The U.S. Supreme Court declined to review a legal challenge brought by Ohio and 16 other states seeking to overturn the U.S. Environmental Protection Agency’s (EPA) decision to restore California’s Clean Air Act waiver, which allows the state to impose vehicle emissions standards that exceed federal requirements. The order did not explain the Court’s reasoning but noted that Justice Clarence Thomas would have granted the petition.

The Court’s denial preserves the U.S. Court of Appeals for the District of Columbia’s unanimous ruling in Ohio v. EPA, which dismissed challenges to the EPA’s 2022 reinstatement of California’s 2013 Clean Air Act preemption waiver for its Advanced Clean Cars regulations. Those rules limit greenhouse gas emissions from new vehicles and require automakers to sell increasing percentages of zero-emission vehicles, culminating in a 2035 target for all new passenger vehicles sold in the state to be zero-emission. (See All New Cars Sold in California Must be ZEVs by 2035.)

In the April 2024 ruling, the D.C. Circuit panel held that neither the state petitioners nor a group of liquid fuels producers had established Article III standing to challenge EPA’s decision. The court concluded that the challengers failed to demonstrate that vacating the waiver would redress their alleged economic or regulatory injuries.

The court also rejected, on the merits, a constitutional claim advanced by the states that the Clean Air Act violates principles of equal state sovereignty by granting California regulatory authority denied to other states. The court found that Congress acted within its constitutional authority in preserving California’s distinct role in regulating motor vehicle emissions.

The litigation stems from the EPA’s March 2022 decision to reinstate a Clean Air Act waiver originally granted to California in 2013 for its Advanced Clean Cars regulations. That action reversed a 2019 Trump administration rule that revoked the waiver as part of the Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule, which sought to establish a single national standard for vehicle fuel economy and greenhouse gas emissions.

California’s Advanced Clean Cars program limits greenhouse gas emissions from new vehicles and requires automakers to sell increasing percentages of zero-emission vehicles over time. Under current regulations, all new passenger vehicles sold in California must be zero-emission by 2035. Seventeen states and the District of Columbia have adopted California’s standards under Clean Air Act provisions allowing other states to opt in.

Under the Clean Air Act, federal regulation of motor vehicle emissions generally preempts state standards, but California is granted a unique exemption reflecting its long-standing air quality challenges and pre-1970 emissions program. The statute directs the EPA to grant a waiver if California demonstrates “compelling and extraordinary circumstances” and shows that the standards are technologically feasible. Other states may then opt to adopt California’s standards. (see California’s Energy Transition—Auto Emissions).

In their Supreme Court petition, the state challengers argued that Congress lacks authority under the Commerce Clause to empower one state to exercise sovereign regulatory power denied to all others, and that California’s standards are preempted by the Energy Policy and Conservation Act.

Separately, in Diamond Alternative Energy LLC v. Environmental Protection Agency, the Court declined to consider the merits of challenges brought by oil and gas interests contesting the lawfulness of EPA’s waiver decision. The justices, however, agreed to review a narrow threshold question concerning whether those industry petitioners have standing to bring their claims. The Court’s order does not address a separate petition filed by the state challengers raising constitutional objections under the Clean Air Act’s equal sovereignty framework.