Researchers have long known that polar bear populations will almost certainly suffer as a result of climate change. But a new study projects that by the end of the century, the bears may exist only in a few subpopulations in the northernmost region of their range.
The study, published Monday in the journal Nature Climate Change, is the first to offer specific estimates of where polar bear numbers are likely to decline over time, and what demographic characteristics may make bears more vulnerable.
"The question we often have gotten is: When will polar bears in different areas begin to decline?" said Steven Amstrup, chief scientist for the conservation group Polar Bears International and a co-author of the study. "So that's what we were able to see in this project, population by population—where we're likely to see the first impacts of a warming climate and melting ice, and where polar bears might hold out the longest."
Of the 19 polar bear subpopulations in the Arctic ring, those located further north, in the Northern Beaufort Sea or Canada's Queen Elizabeth Islands, for example, are less likely to experience devastating effects on survival and reproduction compared to southern populations, like those in Hudson Bay, the study reported.
Polar bear survival is deeply intertwined with climate change because of their dependence on sea ice for hunting seals. In the summer months, the sea ice retreats, and bears are forced onto land, where there is little to eat, leaving them to fast until the sea freezes again.
As the Arctic warms, the number of days without sea ice is increasing, forcing polar bears to survive without food for longer periods. The study analyzed how long a fasting polar bear could live before survival and reproduction are compromised, and aligned this data with sea ice projections to estimate when individual subpopulations might decline.
The researchers looked at how polar bears might fare under two different scenarios. In one scenario, greenhouse gas emissions continue rising on a "business-as-usual" trajectory, leading global temperatures to increase by about 4.5 degrees Celsius by 2100. In the other scenario, some steps are taken to reduce global emissions, and temperatures increase only by 2.5 degrees Celsius by the end of the century. In the study, because climate models for that scenario extend only to 2080, the researchers' projections also went only up to that year. Amstrup said that was a shortcoming of the study.
The study found that under the "business-as-usual" scenario, most polar bear subpopulations would either be certain, or very likely to experience declines in reproduction and cub survival. Under the scenario with reduced emissions and less warming, only a few subpopulations would face such declines.
"You clearly see the difference this is making for bears," said Péter Molnár, a professor at the University of Toronto, Scarborough who also was a co-author of the study. "If you look at the business-as-usual scenario, it's extremely unlikely that you're going to have bears anywhere except perhaps in the high Arctic by the end of the century—most populations would collapse by then. If we manage to mitigate (warming), on the other hand, our results essentially show that you will still lose some subpopulations ... But you will also retain many more populations."
Calculating a Bear's Survival
While the decreasing subpopulations projected by the research are not surprising to scientists who study polar bears, the new study adds the element of time.
"Before this study, we knew bears were going to suffer under climate change, but we could not come up with a reliable estimate about when reproduction and survival would start declining," Molnár said.
The problem was that it was nearly impossible to estimate the future size of polar bear subpopulations. Metrics like survival and reproduction could not be studied through traditional research methods, Amstrup said.
"Those are extremely difficult parameters to get good estimates on," he said. "When you're talking about the future, they're impossible, because the future environment is different and we would only be making educated guesses about what the future reproduction and survival might be."
Instead, the researchers used methods to collect data on polar bears' body mass before a fast and how much energy was expended during a fast. They used that data to calculate how long polar bears could survive while fasting, and estimated how many ice-free days it would take before a bear's survival and reproduction were compromised.
"Even if you just look at these simple components, that's enough to make these calculations," Molnár said.
With this model, the researchers found that the first impacts would be on the number of new cubs born into the population. The survival of cubs between the ages of 1 and 2, which are dependent on their mothers and are more susceptible to stress than older bears, would be the next group whose survival would be jeopardized by longer fasts.
Survival in adult males and females with cubs would decline next, the researchers found, with the number of solitary females being the last to fall. Male bears require more food energy and have lower fat storage capabilities than females, and mother bears need greater fuel reserves to care for their young.
Polar Bear Biology
Biologically, polar bears are built for fasting. They can store enough energy in their fat reserves to last for months, but there are limits to how much energy they can store. Eventually, their fat reserves will run out.
"There's already some evidence that polar bears are starting to lose body mass, especially in the Hudson Bay region," coauthor Cecilia Bitz of the University of Washington, Seattle said. "Polar bears there have always been dealing with summer, but the length of the summer has been increasing."
Bitz said the ice-free period has extended in the Hudson Bay region from about three months to four months, elongating the fasting period.
Some subpopulations across the bears' range are better researched than others, like those in the Hudson Bay and the Southern Beaufort Sea regions of the United States and Canada. Others are not as well studied, but the researchers were able to extrapolate biological data in well-documented populations to populations that are more challenging to monitor.
"Basically, polar bear biology is similar across their range, and taking advantage of that and the energetics modeling allowed us to make projections in places where there was little data and where we're uncertain of the future," Amstrup said.
This study paints a picture of how action on climate change can help slow the decline of polar bear populations, said Andrew Derocher, a biologist at the University of Alberta, who was not involved in the study. One limitation of using models like this, he noted, is sometimes events occur that can't be predicted by the models.
For example, he said, the study projects only mild impacts on the polar bears in the Laptev Sea in Siberia relative to other populations. But Siberia is experiencing an exceptionally warm season.
"It's quite likely that the bears in the Laptev Sea, while not predicted in this paper to show any effects right now, might be actually suffering in 2020 in sort of a very bad year for that single population," Derocher said.
"The models help us frame the question," he added, "but they require us to be on the ground and monitoring in very close to real time to understand what the actual effects are."
This research adds further clarity about how climate change is affecting polar bears, said Tom Smith, a biologist at Brigham Young University, who was not involved with the study.
"This study is another tightening of the resolution," Smith said. "It hasn't changed the storyline, it's added detail."
The researchers said they hope their report will contribute to action on climate change and encourage work on mitigating its effects to the point where more polar bear populations survive and the worst effects of climate change are prevented. In order to do this, Amstrup said, warming in the Arctic must be curtailed early enough for sea ice to stabilize. Emissions reductions would have to take place 25 to 30 years before that, to allow for the delayed effects of the decline of greenhouse gases on the climate.
"The future of polar bears is intricately linked to the future of everything else," Amstrup said. "If society gets their act together and halts global warming to save polar bears, it will benefit the rest of life on Earth, including us."