Up to this point, we have considered only the short-term carbon cycle: exchanges over days, years, and decades driven by the biological processes of photosynthesis and respiration, and altered in recent times by human activities. Geologic evidence, however, tells us that on timescales of thousands to millions of years, CO₂ levels in air have changed even more dramatically. These longer-term changes mean that we must consider additional contributions to the carbon cycle: physical processes, including volcanism and climate change. Indeed, the complete carbon cycle links Earth’s physical and biological processes, providing a foundation for understanding the interconnected histories of life and environment through our planet’s long history.