This paper explores the work of Scottish scientist James Croll (1821-1890), whose interest lay in explaining the underlying principles—not just the physical, geologic remains—of the Ice Ages. In 1875, Croll published Climate and Time in their Geologic Relations: A Theory of the Secular Changes of the Earth’s Climate, a work in which he calculated the variation in eccentricity of the Earth’s orbit over a four-million-year period. Agreeing with astronomers before him, Croll argued that the orbital variation alone could not induce an Ice Age. Crucially, however, Croll suggested that they could trigger positive feedback loops in global environmental systems, specifically ice coverage (the albedo effect) and ocean circulation, that would induce such a dramatic, planetary change. Through a deductive approach based on mathematical calculations and physical laws, Croll both posited a fluctuation in ice coverage over time (glacial and interglacial periods), and emphasized how ice is an essential part of the Earth system as a whole. Croll’s methodology was entirely distinct from the induction and observation of his geologist contemporaries, and offers insight into the important role of physicists in conceptualizing deep time in the nineteenth century.
