Timing, thresholds and processes for complete future Greenland deglaciation

The Greenland ice sheet is currently losing mass at a rate of 0.8 mm of global mean sea level rise (SLR) per year. Here, we simulate its future evolution under an idealized scenario of high greenhouse gas forcing (1% increase per year until four times pre-industrial CO₂).  To this end, we use the newly, bi-directionally coupled Community Earth System Model version 2 and Community Ice Sheet Model version 2 (CESM2-CISM2, Muntjewerf et al, GRL, 2020), that includes an advanced calculation of the surface mass balance in the land component with elevation classes downscaling to CISM. Deglaciation rates increase from 2 mm SLR per year by simulation year 140 (or time of CO₂ stabilization) to 7 mm SLR per year two centuries later as the ablation areas expand and net solar radiation and turbulent (latent, sensible) heat fluxes become the dominant energy sources for melt. The ice sheet retreats to an ice cap in the interior of the northern half of Greenland, that melts completely by simulation year 1,700. We compare the Greenland climate evolution with a CESM2 simulation with fixed topography, and evaluate the role of vegetation, clouds, precipitation, and surface energy fluxes on the relatively fast decay of the ice sheet. In addition, we use a set of CISM2 simulations forced with CESM2 SMB to estimate the global warming/forcing threshold for complete deglaciation.