Historically consistent mass loss projections of the Greenland ice sheet

Abstract. Mass loss from the Greenland ice sheet is presently a significant factor for global sea-level rise and is expected to increase under continued Arctic warming. As sea-level rise is threatening coastal communities worldwide, reducing uncertainties in projections of future sea-level contribution from the Greenland ice sheet is of high importance. In this study we determine sea-level contribution that can be expected from the ice sheet until 2100 by performing an ensemble of stand-alone ice sheet simulations with the Community Ice Sheet Model (CISM). The ice sheet is initialized to resemble the presently observed geometry by calibrating basal friction parameters. We use forcing from various Earth System Models (ESMs), as well as from ERA5 reanalysis for initialization and investigate how this affects the simulated historical mass loss and the projected sea-level contribution until 2100. The observed historical mass loss is generally well reproduced by the ensemble, with a particularly close match with observations when using output from ERA5 reanalysis to force the initialization as well as the historical run. We examine a range of uncertainties, associated with stand-alone ice sheet modeling by prescribing forcing from various ESMs for three different emission scenarios. Atmospheric forcing is downscaled with the regional climate model MAR. Retreat of marine-terminating outlet-glaciers in response to ocean forcing and runoff from the ice sheet is represented by a retreat parameterization and its uncertainty is sampled by considering different sensitivities. Furthermore, we disentangle the relative importance of surface mass balance (SMB) and outlet-glacier retreat forcing, as well as of the SMB-height feedback, on the projected mass loss by performing dedicated single forcing experiments. While discharge from outlet-glaciers remains a substantial factor, the future evolution of the ice sheet is governed by mass loss due to changes in SMB. Assuming a medium sensitivity to outlet-glacier retreat forcing, by 2100, the projections yield a sea-level contribution of 32 to 69 mm under the SSP1-2.6 scenario, 44 to 119 mm under the SSP2-4.5 scenario and 74 to 228 mm under the SSP5-8.5 scenario. With a spread of 154 mm under the SSP5-8.5 scenario climate forcing constitutes the largest source of uncertainty for the projected sea-level contribution, while uncertainty in the retreat forcing account for a spread of 25 mm. We find differences in projected sea-level contribution due to the initial state of the ice sheet and grid resolution to be minor.