Book excerpt: 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. 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--Preface.