Responding to the CO2 Coalition's "Fact #1" on a 140 Million-Year Trend in CO2

CO2 Coalition's "fact #1" is an excellent example of I consider contrarian alarmism. The claim here is essentially that the overall trend over the last 140-million years shows that we are on a long-term trend towards a climate apocalypse until we were saved by carbon emissions from the fossil fuel industry. In fact, while contrarians frequently call those who accurately present the scientific evidence as "alarmist," this so-called "fact" actually qualifies as an alarmist claim.

In the graph above, the CO2 Coalition claims that CO2 levels have been steadily decreasing for 140 million years, such that "CO2 levels fell precipitously & steadily to within about 30 ppm of the 150 ppm 'line of death' below which plants can’t survive. Both the relatively short-term data from ice cores and much longer-term data going back 140 million years... show an alarming downward trend toward CO2 starvation." The data they use comes from Berner 2001, which is a climate model published well over 20 years ago. That model has been updated several times, but the CO2 Coalition appears to be unaware of these updates. Berner's model was a simple "box model" GCM that is useful for filling in gaps in proxy data for CO2, but it doesn't have very good resolution (only about 10 million years), and it doesn't always agree with proxy evidence. Later improvements to this model improve the correlation between the model and proxy data, but we have pretty good proxy data for CO2 over the last 140 million years, so there's no good reason to use a 23-year old model for trends over the last 140 million years when we have perfectly good proxy evidence available. In reality, it would seem that the CO2 Coalition is completely unaware of any paleoclimate research into CO2 concentrations since the publication of Berner 2001. Below I show a comparison of the Berner 2001 climate model to proxy evidence from Judd et al 2024. Clearly Berner's box model is not fully in agreement with the best available proxy data; admittedly Berner's uncertainties were quite large, though not printed on the CO2 Coalition's graph.

The decline in CO2 over the last 140 million years is much less than the CO2 Coalition would have you believe. Below I show data from the Paleo-CO2 Project, which is accumulating proxy evidence for CO2 for the Cenozoic. It has a little more resolution than Berner or Judd, indicating more variability and a long-term decrease in CO2 since the PETM.

Data from the Paleo-CO2 Project

As you can see, there is a trend in decreasing CO2 for the last 56 million years or so, since the beginning of the Eocene, but CO2 levels were generally lower during the Paleocene. The Paleo CO2 Project has also published data for the the last 480 million years or so. In the graph below, the data points with color indicate proxy data that are quantitative enough to be considered reliable. The gray symbols are still being vetted, and they may be either semiquantitative or not currently reliable. The data below does not support the CO2 Coalition's claim of a decreasing trend in CO2 between 140 and 56 million years ago.

Data from the Paleo-CO2 Project

Furthermore, there is no "150 ppm 'line of death' below which plants can’t survive." This lie, as best I can tell, originates from Patrick Moore, and the only support for this I can find is some references to the need to keep greenhouses at elevated CO2 levels. However, greenhouses are enclosed spaces, so as photosynthesis continues throughout the day, the plants will draw down CO2 concentrations inside the greenhouse (a problem that would not occur in conditions outside of greenhouses). At night, CO2 levels will increase due to respiration, but the problem occurs because of the enclosed space of a greenhouse, not because plants can't survive when CO2 levels drop below 150 ppm. I've covered this before, but multiple studies show plants can tolerate CO2 levels below 150 ppm, and in fact, during the early Permian, the earth may have spent about 1 million years at ~100 ppm, and plants survived. I'm not suggesting at all that CO2 <150 ppm would be a good thing for plants; certainly this would cause major stresses for C3 plants in particular, but in point of fact there is no "line of death" at 150 ppm, as the CO2 Coalition claims; that's made up nonsense.

From this, the CO2 Coalition claims that humanity has been spared from a "CO2-related climate apocalypse" due to CO2 levels falling below an imaginary "line of death" because "the use of fossil fuels has allowed humanity to increase concentrations of this beneficial molecule." In other words, the CO2 Coalition wants us to believe that the fossil fuel industry has saved us from a climate apocalypse that was 140-million years in the making through fossil fuel carbon emissions. As best I can tell, there is nothing about this supposed "fact" that qualifies as an honest representation of the available evidence; it does qualify as counterfactual alarmism.

References:

[1] Georg Feulner. Coal formation and global glaciation. Proceedings of the National Academy of Sciences Oct 2017, 114 (43) 11333-11337; DOI: 10.1073/pnas.1712062114.https://www.pnas.org/content/114/43/11333

[2] Montañez, I., McElwain, J., Poulsen, C. et al. Climate, pCO2 and terrestrial carbon cycle linkages during late Palaeozoic glacial–interglacial cycles. Nature Geosci 9, 824–828 (2016). https://doi.org/10.1038/ngeo2822

[3] Montanez, I. P., Tabor, N. J., Niemeier, D., DiMichele, W. A., Frank, T. D., Fielding, C. R., … Rygel, M. C. (2007). CO2-Forced Climate and Vegetation Instability During Late Paleozoic Deglaciation. Science, 315(5808), 87–91. doi:10.1126/science.1134207