Ice Sheet

14,000,000 Leading Edge Experts on the ideXlab platform

Scan Science and Technology

Contact Leading Edge Experts & Companies

Scan Science and Technology

Contact Leading Edge Experts & Companies

The Experts below are selected from a list of 73770 Experts worldwide ranked by ideXlab platform

Richard B. Alley - One of the best experts on this subject based on the ideXlab platform.

  • windblown pliocene diatoms and east antarctic Ice Sheet retreat
    Nature Communications, 2016
    Co-Authors: Reed P Scherer, Robert M. Deconto, David Pollard, Richard B. Alley
    Abstract:

    A long-standing debate regarding the Pliocene history of the East Antarctic Ice Sheet was spurred by the discovery of marine diatoms in the Transantarctic Mountains. Here the authors show that the diatoms were emplaced by wind following a retreat of the Ice Sheet into coastal basins and subsequent isostatic emergence.

  • oceanic forcing of Ice Sheet retreat west antarctica and more
    Annual Review of Earth and Planetary Sciences, 2015
    Co-Authors: Richard B. Alley, David Pollard, Sridhar Anandakrishnan, Knut Christianson, Huw J Horgan, Atsu Muto, Byron R Parizek, Ryan T Walker
    Abstract:

    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...

  • Modeling Ice-Sheet Flow
    Science, 2012
    Co-Authors: Richard B. Alley, Ian Joughin
    Abstract:

    The great Greenland and Antarctic Ice Sheets are the “wild cards” in projections of sea-level change ( 1 ). Early models of the coupled ocean-atmosphere system treated the Ice Sheets as static white mountains. Observations since then have shown that Ice Sheets can change quickly ( 2 ): In some places, the tides strongly modulate coastal Ice flow; in others, warming-induced Ice-shelf loss has caused the flow speed of the subsequently unbuttressed inland Ice to increase almost 10-fold within a few weeks ( 3 , 4 ). A new generation of full-stress Ice-Sheet models incorporates the physics needed to reproduce such processes (see the figure) ( 5 – 7 ). Including full stresses does improve Ice-flow simulations ( 8 ). Well-validated, robust projections of Ice-Sheet behavior under climate change nevertheless remain a challenge, as they will require an ensemble of model Ice Sheets coupled to the rest of the climate system.

  • History of the Greenland Ice Sheet: paleoclimatic insights
    Quaternary Science Reviews, 2010
    Co-Authors: Richard B. Alley, John T. Andrews, Julie Brigham-grette, Garry K. C. Clarke, Kurt M. Cuffey, Joan J. Fitzpatrick, Svend Funder, Shawn J. Marshall, Gifford H. Miller, Jerry X. Mitrovica
    Abstract:

    Abstract Paleoclimatic records show that the Greenland Ice 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.

  • Ice-Sheet and Sea-Level Changes
    Science, 2005
    Co-Authors: Richard B. Alley, Philippe Huybrechts, Peter U. Clark, Ian Joughin
    Abstract:

    Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic Ice Sheets, with the potential to raise sea level ∼70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these Ice Sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to Ice-Sheet behavior, but recently detected, rapid Ice-marginal changes contributing to sea-level rise may indicate greater Ice-Sheet sensitivity to warming than previously considered.

David Pollard - One of the best experts on this subject based on the ideXlab platform.

  • variations of the antarctic Ice Sheet in a coupled Ice Sheet earth sea level model sensitivity to viscoelastic earth properties
    Journal of Geophysical Research, 2017
    Co-Authors: David Pollard, Natalya Gomez, Robert M. Deconto
    Abstract:

    A coupled Ice Sheet-solid Earth-sea level model is applied to long-term variations of the Antarctic Ice Sheet. A set of radially varying viscoelastic profiles in the global Earth model is used to explore feedbacks on Ice-Sheet variability, including one with a very weak upper mantle zone and thin lithosphere representative of West Antarctic regions. Simulations are performed for (1) the deglacial retreat over the last ~20,000 years, (2) the future 5000 years with greenhouse-gas scenario RCP8.5, and (3) the warm Pliocene ~3 Ma. For the deglacial period a large ensemble of 625 simulations is analyzed, with a score computed for each run based on comparisons to geologic and modern data. For each of the five Earth profiles, the top-scoring combinations of the other model parameters in the ensemble are used to perform future and Pliocene simulations. For the last deglacial retreat, the viscoelastic Earth profiles produce relatively small differences in overall Ice volume and equivalent sea level. In contrast, profiles with weak upper mantle and thin lithosphere produce strong negative feedback and less Ice retreat in the future and Pliocene runs. This is due to the faster pace of Ice-Sheet retreat in these runs, leading to greater lags in the viscous bedrock rebound behind the unloading, which allows for greater influence of the viscosity profiles. However, the differences in grounding-line retreat are located primarily in East Antarctic basins, where a weak upper mantle and thin lithosphere may not be realistic, emphasizing the need for lateral heterogeneity in the Earth model.

  • windblown pliocene diatoms and east antarctic Ice Sheet retreat
    Nature Communications, 2016
    Co-Authors: Reed P Scherer, Robert M. Deconto, David Pollard, Richard B. Alley
    Abstract:

    A long-standing debate regarding the Pliocene history of the East Antarctic Ice Sheet was spurred by the discovery of marine diatoms in the Transantarctic Mountains. Here the authors show that the diatoms were emplaced by wind following a retreat of the Ice Sheet into coastal basins and subsequent isostatic emergence.

  • Antarctic bedrock topography uncertainty and Ice Sheet stability
    Geophysical Research Letters, 2015
    Co-Authors: Edward Gasson, Robert M. Deconto, David Pollard
    Abstract:

    Antarctic bedrock elevation estimates have uncertainties exceeding 1 km in certain regions. Bedrock elevation, particularly where the bedrock is below sea level and bordering the ocean, can have a large impact on Ice Sheet stability. We investigate how present-day bedrock elevation uncertainty affects Ice Sheet model simulations for a generic past warm period based on the mid-Pliocene, although these uncertainties are also relevant to present-day and future Ice Sheet stability. We perform an ensemble of simulations with random topographic noise added with various length scales and with amplitudes tuned to the uncertainty of the Bedmap2 data set. Total Antarctic Ice Sheet retreat in these simulations varies between 12.6 and 17.9 m equivalent sea level rise after 3 kyrs of warm climate forcing. This study highlights the sensitivity of Ice Sheet models to existing uncertainties in bedrock elevation and the ongoing need for new data acquisition.

  • oceanic forcing of Ice Sheet retreat west antarctica and more
    Annual Review of Earth and Planetary Sciences, 2015
    Co-Authors: Richard B. Alley, David Pollard, Sridhar Anandakrishnan, Knut Christianson, Huw J Horgan, Atsu Muto, Byron R Parizek, Ryan T Walker
    Abstract:

    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...

  • Ice Sheet model dependency of the simulated Greenland Ice Sheet in the mid-Pliocene
    Climate of the Past, 2015
    Co-Authors: S. J. Koenig, Robert M. Deconto, Daniel J Lunt, David Pollard, B. De Boer, Aisling M. Dolan, Emma J. Stone, Daniel J. Hill, Ayako Abe-ouchi, Aurélien Quiquet
    Abstract:

    The understanding of the nature and behavior of Ice Sheets in past warm periods is important for constraining the potential impacts of future climate change. The Pliocene warm period (between 3.264 and 3.025 Ma) saw global tem- peratures similar to those projected for future climates; nev- ertheless, Pliocene Ice locations and extents are still poorly constrained. We present results from the efforts to simulate mid-Pliocene Greenland Ice Sheets by means of the interna- tional Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP). We compare the performance of existing nu- merical Ice Sheet models in simulating modern control and mid-Pliocene Ice Sheets with a suite of sensitivity experi- ments guided by available proxy records. We quantify equi- librated Ice Sheet volume on Greenland, identifying a po- tential range in sea level contributions from warm Pliocene scenarios. A series of statistical measures are performed to quantify the confidence of simulations with focus on inter- model and inter-scenario differences. We find that Pliocene Greenland Ice Sheets are less sensitive to differences in Ice Sheet model configurations and internal physical quantities than to changes in imposed climate forcing. We conclude that Pliocene Ice was most likely to be limited to the high- est elevations in eastern and southern Greenland as simulated with the highest confidence and by synthesizing available re- gional proxies; however, the extent of those Ice caps needs to be further constrained by using a range of general circulation model (GCM) climate forcings.

Philippe Huybrechts - One of the best experts on this subject based on the ideXlab platform.

  • Simulating the Antarctic Ice Sheet in the late-Pliocene warm period: PLISMIP-ANT, an Ice-Sheet model intercomparison project
    The Cryosphere, 2015
    Co-Authors: B. De Boer, Philippe Huybrechts, Edward Gasson, Nicholas R. Golledge, Aisling M. Dolan, Jorge Bernales, Heiko Goelzer, Johannes Sutter, Gerrit Lohmann, Irina Rogozhina
    Abstract:

    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.

  • Ice Sheet contributions to future sea level change
    Philosophical Transactions of the Royal Society A, 2006
    Co-Authors: Jonathan M. Gregory, Philippe Huybrechts
    Abstract:

    Accurate simulation of Ice-Sheet surface mass balance requires higher spatial resolution than is afforded by typical atmosphere–ocean general circulation models (AOGCMs), owing, in particular, to the need to resolve the narrow and steep margins where the majority of precipitation and ablation occurs. We have developed a method for calculating massbalance changes by combining Ice-Sheet average time-series from AOGCM projections for future centuries, both with information from high-resolution climate models run for short periods and with a 20 km Ice-Sheet mass-balance model. Antarctica contributes negatively to sea level on account of increased accumulation, while Greenland contributes positively because ablation increases more rapidly. The uncertainty in the results is about 20% for Antarctica and 35% for Greenland. Changes in Ice-Sheet topography and dynamics are not included, but we discuss their possible effects. For an annual- and area-average warming exceeding 4:5G0:9 K in Greenland and 3:1G0:8 K in the global average, the net surface mass balance of the Greenland Ice Sheet becomes negative, in which case it is likely that the Ice Sheet would eventually be eliminated, raising global-average sea level by 7 m.

  • Ice-Sheet and Sea-Level Changes
    Science, 2005
    Co-Authors: Richard B. Alley, Philippe Huybrechts, Peter U. Clark, Ian Joughin
    Abstract:

    Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic Ice Sheets, with the potential to raise sea level ∼70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these Ice Sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to Ice-Sheet behavior, but recently detected, rapid Ice-marginal changes contributing to sea-level rise may indicate greater Ice-Sheet sensitivity to warming than previously considered.

  • Modelled Ice-Sheet margins of three Greenland Ice-Sheet models compared with a geological record from Ice-marginal deposits in central West Greenland
    Annals of Glaciology, 1996
    Co-Authors: Frank G. M. Van Tatenhove, Adeline Fabre, Ralf Greve, Philippe Huybrechts
    Abstract:

    Ice-Sheet modelling is an essential tool for estimating the effect of climate change on the Greenland Ice Sheet. The large spatial and long-term temporal scales of the Ice-Sheet model limits the amount of data which can be used to test model results. The geological record is useful because it provides test material on the time-scales typical for the memory of Ice Sheets (millennia). This paper compares modelled Ice-margin positions with a geological scenario of Ice-margin positions since the Last Glacial Maximum to the present in West Greenland. Morphological evidence of Ice-margin positions is provided by moraines. Moraine systems are dated by 14C-dated marine shells and terrestrial peat. Three Greenland Ice-Sheet models are compared. There are distinct differences in modelled Ice-margin positions between the models and between model results and the geological record. Disagreement between models and the geological record in the near-coastal area is explained by the inadequate treatment of marginal processes in a tide-water environment. A smaller than present Ice Sheet around the warm period in the Holocene (Holocene climatic optimum) only occurs if such a period appears in the forcing (Ice-core record) or used temporal resolution. Smoothing of the GRIP record with a 2000 year average eliminates the climatic signal related to the Holocene climatic optimum. This underlines the importance of short-term and medium-term variations (decades, centuries) in climatic variables in determining Ice-margin positions in the past but also in the future.

  • Formation and disintegration of the Antarctic Ice Sheet
    Annals of Glaciology, 1994
    Co-Authors: Philippe Huybrechts
    Abstract:

    A model of the Antarctic Ice Sheet has been used to simulate the Ice Sheet in warmer climates, in order to investigate what kind of Ice-Sheet geometries one can reasonably expect under what kind of climatic conditions and to discover which physical mechanisms may be involved to explain them. The results of these experiments reveal the considerable stability of; in particular, the East Antarctic Ice Sheet. It would require a temperature rise of between 17 and 20 K above present levels to remove this Ice Sheet from the subglacial basins in the interior of the continent and of 25 K to melt down the Antarctic Ice Sheet completely. For a temperature rise below 5 K, the model actually predicts a larger Antarctic Ice Sheet than today as a result of increased snowfall, whereas the west Antarctic Ice Sheet was round not to survive temperatures more than 8–10 K above present values. Furthermore, basal temperature conditions in these experiments point to the problems involved in raising the base of the Ice Sheet to the pressure-melting point over the large areas necessary to consider the possibility of sliding instability. These results bear on a lively debate regarding the late Cenozoic glacial history of Antarctica. Particularly, based on these findings, it is difficult to reconcile a highly variable East Antarctic Ice Sheet until the Pliocene with modest warming recorded in, for instance, the deep-sea records for the late Neogene.

Gael Durand - One of the best experts on this subject based on the ideXlab platform.

  • Progress in Numerical Modeling of Antarctic Ice-Sheet Dynamics
    Current Climate Change Reports, 2017
    Co-Authors: Frank Pattyn, Lionel Favier, Sainan Sun, Gael Durand
    Abstract:

    Numerical modeling of the Antarctic Ice Sheet has gone through a paradigm shift over the last decade. While initially models focussed on long-time diffusive response to surface mass balance changes, processes occurring at the marine boundary of the Ice Sheet are progressively incorporated in newly developed state-of-the-art Ice-Sheet models. These models now exhibit fast, short-term volume changes, in line with current observations of mass loss. Coupling with ocean models is currently on its way and applied to key areas of the Antarctic Ice Sheet. New model intercomparisons have been launched, focusing on Ice/ocean interaction (MISMIP+, MISOMIP) or Ice-Sheet model initialization and multi-ensemble projections (ISMIP6). Nevertheless, the inclusion of new processes pertaining to Ice-shelf calving, evolution of basal friction, and other processes, also increase uncertainties in the contribution of the Antarctic Ice Sheet to future sea-level rise.

  • greenland Ice Sheet contribution to sea level rise from a new generation Ice Sheet model
    The Cryosphere, 2012
    Co-Authors: Fabien Gilletchaulet, Olivier Gagliardini, Thomas Zwinger, Maëlle Nodet, Gael Durand, Hakime Seddik, Catherine Ritz, Ralph Greve, David G Vaughan
    Abstract:

    Over the last two decades, the Greenland Ice Sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise (SLR). The recent increases in Ice loss appear to be due to changes in both the surface mass balance of the Ice Sheet and Ice discharge (Ice flux to the ocean). Rapid Ice flow directly affects the discharge, but also alters Ice-Sheet geometry and so affects climate and surface mass balance. Present-day Ice-Sheet models only represent rapid Ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of Ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic Ice-Sheet model which reproduces the current patterns of rapid Ice flow. This requires three essential developments: the complete solution of the full system of equations governing Ice deformation; a variable resolution unstructured mesh to resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled Ice discharge is in good agreement with observations on the continental scale and for individual outlets. From this initial state, we investigate possible bounds for the next century Ice-Sheet mass loss. We run sensitivity experiments of the GrIS dynamical response to perturbations in climate and basal lubrication, assuming a fixed position of the marine termini. We find that increasing ablation tends to reduce outflow and thus decreases the Ice-Sheet imbalance. In our experiments, the GrIS initial mass (im)balance is preserved throughout the whole century in the absence of reinforced forcing, allowing us to estimate a lower bound of 75 mm for the GrIS contribution to SLR by 2100. In one experiment, we show that the current increase in the rate of Ice loss can be reproduced and maintained throughout the whole century. However, this requires a very unlikely perturbation of basal lubrication. From this result we are able to estimate an upper bound of 140 mm from dynamics only for the GrIS contribution to SLR by 2100.

  • Greenland Ice Sheet contribution to sea-level rise from a new-generation Ice-Sheet model
    The Cryosphere, 2012
    Co-Authors: Fabien Gillet-Chaulet, Ralf Greve, Olivier Gagliardini, Thomas Zwinger, Maëlle Nodet, Gael Durand, Hakime Seddik, Christian Ritz, David G Vaughan
    Abstract:

    Over the last two decades, the Greenland Ice Sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise. The recent increases in Ice loss appear to be due to changes in both the surface mass balance of the Ice Sheet and 5 Ice discharge (Ice flux to the ocean). Rapid Ice flow directly affects the discharge, but also alters Ice-Sheet geometry and so affects climate and surface mass balance. The most usual Ice-Sheet models only represent rapid Ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of Ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, 10 we present a new-generation prognostic Ice-Sheet model which reproduces the current patterns of rapid Ice flow. This requires three essential developments: the complete solution of the full system of equations governing Ice deformation; an unstructured mesh to usefully resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled Ice discharge is in good 15 agreement with observations on the continental scale and for individual outlets. By conducting perturbation experiments, we investigate how current Ice loss will endure over the next century. Although we find that increasing ablation tends to reduce outflow and on its own has a stabilising effect, if destabilisation processes maintain themselves over time, current increases in the rate of Ice loss are likely to continue.

  • Glaciology: Ice-Sheet advance in Antarctica
    Nature, 2010
    Co-Authors: Fabien Gillet-Chaulet, Gael Durand
    Abstract:

    Reliable forecasts of sea-level rise depend on accurately modelling the dynamics of polar Ice Sheets. A numerical framework that better reflects Ice-Sheet basal drag adds greater realism to such models.

Catherine Ritz - One of the best experts on this subject based on the ideXlab platform.

  • A rapidly converging initialisation method to simulate the present-day Greenland Ice Sheet using the GRISLI Ice Sheet model (version 1.3)
    Geoscientific Model Development, 2019
    Co-Authors: Sébastien Le Clec'h, Sylvie Charbit, Aurélien Quiquet, Christophe Dumas, Masa Kageyama, Catherine Ritz
    Abstract:

    Abstract. Providing reliable projections of the Ice Sheet contribution to future sea-level rise has become one of the main challenges of the Ice Sheet modelling community. To increase confidence in future projections, a good knowledge of the present-day state of Ice flow dynamics, which is critically dependent on basal conditions, is strongly needed. The main difficulty is tied to the scarcity of observations at the Ice–bed interface at the scale of the whole Ice Sheet, resulting in poorly constrained parameterisations in Ice Sheet models. To circumvent this drawback, inverse modelling approaches can be developed to infer initial conditions for Ice Sheet models that best reproduce available data. Most often such approaches allow for a good representation of the mean present-day state of the Ice Sheet but are accompanied with unphysical trends. Here, we present an initialisation method for the Greenland Ice Sheet using the thermo-mechanical hybrid GRISLI (GRenoble Ice Shelf and Land Ice) Ice Sheet model. Our approach is based on the adjustment of the basal drag coefficient that relates the sliding velocities at the Ice–bed interface to basal shear stress in unfrozen bed areas. This method relies on an iterative process in which the basal drag is periodically adjusted in such a way that the simulated Ice thickness matches the observed one. The quality of the method is assessed by computing the root mean square errors in Ice thickness changes. Because the method is based on an adjustment of the sliding velocities only, the results are discussed in terms of varying Ice flow enhancement factors that control the deformation rates. We show that this factor has a strong impact on the minimisation of Ice thickness errors and has to be chosen as a function of the internal thermal state of the Ice Sheet (e.g. a low enhancement factor for a warm Ice Sheet). While the method performance slightly increases with the duration of the minimisation procedure, an Ice thickness root mean square error (RMSE) of 50.3 m is obtained in only 1320 model years. This highlights a rapid convergence and demonstrates that the method can be used for computationally expensive Ice Sheet models.

  • Assessment of the Greenland Ice Sheet–atmosphere feedbacks for the next century with a regional atmospheric model coupled to an Ice Sheet model
    The Cryosphere, 2019
    Co-Authors: Sébastien Le Clec'h, Xavier Fettweis, Sylvie Charbit, Aurélien Quiquet, Christophe Dumas, Masa Kageyama, Coraline Wyard, Catherine Ritz
    Abstract:

    In the context of global warming, growing attention is paid to the evolution of the Greenland Ice Sheet (GrIS) and its contribution to sea-level rise at the centennial timescale. Atmosphere-GrIS interactions, such as the temperature-elevation and the albedo feedbacks, have the potential to modify the surface energy balance and thus to impact the GrIS surface mass balance (SMB). In turn, changes in the geometrical features of the Ice Sheet may alter both the climate and the Ice dynamics governing the Ice Sheet evolution. However, changes in Ice Sheet geometry are generally not explicitly accounted for when simulating atmospheric changes over the Greenland Ice Sheet in the future. To account for Ice Sheet-climate interactions, we developed the first two-way synchronously coupled model between a regional atmospheric model (MAR) and a 3-D Ice Sheet model (GRISLI). Using this novel model, we simulate the Ice Sheet evolution from 2000 to 2150 under a prolonged representative concentration pathway scenario, RCP8.5. Changes in surface elevation and Ice Sheet extent simulated by GRISLI have a direct impact on the climate simulated by MAR. They are fed to MAR from 2020 onwards, i.e. when changes in SMB produce significant topography changes in GRISLI. We further assess the importance of the atmosphere-Ice Sheet feedbacks through the comparison of the two-way coupled experiment with two other simulations based on simpler coupling strategies: (i) a one-way coupling with no consideration of any change in Ice Sheet geometry; (ii) an alternative one-way coupling in which the elevation change feedbacks are parameterized in the Ice Sheet model (from 2020 onwards) without taking into account the changes in Ice Sheet topography in the atmospheric model. The two-way coupled experiment simulates an important increase in surface melt below 2000 m of elevation, resulting in an important SMB reduction in 2150 and a shift of the equilibrium line towards elevations as high as 2500 m, despite a slight increase in SMB over the central plateau due to enhanced snowfall. In relation with these SMB changes, modifications of Ice Sheet geometry favour Ice flux convergence towards the margins, with an increase in Ice velocities in the GrIS interior due to increased surface slopes and a decrease in Ice velocities at the margins due to decreasing Ice thickness. This convergence counteracts the SMB signal in these areas. In the two-way coupling, the SMB is also influenced by changes in fine-scale atmospheric dynamical processes, such as the increase in katabatic winds from central to marginal regions induced by increased surface slopes. Altogether, the GrIS contribution to sea-level rise, inferred from variations in Ice volume above floatation, is equal to 20.4 cm in 2150. The comparison between the coupled and the two uncoupled experiments suggests that the effect of the different feedbacks is amplified over time with the most important feedbacks being the SMB-elevation feedbacks. As a result, the experiment with parameterized SMB-elevation feedback provides a sea-level contribution from GrIS in 2150 only 2.5 % lower than the Published by Copernicus Publications on behalf of the European Geosciences Union. 374 S. Le clec'h et al.: Assessment of the Greenland Ice Sheet two-way coupled experiment, while the experiment with no feedback is 9.3 % lower. The change in the ablation area in the two-way coupled experiment is much larger than those provided by the two simplest methods, with an underestimation of 11.7 % (14 %) with parameterized feedbacks (no feedback). In addition, we quantify that computing the GrIS contribution to sea-level rise from SMB changes only over a fixed Ice Sheet mask leads to an overestimation of Ice loss of at least 6 % compared to the use of a time variable Ice Sheet mask. Finally, our results suggest that Ice-loss estimations diverge when using the different coupling strategies, with differences from the two-way method becoming significant at the end of the 21st century. In particular, even if averaged over the whole GrIS the climatic and Ice Sheet fields are relatively similar; at the local and regional scale there are important differences, highlighting the importance of correctly representing the interactions when interested in basin scale changes.

  • The GRISLI Ice Sheet model (version 2.0): calibration and validation for multi-millennial changes of the Antarctic Ice Sheet
    Geoscientific Model Development Discussions, 2018
    Co-Authors: Aurélien Quiquet, Catherine Ritz, Christophe Dumas, Vincent Peyaud, Didier Roche
    Abstract:

    In this paper, we present the GRISLI (Grenoble Ice Sheet and land Ice) model in its newest revision (version 2.0). Whilst GRISLI is applicable to any given Ice Sheet, we focus here on the Antarctic Ice Sheet because it highlights the importance of grounding line dynamics. Important improvements have been implemented in the model since its original version (Ritz et al., 2001). Notably, GRISLI now includes a basal hydrology model and an explicit flux computation at the grounding line based on the analytical formulations of Schoof (2007) or Tsai et al. (2015). We perform a full calibration of the model based on an ensemble of 300 simulations sampling mechanical parameter space using a Latin hypercube method. Performance of individual members is assessed relative to the deviation from present-day observed Antarctic Ice thickness. To assess the ability of the model to simulate grounding line migration, we also present glacial-interglacial Ice Sheet changes throughout the last 400 kyr using the best ensemble members taking advantage of the capacity of the model to perform multi-millennial long-term integrations. To achieve this goal, we construct a simple climatic perturbation of present-day climate forcing fields based on two climate proxies: atmospheric and oceanic. The model is able to reproduce expected grounding line advances during glacial periods and subsequent retreats during terminations with reasonable glacial-interglacial Ice volume changes.

  • Ice Sheet mass balance and climate change
    Nature, 2013
    Co-Authors: Edward Hanna, Xavier Fettweis, Erik R. Ivins, Catherine Ritz, Frank Pattyn, Francisco Navarro, Catia M Domingues, Robert J Nicholls, Ben Smith, Slawek Tulaczyk
    Abstract:

    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.

  • greenland Ice Sheet contribution to sea level rise from a new generation Ice Sheet model
    The Cryosphere, 2012
    Co-Authors: Fabien Gilletchaulet, Olivier Gagliardini, Thomas Zwinger, Maëlle Nodet, Gael Durand, Hakime Seddik, Catherine Ritz, Ralph Greve, David G Vaughan
    Abstract:

    Over the last two decades, the Greenland Ice Sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise (SLR). The recent increases in Ice loss appear to be due to changes in both the surface mass balance of the Ice Sheet and Ice discharge (Ice flux to the ocean). Rapid Ice flow directly affects the discharge, but also alters Ice-Sheet geometry and so affects climate and surface mass balance. Present-day Ice-Sheet models only represent rapid Ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of Ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic Ice-Sheet model which reproduces the current patterns of rapid Ice flow. This requires three essential developments: the complete solution of the full system of equations governing Ice deformation; a variable resolution unstructured mesh to resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled Ice discharge is in good agreement with observations on the continental scale and for individual outlets. From this initial state, we investigate possible bounds for the next century Ice-Sheet mass loss. We run sensitivity experiments of the GrIS dynamical response to perturbations in climate and basal lubrication, assuming a fixed position of the marine termini. We find that increasing ablation tends to reduce outflow and thus decreases the Ice-Sheet imbalance. In our experiments, the GrIS initial mass (im)balance is preserved throughout the whole century in the absence of reinforced forcing, allowing us to estimate a lower bound of 75 mm for the GrIS contribution to SLR by 2100. In one experiment, we show that the current increase in the rate of Ice loss can be reproduced and maintained throughout the whole century. However, this requires a very unlikely perturbation of basal lubrication. From this result we are able to estimate an upper bound of 140 mm from dynamics only for the GrIS contribution to SLR by 2100.