Ron Smith's Yale course The Atmosphere, the Ocean and Environmental Change turns to isotopes as a record of past climate. Smith explains how deuterium and oxygen-18 serve as climate proxies, why lighter isotopes evaporate more readily from the ocean, and how water vapor grows isotopically lighter as it travels to higher latitudes and sheds mass through precipitation. He walks through delta notation and isotopic fractionation before showing how these processes leave a measurable signal in ice cores and deep sea sediment cores. The lecture covers terrestrial and marine sediment records, oxygen isotopes in ocean cores, and closes with the Milankovitch theory of ice ages, linking orbital cycles to the temperature and ice-volume signals recorded in the isotope data. Chapter markers break the hour into discrete topics, making it easy to follow the argument from basic chemistry to planetary climate history.