Soot and Temperatures are the Cause of Greenland's Melting From 1889 to 2012

Rising temperatures and ash from Northern Hemisphere forest fires combined to cause large-scale surface melting of the Greenland ice sheet in 1889 and 2012, contradicting conventional thinking that the melt events were driven by warming alone, a Dartmouth College-led study finds.

The findings suggest that continued climate change will result in nearly annual widespread melting of the ice sheet's surface by the year 2100, and that a positive feedback mechanism may be set in motion. Melting in the dry snow region does not contribute to sea level rise; instead, the meltwater percolates into the snowpack and refreezes, causing lower albedo and leaving the ice sheet surface even more susceptible to future melting. Albedo is the surface's ability to reflect sunlight.

The study appears in the Proceedings of the National Academy of Sciences. A PDF of the study, photos and B-roll are available on request. The study was conducted by the Thayer School of Engineering at Dartmouth and the Desert Research Institute. The research was supported by the National Science Foundation and NASA.

"The widespread melting of the Greenland ice sheet required the combination of both of these effects -- lowered snow albedo from ash and unusually warm temperatures -- to push the ice sheet over the threshold," says Kaitlin Keegan, the study's lead author and a Dartmouth doctoral student. "With both the frequency of forest fires and warmer temperatures predicted to increase with climate change, widespread melt events are likely to happen much more frequently in the future."

The massive Greenland ice sheet experiences annual melting at low elevations near the coastline, but surface melt is rare in the dry snow region in its center. In July 2012, however, more than 97 percent of the ice sheet experienced surface melt, the first widespread melt during the era of satellite observation. Keegan, who added critical information to NASA's announcement of the 2012 melt, studies "firn," the newly deposited layers of snow atop the two-mile-thick ice sheet.

In the new study, the Dartmouth-led team's analysis of six Greenland shallow ice cores from the dry snow region confirmed that the most recent prior widespread melt occurred in 1889. An ice core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from Northern Hemisphere forest fires reduced albedo below a critical threshold in the dry snow region and caused the large-scale melting events in both 1889 and 2012.

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