This story was updated at 12:30 p.m. EST.

The Bush administration must consider regulating carbon dioxide emissions from vehicles, the Supreme Court said today in its first decision addressing legal issues surrounding global warming.

In a 5-4 ruling, the court sided with 11 states and 13 environmental groups that sued U.S. EPA to force the agency to regulate greenhouse gas emissions from new cars and trucks.

The Supreme Court's verdict in Massachusetts v. EPA prompts what could be the first nationwide regulations addressing global warming.

Arguments in the case hinged on the scope of the Clean Air Act and technical legal questions about whether Massachusetts and other state plaintiffs had standing to bring the lawsuit (Greenwire, Nov. 29, 2006).

Writing for the majority, Justice John Paul Stevens today rejected EPA's position that the Clean Air Act does not grant it the authority to issue mandatory regulations addressing climate change -- and that even if EPA held that authority, a federal rule would do little to stem any effects from climate change on Massachusetts or the other petitioners, given that U.S. auto emissions only account for about 7 percent of global fossil fuel emissions and about 30 percent of domestic emissions.

"EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change," Stevens wrote, concluding that the agency's actions were "arbitrary, capricious ... or otherwise not in accordance with law."

Stevens dismissed questions of standing that divided the U.S. Circuit Court of Appeals for the District of Columbia when it considered the case (Greenwire, Oct. 25, 2006).

Sufficient scientific evidence exists to suggest that Massachusetts is already dealing with rising sea levels caused by climate change, a side effect that is only expected to exacerbate as the planet warms, Stevens wrote. "The risk of catastrophic harm, though remote, is real" and warrants granting Massachusetts standing to bring its case against EPA, the justice said.

"That these climate-change risks are 'widely shared' does not minimize Massachusetts' interest in the outcome of this litigation," Stevens added.

Joining him in the majority opinion were Justices Stephen Breyer, Ruth Bader Ginsburg, Anthony Kennedy and David Souter.

Chief Justice John Roberts and Justices Samuel Alito, Antonin Scalia and Clarence Thomas offered two dissents, one written by Roberts and one by Scalia.

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Environmentalists praised the ruling but said it throws decisions on global warming back to Congress.

"This is a very straightforward case and the only drama really was the question of standing," said David Bookbinder, an attorney for Sierra Club, a plaintiff. "The overarching message from all this really is that it's up to Congress to come up for a comprehensive scheme for regulating greenhouse gases."

That is especially true given a series of related cases moving through the federal court system, Bookbinder said.

In one notable case, a federal judge ruled in January to postpone a lawsuit challenging a California law that limits tailpipe GHG emissions, pending the Supreme Court's ruling in Massachusetts v. EPA. Judge Anthony Ishii of U.S. District Court for the Eastern District of California also issued an injunction preventing California from enforcing its law without permission from EPA (Greenwire, Jan. 17).

But pursuing congressional action is likely to be tough even with the Supreme Court win, said Alden Meyer, director of strategy and policy at the Union of Concerned Scientists, also a plaintiff.

The verdict "demolishes" the Bush administration arguments for waiting to take action on climate change and "adds momentum" to an issue already seeing widespread congressional debate, Meyer said. But "it's still a tough road to hoe to get a majority in the House, 60 votes in the Senate and president's signature," he said.

Immediate reaction from industry was muted. The Alliance of Automobile Manufacturers, part of an industry coalition that supported the Bush administration, refused to talk about the specifics of the lawsuit.

But in an e-mailed statement, AAM President Dave McCurdy said his group "believes that there needs to be a national, federal, economy-wide approach to addressing greenhouse gases."

Today's ruling "says that the U.S. Environmental Protection Agency will be part of this process," McCurdy said, adding that AAM "looks forward to working constructively with both Congress and the administration."

Click here for the court's opinion.

Click here for a transcript of the oral arguments.