By Helen Briggs
BBC News Online
The ice covering the Arctic Ocean is getting thinner as summers lengthen, say British scientists.
The habitat of polar bears is under threat
Melting seen in recent years is set to continue, they warn, with the eventual disappearance of ice during the summer months.
It puts the habitat of the polar bear, which relies on the ice to hunt for seals, under increasing threat.
Melting will also increase the effects of global warming in the northern hemisphere, say researchers from University College London (UCL) and the Met Office Hadley Centre for Climate Prediction and Research.
The study, reported in the journal Nature, is based on ice thickness measurements from two European Space Agency (Esa) satellites, ERS-1 and ERS-2.
The team also used microwave images from an American satellite.
Lead researcher Dr Seymour Laxon of UCL said the results from the American satellite showed that the length of summers had increased over the last 25 years.
"When we compared the data from the two satellites we were astonished by the similarity between changes in the ice thickness and the length of the summer melt season," he said.
"This result suggests that if this continues, further melting will occur, leading to the eventual disappearance of the ice during summer."
Ice in the Arctic Ocean has already thinned by 40% in the past 40 years, according to the results of submarine surveys.
Any further loss could spell disaster for the polar bear, which stands to lose its natural habitat.
It also has implications for global warming. As the ice melts, more sunlight is absorbed rather than being reflected back into space, amplifying the effects.
But climate studies are hotly debated and not all researchers agree with this bleak picture.
Computer modelling studies suggest there could be an alternative explanation for the apparent loss of sea ice.
Some experts believe a change in atmospheric circulation patterns has led to the ice being redistributed further north.
It could all be bunched up in the permanent polar pack north of Greenland and Canada, they say.
Martin Doble of the sea ice group at the Scottish Association for Marine Science is sympathetic to this view.
He says the ERS satellites are not designed to study the planet's poles and cannot sample the area beyond 81.5 degrees north.
This means they miss most of the central Arctic pack ice and the area of maximum interest north of Greenland and the Canadian Archipelago.
"One of the major debates in Arctic research at the moment is whether the total volume of sea ice in the Arctic has actually reduced as much as submarine results say that it has," he told BBC News Online.
"This latest paper adds nothing to this debate, since it misses this area entirely."
Settling the argument once and for all may have to wait until the launch of a more advanced Esa satellite - Cryosat - in 2004.
Its primary objective is to test the prediction of thinning Arctic ice due to global warming.