2021, Payne, Antony J., Nowicki, Sophie, Abe‐Ouchi, Ayako, Agosta, Cécile, Alexander, Patrick, Albrecht, Torsten, Asay‐Davis, Xylar, Aschwanden, Andy, Barthel, Alice, Bracegirdle, Thomas J., Calov, Reinhard, Chambers, Christopher, Choi, Youngmin, Cullather, Richard, Cuzzone, Joshua, Dumas, Christophe, Edwards, Tamsin L., Felikson, Denis, Fettweis, Xavier, Galton‐Fenzi, Benjamin K., Goelzer, Heiko, Gladstone, Rupert, Golledge, Nicholas R., Gregory, Jonathan M., Greve, Ralf, Hattermann, Tore, Hoffman, Matthew J., Humbert, Angelika, Huybrechts, Philippe, Jourdain, Nicolas C., Kleiner, Thomas, Munneke, Peter Kuipers, Larour, Eric, Le clec'h, Sebastien, Lee, Victoria, Leguy, Gunter, Lipscomb, William H., Little, Christopher M., Lowry, Daniel P., Morlighem, Mathieu, Nias, Isabel, Pattyn, Frank, Pelle, Tyler, Price, Stephen F., Quiquet, Aurélien, Reese, Ronja, Rückamp, Martin, Schlegel, Nicole‐Jeanne, Seroussi, Hélène, Shepherd, Andrew, Simon, Erika, Slater, Donald, Smith, Robin S., Straneo, Fiammetta, Sun, Sainan, Tarasov, Lev, Trusel, Luke D., Van Breedam, Jonas, Wal, Roderik, Broeke, Michiel, Winkelmann, Ricarda, Zhao, Chen, Zhang, Tong, Zwinger, Thomas

Projections of the sea level contribution from the Greenland and Antarctic ice sheets (GrIS and AIS) rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous Coupled Model Intercomparison Project phase 5 (CMIP5) effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

2021-3-25, Rosier, Sebastian H. R., Reese, Ronja, Donges, Jonathan F., De Rydt, Jan, Gudmundsson, G. Hilmar, Winkelmann, Ricarda

Mass loss from the Antarctic Ice Sheet is the main source of uncertainty in projections of future sea-level rise, with important implications for coastal regions worldwide. Central to ongoing and future changes is the marine ice sheet instability: once a critical threshold, or tipping point, is crossed, ice internal dynamics can drive a self-sustaining retreat committing a glacier to irreversible, rapid and substantial ice loss. This process might have already been triggered in the Amundsen Sea region, where Pine Island and Thwaites glaciers dominate the current mass loss from Antarctica, but modelling and observational techniques have not been able to establish this rigorously, leading to divergent views on the future mass loss of the West Antarctic Ice Sheet. Here, we aim at closing this knowledge gap by conducting a systematic investigation of the stability regime of Pine Island Glacier. To this end we show that early warning indicators in model simulations robustly detect the onset of the marine ice sheet instability. We are thereby able to identify three distinct tipping points in response to increases in ocean-induced melt. The third and final event, triggered by an ocean warming of approximately 1.2 ∘C from the steady-state model configuration, leads to a retreat of the entire glacier that could initiate a collapse of the West Antarctic Ice Sheet.

2021-1-7, De Rydt, Jan, Reese, Ronja, Paolo, Fernando S., Gudmundsson, G. Hilmar

Pine Island Glacier in West Antarctica is among the fastest changing glaciers worldwide. Over the last 2 decades, the glacier has lost in excess of a trillion tons of ice, or the equivalent of 3 mm of sea level rise. The ongoing changes are thought to have been triggered by ocean-induced thinning of its floating ice shelf, grounding line retreat, and the associated reduction in buttressing forces. However, other drivers of change, such as large-scale calving and changes in ice rheology and basal slipperiness, could play a vital, yet unquantified, role in controlling the ongoing and future evolution of the glacier. In addition, recent studies have shown that mechanical properties of the bed are key to explaining the observed speed-up. Here we used a combination of the latest remote sensing datasets between 1996 and 2016, data assimilation tools, and numerical perturbation experiments to quantify the relative importance of all processes in driving the recent changes in Pine Island Glacier dynamics. We show that (1) calving and ice shelf thinning have caused a comparable reduction in ice shelf buttressing over the past 2 decades; that (2) simulated changes in ice flow over a viscously deforming bed are only compatible with observations if large and widespread changes in ice viscosity and/or basal slipperiness are taken into account; and that (3) a spatially varying, predominantly plastic bed rheology can closely reproduce observed changes in flow without marked variations in ice-internal and basal properties. Our results demonstrate that, in addition to its evolving ice thickness, calving processes and a heterogeneous bed rheology play a key role in the contemporary evolution of Pine Island Glacier.