After years of small-scale experiments in using so-called emissions trading to reduce pollution, China is taking steps to set up a nationwide system. In recent months, three cities -- Shanghai, Beijing and Tianjin -- have begun creating emissions exchanges modeled after a system pioneered in the U.S. to reduce emissions that caused acid rain. Known as cap-and-trade, this system sets an overall limit, or cap, on how much an industry can pollute. Individual companies get permits, which can then be traded. A company that invests in cutting its emissions can, for instance, sell its credits to a company that operates less cleanly. Traders can speculate on the future value of the credits. When more production means more pollution, the permits gain value.