WASHINGTON — Lisa P. Jackson, administrator of the Environmental Protection Agency, announced Dec. 7 her agency’s finding that greenhouse gas emissions endangered the public’s health and welfare and may be regulated by the E.P.A. under the Clean Air Act (C.A.A.). In a separate but related finding, the E.P.A. determined greenhouse gas emissions by new light-duty vehicles contribute to the endangerment of the public.

The endangerment finding itself does not impose any requirements on industry or other entities, but minimally, in combination with the finding relating to vehicles, it will allow the E.P.A. to finalize the greenhouse gas emission standards for light-duty vehicles it proposed jointly with the Department of Transportation on Sept. 19.

The endangerment finding also raised the prospect that should Congress not pass climate change legislation, the E.P.A., on its own authority under the C.A.A., might develop regulations requiring actions to reduce U.S. emissions of greenhouse gases.

The endangerment finding was announced as the United Nations world climate change summit was getting under way in Copenhagen, Denmark, a fact not missed by advocates and opponents of the administration’s climate change policy.

"This is a clear message to Copenhagen of the Obama administration’s commitment to address global climate change and a clear message to Congress of the importance of passing comprehensive climate and energy legislation," said Senator John Kerry of Massachusetts, chairman of the Senate Committee on Foreign Relations and co-sponsor of the climate change bill under debate in the Senate.

"If Congress does not pass legislation dealing with climate change, the administration is more than justified to use the E.P.A. to impose new regulations," Mr. Kerry continued. "Imposed regulations by definition will not include the job protections and investment incentives we are proposing in the Senate today. Given the potential for agency regulation, those who now aim to grind the legislative process to a halt would later come running to Congress to secure the kinds of incentives we can pass today. Industry needs the certainty that comes with congressional action on this vital issue."

Senator James M. Inhofe of Oklahoma, ranking Republican member of the Senate Committee on Environment and Public Works and outspoken skeptic about the science asserting climate change, said,

"The administration’s endangerment finding will lead to a wave of new regulations and bureaucracy that will wreak havoc on the American economy, destroy millions of jobs and force consumers to pay more for electricity and gasoline."

The National Association of Manufacturers, while working with Congress to develop "economically and environmentally sound climate action legislation," was adamant in its opposition to E.P.A. regulating greenhouse gases under the C.A.A. Keith McCoy, the NAM’s vice-president of energy and resource policy, said, "Unemployment is hovering at 10%, and many manufacturers are struggling to stay in business. It is doubtful that the endangerment finding will achieve its stated goal, but it is certain to come at a huge cost to the economy."

Jack Gerard, president of the American Petroleum Institute, asserted, "This action poses a threat to every American family and business if it leads to regulation of greenhouses gases under the C.A.A. Such regulation would be intrusive, inefficient and costly. There was no compelling deadline that forced E.P.A.’s hand on this decision. It is a decision that is clearly politically motivated to coincide with the start of the Copenhagen climate summit."

Ms. Jackson acknowledged the announcement "means we arrive at the climate talks in Copenhagen with a clear demonstration of our commitment to facing this global challenge." At the same time, the E.P.A. asserted it did not rush to issue the finding, which the agency said had been in the works since April 2007, when the Supreme Court ruled greenhouse gases are air pollutants covered by the C.A.A. The Court at that time also directed the E.P.A. to determine whether or not emissions of greenhouse gases from new motor vehicles cause or contribute to air pollution that may reasonably be expected to endanger public health or welfare.

In December 2007, in response to the court’s decision, the E.P.A. sent a draft proposal to the White House Office of Management and Budget finding that concentrations of six key greenhouse gases in the atmosphere endangered the public welfare and that emissions from new motor vehicles contribute to the problem. The Bush administration did not consider the proposal, and the E.P.A. subsequently withdrew it.

In July 2008, the E.P.A. issued an advance notice of proposed rulemaking (A.N.P.R.) regarding regulating greenhouses gases under the C.A.A., but further progress on the issue was not made until the new administration was inaugurated.

In April 2009, the E.P.A. under Ms. Jackson issued proposed findings for regulating greenhouse gases under the C.A.A. that updated the A.N.P.R. issued the previous July and took into consideration public comments on that notice. A 60-day comment period on the proposed findings was completed on June 23. More than 380,000 comments were posted.

The final endangerment finding was accompanied by a final technical support document, an 11-volume response to comments document and other supporting details.