“The Obama administration’s actions today,” Mr. Bond said, “will do more to endanger families, farmers and workers with new energy taxes and lost jobs than it does to protect the environment.”

As the E.P.A. begins the process of regulating the climate-altering substances under the Clean Air Act, Congress is writing wide-ranging energy and climate legislation that would alter, combine with or override the actions taken by the agency. Mr. Obama and Ms. Jackson have said they much prefer that Congress address global warming rather than have the E.P.A. tackle it through administrative action that could be subject to lawsuits.

When the agency announced its finding, Mr. Obama was en route from Mexico City to Trinidad and Tobago for a meeting of Western Hemisphere nations. The agency made its decision public in a news release and an 133-page explanation of the scientific and legal basis of its proposed finding.

In 2007, the Supreme Court, in Massachusetts v. E.P.A., ordered the agency to determine whether heat-trapping gases harmed the environment and public health. The case was brought by states and environmental groups to force the E.P.A. to use the Clean Air Act to regulate heat-trapping gases in vehicle emissions.

Agency scientists were virtually unanimous in determining that those gases caused such harm, but top Bush administration officials suppressed their work and took no action.

In his first days in office, Mr. Obama promised to review the case and act quickly if the findings were justified. The announcement Friday was the fruit of that review.

According to the E.P.A. announcement, the finding was based on rigorous scientific analysis of six gases  carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride  that have been widely studied by scientists. The agency said its studies showed that concentrations of the gases were at unprecedented levels as a result of human activity and that it was highly likely that those elevated levels were responsible for an increase in average temperatures and other climate changes.