The Obama administration on Monday declared that greenhouse gases produced by vehicles, power plants and factories were a danger to public health, clearing the way for broad federal limits on climate-warming emissions.
The announcement by the Environmental Protection Agency is a key step in a legal process that would allow the agency to act, without Congress, to develop tough rules to control emissions of carbon dioxide and other gases that scientists blame for global warming.
"The vast body of evidence not only remains unassailable, it's grown stronger, and it points to one conclusion," said EPA Administrator Lisa P. Jackson in announcing the decision. "Greenhouse gases from human activity are increasing at unprecedented rates, and are adversely affecting our environment and threatening our health."
The EPA’s “endangerment finding” came on the opening day of an international two-week climate conference in Copenhagen aimed at hammering out an accord on reducing greenhouse gas emissions.
Though the White House said the timing of the EPA announcement and the conference was a coincidence, the finding still sent a clear message of the administration's resolve to push ahead with emission controls -- with or without Congress.
In making the announcement Monday, Jackson said the administration "will not ignore science or the law any longer."
"Look at the droughts, the flooding, the changes in diseases, the changes in migratory habits, the changes in our water cycle and climate that we now find affect human health and welfare," she said.
The finding might be the latest step in the Obama administration's carrot-and-stick strategy for keeping pressure on Congress to approve a comprehensive climate bill, while giving the president an alternative approach if the legislation bogs down.
The White House has said repeatedly that it would prefer to deal with the complex and emotion-charged issue through congressional action.
The House passed a climate bill in June that proposed a 17% reduction in carbon emissions from 2005 levels. A similar bill is pending in the Senate.
Climate legislation would affect a broad swath of the American economy; it could raise consumer prices and manufacturing costs in at least some areas and faces formidable opposition from business groups, Republican lawmakers and some Democrats.
The challenge of passing a bill is all the greater at a time when Congress is preoccupied with the even more controversial healthcare overhaul and voters seem more concerned about jobs and the economy than about long-term climate change.
As a result, President Obama, who promised action on global warming during his campaign, has moved forward on the alternative track: direct administrative action by the EPA.
Coming on the eve of Obama's trip to the climate summit in Copenhagen, the endangerment announcement gives the White House something positive to point to in the absence of congressional action.
But even with the EPA finding, the White House has not committed to pushing ahead with the regulatory process if climate legislation stalls.
Criticism of the EPA announcement came quickly.
"The elected Congress, not an administrative agency, should write the laws governing the economy's response to climate change," Sen. Lamar Alexander (R-Tenn.) said.
Alexander, chairman of the Senate Republican Conference, added: "The costs of compliance with the EPA's unilateral announcement today could run into hundreds of billions of dollars a year -- costs borne by average Americans through huge increases in their electric bills and at the gas pump. This is an especially bad idea when unemployment is at 10%."
Jeff Holmstead, EPA air administrator in the George W. Bush administration, expressed concern that new paperwork requirements would bring new construction "to a standstill."
"If the agency's eventual regulatory approach is mishandled, it could result in profound consequences for the economy with little environmental benefit to show for it," Holmstead said.
Climate legislation is also strongly opposed by the U.S. Chamber of Commerce, though its position has drawn criticism from some of its prominent members, with some major companies withdrawing from the organization in protest.
The endangerment finding was part of a process prompted by a Supreme Court decision in 2007 that ordered the EPA to review scientific evidence for regulating climate-altering gases under the Clean Air Act.
The Bush administration largely ignored the decision. Obama, however, had promised before taking office that he would address the issue quickly.
The long-anticipated announcement upped the ante for the administration and the Democrats in their push to pass a climate bill.
White House Press Secretary Robert Gibbs, however, cautioned not to read too much into the timing of the announcement.
"This is part of a process that started more than two years ago with a Supreme Court finding that the EPA should regulate greenhouse gases that threaten the public health because it's a pollutant," he said.
Jackson said Monday that the endangerment finding was not intended to pressure Congress and that legislation was still the best way to address climate pollution and move toward clean energy.
In economic terms, complying with new emission regulations would be much more expensive than using the "cap-and-trade" provision in the climate bill, some analysts say.
The cap-and-trade system, under which companies could buy and trade permits to cover the greenhouse gases they release, is designed to minimize costs to emitters. For example, major emitters could use permits to spread the cost of reducing pollutants over time.
If the EPA imposes rules, companies would probably have to move more quickly to make costly changes in their operations.
Even so, Jackson said, "I do not believe this is an 'either-or' proposition. I actually see this as a 'both-and.' I believe the Clean Air Act can complement legislative efforts."
She said the administration still planned to work with Congress to get a climate change bill to the president's desk.