By Tim Radford, Climate News Network
Jun 19, 2013
LONDON—First: the story so far. For a few days in July 2012, almost 97% of the surface of Greenland began suddenly to thaw. This was a melt on an unprecedented scale.
Greenland carries a burden of three million cubic kilometres of ice and even in the summer, most of it stays frozen, partly because of the island’s high latitude and partly because ice reflects sunlight, and tends normally to serve as its own insulator.
The event was so unusual, and so unexpected, and on such a scale that nobody seriously suggested that the dramatic conversion of snow to slush was direct evidence of climate change because of human-induced global warming.
Soot, smoke and heat
At first, climatologists were inclined to see the thaw as a consequence of the record-breaking heat waves and forest fires that afflicted North America last summer: snow could have been darkened by columns of soot and smoke from forest fires, just enough to start absorbing the sunlight, some reasoned.
Then in April a team at the University of Wisconsin-Madison suggested that freak cloud behaviour over Greenland at the time might have caused the melting. Clouds normally block sunlight and keep the terrain below them cool.
But these clouds could have been thin enough to let solar radiation through, but thick enough to trap the consequential infra-red radiation from the ground, and raise the local temperature levels.