"Scientists generally conclude that rising concentrations of greenhouse gases are warming the Earth's climate. Concern about the damage that might result has led policymakers and analysts to consider policies designed to restrict emissions of those gases. One type of policy, a cap-and-trade program, could minimize the cost of achieving a limit, or cap, on emissions by allowing market forces to determine where, how, and to some extent when the cuts in emissions necessary to achieve the cap would be made. (Other options include taxes on emissions and regulatory standards to reduce emissions, or a combination of the various approaches.) A cap-and-trade program would establish increasingly stringent annual limits on greenhouse gas emissions over the course of several decades. The government would distribute rights to emit such gases (allowances) by either selling them, possibly in an auction, or giving them away. Firms would be allowed to trade the allowances after they had been distributed and to shift them over time to some degree by 'banking' unused allowances for future use or by 'borrowing' allowances allocated to future years. The price of allowances would rise to the level necessary to ensure that the limit on cumulative emissions over the life of the policy (implied by the annual caps) was met. That price level would depend crucially on a variety of factors, including the growth of the economy and the development of new technologies to reduce emissions. Because policymakers cannot know in advance how high or low prices will be in any given year, they might consider adding features to the design of a cap-and-trade program that would limit the range of potential allowance prices. This Congressional Budget Office (CBO) study--prepared at the request of the Chairman of the Senate Committee on Energy and Natural Resources--examines the potential effects of features that would help manage allowance prices, and thus the cost of complying with a cap-and-trade program, by altering the number of allowances available to firms at various prices. In keeping with CBO's mandate to provide objective, impartial analysis, the report contains no recommendations." --Preface.