Now we know why polar bears are so vulnerable to global warming

Polar bears are the sentinel species when it comes to global warming. They're the symbol used for fundraising campaigns and initiatives to raise awareness of how human activities, specifically the burning of fossil fuels for energy, is endangering our majestic planet.

We've known for a while that polar bears, which depend on sea ice cover for hunting and breeding, are already seeing their habitats shrink drastically as spring and summer sea ice becomes more scarce.

This is forcing them to swim more, and walk greater distances to find food. But there have been many nagging questions about just how poorly bears might fare in a warming world as the ice melts and the Arctic Ocean gets closer to being seasonally ice-free.

A new study, published Thursday in the journal Science, provides some intriguing answers. By closely tracking nine female polar bears as they roamed about the sea ice cover across the Beaufort Sea, scientists from the U.S. Geological Survey and other research institutions were able to gain new insight into how much energy these apex predators expend while searching for food, compared to their energy intake.

The animals wore collars that recorded rarely seen video, as well as data showing their location and activity levels, for about two weeks. The scientists also made use of trackers to get readings of the bears' energy use.

Data collected by the team shows that many of the polar bears ran an energy deficit. They burned more energy than they took in from prey, like ringed seals, and generally have a higher metabolism than was previously assumed. Polar bears need to eat more than 12,000 calories per day while on sea ice in order to maintain their weight.

This translates to at least one adult ringed seal every 10 to 12 days.

The field metabolic rates the scientists measured averaged more than 50 percent higher than previous studies had predicted. Five of the nine bears in the study lost body mass during the study period, which occurred during the time period when polar bears catch most of their prey and put on most of their body fat for the year.

This is bad news for the species, since it means that polar bears are already having trouble bulking up during the spring and summer, ahead of the harsh winter months when they hibernate.

"As an animal occurring in an extremely remote environment, limited information exists on the behaviors and activity patterns of the bears on the sea ice and in particular, this is the first study to provide quantitative measures of the energy demands of these animals while moving freely on the sea ice," said Anthony Pagano, a study coauthor and research wildlife biologist with the U.S. Geological Survey in Alaska, said in an email.

"The changes in body mass we found highlight the feast or famine lifestyle of these animals. When bears were successful in catching seals, 3 of the 4 bears gained more than 10 percent of their body mass, but when they were unsuccessful, 4 of the 5 bears lost almost 10 percent of their body mass, or about 18 kilograms," Pagano said.

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"We found polar bears have greater energy demands than previously predicted, which reinforces their reliance on access to and availability of their seal prey," Pagano said.

The Arctic region is warming more than twice as fast as the rest of the globe, with sea ice declining significantly at all times of the year. So far in 2018, for example, sea ice in the Chukchi and Beaufort Seas off Alaska was the lowest in the satellite era.

By causing sea ice to retreat earlier in the spring and form later into the fall, polar bears are being forced to travel greater distances to find food. This means they're burning more energy, and then need to eat even more prey to make up for this.

This is increasingly putting them in a bind, with a shortened window during which to bulk up, and a need to travel greater distances to find sufficient quantities of food.