The Supreme Court seemed split 4-4, with Justice Anthony Kennedy on the fence, during spirited arguments on Wednesday in a potentially momentous case on global warming. Memo to Kennedy’s law clerks: The justices should order the Bush administration to come to grips with the need to curb the emissions of carbon dioxide and other greenhouse gases that contribute to what could become catastrophic climate change.

This is not to deny that curbs on emissions might be futile and thus wasteful. The costs of achieving major reductions in the near term might be prohibitive. Meanwhile, soaring greenhouse-gas emissions in China, India, and other rapidly industrializing countries could swamp any government-mandated cuts in America.

But consider the alternative. If the government keeps temporizing on this issue, we may eventually see coastal cities under water, glaciers gone, arable lands turned into desert, massive extinctions of animal and plant species, more killer storms, and other disasters. More to the point for the justices, the Environmental Protection Agency’s head-in-the-sand, we-don’t-want-to-know posture about the dangers of global warming violates the Clean Air Act.

The Court need not decide how grave the danger is, how much should be invested in reducing emissions, or even whether EPA should regulate them at all. It need only require the agency to take its head out of the sand. Specifically, the plaintiffs — Massachusetts, 11 other states, some cities, and others — ask only that the justices order EPA to make an official, yes-or-no finding, after formal consideration, that answers this question: Are greenhouse gases a kind of air pollution that, by contributing to climate change, may — in the words of the Clean Air Act — "reasonably be anticipated to endanger public health or welfare"?

EPA has already admitted that the most likely answer is yes, given the scientific near-consensus that greenhouse-gas emissions contribute to global warming. But EPA knows that an official "yes" would trigger a Clean Air Act mandate that the agency regulate such emissions. President Bush does not want to do that.

Hence the EPA’s we-don’t-want-to-know posture about the greatest environmental threat facing the nation and the world. The Clean Air Act should not be so easily evaded.

To be sure, the danger posed by greenhouse gases was unknown when Congress passed the act more than 30 years ago. But it was designed to be adaptable to the need to combat precisely such previously unknown dangers. The plain language of Section 202(a)(1), concerning emissions from new motor vehicles, makes this clear. (The case before the Court involves only motor vehicles. But the decision will also resolve a similar case involving power plants that is pending in a lower court.)

Section 202(a)(1) directs EPA to "prescribe … standards applicable to the emission of any air pollutant from … new motor vehicles … which in [EPA’s] judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare." Another provision defines "welfare" broadly as including "weather" and "climate."

The natural reading is that EPA must limit emissions unless it finds that they do not "contribute to" changes in climate or weather. Even the Bush EPA would have a hard time finding that. The administration claims three escape hatches from the inexorable logic of this language.

• First, it makes a strained argument that EPA lacks power to regulate greenhouse-gas emissions because they are not "air pollutants." This stance confounds the plain language of Section 302(g) of the Clean Air Act:

"The term ‘air pollutant’ means any air-pollution agent … , including any physical [or] chemical substance or matter which is emitted into or otherwise enters the ambient air." All four greenhouse gases — carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons — are physical or chemical substances emitted into the ambient air. The administration does not dispute this. Instead, it claims that this does not make them "air-pollution agents."

But the definition’s grammatical structure says otherwise. So do other Clean Air Act provisions that explicitly lists carbon dioxide as an air pollutant.

The administration also argues that literal interpretation must give way in cases involving issues of great economic and political consequence that Congress has specifically addressed in more-recent laws. But the six-year-old precedent on which this argument rests involved repeated ratifications by Congress of an agency’s consistent view over 60 years that it lacked statutory power to regulate tobacco products. In the global-warming case, EPA had found before Bush took office that it did have the power to regulate greenhouse-gas emissions. And Congress has never suggested otherwise.

• Second, the administration argues that even if EPA has power to regulate greenhouse-gas emissions, it is free not to do so. And it doesn’t want to, based in large part on three policy objections: Agency restrictions on motor vehicle emissions would be "piecemeal and inefficient"; the only currently available technology is greater fuel economy; and unilateral cuts might leave the U.S. without bargaining chips to induce China and others to cut their own emissions.

This last argument is glaringly inconsistent with Bush’s own foreign policy. He eschews bargaining with other countries to reach mutual, mandatory reductions, preaching voluntary action instead. In any event, the Clean Air Act does not become optional just because the president has policy objections to its requirements. The law says that EPA "shall" regulate any emissions that affect climate or weather (among other things). While giving the agency broad discretion as to how and how much to regulate, it does not allow EPA to refuse even to consider whether a pollutant endangers public health or welfare.

"On EPA’s theory," the plaintiffs point out in their reply brief, "the agency could have right in front of its eyes conclusive evidence that climate change (for example) is causing and will, for the indefinite future, continue to cause an environmental catastrophe, and so long as it did not take a close look at that evidence, it would have absolutely no obligation to do anything to mitigate the threat."

Another reason given by EPA for inaction is the need to know more about the extent of global warming and the options for addressing it. It cites "scientific uncertainties" about the specific effects of greenhouse-gas emissions, quoting from a definitive 2001 report by the National Research Council. But the report’s authors have complained, Justice John Paul Stevens pointed out, that EPA had made misleading use of "selective quotations" and that "there was far less uncertainty than the agency purported to find."

•The administration’s third escape hatch is its argument that even if EPA is violating the Clean Air Act, the justices are powerless to do anything about it. Rather, they must dismiss the lawsuit because no plaintiff is sufficiently harmed by EPA’s inaction to confer legal standing to sue.

This is the issue on which the justices split most visibly during the oral argument. Three conservatives supported the administration’s no-standing position, seeming at times almost to suggest that only proof of imminent cataclysm would persuade them. (Clarence Thomas was silent, as usual.) The four liberals disputed the no-standing position. Justice Kennedy’s comments and questions were hard to read.

The liberals have the better of the argument.

The crux of EPA’s no-standing position is that U.S. motor vehicle emissions are such a small fraction (6 percent) of worldwide greenhouse-gas emissions that restrictions would do Massachusetts and other plaintiffs very little good.

It follows, says EPA, that its refusal to restrict emissions does the plaintiffs very little harm.

This may be true. But U.S. motor vehicle and power-plant emissions together come to 16 percent of worldwide greenhouse-gas emissions. And Supreme Court precedents hold that a showing of some harm to plaintiffs — even very little harm — is enough to confer standing. Massachusetts, for example, plausibly argues that it is already losing land to rising sea levels.

"They don’t have to show that it [EPA regulation] will stop global warming," as Justice David Souter stressed. "Their point is that … it will reduce the degree of global warming and reduce the degree of coastal loss." This should be enough to establish standing.

Legalities aside, the hope is that modest curbs on emissions can be achieved without undue costs; that these curbs will spur technological breakthroughs making possible much larger cuts; that other countries will adopt our technology and follow our example; and that all of this might eventually bring global warming under control.

A long shot? Perhaps. But it makes no sense to keep fiddling while our planet burns.