What do we know about past variations in the air's CO2 content? In answer to this question, we have prepared mini-reviews of The Last 40 Years, The Last 1,000 Years, The Last 10,000 Years, The Last 250,000 Years and The Last 4.5 Billion Years. The first four of these reports contain graphs of atmospheric CO2 concentration over the time periods to which they pertain, as well as actual data files that may be readily accessed; while the last of the five reviews gives a brief written description of what is known about that much longer and more distant time period, followed by references to more detailed accounts that pertain to it.
With respect to specific geologic eras within this last time category, we review the reports of Pagani et al. (1999), dealing with the early to late Miocene (25 to 9 million years ago), and Pearson and Palmer (1999), dealing with the middle Eocene (43 million years ago). In the first of these studies, based on sediment cores extracted from three deep-sea drilling sites, the authors calculate atmospheric CO2 concentrations ranging from 180 to 290 ppm. In the second, data on boron isotope composition in planktonic foraminifera were used to reconstruct the pH-depth profile of ancient seawater, after which the pH results were used to estimate atmospheric CO2 concentrations, which ranged from 180 to 550 ppm, with a best estimate of 385 ppm.
In response to a query about the reason for low atmospheric CO2 concentrations during glacial periods, we note that in addition to the fact that colder water can hold more dissolved CO2, the massive volumes of water that are locked up in glacial ice lead to reduced sea levels, which expose great areas of continental shelf to the atmosphere, leading to a several-fold increase in the arid land area of the globe. The more ferocious winds that are known to prevail during glacial epochs then whip up much greater atmospheric dust loads and transport more iron to the world's oceans, which greatly stimulates phytoplanktonic productivity; and it is the enhanced photosynthetic activity of these minute but multitudinous marine organisms that removes more CO2 from the atmosphere and thereby reduces the air's CO2 concentration.
Finally, in response to another question, we note that CO2 is very well mixed throughout the atmosphere, with a few minor exceptions that are described in more detail in other sections of our web site.
To summarize, then, in the broadest of generalizations, the long-term trend of atmospheric CO2 concentration throughout the entire history of the earth has been basically downward, making the current respite provided by the industrial activities of man appear as a "breath of fresh air" to the planet's biosphere.
References
Pagani, M., Authur, M.A. and Freeman, K.H. 1999. Miocene evolution of atmospheric carbon dioxide. Paleoceanography 14: 273-292.
Pearson, P.N. and Palmer, M.R. 1999. Middle Eocene seawater pH and atmospheric carbon dioxide concentrations. Science 284: 1824-1826.