TY - JOUR A1 - Kreuzer, Moritz A1 - Reese, Ronja A1 - Huiskamp, Willem Nicholas A1 - Petri, Stefan A1 - Albrecht, Torsten A1 - Feulner, Georg A1 - Winkelmann, Ricarda T1 - Coupling framework (1.0) for the PISM (1.1.4) ice sheet model and the MOMS (5.1.0) ocean model via the PICO ice shelf cavity model in an Antarctic domain JF - Geoscientific model development : an interactive open access journal of the European Geosciences Union N2 - The past and future evolution of the Antarctic Ice Sheet is largely controlled by interactions between the ocean and floating ice shelves. To investigate these interactions, coupled ocean and ice sheet model configurations are required. Previous modelling studies have mostly relied on high-resolution configurations, limiting these studies to individual glaciers or regions over short timescales of decades to a few centuries. We present a framework to couple the dynamic ice sheet model PISM (Parallel Ice Sheet Model) with the global ocean general circulation model MOM5 (Modular Ocean Model) via the ice shelf cavity model PICO (Pots-dam Ice-shelf Cavity mOdel). As ice shelf cavities are not resolved by MOM5 but are parameterized with the PICO box model, the framework allows the ice sheet and ocean components to be run at resolutions of 16 km and 3 degrees respectively. This approach makes the coupled configuration a useful tool for the analysis of interactions between the Antarctic Ice Sheet and the global ocean over time spans of the order of centuries to millennia. In this study, we describe the technical implementation of this coupling framework: sub-shelf melting in the ice sheet component is calculated by PICO from modelled ocean temperatures and salinities at the depth of the continental shelf, and, vice versa, the resulting mass and energy fluxes from melting at the ice-ocean interface are transferred to the ocean component. Mass and energy fluxes are shown to be conserved to machine precision across the considered component domains. The implementation is computationally efficient as it introduces only minimal overhead. Furthermore, the coupled model is evaluated in a 4000 year simulation under constant present-day climate forcing and is found to be stable with respect to the ocean and ice sheet spin-up states. The framework deals with heterogeneous spatial grid geometries, varying grid resolutions, and timescales between the ice and ocean component in a generic way; thus, it can be adopted to a wide range of model set-ups. Y1 - 2021 U6 - https://doi.org/10.5194/gmd-14-3697-2021 SN - 1991-959X SN - 1991-9603 VL - 14 IS - 6 SP - 3697 EP - 3714 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 - Feldmann, Johannes A1 - Reese, Ronja A1 - Winkelmann, Ricarda A1 - Levermann, Anders T1 - Shear-margin melting causes stronger transient ice discharge than ice-stream melting in idealized simulations JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Basal ice-shelf melting is the key driver of Antarctica's increasing sea-level contribution. In diminishing the buttressing force of the ice shelves that fringe the ice sheet, the melting increases the ice discharge into the ocean. Here we contrast the influence of basal melting in two different ice-shelf regions on the time-dependent response of an isothermal, inherently buttressed ice-sheet-shelf system. In the idealized numerical simulations, the basal-melt perturbations are applied close to the grounding line in the ice-shelf's (1) ice-stream region, where the ice shelf is fed by the fastest ice masses that stream through the upstream bed trough and (2) shear margins, where the ice flow is slower. The results show that melting below one or both of the shear margins can cause a decadal to centennial increase in ice discharge that is more than twice as large compared to a similar perturbation in the ice-stream region. We attribute this to the fact that melt-induced ice-shelf thinning in the central grounding-line region is attenuated very effectively by the fast flow of the central ice stream. In contrast, the much slower ice dynamics in the lateral shear margins of the ice shelf facilitate sustained ice-shelf thinning and thereby foster buttressing reduction. Regardless of the melt location, a higher melt concentration toward the grounding line generally goes along with a stronger response. Our results highlight the vulnerability of outlet glaciers to basal melting in stagnant, buttressing-relevant ice-shelf regions, a mechanism that may gain importance under future global warming. Y1 - 2022 U6 - https://doi.org/10.5194/tc-16-1927-2022 SN - 1994-0416 SN - 1994-0424 VL - 16 IS - 5 SP - 1927 EP - 1940 PB - Copernicus CY - Göttingen ER - 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 - 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 - 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 - Ciemer, Catrin A1 - Boers, Niklas A1 - Hirota, Marina A1 - Kurths, Jürgen A1 - Müller-Hansen, Finn A1 - Oliveira, Rafael S. A1 - Winkelmann, Ricarda T1 - Higher resilience to climatic disturbances in tropical vegetation exposed to more variable rainfall JF - Nature geoscience N2 - With ongoing global warming, the amount and frequency of precipitation in the tropics is projected to change substantially. While it has been shown that tropical forests and savannahs are sustained within the same intermediate mean annual precipitation range, the mechanisms that lead to the resilience of these ecosystems are still not fully understood. In particular, the long-term impact of rainfall variability on resilience is as yet unclear. Here we present observational evidence that both tropical forest and savannah exposed to a higher rainfall variability-in particular on interannual scales-during their long-term past are overall more resilient against climatic disturbances. Based on precipitation and tree cover data in the Brazilian Amazon basin, we constructed potential landscapes that enable us to systematically measure the resilience of the different ecosystems. Additionally, we infer that shifts from forest to savannah due to decreasing precipitation in the future are more likely to occur in regions with a precursory lower rainfall variability. Long-term rainfall variability thus needs to be taken into account in resilience analyses and projections of vegetation response to climate change. Y1 - 2019 U6 - https://doi.org/10.1038/s41561-019-0312-z SN - 1752-0894 SN - 1752-0908 VL - 12 IS - 3 SP - 174 EP - 179 PB - Nature Publ. Group CY - New York ER - TY - GEN A1 - Ciemer, Catrin A1 - Rehm, Lars A1 - Kurths, Jürgen A1 - Donner, Reik Volker A1 - Winkelmann, Ricarda A1 - Boers, Niklas T1 - An early-warning indicator for Amazon droughts exclusively based on tropical Atlantic sea surface temperatures T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Droughts in tropical South America have an imminent and severe impact on the Amazon rainforest and affect the livelihoods of millions of people. Extremely dry conditions in Amazonia have been previously linked to sea surface temperature (SST) anomalies in the adjacent tropical oceans. Although the sources and impacts of such droughts have been widely studied, establishing reliable multi-year lead statistical forecasts of their occurrence is still an ongoing challenge. Here, we further investigate the relationship between SST and rainfall anomalies using a complex network approach. We identify four ocean regions which exhibit the strongest overall SST correlations with central Amazon rainfall, including two particularly prominent regions in the northern and southern tropical Atlantic. Based on the time-dependent correlation between SST anomalies in these two regions alone, we establish a new early-warning method for droughts in the central Amazon basin and demonstrate its robustness in hindcasting past major drought events with lead-times up to 18 months. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1207 KW - complex networks KW - droughts KW - prediction KW - Amazon rainforest Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-525863 SN - 1866-8372 IS - 9 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 - Ciemer, Catrin A1 - Rehm, Lars A1 - Kurths, Jürgen A1 - Donner, Reik Volker A1 - Winkelmann, Ricarda A1 - Boers, Niklas T1 - An early-warning indicator for Amazon droughts exclusively based on tropical Atlantic sea surface temperatures JF - Environmental Research Letters N2 - Droughts in tropical South America have an imminent and severe impact on the Amazon rainforest and affect the livelihoods of millions of people. Extremely dry conditions in Amazonia have been previously linked to sea surface temperature (SST) anomalies in the adjacent tropical oceans. Although the sources and impacts of such droughts have been widely studied, establishing reliable multi-year lead statistical forecasts of their occurrence is still an ongoing challenge. Here, we further investigate the relationship between SST and rainfall anomalies using a complex network approach. We identify four ocean regions which exhibit the strongest overall SST correlations with central Amazon rainfall, including two particularly prominent regions in the northern and southern tropical Atlantic. Based on the time-dependent correlation between SST anomalies in these two regions alone, we establish a new early-warning method for droughts in the central Amazon basin and demonstrate its robustness in hindcasting past major drought events with lead-times up to 18 months. KW - complex networks KW - droughts KW - prediction KW - Amazon rainforest Y1 - 2019 VL - 15 IS - 9 PB - IOP - Institute of Physics Publishing CY - Bristol 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 - 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 - 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 - 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 - 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 - 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 - JOUR A1 - Levermann, Anders A1 - Albrecht, Tanja A1 - Winkelmann, Ricarda A1 - Martin, Maria A. A1 - Haseloff, Monika A1 - Joughin, I. T1 - Kinematic first-order calving law implies potential for abrupt ice-shelf retreat JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - 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. Y1 - 2012 U6 - https://doi.org/10.5194/tc-6-273-2012 SN - 1994-0416 VL - 6 IS - 2 SP - 273 EP - 286 PB - Copernicus CY - Göttingen 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 - 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 - Albrecht, Tanja A1 - Martin, M. A1 - Haseloff, M. A1 - Winkelmann, Ricarda A1 - Levermann, Anders T1 - Parameterization for subgrid-scale motion of ice-shelf calving fronts JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - A parameterization for the motion of ice-shelf fronts on a Cartesian grid in finite-difference land-ice models is presented. The scheme prevents artificial thinning of the ice shelf at its edge, which occurs due to the finite resolution of the model. The intuitive numerical implementation diminishes numerical dispersion at the ice front and enables the application of physical boundary conditions to improve the calculation of stress and velocity fields throughout the ice-sheet-shelf system. Numerical properties of this subgrid modification are assessed in the Potsdam Parallel Ice Sheet Model (PISM-PIK) for different geometries in one and two horizontal dimensions and are verified against an analytical solution in a flow-line setup. Y1 - 2011 U6 - https://doi.org/10.5194/tc-5-35-2011 SN - 1994-0416 VL - 5 IS - 1 SP - 35 EP - 44 PB - Copernicus CY - Göttingen ER -