One of the great revolutions in environmental policy has been the adoption of the "cap-and-trade" method for controlling air pollution, starting with the 1990 Clean Air Act. The basic idea is to have the government set overall limits and let the market figure out how most efficiently to achieve the goal. And it has been a major success: Over the past 35 years U.S. energy consumption has increased by 48%, and population by 42%, while emissions of the six most common pollutants have decreased by 53%. Yet that achievement is now threatened by federal, state and private lawsuits asking the...