Already facing warmer summers, diminishing habitat and possible pollution-related health problems, the polar bear may not be physiologically equipped to deal with a warming climate, experts say.
With these heightened risk factors, the snow-dwelling bruin is not going into a "hibernation" mode in the hotter months when food is harder for them to come by, according to a study in the journal Science.
Polar bears were thought to enter a hibernation-like state in the summers because "such a strategy could partially compensate for the loss of on-ice foraging opportunities caused by climate change," researchers said in the study published in July.
"That means they use more energy than we thought they were, at the same time they do not acquire any," said Merav Ben-David, professor of zoology and physiology at the University of Wyoming who led the study on polar bear metabolism.
"They are sensitive to the lengthening of the ice melt period in summer," Ben-David told CNBC. In other words, the already-threatened polar bear won't be able to hibernate in order to avert starvation.
The levels of Arctic ice and the state of the bears' natural habitat have been the subject of a fierce battle between environmentalists and a vocal camp of global warming skeptics—both of which hold radically different interpretations of available data. Doubters argue that Arctic melting is being exaggerated, and that polar bears are faring better than the consensus believes.
Nonetheless, most conservationists insist current conditions are endangering the species, which the International Union for Conservation of Nature lists as "vulnerable"—roughly translating into a 3 on the organization's scale of 7 (1 being "least concern" and 7 being "extinct). Some experts, such as evolutionary biologist Susan Crockford, argue that the polar bear population has actually "rebounded remarkably since 1973, and we can say for sure that there are more polar bears now than there were 40 years ago," Crockford wrote in a 2013 study. The union estimates the bears' total global population somewhere north of 25,000, but adds that Arctic bear numbers appear "very low."
Ben-David said that "our findings do not suggest that polar bears are doomed," but they do add to a "huge body of evidence that suggests climate change will bring other changes." Several recent studies have shown that pollution and certain contaminants have made their way up the Arctic and into the bones and tissue of polar bears, resulting in possible neurotoxicity and other health risks.
With those factors in mind, the National Wildlife Foundation pointed out that polar bears were the first ever vertebrates to be listed by the U.S. Endangered Species Act as threatened by extinction solely because of global warming.
In the spring of 2008, the U.S. Fish and Wildlife Service declared its final rule on polar bears and listed it as a "threatened" species, which is defined as one "likely to become" in danger of extinction in the near future. Among the reasons given by the agency is the threat posed to polar bears' habitat by the diminution of sea ice.
Four years later, NASA researchers observed that summer Arctic sea ice had shrunk to half of what it was only three decades earlier. Recent figures from the National Snow and Ice Data Center have shown record-low ice levels, but independent researchers have argued the ice's volume has in fact grown substantially in recent years.
Yet many scientists say environmental changes make the Arctic most vulnerable to climate change. That, in turn, makes the polar bear an early warning system of global changes.
They're the "canary in the cryosphere," said Todd Atwood, a research wildlife biologist at the U.S. Geological Survey, where he's a leader of the polar bear research program. Among other risks, he said the warming environment is also contributing to a shortage of food for the bears.
Atwood points out that some models suggest polar bears may be able to go up to 150 days without food, but anything longer risks starvation and problems with reproduction. Today, some polar bears are already fasting for as many as 120 days, but that could exceed 150 days by mid-century, he said.
"If we mitigate greenhouse gas emissions now, it will likely take at least a decade for the ambient air temperature to stabilize. During that lag, we will likely see the loss of more sea ice habitat and some degree of decline in polar bear abundance," he said.
In an effort to rescue this largest member of the bear family, the federal government has taken recent steps to strengthen the population.
In July, the Fish and Wildlife Service released a draft of its Polar Bear Conservation Management Plan, in which it outlined a seven-prong approach at a total projected cost of nearly $13 million a year. One element called for the reduction of greenhouse gas emissions, which the agency stressed was "the single most important action for conservation and recovery of polar bears." Other objectives included supporting international conservation efforts and minimizing risks of contamination from oil spills.
"These cost estimates are significantly higher than current funding," the plan said, "because some needs currently are being inadequately addressed."
Still, experts say it could be worse.
"The good news is," said Ben-David, "sea ice is declining linearly." Unlike many other ecological systems that suddenly collapse under stress, a gradual decline allows changes to be observed and possibly bucked, or even reversed.
For now, said Atwood, "stabilizing sea ice loss will save the polar bear."