Observed mechanism for sustained glacier retreat and acceleration in response to ocean warming around Greenland

Abstract. The dynamic loss of ice via outlet glaciers around the Greenland Ice Sheet is a major contributor to sea level rise. However, the retreat history and ensuing dynamic mass loss of neighboring glaciers are disparate, complicating projections of sea level rise. Here, we examine the stress balance evolution for three neighboring glaciers prior to, at the onset of, during, and, where possible, after retreat. We find no dynamic or thickness changes precede retreat, implicating a retreat trigger at the ice-ocean boundary. Terminus retreat initiates large-scale changes in the stress state at the terminus. This includes a drop in along-flow resistance to driving stress followed by an increase in lateral drag and associated glacier acceleration. We find that the pre-retreat spatial pattern in stresses along-fjord may control retreat duration and thus the long-term dynamic response of a glacier to terminus retreat. Specifically, glaciers with large regions of low basal drag extending far inland from the terminus permit a chain of stress changes that results in sustained acceleration, increased mass loss, and continued retreat. Our results suggest that vulnerable conditions for prolonged retreat may exist around Greenland, and thus dynamic mass loss may be sustained into the future despite a reduction in ocean forcing.