Precursor of disintegration of Greenland's largest floating ice tongue

Abstract. The largest floating tongue of Greenland’s ice sheet, Nioghalvfjerdsbræ, has so far been relatively stable with respect to areal retreat. Curiously, it experienced significant less thinning and ice flow acceleration than its neighbour Zacharias Isbræ. Draining more than 6 % of the ice sheet, Nioghalvfjerdsbræ might become a large contributor to sea level rise in the future. Therefore, the stability of the floating tongue is a focus of this study. We employ a suite of observational methods to detect recent changes. We found that the calving style has changed at the southern part of the eastern calving front from normal tongue-type calving to a crack evolution initiated at frontal ice rises reaching 5–7 km and progressing further upstream compared to 2010. The calving front area is further weakened by a substantial increase of a zone of fragments and open water at the tongue’s southern margin, leading to the formation of a narrow ice bridge. These geometric and mechanical changes are a precursor of instability of the floating tongue. We complement our study by numerical ice flow simulations to estimate the impact of future break-up or disintegration events on the ice discharge. These idealised scenarios reveal that a loss of the south-eastern area would lead to 1 % of increase of ice discharge at the grounding line, while a sudden collapse of the frontal area (46 % of the floating tongue area) will enhance the ice discharge by 8.3 % due to loss in buttressing.