LOS ANGELES, March 19 (Xinhua) -- A coalition of more than 20 U.S. states, along with 10 dozen cities and counties, filed a legal challenge Thursday against the U.S. Environmental Protection Agency (EPA), contesting the agency's rescission of the 2009 Greenhouse Gas Endangerment Finding, the legal cornerstone of federal climate regulation under the Clean Air Act.

According to the law firm Davis Polk & Wardwell, the case is expected to reach the Supreme Court. The Salata Institute for Climate and Sustainability at Harvard University has separately warned in a briefing that a ruling in the EPA's favor could permanently bar future administrations from regulating greenhouse gas emissions from the transportation sector, the largest single source of such emissions in the country.

The EPA finalized its rescission on Feb. 12, 2026, describing the action in official materials as the "single largest deregulatory action in U.S. history" and projecting cost savings of more than 1.3 trillion U.S. dollars. The rule, published on Feb. 18 and effective April 20, simultaneously repealed all federal greenhouse gas emission standards for light-, medium- and heavy-duty highway vehicles and engines.

The plaintiff coalition argues that the rescission violates both the CAA and the Administrative Procedure Act, relying on a statutory interpretation the Supreme Court previously rejected. The plaintiffs further contend that the agency ignored decades of peer-reviewed scientific evidence affirming that greenhouse gas emissions endanger public health and welfare, including findings from the National Academy of Sciences submitted during the public comment period.

The EPA defended its action by invoking the "major questions doctrine," a legal principle requiring clear congressional authorization for agency actions of vast economic and political significance. The agency argues that the Clean Air Act's provisions governing motor-vehicle emissions were intended to address pollutants with local or regional impacts, rather than global climate phenomena.

The rescission comes as the United States has also withdrawn from the United Nations Framework Convention on Climate Change. ■