I n 2007, the Supreme Court issued its decision in Massachusetts v. EPA , its first pronouncement on climate change and still one of the most important environmental law decisions in the past decade. By a vote of 5-4, the Court held that the Environmental Protection Agency (EPA) in 2003 had improperly denied a petition asking the agency to regulate greenhouse gas (GHG) emissions from new motor vehicles under the Clean Air Act (CAA). Contrary to EPA's position, the Court said that the CAA definition of "air pollutant" was unambiguously broad enough to include GHGs. Accordingly, the Court reversed the lower court decision upholding the petition denial. The Supreme Court decision did not compel EPA to regulate greenhouse gas (GHG) emissions from new motor vehicles, but it did limit the range of options available to the agency so that doing so was its most defensible course of action. This report confines itself to the Massachusetts v. EPA litigation and leaves to other CRS reports the numerous EPA actions taken as a result of the Supreme Court decision. The report traces the events leading up to the Court's decision, describes the decision itself, notes some general implications, and then comments on the decision's continuing force. The saga of Massachusetts v. EPA began in 1999. In that year, 19 environmental and energy organizations petitioned EPA to regulate emissions of GHGs (carbon dioxide [CO 2 ], methane, nitrous oxide, and hydrofluorocarbons) from new motor vehicles. The petition argued that EPA had a mandatory duty to do so under CAA Section 202(a)(1), which directs the EPA Administrator to prescribe "standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles ... which, in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare." In 2003, after receiving about 50,000 comments, EPA denied this petition. Much of the agency's rationale followed a General Counsel memorandum issued the same day. Contrary to a precursor memorandum authored under the Clinton Administration, this new General Counsel memorandum concluded that the CAA does not grant EPA authority to regulate CO 2 and other GHG emissions based on their climate change impacts. Thus, EPA concluded that it had no choice but to reject the petition, though it also described policy reasons for doing so. EPA's denial of the Section 202 petition prompted a lawsuit, Massachusetts v. EPA , in the D.C. Circuit seeking review of the denial. Petitioners were 12 states (California, Connecticut, Illinois, Massachusetts, Maine, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont, and Washington); three cities (New York, Baltimore, and Washington, DC); two U.S. territories (American Samoa and Northern Mariana Islands); and several environmental groups. Opposing the challenge, besides EPA, were 10 state intervenors (Alaska, Idaho, Kansas, Michigan, North Dakota, Nebraska, Ohio, South Dakota, Texas, and Utah), plus several automobile- and truck-related trade groups. In 2005, a split panel of the D.C. Circuit rejected the suit—in effect upholding EPA's denial of the petition. The two judges supporting rejection of the suit, however, did so for different reasons. Judge Randolph concluded that EPA had properly exercised its discretion in choosing not to wield its Section 202 authority. As to this discretion issue, as noted above, CAA Section 202(a)(1) directs the EPA Administrator to prescribe standards for any motor vehicle emissions that "in his judgment" cause harmful air pollution; Judge Randolph read "in his judgment" broadly to allow EPA consideration of not only scientific uncertainty about the effects of GHGs, but also policy considerations that justify not regulating. Thus EPA, in his view, was entitled to rely on such factors as the George W. Bush Administration's policy preference for voluntary GHG control measures, and its belief that regulating motor vehicle emissions was a piecemeal and inefficient approach to dealing with climate change. Judge Sentelle, the other judge supporting rejection of the petition, held that petitioners lacked standing. In dissent, Judge Tatel asserted that Massachusetts had demonstrated standing through past and future loss of shore land as a result of climate-change-induced sea level rise. On the merits, he found that EPA has authority under Section 202(a)(1) to regulate GHG emissions. He further concluded that EPA's 202(a)(1) discretion does not extend to policy considerations, as Judge Randolph held, but relates exclusively to whether the emissions cause harmful air pollution. The Supreme Court agreed to review the D.C. Circuit decision in Massachusetts v. EPA , although the D.C. Circuit majority had not ruled on the key issue of whether CAA Section 202(a)(1) authorizes regulation of GHG emissions. As the Supreme Court stated in its decision, "the unusual importance of the underlying issue persuaded us to grant the writ." The Court ruled 5-4 for petitioner states and environmental groups on all three issues in the case. It held first that at least one petitioner, the Commonwealth of Massachusetts, had standing to sue, so the Court could proceed to the merits. On the merits, it found that the CAA gives EPA authority to regulate GHG emissions from new motor vehicles, and does not give EPA discretion to inject policy considerations into its decision whether to so regulate. The ruling in favor of petitioners was forecast early in the majority opinion by its opening sentences: "A well-documented rise in global temperatures has coincided with a significant increase in the concentration of carbon dioxide in the atmosphere. Respected scientists believe the two trends are related." The dissenters did not dispute these statements. Most of the Supreme Court's decision is devoted to whether plaintiffs had standing to sue, an issue that has recurred in climate change litigation. At the outset, the Court found that petitioners had two factors in their favor. First, the CAA specifically authorizes challenges to agency action unlawfully withheld, such as the Massachusetts suit. A litigant to whom Congress has accorded such a procedural right, said the Court, can assert that right without meeting the normal standards for standing. Second, the Court found it "of considerable relevance" that the petitioner injury on which it focused—Massachusetts's loss of shore land from global-warming-induced sea level rise—was that of a sovereign state rather than a private entity. States are "not normal litigants for the purposes of invoking federal jurisdiction," said the Court, noting their quasi-sovereign duty to preserve their territory. Although the Court described petitioners' favored position with regard to standing, it then undertook a fairly traditional standing analysis. As to the first prong of the black-letter standing test—whether plaintiff has demonstrated actual or imminent "injury in fact" of a concrete and particularized nature—the Court focused on Massachusetts's status as owner of much of the commonwealth's shore land. That this injury may be widely shared with other coastal states does not disqualify this injury, said the Court; it is nonetheless concrete. The second prong of the standing test is causation, requiring that the injury of which the plaintiff complains is fairly traceable to the defendant. EPA did not dispute the existence of a causal relationship between GHG emissions and climate change. It did argue, however, that any reduction in GHG emissions achieved through the current litigation would be too tiny a fraction of worldwide GHG emissions to make a cognizable difference in climate change. In an important ruling that may be of benefit to environmental plaintiffs in other contexts, the Court held that even an agency's refusal to take a "small incremental step," here, one that would result in only a modest reduction in worldwide GHG emissions, is enough for standing purposes. The third and final prong of the standing test is redressability, demanding that the remedy sought by the plaintiff is one likely to redress that plaintiff's injury. In this case, the remedy sought was EPA regulation of GHG emissions from new motor vehicles. The Court found that this remedy satisfied redressability because while it would not by itself reverse climate change, it would nonetheless slow or reduce it. Nor, given the "enormity" of the potential effects of climate change, was it relevant to the Court that the full effectiveness of the remedy would be delayed until existing cars and trucks on the road were largely replaced by new ones. Compared to the large number of pages devoted by the majority opinion to standing, its discussion of the two CAA issues in the case is relatively brief. On the question of EPA's authority to regulate GHG emissions, the Court looked to the CAA's "sweeping" definition of "air pollutant," embracing " any air pollutant ... including any physical, chemical ... substance or matter which is emitted into or otherwise enters the ambient air." Such a broad definition, it said, could not be squared with EPA's position that GHGs are not included. The Court rejected EPA's argument that federal laws enacted following enactment of this statutory definition—laws emphasizing interagency collaboration and research—suggest that Congress meant to curtail EPA's power to use mandatory regulations in addressing air pollutants. Nor was the Court moved by EPA's contention that "air pollutant" in the CAA could not include vehicle GHG emissions because EPA standards for such emissions could be satisfied only by improving fuel economy, a job EPA asserted was assigned solely to the Department of Transportation under a different statute (the Energy Policy and Conservation Act). As to the issue of EPA's discretion, the Court concluded that the phrase "in [the Administrator's] judgment" in CAA Section 202 should be read narrowly. That is, it allows the EPA Administrator, in deciding whether to set emission standards, to consider only whether an air pollutant, in the section's words, "may reasonably be anticipated to endanger public health or welfare." The phrase does not give EPA discretion to factor in its policy preferences.