On July 12, 2012, the Indian Space Research Organization satellite Oceansat-2 detected something odd in Greenland: In just a couple of days, the surface of the interior of the glacier-covered island melted. Although ice still persisted under the melt, about 97 percent of all of Greenland was covered with meltwater.

This was the first time such an occurrence had been detected in the satellite era. And scientists could see from matching refrozen slicks in several ice cores that the last time such an event happened was back in 1889.

At the time of the melt, scientists weren’t sure what was going on. There was unusually warm air sitting over Greenland at the time. And that was a known contributor to the event. But it was black carbon from forest fires in North America and Siberia, as well as from the burning of fossil fuels, that tipped the ice over the edge, [engineering PhD candidate] Kaitlin M. Keegan of Dartmouth and colleagues report today in the Proceedings of the National Academy of Sciences.