California air regulators on September 23 took up the world's most ambitious rules to reduce car emissions that contribute to global warming — an effort that could have a sweeping effect on how the country fights vehicle pollution. The regulations are fiercely opposed by the auto industry, which complains they could lead to substantial increases in car prices. The industry has threatened to challenge the regulations in court if they are adopted. The regulations could have a major impact in two ways: California represents 10 percent of the national auto market and several other states follow California's lead when it comes to adopting their own clean-air standards. California is the only state able to set its own vehicle pollution standards because it began regulating pollution before the federal government. Other states can adopt either federal vehicle pollution standards or California's. The regulations would cut vehicle emissions by as much as 25 percent and call for California to require technological changes in vehicles starting in 2009. The innovations would include better air conditioners, more efficient transmissions and smaller engines. Regulators estimate they would cut exhaust emissions by 25 percent in cars and light trucks and by 18 percent in larger trucks and SUVs. Regulators say the proposals would add hundreds of dollars to the cost of vehicles.