The automakers argue in several pending cases that state regulation of greenhouse gases is illegal, because it amounts to regulating the fuel efficiency of cars, which only the EPA can do.

The Supreme Court ruled on Monday that greenhouse gases can be regulated as air pollutants. For the EPA to regulate, it must first determine that science shows global warming is harmful to human health and welfare.

But even if the EPA decides greenhouse gases should be regulated to protect public health, the agency could still deny California's long-delayed request to implement its own law by saying that the problem is global and not unique to the state, the paper quoted Harvard University environmental law professor Jody Freeman as saying.

"Even if California prevails, Congress could end up passing weaker national legislation that would supersede the state's," the paper said.

"I think it's a very tough call right now," said Freeman. "I don't think the chances are great, because I think there's reason to believe Congress will act before EPA."

To get a waiver, California must show compelling and extraordinary conditions, Freeman said.

"California is special. It's the only state in the country that can set tailpipe standards separate from federal standards," she said. "Everything depends on that waiver."

California has mandated that its emissions standards for cars would begin in 2009 and pledged to cut global warming emissions nearly 30 percent by 2016.

California Governor Arnold Schwarzenegger, who broke with President George W. Bush by endorsing California's Democratic-sponsored emissions law, wrote to the president and the EPA a year ago for asking them to grant the state's request to implement its own law.