CRANKING UP THE HEAT: HOW HIGH WAS PCO2 IN THE EARLY PALEOGENE?

The wide variation in carbon dioxide (CO2) estimates for the Early Paleogene, anywhere from 300 to 3000 ppm, has made the reconstruction of climate during this time period rather difficult. Using the ocean slab version of the National Center for Atmospheric Research (NCAR) Climate System Model (CSM) with Eocene geography and vegetation we examine the sensitivity of climate to a range of CO2 values. We ran three modeling experiments with CO2 levels at 500, 1000, and 2000 ppm. The greatest warming with increasing CO2 occurred in the Northern Hemisphere high latitudes during winter. Compared to the 500 ppm scenario, Arctic wintertime temperatures rose by ~10C for the 1000 ppm scenario, and by ~20C for the 2000 ppm scenario. These higher CO2 experiments produce mean annual and cold month mean temperatures much more compatible with climate interpretations from Eocene flora and fauna. This good match between climate interpretations and the higher CO2 experiments is consistent with the idea that CO2 levels were high during the Early Paleogene, at least 3-4 times the pre-industrial level of 280 ppm.