Last week, the U.S. Supreme Court came face-to-face with what some have called ``the greatest challenge of our generation'' -- global climate change -- and may well blink.

More than two dozen states, cities and non-profit organizations challenged the federal government's refusal to regulate the greenhouse-gas emissions that are heating the Earth's atmosphere. At Wednesday's hearing in Massachusetts vs. EPA, several justices on the high court indicated a serious reluctance to wade into global-warming issues in even the most minimal way. With the future of the planet hanging in the balance, it is now time for Congress to act, again.

Carbon dioxide and other greenhouse gases from motor vehicles and power plants are the main culprit behind the Earth's rapidly rising temperature. These pollutants trap heat in the upper atmosphere and radiate it back to the Earth's surface. Elevated surface temperatures result in accelerated melting of the polar ice, rising sea levels, more chaotic and intense weather systems and, ultimately, potential human, economic and ecological dislocation on a truly massive scale.

There is no longer any serious debate that global warming is both occurring and potentially catastrophic. The only real question is how we, as a nation, will choose to address it. With only 5 percent of the world's population, the United States is responsible for a quarter of all global greenhouse-gas emissions. The lawsuit seeks to force the Bush administration to take the first small steps toward regulating greenhouse-gas pollution from motor vehicles. To date, the federal Environmental Protection Agency has steadfastly resisted action, arguing that it has neither the legal authority nor the scientific support to do so.

The administration is wrong on both counts. Broadly drawn by Congress, the Clean Air Act commands EPA to regulate any ``air pollution which may reasonably be anticipated to endanger public health or welfare.'' In declining to follow this congressional directive, the EPA ignores the overwhelming body of science on the impacts from global climate change. The agency offers continued ``uncertainty'' and a preference for alternative approaches as its justification. These excuses do not stand up to scrutiny.

Until now, the EPA has routinely regulated air pollutants even in the face of residual scientific uncertainty, and has done so with enormous success. For instance, three decades ago, a strenuous scientific debate raged over the question of whether leaded gasoline caused elevated lead levels in humans. The stakes were high because young children are extremely vulnerable to the debilitating neurological effects of lead exposure.

Despite the scientific uncertainty and the industry's vocal concerns about the costs of complying, the EPA acted decisively in the mid-1970s to begin phasing out lead in gasoline. The agency explained, correctly, that Congress intended it to follow a preventive or precautionary approach to protecting the public health and welfare from air pollution. Over the next few years, lead levels in human populations dropped precipitously in lock step with the phase-out.

In much the same way, the EPA administrator under former President Bush used his Clean Air Act mandate to lead the international community in the accelerated phase-out of ozone-depleting substances. Today, the once-frightening hole in the Earth's protective ozone layer is on the mend, although scientists believe that full recovery will take 60 years.

On greenhouse-gas emissions, however, the present administration has changed course dramatically and thrown caution to the wind. It says it prefers a different approach, but offers nothing in the way of alternatives. President Bush has rejected the Kyoto Protocol on global climate change, an international treaty signed by more than 160 other nations. And the administration recently issued new, ludicrously low fuel-economy rules for SUVs and light trucks that dismissed global warming concerns out of hand.

Frustrated states and local governments across the nation have begun to act in the federal void. California, for example, recently adopted regulations to reduce greenhouse-gas emissions from motor vehicles, and just two months ago enacted additional legislation to cut back the state's industrial greenhouse-gas emissions to 1990 levels. Ten other states have followed California's lead in adopting tailpipe emission standards for greenhouse gases, even as the auto industry tries to invalidate California's rules through the courts. Incredibly, the administration has sided with the industry, arguing that only the federal government can regulate greenhouse gases, even though it has refused to do so.

Unfortunately, the Supreme Court seems primed to dismiss Massachusetts vs. EPA on the theory that nobody, not even states with thousands of miles of coastal land at risk from rising sea levels, has ``standing'' to challenge EPA's inaction. But even if the court reaches the merits of the case, the very best that the challengers can hope for is to be sent back to EPA for more years of delay.

Leading climate scientists tell us that ``business as usual'' is not good enough. We stand at a tipping point, on the brink of global calamity. According to James Hansen, director of the NASA Goddard Institute for Space Studies, just 10 more years on the present trajectory will commit us to a bleak future of ``large-scale disastrous climate impacts for humans as well as for other inhabitants of the planet.''

Thus, whatever the outcome of the pending Supreme Court case, one thing has become crystal clear: Congress should act now to reaffirm the basic principles of the Clean Air Act. It should direct EPA to regulate the greenhouse-gas emissions that pose an almost unimaginable threat to our collective future. And it should do so with all deliberate speed.