The CARB limits call for a 30 percent cut in carbon dioxide emissions by 2016. Automakers have fought these California limits as being illegal. They claim only the federal government can regulate fuel economy and think this is essentially the same thing. Considering its size and struggle with pollution in the past, the federal government allowed California to set its own emissions standards, and other states have been given the choice to adopt federal emissions standards or adopt the stricter ones set by CARB. It makes it tough for manufacturers to keep up when there isn't one set of federal rules, but California, for one, thinks moving on their own is the only way to get things to change.
[Source: Automotive News - sub. req.]