Time-dependent rheological behaviour of the solid Earth greatly influence Antarctica's future sea-level contribution.

Over the coming five centuries, bedrock beneath the Antarctic ice sheet is projected to rise by more than one hundred meters as the ice mass continues to decrease depending on the emission scenario. This uplift, known as glacial isostatic adjustment (GIA), is predicted to reduce Antarctica’s contribution to barystatic sea-level rise by up to 20% due to its negative feedback effect on ice-sheet dynamics. The magnitude of this solid Earth response depends on past ice-mass changes and on mantle viscosity.Most GIA models assume that the mantle viscosity is constant in time, or that viscosity varies with stress under the assumption that the material has already reached steady-state, power-law rheological behaviour. However, laboratory experiments on olivine, the dominant mineral in the upper mantle, demonstrate that viscosity evolves in response to changing stress conditions, placing the mantle in a transient state with corresponding lower viscosities and faster deformation rates than predicted based on steady-state rheological behaviours.First, we predict that mantle viscosity beneath the West Antarctic Ice Sheet decreases by several orders of magnitude over the coming centuries by using an ice sheet model (IMAU-ICE) coupled to a spherical 3D GIA model with steady-state, power-law rheological behaviour (FESLA). Next, we extend the power-law behaviour with a laboratory-constrained transient rheological behaviour and implement it as a new flow law in finite element platform GTECTON. Focusing on recent ice-load changes in the Amundsen Sea Embayment, we explore the potential imprint of the extended rheological behaviour in GNSS and satellite altimetry observations. For realistic ice-mass changes, we predict that mantle viscosity may temporarily decrease by one to two orders of magnitude relative to long-term values.Including this time-dependent behavior in GIA models will help to refine projections of future bedrock motion and improve our understanding of how Antarctic ice-mass loss will influence global sea level in the coming centuries.