In Of Montreal and Kyoto: A Tale of Two Protocols, Prof. Cass Sunstein compares the political economy dynamics leading up to the signing and ratification of the Montreal Protocol (governing substances that deplete the ozone layer) and the Kyoto Protocol (governing substances that contribute to global warming). He observes that the United States was a strong and early supporter of the control of ozone-depleting substances but has generally opposed binding controls on greenhouse gases (GHGs). In contrast, Britain was significantly more reluctant to agree to limits on ozone-depleting substances but has actively supported restrictions on GHGs. Professor Sunstein attributes that contrast to differences in the two nations' perceptions of domestic benefits and costs from environmental action, and he concludes that the key to obtaining a global agreement on GHGs will involve raising perceived benefits within the United States from such an agreement while reducing its perceived domestic costs. He suggests that motivating developing countries to agree to emission limits and achieving such reductions through an incentive-based global approach--such as a global tax on carbon dioxide (CO2) emissions or a global cap-and-trade program--are the most promising approaches to altering U.S. perceptions of domestic benefits and costs.