The US government has declared greenhouse gases a human health hazard, meaning Barack Obama can sidestep congress to order cuts in emissions.
The announcement by the US Environmental Protection Agency (Epa) was clearly timed to build momentum towards an agreement at the international conference on climate change that opened in the Danish capital Copenhagen on Monday.
Lisa Jackson, who heads up the Epa, signed orders on Monday declaring six greenhouse gases, including carbon dioxide, to be pollutants that are subject to government regulation.
The Epa "is now authorised and obligated to make reasonable efforts to reduce greenhouse pollutants", Jackson told a news conference in Washington DC.
Boost for Copenhagen
"It means that we arrive at the climate talks in Copenhagen with a clear demonstration of our commitment to facing this global challenge," she added.
The declaration strengthens Obama's hand to meet US pledges on emissions when he heads to the 12-day Copenhagen summit this week, even if his critics in congress derail legislation.
Environmentalists hailed Monday's announcement and welcomed the timing, saying it will help the US president convince delegates at the 192-nation Copenhagen conference that the US is serious about addressing the problem.
Joe Mendelson, the global warming policy director at the National Wildlife Federation, said the Epa move gives Obama "additional authority that if congress doesn't pass climate legislation, the agency can put the country on the path to meet his climate goals".
Obama will pledge at Copenhagen that the US, the world's second largest emitter of greenhouse gases behind China, will cut emissions by about 17 per cent from 2005 levels, by 2020.
World leaders hope to reach an agreement at the meeting on getting rich and developing countries to share the burden in fighting climate change.
Monday's decision paves the way for the Epa to issue standards on how much carbon US factories, buildings and cars can emit, but US business groups said it would hurt the economy and endanger jobs just as the country emerges from a deep recession.
Thomas Donohue, president of the US Chamber of Commerce, the country's leading business lobby which in recent months has been fiercely critical of the Epa's attempt to address climate change, said the move would "choke off growth".
The Epa said scientific evidence clearly shows that greenhouse gases "threaten the public health and welfare of the American people" and that the pollutants - mainly carbon dioxide from burning fossil fuels - should be reduced, if not by congress then by the agency responsible for enforcing air pollution.
Prod for congress
The ruling is the culmination of government studies since April 2007, when five of the nine judges on the Supreme Court agreed that carbon dioxide and other greenhouse gases are pollutants under the Clean Air Act.
|The Epa will be able to issue standards on how much carbon cars can emit [GALLO/GETTY]
The court had said the Epa would have to determine if the pollutants posed a danger to public health and welfare before it could regulate them.
Under the administration of the former US president, George Bush, however, the Epa refused to issue the finding, despite a conclusion by its scientists that it was warranted. It said then that congress was the right place to frame action.
But on Monday Jackson said "there are no more excuses for delaying".
Still, Robert Gibbs, the White House spokesman, said on Monday that Obama "still believes the best way to move forward is through the legislative process".
Legislation by congress would be more palatable politically for Obama, because it would represent a compromise between business, politicians and other interests rather than through an imposed ruling.
If the Epa acts alone it could face a slew of legal challenges, including from business groups who say the action would overstep the administration's authority.
But the administration had pressed the Epa to prod business to support efforts in congress, and to show the world Washington is committed to fighting climate change.
John Kerry, a Democratic senator and a lead author of a climate bill before the US senate, said the Epa move was meant to spur congress to act.
"The message to congress is crystal clear: get moving," he said.