Rate-induced tipping in marine-based regions of the Antarctic ice sheet

The Antarctic ice sheet (AIS) is expected to be the largest contributor to sea-level rise over the coming centuries. As assessed by studies using equilibrium ice-sheet simulations, this is due to crossing a bifurcation point (B-tipping) driven by the marine ice-sheet instability (MISI). However, high forcing rates can lead to tipping at warming levels below the bifurcation point. This effect, called rate-induced tipping (R-tipping), has not been investigated for the AIS yet but is highly relevant given that anthropogenic warming is occurring at high rates. Here we propose that sectors subject to MISI can undergo R-tipping: when forcing rates are high and the bedrock uplift is slow, the grounding-zone retreat is not stabilised as efficiently as in equilibrium simulations, triggering MISI at warming levels below the bifurcation point. We test our hypothesis by using an ice-sheet model coupled to a computationally-efficient glacial isostatic adjustment model. The latter is suited to sample the uncertainties of the mantle viscosity, which governs the timescale of bedrock adjustment. We show that, although the West-Antarctic Ice Sheet is subject to B-tipping, it is unlikely to undergo R-tipping due to the low mantle viscosity of this region. In contrast, the East-Antarctic Subglacial Basins are likely to undergo R-tipping even for rates of warming that are smaller than the present-day ones. Depending on the mantle viscosity, this effect can reduce the critical warming by as much as 0.8 K of global mean temperature anomaly, which can drastically impact mitigation and adaptation strategies.