Widespread surface melting become annual event by the end of this century
FRISCO — Widespread melting on the surface of the Greenland ice sheet was likely caused by a combination of warm temperatures and major depositions of soot and ash from wildfires in the western U.S. and Siberia. The ice sheet last saw a similar melting event in 1889, Dartmouth University scientists concluded in new research published this week in Nature.
“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,” said 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.”
By 2100, widespread surface melting is likely every year, the scientists said, warning of a feedback loop that could make ice sheet more susceptible to melting as it steadily darkens. When the surface melts, the water percolates into the snowpack and refreezes, which darkens the surface even more.
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.
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.
The study didn’t focus explicitly on analyzing the ash to determine the source of the fires, but the presence of a high concentration of ammonium concurrent with the black carbon indicates the ash’s source was large boreal forest fires during the summer in Siberia and North America in June and July 2012.
Air masses from these two areas arrived at the Greenland ice sheet’s summit just before the widespread melt event. As for 1889, there are historical records of testimony to Congress of large-scale forest fires in the Pacific Northwest of the United States that summer, but it would be difficult to pinpoint which forest fires deposited ash onto the ice sheet that summer.
The study appears in the Proceedings of the National Academy of Sciences. It 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.