Whatever the outcome of the lawsuit, California is required by law to slash overall greenhouse gas emissions to 1990 levels by 2020. That will entail 174 million metric tons in reductions, of which 18% is to come from tailpipe emissions, the state Air Resources Board estimates.
Nichols said the board would look at other measures to force reductions from cars, including incentives for consumers to purchase cleaner cars. She declined to say if the board would consider taxing cars that emitted the most greenhouse gas emissions.
"The reductions are going to come from cleaner new vehicles one way or another," she said. "But we do not intend to penalize individual motorists for the failure of the automobile industry to produce cleaner vehicles."
Nationwide, passenger vehicles generate about 20% of carbon dioxide emissions. In California, the proportion is higher: about a third.
Leading scientists worldwide have said that global carbon dioxide emissions must be slashed by about 80% by mid-century if the worst effects of climate change are to be avoided, including rising seas, melting snowcaps, spreading deserts and widespread species extinction.
California is particularly concerned about water shortages if seawater overwhelms levees and the Sierra Nevada snowpack melts.
The 15 states joining California in the lawsuit are Massachusetts, Arizona, Connecticut, Delaware, Illinois, Maine, Maryland, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont and Washington.