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Melting ice in the Beaufort Sea is forcing polar bears to swim long distances, without food or rest for days at a time, with increasing frequency, according to a five-year study that sheds light onto yet another consequence of climate change in the North.
A group of researchers at the University of Alberta monitored polar bears in the Beaufort Sea, north of Alaska and the Yukon, and Hudson Bay from 2007 to 2012.
They found that as sea ice melted, female adult bears and younger bears of both genders were paddling distances greater than 30 miles more often in order to find pieces of ice large enough for them to rest on.
"If you went back in time, even to the 1980s, bears in the Beaufort Sea probably never saw 50 kilometers [30 miles] of open water, and that's the low end of the analysis," said researcher Andrew Derocher, a professor of biological sciences at the University of Alberta.
"For some bears, it is definitely going to cost their life — small animals, old animals, young animals," he said.
Derocher explained that by the end of a swim, a bear's body might cool down dramatically. While "a big, fat adult male with several hundred kilos of body fat is well-insulated," the same journey for a 40-kg cub that was born that year could be fatal.
"It depends on the bear," he said.
The study, published in the latest issue of the journal Econography, used GPS trackers to monitor 115 polar bears — 58 female adults and 18 younger creatures from the Beaufort Sea and 59 adult females from Hudson Bay during seasonal migrations.
Adult males were left out of the study because their necks were too wide to wear the collars, and they'd often take them off.
"Even if their necks are small enough to put a collar around, they're strong, that they'll just rip the collars apart," Derocher explained.
The researchers recorded 115 long-distance swims in total, with 100 of those taking place in the Beaufort Sea, where sea ice levels reached a record low in 2012. This means about 69 percent of the polar bear population in the Beaufort Sea had to perform long-distance swims by 2012, compared to just 25 percent in 2004.
The median length of the swims was about three and a half days, while the longest swim, covering 250 miles, took over nine days, the study said.
Long-distance swims took place more frequently in the Beaufort Sea area, which has gone from having sea ice almost year-round to having almost no sea ice in the summer in the last 30 years. The ice that does exist is several hundred kilometers offshore. By contrast, bears in the Hudson Bay have dealt with melting sea ice in the summer months for hundreds and hundreds of years, and have evolved to feed heavily during the winter and then live off of their fat reserves.
"Not all bears are created equal, so if you're young, if you're old, if you're skinny, those are all factors that are likely to influence your survival rate," said Derocher. "Even if it doesn't kill you, it's so energetically expensive, it's a disadvantage to you in the sense that summer is not a good time for polar bears ever."
The study also found that female polar bears with cubs are less likely to swim long distances.
"If there's any open water, it can be 100 yards across, females will walk around that for miles and miles to avoid taking their cubs into that kind of water," Derocher said.
Ultimately, he said, the findings of the study mean the bears in the Beaufort Sea will have to adopt the same lifestyle as those in the Hudson Bay.
"This is a population that's already in trouble — it'll probably blink out by mid century," he said. "They have to stop swimming and start heading for land."
Follow Tamara Khandaker on Twitter: @anima_tk
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