The bears rely on sea ice for nearly every aspect of their survival: It's where they hunt seals, travel, make dens and mate. So the earlier in the season the ice begins to melt, the less time they have to eat and procreate.
The findings were outlined in a new study published in Ecological Applications.
Researchers found polar bears are becoming thinner and having fewer cubs, and their declining health was tied to melting sea ice.
"Climate-induced changes in the Arctic are clearly affecting polar bears," study author Kristin Laidre, professor of aquatic and fishery sciences at the University of Washington, said in a statement. "They are an icon of climate change, but they're also an early indicator of climate change because they are so dependent on sea ice."
Polar bears are considered a vulnerable species, one level below "endangered." If the climate trends continue, their populations will almost certainly deplete even further. Seeing the effect that a shrinking habitat has on polar bears could convince conservationists to take more immediate action.
And studying how the melting ice affects them could have broader implications for how it'll eventually affect other species. "These bears inhabit a seasonal ice zone, meaning the sea ice clears out completely in summer and it's open water," she said. "Bears in this area give us a good basis for understanding the implications of sea ice loss."
The bears are spending less time on land and losing weight
Laidre and her colleagues tracked the movements of adult female polar bears in Baffin Bay, a body of water off the west coast of Greenland, over two periods of time in the 1990s and 2010s.
Between 2009 to 2015, the bears spent an average of 30 more days on land than they had in the 1991 to 1997 period. That's because sea ice is melting at a faster rate and earlier in the season now than it did even 23 to 29 years ago.
Sea ice waxes and wanes with the seasons, and when there's less sea ice, the polar bears take up residence on the neighboring Baffin Island. They wait there until there's enough sea ice to hunt seals from.
And the more time they spend on land, closer to the shore and unable to hunt, the thinner they become.
Researchers rated the bears' body conditions on a scale of 1 to 3, 1 meaning thin and 3 meaning fat (fat is good and keeps them warm).
Of the 352 bears they analyzed, not even 50 were considered fat.
The availability of sea ice was to blame: Researchers found that in years with less sea ice, the bears weighed less.
"When the bears are on land, they don't hunt seals and instead rely on fat stores," said Laidre. "They have the ability to fast for extended periods, but over time they get thinner."
Less time on the ice is tied to fewer cubs
The bears' worsening health impacted how big their litters were, too.
Female polar bears had more cubs when sea ice was more widely available and when "spring breakup," the period when ice melts and breaks and more water becomes available, occurred later in the season.
Now that the bears are thinner, they're having fewer cubs. Two-cub litters were once the norm for healthy adult female polar bears but could norm could soon "disappear" if sea ice loss continues, Laidre said.
It's the first time this change in litter size has been observed.
Shrinking habitats push bears further inland
The last count of polar bears in Baffin Bay was conducted between 2012 and 2013 -- then, there were over 2,800 bears in the area, according to the IUCN Polar Bear Specialist Group. But the group hadn't recorded how many were there before the most recent count was conducted, so it's difficult to tell whether it's an increase or a decrease.
Nevertheless, the results of the University of Washington study are not promising.
Melting sea ice is partly the reason why "nutritionally stressed" polar bears are spotted in residential areas, stalking neighborhoods they'd never neared before. They're driven there by hunger and a shrinking habitat, and it can lead to increased conflict between humans and bears.
The future of polar bears, the researchers write in their findings, depends on scientists' ability to predict how climate change will continue to impact the bears. The pace of population changes is already startling, so a conservation response will need to come at an equally quick rate, the researchers say.