Polar bears could disappear by 2100 due to shrinking sea ice if climate change continues unchecked: Study
Even if moderate emissions reduction targets are achieved, in which greenhouse gas emissions peak by 2040 and then begin to decline, several populations will collapse
Arctic Sea ice loss from global warming is driving polar bears into extinction. Scientists now predict that if greenhouse gas emissions are not checked, it is likely that all but a few polar bear populations will collapse by 2100.
The new analysis projects when polar bear populations in the Arctic will cross thresholds of food deprivation, leading to rapid declines in reproduction and survival. The authors warn that by the end of the century polar bears could become nearly extinct as a result of shrinking sea ice in the Arctic due to climate change. The researchers found that moderate reductions in emissions may prolong their existence, but are not likely to prevent the extinction of several populations. This underscores the urgent need to tackle global warming and the need for ambitious emissions cuts, says the team.
"Our model captures demographic trends observed during 1979-2016, showing that recruitment and survival impact thresholds may already have been exceeded in some subpopulations. It also suggests that, with high greenhouse gas emissions, steeply declining reproduction and survival will jeopardize the persistence of all but a few high-Arctic subpopulations by 2100. Moderate emissions mitigation prolongs persistence but is unlikely to prevent some subpopulation extirpations within this century," write authors in the study published in Nature Climate Change. The team includes experts from University of Toronto Scarborough, Canada; University of Washington, Seattle; University of Toronto, Canada; National Center for Atmospheric Research, US; University of Colorado Boulder, US; and Polar Bears International, US.
Polar bears were added to the threatened species list when their icy habitat showed steady, precipitous decline because of a warming climate. According to estimates, there are about 25,000 polar bears left, spread out across 19 different subpopulations across four Arctic ecoregions. Previous studies have shown that declining sea ice is likely to decrease polar bear numbers. The new study puts a timeline on when that might happen.
Polar bears depend on sea ice for capturing seals, their primary food source. Due to declining summer sea ice, many bears are forced onto land or they follow the remaining ice as it drifts over deep, unproductive waters. In both cases, polar bears are largely food-deprived. Prolonged fasting periods have been linked to reduced body condition, reproduction and survival in some polar bear populations, explains Polar Bear International.
During seasons when they cannot reach the ice, the bears mostly go without food and can lose about two pounds a day. They draw on energy reserves built up during the winter hunting season to make it through lean summer months on land or time spent on ice in unproductive waters. Previous studies have indicated that the periods when they do not have ice access have increased, and are expected to continue to do so with the current level of greenhouse gas emissions.
In the current study, researchers looked at 13 of the subpopulations, accounting for about 80% of the total population. By modeling the energy use of polar bears, they were able to calculate their endurance limits. "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/or adult survival rates begin to decline," says first author of the study, Dr Péter Molnár, University of Toronto, Scarborough. A male bear, which is 20% below its normal body weight when fasting begins, for example, will only have enough stored energy to survive about 125 days rather than 200 days.
"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," writes the second author of the study, Dr Cecilia Bitz, a climate scientist with the University of Washington, Seattle.
The authors analyzed how the bears will be affected by two different greenhouse gas emissions scenarios. Under a high greenhouse gas emissions scenario, all but a few high Arctic subpopulations will likely disappear by 2100. Even if moderate emissions reduction targets are achieved, in which greenhouse gas emissions peak by 2040 and then begin to decline, several populations will still be wiped out, shows analysis.
"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," says study author Dr Steven C Amstrup, chief scientist at Polar Bears International, US, and an adjunct professor at the University of Wyoming, Laramie, US. Dr Amstrup adds, "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."