内容摘要：In the 1980s, polar bear biologist Andrew Derocher never saw weak, gaunt bear cubs sauntering along

In the 1980s, polar bear biologist Andrew Derocher never saw weak, gaunt bear cubs sauntering along the shores of Canada's frigid Hudson Bay. Then, beginning around 2000, he sometimes spotted malnourished polar bear cubs "in a walking dead state."

"They were just walking skeletons," Derocher, a professor at the University of Alberta, said. Since the nineties, the Western Hudson Bay polar bear population, one of 19 subpopulations, has dropped by 30 percent.

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The current decline of polar bear populations in some, though not yet all, Arctic regions is straightforward: Arctic sea ice is in sharp decline, and bears require sea ice to expertly hunt seals. Polar bears cannot catch seals on open water. Now, research published Monday in the journal Nature Climate Change identifies how long bears can fast in different Arctic regions before their populations start falling, perhaps rapidly.

"It has been clear for a long time that polar bears are going to suffer immensely under climate change, but it was not clear when we should start seeing declines," said Péter Molnár, an ecologist at the University of Toronto and lead author of the research.

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The study's conclusions range between bad and dire. The researchers tested how polar bear populations would fare by measuring how long bears could fast under two different carbon emission scenarios this century, both of which would further deplete Arctic sea ice. The research demonstrates the importance of slashing heat-trapping carbon emissions — even if humanity cannot meet the extremely ambitious cuts agreed to by world nations at the 2015 historic Paris climate pact. The Trump administration has since abandoned this agreement.

As the planet heats up in the coming decades and beyond, the researchers found that under a warming scenario where greenhouse gas emissions moderately fall (starting around mid-century), 10 of the 19 polar bear subpopulations are still likelyor very likelyto suffer "reproductive failure" by 2080. Reproductive failure is the point where the survival of cubs is dramatically reduced, largely because their mother runs out of energy to nurse her helpless cubs.

This intermediate emissions scenario (labeled below as "moderately mitigated" and formally called RCP 4.5) entails civilization bringing emissions into a slow decline and eventually stabilizing the climate at some 2.4 degrees Celsius (4.3 degrees Fahrenheit) above pre-Industrial levels. In such a future, polar bear experts still expect many bears to die.

"You're still going to lose some southern populations," explained Molnár. "So it's not all great news."

Under the most extreme (though more unlikely) warming scenario — wherein our civilization with rising global energy demands increases coal burning by fivefoldthis century — nearly all polar bear populations are likely to experience reproductive failure. For two populations, it would be all but guaranteed. (This scenario is often labeled "business as usual" or BAU, but many climate scientists now say that a still catastrophic, though less extreme, emissions scenario better describes BAU).

Where polar bear populations are threatened.Credit: Polar Bears InternationalPolar bears on the sea ice.Credit: KT miller / polar bears international

The decline of polar bears won't be a simple story. Things will play out differently in disparate places. That's because the Arctic is a diverse, expansive place with islands, bays, straits, and beyond.

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"Polar bears occur all over the top of the globe," said Steven Amstrup, a study coauthor and chief scientist for Polar Bears International, a conservation organization based in Churchill, Canada that actively researches polar bears. "The ecological and sea ice conditions vary a lot, so we would expect different populations to respond in different ways at different times," noted Amstrup, who previously led polar bear research for the U.S. Geological Survey in Alaska for 30 years.

Scientists have a good understanding of how long polar bears can fast before their fat stores deplete and they can't survive or support cubs. Bears typically go through fasting periods each summer as sea ice declines and finding food becomes difficult.

"Polar bears are amazing at fasting," said Molnár, noting that bears can fast for months. "But ultimately every organism has its limits."

"Our goal was to use physiology to explore the limits of what bears can do," he said.

"Every organism has its limits."

For example, researchers know a healthy female bear with newly born cubs can fast for 117 days before her cubs' survival begins decreasing with each passing day (researchers use the body conditions of polar bears in the well-studied Western Hudson Bay subpopulation of the early 1990s as a reference point). To determine how bear populations would fare, Molnár and his team paired these survival expectations, along with the survival expectations of bears in both thinner and fatter states, to how ice is projected to vanish in different regions in the coming decades.

In some polar bear populations — like in the Barents Sea, Davis Strait, Western Hudson Bay, and Chukchi Sea — it will become difficult for bears to persist: With more ice free days, mothers must fast beyond the point they can either support cubs or themselves, leading to a population that can't endure.

"We're going to have populations of polar bears that are not self-sustaining anymore," said the bear biologist Derocher, who had no role in the study. It comes down to physical energy limits. "They can only get so fat, and they can only use that fat for so long," he said.

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Polar cub bears might still be born in struggling populations, but if they can't survive, the population will diminish, and perhaps disappear. It's already starting to happen. In Western Hudson Bay, home to the "polar bear capital of the world," the proportion of cubs in their second year of life, called yearlings, is falling, said Amstrup.

However grim these polar bear population projections are, Amstrup emphasizes they might be "overly optimistic." That's because as the Arctic continues warming and sea ice diminishes, polar bears are more likely to be skinnier when the low-ice summer season begins. "If bears are lighter, they cannot fast as long," Amstrup said. So bears may pass their fasting thresholds sooner than these new projections show. This could be why the polar bear population in the Southern Beaufort Sea dropped starkly by around 40 percentbetween 2001 and 2010.

What's more, if humanity radically cuts carbon emissions, the planet will take at least many decades to stabilize (whether the temperature rise is 2 C, 2.5 C, 3 C, or more above pre-Industrial levels), because there's still bounties of heat left in the climate system. Stabilizing Arctic sea ice, even at levels much lower than today, won't happen for another quarter-century or so after the planet stops warming. Conserving polar bear habitat, then, demands much foresight.

"If we wait until the projected events are upon us, it will be too late," said Amstrup.

Though many polar bear populations might still be saved from depletion — if humanity slashes carbon emissions, that is — the data spells a dark future. Arctic sea ice is plummeting.

"It's not a good news story for polar bears," said Derocher.