Scientists link greenhouse gas emissions to polar bear population declines

Planet-warming greenhouse gas emissions pose a direct threat to the survival of polar bears — by limiting their access to the sea ice that serves as their hunting grounds, a new study has found.

During ice-free summer months, the bears must fast, which in worst-case scenarios mean adults could die and, before then, lose the ability to successfully raise cubs, according to the study, published Thursday in the journal Science.

The first-of-its-kind research quantifies the amount of  ice-free days caused by specific amounts of emissions, as well as associated polar bear survival rates and declining trends in some subpopulations.

“We’ve known for decades that continued warming and sea ice loss ultimately can only result in reduced distribution and abundance of polar bears,” lead author Steven Amstrup, chief scientist emeritus at Polar Bears International, said in a statement. 

“Until now, we’ve lacked the ability to distinguish impacts of greenhouse gases emitted by particular activities from the impacts of historic cumulative emissions,” added Amstrup, who is also an adjunct professor at the University of Wyoming.

Amstrup and his colleagues were able to connect ice-free days and polar bear fasting limits to cumulative greenhouse gas emissions — conducting a data analysis that establishes a direct link between these circumstances.

They found that the hundreds of power plants across the U.S. will emit more than 60 gigatons of greenhouse gases over their 30-year lifespans — reducing polar bear cub survival by 4 percent in the southern Beaufort Sea, north of Alaska.

“Polar bears are beautiful creatures, and I hope they survive global warming,” co-author Cecilia Bitz, a University of Washington professor of atmospheric sciences, said in a statement.

“All of us have experienced heat extremes in the last few years. The harm is inescapable,” she added.

Although polar bears were listed under the Endangered Species Act in 2008 because of climate-induced sea ice loss, federal officials at the time issued a legal opinion indicating that impacts of emissions need not be considered when evaluating infrastructure projects that touch on polar bear habitats.

This decision, known as the Bernhardt Opinion, required specific proof as to how a project’s emissions would affect a population’s survival, while arguing that such pollution could not be separated from the greenhouse gas releases that have occurred since the beginning of the Industrial Revolution.

“Overcoming the challenge of the Bernhardt Opinion is absolutely in the realm of climate research,” Bitz said.

While scientists in 2008 could not quantify how emissions equated to the plunge in polar bear populations, this is no longer the case, Bitz explained. The new findings, the authors contended, provide the Department of Interior with the evidence needed to repeal the Bernhardt Opinion.

Bitz expressed hopes that the U.S. government “fulfills its legal obligation to protect polar bears by limiting greenhouse gas emissions from human activity.”

“I hope investments are made into fossil fuel alternatives that exist today, and to discover new technologies that avoid greenhouse gas emissions,” she added.