The fight against global climate change requires more effort as evidenced by one of its worst consequences – the possible extinction of polar bears.
New research published last week in the British magazine Nature Climate Change suggests that in 80 years, polar bears will be wiped out completely if greenhouse gas emissions remain at their current levels. According to the study, even if emissions are vastly reduced, the majority of polar bears will disappear.
Steven Amstrup, chief scientist from Polar Bears International, made a statement to BBC News: “What we've shown is that, first, we'll lose the survival of cubs, so cubs will be born, but the females won't have enough body fat to produce milk to sustain them through the ice-free season.”
"The bears face an ever-longer fasting period before the ice refreezes and they can head back out to feed," he said, adding that their dwindling bodyweight greatly undermines their chances of surviving Arctic winters.
Science correspondent Victoria Gill also underlined the urgency of the problem: “Polar bears have become the poster species for climate change because their survival is linked to sea ice, which they use as platforms to hunt for seals.”
On current trends, the study concluded, polar bears in 12 of 13 subpopulations analyzed will have been decimated within 80 years by the galloping pace of change in the Arctic, which is warming twice as fast as the planet as a whole.
"By 2100, recruitment (new births) will be severely compromised or impossible everywhere except perhaps in the Queen Elizabeth Island subpopulation" in Canada's Arctic Archipelago, Amstrup said.
That scenario foresees Earth's average surface temperature rising 3.3 degrees Celsius above the preindustrial benchmark. One degree of warming so far has triggered a crescendo of heatwaves, droughts and superstorms made more destructive by rising seas.
But even if humanity were able to cap global warming at 2.4 degrees Celsius – about half-a-degree above Paris Agreement targets, but hugely ambitious all the same – it would probably only delay the polar bears' collapse.
The melting rate of sea ice in the Arctic has increased 13% per decade since data collection with satellites began in the late 1970s.
Timeline of demise
"That is still way above anything polar bears have faced during 1 million years of evolutionary history," said Amstrup.
The threat is not rising temperatures per se but the top-of-the-food-chain predators' inability to adapt to a rapidly shifting environment.
"If somehow, by magic, sea ice could be maintained even as temperatures increase, polar bears might be fine," Amstrup said by email.
"The problem is that their habitat is literally melting."
Half of Earth's land-based megafauna are classified as threatened with extinction, but only polar bears are endangered primarily by climate change.
But that status may not be unique for long and should be seen as a harbinger of how climate will impact other animals in the coming decades, the authors warned.
There are approximately 25,000 polar bears left in the wild today.
The challenge to their survival has long been understood, but the new study – building on pioneering work by Amstrup a decade ago – is the first to put a timeline on their likely demise.
The new approach overlays two sets of data.
One is the expanding fasting period, which varies across regions and can last for half a year or more.
The other is a pair of climate change projections tracking the decline of sea ice until the end of the century, based on scenarios from the United Nation's Intergovernmental Panel on Climate Change (IPCC).
In a free fall
"By estimating how thin and how fat polar bears can be, and modeling their energy use, we were able to calculate the threshold number of days that polar bears can fast before cub and adult survival rates begin to decline," said lead author Peter Molnar, a professor at the University of Toronto.
A male bear, for example, in the West Hudson Bay population that is 20% below its normal body weight when fasting begins will only have enough stored energy to survive about 125 days rather than 200 days.
Newborn cubs are even more exposed, according to the study, especially when mothers have not fattened up enough to provide nourishing milk.
Females without offspring, however, have the greatest capacity to withstand long periods without food.
The polar bear's "vulnerable" status on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species – less severe that "endangered" or "critically endangered" – does not accurately reflect their plight, the authors argue.
Catagories established by the IUCN are based mainly on threats such as poaching and habitat encroachment that can be addressed with local action on the ground.
"But we cannot build a fence to protect sea ice from rising temperatures," Amstrup said.
"Think of it this way: If I were to push you off of the roof of a 100-story building, would your risk level be 'vulnerable' until you pass by the 10th floor?" he told Agence France-Presse (AFP).
"Or would you be 'endangered' all the way down?"
Dire predictions for polar bears has led to the mulling of alternative solutions, such as captive breeding programs or airlifts to Antarctica, but there is no plan B.
"The only way to save them is to protect their habitat by halting global warming," Amstrup said.