The Clean Air Act Amendments of 1990 set as its primary goal the reduction of annual SO2 emissions by 10 million tons below 1980 levels of about 18.9 million tons. To achieve these reductions by 2000, when a nationwide sulfur dioxide emissions cap of 8.95 million tons per year began, the law required a two phase tightening of operating restrictions placed on fossil fuel fired (e.g., coal, oil, natural gas) power plants. The operation and pricing of a market for emissions allowances would not be viable in the absence of an effective regulatory cap on the total number of allowances available.