The Importance of Long-Term Temperature and CO2 Data
Drake, B.L. 2012. The influence of climatic change on the Late Bronze Age Collapse and the Greek Dark Ages. Journal of Archaeological Science 39: 1862-1870.
Yet what is of most interest to this review is something that has been known by researchers about Earth's climate for quite some time now; and that is the long-term temperature record produced by the Greenland Ice Sheet Project (GISP2), which is depicted below, as plotted by Drake but based on data reported by Alley (2004).
Figure 1. The past 5,000 years of the GISP2 temperature history of the Greenland Ice Sheet, adapted from Drake (2012), who denoted the general locations of the Late Bronze Age (LBA), the Roman Warm Period (RWP) and the Medieval Warm Period (MWP) in their original work.
In viewing this history, it is most interesting to note (and know) that over its first 4,800 years (that's 96% of the record) - when the temperature varied all over the place - the atmosphere's CO2 concentration was extremely stable, hovering between about 275 and 285 ppm; while over the last 200 years (the remaining 4% of the record), when the temperature shows but a fraction of a degree warming, the air's CO2 concentration rose by well over an extra 100 ppm.
Clearly, the air's CO2 content is not a major driver of Earth's temperature. In fact, it may not even be a minor driver.
Alley, R.B. 2004. GISP2 Ice Core Temperature and Accumulation Data. In: Data Contribution Series #2004-013. NOAA/NGDC Paleoclimatology Program. IGBP PAGES World Data Center for Paleoclimatology, Boulder, Colorado, USA.