The increased atmospheric CO₂ level is widely recognized as a primary driver of global greening (a 30% increase in GPP since 1900). It raises the question whether such an increased CO₂ level is also a necessary condition for a large GPP. This paper evaluates whether CO₂ levels during historical periods of similar or more greenness as today, are consistent with the widely held view that CO₂ levels remained below 300 ppm over the past 800,000 years, as indicated by Antarctic ice core records. 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. Natural fluctuations of the atmospheric CO₂ concentration can be well explained, based on the strong temperature dependence of the degeneration of carbon compounds that are stored in large quantities in the soil and the oceans.
