Tidewater-glacier response to supraglacial lake drainage

The flow speed of the Greenland Ice Sheet changes dramatically in inland regions when surface meltwater drains to the bed. But ice-sheet discharge to the ocean is dominated by fast-flowing outlet glaciers, where the effect of increasing surface melt on annual discharge is unknown. Observations of a supraglacial lake drainage at Helheim Glacier, and a consequent velocity pulse propagating down-glacier, provide a natural experiment for assessing the impact of changes in injected meltwater, and allow us to interrogate the subglacial hydrological system. We find a highly efficient subglacial drainage system, such that summertime lake drainage has little net effect on ice discharge. Our results question the validity of common remote-sensing approaches for inferring subglacial conditions, knowledge of which is needed for improved projections of sea-level rise. The effect of increasing surface melt on annual discharge is unknown for the Greenland Ice Sheet. Here, the authors find that Greenland’s largest single-glacier contributor to sea-level rise accommodates basal floods following supraglacial lake-drainage events with limited impact on ice flow.