President Barack Obama's directive on cars and climate change Monday sent the clearest signal yet the administration plans to regulate the emissions blamed for global warming.
The Bush administration repeatedly refused to use existing law to control greenhouse gases, despite increasing scientific evidence that the Earth is warming and court rulings that said the government has the authority to act.
But Obama changed direction by ordering the Environmental Protection Agency to reconsider California's request for permission to reduce greenhouse gases from automobiles.
Federal regulations on carbon dioxide from vehicles could soon follow, then limits on emissions from refineries and industrial plants.
"There is little question that this is heading in the direction of federal regulation of carbon dioxide," said Michael Gerrard, director of the Center for Climate Change Law at Columbia University.
Greenhouse gases from automobiles currently are not regulated by the federal government, and the Bush administration opposed state efforts to set their own restrictions, despite requests dating back to 1999.
The Bush administration instead focused on improvements to fuel economy as a way to reduce greenhouse gas emissions, since cars and trucks burning less fuel would generate less pollution.
On Monday, in a separate action, Obama directed the Transportation Department to set new fuel economy standards for 2011 model-year automobiles.
Both actions will reduce greenhouse gases but could put the states and the federal government on slightly different courses.
The federal Clean Air Act has always allowed states to set stricter standards than the federal government for refineries, factories and other stationary sources of pollution. However, it bars states from setting more stringent pollution standards for motor vehicles because of the problems it might cause automakers.
California's standard, which requires a waiver from the EPA under the Clean Air Act, would require SUVs, minivans and cars starting in model year 2009 to reduce their emissions of greenhouse gases by 30 percent by 2016.
Initially, the mandate could be achieved by using different paints and installing electric steering, but eventually fuel economy would have to be improved. To achieve the reductions, the fleetwide average for the state would have to be 35.7 miles per gallon by 2016, and 42.5 miles per gallon in 2020.
The 2007 energy bill signed into law by President George W. Bush set a 35 miles per gallon fuel economy target by 2020.
Industry representatives said if EPA grants the waiver it would trigger the agency to regulate greenhouse gases under the Clean Air Act. The Bush administration refused to use the law, saying that it was the wrong tool to address global warming and would cripple the economy.
"Such a move would put the EPA one step closer to making carbon dioxide 'subject to regulation' under the Act," said William Kovacs, vice president of environment, technology and regulatory affairs at the U.S. Chamber of Commerce. "This would almost certainly extend well beyond cars and trucks."
Obama's announcement Monday was welcomed by environmentalists and Democrats who had accused the Bush administration of reversing the recommendation of its own scientists when it denied California's waiver request last year.
"When the California waiver is approved, as it is almost certain to be, states will be free to apply the letter and the spirit of the Clean Air Act in the manner in which it was intended," said John DeCock, president of Clean Water Action.
Sixteen states and the District of Columbia already have adopted or are considering adopting California's standards. The states are Arizona, Connecticut, Florida, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Washington, Utah and Colorado.
Even if most of these states adopt the California standard, it will reduce U.S. greenhouse gas emissions by about 3 percent and worldwide greenhouse gas emissions by only 0.6 percent, according to a 2007 report by the Congressional Research Service.
Environmentalists already are pressing the EPA to regulate greenhouse gases on other fronts, including emissions from ships and offroad vehicles, such as ATVs and snowmobiles.