Low-level clouds usually reflect solar energy back into space, as does the white coverage of snow. The albedo of cloud and snow — it’s ability to reflect sunlight back into space — is vitally important for minimising the level of solar energy wandering around inside our atmosphere, heating up our planet. One of the fears of ice melt — and conversely, one of the causes of ice melt — is the decrease in albedo when snow melts away and allows the darker heat-absorbent ground beneath to show.
New research has found that the incredible rate of melt across the Greenland ice sheet during last summer was a result of specific conditions that allowed enough heat to reach down to the ground and stay.
In fact, this specific confluence of events allowed for melting which scientists say happens only once every 150 years or so across the past four thousand years.
“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. “Of course, there is more than one cause for such widespread change. We focused our study on certain kinds of low-level clouds.”
Bennartz and colleagues describe in an article published in the April 4 issue of the journal Nature the conditions that led to the extraordinary melt of 2012, a phenomena observed from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit (ICECAPS) experiment funded by the National Science Foundation and run by UW-Madison and several partners atop the Greenland ice sheet.
“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 National Severe Storms Laboratory. “In our paper we show that low-level clouds were instrumental in pushing temperatures up above freezing.”
As mentioned, low-level clouds are normally useful in minimising the heat entering our atmosphere, but in this particular instance the clouds were both thin enough to allow heat through and dense enough to trap the heat even as it was reflected by the snow.
Working in combination with a mixture of factors — including wind speed, turbulence, and humidity — the low-level cloud managed to push temperatures above freezing.
“We know that these thin, low-level clouds occur frequently,” Bennartz says. “Our results may help to explain some of the difficulties that current global climate models have in simulating the Arctic surface energy budget.”
As with many other scientific revelations of this sort, the impact of this study plays directly upon future climate models. The authors of the report note that these specific types of clouds — which occur more frequently than once thought — are not given much weight in current climate models.
“Above all, this study highlights the importance of continuous and detailed ground-based observations over the Greenland ice sheet and elsewhere,” he says. “Only such detailed observations will lead to a better understanding of the processes that drive Arctic climate. “