Steve Brown, the head of the Environmental Council of the States, has a smart-mouth answer to the question of what the U.S. Supreme Court's recent decision on greenhouse gases means for states. "There are two ways to answer that," he says. "One is that it doesn't mean anything, and the other is that it's a very big deal."

It seems to us he's closer the second time. What makes the 5-to-4 court decision a very big deal is its impact on a vehicle-emissions bill passed by California in 2002. The Supreme Court determined that carbon dioxide was indeed an air pollutant according to the federal Clean Air Act. Under that law, California has the authority to regulate any pollutant, as long as it can get a waiver from the Environmental Protection Agency. EPA was unwilling to give it before the court acted in early April; now, it is moving. It would appear to have little choice.

The court decision also strengthens California's hand in lawsuits that have so far blocked its emissions statute from taking effect. As long as EPA could claim that carbon dioxide wasn't a Clean Air Act pollutant, the auto companies suing California had reason to hope for a favorable judicial ruling. Now the Supreme Court appears to have wiped out their main argument. Amy Royden-Bloom, of the National Association of Clean Air Agencies, puts it flatly: "This settles that issue," she says.

The automakers don't agree: They're still contesting the case. But there's no question that the balance of expectations has shifted. What's more, any other state can follow California's lead on pollutants. Ten of them already have passed similar legislation regarding tailpipe emissions. More are likely to come.

The reason Brown says the decision might not mean so much to states is that, regardless of the Clean Air Act, many of them were moving to address climate change through their own legislation and regional pacts. The Supreme Court decision has no direct effect on such action. But it is likely to embolden more states to act and offer some cover to legislators who may have been wavering about the issue. "It lends credence," Brown admits. "The fact that the Supreme Court recognizes that it's a pollutant legitimizes the issue in states politically."

In that regard, and in breaking the five-year legal stalemate that has kept California and its followers from implementing their auto- emission regulations, the decision is clearly a very big deal indeed.