The CO2 content of the atmosphere has varied cyclically between ~180 and ~280 ppmv over the last 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, pCO2 is much less certain and the relationship between pCO2 and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO2 during major climate transitions of the last 20 million years (myr). During the Middle Miocene, when temperatures were ~3 to 6°C warmer and sea level 25 to 40 meters higher than present, pCO2 was similar to modern levels. Decreases in pCO2 were synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago; Ma) and Late Pliocene (~3.3 to -2.4 Ma).
Tripati, A. K., Roberts, C. D. & Eagle, R. A., 2009. Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years. Sciance DOI: 10.1126/science.1178296. Article (subscription required).
