A US federal appeals court has delivered a decisive defeat to states and industry groups that had challenged the scientific and legal underpinnings of the Environmental Protection Agency's (EPA's) decision to regulate greenhouse gases as pollutants under the federal Clean Air Act.
In a unanimous decision, a three-judge panel ruled that the EPA had relied on sound science in deciding that greenhouse gases potentially "endangered" public health and welfare.
It also said that the agency had followed proper procedures in developing a series of regulations aimed at curbing emissions from cars and industrial facilities.
Discussions of climate science are sprinkled liberally throughout the 82-page opinion from the US Court of Appeals for the District of Columbia Circuit.
"We begin with a brief primer on greenhouse gases," judges David Sentelle, Judith Rogers, and David Tatel wrote in an opening section that outlines the science of climate change.
Later, in a one-liner likely to be widely repeated, the judges wrote that the "EPA is not required to re-prove the existence of the atom every time it approaches a scientific question".
The plaintiffs had complained that the EPA had improperly "delegated" its scientific judgment on greenhouse gases by relying on climate change assessments developed by the Intergovernmental Panel on Climate Change, the US Global Change Research Program, and the National Research Council of the US National Academies. But the appellate court soundly rejected their logic.
"This argument is little more than a semantic trick," the judges wrote.
"EPA did not delegate, explicitly or otherwise, any decision-making to any of those entities. EPA simply did here what it and other decision makers often must do to make a science-based judgment: It sought out and reviewed existing scientific evidence to determine whether a particular finding was warranted.
"It makes no difference that much of the scientific evidence in large part consisted of 'syntheses' of individual studies and research. Even individual studies and research papers often synthesise past work in an area and then build upon it. This is how science works. EPA is not required to re-prove the existence of the atom every time it approaches a scientific question."
The judges also dismissed claims that the science was tainted by too much uncertainty, and that the agency had violated procedures by not specifying the exact atmospheric concentrations at which greenhouse gases become a threat.
"EPA relied on a substantial record of empirical data and scientific evidence," they noted.
But "EPA's failure to distil this ocean of evidence into a specific number at which greenhouse gases cause 'dangerous' climate change" is "not a sign of arbitrary or capricious decision-making".
"In the end, petitioners are asking us to re-weigh the scientific evidence before EPA and reach our own conclusion," the judges also noted. "This is not our role."
Today's ruling has its roots in a 2007 US Supreme Court decision that found that EPA had the power to regulate carbon dioxide and other greenhouse gases under the Clean Air Act - if the agency could show that they threatened the public.
EPA issued such an "endangerment finding" in 2009, followed by rules limiting emissions from power plants, factories, and vehicles. Those rules, and the original endangerment finding, drew dozens of legal challenges that were consolidated into the current case.