Monday, May 19, 2014

Forest Fires Cause Melting of Greenland Ice Sheet

Kaitlin Keegan, of Dartmouth College, in a snow pit on the Greenland ice sheet. The pit shows layers of annual accumulation.  Credit: NSF photo
NASA and National Science Foundation (NSF)-funded researchers at Dartmouth College and the Desert Research Institute say they have found that a combination of rising temperatures and ash from Northern Hemisphere forest fires caused the large-scale surface melting of the Greenland ice sheet in 1889 and 2012. 

Melting in the dry snow region of the Greenland ice sheet does not contribute to sea level rise.  Instead, the meltwater percolates into the snowpack and refreezes, causing less sunlight to be reflected --which scientists refer to as lower albedo-- and leaving the ice-sheet surface even more susceptible to future melting. 

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 presence of a high concentration of ammonium concurrent with the black carbon indicates the ash's source was large boreal forest fires 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. 

For several days during the month of July 2012, Greenland's surface ice cover melted over a larger area than at any time in more than 30 years of satellite observations.  Nearly the entire ice cover of Greenland, from its thin, low-lying coastal edges to its 2-mile-thick center, experienced some degree of melting at its surface, according to measurements from three independent satellites analyzed by NASA and university scientists.

"Forest fires burning far from Greenland provided the ash that, along with the warm temperatures, caused widespread melting of the Greenland Ice Sheet," said Kaitlin Keegan, the lead author of the paper, which appears today in the Proceedings of the National Academy of Sciences

"It 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 Keegan. "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." 


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