When historians finally take stock, Vermont may look like the mouse that roared – the tiny state that brought the nation's mighty auto industry to heel by requiring cars that emit fewer greenhouse gases

This is one scenario that could unfold following a federal judge's ruling Wednesday, which upheld a Vermont law patterned after California's mandate that the carbon-dioxide emissions of cars sold in the state must be slashed 30 percent by 2016.

The judge's finding – that federal fuel-economy laws are not in conflict with state emissions laws – is particularly significant, coming on the heels of a US Supreme Court decision in April. That ruling found that the Environmental Protection Agency has the authority to regulate greenhouse-gas emissions, legal experts say.

On the one hand, Wednesday's decision strengthens the hand of states that want to take action against global warming. But in the longer term, the impact from the ruling could lead to one nationwide standard, which is already expected by many.

By Mark Clayton