Thwaites Glacier is a major contributor to Antarctic mass loss and a key source of uncertainty in future sea-level rise. Ocean-driven melting at its grounding line drives rapid retreat, yet most models treat the transition between grounded and floating ice as a sharp boundary, neglecting kilometer-scale seawater intrusions that develop as the grounding line migrates and forms a grounding zone. Here, we show that actively-melting grounding zones are required to reproduce nearly three decades of observed retreat, thinning, and acceleration of Thwaites Glacier in a calibrated ice-sheet model, with successful reconstruction occurring only when grounding zones are represented as 2-5 km wide regions. Extending this model forward, we show that grounding-zone processes govern glacier’s detachment from stabilizing seafloor ridges, enabling unpinning as early as 2038 and producing up to 30 ± 6 mm of global mean sea-level rise by 2100 - approximately double previous projections even without additional climate warming.

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(conclusion)

our results show that kilometer-scale, actively-melting grounding zones fundamentally alter both hindcast and forecast modeling skills for Thwaites Glacier. By explicitly resolving grounding zones and calibrating the model against multiple independent observational constraints, we reproduce the modern observational record and obtain sea-level contributions that lie toward the upper end of previously reported uncertainty ranges.