Positive feedbacks drive the Greenland ice sheet evolution in millennial-length MAR–GISM simulations under a high-end warming scenario

Abstract. Understanding the complex interactions between the Greenland ice sheet (GrIS) and the atmosphere is crucial for projecting its future sea level contribution. However, studying these interactions remains challenging, as it requires high-resolution climate or atmospheric models to be run over extended timescales before their influence on the ice sheet–climate system becomes evident. Therefore, in this study, we coupled an ice sheet model (GISM) with a regional climate model (MAR) and conducted millennial-length simulations. The simulations consist of a zero-way, a one-way, and a two-way coupled configuration, which were forced by the IPSL-CM6A-LR global climate model output under the SSP5-8.5 scenario until 2300 and extended until the year 3000 by randomly sampling the last 51 years of forcing. They represent the first coupled simulations of an ice sheet model (ISM) and regional climate model (RCM) that extend beyond the centennial timescale and allow us to assess the evolving role of ice sheet–atmosphere feedbacks. Our results reveal that the ice sheet evolution is determined by positive as well as negative feedback mechanisms, that act over different timescales. The main observed negative feedback in our simulations is related to changing wind speeds at the ice sheet margin, due to which the integrated ice mass loss differs by only 2.4 % by 2300 between the two- and one-way coupled simulations, regardless of the differently evolving ice sheet geometries. Beyond this time however, positive feedback mechanisms related to decreasing surface elevation, namely the melt–elevation feedback and changes in cloudiness and orographic precipitation, dominate the ice sheet–climate system and strongly accelerate the integrated ice mass loss in the two-way coupled simulation. As a result, the ice sheet has almost entirely disappeared by the end of the two-way coupled simulation, with a sea level contribution of 7.135 m s.l.e., compared to significantly smaller contributions of 5.635 and 5.122 m s.l.e. for the one-way and zero-way coupled simulations, respectively. This highlights the importance of accurately representing ice sheet–atmosphere interactions for long-term assessments of the Greenland ice sheet and climate.