The Supreme Court stepped gingerly into the national debate over global warming on Wednesday, asking how much harm would occur if the Environmental Protection Agency continues its refusal to regulate greenhouse gases from new vehicles.

In the first case about global warming to reach the high court, a lawyer for 12 states and 13 environmental groups pressed the justices to make the government act, saying the country faces grave environmental harm.

Inaction is like lighting "a fuse on a bomb," said James Milkey, an assistant attorney general for the state of Massachusetts.

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Opening up an hour of arguments, Justice Antonin Scalia asked, "When is the predicted cataclysm?"

It's not cataclysmic, but rather "ongoing harm," Milkey replied.

Chief Justice John Roberts pointed out that regulating carbon dioxide emissions from new vehicles addresses just one aspect of an issue of global dimensions.

The argument by those pushing for EPA action on vehicle emissions might or might not be valid, but it "assumes everything else is going to remain constant," Roberts observed.

Several justices questioned whether the states and environmental groups have met their legal burden to show they will be harmed by continued EPA inaction. Petitioners to courts must meet that threshold before the merits of a case may be addressed.

The Bush administration argued in court papers that the EPA lacks the power to regulate carbon dioxide as a pollutant under the Clean Air Act. Even if it had such authority, the EPA still would not use it at this point because of uncertainty surrounding the issue of global warming, the administration said.

Global climate change is "a controversial phenomenon that is far from fully understood or defined," trade associations for car and truck makers and automobile dealers said in a court filing signed by former Solicitors General Theodore Olson and Kenneth Starr. They backed the administration position.

Twelve states, mainly along the nation's Atlantic and Pacific coasts, three cities, a U.S. territory and 13 environmental groups are arguing that the EPA ignored the clear language of the Clean Air Act. Under the 1970 law, carbon dioxide is an air pollutant that threatens public health and the EPA must regulate it, they said.

Carbon dioxide is produced when fossil fuels such as oil and natural gas are burned. It is the principal "greenhouse" gas that many scientists believe is flowing into the atmosphere at an unprecedented rate, leading to a warming of the Earth and widespread ecological changes. One way to reduce those emissions is to have more fuel-efficient cars.

"There are compelling reasons for the court to join the issue now," Massachusetts Attorney General Thomas Reilly said in a brief on behalf of the states, cities and environmental groups.

A federal appeals court in Washington, in a fractured decision in 2005, upheld the administration's position. The Supreme Court decided to take the case in June and is expected to rule before July 2007.

The court's decision could have far-reaching effects. A separate case involving the EPA's claim that the Clean Air Act similarly does not give it authority to regulate greenhouse gas emissions from power plants also is making its way through the federal courts.

Together, U.S. power plants and vehicles account for 15 percent of the world output of greenhouse gases, said David Doniger, counsel for the Natural Resources Defense Council, an environmental group involved in the Supreme Court case.

An association of electric utilities, the Utility Air Regulatory Group, opposes greenhouse gas regulation. But two individual power companies, Calpine Corp. and Entergy Corp., are on the other side.

"This case makes for strange bedfellows," Entergy said in its brief. The company said it has to be able to make plans 25 years in advance and that the EPA's current rules will not "stand the test of time."

Michigan, home of the U.S. auto industry, and eight other states are backing the EPA.

The case is Massachusetts v. Environmental Protection Agency, 05-1120.