California has passed tough new legislation to cut greenhouse gas emissions under a deal reached by Governor Arnold Schwarzenegger.
California has taken a lead on environmental issues
It makes California the first US state to impose a cap on expulsion of carbon dioxide and other gases.
Mr Schwarzenegger reached a deal with the Democrats who control the state legislature, defying the opposition of his fellow Republicans.
He called it "a historic agreement... to combat global warming".
The bill cleared its final hurdle on Thursday, when it was approved in California's State Assembly by 43 votes to 31, with Mr Schwarzenegger's Republicans voting against it.
The Global Warming Solutions Act cleared its last legislative hurdle in the State Assembly in a 46-31 vote, with opposition from Schwarzenegger's own Republican Party.
But the measures may be too little, too late, according to the warnings of a leading US scientist.
Professor John Holdren, the president of the American Association for the Advancement of Science, told the BBC that climate change due to human influence was happening faster than predicted.
"We're already experiencing dangerous human disruption of the global climate and we're going to experience more," he said.
A catastrophic rise in sea level of 4m could take place within this century, he stated.
Under the law, major industries will be required to cut their output of greenhouse gases and will be able to trade emissions credits.
Overall, California's emissions should be cut by 25% by 2020.
"We can now move forward with developing a market-based system that makes California a world leader in the effort to reduce carbon emissions," the governor said in a statement.
The state, the most populous in the US, is the world's 12th largest emitter of greenhouse gases.
Tony Blair and Arnold Schwarzenegger did a deal
It has taken a lead in environmental issues in the US, and Mr Schwarzenegger has touted his environmental record in his bid for re-election in November.
Last month, he signed an accord with British Prime Minister Tony Blair establishing joint research into cleaner-burning fuels and technologies.
But the governor's commitment to emissions caps puts him at odds with the White House.
In the California legislature, too, Republicans demanded a national, not state-by-state approach, to climate change.
"Adopting costly and unattainable regulations will drive businesses and jobs out of California into other states and even into other countries with no commitment to improve air quality," said Assembly Republican leader George Plescia.