U.S. EPA appears to be on the verge of proposing an "endangerment" finding for carbon dioxide (and likely other greenhouse gases) under the Clean Air Act in response to the Supreme Court's holding in Massachusetts v. EPA, 549 U.S. 497 (2007), that U.S. EPA has authority under the Clean Air Act to regulate carbon dioxide emissions from new motor vehicles if U.S. EPA finds that such emissions "endanger public health or welfare." Under the Bush Administration, U.S. EPA issued an Advanced Notice of Proposed Rulemaking ("ANPR") that explored the issue of endangerment, as required by the Supreme Court, but also questioned whether U.S. EPA could effectively regulate such emissions under the Clean Air Act's existing programs.

New U.S. EPA Administrator Lisa Jackson sent a proposed endangerment finding to the Office of Management and Budget on March 20, 2009. The text of the proposed endangerment finding has not been made public, and U.S. EPA so far has refused to provide further details. An internal U.S. EPA document leaked in February 2009 suggests that the Agency intends to find that endangerment exists on both public health and welfare grounds. However, the finding, as described in the leaked document, would not take the next step of actually proposing to limit greenhouse gas emissions from specific sources.

As demonstrated in the pre-Obama ANPR, any regime to regulate greenhouse gases without new legislation would be fraught with legal and technical uncertainty. With respect to stationary sources of air pollution, the problems arise primarily under four programs established by the Clean Air Act: the National Ambient Air Quality Standards, the New Source Performance Standards, the Prevention of Significant Deterioration requirements, and the Title V permitting program for major stationary emission sources. These programs are ill-suited to addressing pollutants such as carbon dioxide that are emitted in significant quantities by a large number of sources and distributed equally throughout the global atmosphere.

According to the internal U.S. EPA document, once the proposed endangerment finding receives approval from the Office of Management and Budget, the U.S. EPA Administrator intends to publish a proposed rule in the Federal Register. At that point, companies and other interested parties would have 60 days to review the proposal in detail and submit comments to U.S. EPA.