The study of the bears in Western Hudson Bay also draws attention to the increasing numbers of hungry bears wandering into the northern Canadian community of Churchill, Manitoba.

Jane Ball reports on the threat to the habitat of the polar bears

Executive Director of the Churchill Northern Study Center, Harvey Lemelin said: "Bears now have to be moved away from property using everything from dogs to vehicles to cracker shells. In the last three years we've gone from 20 encounters to 36 and we're not done with the season yet."

Seals less accessible

The study, by Ian Stirling and colleagues is published in the journal Arctic. It finds that the bears' main food source, ringed seals, are becoming less accessible. The seals live on the ice of Hudson Bay but this ice is breaking up earlier and earlier.

The polar bears therefore have less time to hunt and are returning to land in poorer condition. Weight for both male and female polar bears is declining and female bears are having fewer cubs. However, significant population decline has not yet begun.

The study says that the sea ice season in western Hudson Bay has been reduced by about three weeks over the last 20 years. The scientists say the shorter season could be caused by a long-term warming trend.

'Cut greenhouse gases'

A previous Nasa study found a 2.9% decline in total Arctic sea ice extent over the last decade.

"The plight of the Hudson Bay polar bears is a warning that that climate change is not something that will happen far off in the future, it's happening now," said Stephanie Tunmore, climate campaigner with green pressure group Greenpeace.

"World governments must intensify negotiations to cut greenhouse gas emissions while simultaneously beginning the phase-out of fossil fuels and switch to renewables," she added.