For fifteen years, Alaska’s polar bears have been caught in Endangered Species Act limbo.
In 2008 the animals were given federal protection because global warming is eating away at the sea ice on which they rely to hunt. While it might seem this would open the door to scrutiny of the climate impacts of federal actions, that hasn’t been the case. That’s because it was impossible to tie greenhouse gas pollution from a single source to the fate of a species thousands of miles away. Until now.
Two scientists last week reported in the prominent journal Science that they have developed a way to quantify the toll on polar bears from an isolated dose of greenhouse gases. If the research holds up, it could force closer scrutiny of federal activities—such as allowing oil drilling on public lands—on the grounds that they are hurting polar bears.
The approach 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,” said Steven Amstrup, the paper’s lead author and a biologist who spent three decades studying polar bears at the U.S. Geological Survey before moving to the conservation group Polar Bears International.
Polar bears, enormous ivory-colored masters of the Arctic, have become emblems for the toll of climate change. Arctic temperatures are rising nearly four times faster than the global average, chiefly because sea ice that reflects sunlight is giving way to darker water that absorbs it. The decline of sea ice has also stranded polar bears, removing the platforms they use to reach icy hunting grounds. The amount of sea ice at the end of the Arctic summer has shrunk by nearly 13% per decade over the last 40 years. It is breaking up earlier in the spring and forming later in the fall. As a consequence, researchers have found that polar bears are going longer without food, having fewer cubs and spending less time on the ice. Scientists have warned that thanks to climate change, polar bears could disappear from much of the Arctic by mid-century.
This grim picture prompted the U.S. Fish and Wildlife Service to grant the bears endangered species protection in 2008. But that same year, a top lawyer in the George W. Bush Administration, David Bernhardt, issued a memo stating that the fate of polar bears couldn’t be factored into federal actions that might cause climate pollution, because the bear’s woes are tied to cumulative pollution stretching back decades and “cannot be attributed to the emissions from any particular source.”
Amstrup and University of Washington Arctic sea ice expert Cecilia Bitz set out to prove Bernhardt wrong. The two dove into the scientific literature, looking for links in a chain connecting the smokestack of a coal fired power plant (for example) to the fate of a bear cub in northern Alaska. They emerged with a publication that reads like one part jargon-filled research and one part impassioned advocacy.
“Overcoming the challenge of the Bernhardt Memo is absolutely in the realm of climate research,” said Bitz. “Our study shows that not only sea ice, but polar bear survival, can be directly related to greenhouse gas emissions.”
It all comes down to sea ice.
On the bear’s side, recent research has quantified relationships between sea ice and polar bear health. In 2020, a group of Canadian and U.S. scientists published research showing that when bears in a particular area went without food for a certain number of days—a phenomenon linked to sea ice breaking up—then survival of adults and new cubs started to decline. Using energy budgets estimating bears’ metabolism and energy stores, the researchers found that cub survival starts to suffer the first time bears go without food for more than 117 days for three of five years in a row.
Other research, meanwhile, found a linear relationship between rising carbon emissions and loss of Arctic sea ice.
Using the relationships between sea ice and emissions and sea ice and bear cub health, Bitz and Amstrup connected all three. They emerged with an estimate of how much greenhouse gas pollution it would take to add another day of starvation for polar bears living in various parts of the Arctic, and how that translates into cub survival.
The findings point to the importance of scrutinizing a large collection of federal actions, rather than scrutinizing them one at a time, the scientists caution. While an individual oil well or coal-fired power plant might not make an appreciable difference, the duo calculated that new activities planned on public lands in the U.S. by 2050 could release enough greenhouse gases to decrease bear cub production by 2.7% in the Chukchi Sea. All the emissions from U.S. coal plants over 30 years can be expected to cut cub populations in the Beaufort Sea by around 4%.
The new findings mean the federal government “has the scientific justification and duty to rescind” the Bernhardt memo, the researchers wrote—potentially opening the door to new scrutiny for the climate impacts of activities involving the federal government. That could range from direct government projects such as new highways to the issuance of government permits for things like power plants.
A menagerie of other species could be following in the polar bear’s footsteps. Scientists could calculate the relationship between climate pollution, rising sea levels and the loss of beach habitat for nesting birds and sea turtles, Amstrup and Bitz noted.
“We are confident that our polar bear example will lead other investigators to uncover parallels in their data,” the scientists wrote, “providing numerous other species with previously unavailable ESA protections.”
Amstrup, et. al. “Unlock the Endangered Species Act to address GHG emissions.” Science. Aug. 31, 2023.
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