Response of Thwaites Glacier's Shear Margins to Ice Sheet Thinning and Surface-Slope Steepening

Thwaites Glacier is a major contributor to uncertainty in near-term sea level estimates world wide. Current projections of the rate of mass flux, and possible collapse, of Thwaites Glacier depend not only on the ice speeds, but also on the width of the region of rapidly sliding ice. The lateral bounds of this rapidly sliding ice, termed shear margins, appear to be weakly constrained on the eastern side, raising the possibility of margin migration.

The goal of this study is to evaluate the possibility of shear margin migration in response to ice sheet thinning and steepening of the ice surface as thinning is accelerated towards the coastal margin. We hypothesize that ice sheet thinning will drive inwards margin migration, while ice sheet steepening will drive outwards margin migration. We test our hypothesis by developing a depth-averaged, thermomechanical free-boundary model that captures the complex topography underneath the glacier and solves for both the ice velocity and for the position of the shear margins. Our free-boundary approach enables a minimally parameterized representation of shear margin behavior, avoiding regularization of the Coulomb-Plastic sliding law.

We test the impact of ice sheet thinning alone by imposing uniform thinning across the entire domain. We then test the impact of steepening and thinning combined by applying non-uniform thinning across the domain, based on thinning observations from satellite measurement over the past decades. We find that while both shear margins are prone to inwards migration due to thinning alone, steepening consistent with observed thinning rates negates this inwards migration, explaining the stable margins observed over the satellite era. To test the sensitivity of our findings to basal conditions, we consider multiple instances of basal strength, including an inversion and several forward models of basal strength. We discuss how ongoing field efforts can be used to distinguish between these different scenarios.