Emission trading is a well-established method for managing certain kinds of environmental problems. Also called cap-and-trade, emission trading was used to eliminate lead from gasoline in the 1980s, and to control acid rain in the 1990s. The basic idea is simple: Experts assess how much of a given pollutant can be safely put into an environmental medium on an annual basis (that’s the cap), and shares (called permits or allowances) equaling that amount are given or sold to those who have historically emitted the pollutant.