But environmentalists hope a ruling in their favor would force changes in the policies of President George W. Bush's administration, which has refused to ratify the Kyoto Protocol on reducing greenhouse gas emissions blamed for global warming.

The plaintiffs in the case include several US state governments who argue that the EPA should use its authority to regulate greenhouse gas emissions under the 1963 Clean Air Act.

But the federal agency has refused, arguing that the act, which does not address global warming, does not require it to regulate greenhouse gas emissions.

The plaintiffs, who have lost in a lower court and on appeal, are joined by religious groups, scientists, ski resorts, businesses as well as former EPA officials, who say the stakes are high.

Auto makers are backing the EPA and have been joined by nine other states.

At Wednesday's hearing, the Supreme Court's conservative members seemed to recoil at the idea of government regulation, while their liberal colleagues openly embraced it. Conservative Justice Antonin Scalia insisted that the greenhouse gases in question represent "only" six percent of the world total and that they pose no "imminent harm," agreeing with Deputy Attorney General Gregory Garre, who represented the EPA.

Liberal Justice David Souter slammed the government's contention that cutting greenhouse gas emissions on new US-made cars would only have a very marginal effect on global warming.

"Let's assume the rest of the world doesn't do anything about it, which I think is not very reasonable ... the question is not to stop global warming but to reduce it," Souter said.

In a more technical argument, Scalia said greenhouse gas emissions affected the upper atmosphere and were not part of lower atmosphere pollutants the EPA was charged with regulating.

Massachussetts deputy attorney general James Milkey, who represented the plaintiffs, dismissed the distinction noting that the EPA regulates emissions that cause acid rain, which does not pollute the lower atmosphere but causes harm upon reaching the ground.

Milkey went on to say that any limitation of greenhouse gas emissions was beneficial, because his state stood to lose more than 4.5 meters (15 feet) of land all along its coastline if the sea level should rise 30 centimeters (one foot).

Over the next few months of deliberations, it appears the Supreme Court's decision will once again depend on Justice Anthony Kennedy, who holds the moderate-conservative swing vote and who did not voice an opinion Wednesday.

Law professors close to the administration said in an amicus brief that global warming could not be resolved with a judicial ruling, while climatologists argued that the effects of greenhouse gases on the climate have not been proven.

In another brief, climatologists close to the Bush administration said: "The mere existence of human-caused climate change itself would not compel regulation. Rather, it is necessary to demonstrate that regulation will not cause more harm than good to human health and welfare."

The United States is the world's biggest single contributor to man-made global warming, accounting for a quarter of global emissions of greenhouse gases.

Bush has refused to ratify the Kyoto Protocol, saying it would hurt the US economy, and is strongly opposed to its approach of binding cuts, also called caps, in emissions.

Instead, he has promoted voluntary action, backed by some incentives for cleaner energy sources and gains in energy efficiency.

Kyoto commits industrialized countries that have signed and ratified the treaty to trim greenhouse gases by 2012, as compared with a 1990 benchmark. It does not require larger developing countries to make these pledges, an approach that Bush says is unfair.

Some US states have implemented their own regulations, such as California, which in September mandated a 25 percent reduction in carbon dioxide emissions by 2020 and sued six US and Japanese auto makers for their alleged contribution to global warming.

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