22nd century tipping point in refreezing accelerates Greenland ice sheet mass loss

Abstract:

On the Greenland ice sheet, half of the surface meltwater is currently prevented to run off through refreezing in firn, i.e. the layer of compressed snow covering ~90% of the ice sheet in the interior accumulation zone. Greenland will cross a tipping point that accelerates surface mass loss when the firn buffer collapses, increasing runoff from all elevations. To study the response of the Greenland firn buffer to atmospheric warming, we dynamically downscale a high-end warming projection throughout the 21^st century from the Community Earth System Model (CESM2) using the Regional Atmospheric Climate Model (RACMO2), further statistically downscaled to 1 km spatial resolution. To sample other warming trajectories, the product is complemented by 12 CESM2 historical simulations (1950-2014) and 22 climate projections (2015-2100) for multiple warming scenarios, all statistically downscaled to 1 km. Using these 35 high-resolution climate simulations, we predict that Greenland firn will cross a tipping point in refreezing in the mid 22^nd century under a high-end warming scenario (SSP5-8.5). Under lower emission scenarios, this threshold is delayed to the late 22^nd century (SSP3-7.0) or further postponed to the mid 23^rd century (SSP2-4.5). Strong mitigation likely prevents the collapse of firn refreezing (SSP1-2.6). Passing this tipping point, Greenland's contribution to global sea level rise would increase more than 30-fold compared to the present-day.