Abstract: If one nation damages another, what are its obligations? This question can be approached and understood in diverse ways, but it is concretized in debates over the social cost of carbon, which is sometimes described as the linchpin of national climate policy. The social cost of carbon, meant to capture the damage done by a ton of carbon emissions, helps to determine the stringency of regulations in many domains, including emissions limits on motor vehicles and on stationary sources. In determining the social cost of carbon, agencies must decide whether to use the global number (as chosen by Presidents Barack Obama and Joe Biden) or instead the domestic number (as chosen by President Donald Trump). Use of the global number should be seen as a form of climate change cosmopolitanism, whether the grounding is moral or otherwise. There are four central arguments in favor of using the global figure. (1) The epistemic argument: Experts do not know a great deal about the purely domestic harms from climate change, which makes it impossible to generate a purely domestic number. (2) The interconnectedness argument: Harms done by domestic emissions are not limited to those done by the incremental increase in temperatures in the United States; they include harms to U.S. citizens living abroad and harms to U.S. citizens and interests that come from the cascading effects of harm done to foreigners (including governments, companies, and individuals), which are ultimately felt in the United States. (3) The moral cosmopolitan argument: In deciding on the scope of its regulations, the United States should account of the harms it does to non-Americans. (4) The prisoner’s dilemma argument: If all nations used a domestic figure, all nations would lose; a successful approach to the climate problem requires nations to treat greenhouse gas emissions as a global, and not merely domestic, externality. Neither the epistemic argument nor the incompleteness argument justifies choice of the global number. The moral cosmopolitan and prisoner’s dilemma arguments stand on much stronger grounds.