Extending the range and reach of physically-based Greenland ice sheet sea-level projections

Abstract. We present an ensemble of ice sheet model projections for the Greenland ice sheet (GrIS) that was produced as part of the European project PROTECT. The work makes use of ice sheet model (ISM) projections forced by high-resolution regional climate model (RCM) output and other climate model forcing, including a parameterisation for the retreat of marine-terminating outlet glaciers. The focus is on providing extended physically-based projections that improve our understanding of the range of GrIS future sea-level contributions and the inherent uncertainties over decadal to multi-centennial timescales. The experimental design builds on the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) protocol and extends it to more fully account for some of the uncertainties in sea-level projections. We include a wider range of CMIP6 climate model output, more climate change scenarios, several climate downscaling approaches, a wider range of sensitivity to ocean forcing and we extend projections schematically beyond the year 2100 up to year 2300, including idealised overshoot scenarios. GrIS sea-level rise contributions range from 16 to 353 mm in the year 2100 (relative to 2014), with strong dependency on the applied climate forcing. Contributions reach 49 to 3127 mm in 2300, indicative of large uncertainties and a potentially very large long-term response. We also extend the ISMIP6 forcing approach backwards over the historical period and successfully produce consistent simulations in both past and future for three of the four ISMs. The ensemble design of ISM experiments is geared towards the subsequent use of emulators to facilitate statistical interpretation of the results and produce probabilistic projections of the GrIS contribution to future sea-level rise.