Thomas R Chudley, Ian M Howat, Michalea D King, Adelaide Negrete

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—2021, retreated ~7 km, thinned ~20%, doubled in discharge, and quadrupled in flow speed. This rate of acceleration is unprecedented amongst Greenland’s glaciers, and now places Steenstrup in the top 10% of glaciers by contribution to Greenland’s 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, responding instead to a large anomaly in deeper Atlantic Water (AW) in 2018. By 2021, a rigid pro-glacial mélange had developed alongside notable seasonal variability. Steenstrup’s behaviour highlights that even apparently long-term stable glaciers with high sills are vulnerable to sudden and rapid retreat from warm AW intrusion.