Atmospheric CO2 over the Past 66 Million Years from Marine Archives

Throughout Earth's history, CO 2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO 2 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 CO 2 and climate throughout the Cenozoic, with peak CO 2 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 CO 2 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 CO 2 record highlights the truly geological scale of anthropogenic CO 2 change: Current CO 2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO 2 levels of the Miocene or Eocene in the coming century.