A nine-year record of slush on the Greenland Ice Sheet

Abstract. Surface melt on the Greenland Ice Sheet (GrIS) has intensified in recent decades, accelerating GrIS mass loss. Slush (i.e., water-saturated firn or snow) is a key component of this system, yet its areal extent and behaviour remains poorly constrained. Here, we present the first GrIS-wide classification of slush, using over 300,000 Sentinel-2 images and a supervised Random Forest classifier in Google Earth Engine. We generate a nine-year (2016–2024), high spatial resolution (10 m) dataset, which we then use to perform an ice sheet wide systematic assessment of slush distribution across all six major drainage basins. On average, slush covers ~2.9% (~50,400 km²) of the entire GrIS each summer, with around 40% (~29,300 km²) of this area in regions with known low-permeability subsurface structures (e.g., ice slabs and firn aquifers). Slush shows marked interannual variability that mirrors variability in melt intensity, ranging from a maximum slush areal extent of 1.2% (20,500 km²) of the ice sheet in 2018, the lowest melt year of the time series, to a maximum of 5.2% (90,300 km²) in 2019, the highest melt year. When our results are evaluated alongside those from other studies, we find that slush is the dominant meltwater feature in terms of spatial coverage on the GrIS each melt season: the maximum slush area was nine and four times greater than the combined area of supraglacial lakes and streams in 2019 and 2018, respectively. As climate change drives more frequent and prolonged extreme melt seasons on the GrIS, slush is likely to increase in area. Given the influence of slush on lateral meltwater transport, firn-air depletion, and melt–albedo feedback, its incorporation into energy balance and hydrological models will help better constrain ice-sheet mass balance projections.