An unusual blast of warm air has caused an unprecedented thaw across the entire Greenland ice sheet.
On July 8, Air Force and NASA satellites showed melting on about 40 percent of the ice sheet's surface. Just four days later, that number rocketed up to an astonishing 97 percent, covering a larger area than any melting event since satellite records began 30 years ago.
Experts said one in a series of strong high-pressure ridges that have moved waves of warm air over Greenland since late May appears to be responsible for the unusual event, which persisted for several days. Whether man-made climate change helped produce those ridges isn't clear.
"Each [ridge] has been stronger than the previous one until we saw this exceptionally strong ridge that moved across Greenland in mid-July," said Tom Mote, a climatologist at the University of Georgia.
Extent of surface melt over Greenland's ice sheet on July 8 (left) and July 12 (right). Measurements from three satellites showed that on July 8, about 40 percent of the ice sheet had undergone thawing at or near the surface. In just a few days, the melting had dramatically accelerated, and an estimated 97 percent of the ice sheet surface had thawed by July 12. Click the map for a larger version. Map courtesy of NASA.
Marco Tedesco, a glaciologist at the City University of New York who helped analyze satellite data that confirmed the unusual surface thaw, said conditions have been "very warm" during his weeklong stay in Greenland. He spoke to ClimateWire yesterday from the U.S. research hub in Kangerlussuaq, near the ice sheet's western edge.
Just outside of town, the Watson River "has been roaring for the past couple of weeks," he said, fed by melting snow and ice. Unusually high flows have wiped out a bridge ClimateWire visited last year with researchers trying to track runoff from nearby Russell Glacier (see special report).
More surprising was the pronounced melting in the ice sheet's interior, at its highest, coldest point, where measurable surface thawing is rare. Air temperatures at the National Science Foundation's Summit Camp -- a research station perched on ice roughly 2 miles thick -- stayed close to, or just above, freezing for several hours July 11 and 12, according to a federal weather station there.
That was enough to cause the most significant surface melting near the summit -- and across the entire ice sheet -- since 1889, according to an analysis of ice core records by Dartmouth College graduate student Kaitlin Keegan.
"The warm air was able to climb up for a few days to Summit, and then the melt was extensive," Tedesco said.
Scientists cautious about climate change's influence
But he and other researchers are cautious about pinning the blame for that thaw on man-made climate change. The ice core study suggests that Greenland's summit experiences significant surface melting every 150 years or so, and scientists aren't sure why.
"I don't think there is any way that we can claim now that [the warming at Summit] is connected to a long-term trend, not because we don't think so, just because we don't know it," Tedesco said. "According to the record we have, the melt at Summit fits within what has been observed to happen in the past."
But if melting at the ice summit becomes stronger or more frequent, that could change the picture, he added.
What is true is that rising temperatures have increased melting at lower elevations on the ice sheet, experts said, and what has happened this summer fits with that long-term trend.
And as air over the ice warms, it is changing the structure of snow crystals and in some cases melting away snow to reveal bare ice. Both types of changes make the ice surface less reflective, so it absorbs more heat, which further increases surface melting.
Tedesco compared the situation to a train racing downhill. Warming air keeps the train running. When ice becomes less reflective, it's like sending that train downhill, accelerating surface melting.
If fresh snow falls, that can make the ice sheet more reflective, slowing the train. But that is not what has happened the past few years.
Jason Box, a glaciologist at Ohio State University, said satellite data show the Greenland ice sheet is less reflective this year than it has been since record-keeping began in 2000.
That likely helped intensify the effect of the ridge of warm air that spurred the recent, widespread surface melting. "The heat that was delivered by this ridge could go more quickly to melt the ice, because the ice surface had been heated already," he said.
ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500. E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.
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