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KAI RYSSDAL: And it's official. Carbon dioxide is a pollutant. The Supreme Court says so. The Bush Administration had been arguing the Environmental Protection Agency doesn't have the authority to regulate greenhouse gases. Like carbon dioxide when it comes out of automobile engines.

But five members of the High Court read the Clean Air Act differently. Marketplace's Steve Henn reports the EPA case was just one of two environmentally-friendly rulings the Justices handed down today.

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STEVE HENN: Justice may be blind, but the Supreme Court is seeing green. Environments say this decision was a long time coming.

NORMAN DEAN: I think this is one of the most important environmental decisions in the past quarter-century.

Norman Dean is executive director of Friends of the Earth, one of the original plaintiffs in the suit.

DEAN: Back in 1999, we petitioned EPA to issue a rule regulating greenhouse gas emmissions from motor vehicles.

For years, the Bush Administration had argued the EPA didn't have the right to limit auto-emissions of carbon dioxide under the Clean Air Act. In today's 5 to 4 decision, the high court disagreed. Justice Stevens wrote the Bush Administration's claims were quote "arbitrary, capricious, or otherwise not in accordance with law."

But will the EPA regulate greenhouse gases? Scott Segal, a lobbyist for the Electric utilities and petroleum refiners, notes today's decision stops short of ordering the EPA to do so.

SCOTT SEGAL: The case does not mandate that the Environmental Protection Agency regulate carbon dioxide. What it says is that the agency did not do a very good job of explaining itself.

Still, both sides agree this decision increases pressure on the Bush Administration to act. Score one for the greens.

In another potential victory for environmentalists, the high court also sided with federal regulators trying to force Duke energy to clean up old, coal-burning power plants.

In Washington, I'm Steve Henn for Marketplace.