Two decades have passed since the Clean Air Act Amendments of 1990 launched a grand experiment in market-based environmental policy: the SO₂ cap-and-trade system. That system performed well but created four striking ironies: First, by creating this system to reduce SO₂ emissions to curb acid...

The most ambitious application ever attempted of a market-based approach to environmental protection has been for the control of acid rain under the Clean Air Act amendments of 1990, which established a sulfur dioxide allowance trading program. This essay identifies lessons that can be learned...