Competing processes determine the long-term impact of basal friction parameterizations for Antarctic mass loss

Abstract. An often-mentioned source of uncertainty when projecting future sea level rise with ice sheet models is the choice of basal friction law. Previous studies do not agree on whether this choice causes significantly different projections. We use the Community Ice Sheet Model (CISM) to show that the sensitivity of the projected sea level rise to the choice of basal friction law depends on the geometric setting and the inversion procedure: CISM can be tuned to be sensitive to the choice of basal friction law or not. We find a geometry-driven connection between buttressing and basal sliding in the Amundsen Sea Embayment. When Thwaites Glacier collapses, it creates a grounding line flux large enough to sustain an ice shelf that provides buttressing and reduces the importance of basal friction. This is not the case, however, when Pine Island Glacier retreats significantly. Thus, a collapsing Pine Island glacier is sensitive to the choice of basal friction law, but a collapsing Thwaites Glacier is not. Which glacier collapses first depends on the inversion procedure. This study highlights the importance of the initialization procedure, and the underdetermined nature of ice sheet modelling. The latter makes it difficult to base general claims on ice sheet modelling results.