The Environmental Protection Agency took a big step in that direction, concluding that carbon dioxide and five other greenhouse gases are a major hazard to Americans' health. That was a reversal from the Bush administration, which resisted such a conclusion and said it would be costly for companies to meet new emission limits and therefore could harm the national economy.
"In both magnitude and probability, climate change is an enormous problem (and) the greenhouse gases that are responsible for it endanger public health and welfare," said the EPA, concluding the dangers warrant action under federal air pollution laws.
It was the first time the federal government had said it was ready to use the Clean Air Act to require power plants, cars and trucks to curtail their release of climate-changing pollution, especially carbon dioxide from the burning of fossil fuels.
The agency said the science pointing to man-made pollution as a cause of global warming is "compelling and overwhelming." It also said tailpipe emissions from motor vehicles contribute.
EPA Administrator Lisa Jackson cautioned that regulations are not imminent and made clear that the Obama administration would prefer that Congress address the climate issue through a broader "cap-and-trade" program that would limit heat-trapping pollution.
But she said it was clear from the EPA analysis "that greenhouse gas pollution is a serious problem now and for future generations" and steps are needed to curtail the impact.
Even if actual regulations are not imminent, the EPA action was seen as likely to encourage action on Capitol Hill.
It's "a wake-up call for Congress" - deal with it directly through legislation or let the EPA regulate, said Sen. Barbara Boxer, D-Calif., who chairs the Senate committee dealing with climate legislation. If Congress doesn't move, Boxer said she would press EPA to taker swift action.
Rep. Ed Markey, D-Mass., whose House Energy and Commerce subcommittee hopes to craft legislation in the coming weeks, called the EPA action "a game changer."
"It now changes the playing field with respect to legislation. It's now no longer doing a bill or doing nothing. It is now a choice between regulation and legislation," said Markey.
Republicans and some centrist Democrats have been critical of proposed cap-and-trade climate legislation, arguing it would lead to much higher energy prices. Such a measure could impose an economy-wide limit on greenhouse gas emissions but let individual companies or plants trade emission allowances among each other to mitigate costs.
House Republican leader John Boehner of Ohio called EPA's move toward regulation "a backdoor attempt to enact a national energy tax that will have a crushing impact on consumers, jobs and our economy."
But environmentalists called the EPA action a watershed in addressing climate change.
