A significant amount of melt water from the Greenland ice sheet is stored in old compacted snow, according to a study published online in Nature Geoscience. This hidden aquifer delays the journey of melt water from the ice sheet towards the ocean, where it contributes to sea-level rise.

Richard Forster and colleagues drilled through snow layers in the southern Greenland ice sheet in the spring of 2011 to measure the thicknesses of snow layers. Surprisingly, they found liquid water in firn - old compacted snow - that had survived the winter. They used ground and airborne radar to trace the extent of the water layer across a widespread region of southern Greenland. They estimate that the hidden aquifer in watery firn covers an area the size of Ireland (about 70,000 km2).

In an accompanying News and Views article, Joel Harper writes that the discovery of widespread water storage in firn “has uncovered a fundamental, but previously overlooked, component of meltwater runoff processes in Greenland.”