Atmospheric CO ₂ over the Past 66 Million Years from Marine Archives

Abstract

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.

▪ CO ₂ reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪ CO ₂ estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO ₂ system over the Cenozoic. ▪ CO ₂ 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 CO ₂ to levels not seen since the much warmer climates of Earth's distant past.