A three-judge panel of the U.S. Court of Appeals for the District of Columbia said the Environmental Protection Agency acted properly when it rejected a nonprofit group's petition. The group had asked EPA to impose new controls on carbon dioxide and other automobile pollutants that scientists say trap heat in the atmosphere like a greenhouse.

Tom Reilly, the Massachusetts attorney general, said the states probably would ask for the full appeals court to hear the case.

"With each day of inaction, the problem of global warming worsens," Reilly said. "We will continue to fight to compel the federal government to use its legal authority to address this serious problem."

After EPA rejected the group's petition, 12 states and several cities argued that EPA should regulate those gases under the Clean Air Act and hadn't justified its decision not to. The three federal appeals judges ruled otherwise.

Judge A. Raymond Randolph, writing for the panel, and Judge David Santelle, who disagreed with Randolph on some of the issues in the case, each sidestepped the larger question of whether EPA lacks authority to order reductions in greenhouse gases.

In August 2003, the EPA's top lawyer issued an opinion that the agency lacked that authority, a reversal of a Clinton administration legal opinion that the gases should be regulated.

The Bush administration used that opinion to justify denying the request from the nonprofit group, International Center for Technology Assessment, and other groups seeking new federal controls on auto emissions.

Randolph said the court assumed for the sake of argument that EPA has the authority to regulate greenhouse gases as air pollutants but that "the question we address is whether EPA properly declined to exercise that authority."

Randolph said the court should defer to a federal agency's judgment in policy questions that are "on the frontiers of scientific knowledge."

Judge David Tatel also disagreed with Randolph on some of the issues. Most notably, he found that greenhouse gases "plainly fall within the meaning" of air pollutants that should be regulated.

If the EPA administrator finds that the gases contribute to air pollution that puts the public's health or welfare in danger, Tatel wrote, "then EPA has authority — indeed, the obligation — to regulate their emissions from motor vehicles."

States challenging the EPA's rejection of that petition were California, Connecticut, Illinois, Maine, Massachusetts, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington, along with the U.S. territory of American Samoa and the cities Baltimore, New York and Washington, D.C.

Reilly said the states were disappointed by the ruling but "heartened" that Tatel, the only judge to look at the issue of EPA's legal authority on greenhouse gases, "firmly rejected EPA's claim."