QUIQUET Aurelien

The land ice contribution to global mean sea level rise has not yet been predicted1 using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models2–8, but primarily used previous-generation scenarios9 and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12 using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

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The Antarctic ice sheet's contribution to global sea level rise over the 21st century is of primary societal importance and remains largely uncertain as of yet. In particular, in the recent literature, the contribution of the Antarctic ice sheet by 2100 can be negative (sea level fall) by a few centimetres or positive (sea level rise), with some estimates above 1 m. The Ice Sheet Model Intercomparison Project for the Coupled Model Intercomparison Project-phase 6 (IS-MIP6) aimed at reducing the uncertainties in the fate of the ice sheets in the future by gathering various ice sheet models in a common framework. Here, we present the GRISLI-LSCE (Grenoble Ice Sheet and Land Ice model of the Laboratoire des Sciences du Climat et de l'Environnement) contribution to ISMIP6-Antarctica. We show that our model is strongly sensitive to the climate forcing used, with a contribution of the Antarctic ice sheet to global sea level rise by 2100 that ranges from −50 to +150 mm sea level equivalent (SLE). Future oceanic warming leads to a decrease in thickness of the ice shelves, resulting in grounding-line retreat, while increased surface mass balance partially mitigates or even overcompensates the dynamic ice sheet contribution to global sea level rise. Most of the ice sheet changes over the next century are dampened under low-greenhousegas-emission scenarios. Uncertainties related to sub-ice-shelf melt rates induce large differences in simulated groundingline retreat, confirming the importance of this process and its representation in ice sheet models for projections of the Antarctic ice sheet's evolution.

During the last deglaciation (21–7 kaBP), the gradual retreat of Northern Hemisphere ice sheet margins produced large proglacial lakes. While the climatic impacts of these lakes have been widely acknowledged, their role on ice sheet grounding line dynamics has received very little attention so far. Here, we show that proglacial lakes had dramatic implications for the North American ice sheet dynamics through a self‐sustained mechanical instability which has similarities with the known marine ice sheet instability consequently providing fast retreat of large portions of the ice sheet over the continent. This instability mechanism is likely important in contributing to deglaciation of terrestrial glaciers and ice sheets with proglacial lakes at their margins as it can substantially accelerate the mass loss. Echoing our knowledge of Antarctic ice sheet dynamics, proglacial lakes are another manifestation of the importance of grounding line dynamics for ice sheet evolution.

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Understanding the ocean circulation changes associated with abrupt climate events is key to better assessing climate variability and understanding its different natural modes. Sedimentary Pa=Th, benthic δ13C and Δ114C are common proxies used to reconstruct past circulation flow rate and ventilation. To overcome the limitations of each proxy taken separately, a better approach is to produce multiproxy measurements on a single sediment core. Yet, different proxies can provide conflicting information about past ocean circulation. Thus, modelling them in a consistent physical framework has become necessary to assess the geographical pattern and the timing and sequence of the multiproxy response to abrupt circulation changes. We have implemented a representation of the 231Pa and 230Th tracers into the model of intermediate complexity iLOVECLIM, which already included δ13C and Δ114C. We have further evaluated the response of these three ocean circulation proxies to a classical abrupt circulation reduction obtained by freshwater addition in the Nordic Seas under preindustrial boundary conditions. The proxy response is shown to cluster in modes that resemble the modern Atlantic water masses. The clearest and most coherent response is obtained in the deep (>. 2000 m) northwest Atlantic, where δ13C and Δ114C significantly decrease, while Pa=Th increases. This is consistent with observational data across millennial-scale events of the last glacial. Interestingly, while in marine records, except in rare instances, the phase relationship between these proxies remains unclear due to large dating uncertainties, in the model the bottom water carbon isotope (δ13C and Δ114C) response lags behind the sedimentary Pa=Th response by a few hundred years.

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This study investigates the evolution of Greenland ice sheet flux focusing on five of the main fast flowing regions (Petermann glacier, North East Greenland Ice Stream, Kangerdlugssuaq glacier, Helheim glacier and Jakobshavn glacier) in response to 20th and 21st century climate change. A hybrid (shallow ice and shallow shelf) ice-sheet model (ISM) is forced with the combined outputs of a set of seven CMIP5 models and the regional climate model MAR. The ISM simulates the present-day ice velocity pattern, topography and surface mass balance (SMB) in good agreement with observations. Except for the Kangerdlugssuaq glacier, over the 21st century all the fast-flowing areas have exhibited a decrease in ice flux as a result of a negative SMB rather than dynamical changes. Only the fronts of Kangerdlugssuaq and Helheim glaciers have shown an interannual variability driven by dynamical rather than climate changes. Finally, the results predict a substantial inland ice margin retreat by the end of the 21st century, especially along the northern coasts.