Can the time scale of glacial isostatic adjustment condition rate-induced tipping of the West-Antarctic Ice Sheet?

In the context of anthropogenic global warming, the West-Antarctic Ice Sheet (WAIS) is expected to be the greatest contributor to sea-level rise in the coming centuries, thus posing a significant threat for high population densities in coastal regions. Equilibrium simulations of regional warming have shown that the WAIS can tip to a largely collapsed state at forcing levels that will be reached even for relatively low emission paths (SSP2), a process that can be mathematically understood as crossing a bifurcation. It has been shown that some systems might undergo tipping before reaching their bifurcation point due to large rates of the forcing. This process is known as rate-induced tipping (R-tipping) and typically involves competing timescales between negative and positive feedbacks. In the case of the WAIS, the main positive feedback is known as the marine ice-sheet instability, which consists in an increase of the outflow of ice at the grounding line as it retreats on a retrograde bedrock, possibly giving rise to a runaway process. In some cases, such a retreat can however be stabilised by the land uplift and the local drop in sea-surface height following an ice thinning. This process is commonly called glacial isostatic adjustment (GIA) and presents an important negative feedback on marine-terminating ice sheets. Due to a regional anomaly of low viscosities of the upper mantle, the GIA timescale in West-Antarctica has been proven to be one of the shortest observed globally, which could prevent the occurrence of R-tipping. The upper-mantle viscosity of West-Antarctica is however subject to large uncertainties due to the sparsity of seismic data available. To assess how the upper-mantle viscosity can condition the occurrence of R-tipping of the WAIS, we present coupled ice-sheet/solid-Earth simulations that sample the range of plausible viscosity fields under various rates of climatic forcing.