Data-driven bed rheology enhances retreat on Thwaites Glacier

Field data suggest that Thwaites Glacier's bed is composed of alternating areas of hard and soft material with correlated topographic variations.Prior modeling shows that forced response differs strongly for simulations on endmember beds, suggesting that under some circumstances a mosaicked bed can exhibit response that falls outside endmembers.We explore the impact of spatially-variable bed rheology draped on the measured topography beneath Thwaites under sustained sub-ice-shelf melting.Experiments utilize the Ice-Sheet & Sea-Level System Model to assign the basal sliding law exponent and associated friction coefficient on an element-wise basis.In mixed-rheology models, thinning is localized over harder basal topographic ridges (bedrock), with widespread drawdown over flatter areas comprising a softer material (deforming till).This dual dynamic signature creates rapid retreat under sustained forcing, as enhanced driving stresses over ridges upstream of extended areas of retrograde bed allow faster retreat across a more-thinned catchment.For more-moderate forcing that allows this pattern to develop, retreat can be faster than for either endmember bed.Findings indicate a need for additional data and more-informed inversions for basal friction in order to constrain and simulate basal conditions beneath Thwaites, thereby reducing uncertainty in predictive models of sea-level rise from the WAIS. IntroductionThwaites Glacier has been actively retreating in response to sub-shelf melt from enhanced circumpolar deep-water intrusion onto the Antarctic continental shelf (e.g., Pritchard and others, 2012; Sutterley and others, 2013; Rignot and others, 2014; Mouginot and others, 2014; Joughin and others, 2014).This incursion trend is expected to continue and, as past warmer periods have produced rapid retreat on Thwaites (Graham and others, 2022), it is likely that continued sub-shelf melt will maintain or accelerate mass-loss and associated sea-level rise.Given its unique position as a climatic tipping point situated in the "weak underbelly" (Hughes, 1981) of the West Antarctic Ice Sheet (WAIS), it is critical to reduce uncertainty in estimates of Thwaites' potential timelines of retreat.Predictive models seeking to anticipate sea-level contributions from the Amundsen Sea Embayment (ASE) sector of the WAIS tend to diverge (e.g., Yu and others, 2018, Hoffman andothers, 2019;, Robel and others, 2019).In addition to uncertainties inherent in future climate prescription, differences among models of Thwaites and the surrounding region are largely due to ill-constrained physics and approximations therein, and point to an incomplete understanding of controlling processes and their dynamic interplay (e.g., Parizek, 2024).A recent intercomparison of the ISMIP6 models found that the choice of model is the primary source of uncertainty in sea-level projections from the WAIS as a whole under strong warming (Seroussi and others, 2024).In turn, the assumed basal sliding relation is important or dominant in the model differences (e.g., Sun and others, 2020).Thwaites continues to lose buttressing from ice-shelf contact with its current pinning point, and to experience grounding-zone retreat as it is melted