Greenland's ice sheet is melting faster than expected, and this has been accelerating over the past two decades. It is now the biggest single contributor to global sea level rise, accounting for 25 percent of the total. But besides warming climes, there is another culprit for the melt: sunnier days in fair Greenland.
A paper published June 28 in the journal Science Advances shows that cloud cover has decreased by 14 percent from 1994 to 2009, at an average of just under 1 percent per year. That may not sound like much, but for ice, it's a big deal. The researchers show that for every 1 percent drop in cloud cover, the amount of ice melt has increased by 27 gigatons. That's a vast amount of water, approximately equivalent to the domestic water supply of the United States.
The more plentiful sunshine is actually now the leading cause of the increased melting, says co-author Jonathan Bamber, a researcher with the University of Bristol, which led the study. Of course, increasing temperatures also are important; Greenland, along with the rest of the Arctic, is warming at about twice the global average rate. "But while the changing temp is important, more important is the impact of cloud cover."
To measure the amount of clouds of Greenland, researchers used both satellite observations, as well as a complex climate model that re-analyzed past weather patterns. Both came up with similar numbers. Besides the overall drop in cover, the decline in incredibly thick summertime clouds was striking: These dropped by two-thirds since 2002, and by more than 80 percent from 1982 to 2009.
Although it's not yet entirely clear why the number of clouds is dropping, it may be influenced by the amount of sea ice in the Arctic. Bamber explains that as sea ice declines with warming temperatures, there is more exchange of heat and moisture between the water and the air, leading to more clouds off shore. This changes weather patterns in a way that decreases clouds over Greenland.
James Overland, a researcher with the National Oceanic and Atmospheric Administration in Seattle who wasn't involved in the study, says that less sea ice also may help change atmospheric patterns so that high pressure systems predominate over the Greenland ice sheet, and with higher pressure comes fewer clouds. However, this strength and importance of this connection remains a matter of some debate, he says.
Regardless, "this study helps explains why Greenland sea ice is melting faster than expected," Overland says.
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