WASHINGTON In a stinging defeat for the Bush administration, the Supreme Court ruled on Monday that U.S. environmental officials have the power to regulate greenhouse gas emissions that spur global warming.
By a 5-4 vote, the nation's highest court told the U.S. Environmental Protection Agency to reconsider its refusal to regulate carbon dioxide and other emissions from new cars and trucks that contribute to climate change.
The high court ruled that such greenhouse gases from motor vehicles fall within the law's definition of an air pollutant.
The ruling in one of the most important environmental cases to reach the Supreme Court marked the first high court decision in a case involving global warming.
President George W. Bush has opposed mandatory controls on greenhouse gases as harmful to the U.S. economy, and the administration instead has called for voluntary programs.
In 2003, the EPA refused to regulate the emissions, saying it lacked the power to do so. Even if it had the power, the EPA said it would be unwise to do it and would impair Bush's ability to negotiate with developing nations to cut emissions.
The states and environmental groups that brought the lawsuit hailed the ruling.
"As a result of today's landmark ruling, EPA can no longer hide behind the fiction that it lacks any regulatory authority to address the problem of global warming," Massachusetts Attorney General Martha Coakley said.
Greenhouse gases occur naturally and also are emitted by cars, trucks and factories into the atmosphere. They can trap heat close to Earth's surface like the glass walls of a greenhouse.
Such emissions have risen steeply in the past century and many scientists see a connection between the rise, an increase in global average temperatures and a related increase in extreme weather, wildfires, melting glaciers and other damage to the environment.
Democrats in Congress predicted the ruling could add pressure on lawmakers to push forward with first-ever caps on carbon dioxide emissions. The United States is the world's biggest emitter of such gases.
The ruling also could make it easier for California and 13 other states to put in place mandatory emission caps, officials in that state said.
Writing for the court majority, Justice John Paul Stevens said the EPA's decision in 2003 was "arbitrary, capricious or otherwise not in accordance with law."
In sending the case back for further proceedings, Stevens said the EPA could avoid regulation only if it determined that the gases do not contribute to climate change or if it provided a reasonable explanation.
Stevens said the EPA could not avoid its legal obligation by noting the scientific uncertainty surrounding some features of climate change and concluding it would be better not to regulate at this time.
White House spokeswoman Dana Perino said of the ruling, "We're going to have to take a look and analyze it and see where we go from there."
The EPA said the administration was committed to reducing greenhouse gases and it was "reviewing the court's decision to determine the appropriate course of action."
The court's four most conservative members -- Chief Justice John Roberts and Justice Samuel Alito, both Bush appointees, and Justices Antonin Scalia and Clarence Thomas -- dissented.
They said the environmental groups and the states lacked the legal right to bring the lawsuit in the first place.
"No matter how important the underlying policy issues at stake, this court has no business substituting its own desired outcome for the reasoned judgment of the responsible agency," Scalia wrote.
(Additional reporting by Chris Baltimore)