Earth history provides valuable information about the relationship between climate, global temperatures, and CO2 levels. In the most recent special issue of Philosophical Transactions of Royal Society A entitled “Warm climates of the past – a lesson for the future”, a series of articles on warm periods in Earth history have been published. One study carried out by Yale graduate student Yige Zhang and YCEI director Mark Pagani (Geology and Geophysics) provides estimates of atmospheric CO2 for the past 40 million years using isotopic compositions of organic carbon in marine sediments. This CO2 record shows a general long-term decreasing trend, consistent with the long-term global cooling during the past 40 million years. Overlaying this trend are several major warming and cooling events that are also accompanied by increasing and decreasing CO2 levels — highlighting the role of CO2 in controlling global climate. However, there are also some time intervals when CO2 and temperature records are not tightly coupled suggesting that either the methodology used to reconstruct CO2 is flawed or that other climate forcing (albedo, aerosol, other greenhouse gases like methane) might be playing more important roles during these times.
Zhang, Y.G., Pagani, M., Liu, Z., Bohaty, S., and DeConto, R., 2013: A 40‐million‐year history of atmospheric CO2: Philosophical Transactions of The Royal Society A, 371, 20130096.