ALBERT

Description: Projections of the sea level contribution from the Greenland and Antarctic ice sheets (GrIS and AIS) rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous Coupled Model Intercomparison Project phase 5 (CMIP5) effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

Description: The land ice contribution to global mean sea level rise has not yet been predicted with ice sheet and glacier models for the latest set of socio-economic scenarios, nor with coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using 100 multiple models, but mostly used previous generation scenarios and climate models, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios using statistical emulation of the ice sheet and glacier models, and find that limiting global warming to 1.5°C since preindustrial would halve the land ice contribution to sea level rise this century, relative to 105 predictions for current climate pledges under the Paris Agreement: the median 4 decreases from 25 to 13 cm sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The Antarctic contribution does not show a clear response to emissions scenario, due to competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing 110 the median land ice contribution to 42 cm SLE under current policies and pledges, with the upper end (95th percentile) exceeding half a metre even under 1.5°C warming. This would severely limit the possibility of mitigating future coastal flooding. Until climate policies and the Antarctic response are further constrained, adaptation must therefore plan for a factor 115 of three uncertainty in the land ice contribution to global mean sea level rise.