TY - JOUR A1 - Zeitz, Maria A1 - Reese, Ronja A1 - Beckmann, Johanna A1 - Krebs-Kanzow, Uta A1 - Winkelmann, Ricarda T1 - Impact of the melt-albedo feedback on the future evolution of the Greenland Ice Sheet with PISM-dEBM-simple JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Surface melting of the Greenland Ice Sheet contributes a large amount to current and future sea level rise. Increased surface melt may lower the reflectivity of the ice sheet surface and thereby increase melt rates: the so-called melt-albedo feedback describes this self-sustaining increase in surface melting. In order to test the effect of the melt-albedo feedback in a prognostic ice sheet model, we implement dEBM-simple, a simplified version of the diurnal Energy Balance Model dEBM, in the Parallel Ice Sheet Model (PISM). The implementation includes a simple representation of the melt-albedo feedback and can thereby replace the positive-degree-day melt scheme. Using PISM-dEBM-simple, we find that this feedback increases ice loss through surface warming by 60 % until 2300 for the high-emission scenario RCP8.5 when compared to a scenario in which the albedo remains constant at its present-day values. With an increase of 90 % compared to a fixed-albedo scenario, the effect is more pronounced for lower surface warming under RCP2.6. Furthermore, assuming an immediate darkening of the ice surface over all summer months, we estimate an upper bound for this effect to be 70 % in the RCP8.5 scenario and a more than 4-fold increase under RCP2.6. With dEBM-simple implemented in PISM, we find that the melt-albedo feedback is an essential contributor to mass loss in dynamic simulations of the Greenland Ice Sheet under future warming. Y1 - 2021 U6 - https://doi.org/10.5194/tc-15-5739-2021 SN - 1994-0416 SN - 1994-0424 VL - 15 IS - 12 SP - 5739 EP - 5764 PB - Copernicus CY - Katlenburg-Lindau ER - TY - JOUR A1 - Zeitz, Maria A1 - Levermann, Anders A1 - Winkelmann, Ricarda T1 - Sensitivity of ice loss to uncertainty in flow law parameters in an idealized one-dimensional geometry JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Acceleration of the flow of ice drives mass losses in both the Antarctic and the Greenland Ice Sheet. The projections of possible future sea-level rise rely on numerical ice-sheet models, which solve the physics of ice flow, melt, and calving. While major advancements have been made by the ice-sheet modeling community in addressing several of the related uncertainties, the flow law, which is at the center of most process-based ice-sheet models, is not in the focus of the current scientific debate. However, recent studies show that the flow law parameters are highly uncertain and might be different from the widely accepted standard values. Here, we use an idealized flow-line setup to investigate how these uncertainties in the flow law translate into uncertainties in flow-driven mass loss. In order to disentangle the effect of future warming on the ice flow from other effects, we perform a suite of experiments with the Parallel Ice Sheet Model (PISM), deliberately excluding changes in the surface mass balance. We find that changes in the flow parameters within the observed range can lead up to a doubling of the flow-driven mass loss within the first centuries of warming, compared to standard parameters. The spread of ice loss due to the uncertainty in flow parameters is on the same order of magnitude as the increase in mass loss due to surface warming. While this study focuses on an idealized flow-line geometry, it is likely that this uncertainty carries over to realistic three-dimensional simulations of Greenland and Antarctica. Y1 - 2020 U6 - https://doi.org/10.5194/tc-14-3537-2020 SN - 1994-0416 SN - 1994-0424 VL - 14 IS - 10 SP - 3537 EP - 3550 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Zeitz, Maria A1 - Haacker, Jan M. A1 - Donges, Jonathan A1 - Albrecht, Torsten A1 - Winkelmann, Ricarda T1 - Dynamic regimes of the Greenland Ice Sheet emerging from interacting melt-elevation and glacial isostatic adjustment feedbacks JF - Earth system dynamics N2 - The stability of the Greenland Ice Sheet under global warming is governed by a number of dynamic processes and interacting feedback mechanisms in the ice sheet, atmosphere and solid Earth. Here we study the long-term effects due to the interplay of the competing melt-elevation and glacial isostatic adjustment (GIA) feedbacks for different temperature step forcing experiments with a coupled ice-sheet and solid-Earth model. Our model results show that for warming levels above 2 degrees C, Greenland could become essentially ice-free within several millennia, mainly as a result of surface melting and acceleration of ice flow. These ice losses are mitigated, however, in some cases with strong GIA feedback even promoting an incomplete recovery of the Greenland ice volume. We further explore the full-factorial parameter space determining the relative strengths of the two feedbacks: our findings suggest distinct dynamic regimes of the Greenland Ice Sheets on the route to destabilization under global warming - from incomplete recovery, via quasi-periodic oscillations in ice volume to ice-sheet collapse. In the incomplete recovery regime, the initial ice loss due to warming is essentially reversed within 50 000 years, and the ice volume stabilizes at 61 %-93 % of the present-day volume. For certain combinations of temperature increase, atmospheric lapse rate and mantle viscosity, the interaction of the GIA feedback and the melt-elevation feedback leads to self-sustained, long-term oscillations in ice-sheet volume with oscillation periods between 74 000 and over 300 000 years and oscillation amplitudes between 15 %-70 % of present-day ice volume. This oscillatory regime reveals a possible mode of internal climatic variability in the Earth system on timescales on the order of 100 000 years that may be excited by or synchronized with orbital forcing or interact with glacial cycles and other slow modes of variability. Our findings are not meant as scenario-based near-term projections of ice losses but rather providing insight into of the feedback loops governing the "deep future" and, thus, long-term resilience of the Greenland Ice Sheet. Y1 - 2022 U6 - https://doi.org/10.5194/esd-13-1077-2022 SN - 2190-4979 SN - 2190-4987 VL - 13 IS - 3 SP - 1077 EP - 1096 PB - Copernicus Publ. CY - Göttingen ER - TY - JOUR A1 - Wunderling, Nico A1 - Willeit, Matteo A1 - Donges, Jonathan A1 - Winkelmann, Ricarda T1 - Global warming due to loss of large ice masses and Arctic summer sea ice JF - Nature Communications N2 - Several large-scale cryosphere elements such as the Arctic summer sea ice, the mountain glaciers, the Greenland and West Antarctic Ice Sheet have changed substantially during the last century due to anthropogenic global warming. However, the impacts of their possible future disintegration on global mean temperature (GMT) and climate feedbacks have not yet been comprehensively evaluated. Here, we quantify this response using an Earth system model of intermediate complexity. Overall, we find a median additional global warming of 0.43 degrees C (interquartile range: 0.39-0.46 degrees C) at a CO2 concentration of 400 ppm. Most of this response (55%) is caused by albedo changes, but lapse rate together with water vapour (30%) and cloud feedbacks (15%) also contribute significantly. While a decay of the ice sheets would occur on centennial to millennial time scales, the Arctic might become ice-free during summer within the 21st century. Our findings imply an additional increase of the GMT on intermediate to long time scales. The disintegration of cryosphere elements such as the Arctic summer sea ice, mountain glaciers, Greenland and West Antarctica is associated with temperature and radiative feedbacks. In this work, the authors quantify these feedbacks and find an additional global warming of 0.43 degrees C. Y1 - 2020 U6 - https://doi.org/10.1038/s41467-020-18934-3 SN - 2041-1723 VL - 11 IS - 1 PB - Nature Publishing Group CY - Berlin ER - TY - JOUR A1 - Winkelmann, Ricarda A1 - Martin, Maria A. A1 - Haseloff, Monika A1 - Albrecht, Torsten A1 - Bueler, Ed A1 - Khroulev, C. A1 - Levermann, Anders T1 - The Potsdam parallel ice sheet model (PISM-PIK) - Part 1: Model description JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - 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). Y1 - 2011 U6 - https://doi.org/10.5194/tc-5-715-2011 SN - 1994-0416 VL - 5 IS - 3 SP - 715 EP - 726 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Winkelmann, Ricarda A1 - Levermann, Anders A1 - Martin, Maria A. A1 - Frieler, Katja T1 - Increased future ice discharge from Antarctica owing to higher snowfall JF - Nature : the international weekly journal of science N2 - 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. Y1 - 2012 U6 - https://doi.org/10.1038/nature11616 SN - 0028-0836 VL - 492 IS - 7428 SP - 239 EP - + PB - Nature Publ. Group CY - London ER - TY - THES A1 - Winkelmann, Ricarda T1 - The future sea-level contribution from antartica: projections of solid ice discharge Y1 - 2012 CY - Potsdam ER - TY - JOUR A1 - Steffen, Will A1 - Röckstrom, Johan A1 - Richardson, Katherine A1 - Lenton, Timothy M. A1 - Folke, Carl A1 - Liverman, Diana A1 - Summerhayes, Colin P. A1 - Barnosky, Anthony D. A1 - Cornell, Sarah E. A1 - Crucifix, Michel A1 - Donges, Jonathan A1 - Fetzer, Ingo A1 - Lade, Steven J. A1 - Scheffer, Marten A1 - Winkelmann, Ricarda A1 - Schellnhuber, Hans Joachim T1 - Trajectories of the Earth System in the Anthropocene JF - Proceedings of the National Academy of Sciences of the United States of America N2 - We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a "Hothouse Earth" pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System-biosphere, climate, and societies-and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values. KW - Earth System trajectories KW - climate change KW - Anthropocene KW - biosphere feedbacks KW - tipping elements Y1 - 2018 U6 - https://doi.org/10.1073/pnas.1810141115 SN - 0027-8424 VL - 115 IS - 33 SP - 8252 EP - 8259 PB - National Acad. of Sciences CY - Washington ER - TY - JOUR A1 - Seroussi, Helene A1 - Nowicki, Sophie A1 - Simon, Erika A1 - Abe-Ouchi, Ayako A1 - Albrecht, Torsten A1 - Brondex, Julien A1 - Cornford, Stephen A1 - Dumas, Christophe A1 - Gillet-Chaulet, Fabien A1 - Goelzer, Heiko A1 - Golledge, Nicholas R. A1 - Gregory, Jonathan M. A1 - Greve, Ralf A1 - Hoffman, Matthew J. A1 - Humbert, Angelika A1 - Huybrechts, Philippe A1 - Kleiner, Thomas A1 - Larourl, Eric A1 - Leguy, Gunter A1 - Lipscomb, William H. A1 - Lowry, Daniel A1 - Mengel, Matthias A1 - Morlighem, Mathieu A1 - Pattyn, Frank A1 - Payne, Anthony J. A1 - Pollard, David A1 - Price, Stephen F. A1 - Quiquet, Aurelien A1 - Reerink, Thomas J. A1 - Reese, Ronja A1 - Rodehacke, Christian B. A1 - Schlegel, Nicole-Jeanne A1 - Shepherd, Andrew A1 - Sun, Sainan A1 - Sutter, Johannes A1 - Van Breedam, Jonas A1 - van de Wal, Roderik S. W. A1 - Winkelmann, Ricarda A1 - Zhang, Tong T1 - initMIP-Antarctica BT - an ice sheet model initialization experiment of ISMIP6 JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMlP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMlP-Greenland, initMlP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMlP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue. Y1 - 2019 U6 - https://doi.org/10.5194/tc-13-1441-2019 SN - 1994-0416 SN - 1994-0424 VL - 13 IS - 5 SP - 1441 EP - 1471 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Seroussi, Helene A1 - Nowicki, Sophie A1 - Payne, Antony J. A1 - Goelzer, Heiko A1 - Lipscomb, William H. A1 - Abe-Ouchi, Ayako A1 - Agosta, Cecile A1 - Albrecht, Torsten A1 - Asay-Davis, Xylar A1 - Barthel, Alice A1 - Calov, Reinhard A1 - Cullather, Richard A1 - Dumas, Christophe A1 - Galton-Fenzi, Benjamin K. A1 - Gladstone, Rupert A1 - Golledge, Nicholas R. A1 - Gregory, Jonathan M. A1 - Greve, Ralf A1 - Hattermann, Tore A1 - Hoffman, Matthew J. A1 - Humbert, Angelika A1 - Huybrechts, Philippe A1 - Jourdain, Nicolas C. A1 - Kleiner, Thomas A1 - Larour, Eric A1 - Leguy, Gunter R. A1 - Lowry, Daniel P. A1 - Little, Chistopher M. A1 - Morlighem, Mathieu A1 - Pattyn, Frank A1 - Pelle, Tyler A1 - Price, Stephen F. A1 - Quiquet, Aurelien A1 - Reese, Ronja A1 - Schlegel, Nicole-Jeanne A1 - Shepherd, Andrew A1 - Simon, Erika A1 - Smith, Robin S. A1 - Straneo, Fiammetta A1 - Sun, Sainan A1 - Trusel, Luke D. A1 - Van Breedam, Jonas A1 - van de Wal, Roderik S. W. A1 - Winkelmann, Ricarda A1 - Zhao, Chen A1 - Zhang, Tong A1 - Zwinger, Thomas T1 - ISMIP6 Antarctica BT - a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and assess the mass loss that would contribute to future sea level rise. However, there is currently no consensus on estimates of the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes, forcings employed and initial states of ice sheet models. This study presents results from ice flow model simulations from 13 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015-2100 as part of the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). They are forced with outputs from a subset of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climate model results. Simulations of the Antarctic ice sheet contribution to sea level rise in response to increased warming during this period varies between 7:8 and 30.0 cm of sea level equivalent (SLE) under Representative Concentration Pathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment with constant climate conditions and should therefore be added to the mass loss contribution under climate conditions similar to present-day conditions over the same period. The simulated evolution of the West Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica mass change varies between 6 :1 and 8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional simulated mass loss of 28mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared to simulations done under present-day conditions for the two CMIP5 forcings used and display limited mass gain in East Antarctica. Y1 - 2020 U6 - https://doi.org/10.5194/tc-14-3033-2020 SN - 1994-0416 SN - 1994-0424 VL - 14 IS - 9 SP - 3033 EP - 3070 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Schlemm, Tanja A1 - Feldmann, Johannes A1 - Winkelmann, Ricarda A1 - Levermann, Anders T1 - Stabilizing effect of melange buttressing on the marine ice-cliff instability of the West Antarctic Ice Sheet JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Owing to global warming and particularly high regional ocean warming, both Thwaites and Pine Island Glaciers in the Amundsen region of the Antarctic Ice Sheet could lose their buttressing ice shelves over time. We analyse the possible consequences using the parallel ice sheet model (PISM), applying a simple cliff-calving parameterization and an ice melange-buttressing model. We find that the instantaneous loss of ice-shelf buttressing, due to enforced ice-shelf melting, initiates grounding-line retreat and triggers marine ice sheet instability (MISI). As a consequence, the grounding line progresses into the interior of the West Antarctic Ice Sheet and leads to a sea level contribution of 0.6 m within 100 a. By subjecting the exposed ice cliffs to cliff calving using our simplified parameterization, we also analyse marine ice cliff instability (MICI). In our simulations it can double or even triple the sea level contribution depending on the only loosely constrained parameter that determines the maximum cliff-calving rate. The speed of MICI depends on this upper bound of the calving rate, which is given by the ice melange buttressing the glacier. However, stabilization of MICI may occur for geometric reasons. Because the embayment geometry changes as MICI advances into the interior of the ice sheet, the upper bound on calving rates is reduced and the progress of MICI is slowed down. Although we cannot claim that our simulations bear relevant quantitative estimates of the effect of ice-melange buttressing on MICI, the mechanism has the potential to stop the instability. Further research is needed to evaluate its role for the past and future evolution of the Antarctic Ice Sheet. Y1 - 2022 U6 - https://doi.org/10.5194/tc-16-1979-2022 SN - 1994-0416 SN - 1994-0424 VL - 16 IS - 5 SP - 1979 EP - 1996 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Rockström, Johan A1 - Kotzé, Louis A1 - Milutinović, Svetlana A1 - Biermann, Frank A1 - Brovkin, Victor A1 - Donges, Jonathan A1 - Ebbesson, Jonas A1 - French, Duncan A1 - Gupta, Joyeeta A1 - Kim, Rakhyun A1 - Lenton, Timothy A1 - Lenzi, Dominic A1 - Nakicenovic, Nebojsa A1 - Neumann, Barbara A1 - Schuppert, Fabian A1 - Winkelmann, Ricarda A1 - Bosselmann, Klaus A1 - Folke, Carl A1 - Lucht, Wolfgang A1 - Schlosberg, David A1 - Richardson, Katherine A1 - Steffen, Will T1 - The planetary commons BT - a new paradigm for safeguarding earth-regulating systems in the Anthropocene JF - Proceedings of the National Academy of Sciences of the United States of America N2 - The Anthropocene signifies the start of a no- analogue tra­jectory of the Earth system that is fundamentally different from the Holocene. This new trajectory is characterized by rising risks of triggering irreversible and unmanageable shifts in Earth system functioning. We urgently need a new global approach to safeguard critical Earth system regulating functions more effectively and comprehensively. The global commons framework is the closest example of an existing approach with the aim of governing biophysical systems on Earth upon which the world collectively depends. Derived during stable Holocene conditions, the global commons framework must now evolve in the light of new Anthropocene dynamics. This requires a fundamental shift from a focus only on governing shared resources beyond national jurisdiction, to one that secures critical functions of the Earth system irrespective of national boundaries. We propose a new framework—the planetary commons—which differs from the global commons frame­work by including not only globally shared geographic regions but also critical biophysical systems that regulate the resilience and state, and therefore livability, on Earth. The new planetary commons should articulate and create comprehensive stewardship obligations through Earth system governance aimed at restoring and strengthening planetary resilience and justice. KW - anthropocene KW - earth system governance KW - global commons KW - international law KW - planetary boundaries Y1 - 2024 U6 - https://doi.org/10.1073/pnas.2301531121 SN - 1091-6490 SN - 1877-2014 VL - 121 IS - 5 PB - National Academy of Sciences CY - Washington, DC ER - TY - JOUR A1 - Reese, Ronja A1 - Winkelmann, Ricarda A1 - Gudmundsson, Gudmundur Hilmar T1 - Grounding-line flux formula applied as a flux condition in numerical simulations fails for buttressed Antarctic ice streams JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Currently, several large-scale ice-flow models impose a condition on ice flux across grounding lines using an analytically motivated parameterisation of grounding-line flux. It has been suggested that employing this analytical expression alleviates the need for highly resolved computational domains around grounding lines of marine ice sheets. While the analytical flux formula is expected to be accurate in an unbuttressed flow-line setting, its validity has hitherto not been assessed for complex and realistic geometries such as those of the Antarctic Ice Sheet. Here the accuracy of this analytical flux formula is tested against an optimised ice flow model that uses a highly resolved computational mesh around the Antarctic grounding lines. We find that when applied to the Antarctic Ice Sheet the analytical expression provides inaccurate estimates of ice fluxes for almost all grounding lines. Furthermore, in many instances direct application of the analytical formula gives rise to unphysical complex-valued ice fluxes. We conclude that grounding lines of the Antarctic Ice Sheet are, in general, too highly buttressed for the analytical parameterisation to be of practical value for the calculation of grounding-line fluxes. Y1 - 2018 U6 - https://doi.org/10.5194/tc-12-3229-2018 SN - 1994-0416 SN - 1994-0424 VL - 12 IS - 10 SP - 3229 EP - 3242 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Reese, Ronja A1 - Levermann, Anders A1 - Albrecht, Torsten A1 - Seroussi, Helene A1 - Winkelmann, Ricarda T1 - The role of history and strength of the oceanic forcing in sea level projections from Antarctica with the Parallel Ice Sheet Model JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Mass loss from the Antarctic Ice Sheet constitutes the largest uncertainty in projections of future sea level rise. Ocean-driven melting underneath the floating ice shelves and subsequent acceleration of the inland ice streams are the major reasons for currently observed mass loss from Antarctica and are expected to become more important in the future. Here we show that for projections of future mass loss from the Antarctic Ice Sheet, it is essential (1) to better constrain the sensitivity of sub-shelf melt rates to ocean warming and (2) to include the historic trajectory of the ice sheet. In particular, we find that while the ice sheet response in simulations using the Parallel Ice Sheet Model is comparable to the median response of models in three Antarctic Ice Sheet Intercomparison projects - initMIP, LARMIP-2 and ISMIP6 - conducted with a range of ice sheet models, the projected 21st century sea level contribution differs significantly depending on these two factors. For the highest emission scenario RCP8.5, this leads to projected ice loss ranging from 1:4 to 4:0 cm of sea level equivalent in simulations in which ISMIP6 ocean forcing drives the PICO ocean box model where parameter tuning leads to a comparably low sub-shelf melt sensitivity and in which no surface forcing is applied. This is opposed to a likely range of 9:1 to 35:8 cm using the exact same initial setup, but emulated from the LARMIP-2 experiments with a higher melt sensitivity, even though both projects use forcing from climate models and melt rates are calibrated with previous oceanographic studies. Furthermore, using two initial states, one with a previous historic simulation from 1850 to 2014 and one starting from a steady state, we show that while differences between the ice sheet configurations in 2015 seem marginal at first sight, the historic simulation increases the susceptibility of the ice sheet to ocean warming, thereby increasing mass loss from 2015 to 2100 by 5% to 50 %. Hindcasting past ice sheet changes with numerical models would thus provide valuable tools to better constrain projections. Our results emphasize that the uncertainty that arises from the forcing is of the same order of magnitude as the ice dynamic response for future sea level projections. Y1 - 2020 U6 - https://doi.org/10.5194/tc-14-3097-2020 SN - 1994-0416 SN - 1994-0424 VL - 14 IS - 9 SP - 3097 EP - 3110 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Reese, Ronja A1 - Gudmundsson, Gudmundur Hilmar A1 - Levermann, Anders A1 - Winkelmann, Ricarda T1 - The far reach of ice-shelf thinning in Antarctica JF - Nature climate change N2 - Floating ice shelves, which fringe most of Antarctica’s coastline, regulate ice flow into the Southern Ocean1,2,3. Their thinning4,5,6,7 or disintegration8,9 can cause upstream acceleration of grounded ice and raise global sea levels. So far the effect has not been quantified in a comprehensive and spatially explicit manner. Here, using a finite-element model, we diagnose the immediate, continent-wide flux response to different spatial patterns of ice-shelf mass loss. We show that highly localized ice-shelf thinning can reach across the entire shelf and accelerate ice flow in regions far from the initial perturbation. As an example, this ‘tele-buttressing’ enhances outflow from Bindschadler Ice Stream in response to thinning near Ross Island more than 900 km away. We further find that the integrated flux response across all grounding lines is highly dependent on the location of imposed changes: the strongest response is caused not only near ice streams and ice rises, but also by thinning, for instance, well-within the Filchner–Ronne and Ross Ice Shelves. The most critical regions in all major ice shelves are often located in regions easily accessible to the intrusion of warm ocean waters10,11,12, stressing Antarctica’s vulnerability to changes in its surrounding ocean. Y1 - 2017 U6 - https://doi.org/10.1038/s41558-017-0020-x SN - 1758-678X SN - 1758-6798 VL - 8 IS - 1 SP - 53 EP - 57 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Reese, Ronja A1 - Albrecht, Torsten A1 - Mengel, Matthias A1 - Asay-Davis, Xylar A1 - Winkelmann, Ricarda T1 - Antarctic sub-shelf melt rates via PICO JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Ocean-induced melting below ice shelves is one of the dominant drivers for mass loss from the Antarctic Ice Sheet at present. An appropriate representation of sub-shelf melt rates is therefore essential for model simulations of marine-based ice sheet evolution. Continental-scale ice sheet models often rely on simple melt-parameterizations, in particular for long-term simulations, when fully coupled ice-ocean interaction becomes computationally too expensive. Such parameterizations can account for the influence of the local depth of the ice-shelf draft or its slope on melting. However, they do not capture the effect of ocean circulation underneath the ice shelf. Here we present the Potsdam Ice-shelf Cavity mOdel (PICO), which simulates the vertical overturning circulation in ice-shelf cavities and thus enables the computation of sub-shelf melt rates consistent with this circulation. PICO is based on an ocean box model that coarsely resolves ice shelf cavities and uses a boundary layer melt formulation. We implement it as a module of the Parallel Ice Sheet Model (PISM) and evaluate its performance under present-day conditions of the Southern Ocean. We identify a set of parameters that yield two-dimensional melt rate fields that qualitatively reproduce the typical pattern of comparably high melting near the grounding line and lower melting or refreezing towards the calving front. PICO captures the wide range of melt rates observed for Antarctic ice shelves, with an average of about 0.1 ma(-1) for cold sub-shelf cavities, for example, underneath Ross or Ronne ice shelves, to 16 ma(-1) for warm cavities such as in the Amundsen Sea region. This makes PICO a computationally feasible and more physical alternative to melt parameterizations purely based on ice draft geometry. Y1 - 2018 U6 - https://doi.org/10.5194/tc-12-1969-2018 SN - 1994-0416 SN - 1994-0424 VL - 12 IS - 6 SP - 1969 EP - 1985 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Martin, Maria A. A1 - Winkelmann, Ricarda A1 - Haseloff, M. A1 - Albrecht, Tanja A1 - Bueler, Ed A1 - Khroulev, C. A1 - Levermann, Anders T1 - The Potsdam parallel ice sheet model (PISM-PIK) - Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - 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. Y1 - 2011 U6 - https://doi.org/10.5194/tc-5-727-2011 SN - 1994-0416 VL - 5 IS - 3 SP - 727 EP - 740 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Levermann, Anders A1 - Winkelmann, Ricarda A1 - Nowicki, S. A1 - Fastook, J. L. A1 - Frieler, Katja A1 - Greve, R. A1 - Hellmer, H. H. A1 - Martin, M. A. A1 - Meinshausen, Malte A1 - Mengel, Matthias A1 - Payne, A. J. A1 - Pollard, D. A1 - Sato, T. A1 - Timmermann, R. A1 - Wang, Wei Li A1 - Bindschadler, Robert A. T1 - Projecting antarctic ice discharge using response functions from SeaRISE ice-sheet models JF - Earth system dynamics N2 - 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 values increase to 0.09 m (66% range: 0.04-0.17 m; 90% range: 0.02-0.25 m) for RCP-2.6 and 0.15 m (66% range: 0.07-0.28 m; 90% range: 0.04-0.43 m) for RCP-8.5. All probability distributions are highly skewed towards high values. The applied ice-sheet models are coarse resolution with limitations in the representation of grounding-line motion. Within the constraints of the applied methods, the uncertainty induced from different ice-sheet models is smaller than that induced by the external forcing to the ice sheets. Y1 - 2014 U6 - https://doi.org/10.5194/esd-5-271-2014 SN - 2190-4979 SN - 2190-4987 VL - 5 IS - 2 SP - 271 EP - 293 PB - Copernicus CY - Göttingen ER - TY - GEN A1 - Levermann, Anders A1 - Winkelmann, Ricarda T1 - A simple equation for the melt elevation feedback of ice sheets T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - 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 slow-down 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 degrees C above the threshold and 500 years for 5 degrees 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. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 529 KW - sea-level rise KW - mass-balance KW - climate-change KW - Greenland KW - model KW - glacier KW - projections KW - dynamics KW - impact KW - 21st-Century Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-409834 SN - 1866-8372 IS - 529 ER - TY - JOUR A1 - Levermann, Anders A1 - Winkelmann, Ricarda T1 - A simple equation for the melt elevation feedback of ice sheets JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - 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 slow-down 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 degrees C above the threshold and 500 years for 5 degrees 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. Y1 - 2016 U6 - https://doi.org/10.5194/tc-10-1799-2016 SN - 1994-0416 SN - 1994-0424 VL - 10 SP - 1799 EP - 1807 PB - Copernicus CY - Göttingen ER -