The impact of Earth structure on Antarctic ice sheet tipping thresholds

Reducing uncertainties in the projections of the future contribution of the Antarctic Ice Sheet (AIS) to global sea-level rise is crucial for coastal communities and policymakers worldwide. Self-amplifying feedback mechanisms can lead to accelerated and irreversible ice loss once certain temperature regimes are crossed. Such tipping behaviour will ultimately lead to a new equilibrium state, even if boundary conditions remain constant. In contrast, negative feedback loops, such as the sea-level feedback due to glacial isostatic adjustment (GIA), potentially slow down the rate of ice loss by reducing the local water depth at the grounding line. The rebound rate of the bedrock following a reduction in ice mass depends heavily on the Earth structure beneath Antarctica, with mantle viscosities and corresponding response timescales that can vary laterally by two to three orders of magnitude. Yet, it is unclear whether GIA feedbacks can shift ice sheet tipping points or even prevent tipping as a result of path dependency, as bifurcation-tipping theory considers stationary states only, where the ice sheet load and solid Earth deformation are in isostatic equilibrium.

By employing different Earth structures in (quasi-)equilibrium simulations and varying temperature forcing rates, using a 3D coupled ice sheet–GIA model (PISM-VILMA), we explore their influence on the AIS's stability and tipping thresholds, focusing on the West Antarctic Ice Sheet and the Wilkes Subglacial Basin. Our simulations demonstrate that the Earth structure significantly affects both the temperature threshold at which self-sustained retreat of the AIS is initiated and the long-term committed contribution to global sea-level rise; this as a result of path dependency.
Moreover, our results highlight the competing timescales of ice sheet and solid Earth dynamics. We find that the rate of temperature increase represents a crucial parameter. Rate-induced tipping can lead to abrupt changes at lower thresholds than in the quasi-equilibrium case, in particular for stronger Earth structures, leading to higher sea-level contribution for the same warming levels.