Writing for the Court, Justice John Paul Stevens held that Massachusetts standing under Article III to challenge the EPA’s refusal to regulate greenhouse gas emissions. Stevens began by establishing that the “case or controversy” requirement of Article III is satisfied when a plaintiff demonstrates a personal stake in the outcome through an injury that is fairly traceable to the defendant and likely to be redressed by a ruling in their favor. However, Stevens emphasized that when a sovereign State is the party seeking protect its quasi-sovereign interests, such as greenhouse gas emissions, it’s entitled to “special solicitude” in the standing analysis.

Regarding the injury-in-fact, Stevens rejected the argument that the widespread nature of climate change harms precluded standing. Stevens agreed with Massachusetts argument that the rise in sea levels had already begun to “swallow” coastal land owned by Massachusetts, creating a risk of injury that was both actual and imminent. Stevens argued that the severity of this injury would only increase, with significant fractions of coastal property facing permanent loss through inundation. Stevens found that such an impact constituted a concrete and palpable injury to the Massachusetts’ interests.

Regarding causation and redressability, Stevens concluded that the EPA’s refusal to regulate emissions contributed to Massachusetts’ injuries. Stevens argued that while a single regulatory act might not reverse global warming entirely, a reduction in domestic emissions would “slow the pace of global emissions increases,” which would likely redress the harm to Massachusetts. Ultimately, the Court held that the petitioners met the standing requirements of Article III, permitting judicial review of the EPA’s action.

On the merits, Stevens rejected the EPA’s argument that it lacked statutory authority. Stevens pointed to the broad definition of “air pollutant” in the Act, which includes “any physical, chemical... substance or matter which is emitted into or otherwise enters the ambient air.” Stevens argued that the statute is unambiguous and that greenhouse gases “fit well within the Clean Air Act’s capacious definition.” Regarding the EPA’s argument that it was unwise to regulate even if it had the authority, Stevens found it to be "divorced from the statutory text.” He explained that while the statute conditions action on the formation of a “judgment”, that discretion is not “a roving license to ignore the statutory text.” Stevens noted that the EPA’s list of policy reasons, such as foreign policy concerns and a preference for voluntary programs “have nothing to do with whether greenhouse gas emissions contribute to climate change.” Stevens concluded that the EPA must “ground its reasons for action or inaction in the statute.” He emphasized that if “scientific uncertainty is so profound that it precludes EPA from making a reasoned judgment,” it must explicitly say so rather than relying on “impermissible considerations.” As a result, the Court reversed the lower court’s judgment and remanded the case, requiring the EPA to provide a reasoned explanation for its decision based on the statutory “endangerment” standard.