In oceanographer Wallace Broecker’s landmark article ‘Climatic Change: Are We on the Brink of a Pronounced Global Warming’, published in Science in 1975, ice played a crucial, yet somewhat opaque, role. By using data from the 1966 Camp Century ice core, Broecker made claims about future global warming. The ice core itself was not the heart of the inquiry for Broecker; rather it served as a frame of reference in the making of a different object of knowledge: the warming planet. Ice cores have increasingly become ‘planetary archives’ of interest to scientists beyond glaciology and they have ventured from a remote existence in the cryosphere into the models of Earth system scientists. This chapter aims to situate ice core drilling within a broader history of the making of planetary-scale environmental knowledge. By tracing early applications of ice core data in scientific practices beyond glaciology, the work of the ice core can be located in a process of temporalization of the planetary environment. During the 1970s and 1980s, ice cores became crucial elements in the efforts to synchronize multiple paleoarchives into a coherent understanding of planetary dynamics. By conceptualizing ice cores as environing media, this chapter points to the multiple stages of mediation ice cores have undergone during the postwar era and their subsequent rise as a key technology to produce planetary-scale environmental knowledge.