Polar Bears Can’t Adapt to a Warming World

A groundbreaking study shows that the animals’ metabolism won't allow them to survive on land as sea ice melts because of climate change.

(Photo: John Shaw/Getty Images)

Jul 16, 2015· 2 MIN READ
David Kirby has been a professional journalist for 25 years. His third book, Death at Seaworld, was published in 2012.

Polar bears will not be able to surmount the impacts of global warming on their Arctic habitat, according to scientists, because they will not be able to adjust their metabolism to survive on land-based prey during longer and longer periods with no sea ice.

The finding, published Thursday in the journal Science, has grave implications for the survival of the species, as warming temperatures owing to global climate change continue to shrink Arctic sea ice.

The study, conducted by researchers at the University of Wyoming and others, offered hard data to refute earlier suggestions that polar bears might be able to enter a state of “walking hibernation” while on land, drastically reducing their daily calorie needs and enabling them to get by during the summer, when they cannot use floating sea ice as a platform to hunt calorie-rich seals.

“To reduce the loss of body condition during summer food deprivation, shore bears purportedly enter a state of lowered activity and resting metabolic rate similar to winter hibernation but without denning,” the authors wrote. “Our data indicate that bears cannot use a hibernation-like metabolism to meaningfully prolong their summer period of fasting and reliance on energy stores.”

That suggests that many polar bears will likely starve to death in coming decades as their time on land increases “with continued ice loss and lengthening of the ice melt period,” the paper stated.

So, Why Should You Care? The United States listed the polar bear as a threatened species in 2008, largely because the impacts of climate change on its Arctic habitat threaten to drive the species extinct in the wild.

Polar bears rely on sea-ice pack for access to ringed seals, their main prey. But as continued burning of fossil fuels increases global temperatures, the sea ice has been retreating earlier in the year and returning later. This is forcing polar bears to spend significantly more time on land, where there is little prey as nutritious as seals.

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“Many people would have you believe that polar bears will somehow escape the negative impacts of sea ice disappearance,” wrote coauthor Steve Amstrup, chief scientist at Polar Bears International, in a written summary of the study. “This evidence that they have no special ‘metabolic escape’ further confirms that retreating sea ice can only negatively affect polar bears.”

To perform the complex study, researchers monitored the body temperatures and movements of 26 polar bears in the Beaufort Sea during the summers of 2008 and 2009, using satellite collars and implanted tracking devices. While the bears’ body temperature and activity level declined, they were still well above those found during typical bear hibernation. Instead, the authors wrote, the bears showed signs of “mammalian fasting.”

“The project was very expensive, required collaboration of multiple agencies, and had such complicated logistics that it will be very hard to duplicate,” coauthor Merav Ben-David, professor of zoology and physiology at the University of Wyoming, said in an email.

That doesn’t reduce the study’s validity, she said. “If because of prohibitive costs only one mission is launched to explore Pluto,” Ben-David said, “does that mean the results from this effort are invalid? Not!”

Other studies have shown that for polar bears, increased time on land means decreased survival. A 2010 study found that the southern Beaufort Sea continental shelf was free of ice about 101 days each year in 2001–2003, and adult female polar bear survival was high. But in 2004 and 2005, “the ice-free period was longer (mean 135 days) and adult female survival was low,” the study said.

A 2012 study, meanwhile, found that polar bear populations along the southern Beaufort Sea plunged by 40 percent between 2001 and 2010.

Melting ice is not the only thing affecting the species. Increased human activity in the warming Arctic is causing polar bears to expend more energy than usual during the ice-free period, making walking hibernation impossible, according to Sybille Klenzendorf, a senior wildlife specialist with the World Wildlife Fund.

“Other bears do that, with a resting time when they lower their body metabolism and activity,” Klenzendorf said. “But in the summer, when polar bears have to spend time on land, it’s difficult for them to find shelter that’s undisturbed.” Those stresses, she said, “prevent them from [going into] walking hibernation, which requires them to settle down and not move around, with a lower metabolism.”

Some studies have suggested that polar bears are now preying on bird eggs, geese, and caribou in the summer, and bears in Norway have been photographed eating dolphins. But it is too soon to tell if these foods will compensate for fewer seals in the bears’ diet.