Some populations have likely already crossed the key thresholds that will hinder their survival, and may be impossible, according to the findings of the study published Monday in the journal Nature Climate Change.
However, there is a ray of hope that some bears can be saved.
Although the number of polar bears is likely to drop considerably in some regions, moderate cuts in heat-trapping gas emissions in the coming decades may allow them to persist in some pockets of the Arctic.
As Arctic sea ice goes, so do polar bears
Polar bears are the largest land carnivores on Earth, but their fate is closely related to what happens to Arctic sea ice.
They rely on ice as a platform to catch seals, their preferred prey, because they are not skilled enough swimmers to catch them in open water, says Péter K. Molnár, assistant professor at the University of Toronto Scarborough and one of the study’s authors.
The life of a polar bear has always been characterized by periods of partying and famine. In winter, when sea ice is at its greatest extent, bears try to gorge themselves on seals to build energy reserves to survive the summer months when the ice melts and they are forced to land.
But as the region’s sea ice declines as it has in recent decades, bear populations in some parts of the Arctic are forced to live increasingly without food.
“Finally, bears need food and to have food they need ice,” Molnár said. “But for them to have ice, we need to control climate change.”
The researchers’ first challenge was determining how long bears can fast in different regions before adult reproduction and survival deteriorate.
The study found that how long a bear can survive without food varies by bear region and condition, but that cubs are the first to be affected by prolonged fasting.
Adult female cubs are generally the second most vulnerable, followed by adult males, and finally solitary females, some of whom can fast for a staggering 255 days before their chances of survival abruptly decrease, the researchers estimate.
The researchers then looked at projections of Arctic sea ice in two different climate change scenarios to see how long the bears might be forced to run out of food in the future.
The last 13 years, from 2007 to 2019, have been the lowest 13 years for summer sea ice in the satellite record, dating back to 1979, according to Walt Meier, senior research scientist at the National Snow and Ice Data Center .
“The question is not ‘if’ we will see an ice-free summer in the Arctic, it is ‘when’,” Meier said.
‘… They are not completely condemned if we change our behavior’
By combining their estimates of how long polar bears can fast with projections of Arctic sea ice, the researchers were able to estimate for the first time when polar bears in 13 different regions might face problems in the future.
Some populations, such as those in Canada’s Hudson Bay, may have crossed thresholds that will affect reproduction and survival.
And the future will likely look much worse for bears without efforts to curb global warming.
Under a “business as usual” climate scenario, where humans fail to reduce greenhouse gas emissions, almost all polar bears in the Arctic, outside of perhaps a few groups in Canada’s remote Queen Elizabeth Islands, can fight to survive by 2100, the study found.
However, if humans can achieve moderate cuts to global emissions, the chances that bears in other regions will persist beyond the end of this century increase.
The authors say that ultimately these findings show that the fate of polar bears is in our hands.
“I am well aware that the story we are telling is bleak,” said Molnár. “But there is also an element of hope that they are not completely doomed if we change our behavior.”
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