April 4, 2013
Greenland’s 2012 Record Ice Melt Due To Thin Cloud Cover
April Flowers for redOrbit.com - Your Universe Online
Global sea levels would rise by 24 feet if the sheet of ice covering Greenland were to melt in its entirety tomorrow. But it is very unlikely that nearly two million cubic miles of ice will wash into the ocean overnight.
Researchers have been tracking increasing melt rates since 1979. Last summer the melt was so extensive that ice core records show similar events only about once every 150 years 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," says Ralf Bennartz, professor of atmospheric and oceanic sciences and scientist at the University of Wisconsin—Madison's Space Science and Engineering Center (SSEC). "Of course, there is more than one cause for such widespread change. We focused our study on certain kinds of low-level clouds."
Bennatz and his colleagues describe the moving parts that led to the melt in a recent issue of Nature. The melt was observed from the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) experiment atop the Greenland ice sheet. ICECAPS is funded by the National Science Foundation (NSF) and run by UW Madison and its partners. ICECAPS is part of the Arctic Observing Network (AON), which encompasses physical, biological and human observations of the land, ocean and atmosphere.
"The July 2012 event was triggered by an influx of unusually warm air, but that was only one factor," says Dave Turner, physical scientist at the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory (NSSL). "In our paper we show that low-level clouds were instrumental in pushing temperatures up above freezing."
"It's kind of like the story of Goldilocks," Von P. Walden, principal investigator for the ICECAPS project, said in an NSF statement.
"If the sky had no clouds on July 11th, it would have been too clear and cold to melt the surface. But if the clouds were too thick, it also would have been too cloudy and cold. The thin, liquid-water clouds were just right for melting the surface snow," he said.
Typically, low-level clouds and snow cover reflect solar energy back into space from the surface of the Earth. 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 under particular temperature conditions.
Low, thin cloud cover is just one element within a complex interaction of wind speed, turbulence and humidity. However, the extra heat energy trapped by these clouds can push temperatures above freezing, which is exactly what happened in July 2012 over large parts of the Greenland ice sheet. Similar conditions may answer climate mysteries elsewhere as well.
"We know that these thin, low-level clouds occur frequently," Bennartz said. "Our results may help to explain some of the difficulties that current global climate models have in simulating the Arctic surface energy budget."
The occurrence of such clouds tends to be underestimated by current climate models, limiting their ability to predict cloud response to Arctic climate change and possible feedback like spiking rates of ice melt.
"Clouds are still one of the greatest uncertainties in climate models. Even though the current climate models are generally correct, we need better measurements to improve them," said Walden, a professor in the geography department at the University of Idaho. "We're doing this to avoid future surprises, and we need to expand our knowledge of the details of how the climate operates."
The study uses a combination of surface-based observations, remote sensing data, and surface energy-balance models to delineate the effects of clouds on ice melting. It also shows that this type of cloud is common over both Greenland and across the Arctic, occurring between 30 and 50 percent of the time in summer.
"The cloud properties and atmospheric processes observed with the Summit Station instrument array provide a unique dataset to answer the large range of scientific questions we want to address," said Turner. "Clouds play a big role in the surface mass and energy budgets over the Greenland Ice Sheet. Melting of the world's major ice sheets can significantly impact human and environmental conditions via its contribution to sea-level rise."
Climate models can be improved with a better understanding of the interactions of such clouds.
"Our results may help to explain some of the difficulties that current global climate models have in simulating the Arctic surface energy budget, including the contributions of clouds," said Bennartz. "Above all, this study highlights the importance of continuous and detailed ground-based observations over the Greenland ice sheet and elsewhere. Only such detailed observations will lead to a better understanding of the processes that drive Arctic climate."