The US Supreme Court ruled Monday that the Environmental Protection Agency (EPA) has the authority under the Clean Air Act to regulate the emission of "greenhouse gases," such as carbon dioxide, by automobiles. In Massachusetts v. EPA, 12 states and several environmental groups sued the EPA arguing that the agency had, according to the court, "abdicated its responsibility under the Clean Air Act" to regulate greenhouse gas emissions. The Court first agreed that the plaintiffs had standing to bring the lawsuit and went on to rule that "Because greenhouse gases fit well within the Clean Air Act's capacious definition of 'air pollutant,' we hold that EPA has the statutory authority to regulate the emission of such gases from new motor vehicles." The Court reversed and remanded the federal appeals court decision, saying that the EPA improperly "offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change" and holding that the "EPA must ground its reasons for action or inaction in the statute." Read the Court's 5-4 opinion per Justice Stevens, along with a dissent from Chief Justice Roberts and a second dissent from Justice Scalia.

In a second decision handed down Monday, the Court ruled in Environmental Defense Fund v. Duke Energy Corp. that the US Court of Appeals for the Fourth Circuit improperly interpreted Prevention of Significant Deterioration (PSD) regulations under the Clean Air Act. The appeals court ruled that the Clean Air Act requires the EPA to conform PSD regulations to their New Source Performance Standards (NSPS) counterparts, and the Supreme Court held that "the Court of Appeals's reading of the 1980 PSD regulations, intended to align them with NSPS, was inconsistent with their terms and effectively invalidated them; any such result must be shown to comport with the Act's restrictions on judicial review of EPA regulations for validity." Read the Court's opinion per Justice Souter, along with a concurrence from Justice Thomas.