Polar bears have been one of the species hit hardest by climate change over the last decade, experiencing population declines up to 40 percent in some areas. Even so, scientists have long held out hope the mammals might adapt in some way to their melting Arctic habitat and rebound––or at least stabilize in numbers––as the world continues to warm.
But new research published Thursday in the peer-reviewed journal Science deals a blow to that optimism.
Eight U.S.-based scientists tested a leading hypothesis that polar bears could possibly lower their metabolic rates enough to survive as food resources became increasingly scarce due to declining sea ice during the summer months. The bears proved capable of lowering their metabolic rates slightly, the scientists found, but not enough to sustain them over the long term.
"Polar bears are amazing animals, but this study shows there is no amazing solution to the loss of sea ice and habitat," said Andrew Derocher, a biologist at the University of Alberta in Canada. Derocher, who has studied polar bears for 30 years, was not involved in the latest Science research.
Polar bears will "experience a slow wasting away" as the platform they use to hunt disappears, he said.
The Arctic is warming twice as fast as the rest of the world due to the buildup of greenhouse gases in the atmosphere. Temperatures in the region have risen 7 degrees Fahrenheit over the last half-century. The region's sea ice has declined 40 percent since the 1970s, melting earlier every spring and taking longer to freeze every fall, a trend that has disrupted the feeding and breeding cycles of hundreds of Arctic species.
Led by John Whiteman, a physiological ecologist at the University of Wyoming who studies how species adapt, the scientists tracked the movements and internal temperatures, a measure of metabolic rate, of approximately 30 polar bears in the Beaufort Sea off the northern coast of Alaska and Canada. They recorded data from April to October (2008-2010), the season when bears bulk up on food before lying low during the Arctic's harsh winter months.
The bears' core body temperatures slightly decreased from an average 99.1 degrees Fahrenheit in May to 97.9 degrees Fahrenheit in September, as did the bears' activity levels. The trends indicate the species is reducing the amount of energy it uses in response to dwindling available food supplies, but this decrease wasn't enough to prevent a gradual wasting away of the bears, the research found.
The scientists also discovered that polar bears have the ability to cool down their outer skin and tissues to keep their vital organs warm when submerged in cold water. This varying body temperature may seem like good news at first because it could theoretically help delay the onset of hypothermia as polar bears are forced to swim farther and farther between ice shelves, said Steven Amstrup, a co-author of the study and chief scientist at the research and advocacy group Polar Bears International.
"But it doesn't necessarily mean they can swim forever," Amstrup said. "We had one bear in our study who swam 500 miles over nine days. She lost 22 percent of her body weight and her cub. This is not something that can be done indefinitely without cost."
Amstrup said the Arctic research community is now racing to figure out just how long polar bears have until their population declines beyond the point of recovery. While some studies have indicated this bottoming out could happen as soon as mid-century, not all hope is lost for the species, he said.
"We already know what we need to do to stop this process," Amstrup said. "If we cut greenhouse gas emissions, global temperatures could stabilize in 10 years, and sea ice could stabilize in 20-25 years. If we don't, those of us who study polar bears could soon become polar bear historians."