This Essay concludes, based on a political economy analysis, that many states, cities, and other sub-national authorities (SNAs) will continue to take strong independent climate regulatory initiatives even after the federal government adopts a broad cap-and-trade system. Should federal law allow SNAs to do so? In order to answer this question, this Essay examines unitary and plural models of climate regulation. It concludes that, notwithstanding the advantages in principle of a unitary approach, a plural model that allows for multiple regulatory systems is preferable because in practice it is more likely than a unitary model to advance climate protection. Because independent SNA regulation furthers with the plural approach, federal law should presumptively allow it, subject to congressional legislation to prevent serious conflicts with the federal regulatory scheme that it adopts.