In this article I explore how, in the League of Nations’ emerging anti-trafficking regime of the 1920s and 1930s, one category of race science — climate — played a prominent role in positing natural hierarchies between nations. My purpose is twofold: (1) to explain the currency of climate at this moment and to examine the trajectory of climate as an explanatory device in the intellectual history of ‘race’; and (2) to reflect on the biopolitical implications of explanations rooted in climate. (...) The article begins with a description of how League of Nations delegates used climate as shorthand to refer to differences between the sexual mores of various nations. I then reflect more broadly on the emergence, submergence, and reemergence of climate in the history of race science, and its effects in practical settings. I move to a discussion of the significance of the age of consent as a category, and analyse the League of Nations-sponsored efforts to track ages of consent across countries as a biopolitical project. My overarching argument is that references to climate performed important ideological work in naturalizing hierarchical relations between nations. In arenas where diplomats sought to arrive at a consensus, such references rendered them more palatable and less disputable. (shrink)