@article{MartinWinkelmannHaseloffetal.2011, author = {Martin, Maria A. and Winkelmann, Ricarda and Haseloff, M. and Albrecht, Tanja and Bueler, Ed and Khroulev, C. and Levermann, Anders}, title = {The Potsdam parallel ice sheet model (PISM-PIK) - Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet}, series = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, volume = {5}, journal = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, number = {3}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1994-0416}, doi = {10.5194/tc-5-727-2011}, pages = {727 -- 740}, year = {2011}, abstract = {We present a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). The simulation is initialized with present-day conditions for bed topography and ice thickness and then run to steady state with constant present-day surface mass balance. Surface temperature and sub-shelf basal melt distribution are parameterized. Grounding lines and calving fronts are free to evolve, and their modeled equilibrium state is compared to observational data. A physically-motivated calving law based on horizontal spreading rates allows for realistic calving fronts for various types of shelves. Steady-state dynamics including surface velocity and ice flux are analyzed for whole Antarctica and the Ronne-Filchner and Ross ice shelf areas in particular. The results show that the different flow regimes in sheet and shelves, and the transition zone between them, are captured reasonably well, supporting the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice. This approach also leads to a natural emergence of sliding-dominated flow in stream-like features in this new 3-D marine ice sheet model.}, language = {en} } @article{WinkelmannMartinHaseloffetal.2011, author = {Winkelmann, Ricarda and Martin, Maria A. and Haseloff, Monika and Albrecht, Torsten and Bueler, Ed and Khroulev, C. and Levermann, Anders}, title = {The Potsdam parallel ice sheet model (PISM-PIK) - Part 1: Model description}, series = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, volume = {5}, journal = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, number = {3}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1994-0416}, doi = {10.5194/tc-5-715-2011}, pages = {715 -- 726}, year = {2011}, abstract = {We present the Potsdam Parallel Ice Sheet Model (PISM-PIK), developed at the Potsdam Institute for Climate Impact Research to be used for simulations of large-scale ice sheet-shelf systems. It is derived from the Parallel Ice Sheet Model (Bueler and Brown, 2009). Velocities are calculated by superposition of two shallow stress balance approximations within the entire ice covered region: the shallow ice approximation (SIA) is dominant in grounded regions and accounts for shear deformation parallel to the geoid. The plug-flow type shallow shelf approximation (SSA) dominates the velocity field in ice shelf regions and serves as a basal sliding velocity in grounded regions. Ice streams can be identified diagnostically as regions with a significant contribution of membrane stresses to the local momentum balance. All lateral boundaries in PISM-PIK are free to evolve, including the grounding line and ice fronts. Ice shelf margins in particular are modeled using Neumann boundary conditions for the SSA equations, reflecting a hydrostatic stress imbalance along the vertical calving face. The ice front position is modeled using a subgrid-scale representation of calving front motion (Albrecht et al., 2011) and a physically-motivated calving law based on horizontal spreading rates. The model is tested in experiments from the Marine Ice Sheet Model Intercomparison Project (MISMIP). A dynamic equilibrium simulation of Antarctica under present-day conditions is presented in Martin et al. (2011).}, language = {en} } @article{LevermannAlbrechtWinkelmannetal.2012, author = {Levermann, Anders and Albrecht, Tanja and Winkelmann, Ricarda and Martin, Maria A. and Haseloff, Monika and Joughin, I.}, title = {Kinematic first-order calving law implies potential for abrupt ice-shelf retreat}, series = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, volume = {6}, journal = {The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union}, number = {2}, publisher = {Copernicus}, address = {G{\"o}ttingen}, issn = {1994-0416}, doi = {10.5194/tc-6-273-2012}, pages = {273 -- 286}, year = {2012}, abstract = {Recently observed large-scale disintegration of Antarctic ice shelves has moved their fronts closer towards grounded ice. In response, ice-sheet discharge into the ocean has accelerated, contributing to global sea-level rise and emphasizing the importance of calving-front dynamics. The position of the ice front strongly influences the stress field within the entire sheet-shelf-system and thereby the mass flow across the grounding line. While theories for an advance of the ice-front are readily available, no general rule exists for its retreat, making it difficult to incorporate the retreat in predictive models. Here we extract the first-order large-scale kinematic contribution to calving which is consistent with large-scale observation. We emphasize that the proposed equation does not constitute a comprehensive calving law but represents the first-order kinematic contribution which can and should be complemented by higher order contributions as well as the influence of potentially heterogeneous material properties of the ice. When applied as a calving law, the equation naturally incorporates the stabilizing effect of pinning points and inhibits ice shelf growth outside of embayments. It depends only on local ice properties which are, however, determined by the full topography of the ice shelf. In numerical simulations the parameterization reproduces multiple stable fronts as observed for the Larsen A and B Ice Shelves including abrupt transitions between them which may be caused by localized ice weaknesses. We also find multiple stable states of the Ross Ice Shelf at the gateway of the West Antarctic Ice Sheet with back stresses onto the sheet reduced by up to 90 \% compared to the present state.}, language = {en} } @article{NowickiBindschadlerAbeOuchietal.2013, author = {Nowicki, Sophie and Bindschadler, Robert A. and Abe-Ouchi, Ayako and Aschwanden, Andy and Bueler, Ed and Choi, Hyeungu and Fastook, Jim and Granzow, Glen and Greve, Ralf and Gutowski, Gail and Herzfeld, Ute and Jackson, Charles and Johnson, Jesse and Khroulev, Constantine and Larour, Eric and Levermann, Anders and Lipscomb, William H. and Martin, Maria A. and Morlighem, Mathieu and Parizek, Byron R. and Pollard, David and Price, Stephen F. and Ren, Diandong and Rignot, Eric and Saito, Fuyuki and Sato, Tatsuru and Seddik, Hakime and Seroussi, Helene and Takahashi, Kunio and Walker, Ryan and Wang, Wei Li}, title = {Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project II Greenland}, series = {Journal of geophysical research : Earth surface}, volume = {118}, journal = {Journal of geophysical research : Earth surface}, number = {2}, publisher = {American Geophysical Union}, address = {Washington}, issn = {2169-9003}, doi = {10.1002/jgrf.20076}, pages = {1025 -- 1044}, year = {2013}, abstract = {The Sea-level Response to Ice Sheet Evolution (SeaRISE) effort explores the sensitivity of the current generation of ice sheet models to external forcing to gain insight into the potential future contribution to sea level from the Greenland and Antarctic ice sheets. All participating models simulated the ice sheet response to three types of external forcings: a change in oceanic condition, a warmer atmospheric environment, and enhanced basal lubrication. Here an analysis of the spatial response of the Greenland ice sheet is presented, and the impact of model physics and spin-up on the projections is explored. Although the modeled responses are not always homogeneous, consistent spatial trends emerge from the ensemble analysis, indicating distinct vulnerabilities of the Greenland ice sheet. There are clear response patterns associated with each forcing, and a similar mass loss at the full ice sheet scale will result in different mass losses at the regional scale, as well as distinct thickness changes over the ice sheet. All forcings lead to an increased mass loss for the coming centuries, with increased basal lubrication and warmer ocean conditions affecting mainly outlet glaciers, while the impacts of atmospheric forcings affect the whole ice sheet.}, language = {en} } @article{WinkelmannLevermannMartinetal.2012, author = {Winkelmann, Ricarda and Levermann, Anders and Martin, Maria A. and Frieler, Katja}, title = {Increased future ice discharge from Antarctica owing to higher snowfall}, series = {Nature : the international weekly journal of science}, volume = {492}, journal = {Nature : the international weekly journal of science}, number = {7428}, publisher = {Nature Publ. Group}, address = {London}, issn = {0028-0836}, doi = {10.1038/nature11616}, pages = {239 -- +}, year = {2012}, abstract = {Anthropogenic climate change is likely to cause continuing global sea level rise(1), 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(2,3) and ocean expansion(4). Uncertainties exist in modelled snowfall(5), but even larger uncertainties exist in the potential changes of dynamic ice discharge from Antarctica(1,6) 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(7) 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.}, language = {en} } @article{BindschadlerNowickiAbeOuchietal.2013, author = {Bindschadler, Robert A. and Nowicki, Sophie and Abe-Ouchi, Ayako and Aschwanden, Andy and Choi, Hyeungu and Fastook, Jim and Granzow, Glen and Greve, Ralf and Gutowski, Gail and Herzfeld, Ute and Jackson, Charles and Johnson, Jesse and Khroulev, Constantine and Levermann, Anders and Lipscomb, William H. and Martin, Maria A. and Morlighem, Mathieu and Parizek, Byron R. and Pollard, David and Price, Stephen F. and Ren, Diandong and Saito, Fuyuki and Sato, Tatsuru and Seddik, Hakime and Seroussi, Helene and Takahashi, Kunio and Walker, Ryan and Wang, Wei Li}, title = {Ice-sheet model sensitivities to environmental forcing and their use in projecting future sea level (the SeaRISE project)}, series = {Journal of glaciology}, volume = {59}, journal = {Journal of glaciology}, number = {214}, publisher = {International Glaciological Society}, address = {Cambridge}, issn = {0022-1430}, doi = {10.3189/2013JoG12J125}, pages = {195 -- 224}, year = {2013}, abstract = {Ten ice-sheet models are used to study sensitivity of the Greenland and Antarctic ice sheets to prescribed changes of surface mass balance, sub-ice-shelf melting and basal sliding. Results exhibit a large range in projected contributions to sea-level change. In most cases, the ice volume above flotation lost is linearly dependent on the strength of the forcing. Combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments, suggesting that nonlinear feedbacks are modest. Our models indicate that Greenland is more sensitive than Antarctica to likely atmospheric changes in temperature and precipitation, while Antarctica is more sensitive to increased ice-shelf basal melting. An experiment approximating the Intergovernmental Panel on Climate Change's RCP8.5 scenario produces additional first-century contributions to sea level of 22.3 and 8.1 cm from Greenland and Antarctica, respectively, with a range among models of 62 and 14 cm, respectively. By 200 years, projections increase to 53.2 and 26.7 cm, respectively, with ranges of 79 and 43 cm. Linear interpolation of the sensitivity results closely approximates these projections, revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice-sheet response to complicated forcings over 200 years can be linearized.}, language = {en} }