Forget gold to back up your greenbacks, how about carbon? Should the U.S. and other major emitters follow the E.U.'s lead and establish carbon markets the resulting value transfer will number in the trillions--and may even reach or surpass the scale of the international trade in currencies.

An analysis by New Energy Finance finds that even if the U.S. goes it alone and creates only a domestic carbon market, that market would be worth $1 trillion by 2020. Not bad for a colorless, odorless gas that we breathe out every second.

The downside of that domestic only program would be higher costs. Without cheap Brazilian (or Chinese or Indian) emission credits, the U.S. would end up paying more for less.

Still, that's an awfully fat market for brokers to pass up. Think of the fees! Too bad a carbon tax would work better.