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

Discharge from Greenland’s marine-terminating glaciers contribute to half of all mass loss from the ice sheet, but the factors forcing their retreat are complex and contested. Here, we examine K.I.V Steenstrups Nordre Bræ (‘Steenstrup’), which, between 2018—2021, retreated ~7 km, thinned by ~20%, doubled in ice 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 an extreme anomaly in deeper Atlantic Water (AW) in 2018. Steenstrup’s behaviour highlights that, as AW intrusions occur at increasingly shallow depths, even apparently long-term stable glaciers with high sills are vulnerable to sudden and rapid retreat.