Atlantic water intrusion triggers rapid retreat and regime change at previously stable Greenland glacier

Ice discharge from Greenland's marine-terminating glaciers contributes to half of all mass loss from the ice sheet, with numerous mechanisms proposed to explain their retreat. Here, we examine K.I.V Steenstrups Nordre Bræ ('Steenstrup') in Southeast Greenland, which, between 2018 and 2021, retreated ~7 km, thinned ~20%, doubled in discharge, and accelerated ~300%. This rate of change is unprecedented amongst Greenland's glaciers and now places Steenstrup in the top 10% of glaciers by contribution to ice-sheet-wide discharge. In contrast to expected behaviour from a shallow, grounded tidewater glacier, Steenstrup was insensitive to high surface temperatures that destabilised many regional glaciers in 2016, appearing instead to respond to a >2 °C anomaly in deeper Atlantic water (AW) in 2018. By 2021, a rigid proglacial mélange had developed alongside notable seasonal variability. Steenstrup's behaviour highlights that even long-term stable glaciers with high sills are vulnerable to sudden and rapid retreat from warm AW intrusion.

Thermodynamics of a fast-moving Greenlandic outlet glacier revealed by fiber-optic distributed temperature sensing

Measurements of ice temperature provide crucial constraints on ice viscosity and the thermodynamic processes occurring within a glacier. However, such measurements are presently limited by a small number of relatively coarse-spatial-resolution borehole records, especially for ice sheets. Here, we advance our understanding of glacier thermodynamics with an exceptionally high-vertical-resolution (~0.65 m), distributed-fiber-optic temperature-sensing profile from a 1043-m borehole drilled to the base of Sermeq Kujalleq (Store Glacier), Greenland. We report substantial but isolated strain heating within interglacial-phase ice at 208 to 242 m depth together with strongly heterogeneous ice deformation in glacial-phase ice below 889 m. We also observe a high-strain interface between glacial- and interglacial-phase ice and a 73-m-thick temperate basal layer, interpreted as locally formed and important for the glacier's fast motion. These findings demonstrate notable spatial heterogeneity, both vertically and at the catchment scale, in the conditions facilitating the fast motion of marine-terminating glaciers in Greenland.