Al Gore might have been partly right after all. Melting sea ice could indeed be contributing to the death of polar bears, but the cause is more likely starvation than drowning*.
Researchers analysing 20 years of population data of polar bears in Canada's Hudson Bay report a correlation between yearly survival rates of the very young and old polar bears, and the date the pack ice began breaking up.
The problem lies in the dependence of the bears on the ice for hunting. The bears have to gorge themselves on seal pups as soon as they emerge from hibernation to store enough energy to make it through the summer, when the sea ice breaks up. The earlier the ice breaks, the tougher the summer will be on the less able hunters - the cubs and the very old.
This means the more vulnerable bears are not fattening up properly before winter, so more perish during the cold months of hibernation.
"Survivorship has dropped in the cubs, sub-adults and very old animals and is directly related to the date of break-up," Ian Stirling, a biologist with the Canadian Wildlife Service in Edmonton, Alberta, and an author on the report, told Nature's news site.
The team captures and tags bears during the hunting season, tracking and, where possible, recapturing them the following year to see how well they have survived the winter. In the time the census has been running (since 1984) the polar bear population has fallen 20 per cent.
The results are not entirely surprising. Other studies have found bears are getting thinner. A survey by Martyn Obbard, who works for the Ontario Ministry of Natural Resources, found that bears captured in the last five years weighed 15 per cent less, on average, than they did 20 years ago.
Hudson Bay is at the very southern tip of normal polar bear hunting grounds (the polar cap) and as such is seen as an indicator of what could happen to the rest of the ice cap, if the planet gets warmer, as many scientists expect it will.
The timing of the break-up of the sea ice does vary, the researchers say, but the overall trend is towards longer periods of open seas. The ice now clears in the bay roughly three weeks earlier than it did 30 years ago, the scientists say.
The findings are reported in the November issue of the Journal of Wildlife Management. ®
Polar bears that are not drowning or starving thanks to melting ice are instead being driven south, interbreeding with local grizzlies, and being shot dead. It is a tough life, it seems, being a bear.
*In his film, An Inconvenient Truth, Gore asserted polar bears are drowning on long swims between icebergs, thanks to global warming. Although there is some anecdotal evidence to support the claim of drowning bears, the idea that climate change is causing this is not backed by mainstream science. A UK judge ruled that teachers showing the film to children must make this clear.