Problem is, the U.S. Environmental Protection Agency won't allow California to use the law. Twenty other states have either adopted California's regulations or are in the process of doing so. But they won't be able to enforce the regulations either.

California has sued EPA in federal court, hoping to overturn the agency's ruling.

Meanwhile, the argument over whether to even regulate greenhouse gas emissions went below the radar. The proposed fuel economy standards issued by the National Highway Transportation Safety Administration last month included an attempt to prevent California and other states from enforcing standards on greenhouse gas emissions from cars.