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zondag 2 juli 2017

Greenland's shrinking cloud cover

Greenland's shrinking cloud cover 
speeds melt

More sunshine penetrating Greenland’s shrinking cloud cover explains why its icy mountains are turning increasingly slushy.
By Tim Radford

LONDON, 2 Jul
y, 2017
 – British and Belgian scientists think they understand why part of the Arctic is melting at a faster rate. For once, they don’t blame global atmospheric warming. But they have found a marked change during the last two decades, with Greenland’s shrinking cloud cover letting more sunshine reach the surface. So the link with climate change remains.
Their thesis, reported in the journal Science Advances, is that a drop in cloud cover and more summer sunshine means that more radiation hits the snows to deliver more energy for melting.
And they calculate that just a 1% reduction in summer cloud cover means an extra 27 billion metric tons of melting ice at the island’s surface. This is, roughly, the annual domestic water supply of the United States.
Their conclusion may not be the end of the matter, though. That Greenland’s ice is melting at an increasing rate is not in doubt.

Varied explanations

But previous studies have linked the acceleration to a darkening of the snows, to the insulating effects of cloud cover, and even to the temperature of the bedrock below the glaciers. 
But Greenland has lost 4,000 billion tons of ice since 1995 and, for the moment, scientists think they can attribute much of this to an increase in direct summer sunlight.
“The impact of increased sunshine during summer is large, it explains about two-thirds of Greenland's melting signal in recent decades,” said Stefan Hofer, a doctoral student at the University of Bristol, UK, who led the study.
“Until now we thought that the recent Greenland melt is caused almost exclusively by higher temperatures and the resulting feedbacks. Our study shows that there is more to the story than the local increase in temperatures. And the change in cloud cover isn't just a blip, it's been happening for the last two decades. That was a big surprise.”

“We are seeing changes in the large-scale circulation patterns, which leads to more frequent sunshine and higher amounts of solar energy reaching the surface of the ice sheet”

That, however, is not the only contributing factor. The scientists found that although summer cloud cover decreased by an average of 0.9% a year between 1995 and 2009, this might be connected to a meteorological phenomenon called the North Atlantic Oscillation, a natural cycle that four years ago was linked to dramatic melting in Greenland.
“We are seeing changes in the large-scale circulation patterns, which leads to more frequent sunshine and higher amounts of solar energy reaching the surface of the ice sheet,” said Jonathan Bamber, a geographer at the University of Bristol and president of the European Geosciences Union.
“These changes in large-scale circulation patterns during summer are especially pronounced over the Arctic and the North Atlantic. The state shift in atmospheric circulation is unprecedented in the observational record, which goes back as far as 1850.”
Professor Bamber continued: “This highly unusual state of the atmosphere has been linked to record low sea ice cover during summer over the Arctic Ocean. This highlights the coupled nature of the climate system and the consequences of changes in one component on another.” – Climate News Network
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