However, omitted in these articles are discussions of climate change in the pre-industrial past, particularly in the last two millennia, for which there is significant source material. The reason this neglect is so critical is that it is commonly assumed that atmospheric CO2 is the only meaningful variable behind climate change. Since pre-industrial levels of CO2 appear to have consistently been below 300ppm with little variation, and levels since the mid-twentieth century have steadily risen to about 400ppm today, atmospheric CO2 is an obvious anthropogenic variable. And, the reasoning goes, since both CO2 levels and global temperature averages have increased together, man’s fossil fuel usage must be causing global warming. The science is settled. We can move on.
Well, let’s continue this line of reasoning and see where it goes. If atmospheric CO2 levels and global temperature are truly correlated, then they ought to be as correlatedbefore the industrial revolution as they are afterward. Pre-industrial CO2 levels remained relatively constant. Did pre-industrial global temperature levels also remain relatively constant? No, they did not. Over the last thousand years, for example, there was the well-attested Medieval Warm Period from about 900–1300 followed by the equally well-attested Little Ice Age from about 1350–1800.