NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1

news & views At intermediate elevations along the flanks of the Greenland ice sheet, strong surface melting can occur during summer months. Nevertheless, over the course of a year more snow falls than melts. As snow layers are buried by the accumulating snowfall, they compact and densify. Eventually the snow turns to firn, which can extend up to 100 metres in depth before fully transforming into glacial ice. An important source of uncertainty in determining Greenland’s runoff — and how it might change in the future — has been in understanding how much of the melt water from firn-covered regions is partitioned into runoff, and how much is lef