Ron Smith, professor of geology and geophysics at Yale, delivers this lecture from the course The Atmosphere, the Ocean and Environmental Change (GG 140). He examines how greenhouse gas emissions, particularly carbon dioxide, drove climate change over the latter half of the twentieth century. Smith walks through the carbon cycle, tracing exchanges between the atmosphere, terrestrial biomass, and ocean, and explains sources like fossil fuel combustion and respiration alongside sinks like photosynthesis and ocean uptake. He calculates the residence time of atmospheric carbon dioxide at a few hundred years, then reviews measured concentration trends since the 1950s and longer ice core records. Later segments cover carbon isotopes as tracers and use the Holocene as a climatic reference period for judging recent change. Recorded in fall 2011 as part of Yale's Open Courses series, the lecture is data driven throughout, building its argument from measurement records rather than general claims.