April 5, 2013, by Tim Radford
FOR IMMEDIATE RELEASE
Last year’s brief but startlingly rapid melting of the Greenland ice sheet probably had nothing to do with climate change. What it did reveal was the limits of our current knowledge.
LONDON, 5 April – US scientists think they can explain why the Greenland ice sheet started melting at an unexpected and alarming rate in the summer of 2012. They blame it on unusual clouds.
In four days during July last year, Nasa satellite measurements revealed that 97% of the surface of the Greenland ice sheet had begun to thaw. The slush was even recorded at the summit of the icecap, more than three kilometres above sea level. This sudden, dramatic thaw was brief, but without precedent.
Greenland is home to three million cubic kilometres of ice. If all of it melted, sea levels globally would rise by more than seven metres. So climate scientists have for decades taken a keen interest in Greenland, and report that such sudden periods of dramatic melting occur roughly only once in 150 years.
In July 2012, observers blamed the record North American heat waves, and even wild fires in the tundra that might have sent columns of sunlight-absorbing soot to darken the snow.
But now Ralf Bennartz of the University of Wisconsin-Madison thinks he has the answer. He and colleagues report in Nature that while there would be more than one cause for such a huge change in the pattern of summer thaw, they focused on the role of low-level clouds.
Snow keeps itself cool by reflecting sunlight back into space. Low-level clouds, too, should keep land masses cool, by reflecting sunlight back into space.
But the scientists calculated that, under particular temperature conditions, clouds could be thin enough to permit solar radiation to filter through, but thick enough to trap some of the Sun’s energy as infra-red radiation even if it was reflected by the snow and ice on the ground. The extra heat trapped close to the ice surface was enough to push temperatures above freezing.
There would have been other factors to consider: air pressure, regional temperatures, wind speeds, turbulence, ocean currents and so on. Nobody last year was inclined to blame global warming for such an entirely unexpected phenomenon.
The short, sudden and very unusual event was just that, an unusual event, to be reconstructed months later by a combination of observations on the ground, remote sensing data and computer models.
But it told meteorologists and climate scientists something about the complexities of the interplay of light, land, air, water and ice in those latitudes.
“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”, said Professor Bennartz.
“Above all, this study highlights the importance of continuous and detailed ground-based observations over the Greenland ice sheet and elsewhere.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.