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 - GEN A1 - Frieler, Katja A1 - Mengel, Matthias A1 - Levermann, Anders T1 - Delaying future sea-level rise by storing water in Antarctica T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Even if greenhouse gas emissions were stopped today, sea level would continue to rise for centuries, with the long-term sea-level commitment of a 2 degrees C warmer world significantly exceeding 2 m. In view of the potential implications for coastal populations and ecosystems worldwide, we investigate, from an ice-dynamic perspective, the possibility of delaying sea-level rise by pumping ocean water onto the surface of the Antarctic ice sheet. We find that due to wave propagation ice is discharged much faster back into the ocean than would be expected from a pure advection with surface velocities. The delay time depends strongly on the distance from the coastline at which the additional mass is placed and less strongly on the rate of sea-level rise that is mitigated. A millennium-scale storage of at least 80% of the additional ice requires placing it at a distance of at least 700 km from the coastline. The pumping energy required to elevate the potential energy of ocean water to mitigate the currently observed 3 mmyr(-1) will exceed 7% of the current global primary energy supply. At the same time, the approach offers a comprehensive protection for entire coastlines particularly including regions that cannot be protected by dikes. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 533 KW - carbon-dioxide emissions KW - ice-sheet KW - climate-change KW - model KW - collapse KW - commitment KW - Greenland KW - discharge KW - project KW - surface Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-410234 SN - 1866-8372 IS - 533 ER - TY - GEN A1 - Schleussner, Carl-Friedrich A1 - Lissner, Tabea Katharina A1 - Fischer, Erich M. A1 - Wohland, Jan A1 - Perrette, Mahé A1 - Golly, Antonius A1 - Rogelj, Joeri A1 - Childers, Katelin A1 - Schewe, Jacob A1 - Frieler, Katja A1 - Mengel, Matthias A1 - Hare, William A1 - Schaeffer, Michiel T1 - Differential climate impacts for policy-relevant limits to global warming BT - the case of 1.5 °C and 2 °C T2 - Earth System Dynamics N2 - Robust appraisals of climate impacts at different levels of global-mean temperature increase are vital to guide assessments of dangerous anthropogenic interference with the climate system. The 2015 Paris Agreement includes a two-headed temperature goal: "holding the increase in the global average temperature to well below 2 degrees C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 degrees C". Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing. Here we provide an assessment of key impacts of climate change at warming levels of 1.5 degrees C and 2 degrees C, including extreme weather events, water availability, agricultural yields, sea-level rise and risk of coral reef loss. Our results reveal substantial differences in impacts between a 1.5 degrees C and 2 degrees C warming that are highly relevant for the assessment of dangerous anthropogenic interference with the climate system. For heat-related extremes, the additional 0.5 degrees C increase in global-mean temperature marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions. Similarly, this warming difference is likely to be decisive for the future of tropical coral reefs. In a scenario with an end-of-century warming of 2 degrees C, virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards. This fraction is reduced to about 90% in 2050 and projected to decline to 70% by 2100 for a 1.5 degrees C scenario. Analyses of precipitation-related impacts reveal distinct regional differences and hot-spots of change emerge. Regional reduction in median water availability for the Mediterranean is found to nearly double from 9% to 17% between 1.5 degrees C and 2 degrees C, and the projected lengthening of regional dry spells increases from 7 to 11%. Projections for agricultural yields differ between crop types as well as world regions. While some (in particular high-latitude) regions may benefit, tropical regions like West Africa, South-East Asia, as well as Central and northern South America are projected to face substantial local yield reductions, particularly for wheat and maize. Best estimate sea-level rise projections based on two illustrative scenarios indicate a 50cm rise by 2100 relative to year 2000-levels for a 2 degrees C scenario, and about 10 cm lower levels for a 1.5 degrees C scenario. In a 1.5 degrees C scenario, the rate of sea-level rise in 2100 would be reduced by about 30% compared to a 2 degrees C scenario. Our findings highlight the importance of regional differentiation to assess both future climate risks and different vulnerabilities to incremental increases in global-mean temperature. The article provides a consistent and comprehensive assessment of existing projections and a good basis for future work on refining our understanding of the difference between impacts at 1.5 degrees C and 2 degrees C warming. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 426 KW - sea-level rise KW - Greenland ice-sheet KW - coral-reefs KW - precipitation extremes KW - West Antarctica KW - pine Island KW - model KW - projections KW - temperature KW - scenarios Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-410258 ER - TY - JOUR A1 - Schleussner, Carl-Friedrich A1 - Lissner, Tabea K. A1 - Fischer, Erich M. A1 - Wohland, Jan A1 - Perrette, Mahe A1 - Golly, Antonius A1 - Rogelj, Joeri A1 - Childers, Katelin A1 - Schewe, Jacob A1 - Frieler, Katja A1 - Mengel, Matthias A1 - Hare, William A1 - Schaeffer, Michiel T1 - Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 degrees C and 2 degrees C JF - Earth system dynamics N2 - Robust appraisals of climate impacts at different levels of global-mean temperature increase are vital to guide assessments of dangerous anthropogenic interference with the climate system. The 2015 Paris Agreement includes a two-headed temperature goal: "holding the increase in the global average temperature to well below 2 degrees C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 degrees C". Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing. Here we provide an assessment of key impacts of climate change at warming levels of 1.5 degrees C and 2 degrees C, including extreme weather events, water availability, agricultural yields, sea-level rise and risk of coral reef loss. Our results reveal substantial differences in impacts between a 1.5 degrees C and 2 degrees C warming that are highly relevant for the assessment of dangerous anthropogenic interference with the climate system. For heat-related extremes, the additional 0.5 degrees C increase in global-mean temperature marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions. Similarly, this warming difference is likely to be decisive for the future of tropical coral reefs. In a scenario with an end-of-century warming of 2 degrees C, virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards. This fraction is reduced to about 90% in 2050 and projected to decline to 70% by 2100 for a 1.5 degrees C scenario. Analyses of precipitation-related impacts reveal distinct regional differences and hot-spots of change emerge. Regional reduction in median water availability for the Mediterranean is found to nearly double from 9% to 17% between 1.5 degrees C and 2 degrees C, and the projected lengthening of regional dry spells increases from 7 to 11%. Projections for agricultural yields differ between crop types as well as world regions. While some (in particular high-latitude) regions may benefit, tropical regions like West Africa, South-East Asia, as well as Central and northern South America are projected to face substantial local yield reductions, particularly for wheat and maize. Best estimate sea-level rise projections based on two illustrative scenarios indicate a 50cm rise by 2100 relative to year 2000-levels for a 2 degrees C scenario, and about 10 cm lower levels for a 1.5 degrees C scenario. In a 1.5 degrees C scenario, the rate of sea-level rise in 2100 would be reduced by about 30% compared to a 2 degrees C scenario. Our findings highlight the importance of regional differentiation to assess both future climate risks and different vulnerabilities to incremental increases in global-mean temperature. The article provides a consistent and comprehensive assessment of existing projections and a good basis for future work on refining our understanding of the difference between impacts at 1.5 degrees C and 2 degrees C warming. Y1 - 2016 U6 - https://doi.org/10.5194/esd-7-327-2016 SN - 2190-4979 SN - 2190-4987 VL - 7 SP - 327 EP - 351 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Mengel, Matthias A1 - Levermann, Anders A1 - Schleussner, Carl-Friedrich A1 - Born, Andreas T1 - Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model JF - Earth system dynamics N2 - Direct observations, satellite measurements and paleo records reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its centre. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying, but small amplitude and multi-decadal to centennial periodicities and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multi-decadal scales. Y1 - 2012 U6 - https://doi.org/10.5194/esd-3-189-2012 SN - 2190-4979 SN - 2190-4987 VL - 3 IS - 2 SP - 189 EP - 197 PB - Copernicus CY - Göttingen ER - TY - THES A1 - Mengel, Matthias T1 - Forcing Earth’s sea level BT - instabilities and linear responses Y1 - 2015 ER - TY - JOUR A1 - Mengel, Matthias A1 - Levermann, Anders T1 - Ice plug prevents irreversible discharge from East Antarctica JF - Nature climate change N2 - Changes in ice discharge from Antarctica constitute the largest uncertainty in future sea-level projections, mainly because of the unknown response of its marine basins(1). Most of West Antarctica's marine ice sheet lies on an inland-sloping bed(2) and is thereby prone to a marine ice sheet instability(3-5). A similar topographic configuration is found in large parts of East Antarctica, which holds marine ice equivalent to 19 m of global sea-level rise(6), that is, more than five times that of West Antarctica. Within East Antarctica, the Wilkes Basin holds the largest volume of marine ice that is fully connected by subglacial troughs. This ice body was significantly reduced during the Pliocene epoch(7). Strong melting underneath adjacent ice shelves with similar bathymetry(8) indicates the ice sheet's sensitivity to climatic perturbations. The stability of the Wilkes marine ice sheet has not been the subject of any comprehensive assessment of future sea level. Using recently improved topographic data(6) in combination with ice-dynamic simulations, we show here that the removal of a specific coastal ice volume equivalent to less than 80 mm of global sea-level rise at the margin of the Wilkes Basin destabilizes the regional ice flow and leads to a self-sustained discharge of the entire basin and a global sea-level rise of 3-4 m. Our results are robust with respect to variation in ice parameters, forcing details and model resolution as well as increased surface mass balance, indicating that East Antarctica may become a large contributor to future sea-level rise on timescales beyond a century. Y1 - 2014 U6 - https://doi.org/10.1038/NCLIMATE2226 SN - 1758-678X SN - 1758-6798 VL - 4 IS - 6 SP - 451 EP - 455 PB - Nature Publ. Group CY - London 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 - Mengel, Matthias A1 - Feldmann, Johannes A1 - Levermann, Anders T1 - Linear sea-level response to abrupt ocean warming of major West Antarctic ice basin JF - Nature climate change N2 - Antarctica’s contribution to global sea-level rise has recently been increasing1. Whether its ice discharge will become unstable and decouple from anthropogenic forcing2,3,4 or increase linearly with the warming of the surrounding ocean is of fundamental importance5. Under unabated greenhouse-gas emissions, ocean models indicate an abrupt intrusion of warm circumpolar deep water into the cavity below West Antarctica’s Filchner–Ronne ice shelf within the next two centuries6,7. The ice basin’s retrograde bed slope would allow for an unstable ice-sheet retreat8, but the buttressing of the large ice shelf and the narrow glacier troughs tend to inhibit such instability9,10,11. It is unclear whether future ice loss will be dominated by ice instability or anthropogenic forcing. Here we show in regional and continental-scale ice-sheet simulations, which are capable of resolving unstable grounding-line retreat, that the sea-level response of the Filchner–Ronne ice basin is not dominated by ice instability and follows the strength of the forcing quasi-linearly. We find that the ice loss reduces after each pulse of projected warm water intrusion. The long-term sea-level contribution is approximately proportional to the total shelf-ice melt. Although the local instabilities might dominate the ice loss for weak oceanic warming12, we find that the upper limit of ice discharge from the region is determined by the forcing and not by the marine ice-sheet instability. Y1 - 2016 U6 - https://doi.org/10.1038/NCLIMATE2808 SN - 1758-678X SN - 1758-6798 VL - 6 SP - 71 EP - + PB - Nature Publ. Group CY - London 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 -