Response of ice sheets, sea-ice and sea level in climate stabilisation and reversibility simulations using a state-of-the-art Earth System Model

We have conducted an ensemble of idealised climate overshoot simulations in a state-of-the-art Earth System Model in which global mean temperature is increased at a constant rate to global warming levels (GWLs) from 1.5 to 5 °C by prescribing constant CO₂ emissions, followed by a 50 year period of zero  CO₂ emissions started at different GWLs, and then a period of cooling in which  CO₂ is removed from the atmosphere. We give an overview of the ice sheet, sea-ice and sea level responses in these simulations highlighting long term responses at different GWLs and discuss the degree to which those responses can be simply reversed as global surface temperature cools. We show a broad divide between the two hemispheres, in which Northern Hemisphere polar processes have larger direct responses to warming which are more simply reversible than those in the Southern Hemisphere.

Cessation of  CO₂ emissions at most GWLs stabilises surface temperatures at high northern latitudes, although a slow warming trend continues at high southern latitudes. Northern Hemisphere sea-ice extent and Greenland surface mass balance both stabilise under zero  CO₂ emissions and appear to return toward preindustrial levels along with surface temperatures when  CO₂ is sequestered, but Southern Hemisphere sea-ice continues to decline under zero emissions and does not simply increase again as global temperatures cool. Likewise, the Antarctic circumpolar westerlies exhibit strengthening and shift poleward under global warming, but do not simply return to their preindustrial state when the climate is cooled, with implications for ocean circulation and Antarctic surface mass balance. The thermosteric contributions to global mean sea level from ocean heat uptake and the Greenland ice sheet continue when  CO₂ emissions are ceased or reversed, at rates which slowly decline on centennial timescales. The net mass balance of Antarctica and its contribution to sea level do not simply scale with global temperature; they result from a complex interaction between the basal and surface mass balances, the ice-dynamic response to those forcings and significant trends inherent to the initialisation of our ice sheet model state.