The Supreme Court cleared the way Monday for a more aggressive attack by government on global warming, which could include the first national rules to limit carbon dioxide emissions from new cars, trucks and power plants.
In a 5-4 decision, the high court rebuked the Bush administration and ruled that so-called greenhouse gases -- like carbon dioxide -- were air pollutants subject to federal regulation.
President Bush and his aides, allied with automakers, argued that federal officials did not have the power to set mandatory limits on greenhouse gas emissions.
The court's ruling knocked down a legal barrier that kept California and other states from requiring reduced carbon emissions by new vehicles starting in 2009.
Gov. Arnold Schwarzenegger praised the decision and said he was "very encouraged."
Under the Clean Air Act, California won the right to adopt its own regulations limiting emissions by new vehicles, but only if the Environmental Protection Agency issued a waiver saying the state's rules complied with the law. No waiver has been issued.
Schwarzenegger said he expected the EPA "to move quickly now in granting our request for a waiver, which will allow California and [the] other states that have adopted our standards to set tougher vehicle emissions levels."
Auto industry lawyers have sued in federal court in Fresno, arguing that California's emissions rules conflicted with the Clean Air Act. The case was put on hold, awaiting the Supreme Court decision issued Monday.
"We think this is the end of the automakers' case," said David Bookbinder, a Sierra Club lawyer.
Scientists have linked the rise in greenhouse gas emissions to a steady and potentially catastrophic increase in air temperatures. The administration maintained that the gases were not air pollutants as defined by the Clean Air Act.
The measure, passed in the 1970s, targeted specific pollutants, such as lead.
In its ruling, the Supreme Court agreed that global warming represented a different kind of air pollution problem. Gases such as carbon dioxide, once released into the atmosphere, "act like a ceiling of a greenhouse, trapping solar energy and retarding the escape of reflected heat," the court said.
The majority opinion, written by Justice John Paul Stevens, said that under the Clean Air Act, the EPA was required to regulate the emission of "any air pollutant" that was likely "to endanger public health or welfare."
He said the word "welfare" was defined broadly to include "effects on the climate and weather."
In scolding the EPA for not moving to regulate greenhouse gases, he said the emissions fit well within the law's definition of air pollutants and that the agency had "the statutory authority to regulate the emission of such gases from new motor vehicles."
The court did not say that the EPA must set national emissions standards for motor vehicles. But it made clear the agency must make its case if it chooses not to act.
"Under the clear terms of the [law], EPA can avoid taking further action only if it determines that greenhouse gases do not contribute to climate change or it provides some reasonable explanation" why regulations are not needed, Stevens said.
New regulations limiting greenhouse gases would probably force automakers to produce vehicles that burn less gasoline.
Roberts, in his dissent, said that even though global warming might be "the most pressing environmental issue of our time," how to deal with it should be resolved by Congress and the president, not the court.
A former Bush administration lawyer at the EPA said she was disappointed by the ruling.
"I agree with Chief Justice Roberts that the court stepped into a policy issue that is better left to Congress," Ann R. Klee said. "We need a global political solution to the global warming problem."
Automakers have said they are producing more fuel-efficient vehicles, and that federal limits on emissions would put them at a competitive disadvantage.
Dave McCurdy, president of the Alliance of Automobile Manufacturers, said the industry wanted to work "constructively" with Congress and the administration on a national "economy-wide approach to addressing greenhouse gases."
Environmentalists praised the court's decision.
"Today's ruling is a watershed moment in the fight against global warming," said Carl Pope, the Sierra Club's executive director. It "sends a clear signal to the markets that the future lies not in the dirty, outdated technologies of yesterday, but in the clean energy solutions that will fuel the economy of tomorrow."
Stevens, in his majority opinion, rejected the "laundry list of reasons not to regulate" that the administration had put forth.
They included the assertion that regulations on emissions by new vehicles "might impair the president's ability to negotiate with 'key developing nations' to reduce emissions."
Stevens said, "While the president had broad authority in foreign affairs, that authority does not extend to the refusal to execute domestic laws."
He said the EPA "offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change." He said the agency's position was "arbitrary, capricious or otherwise not in accordance with law."
The ruling arose from an unusual lawsuit. Twelve states -- led by Massachusetts and California -- sued the administration after the EPA refused to take action on greenhouse gases.
The other states were Connecticut, Illinois, Maine, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington. An array of environmental groups also joined the lawsuit.
Roberts said the suit should have been thrown out because the states had no standing to sue the EPA.
Lawyers for Massachusetts said the state was slowly losing coastal land because the ocean was rising. Roberts called it "pure conjecture" that did not establish the state's right to bring the suit.
Scalia, in his dissent, said the court should have upheld the EPA's judgment that regulations on greenhouse gas emissions were not warranted.
"The court's alarm over global warming may or may not be justified," Scalia said, but "this court has no business substituting its own desired outcome for the reasoned judgment of the responsible agency."
Jennifer Wood, a spokeswoman for the EPA, said the agency was "reviewing the court's decision to determine the appropriate course of action."
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The carbon dioxide picture
The Supreme Court ruled that greenhouse gases, such as carbon dioxide, were air pollutants subject to federal regulation. California has been a leader in trying to regulate gas emissions. Carbon dioxide (CO2) makes up the bulk of greenhouse gas emissions and is largely the result of fossil fuel consumption.
Greenhouse gas emissions, 2005
Carbon dioxide: 84%
Nitrous oxide: 7%
HFCs, PFCs, SF6*: 2%
*Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)
Man-made CO2 sources from fossil fuel combustion, 2005
Does not add to 100% due to rounding
Sources: EPA, Pew Center on Global Climate Change
Graphics reporting by Julie Sheer