Legal breakthrough as greenhouse gas emissions linked to polar bear decline

New research linking greenhouse gas emissions to polar bear population declines will enable greater protections for the species under the US Endangered Species Act.

A paper, published in the journal Science, combines past research and new analysis to measure the link between greenhouse gas (GHG) emissions, sea ice loss, and polar bear cub survival rates.

As a result of human-induced climate change, a warming Arctic is limiting polar bears’ access to the sea ice they use as a hunting platform. This means they have to fast in the summer months, losing nearly a kilogram of body mass per day. They may lose the ability to successfully raise cubs, and in the worst case scenario, die.

“We’ve known for decades that continued warming and sea ice loss ultimately can only result in reduced distribution and abundance of polar bears,” says lead author Steven Amstrup, chief scientist emeritus at the non-profit advocacy group Polar Bears International, and adjunct professor at the University of Wyoming, US.

“Until now, we’ve lacked the ability to distinguish impacts of greenhouse gases emitted by particular activities from the impacts of historic cumulative emissions. In this paper, we reveal a direct link between anthropogenic greenhouse gas emissions and cub survival rates.”

A polar bear on sea ice
A polar bear photographed in the Greenland Sea in September 2012. Credit: Cecilia Bitz/University of Washington

In 2008, polar bears (Ursus maritimus) became the first species to be listed under the US Endangered Species Act (ESA) due to threats from human-caused climate warming. Section 7 of the ESA requires that any government-authorised projects, including those they fund or authorise, do not endanger any listed species.

But a document released by the US Department of the Interior in 2008, known as the Bernhardt Opinion, required specific proof of how a proposed project’s GHG emissions would affect a species’ survival before the ESA could be fully implemented.

According to the authors of the new study, it claims that “the consequences from current anthropogenic GHG emissions cannot be separated from natural sources or the past accumulation of anthropogenic emissions”.

A closeup of a polar bear's face
A polar bear photographed in Churchill, Canada, in November 2021. Credit: Erinn Hermsen/Polar Bears International

This new research fills that knowledge gap by linking ice-free days and polar bear fasting limits to cumulative greenhouse gas emissions.

“Overcoming the challenge of the Bernhardt Opinion is absolutely in the realm of climate research,” says second author Cecilia Bitz, professor of atmospheric sciences at the University of Washington, US.

“When the memo was written in 2008, we could not say how human-generated 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.

“Our study shows that not only sea ice, but polar bear survival, can be directly related to our greenhouse gas emissions.”

The implications of this new study reach beyond polar bears and sea ice, the authors say, as the same method of analysis can be adapted for other species and species’ habitats with direct connections to global warming, such as coral reefs or beach-nesting species effected by rising sea levels.

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