"We had to check that our models were still working properly" Peter Langen, a climate scientist at DMI, told Polar Portal earlier this month. The early-season melting of Greenland ice has scientists worried. Credit:Daniel Beltra, via Catherine Edelman Gallery (Chicago) And they were. Warm air sweeping in from the south-west of Greenland had prompted more than 12 per cent of the ice sheet to register melting.

Weather stations 1840 metres above sea-level reported temperatures of above 3 degrees, conditions that would be considered a warm day in July, let alone April. "Everything is melting", said Aqqaluk Petersen, a resident of Nuuk, Greenland's capital. Water lies on part of the glacial ice sheet that covers about 80 per cent of Greenland. Credit:Getty Images The heatwave, Greenland style, added to other evidence that the top of the world continues to warm about twice as fast as the rest of the planet. 'The big show'

"Greenland is really the big show when it comes to ice melt," said Matt King, Professor of Polar Geodesy and an ARC Future Fellow at the University of Tasmania. "It's probably losing as much ice as all the small glaciers around the world combined, and probably more than Antarctica. Meltwater flows along a glacial river on the Greenland ice sheet last July. Credit:New York Times "Greenland is being eaten away from away from above and from the edges." Arctic air temperatures have risen about two degrees since the 1960s. Ocean temperatures are also warming, thawing Greenland glaciers in contact with surrounding seas. Since satellite records date only from the 1970s, some natural fluctuations may be in play, he said. Still, Greenland's early April warmth was consistent with other signals of a warming planet.

"Such a big spike in melting so early is in complete agreement with what you'd expect when we heat the atmosphere so much," Professor King said, referring to the impact from humans burning fossil fuels and releasing other greenhouse gases. The Arctic sea ice, for instance, also set a record low maximum range this year, setting up a shorter-than-usual melt season in spring and summer, according to the US National Snow and Ice Data Center. Less ice also fosters more warming since there is less reflection of solar radiation back into space, with more of the warmth absorbed by the now ice-free waters. Warmer conditions mean there are more days when polar temperatures are positive, meaning that they is more melting of surface ice. Sea-level impact While April's early warm spurt in Greenland eased back, the loss of ice mass has continued, running at about two months ahead of the average for the period since 1990, according to the DMI.

The loss of a gigatonne of ice per day amounts to about one cubic kilometre, or one billion tonnes, of water. For the 2003-2011 period, Greenland net annual ice loss was 234 cubic km of water. That's enough to lift global sea levels by an average of 0.65 mm, the DMI said. "This process of mass loss started around 1990 and has accelerated since the year 2000," the Polar Portal said. "The mass loss in recent years is approximately four times greater than it was before 2000." Professor King said that one effect of early melting is that the surface snow turns to water, exposing the darker glacial ice below. That ice has a lower albedo effect, trapping in more warmth and adding to the melting trend.

The warm start to April, meanwhile, has continued for much of the month, leaving Greenland on course for a month well-above normal temperatures: