97% of Greenland surface ice turns to slush

The surface of Greenland has turned to slush. Satellite data shows that a warm spell earlier this month melted nearly the entire surface of the nation's ice cap. The melt is unusual: normally about half of the ice sheet melts at the surface during summer, mostly at low elevations.

This year the thaw was stunningly swift and widespread, and extended high up the nation's peaks.

On 8 July, 40 per cent of the ice surface was wet; by 12 July, the fraction of wet ice had soared to 97 per cent (see image). The snowpack turned to slush even at the 3.2-kilometre-high Summit Station, at the apex of Greenland's ice sheet. Kaitlin Keegan of Dartmouth College says she found a layer of melted ice at the surface when she arrived at the station on 13 July. The weather then grew colder, and the melt layer formed an icy crust.

Ice cores show that this last happened in 1889.

"We were all surprised," says Thomas Mote, a climatologist at the University of Georgia. In 30 years of satellite observations, the greatest extent of surface melt was recorded a decade ago, when about 75 per cent of the surface was wet.

No cause for concern

Nonetheless, climatologists say there is no cause for immediate concern. Long-term ice core records show that ice at Summit Station melts about once every 150 years, so widespread melting in one year is not entirely unusual. What would be troubling, says Mote, is if it were to happen again within a decade ? this would threaten the stability of the ice sheet.

The discovery of widespread melting came after hydrologist ?sa K. Rennermalm of Rutgers University, New Jersey, noticed that stream runoffs at her field site in west Greenland were unusually heavy. She contacted Mote to find out if this was widespread across the region. Together with collaborators, Mote found that three independent datasets (microwave, infrared and radar data) showed liquid water was widespread across much of the island nation.

Mote blames the melting on a pocket of warm air that sat above Greenland in a weather pattern similar to the one blamed for the ongoing drought in central US. Earlier in the summer, a strong southerly jet blew anomalously warm air from the North Atlantic over Greenland. The jet stream then pinched off the flow and left the warm air behind to stagnate, Mote says.

Greenland is losing mass from more extensive runoff at lower elevations. The threatened floating Petermann Glacier in northwest Greenland, which has been losing mass lost another giant iceberg the size of Manhattan on 16 July, just after the surface melting peaked. That event had nothing to do with the warm air, says Andreas Muenchow of the University of Delaware. "Eighty to 90 per cent of this glacier is being melted from below," he says.

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