The Paleogene atmospheric CO2 concentrations reconstructed using stomatal analysis of fossil Metasequoia needles

During the Paleogene, the Earth experienced a global greenhouse climate, which was much warmer and more humid than the present climate. The present global warming is ascribed to increasing levels of atmospheric CO₂ caused by human activity since the industrial revolution; therefore, knowledge of the role of atmospheric CO₂ in the thermal climate during the Paleogene will be helpful for understanding current and future climate. However, unlike for the late Cenozoic, atmospheric CO₂ reconstructions for the Paleogene are still inconsistent and vary between preindustrial-level to values over 4000 ppmv. In this study, we reconstructed the levels of atmospheric CO₂ in the early and middle Paleocene and middle Eocene based on the stomatal index of fossil Metasequoia needles collected from four fossil sites in Canada and Japan. We found the atmospheric CO₂ levels during the early and middle Paleocene to be similar to that of the present, and up to twice the present atmospheric CO₂ level was found during the middle Eocene. Our estimated atmospheric CO₂ level supports the hypothesis that the climate changes during the Paleogene cannot be explained merely by atmospheric CO₂ variations, which suggests that atmospheric CO₂ might not have always played a critical role in climate change during these ancient epochs and therefore cannot be a direct analogy for the current global warming.