Figure 2. The concentration of CO2 in the atmosphere, measured over the past 800,000 years. It never came close to 400 ppm. Present day is on the right of the curve.
In December 2009, a research team from UCLA published a paper in Science that suggested we would have to go back at least 15 million years to find carbon dioxide levels approaching today’s levels. This research used isotopic analysis of shells in deep sea sediments and reported that CO2concentrations may not have exceeded 400 parts per million since the Mid-Miocene Climatic Optimum (MMCO) – between 16 and 14 million years ago. The MMCO was associated with reduced planetary ice volumes, global sea levels a huge 25 to 40 meters higher than today, and warmer ocean temperatures. Decreasing CO2 concentrations after that were associated with substantial global cooling, glaciations, and dropping sea levels.
Gavin Schmidt of NASA’s GISS has pointed me to research in a December 2011 article in the journal Paleoceanography by Gretta Bartoli, Bärbel Hönisch, and Richard E. Zeebe reporting on paleoclimatic records that suggest that CO2 concentrations (at least in the Northern Hemisphere) may have been around 400 ppm between 2 and 4.6 million years ago. This evidence comes from isotopes measured in planktic foraminifer shells spanning 2.0 to 4.6 million years ago and indicates that atmospheric CO2 estimates during the Pliocene gradually declined from just above 400 ppm to around 300 ppm in the early Pleistocene 2 million years ago.
800,000 years ago? Three million years ago? 15 million years ago? More research will continue to clarify the variability of Earth’s atmospheric composition over time, as well as the impacts for the planet as a whole of screwing with it. [That’s a technical term…]
But the more important point to remember is that never in the history of the planet have humans altered the atmosphere as radically as we are doing so now. And the climatic consequences for us are likely to be radical as well, on a time-scale far faster than humans have ever experienced.