Responding to the CO2 Coalition's "Facts #6 and #7" on CO2 Concentrations on Geologic Time Scales

CO2 Coalition's "fact #6" and "fact #7" are essentially identical, and both are largely just a rehash of statements made in previous "facts." Here we're told that "our current geologic period, the Quaternary, has seen the lowest average levels of carbon dioxide since the Precambrian." To support this claim, they took the data from the climate model in Berner 2001 and averaged all the CO2 concentration values for each Geologic period, ignoring the confidence intervals and the length of the period, and plotted the following graph. Note that each geologic period is shown as if they were the same length.

As I've already shared, CO2 proxies often paints a somewhat different picture from Berner 2001's climate model. The model has been updated several times since 2001, but the CO2 Coalition appears to be completely unaware of this. Here again is the proxy evidence we have for CO2. It's pretty sparse before the proliferation of vascular plants on land. 

CO2 Coalition's chart is also pretty disingenuous. The Quaternary has lasted only about 2.6 million years, while the Cretaceous lasted 79 million years. It's hardly a valid comparison to treat the Cretaceous and Quaternary equally. In fact, their "fact #7" nicely illustrates how short the Quaternary has been compared to other Geologic periods.

Has there been a Quaternary-length time frame in which CO2 levels dropped below the average CO2 concentrations of the Quaternary? Probably. During the Permian (which lasted 47 million years), CO2 levels dropped to ~100 ppm for about 1 million years in the early Permian and were likely ~200 ppm or lower for a longer period of time. The graph below shows how this time period compares to the range of CO2 values during the Quaternary glaciations (indicated to the right of the graph).

Of course, pretty much none of this has any relevance to current warming. The mere fact that throughout the Phanerozoic CO2 levels averaged higher than the Quaternary doesn't mean that the current increase in CO2 isn't harmful to human civilization. We built our civilizations during the relatively stable climate of the Holocene (the last 11,700 years of the Quaternary), where CO2 levels varied between 260 and 280 ppm. Human carbon emissions have increased this by 50% to 420 ppm, and the climate is quickly responding. Global climate is headed into territory that the Earth has not seen in the last 3 million years. Its the disruption of stability that is harmful, not the absolute value of CO2.