The Carbon Conundrum: Did Earth’s Greenest Periods Demand High CO2?

A “green Earth” is inextricably linked to high atmospheric CO₂ concentrations

A new paper by Dutch independent scientist Frans J. Schrijver, Historical CO₂ levels in periods of global greening^[1], looks at CO2 and increased global greening. 

The Earth is getting greener. Satellite data confirms a significant increase in vegetation—or global greening—over the past century, with Gross Primary Production (GPP) –the rate of carbon fixation by plants– rising by over 30% since 1900.

The prevailing scientific consensus attributes this surge primarily to the CO₂ fertilization effect, meaning higher atmospheric carbon dioxide allows plants to photosynthesize more effectively.

How could planet be greener with less CO2

Historical records suggest that today’s level of greenness is not unique. For instance, approximately 10,000 years ago, Earth’s forest area was an estimated 50% greater than it is today. This presents a fundamental puzzle: How could the planet be so much greener in the past if, as ice core records suggest, atmospheric CO₂ concentrations were significantly lower—typically around 280 ppm—and remained below 300 ppm for the past 800,000 years?

The law of diminishing returns: A green Earth’s true cost

A recent publication applied Mitscherlich’s Law—also known as the Law of Diminishing Returns in Agriculture—to model the relationship between global GPP and atmospheric CO₂ levels.

The core idea was that while CO₂ fuels initial growth, its “fertilization effect” eventually slows down because other essential factors, like nutrient availability (e.g., nitrogen and phosphorus) and water, become limiting constraints. The model found this diminishing return effect provides a strong fit to recent GPP data.

This scientific model reveals a critical consequence: the diminishing returns lead to an increasingly longer average residence time for CO₂ in the atmosphere as GPP rises. Because the total mass of CO₂ in the atmosphere is directly proportional to both the rate of carbon uptake (the down flux, largely GPP) and its residence time, the study concludes that a “green Earth” with high GPP is inextricably linked to high atmospheric CO₂ concentrations.

Challenging the Historical Narrative

Applying this model to the past yields a powerful contradiction to the conventional view:

Historical Greenness Requires High CO₂: Since the estimated global GPP 10,000 years ago was at least as high as today (potentially 4.4% larger than today, even accounting for modern-era land use change), the average CO₂ residence time must have been at least as long as the current 4.1 years. Therefore, high CO₂ levels, comparable to or exceeding current levels, were necessary to support periods of similar greenness in history.

The Low CO₂ Impossibility: If CO₂ levels were truly the accepted low value of 280 ppm during such a highly green period, the CO₂ residence time would have been unnaturally short (around 2.7 years). The authors argue this is only possible if nature’s response to CO₂ was fundamentally different then, with other growth constraints (like nutrients and water) being significantly more favorable, which they deem very unlikely.

The core conclusion is that the assumption of low historical CO₂ levels—as indicated by ice core proxies—is inconsistent with periods of high global greenness.

Temperature, the other driver

The paper further suggests an alternative causality for natural CO₂ fluctuations that is independent of human emissions.

While atmospheric CO₂ is the main driver for the down flux (photosynthesis/greening), temperature is identified as the most important driver for the up flux (natural emissions).

Large quantities of organic carbon are stored in the soil and oceans. The process of decomposition and respiration of this organic material (the source of the up flux) is strongly dependent on temperature.

Therefore, a period of rising temperatures would lead to:

1. More respiration/outgassing from the soil and oceans (up flux).
2. A higher atmospheric CO₂ level.
3. More greening (down flux).

In this view, greening is not the cause of high CO₂ but rather its result. This supports other studies suggesting temperature is the primary driver of natural atmospheric CO₂ fluctuations.

The findings present a significant challenge to the conventional understanding of the Earth’s carbon cycle history, suggesting that periods of natural abundance and high GPP may have always been tied to atmospheric CO₂ levels comparable to or higher than those observed today.