Thunder? It's the sound of Greenland melting

Swiss-born Steffen is one of dozens of scientists who have peppered the Greenland ice cap with instruments to measure temperature, snowfall and the movement, thickness and melting of the ice.

Since 1990, Steffen has spent two months a year at Swiss Camp, a wind-swept outpost of tents on the ice cap, where he and other researchers brave temperatures of minus 30 Celsius (minus 22 Fahrenheit) to scrutinize Greenland's climate change clues.

The more the surface melts, the faster the ice sheet moves towards the ocean. The glacier Swiss Camp rests on has doubled its speed to about 15 km (9 miles) a year in the last 12 years, just as its tongue retreated 10 km into the fjord.

"It is scary," said Steffen. "This is only Greenland. But Antarctica and glaciers around the world are responding as well."