A polar bear in the Arctic. Credit:Peregrine Adventures "Ice is changing so quickly that we're finding the bears are getting caught in places where they're finally coming to the realisation, 'I just can't stay here'," Dr Derocher said. "These kinds of long-distance swims are not what they evolved to undergo." The results of Dr Derocher's study, published in the most recent issue of the journal Ecography, show a dramatic increase in the number of polar bears paddling across vast expanses of ocean to find suitable ice to stand on. In 2004, just a quarter of the bears monitored performed a long-distance swim (defined as more than 50 kilometres). By 2012, that proportion had ballooned to 69 per cent. The number of bears making such a swim was directly proportional to the loss of sea ice in the area, Dr Derocher said. Though they're good swimmers, polar bears re not adapted to long trips and can only paddle about 2 kilometres an hour. A 50-kilometre journey to find new ice takes an entire day, during which the bears can't eat or rest. Adult bears are likely to lose weight, and their cubs tend to get hypothermic.

Polar bear cub Lili and mother Valeska at a zoo in Bremerhaven, northern Germany this month. Credit:AP In 2009, a mother bear who swam for nine days straight off the coast of Alaska (who was not part of Dr Derocher's study) lost 22 per cent of her body weight, biologists with the US Geological Survey reported. Her year-old cub died during the journey. "With cubs, if they have to undergo a long distance swim, it's basically a death sentence," Dr Derocher said. A polar bear off the coast of Svalbard, Norway. Credit:Steven Kazlowski Mothers with cubs were much less likely to swim, he found. Instead, "they will walk for hundreds of kilometres to keep their cubs out of water".

In part because of the difficulty of monitoring bears (collars can fall off or malfunction, bears seem to drop off the map), the amount of data Dr Derocher and his colleagues collected varied from year to year. But the trend is pretty clear, he said. In the 1980s, when he first began studying polar bears, it would have been unheard of for any bears to make a long distance swim, let alone dozens. In those days there was no need - the Beaufort Sea and Hudson Bay (the two areas covered by the study) were clogged with ice even in the height of summer. That's changed now, especially in the Beaufort Sea above Alaska and the Yukon, which has seen an especially large decline in sea ice compared with the Hudson Bay. Satellite images taken earlier this month show that the ice there is already beginning to break up. That's bad news during prime hunting season for polar bears, which rely on sea ice as a platform from which to dive for seals and other prey. "None of this is what I would call a smoking gun as to what is happening with polar bear abundance, but the signs are all pointing in the same direction," Dr Derocher said. "We're seeing bears with lower body fat, fewer cubs, changing hunting behaviour." Pair that with a 2014 study that found that the polar bear population in the southern Beaufort Sea dropped between 25 and 50 per cent from 2001 to 2010, and the picture becomes pretty clear.

"The Beaufort population is one of the ones that will more than likely be extirpated probably by mid-century," Dr Derocher said. The best-case scenario is that these bears will travel south and find a way to hunt on land during the northern summers, the way some of their Hudson Bay cousins do. So much depends on their ability to adapt. "We're really changing the rules on the bears with the warming that we're observing," he said. Washington Post