maximum grounding line.

  18. Linking climate history and ice crystalline fabric evolution in polar ice sheets

    NASA Astrophysics Data System (ADS)

    Kennedy, Joseph Huston

    An ice sheet consists of an unfathomable number of grains that typically have a preferred orientation of the crystalline lattices, termed fabric. At the surface of ice sheets, the microstructural processes which control the grain structure and fabric evolution are influenced by climate variables. Layers of firn, in different climate regimes, may have an observable variation in fabric which can persist deep into the ice sheet; fabric may have 'memory' of these past climate regimes. To model the evolution of a subtle variation in fabric below the firn-ice transition, we have developed and released an open-source Fabric Evolution with Recrystallization (FEvoR) model. FEvoR is an anisotropic stress model that distributes stresses through explicit nearest-neighbor interaction. The model includes parameterizations of grain growth, rotation recrystallization and migration recrystallization which account for the major recrystallization processes that affect the macroscopic grain structure and fabric evolution. Using this model, we explore the evolution of a subtle variation in near-surface fabric using both constant applied stress and a stress-temperature history based on data from Taylor Dome, East Antarctica. Our results show that a subtle fabric variation will be preserved for ≈200 ka in compressive stress regimes with temperatures typical of polar ice-sheets. The addition of shear to compressive stress regimes preserves fabric variations longer than in compression-only regimes because shear drives a positive feedback between crystal rotation and deformation. We find that temperature affects how long the fabric variation is preserved, but does not affect the strain-integrated fabric evolution profile except when crossing the thermal-activation-energy threshold (≈ -10°C). Even at high temperatures, migration recrystallization does not rid the fabric of its memory under most conditions. High levels of nearest-neighbor interactions between grains will rid the fabric

  19. Ross Ice Shelf Seismic Survey and Future Drilling Recommendation

    NASA Astrophysics Data System (ADS)

    van Haastrecht, Laurine; Ohneiser, Christian; Gorman, Andrew; Hulbe, Christina

    2016-04-01

    The Ross Ice Shelf (RIS) is one of three gateways through which change in the ocean can be propagated into the interior of West Antarctica. Both the geologic record and ice sheet models indicate that it has experienced widespread retreat under past warm climates. But inland of the continental shelf, there are limited data available to validate the models. Understanding what controls the rate at which the ice shelf will respond to future climate change is central to making useful climate projections. Determining the retreat rate at the end of the last glacial maximum is one part of this challenge. In November 2015, four lines of multi-channel seismic data, totalling over 45 km, were collected on the Ross Ice Shelf, approximately 300 km south of Ross Island using a thumper seismic source and a 96 channel snow streamer. The seismic survey was undertaken under the New Zealand Antarctic Research Institute (NZARI) funded Aotearoa New Zealand Ross Ice Shelf Programme to resolve bathymetric details and to image sea floor sediments under a proposed drilling site on the ice shelf, at about 80.7 S and 174 E. The thumper, a purpose-built, trailer mounted, weight-drop seismic source was towed behind a Hägglund tracked vehicle to image the bathymetry and sediments underneath the RIS. Seismic data collection on an ice shelf has unique challenges, in particular strong attenuation of the seismic energy by snow and firn, and complex multiple ray paths. The thumper, which consists of a heavy weight (250kg) that is dropped on a large, ski mounted steel plate, produced a consistent, repeatable higher energy signal when compared to sledge hammer source and allowed for a greater geographic coverage and lower environmental impact than an explosive source survey. Our survey revealed that the seafloor is smooth and that there may be up to 100 m of layered sediments beneath the seafloor and possibly deeper, more complex structures. A multiple generated by internally reflected seismic energy

  20. 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

  1. Inter-annual and geographical variations in the extent of bare ice and dark ice on the Greenland ice sheet derived from MODIS satellite images

    NASA Astrophysics Data System (ADS)

    Shimada, Rigen; Takeuchi, Nozomu; Aoki, Teruo

    2016-04-01

    Areas of dark ice have appeared on the Greenland ice sheet every summer in recent years. These are likely to have a great impact on the mass balance of the ice sheet because of their low albedo. We report annual and geographical variations in the bare ice and dark ice areas that appeared on the Greenland Ice Sheet from 2000 to 2014 by using MODIS satellite images. The July monthly mean of the extent of bare ice showed a positive trend over these 15 years, and large annual variability ranging from 89,975 km2 to 279,075 km2, 5% and 16% of the entire ice sheet, respectively. The extent of dark ice also showed a positive trend and varied annually, ranging from 3,575 km2 to 26,975 km2, 4% and 10% of the bare ice extent. These areas are geographically varied, and their expansion is the greatest on the western side, particularly the southwestern side of the ice sheet. The bare ice extent correlates strongly with the monthly mean air temperature in July, suggesting that the extent was determined by snow melt. The dark ice extent also correlates with the air temperature; however, the correlation is weaker. The dark ice extent further correlates negatively with solar radiation. This suggests that the extent of dark ice is not only controlled by snow melt on the ice, but also by changes in the surface structures of the bare ice surface, such as cryoconite holes, which are associated with impurities appearing on the ice surface.

  2. The effects of mountain building in Greenland on the initiation of the ice sheet

    NASA Astrophysics Data System (ADS)

    Solgaard, Anne; Bonow, Johan; Langen, Peter; Japsen, Peter; Hvidberg, Christine

    2013-04-01

    We investigate the effects of a new hypothesis about mountain building in Greenland on ice sheet initiation using an ice sheet model in combination with a climate model. According to this hypothesis, the present-day mountains formed after two phases of uplift since the Late Miocene where the Greenland topography was close to sea level. Ice sheet initiation is studied using the Greenland topography before uplift and after each phase of uplift by applying different forcing conditions relevant for the time period. We find a large response of ice sheet growth to the cooling and precipitation increase related to the local effect of elevation change, but also that a Föhn effect is induced which inhibits ice sheet expansion into the interior Greenland. Despite this, the second phase of uplift facilitates ice-sheet initiation. The area of main ice sheet nucleation is relocated from the northern tip of Greenland to the eastern coastal mountains following the second phase of uplift. The period where uplift occurred was a time characterized by a long term cooling trend but with cold and warm excursions superimposed on this. We compare our results with observations of ice sheet extent over this period. We find that they represent conditions well, and that the mountain building history augments the effect in Greenland of the climatic deterioration leading to the Northern Hemisphere glaciations.

  3. Rapid sea-level rise soon from West Antarctic ice sheet collapse?

    SciTech Connect

    Bentley, C.R.

    1997-02-21

    This article presents a commentary on the possibility of a global sea-level rise because of shrinkage of the west antarctic ice sheet (WAIS). The WAIS is the focus of attention because of general agreement among glaciologists that only a marine ice sheet is likely to undergo rapid change, and WAIS appears to be the most vulnerable. 14 refs., 1 fig.

  4. 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.

  5. Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet