Throughout Earth's history, CO₂ is thought to have exerted a fundamental control on environmental change. Here we review and revise CO₂ reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoic—the past 66 million years. We find close coupling between CO₂ and climate throughout the Cenozoic, with peak CO₂ levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO₂ drawdown is explained by an increase in the ratio of ocean alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO₂ record highlights the truly geological scale of anthropogenic CO₂ change: Current CO₂ levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO₂ levels of the Miocene or Eocene in the coming century.

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