Polar bears could become nearly extinct by the end of the century if global warming continues unabated, according to a new study.
The study by Polar Bears International - a conservation organization dedicated to polar bears - and a team of scientists from leading universities was published in the journal Nature Climate Change on Monday, July 20.
According to the study, if humans fail to reduce greenhouse gas emissions, a majority of the polar bear population could struggle to survive beyond 2100.
"With business-as-usual greenhouse gas emissions, steeply declining reproduction and survival will jeopardize persistence of all but a few high Arctic subpopulations by 2100," the study said.
Polar bears, the largest terrestrial carnivores on Earth, rely on ice as a platform to catch seals, as they are not skilled swimmers to catch them in the open water. The researchers said that nearly all of the 19 subpopulations of polar bears will face extinction as the loss of sea ice would force the animals onto land, keeping them away from their food supplies for longer periods.
"Ultimately, the bears need food and in order to have food, they need ice. But in order for them to have ice, we need to control climate change," Dr Péter Molnár, an assistant professor at the University of Toronto Scarborough and one of the authors of the study, told CNN.
The study found that prolonged fasting, and reduced nursing of cubs by mothers, would lead to rapid declines in reproduction and survival of polar bear populations.
"There is very little chance that polar bears would persist anywhere in the world, except perhaps in the very high Arctic in one small sub-population," said Dr Molnár.
According to the study, there are about 25,000 polar bears in the Arctic. The ice in the region grows in the winter and melts and retreats in spring and summer. During winter, when the ice is at it its greatest extent, the bears extensively feed on seals to build up energy stores, which will help them survive summer when they will be forced on to land.
However, with the rapid global warming in recent times, the ice extent in summer has declined by about 13% per decade compared to the 1981-2010 average. The last 13 years, between 2007 and 2019, have been the lowest 13 years for summer sea ice in the satellite record. This means that the bears are forced to go longer without food.
According to Walt Meier, a senior research scientist at the National Snow and Ice Data Center, since the mid-1990s, the Arctic has experienced levels of warming that are more than double the global average. Such exceptionally high air temperature has led to the declining sea ice in the Arctic. A major driving force behind the high air temperatures are people burning fossil fuels.
Several studies have suggested that the Arctic could be ice-free in summer of 2040 or 2050.
"The question is not 'if' we'll see an ice-free summer in the Arctic -- it's 'when,'" Meier said.
"Previously, we knew that polar bears would ultimately disappear unless we halt greenhouse gas rise. But knowing when they will begin to disappear in different areas is critical for informing management and policy—and inspiring action," said Dr Steven Amstrup, who conceived the project and is a co-author on the study.
The study said that already some populations, like those in Canada's Hudson Bay, have likely crossed key thresholds that will make their survival difficult, and even impossible. However, some bears can persist in a few pockets of the Arctic if humans are able to reduce heat-trapping gas emissions in the coming decades.
"I'm well aware that the story we're telling is a grim one. But there is also an element of hope that they are not completely doomed if we change our behaviour," Dr Molnár said.
The researchers said that while moderate emissions can prolong persistence, it is unlikely to prevent some population extirpations in this century.
"We found that moderate emissions reductions may prolong global persistence, but are not likely to prevent the extirpation of several populations, emphasizing the urgency of more ambitious emissions cuts," said Dr Amstrup, who is also the chief scientist of Polar Bears International and an adjunct professor in the University of Wyoming Department of Zoology and Physiology.
The study found that the length of time a bear can survive without food varies by region and the condition of the bear. While cubs are most vulnerable to a lengthy fast, adult females with cubs are second-most vulnerable. This is followed by adult males and solitary females -some of which can last for up to 255 days.
"By estimating how thin and how fat polar bears can be, and modelling their energy use, we were able to calculate the threshold number of days that polar bears can fast before cub and/or adult survival rates begin to decline," Dr Molnár said.
To see how long the bears could be forced to go without food in the future, the researchers looked at projections for Arctic sea ice under two different climate change scenarios.
"Intersecting these fasting impact thresholds with the projected future number of days that sea ice will be absent, we were able to project when fasting impact thresholds will be exceeded in different parts of the Arctic," said Dr Cecilia Bitz, a climate scientist with the University of Washington, Seattle and second author of the study.
"Polar bears have long been considered messengers of the climate change symptoms that will impact all life, including humans. We know that floods, droughts, and wildfires will become more frequent and severe as the world continues to warm, but timelines for such events are hard to predict," Dr Amstrup said.
"The coronavirus pandemic is a reminder of how vital it is for our governments to take the actions needed, even when the timeline of the threat feels uncertain. Showing how imminent the threat is for different polar bear populations is another reminder that we must act now to head off the worst of future problems faced by us all," Dr Amstrup added.