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Cloud Cover Seen as Cause in Greenland Glacier Meltdown
April 5, 2013

Summer melting created this pool in a Greenland glacier.

April 4, 2013

The melting of Greenland’s glaciers in the summer of 2012 was as extensive as it has been in more than a century, and U.S. scientists now conclude that a very particular type of cloud formation made the difference.

Greenland’s ice sheet is the world’s second largest, after Antarctica. What happens to that ice in periods of climate change could affect sea levels worldwide, scientists project.

“To understand the region’s future, you’ll need to understand its clouds,” said Matthew Shupe, a research meteorologist involved in the study backed by the National Oceanic and Atmospheric Administration (NOAA).

The research team found that July 2012 cloud conditions over Greenland’s ice sheet caused just enough surface warming and retention of planetary heat to cause an abnormally high level of warming on the world’s largest island. Published April 3 in Nature, the research reveals that thin, low-lying clouds allowed the sun’s energy to warm the surface of the ice, and they were also just thick enough to trap the heat on the ice and create conditions for melting.

More than 97 percent of the surface of Greenland’s ice sheet showed some degree of melting in July 2012, even at a scientific station on the glacier’s high point. Ice core records indicate the last time the surface melted at the National Science Foundation’s Summit Station was in 1889.

“Our finding has implications for the fate of ice throughout the Arctic,” Shupe said in a NOAA press release. Shupe is with NOAA’s Cooperative Institute for Research in Environmental Sciences at the University of Colorado. Scientists from the universities of Wisconsin and Idaho were also members of the research team.

“The July 2012 ice melt was triggered by an influx of unusually warm air sweeping in from North America, but that was only one factor,” said David Turner, research meteorologist with the NOAA National Severe Storms Laboratory and one of the lead authors. “In our paper, we show that low-lying clouds containing a low amount of condensed water were instrumental in pushing surface air temperatures up above freezing and causing the surface ice to melt.”

Whether clouds cool the surface by deflecting solar energy, or warm the surface by retaining heat depends on factors such as wind speed, turbulence, humidity and cloud “thickness” or liquid water content. The conditions that brought on the July 2012 ice melt occur 30 percent to 50 percent of the time in Greenland and Arctic summers. The NOAA press release says climate modeling techniques currently in use underestimate the occurrence of these cloud formations in the Arctic, thus limiting their accuracy.

“Clouds play a big role in the surface mass and energy budgets over the Greenland ice sheet,” said Turner. “Melting of the world’s major ice sheets can significantly impact human and environmental conditions via its contribution to sea-level rise.”

To better understand how that process may unfold, “continuous and detailed ground-based observations over the Greenland ice sheet and elsewhere” will be needed, according to Ralf Bennartz, lead author for the study and a professor at the University of Wisconsin-Madison. “Only such detailed observations will lead to a better understanding of the processes that drive Arctic climate.”