Landmark study quantifies the impact of greenhouse gas emissions on polar bears, removing an obstacle that prevented climate action

In a new paper, scientists with Polar Bears International, the University of Washington, and the University of Wyoming have, for the first time, quantified a direct link between greenhouse gas (GHG) emissions and polar bear survival. Published today in Science, the report, “Unlock the Endangered Species Act to address GHG emissions,” provides a template for estimating the demographic impact of proposed GHG-emitting actions on polar bears—overcoming a loophole in the Endangered Species Act (ESA) that has historically blocked climate considerations. The approach outlined in the paper connects the dots between greenhouse gas emissions, the number of ice-free days caused by specific amounts of emissions, and polar bear survival rates. It also explains the recent declining trends observed in some polar bear subpopulations.

Although polar bears were listed under the Endangered Species Act (ESA) in 2008 because of sea ice loss caused by climate warming, then-Solicitor of the Department of the Interior, David Bernhardt, issued a legal opinion stating that ESA considerations of emissions would not be required unless the impact of emissions from considered projects could be separated from the impact of all historic global emissions. The inability to make that separation and measure the impact of a specific project meant that climate change—the very reason polar bears were listed—was blocked from inclusion in ESA evaluations.

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“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 Dr. Steven Amstrup, chief scientist emeritus at Polar Bears International and an adjunct professor at the University of Wyoming, adding, “But 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 GHG emissions and cub survival rates. The methodology, for the first time, allows us to parse the impact of emissions by source.  Importantly, the approach we describe, using regression analysis to connect GHG emissions to habitat and demographic changes, also has broad application beyond polar bears to other ecosystems and species—and could be used by managers and policymakers around the world when evaluating development projects.”

Building on the foundation established in a 2020 report linking projected polar bear survival against summer fasting duration caused by global warming, this paper takes the additional step of quantifying the number of ice-free/fasting days caused by a specific amount of CO2-eq emissions, thus allowing for a direct calculation of the impact of a project’s emissions on future polar bear cub recruitment. For example, the hundreds of power plants in the U.S. together will emit 60+ Gt emissions over 30+ year lifespans, which reduce cub recruitment in the Southern Beaufort Sea population by ~4%.

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In 2008, based on projections that up to two-thirds of the world’s polar bears could disappear by mid-century, polar bears became the first species listed under the ESA due to threats from human-caused climate warming. Section 7 of the ESA provides a process ensuring that government-authorized projects (including oil and gas leases) do not further endanger any ESA-listed species. Shortly after polar bears were listed, however, then-Solicitor of the Department of Interior David Bernhardt issued Memo M-37017, claiming impacts of emissions from any individual action or group of actions being considered could not be separated from the impact of historic emissions that have been accumulating since the beginning of the Industrial Revolution. The limitation described in the Bernhardt Memo has prevented inclusion of global warming emissions from oil and gas leasing, and other GHG-emitting activities, in ESA Section 7 reviews—even though global warming, resulting from GHG emissions, was the reason polar bears were listed in the first place. This new report, published during the 50th-year anniversary of the ESA and 15th-year anniversary of polar bears being listed under the ESA, directly fills the knowledge gap identified in M-37017, allowing GHG emissions from any action to be parsed from historic emissions. This will allow rescission of the Bernhardt Memo and inclusion of GHG pollution in the review of future actions.

“Overcoming the challenge of the Bernhardt Memo is absolutely in the realm of climate research,” says co-author Dr. Cecilia Bitz, professor of atmospheric sciences at the University of Washington, noting, “When the 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. Our study shows that not only sea ice, but polar bear survival, can be directly related to greenhouse gas emissions.”

IMAGE CREDIT: Erinn Hermsen / Polar Bears International

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