Greenland's snowy surface is now darker in the spring than it was before 2009, suggesting that the ice sheet's melting and contribution to sea-level rise may be larger than expected, according to a new study.

The research in Nature Geoscience this week adds to a body of recent research finding that the Arctic's albedo, or reflectiveness, may decline more sharply than original estimates. When snow is darker, it absorbs more solar energy and melts more easily. In the new study, French scientists pinpoint greater amounts of dust—driven by earlier melt seasons—as a chief culprit for the lower spring albedo of Greenland snow.

"This effect is most likely to accelerate the current rate at which Greenland is losing mass," said Marie Dumont, a scientist at the French National Centre for Meteorological Research and author of the paper.

While it's impossible to know how much the melt rate could go up, she said a minimum estimate likely is an additional 2 centimeters of sea-level rise from Greenland by 2100. The Greenland ice sheet is expected to raise global sea level by approximately 20 centimeters or more by 2100, according to the paper.

Scientists have known that the Greenland surface is darker than before, but they had less understanding of the seasonal variations and drivers of the changing reflectiveness, said Dumont. She and other French scientists analyzed satellite data to determine that albedo between May and June began a significant decline in 2009, a year coinciding with the beginning of more severe snow cover retreat.

"Satellite observations show that since 2009, albedo values even in springtime at high elevations have been lower than the 2003-2008 average," the paper states.

Higher levels of windborne dust
The satellite signal showed that impurities in the ice sheet causing the darkening were most likely colored, and not black like soot or other impurities, said Dumont.

Via additional analysis, they concluded that dust carried by the wind likely was the main source of the impurities in the snow. On-ground measurements from Northern Greenland showing much higher levels of dust than soot backed the conclusion, according to the paper. An increase in dust is consistent with climate change, Dumont said.

"Our hypothesis is that now that seasonal snow cover in the Arctic is retreating earlier than before, bare soil is available earlier in the Spring for dust transport," said Dumont in an email. While it will take on-the-ground measurements to determine the dust link definitively, it appears to be the most likely explanation, she said.

Other light-absorbing impurities in snow such as black carbon from industrial pollution, and microorganisms like algae, also may be contributing to the surface darkening, she said. A greater number of microorganisms on Greenland is consistent with melt trends after 2009, as a greater frequency of liquid water occurring in the snowpack allows the thriving of microorganisms. Dust also contains nutrients for microorganisms.

"Consequently, as dust content increases, microorganisms are likely to increase, too," Dumont said.

The study follows a Proceedings of the National Academy of Sciences paper this year reporting that declining Arctic albedo between 1979 and 2011 constituted one-quarter of the heating effect from carbon dioxide in the same time frame (ClimateWire, Feb. 18).

Jason Box, an ice sheet expert and professor at the Geological Survey of Denmark and Greenland not involved with the research, said in an email that the paper was a "fine study" highlighting how small changes can have important effects downstream. He said his calculations show that a 1 percent reduction in surface reflectivity can more than double to 2 percent as time goes on, further facilitating melt.

The dust will spur melt earlier in the year, and not just at upper elevations, said Box, who is leading a crowdfunded science expedition called the "Dark Snow Project" to assess Greenland melt.

"Models used to project climate don't include this [dust factor] and several other feedbacks, so the increase in melt from Greenland is probably faster than the models project," Box said.
