Long-term future projections for the Greenland and Antarctic ice sheets with the model SICOPOLIS

The Coupled Model Intercomparison Project Phase 6 (CMIP6) is a major international climate modelling initiative. As part of it, the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) was devised to assess the likely sea-level-rise contribution from the Greenland and Antarctic ice sheets until the year 2100. This was achieved by defining a set of future climate scenarios by evaluating results of CMIP5 and CMIP6 global climate models (GCMs, including MIROC) over and surrounding the Greenland and Antarctic ice sheets. These scenarios were used as forcings for a variety of ice-sheet models operated by different working groups worldwide (Goelzer et al. 2020, doi: 10.5194/tc-14-3071-2020; Seroussi et al. 2020, doi: 10.5194/tc-14-3033-2020).

Here, we use the model SICOPOLIS to carry out extended versions of the ISMIP6 future climate experiments for the Greenland and Antarctic ice sheets until the year 3000. For the atmospheric forcing (anomalies of surface mass balance and temperature) beyond 2100, we sample randomly the ten-year interval 2091-2100, while the oceanic forcing beyond 2100 is kept fixed at 2100 conditions. We conduct experiments for the pessimistic, "business as usual" pathway RCP8.5 (CMIP5) / SSP5-8.5 (CMIP6), and for the optimistic RCP2.6 (CMIP5) / SSP1-2.6 (CMIP6) pathway that represents substantial emissions reductions. For the unforced, constant-climate control runs, both ice sheets are stable until the year 3000. For RCP8.5/SSP5-8.5, they suffer massive mass losses: For Greenland, ~1.7 m SLE (sea-level equivalent) for the 12-experiment mean, and ~3.5 m SLE for the most sensitive experiment. For Antarctica, ~3.3 m SLE for the 14-experiment mean, and ~5.3 m SLE for the most sensitive experiment. For RCP2.6/SSP1-2.6, the mass losses are limited to a two-experiment mean of ~0.26 m SLE for Greenland, and a three-experiment mean of ~0.25 m SLE for Antarctica. Climate-change mitigation during the next decades will therefore be an efficient means for limiting the contribution of the ice sheets to sea-level rise in the long term.