Sometime, about one year from now, the front pages of whatever decent newspapers are left will carry a headline like the one above, announcing that for the first time in human existence (or in nearly a million years, or 3 million years, or 15 million years), the global atmospheric concentration of carbon dioxide – the principal gas causing climate change – will have passed 400 parts per million.
That’s a significant and shocking figure. Unfortunately, it is only a temporary marker on the way to even higher and higher levels. Here (Figure 1 below) are the most recent (March 2013) data from the Mauna Loa observatory showing the inexorable increase in atmospheric CO2 and the rapid approach to 400 ppm.
There is a range of estimates around the detailed time record of atmospheric composition, and the study of changes in atmospheric carbon dioxide concentrations over the billions of years of the Earth’s existence is an exciting area for research. A commonly cited figure with strong evidence comes from measurements of air trapped in ancient ice cores obtained from Antarctic ice. We now have a detailed 800,000 year record, which shows clearly that atmospheric CO2 levels never approached 400 ppm during this period (as shown in Figure 2).
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 CO2 concentrations 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.