By Tim Radford, Climate News NetworkThis piece first appeared at Climate News Network.

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.

Now Edward Hanna and colleagues at Sheffield report in the International Journal of Climatology that they have another explanation. Unusual atmospheric circulation and changes in the jet stream – the same changes that almost washed away summer in England – sent a blister of warm air sweeping over the ice sheet.

Hanna and his team analysed all the weather data collected by the Danish Meteorological Institute and by US researchers, and then employed satellite readings and a computer simulation called SnowModel to reconstruct the strange turn of events. And climate change may after all be a suspect.

High melt years

The Greenland Ice Sheet is a highly sensitive indicator of regional and global change, and, says Prof Hanna, been undergoing rapid warming, and losing ice, for at least the last five years and probably the last 20.

“Our research found that a ‘heat dome’ of warm southerly winds over the ice sheet led to widespread surface melting.” This was not predicted by the climate models used by the Intergovernmental Panel on Climate Change, and perhaps that indicated a deficiency in those models, he suggested.

The event seemed to be linked to changes in a phenomenon known to oceanographers and meteorologists as the summer North Atlantic Oscillation (NAO), another well-observed high pressure system called the Greenland Blocking Index, and the polar jet stream, all of which sent warm southerly winds sweeping over Greenland’s western coast.

“The next five to 10 years will reveal whether or not 2012 was a rare event resulting from natural variability of the NAO or part of an emerging pattern of new extreme high melt years.” It was hard to predict future changes in the Greenland climate in the current state of knowledge, but important to keep on trying.

There is an awful lot of ice on top of Greenland. Once it starts to melt, it is likely to be, say the Sheffield scientists, “dominant contributor to global sea level change over the next 100 to 1,000 years.”

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