ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century

Abstract

Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and inform on the mass loss that would contribute to future sea level rise. However, there is currently no consensus on estimated the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes and the forcings employed. This study presents results from 18 simulations from 15 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015-2100, forced with different scenarios from the Cou-5 pled Model Intercomparison Project Phase 5 (CMIP5) representative of the spread in climate model results. The contribution of the Antarctic ice sheet in response to increased warming during this period varies between -7.8 and 30.0 cm of Sea Level Equivalent (SLE). The evolution of the West Antarctic Ice Sheet varies widely among models, with an overall mass loss up to 21.0 cm SLE in response to changes in oceanic conditions. East Antarctica mass change varies between -6.5 and 16.5 cm SLE, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario 10 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 mass loss of 8 mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from 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 AOGCMs show an overall mass loss of 10 mm SLE compared to simulations done under present-day 15 conditions, with limited mass gain in East Antarctica.

Could the Last Interglacial Constrain Projections of Future Antarctic Ice Mass Loss and Sea‐Level Rise?

“…Whereas our methodology is developed with a generalizable emulation and calibration framework, quantitative results in section 3 apply only to this specific ice‐sheet model. The LIG could inform additional or alternative physical processes (see section 4) not considered here, and the emulation and calibration framework could be extended to include assessments of LIG constraints on the Greenland ice sheet (GrIS), calibration of other ice‐sheet models or ensembles (e.g., ISMIP6; Goelzer et al, 2018; Nowicki et al, 2016; Seroussi et al, 2020), calibration of different parameters or regions of parameter space, or calibration with different paleo sea‐level constraints (e.g., the Pliocene).…”

Diverging future surface mass balance between the Antarctic ice shelves and grounded ice sheet

“…CESM2 and CNRM-CM6-1 (ssp585) to the ISMIP6-derived SMB used to predict the future Antarctic sea-level contribution (Seroussi et al, 2020) by interpolating the 32 km SMB fields built by ISMIP6 on the 35 km MAR grid.…”

“…As the integrated differences cumulated over 1995-2100 can be larger or as large as the differences between CMIP5-RCP8.5 and CMIP6-ssp585, or between CMIP6-ssp126 and CMIP6-ssp245 ( Fig. 11), this also raises the question of the sensitivity to the forcing of ISMIP6 projections, where the SMB is used as an input for performing projections of the total AIS mass balance (Seroussi et al, 2020).…”

Ocean-induced melt volume directly paces ice loss from Pine Island Glacier

“…Although observations ( 2 , 6 ) and models ( 7 – 9 ) reveal that melt-induced ice-shelf thinning increases ice discharge, challenges remain in fully coupling ice and ocean models to resolve the spatiotemporal evolution of melt ( 10 , 11 ). Consequently, several studies have relied on depth-parameterized melt rates to drive ice-flow models ( 7 , 8 , 10 , 12 , 13 ), which, although often tuned to the initial state, may not hold as ice-shelf cavities evolve ( 14 , 15 ).…”

“…While several studies suggest that melt’s spatial distribution helps govern ice-sheet loss ( 13 , 16 – 19 ), it is not immediately clear that this should be so. Ice-shelf mass balance is determined by grounding-line flux and surface mass balance (SMB) (net snowfall after surface ablation) gains, which are countered by losses from subshelf melting and iceberg calving.…”