Atmospheric CO2 over the Past 66 Million Years from Marine Archives

摘要

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 similar to 1,500 ppm in the Eocene greenhouse, decreasing to similar 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 onethird 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.