The assumption that African-descended people were more able than European-descended people to labor in the “hot” climates of southern North America and the Caribbean was an important justification for slavery in the British Empire since at least the early eighteenth century, and it continued to be invoked by both British and American enslavers after the Revolution. However, pro-slavery writers were not alone in making claims about the association of race and climate. A surprising number of anti-slavery activists agreed with their opponents that racial difference made people of African descent more able to live and work in hot climates. When the association between climate and race was challenged, it was almost always by black activists. Seemingly, even well-meaning, radical abolitionists overlooked the damage done by repeating racial assumptions. This dissertation traces the various ways that climatic-racial arguments were used to attack and bolster slavery. It argues that the persistent and shared usage of these arguments demonstrates the flexibility of these widely shared ideas. This flexibility encouraged their spread and acceptance and enabled the application of climatic-racial ideas beyond slavery itself into debates over citizenship, subjecthood, and social and cultural belonging. This dissertation tracks the use of climatic-racial arguments in political debates related to slavery to reveal the many ways they could be deployed from the 1730s through the 1840s. However, this is not a history of abolition or emancipation. Instead, case studies related to abolition show the persistence of climatic-racial argumentation. Focusing on the use of climatic-racial thinking also creates a new perspective from which to view the connections between race and abolition. This perspective puts specific elements of pro- and anti-slavery thought into closer proximity, demonstrating their shared premises. It also enables comparisons between how these ideas were varyingly deployed in the United States and Great Britain as well as their use by black and white activists. Additionally, it highlights how interpretations of nature and climate always reflect human cultural and political biases, and it explains how a specific kind of thought could change and persist across a period of dramatic change, such as abolition.