Increasing surface runoff from Greenland's firn areas

Most of the Greenland ice sheet (∼78% to 92%) is underlain by porous snow and firn, into which meltwater can percolate and refreeze, or run off. However, the fate of meltwater in firn areas is poorly constrained. We identified the ice sheet’s annual visible runoff limits by mapping surface hydrological features in >25,000 Landsat satellite scenes. Between 1985–1992 and 2013–2020, the visible runoff limits along the west and north margins rose by 58–329 metres, expanding the visible runoff area by ~29%. Estimates using two different regional climate models suggest that the enlarged area has contributed from 190–264 Gt to 320–491 Gt of runoff since 1985, equivalent to as much as ∼10% of recent annual runoff from the west and north margins. However, the spread highlights that runoff processes in the percolation zone are a source of considerable uncertainty among the major models. We demonstrate that sustained excess melting since the 1990s has provided favourable conditions for anomalous near-surface firn densification. Much of the expanded visible runoff area is underlain by relatively impermeable and persistent ice slabs that have previously been identified by airborne radar campaigns. These slabs have recently enabled sustained runoff from higher elevations even in cooler summers. Our findings highlight that lateral runoff over densified near-surface firn is pervasive in several sectors of the ice sheet and therefore must be incorporated into future runoff projections.