Atmospheric CO₂ over the past 66 million years from marine archives

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 major climate transition of 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 ∼550 ppm in the Miocene, and falling further into ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO₂ drawdown is explained by an increase in the ratio of 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.
▪ CO₂ reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated.
▪ CO₂ estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO₂ system over the Cenozoic.
▪ CO₂ and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity.
▪ Twenty-first-century carbon emissions have the potential to return CO₂ to levels not seen since the much warmer climates of Earth's distant past.