500 Million Years of CO2: What is Normal?

Low Temperature Eras Designated by Gray Bars at Top

How do modern levels of CO2 rank on the geologic time scale? Based upon the figure above, from PNAS (7), April 2, 2002, pp. 4167-4171 authored by MIT's Daniel H. Rothman, modern CO2 levels are somewhat lower than typical levels over the past 500 million years. Among the author's conclusions:

Where carbon dioxide is concerned, we remain at a low ebb. Moreover, if for long geological periods the ‘null hypothesis’ that pCO2 and climate are unrelated cannot be rejected on the basis of this evidence, how certain are we that such constraints might not apply at much shorter time scales too? I only ask?____Source of quote

More on this intriguing perspective at Global Warming Politics

H/T Tom Nelson