The Holocene CO₂ rise: Anthropogenic or natural?

In view of the wide attention received by the suggestion that the rise in atmospheric carbon dioxide (CO₂) over the last 8000 years is anthropogenic rather than natural in origin [Ruddiman, 2003], this claim should be carefully examined.The basis for the claim is that following each of the three preceding glacial terminations, the CO₂ content of the atmosphere peaked early on and then underwent a steady decline. By contrast, following the end of the last glacial period, while it also peaked early the decline bottomed out around 8000 years ago, and since then the atmospheric CO₂ content has steadily risen.

By analogy with previous interglaciations, Ruddiman estimates that in the absence of human activity, the CO₂ content of the atmosphere would have dropped to 240 ppm. Instead it has risen to 280 ppm. In a recent article, Ruddiman [2005] proposes that this 40 ppm human-induced rise prevented the onset of another ice age.