From No Trick Zone

By Kenneth Richard

It is widely acknowledged that the enhanced CO2 fertilization effect due to the modern rise in atmospheric CO2 is the predominant driver of the dramatic increases in global greening, or “gross primary production” (GPP). Global greening attribution estimates for CO2 fertilization range from 70% (Zhu et al., 2016) to 86% (Haverd et al., 2020).

As noted in a new study, Earth is thought to have been much greener than today during the Early Holocene (10,000 years ago), with over 50% more forest cover.

Since an increase in the CO2 concentration is recognized as the most dominant factor in greening, it would not be consistent with what we know about the requisite physics of GPP (CO2 residence time, water and nutrient availability, the diminishing returns effect of CO2 fertilization) to assume the lower levels of CO2 (under 300 ppm) suggested by Antarctic ice core records are accurate reflections of the actual CO2 levels from 10,000 years ago.

To achieve the high levels of Early Holocene greenness, CO2 levels needed to be as high or higher than today’s 420 ppm at that time too. The Earth could not have been as green as it was back then with atmospheric CO2 concentrations of only 280 ppm or less.

“Employing Mitscherlich’s Law, the research models the global GPP response to increasing CO₂, based on the mean value of eight different long-term GPP datasets. It illustrates a diminishing return of vegetation associated with rising CO₂, as additional factors such as nutrient and water availability impose constraints on the fertilization effect. Due to this diminishing return the average residence time of CO₂ in the atmosphere increases significantly with higher GPP values. High CO₂ levels, similar to today’s, were therefore necessary for comparable GPP during green periods like 10,000 years ago. A CO₂ concentration of 280 ppm would only be possible if nature’s response to CO₂ were fundamentally different from what we observe today, with other constraining factors exceptionally more favorable.”

Interestingly, a chart from the paper uses the IPCC’s carbon cycle estimations to asses nature’s CO2 emission rates have risen 4 times more (+44 GtC/year net, or from +166 GtC/year in 1750 to +210 GtC/year in 2022) than human emissions rates have (from 0 GtC/year in 1750 to +11 GtC/year today). Since Earth’s natural sinks don’t distinguish between natural vs. human CO2 emissions, the bulk of the modern increase in CO2 can be said to have been derived from rising natural CO2 emissions, not human activity.