WASHINGTON -- The Supreme Court unanimously ruled out yesterday a federal lawsuit by states and conservation groups trying to force cuts in greenhouse gas emissions from power plants.

The court said that the authority to seek reductions in emissions rests with the Environmental Protection Agency, not the courts.

EPA said in December it would issue new regulations by next year to reduce power plants' emissions of carbon dioxide, the chief greenhouse gas. The Obama administration already started controlling heat-trapping pollution from autos and from some of the largest, and most polluting, industrial plants.

But the administration's actions have come under criticism in Congress, where the Republican-controlled House has passed a bill to strip the EPA from using the Clean Air Act to regulate global warming gases. The measure failed in the Senate, but a majority there indicated they would back reining in EPA in some way.

In pushing to curtail EPA's work, Republicans have accused the administration of acting unilaterally after failing to get a bill passed to deal with the problem. The administration has said the overwhelming scientific evidence has compelled them to act under existing law.

Still, in this case, the administration sided with the power companies. Justice Ruth Bader Ginsburg, writing for the court, said the Clean Air Act gives the EPA authority to regulate carbon-dioxide emissions from power plants.

The landmark environmental law leaves no room for what Ginsburg described as a parallel track, "control of greenhouse gas emissions by federal judges."

On the other hand, Ginsburg said that the states and conservation groups can go to federal court under the Clean Air Act if they object to EPA's decision.

California Attorney General Kamala Harris said her state, one of those that sued, would watch the EPA closely. The "decision reaffirms the U.S. Environmental Protection Agency's responsibility to regulate dangerous carbon pollution," Harris said.

David Doniger, the Natural Resources Defense Council lawyer who represented the conservation groups, called on EPA to impose new regulations "without delay."

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