In a 5-4 decision, the court said that the agency considered improper factors in 2003 when it decided not to order cuts in carbon emissions from new cars and trucks, and that the agency has the authority to regulate greenhouse gases.

The ruling doesn’t require the agency to impose new regulations, but it is a setback to automakers challenging the regulations mandating reductions in tailpipe CO₂ implemented by California and adopted by other states.

Part of the argument made against the tailpipe regulations  was the contention that carbon dioxide is not an air pollutant subject to regulation under the Clean Air Act. The Supreme Court decision rejected that interpretation.

“EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change,” Justice John Paul Stevens wrote for the majority.

“Greenhouse gases fit well within the Clean Air Act’s capacious definition of ‘air pollutant,’” Stevens wrote.

Resources:

• Opinion in Massachusetts v. EPA, 05-1120

• Docket for Massachusetts v. EPA, 05-1120

• Transcript of Arguments in Massachusetts v. EPA, 29 Nov 06