The Environmental Protection Agency on Friday unveiled a finding that greenhouse gases endanger human health and welfare, opening the door to federal regulation of carbon dioxide as a pollutant.

The finding does not automatically trigger new carbon rules but could allow the EPA to move forward with limiting greenhouse gas emissions under the federal Clean Air Act.

Rising levels of greenhouse gases are the unambiguous result of human emissions, and are very likely the cause of the observed increase in average temperatures and other climatic changes, the EPA proposal said.

Environmentalists applauded the decision, while affected industries expressed concern.

* With this step, Administrator Lisa Jackson and the Obama administration have gone a long way to restore respect for both science and law, David Doniger of the Natural Resources Defense Council said in a statement.

* Business groups, including the U.S. Chamber of Commerce, have warned that the U.S. economy could grind to a halt if the EPA were to begin regulating carbon.

Bruce Braine, a vice president for policy analysis at American Electric Power, the country's largest burner of coal for electricity generation, said that AEP would rather see greenhouse gases regulated by legislation.

The Clean Air Act was never really designed with carbon in mind, it was designed for air pollutants in the conventional sense, like acid rain and smog components and mercury and lead that are easier to control on a local basis, he said.

* Joe Mendelson, global warming policy director at the National Wildlife Federation, said the EPA decision is historic and a game-changer for climate policy that will have political and policy repercussions domestically and abroad.

This is the single largest step the federal government has taken to fight climate change, he said.

* Charles Drevna, president of the National Petrochemical and Refiners Association, said such regulation would have an enormous impact on every facet of the economy, businesses large and small, as well as on the general population.

Before moving forward with regulation, the United States must ensure that other major global contributors are similarly committed to reducing their ambient greenhouse gas concentrations, he added.

* The move to regulate carbon through the EPA will likely place pressure on Congress to pass legislation to limit greenhouse gases. The House Energy and Commerce Committee hopes to clear such a bill by the end of May.

With the economy faltering, such legislation faces an uphill battle as detractors say it will raise energy costs for businesses and consumers.

* The White House has said it prefers for Congress to pass a bill that caps carbon emissions and requires companies to acquire permits to release carbon into the atmosphere.

Today's action by EPA should give the Obama administration more ammunition to get Congress to do its job correctly, said Frank O'Donnell, president of environmental group Clean Air Watch. The Obama administration now has the legal equivalent of a .44 Magnum (firearm). The bullets aren't loaded yet, but they could be.

* The EPA will accept public comments on the finding for 60 days. The agency will also hold two hearings on the proposal.