Deeply incised submarine glacial valleys beneath the Greenland ice sheet

The bed topography beneath the Greenland ice sheet controls the flow of ice and its discharge into the ocean. Outlet glaciers move through a set of narrow valleys whose detailed geometry is poorly known, especially along the southern coasts(1-3). As a result, the contribution of the Greenland ice sheet and its glaciers to sea- level change in the coming century is uncertain(4). Here, we combine sparse ice-thickness data derived from airborne radar soundings with satellite derived high- resolution ice motion data through a mass conservation optimization scheme(5). We infer ice thickness and bed topography along the entire periphery of the Greenland ice sheet at an unprecedented level of spatial detail and precision. We detect widespread ice-covered valleys that extend significantly deeper below sea level and farther inland than previously thought. Our findings imply that the outlet glaciers of Greenland, and the ice sheet as a whole, are probably more vulnerable to ocean thermal forcing and peripheral thinning than inferred previously from existing numerical ice- sheet models.