Thin clouds drove Greenland's record-breaking 2012 ice melt

Thin, low-level clouds were instrumental in driving Greenland's record-shattering ice melt last year, a new study has found.
If the sheet of ice covering Greenland were to melt in its entirety, global sea levels would rise by 24 feet.
Three million cubic kilometres of ice won't wash into the ocean overnight, but researchers have been tracking increasing melt rates since at least 1979.
Last summer, however, the melt was so large that similar events show up in ice core records only once every 150 years or so over the last four millennia.
"In July 2012, a historically rare period of extended surface melting raised questions about the frequency and extent of such events," said Ralf Bennartz, professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison's Space Science and Engineering Center.

"Of course, there is more than one cause for such widespread change. We focused our study on certain kinds of low-level clouds," Bennartz said in a statement.

In the study published in the journal Nature, Bennartz and collaborators describe the moving parts that led to the melt.
"The July 2012 event was triggered by an influx of unusually warm air, but that was only one factor," said Dave Turner, physical scientist at National Oceanic and Atmospheric Administration's National Severe Storms Laboratory.
"In our paper we show that low-level clouds were instrumental in pushing temperatures up above freezing," he said.
Low-level clouds typically reflect solar energy back into space, and snow cover also tends to bounce energy from the Sun back from the Earth's surface.

Under particular temperature conditions, however, clouds can be both thin enough to allow solar energy to pass through to the surface and thick enough to "trap" some of that heat even if it is turned back by snow and ice on the ground.
While low, thin cloud cover is just one element within a complex interaction of wind speed, turbulence and humidity, the extra heat energy trapped close to the surface can push temperatures above freezing.
That is exactly what happened in July 2012 over large parts of the Greenland ice sheet, and similar conditions may help answer climate conundrums elsewhere.

"We know that these thin, low-level clouds occur frequently. Our results may help to explain some of the difficulties that current global climate models have in simulating the Arctic surface energy budget," Bennartz said.