Topographically constrained tipping point for complete Greenland Ice Sheet melt

A major impact of anthropogenic climate change is the potential triggering of tipping points, such as the complete loss of the Greenland Ice Sheet (GrIS). Currently, the GrIS is losing mass at an accelerated pace, mainly due to a steep decrease in its Surface Mass Balance (SMB, snow accumulation minus surface ablation from melt and associated runoff). Here, we investigate a potential SMB threshold for complete GrIS melt, the processes that control this threshold, and whether it exhibits characteristics commonly associated with tipping points, such as a non-linear response to external forcings. To do this, we adopt a semi-coupled approach, forcing the Community Ice Sheet Model v.2 (CISM2) with different SMB levels previously calculated at multiple elevation classes with the Community Earth System Model v.2 (CESM2). The SMB calculation in CESM2 and the elevation class method allow us to account for the SMB-elevation feedback based on snow/firn processes and energy fluxes at the ice sheet surface. We find a positive SMB threshold for complete GrIS melt of 230±84 Gt/yr, corresponding to a 60% decrease from the GrIS simulated pre-industrial SMB. The ice sheet shows a highly non-linear response to sustained melt, and its final state is determined by the effect of the SMB-height feedback in response to surface melt and Glacial Isostatic Adjustment (GIA). The GrIS is tipping from nearly 50% equilibrium volume towards complete melt when the ice margin in the central west unpins from a coastal region with high bedrock elevation and SMB. We find that this relatively small coastal region is important to determine the ice sheet stability in response to sustained warming. Based on the ice sheet geometry in previous modelling studies of the GrIS during the last interglacial, we suggest that a stabilizing effect of the bedrock topography in the central West might have occurred in the past.