Abstract: Do foreign lives matter? When? How much? If one nation damages another, what are its obligations, as a matter of law and policy? These questions can be approached and understood in diverse ways, but they are 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, strategic, or otherwise. Within the constraints of governing statutes, 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 to U.S. citizens by domestic emissions are not limited to those directly brought about by the incremental increase in temperatures within the territorial boundaries of the United States; they include an assortment of harms to U.S. citizens living abroad and harms to U.S. citizens and interests that come as a result of the cascading effects of harm done to foreigners (including governments, companies, and individuals), which are ultimately felt by U.S. citizens or within the United States. (3) The moral cosmopolitan argument: in deciding on the scope of its regulations, the United States has a moral obligation to take account of the harms it does to non-Americans. (4) The reciprocity 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 the choice of the global number. The moral cosmopolitan and reciprocity arguments stand on much stronger grounds, though they both run into plausible objections.