April 2 (Bloomberg) -- The U.S. Supreme Court ordered Bush administration environmental officials to reconsider their refusal to regulate greenhouse-gas emissions, giving a boost to advocates of stronger action against global warming.

The justices, voting 5-4, today said the Environmental Protection Agency didn't follow the requirements of the Clean Air Act in 2003 when it opted not to order cuts in carbon emissions from new cars and trucks.

``EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change,'' Justice John Paul Stevens wrote for the majority.

The ruling doesn't necessarily mean the EPA will have to impose new regulations. Still, it adds to growing pressure on the administration, which has resisted mandatory limits on carbon emissions. The decision is a setback for General Motors Corp. and other automakers and for utilities with coal-fired plants, including American Electric Power Co. and Southern Co.

Environmentalists and 12 states, including California and Massachusetts, are seeking to force the federal agency to limit emissions from new cars and trucks. New York is leading a separate state effort to curb power-plant emissions.

The decision also helps efforts by states including California to enact their own climate-change regulations. In fighting those rules, automakers have pointed to the EPA's conclusion that carbon dioxide isn't an ``air pollutant'' subject to regulation under the Clean Air Act. The majority today rejected the agency's interpretation, saying greenhouse gases are air pollutants.

`Blow for GM'

``States are fighting on multiple fronts against General Motors and this definitely is a blow for GM,'' California Attorney General Jerry Brown said in an interview.

The Alliance of Automobile Manufacturers, which represents GM and backed the Bush administration in the case, said in a statement that it will work with Congress and the administration for a ``national economy-wide approach to addressing greenhouse gases.''

White House spokeswoman Dana Perino said the administration is working to make cars more efficient and to mandate alternative fuels to reduce gasoline consumption by 20 percent in 10 years.

``In that regard, we are regulating the vehicle sector,'' she told reporters.

Environmentalists hailed the decision. ``The ruling is a total rejection of the Bush administration's refusal to use its existing authority to meet the challenge posed by global warming,'' Carl Pope, the Sierra Club's executive director, said in a statement.

Four Dissenters

Chief Justice John Roberts and Justices Antonin Scalia, Clarence Thomas and Samuel Alito dissented.

Roberts said the court lacked constitutional power to second-guess the agency at the behest of states and environmental groups. The majority's reasoning ``has caused us to transgress the proper -- and properly limited -- role of the courts in a democratic society,'' he wrote.

Scalia said the court ``has no business substituting its own desired outcome for the reasoned judgment of the responsible agency.''

Justices Anthony Kennedy, Ruth Bader Ginsburg, David Souter and Stephen Breyer joined Stevens in the majority. Kennedy has become the court's swing vote on environmental cases. Last year, he cast the deciding vote in a ruling that limited the scope of the Clean Water Act.

EU Criticism

Though having less than 5 percent of the world's population, the U.S. produces 22 percent of the planet's man-made carbon- dioxide emissions, more than any other country, according to Energy Department figures.

Earlier this year a United Nations panel concluded that global warming is ``unequivocal'' and that human activities are ``very likely'' the primary cause. The report forecast a rise in sea levels of 7 to 23 inches by 2100.

Separately today, European Union Environment Commissioner Stavros Dimas blasted the U.S. for ``having a negative attitude in international negotiations'' on climate change. ``It is absolutely necessary that they move,'' Dimas said.

Congressional Democrats are pushing legislation to cap emissions. In January, five other U.S. power producers endorsed a proposal that would reduce emissions from electricity providers by 25 percent of projected levels by 2020.

``Today's Supreme Court decision leaves no doubt that the federal government has the authority and duty to act to address global warming,'' Democratic Senator Jeff Bingaman of New Mexico, the chairman of the Energy and Natural Resources Committee, said in a statement.

Science Debate

The EPA said in 2003 that it wouldn't regulate greenhouse- gas emissions from new vehicles, pointing to ``substantial scientific uncertainty'' about the effects of climate change on human health and the environment and about the best means to address the issue.

The Supreme Court today said that reasoning was insufficient to meet the statute's requirements. The Clean Air Act instructs the EPA to regulate air pollutants that ``may reasonably be anticipated to endanger public health or welfare.''

``If the scientific uncertainty is so profound that it precludes EPA from making a reasoned judgment as to whether greenhouse gases contribute to global warming, EPA must say so,'' Stevens wrote.

The agency also said unilateral EPA regulation would undermine the president's ability to persuade developing countries to cut their emissions.

Stevens said foreign policy considerations were for the State Department to assess, not the EPA.

Standing to Sue

The Bush administration said the Clean Air Act confers broad discretion on the EPA to decide whether to regulate. The government also contended that the states lack the legal standing to sue because they can't show they would reap any environmental benefit from EPA regulation.

Stevens rejected that argument. He pointed to Massachusetts's contention that its coastline is already being damaged by rising sea levels.

``A reduction in domestic emissions would slow the pace of global emissions increases, no matter what happens elsewhere,'' Stevens wrote.

The case is Massachusetts v. EPA, 05-1120.

To contact the reporter on this story: Greg Stohr in Washington at gstohr@bloomberg.net.

Last Updated: April 2, 2007 16:14 EDT