Continuous and self-consistent CO2 and climate records over the past 20 Myrs

The gradual cooling of the climate during the Cenozoic has generally been attributed to a decrease in CO2 concentration in the atmosphere. The lack of transient climate models and in particular the lack of high-resolution proxy records of CO2, beyond the ice-core record prohibit however a full understanding of the inception of the Northern Hemisphere glaciation, as well as the mid-Pleistocene transition. Here we elaborate on an inverse modeling technique to reconstruct a continuous high-resolution CO2 record over the past 20 Ma, by decomposing the global deep-sea benthic δ18O record into a mutually consistent temperature and sea-level record, using a set of 1-D models of the major Northern and Southern Hemisphere ice sheets. We subsequently compared the modeled temperature record to ice core and proxy-derived CO2 data to reconstruct a continuous CO2 record over the past 20 Myrs. Results show a gradual decline from 450 ppmv around 15 Myrs ago to 280 ppmv for pre-industrial conditions, coinciding with a gradual cooling of the Northern Hemisphere land temperatures by approximately 12 K, whereas there is no long-term sea-level variation caused by ice-volume changes between 13 to 3 Myrs ago. We find no evidence for a change in climate sensitivity other than the expected decrease following from saturation of the absorption bands for CO2. The reconstructed CO2 record shows that the Northern Hemisphere glaciation starts once the average CO2 concentration drops below 265 ppmv after a period of strong decrease in CO2. Finally it might be noted that we observe only a small long-term change (23 ppmv) for CO2 during the mid-Pleistocene transition.