The melting that occurred across 98.6% of the Greenland ice sheet this summer was so rare that the most recent equivalent melt was in 1889.

Ordinarily, melting occurs over about half of the surface area of Greenland's ice during summertime. Son Nghiem of NASA's Jet Propulsion Laboratory in Pasadena, California, and his colleagues detected this year's extreme melt using three satellite sensors that differ in their resolution, spatial coverage and their ability to detect thawing ice. In combination, the sensors' data revealed the extent of the melt — a result verified by temperature data from weather stations and on-the-ground observations.

The melting event coincided with an unusually warm ridge of air hovering over the ice sheet, the authors say.

Geophys. Res. Lett. http://dx.doi.org/10.1029/2012GL053611 (2012)