Abstract
Abstract. The future retreat rate for marine-based regions of the Antarctic Ice Sheet is one of the largest uncertainties in sea-level projections. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) aims to improve projections and quantify uncertainties by running an ensemble of ice sheet models with atmosphere and ocean forcing derived from global climate models. Here, the Community Ice Sheet Model (CISM) is used to run ISMIP6-based projections of ocean-forced Antarctic Ice Sheet evolution. Using multiple combinations of sub-ice-shelf melt parameterizations and calibrations, CISM is spun up to steady state over many millennia. During the spin-up, basal friction parameters and basin-scale thermal forcing corrections are adjusted to optimize agreement with the observed ice thickness. The model is then run forward for 550 years, from 1950–2500, applying ocean thermal forcing anomalies from six climate models. In all simulations, the ocean forcing triggers long-term retreat of the West Antarctic Ice Sheet, especially in the Filchner–Ronne and Ross sectors. Mass loss accelerates late in the 21st century and then rises steadily for several centuries without leveling off. The resulting ocean-forced sea-level rise at year 2500 varies from about 150 to 1300 mm, depending on the melt scheme and ocean forcing. Further experiments show relatively high sensitivity to the basal friction law, moderate sensitivity to grid resolution and the prescribed collapse of small ice shelves, and low sensitivity to the stress-balance approximation. The Amundsen sector exhibits threshold behavior, with modest retreat under many parameter settings but complete collapse under some combinations of low basal friction and high thermal forcing anomalies. Large uncertainties remain, as a result of parameterized sub-shelf melt rates, simplified treatments of calving and basal friction, and the lack of ice–ocean coupling.
Highlights
The Antarctic Ice Sheet has been losing mass at an increasing rate for the past several decades (Shepherd et al, 2018; Rignot et al, 2019)
Much of the ice loss has been driven by increased access of warm Circumpolar Deep Water (CDW) to marine-based parts of the West Antarctic Ice Sheet (WAIS), likely caused in part by radiatively forced changes in wind patterns (Thomas et al, 2004; Jenkins et al, 2010; Rignot et al, 2013; Holland et al, 2019; Gudmundsson et al, 2020)
Many WAIS glaciers lie on reverse-sloping beds, making these glaciers vulnerable to the marine ice sheet instability (MISI; Weertman, 1974; Mercer, 1978; Schoof, 2007)
Summary
The Antarctic Ice Sheet has been losing mass at an increasing rate for the past several decades (Shepherd et al, 2018; Rignot et al, 2019). Despite recent advances in ice sheet modeling (Pattyn, 2018), projections of 21st century Antarctic Ice Sheet retreat and resulting sealevel rise (SLR) are highly uncertain, ranging from modest (∼ 10 cm; Ritz et al, 2015) to large and abrupt (> 1 m; Pollard and DeConto, 2016). Recent work (Edwards et al, 2019) suggests that the most extreme projections may overestimate the rate of SLR, much of the WAIS is likely vulnerable to long-term, self-sustaining retreat
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