Atmospheric carbon dioxide and early Eocene climate: A general circulation modeling sensitivity study

The early Eocene was the warmest time interval of the past 65 million years. We have investigated the hypothesis that the warm climate of the early Eocene may have been caused by higher-than-present levels of atmospheric carbon dioxide (CO2). We have used an atmospheric general circulation model, the NCAR genesis model, coupled to a land surface model and a mixed layer ocean, to examine the sensitivity of early Eocene climate to varying levels of atmospheric CO2. Model cases contained a description of the early Eocene Earth and included either 300, 560, or 1680 ppm atmospheric CO2. Climate responses to increased atmospheric CO2 include (1) increased mean annual global temperature by 2–6°C, and decreased latitudinal surface temperature gradients; (2) maximum warming at high latitudes, especially in the winter season; (3) increased mean global precipitation; (4) increased high latitude clouds. In comparison with geological paleoclimate data, the climate response to increased CO2 does not appear to explain completely the early Eocene latitudinal surface temperature gradient; for example, with the 1680 ppm CO2 case, high latitude oceans and continents are minimally warm enough to match paleoclimate data, while low latitude regions are overly warm when compared to sea surface temperature reconstructions. We conclude that CO2 was very likely at least double the present level during the early Eocene and contributed to the overall character of early Eocene climate. However, consideration of increased poleward oceanic heat transport and/or other factors, may be required to produce model results that more closely agree with marine and terrestrial paleoclimate data, especially at low latitudes.