Ice slabs thickening drives surface runoff expansion from the Greenland Ice Sheet’s percolation zone

Abstract

Abstract Firn – an interannual layer made of a seasonal snow – covers the vast majority of the Greenland Ice Sheet. It holds the potential to buffer meltwater runoff contribution to sea level rise by storing it in its porous space. However, the ice sheet has recently experienced record summer melting, favouring the development of several meters thick low-permeability ice slabs when meltwater refreezes in the shallow firn. At the same time, surface runoff from the ice sheet has occurred at increasingly higher elevations. Using runoff limits retrieved from Landsat imagery and ice slab thickness derived from airborne Accumulation Radar onboard NASA’s Operation Ice Bridge, we show that the maximum visible runoff limit is controlled by the structure of the underlying firn. We demonstrate that there is heterogeneity in ice slab thickness which can be explained by lateral meltwater flow in the slush matrix and surface rivers overlying the slab, causing surface meltwater to refreeze some distance downslope of where it melted. Finally, comparing ice slab thickness with ice-sheet-wide Sentinel-1 Synthetic Aperture Radar Horizontal-Vertical polarisation signal strength, we show that the onset of visible meltwater runoff is associated with 2–4 m thick ice slabs, constraining the minimum ice thickness needed to sustain runoff. Our findings therefore provide a basis for making improvements to the parametrization of partitioning between meltwater refreezing and runoff in firn models, in turn reducing uncertainty in future sea level rise.