## Facts

- Private organizations petitioned the Environmental Protection Agency (EPA) in 1999 to regulate greenhouse-gas emissions from new motor vehicles under Clean Air Act (CAA) § 202(a)(1).
- Section 202(a)(1) directs EPA to set standards for “any air pollutant” from new motor vehicles that, in the Administrator’s judgment, “cause[s], or contribute[s] to, air pollution” reasonably anticipated to endanger public health or welfare.
- In 2003, EPA denied the petition, stating that the CAA did not authorize regulation of greenhouse gases for climate-change purposes and that, even if it had authority, it would be imprudent to regulate due to scientific uncertainty and policy considerations (including diplomatic and broader executive-branch strategies).
- Massachusetts, other states, local governments, and environmental organizations sought judicial review in the D.C. Circuit.
- The D.C. Circuit denied review in a split decision.
- The Supreme Court granted certiorari to address standing, statutory authority, and EPA’s reasons for declining to act.

## Issues

1. Whether Massachusetts had Article III standing to challenge EPA’s denial of the rulemaking petition.
2. Whether greenhouse gases, including carbon dioxide, fall within the CAA’s definition of “air pollutant” applicable to § 202(a)(1).
3. Whether EPA may refuse to regulate motor-vehicle greenhouse-gas emissions based on policy considerations not grounded in § 202(a)(1)’s endangerment framework.

## Decision

- The Court reversed the D.C. Circuit and remanded.
- Massachusetts had standing based on threatened loss of coastal territory from sea-level rise, with states entitled to “special solicitude” in the standing inquiry.
- Greenhouse gases are “air pollutants” under the CAA’s broad definition covering physical and chemical substances emitted into ambient air.
- EPA may not decline to act for policy reasons unrelated to whether the emissions cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare.
- If EPA declines to regulate, it must ground the decision in the statute—e.g., a determination that emissions do not contribute to the relevant endangerment or that the science is too uncertain to make the required judgment.

## Legal Principles

- A state may establish Article III standing to challenge federal regulatory inaction where it shows a concrete injury to its territory, and incremental risk reduction can satisfy redressability.
- The Clean Air Act’s definition of “air pollutant” is sufficiently broad to include greenhouse gases emitted into the ambient air.
- When a statute directs an agency to make a science-based endangerment judgment, the agency cannot substitute policy preferences or nonstatutory considerations for that required determination.
- Agency discretion under § 202(a)(1) is constrained by the statutory criteria; refusal to regulate must be explained on the statutory grounds tied to endangerment.

## Conclusion

The Court held that Massachusetts could sue EPA for denying a rulemaking petition, that greenhouse gases qualify as “air pollutants” under the Clean Air Act, and that EPA must justify any decision not to regulate motor-vehicle greenhouse-gas emissions by reference to the statute’s endangerment-based criteria rather than freestanding policy considerations.


Massachusetts v. EPA, 549 U.S. 497 (2007)