California regulators yesterday approved the nation's first limits on greenhouse gas produced by automobiles, requiring an average 30 percent cut in carbon dioxide emissions from new cars and light trucks by 2016.

The regulation could have a significant effect on the auto industry. California accounts for more than 10 percent of the cars and trucks on U.S. roadways, and several other states follow its lead on auto emissions standards, so changes required there could affect vehicle prices and designs everywhere.

But manufacturers hotly oppose the new regulation and have threatened to challenge it in court.

Carbon dioxide is thought to be a leading contributor to global warming. Because the gas is a natural byproduct of internal combustion engines, the only way to limit it in traditionally powered cars is to make them go farther on a gallon of gasoline.

Only the federal government is allowed by law to regulate fuel efficiency, but California regulators say the greenhouse gas regulation is justified under the state's authority to regulate air quality.

"This landmark decision sets a course for California that is likely to be copied throughout the U.S. and other countries," California Air Resources Board Chairman Alan C. Lloyd said in a news release after the board's unanimous vote.

Auto manufacturers have said the California requirement would carry great cost and offer little benefit to the environment.

"Certainly, Sacramento regulators are well-intentioned, but their plan calls for extreme, extensive changes for these vehicles," said Gloria Bergquist, spokeswoman for the Alliance of Automobile Manufacturers, which represents most major automakers. "At the same time, regulators have been hazy on how their plan would improve California's climate or reduce California's temperatures."

While the state's regulators have calculated that complying with the new requirement would cost about $1,050 per vehicle by 2016, the alliance says the cost would be more than $3,000 per vehicle.

Automakers have said they might challenge the rule in court on the grounds that a state can't regulate gas mileage. "Right now we're looking at our options," Bergquist said.

The Air Resources Board will report the regulation to the California legislature, which called last year for the greenhouse gas standard. The legislature will hold public hearings, and the new standard is scheduled to be phased in beginning in 2009.