DEBORAH SMITHThe Age, April 29, 2010
In the same way, the melting of sea ice in the Arctic, revealing the dark water below, has been shown by Australian scientists to be the main cause of unusually rapid warming at the top of the world.
Confirmation of this "feedback loop" means the region is likely to continue to warm strongly, with greater loss of sea ice and possible melting of the ice sheets.
James Screen and Ian Simmonds, of the University of Melbourne, said the rise in surface temperatures in the Arctic in the past 20 years had been more than double the global average.
The reasons for this enhanced warming have been "hotly debated" by scientists, with factors such as changes in cloud cover, and ocean and atmospheric circulation suggested as playing a role, along with sea ice loss, Dr Screen said.
To test these ideas, the researchers looked at the latest detailed atmospheric information, modelling and satellite measurements, including warming at different heights from the surface, for the period 1989 to 2008.
"Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover," said Dr Screen, whose study is published in the journal Nature.
Cloud cover did not have a big effect on recent warming, although increases in water vapour may have an impact in the lower part of the atmosphere in summer and early autumn.
Dr Screen said white ice reflects a lot of sunlight, but as it melts due to man-made warming from greenhouse gas emissions, the dark water that is exposed absorbs more heat, which in turn, melts more ice, and so on.
The amount of Arctic sea ice was at a record low in the summer of 2007, down about 40 per cent.
Although it has recovered slightly since, the long-term trend is down, he said. "We're heading towards a situation where the Arctic Sea will be ice-free in summer."