Polar bears were listed under the Endangered Species Act in 2008 but have not benefited from emissions-related protections.
CNN  — 

Scientists say they have found a link between human-related greenhouse gas emissions and polar bear reproduction and survival rates for the first time in a new study, potentially overcoming a barrier to protecting the species.

Polar bears live in 19 populations across the Arctic and are found in Canada, the United States, Russia, Greenland and Norway, according to conservation organization Polar Bears International.

The populations live under distinct and varying circumstances, but all depend upon ice sheets to access their main prey, two species of seal, said study coauthor Steven Amstrup, chief scientist emeritus at Polar Bears International.

When sea ice melts, polar bears are forced onto land where they are deprived of food and must survive on fat reserves that they have accumulated beforehand.

Polar bears mainly feed on their prey from the surface of ice sheets, which are increasingly declining due to climate warming caused by human activity.

Climate change caused by human activity is accelerating sea ice loss, giving polar bears less time to feed and build up their fat reserves, and more days where they are forced to go without food. This leads ultimately to a decline in their population.

Researchers from Polar Bears International, the University of Washington and the University of Wyoming have quantified the connection between the number of ice-free days a population of polar bears has to endure and the amount of planet-warming pollution released into the atmosphere, as well as corresponding polar bear survival rates in some populations, according to the study published Thursday in the journal Science.

Polar bears were listed as “threatened” due to human-caused climate warming under the US Endangered Species Act, or ESA, in 2008. But the US Department of Interior said at the time that, because threat to a particular species couldn’t be directly linked to a specific source of greenhouse gases, federal agencies don’t have to consider emissions when approving projects.

The researchers said the new study provides evidence of that direct link.

Impact of emissions

Researchers looked at polar bear subpopulations that had experienced at least 10 years with ice-free seasons from 1979 — when satellite imagery of sea ice first became available — to 2020.

They found that the number of days polar bears were forced to go without food increased as greenhouse gas emissions accumulated.

For example, polar bears in the Chukchi Sea in the Arctic Ocean were forced to go without food for about 12 days in 1979. It increased to about 137 days in 2020, with another day of fasting being added for every 14 gigatons of greenhouse gas emissions released into the atmosphere.

Subpopulations in areas where ice sheets would usually remain during the summer have experienced a "sudden shift" to more ice-free days without food, according to study coauthor Steven Amstrup.

Subpopulations in areas where ice sheets would typically melt entirely during the summer — such as in Hudson Bay — saw slower increases in the days they were forced to go without food over time, compared with subpopulations in areas where ice sheets would usually remain during the summer, such as in the South Beaufort Sea.

Bears in areas where ice sheets used to persist for longer in the summer are now experiencing “a sudden shift” in their environment, requiring a significant adjustment where they are having forced fast days that they did not have before, Amstrup told CNN.

A 2020 study that Amstrup was involved in found that the number of days a bear can survive without food varies by region and the condition of the animal, but the more ice-free days experienced, the steeper the decline in reproduction and survival.

The researchers behind the new study combined the relationship they found between the number of forced fast days polar bears had to endure and cumulative greenhouse gas emissions with the connection found in 2020 between the number of forced fast days and declining survival rates. That allowed them to calculate the impact of cumulative emissions on the rate of polar bear survival.

“When the (Department of the Interior) memo was written in 2008, we could not say how greenhouse gas emissions equated to a decline in polar bear populations. But within a few years we could directly relate the quantity of emissions to climate warming and later to Arctic sea ice loss as well,” said study coauthor Cecilia Bitz, professor of atmospheric sciences at the University of Washington, in the news release. “Our study shows that not only sea ice, but polar bear survival, can be directly related to greenhouse gas emissions.”

For example, the study noted that each of the hundreds of power plants in the US might make a relatively small contribution to emissions, but collectively the power plants emit nearly 2 gigatons of greenhouse gases each year. This would be about 60-plus gigatons over the 30-plus year life span of a polar bear in the southern Beaufort Sea.

In this subpopulation, bears endure an extra forced fasting day for every 23 gigatons of emissions added to the atmosphere, so their survival rate would be reduced by about 4% by the power plant emissions over that period, according to the study.

The US Department of the Interior declined to comment on the findings.

Applications for other species

Kirsten Zickfeld, distinguished professor of climate science at Simon Fraser University in Canada, said the new research shows a shift in what’s possible when it comes to demonstrating the effects of climate change.

“Previous research showed that the more carbon dioxide we produce from human activities, the more our climate changes. The new study by Amstrup and Bitz takes this idea further by showing that this direct link also applies to ecosystem impacts,” said Zickfeld, who was not involved in the study. “They found that when we emit more carbon dioxide, it directly affects how many polar bear cubs can survive. This finding allows us to connect the decline in polar bear populations to individual sources of carbon emissions.”

Amstrup said he hopes the study can be used as a template by other researchers to get other species affected by human-caused climate change protected by the ESA.