The EPA, along with 10 states, four motor vehicle trade associations and two coalitions of utility companies and other industries, maintain the agency lacks the authority to limit emissions of greenhouse gases such as carbon dioxide.

Even if EPA did have this authority, the science on global warming is so uncertain that no regulation should be made, the government's lawyer argued before the court.

At the heart of the case is a dispute over whether greenhouse gases fit the federal Clean Air Act's definition of a pollutant. The plaintiffs argue that if they do, the EPA then has the power to regulate them.

But Gregory Garre, the US deputy solicitor general who argued the government's case, said the EPA has never determined that carbon dioxide, one of the greenhouse gases emitted by motor vehicles, endangers Americans by causing global warming.

"There is substantial scientific uncertainty surrounding global climate change," Garre repeatedly told the justices.

James Milkey, a Massachusetts assistant attorney general who argued the plaintiffs' case, said the dangers from global climate change were particularly keenly felt in his state.

IMMINENT HARM

Global warming has been blamed for rising seas, which could affect 200 miles (322 km) of Massachusetts coastline, Milkey said.

"Is this harm imminent?" Justice Antonin Scalia asked. "When is the predicted cataclysm?"

"It's not so much a cataclysm, it's more like ongoing harm," Milkey replied.

Emissions have risen steeply over the past century and many scientists see a connection between this rise and an increase in global average temperatures and a related increase in extreme weather, wildfires, melting glaciers and other damage to the environment.

This case is not a debate about whether these emissions are linked to global warming. The Bush administration, and in fact President George W. Bush himself, have acknowledged this link, and Bush told a summit of industrialized nations this year that human activities play a role in world climate change.

At issue is whether the US government has the power to cap these emissions. Industry groups argue that it doesn't, and that carbon dioxide is a naturally occurring gas that does not fit the US Clean Air Act's definition of a pollutant.

The Bush administration has consistently rejected capping greenhouse gas emissions as bad for business and US workers.

Scalia prompted laughter when he questioned whether carbon dioxide was an air pollutant or a stratosphere pollutant.

"Respectfully, your honor, it is not the stratosphere. It's the troposphere," said Milkey, referring to atmospheric layers that can be affected by pollution.

"Troposphere, whatever," Scalia said as the normally silent gallery erupted in chuckles. "I told you before I'm not a scientist. That's why I don't want to have to deal with global warming."