Stabilizing feedbacks allow for multiple states of the Greenland Ice Sheet in a fully coupled Earth System Model

Abstract. The Greenland Ice Sheet (GrIS) will experience substantial mass loss and might even disappear if elevated global-mean temperatures are maintained over the next millennia. Previous studies indicated that once melted, the GrIS might not regrow even under subsequently lowered temperatures.
Here, we use a newly developed complex fully-coupled climate-ice sheet model to explore a potential multistability of the GrIS. This model system is more complex and includes more critical feedbacks relevant for the stability of the GrIS than previously used models. In a set of steady state simulations, we find that at least four steady states exist under a pre-industrial (PI) climate: Besides a state with a large GrIS that is similar to the PI state, we find steady states with GrIS volumes of about 48 %, 28 % and 19 % of the PI volume. These steady states are stabilized through several feedback processes, such as the melt-elevation and melt-albedo feedback. In the smaller states, ice sheet expansion is further limited by a redistribution of precipitation, a Föhn effect and additional warming driven by atmospheric circulation changes due to the reduced blocking of a smaller GrIS. The southern part of the GrIS is controlled by alterations of the sea-surface temperature of the Irminger Sea and the Nordic Seas. We also show that interactions between the GrIS and the Antarctic Ice Sheet (AIS) impact the transient behavior of the GrIS. Our results highlight the importance of climate-ice sheet feedbacks in maintaining multiple steady states of the GrIS. Such multistability has implications for assessing the consequences of global warming. Our simulations indicate that if the GrIS volume drops below a critical threshold of 83–70 % of its PI volume, at least half of its current volume will be irreversibly lost even if we return to global PI temperatures through a reduction in CO₂ concentrations.