Exploring ice sheet model sensitivity to ocean thermal forcing using the Community Ice Sheet Model (CISM)

Abstract

Abstract. Multi-meter sea level rise (SLR) is thought to be possible within a century or two, with most of the uncertainty originating from the Antarctic land ice contribution. One source of uncertainty relates to the ice sheet model initialization. Since ice sheets have a long response time (compared to other Earth system components such as the atmosphere), ice sheet model initialization methods can have significant impacts on how the ice sheet responds to future forcings. To assess this, we generated 25 different ice sheet spin-ups, using the Community Ice Sheet Model (CISM) at 4 km resolution. During each spin-up we varied two key parameters known to impact the sensitivity of the ice sheet to future forcing: One related to the sensitivity of the ice-shelf melt rate to ocean thermal forcing, and the other related to the basal friction. The spin-ups all nudge toward observed thickness and enforce a no-advance calving criterion, such that all final spun-up states resemble observations but differ in their melt and friction parameter settings. Each spin-up was then forced with future ocean thermal forcings from 13 different CMIP6 models under the SSP5-8.5 emissions scenario, and modern climatological surface mass balance data. Our results show that the effects of the ice sheet and ocean parameter settings used during the spin-up are capable of impacting multi-century future SLR predictions by as much as 2 m. By the end of this century, the effects of these choices are more modest, but still significant, with differences of up to 0.2 m of SLR. We have identified a combined ocean and ice parameter space that leads to widespread mass loss (low friction & high melt rate sensitivity). To explore temperature thresholds, we also ran a synthetically-forced CISM ensemble that is focused on the Amundsen region only. We find that given certain ocean and ice parameter choices, Amundsen mass loss can be triggered with thermal forcing anomalies between 1.5 and 2 °C. Our results emphasize the critical importance of considering ice sheet/ocean parameter choices during spin-up for sea level rise predictions.

Present-day mass loss rates are a precursor for West Antarctic Ice Sheet collapse

“…GIA with a very short relaxation time of 100 yr delays the collapse by several centuries but does not prevent it. These findings are similar to the ones in Berdahl et al (2023). The collapse timing is sensitive to model choices related to basal melting.…”

“…The Community Ice Sheet Model (CISM; (Lipscomb et al, 2019) is a thermo-mechanical higher-order ice sheet model, which is part of the Community Earth System Model version 2 (CESM2, Danabasoglu et al (2020)). This study primarily uses CISM v2.1 and builds on earlier applications of CISM to Antarctic Ice Sheet retreat (Lipscomb et al, 2021, Berdahl et al, 2023. CISM is run at 4 km resolution with a vertically integrated higher-order approximation to the momentum balance (Goldberg, 2011, Robinson et al, 2022, with a regularized Coulomb sliding law (Zoet and Iverson, 2020) as basal boundary condition.…”

“…The collapse timing is sensitive to model choices related to basal melting. A weaker/stronger dependence of the basal melt on the thermal forcing at the ice draft depth (Jourdain et al, 2020) delays/advances the start of the collapse (Berdahl et al 2023).…”

Antarctic climate response in Last-Interglacial simulations using the Community Earth System Model (CESM2)

“…In their ice sheet model experiments, for example, Turney et al (2020) achieve WAIS collapse (4.5m sea level equivalent mass loss) under only 1 • C ocean temperature forcing within two millennia. Berdahl et al (2023) found that under ocean conditions comparable to today in the Amundsen sector, ice shelves lifted off key pinning points and unfettered retreat was initiated within five centuries. Given that today's Amundsen sector is likely warmer than PI by a few tenths of a degree, we argue that the modest CESM2 warming seen in the lig127ka run provides a reasonable estimate of actual LIG warming.…”

“…Some ice-sheet modelling studies suggest that a relatively high TF threshold must be reached before WAIS collapse is likely. For example, Sutter et al (2016) found that ocean warming of at least 2-3 • C compared to PI was a prerequisite for collapse, while DeConto and Pollard (2016) found that, even with the inclusion of sensitive ice-calving physics and atmospheric feedbacks, the WAIS does not collapse under simulated LIG conditions without ocean warming of 3 • C. However, other studies suggest that widespread WAIS mass loss could be triggered even with smaller TF (1-2 • C) given the right ice sheet conditions and enough time (Berdahl et al, 2023;Lipscomb et al, 2021;Garbe et al, 2020).…”