ALMOST the entire surface layer of ice over Greenland melted in the space of four days this month – faster than at any time in the satellite era – in an event that has stunned and alarmed NASA scientists.
The rapid melting occurred over 97 per cent of the surface of Greenland, deepening fears about the pace and future consequences of climate change. In a statement on NASA's website, scientists said the satellite data was so striking they thought at first there had to be a mistake. "This was so extraordinary that at first I questioned the result: was this real or was it due to a data error?" said Son Nghiem, of NASA's Jet Propulsion Laboratory in California. The data came from three satellites.
He consulted several colleagues, who confirmed his findings. Dorothy Hall, who studies the surface temperature of Greenland at Goddard space flight centre in Maryland, confirmed that the area experienced unusually high temperatures in mid-July, and there was widespread melting over the surface of the ice sheet.
Climatologists Thomas Mote, at Georgia University, and Marco Tedesco, of the City University of New York, also confirmed the melt.
However, scientists are still coming to grips with the images. "I think it's fair to say this is unprecedented," said Jay Zwally, a Goddard glaciologist.
The images released by NASA show a rapid thaw between July 8 and July 12, when measurements from three satellites showed a swift expansion of the area of melting ice, from about 40 per cent of the ice sheet surface to 97 per cent.
Scientists attributed the sudden melt to a heat dome, or a burst of unusually warm air, which hovered over Greenland from July 8-16. The country had returned to more typical summer conditions by July 21 or 22, Mote said. He added that the event, while exceptional, should be viewed alongside other compelling evidence of climate change, including on the ground in Greenland.
"What we are seeing at the highest elevations may be a sort of sign of what is going on across the ice sheet," he said. "At lower elevations on the ice sheet, we are seeing earlier melting, melting later in the season and more frequent melting over the last 30 years, and that is consistent with what you would expect with a warming climate."
Jason Box, a glaciologist at Ohio State University, had predicted a big melt year for 2012, because of earlier melt and a decline in summer snow flurries.
He said the heat dome was not necessarily a one-off. "This is the seventh summer in a row with this pattern of warm air being lifted on to the ice sheet. What is surprising is how persistent this circulation anomaly is." He added that surfaces at higher elevation, now refrozen, could be more prone to future melting, because of changes in the structure of the snow crystals. He expected melting to continue at the lower elevations.
About half of Greenland's surface ice sheet melts during a typical summer, but Zwally said he and other scientists had recorded an acceleration of that melting process over the last few decades. This year his team had to rebuild their camp, at Swiss Station, when the snow and ice supports melted.
He has never seen such a rapid melt over his three decades of almost yearly trips to the Greenland ice sheet. He was most surprised to see indications of melting even around the area of Summit Station, about two miles above sea level.
It was the second unusual event in Greenland within days, after an iceberg the size of Manhattan broke off from the Petermann Glacier. But the rapid melt was viewed as more serious.
"If you look at the July 8 image, that might be the maximum extent of warming you would see in the summer," Zwally said. "There have been periods when melting might have occurred at higher elevations briefly ... but to have it cover the whole of Greenland like this is unknown, certainly in the time of satellite records."
Goddard glaciologist Lora Koenig told NASA similar rapid melting occurs about every 150 years, according to ice core records. But she warned there were wide-ranging implications from this year's thaw.
The most immediate consequences are sea level rise and a further warming of the Arctic. In the centre of Greenland, the ice remains up to 3000 metres deep. On the edges, the ice is much, much thinner and has been melting into the sea.
Scientists attribute about one-fifth of the annual sea level rise, which is about three millimetres, to the melting of the Greenland ice sheet. If the entire ice sheet melted that would have a catastrophic impact, raising sea level by several metres, but even the most pessimistic climate models do not suggest that is imminent.
