The role of Greenland ice sheet – climate interactions from 1000-year coupled simulations with MAR-GISM

As the second largest ice body on Earth, comprising an ice volume of 7.4 m sea level equivalent, the Greenland ice sheet is one of the main contributors to global sea level rise. Though observational and modelling efforts have increased substantially in recent years, major uncertainties remain regarding the ice sheet – climate interactions and feedback mechanisms that drive the ice sheet’s long-term mass loss. To improve sea level projections and the representation of such interactions in model simulations, efforts are currently emerging to couple ice sheet and regional climate models. However, so far, only a few coupled ice sheet – regional climate model simulations have been performed, and these do not extend beyond the centennial timescale. They therefore provide limited insights into the evolution and critical thresholds of the ice sheet – climate system over longer timescales.

As such, to obtain a better understanding of the ice sheet – climate interactions and potential feedback mechanisms over Greenland, we coupled our Greenland Ice Sheet Model (GISM) with a high-resolution regional climate model, the Modèle Atmosphérique Régional (MAR), and performed millennial-length simulations. The global climate model forcing for MAR during these simulations consisted of the IPSL-CM6A-LR model output under the SSP5-8.5 scenario, which was available until 2300. After this date, the climate was held constant, and we prolonged our coupled simulations until the year 3000.

Specifically, we performed three coupled simulations for the period 1990-3000 with differing coupling complexity: full two-way coupling, one-way coupling and zero-way coupling. In the two-way coupled set-up, the ice sheet topography and surface mass balance were communicated yearly between both models, such that ice sheet – climate interactions were fully captured. In the one-way coupled set-up only the surface mass balance – elevation feedback was considered, through interpolation of the yearly SMB onto the changing ice sheet topography. And lastly, in the zero-way coupled set-up the ice sheet – climate interactions were entirely omitted.

The results show 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 remains fairly similar for all simulations up to 2300, regardless of the differently evolving ice sheet geometries. Beyond this time however, positive feedback mechanisms related to decreasing surface elevation and changing precipitation patterns dominate the ice sheet – climate system and strongly accelerate the integrated ice mass loss. Hence, over longer timescales and for a realistic representation of the evolving ice sheet geometry, it is indispensable to account for ice sheet – climate interactions as was done in our two-way coupled ice sheet – regional climate model set-up.