To cut back on carbon, environmentalists are using the force of the free market. In carbon-emissions trading, the government puts a cap on how much carbon an industry is allowed to emit from power plants, factories and cars. Innovative companies could meet those caps through actual reductions and earn carbon "credits," which they could sell to industry laggards. Connecticut, Delaware, Maine, New Hampshire, New Jersey, New York and Vermont have agreed on a regional cap-and-trade system. Arizona, California, New Mexico, Oregon and Washington have signed a similar pact. New emissions-reduction technology is sexier, but old-fashioned horse trading might just be more effective.