This paper traces the development of numerical climate models in the United Kingdom from 1963, when the U.K.’s Meteorological Office first took up climate modelling, to the mid-to-late 1970s, when climate change became politicized in the United Kingdom. The central question posed is how U.K. climate modellers developed rhetoric, managed expectations, and weighed their professional and political responsibilities in the face of growing political interest in climate change. Whilst the modellers were reluctant to allow the modelling results to be used for political ends, U.K. civil servants saw climate modelling as a modern tool for a new problem. As scientific and political agendas diverged, the director of the Meteorological Office, John Mason, found himself caught between his position as a government employee in a service organization and his responsibility as a gatekeeper between climate models and their potential uses. Ultimately, as Mason and his modellers were forced to admit, their climate models became cultural and political as well as scientific objects.