A severe thinning of sea ice across the Arctic region has been confirmed for the first time by a satellite study, which predicts the imminent demise of the only natural habitat for the polar bear.
Climate scientists said yesterday they had established an unambiguous link between the melting of the Arctic and rising temperatures caused by global warming.
There will be little or no ice left for the polar bear to use as its vital hunting ground within 100 years if trends continue, the researchers say.
Previous studies on the extent of the Arctic sea ice have had to rely on sporadic measurements taken by the sonar instruments of military submarines. Those studies showed that, in some regions, the ice had thinned by about 40 per cent between the 1960s and the early 1990s but there was a suspicion that the ice might simply have been blown from one region to another.
The latest study, published in the journal Nature, uses a far more comprehensive set of measurements taken by satellite-mounted radar.
It has confirmed that the melting has continued in the past eight years and that it has taken place across the entire region.
Seymour Laxon, a climate researcher at University College London, said the melting of the Arctic sea ice had been proved to be a universal phenomenon so there was almost no doubt that it was caused by rising sea temperatures.
Dr Laxon said: "For the first time we've been able to measure the thickness of the ice from space across the entire Arctic and for the first time we've made a clear link between longer Arctic summers, rising temperatures and the thinning of the ice."
The sea ice in the Arctic is about 2.7 metres (nine feet) thick on average andhas thinned by about 1.3 metres in the past 30 years. The latest study shows that, in the eight years to 2001, the ice thinned by a further 30cm.
Ice-cover in the Arctic goes through dramatic changes over the yearly cycle:freezing during the winter months and melting during the long summer days.
A previous satellite study revealed that the length of the Arctic summer - the period from when the ice begins to melt to when it begins to freeze again - had gradually lengthened in the past 25 years, with summer getting longer by about five days every decade, Dr Laxon said.
In the winter months, and especially in the early spring, polar bears need the ice to hunt for seals. Bernard Stonehouse, from the Scott Polar Research Institute in Cambridge, said that as the ice retreated, then so did their opportunity to find food.
"The ice varies a great deal from year to year. The most likely consequence of this is that there will be more bad ice years than good ones. It's also more likely to affect the seals who live on the ice," Dr Stonehouse said.
Dr Laxon said that the disappearing sea ice in the Arctic could affect ocean currents, in particular the warm Gulf Stream, which keeps winters in Britain relatively mild.
Dr Laxon said: "Sea ice is sometimes called the clouds of the ocean because it picks up and transports fresh water from the Arctic. There are suggestions that, if you change the amount of ice cover, you'll change the structure of the Atlantic ocean.
"If you remove the ice it could well have the effect of moving the Gulf Stream further south."
If this happens, then winters in Britain will become more similar to the freezing conditions seen during winter in Newfoundland, which is at the same latitude but does not benefit from the Gulf Stream.
The latest research was in collaboration with the Met Office's Hadley Centre for Climate Prediction and Research, and was funded by the Govern-ment's Natural Environment Research Council.