What would Rawls have done to his theory of justice had he considered the problems of climate change? In this paper, I argue that he would or should have added a new principle of international intergenerational justice that guarantees the long-term stability of the Society of Peoples. More specifically, I argue that climate change poses an unforeseen stability problem to the liberal peoples whose national economy is dependent on climate-sensitive resources and barely sufficient to maintain a just basic structure. Rawls’s principles of justice, such as the duty of assistance and the just savings principle, do not properly prevent negative impacts of climate change from destabilizing these climate-vulnerable peoples and thus threatening the stability of other peoples. A precautionary principle requires well-ordered peoples to establish an international institution and secure the environmental background conditions in which each of them can internally maintain just or decent basic institutions over time. The total amount of global greenhouse gas emissions is determined by the optimal portfolio of mitigation, adaptation, and geoengineering that guarantees the long-term stability of the Society of Peoples and the well-ordered peoples therein. The allowable emissions are distributed among peoples in proportion to how much emissions each people needs to maintain its basic institutions, and this is affected by, among others, its population size. The resources for adaptation are shared across nations and generations. Protecting stability against climate change is not merely a matter of mutual advantage, but a matter of justice and equality among free and independent peoples, so international sanctions may be imposed on uncooperative countries. Indeed, in extreme cases, they forfeit membership in good standing of the Society of Peoples.