The effect of the present-day imbalance on schematic and climate forced simulations of the West Antarctic Ice Sheet collapse

Abstract. Recent observations reveal that the West Antarctic Ice Sheet is rapidly thinning, particularly at its two largest outlet glaciers, Pine Island Glacier and Thwaites Glacier, while East Antarctica remains relatively stable. Ice sheet model projections over the next few centuries give a mixed picture, some ice sheet models forced by climate models project mass gain by increased surface mass balance, while most models project moderate to severe mass loss by increasing ice discharge. In this study, we explore the effect of present-day ice thickness change rates on forced future simulations of the Antarctic Ice Sheet using the Community Ice Sheet Model (CISM). We start with a series of schematic, uniform ocean temperature perturbations in the Amundsen Sea Embayment (ASE) to probe the sensitivity of the modelled present-day imbalance to ocean warming. We then apply ocean and atmospheric forcing from seven datasets produced by five Earth System Models (ESMs) from the CMIP5 and CMIP6 ensemble to simulate the Antarctic Ice Sheet from 2015 to 2500. The schematic experiments suggest the presence of an ice-dynamical limit; Thwaites Glacier (TG) does not collapse in these experiments (i.e., with accelerated deglaciation leading to considerable grounded ice mass loss) before ∼2100 without more than 2° of ocean warming. Meanwhile, the maximum rate of Global Mean Sea Level rise (GMSLr) from the ASE during the collapse increases linearly with ocean temperature, indicating that while earlier collapse timing shows diminishing returns, the rate of sea-level rise keeps on intensifying with stronger forcing. The relative importance of including the observed present-day mass loss rates decreases for larger (ocean) warming under climate forcing, and decreases over time. For the East Antarctic Ice Sheet on shorter timescales (until 2100), adding the present-day observed mass change rates doubles its global mean sea level rise contribution. On longer timescales (2100–2500), the effect of the present-day observed mass change rates is smaller. Thinning of the West Antarctic Ice Sheet induced by the present-day imbalance is partly compensated by present-day ice sheet thickening of the East Antarctic Ice Sheet over the coming centuries, which persists in our simulations. These deviations are overshadowed by the mass losses induced by the projected ocean warming.