Future sea levels depend on how much and how quickly the massive ice sheet covering Greenland melts. Satellite-based measurements have revealed that Greenland's glaciers are melting and on the move—and the ice sheet has lost some 36 billion metric tons of ice each year in recent years from its northwestern flank. Thanks to weird weather, nearly the entire ice-covered surface of the world's largest island melted for a period this year.

View a slide show of Greenland's meltdown.

Now aerial photographs reveal that this loss of ice is driven more by the accelerated breakdown of ice sheets where they reach the sea (and a subsequent speed up in outflowing ice) than the difference between snowfall and such surface melt. Danish scientists identified two periods of ice loss—1985 to 1993 and 2005 to 2010—that have been responsible for the bulk of Greenland's meltdown. The research appears in Science on August 3.

In fact, outlet glaciers have receded by more than 100 meters across the northwestern edge of the ice sheet. For example, the Sverdrup Glacier retreated by 1,000 meters between 2005 and 2010, also losing 80 meters in height. Roughly 80 percent of the ice loss seems to be attributable to this kind of loss at glacier outlets rather than the glacier as a whole. The cause may be warmer ocean temperatures, although sea-surface temperature measurements are lacking to definitely prove that hypothesis.

Aerial maps that extend the record back as far as the 1930s may help further refine scientists' understanding of this meltdown in the north—and its implications for how much the seas will rise in coming decades.