The impact of global greening
A central element in the present thinking about atmospheric CO₂ is the belief that there used to be a stable natural CO₂ concentration of around 280 ppm.
However, it is easy to see that there is no single natural level, but that this level depends on the greenness of the Earth. The concentration is directly related to the total amount of photosynthesis from plants and plankton.
No single natural level
- On average CO₂ remains for about 4 years in the atmosphere. This is called the Residence Time (RT). With a RT of 4 years, every year about a quarter leaves the atmosphere. CO₂ leaves the atmosphere in a downward direction, so the Down flux = CO₂ atmosphere / RT, or: CO₂ atmosphere = RT x Down flux. This means that the concentration is proportional to the down flux and the residence time. This is undisputed for natural fluxes. You can find it in every IPCC-report and a simple animation is added at the end of this article. So, regardless of why the fluxes have changed, the present CO₂ level is consistent with the present down flux and residence time.
- Both the down flux and the residence time have increased since 1750 as a result of the greening of the Earth. More vegetation means more photosynthesis, and so also more down flux as almost all the down flux is the result of photosynthesis. Also, the change in RT can be explained by natural causes. The down flux has increased with 29% and the residence time by 16% as can be seen in Figure 1. Together they fully explain the 50% increase of the CO₂ concentration in the atmosphere. This data is sourced from the IPCC (AR6) and the Global Carbon Project 2023.
This means that the present CO₂ level in the atmosphere can be regarded as a natural level. It is directly related to the greenness of the Earth in terms of Gross Primary Production (GPP, the sum of all photosynthesis). If due to whatever reason the global biological activities (photosynthesis and respiration) increase, both up and down fluxes will increase, leading to a higher CO₂ level in the atmosphere (and vice versa). More GPP goes together with a higher atmospheric concentration (whatever the cause).
The two-way relationship of greening and CO₂
The changes in fluxes and residence time are related to increased biological activity. Long-term satellite records revealed a significant global greening of vegetated areas. Due to this greening there is more photosynthesis which is measured in 'gross primary production' (GPP). The global terrestrial GPP has gone up by more than 30% since 1900 (Haverd, V. et al. (2020), Lai, J. et al. (2024)). See Figure 2. A similar effect has been observed in the oceans, where increased levels of dissolved CO₂ lead to more photosynthesis by phytoplankton.
The greening of the Earth is for a significant part the result of the increased CO₂ concentration. Other factors include climate warming and nitrogen deposition. This means that the CO₂ concentration can be characterized as positive feedback for greening. More CO₂ leads to more greening, but greening is also conditional for a higher atmospheric concentration. We can compare it with the two-way relationship between humidity and plants: while humidity is crucial for plant growth, plants also contribute to the humidity in their environment.
Longer residence time
The 16% change in residence time from 3.5 to 4.1 years is also the result of natural factors. A longer residence time indicates that a larger concentration is needed for the same uptake rate. There could be many causes for this behavior, but satellite observations indicate that it can be related to constraining factors in the fertilization. CO₂ is just one limiting factor of photosynthesis; other factors include phosphorus, nitrogen, and water availability. With increasing CO₂ levels, the fertilization may be slowed down, due to the constraining effect of soil nutrients and soil water (Wang, S. et al. (2020)). So, the increase in residence time can at least partly be linked to the increased greenness of the Earth. The residence time is also influenced by global warming. Several studies have suggested that higher temperatures result in not just more emissions, but also reduced absorption with a consequently longer residence time (Harde, H. (2017a)).
Uncertainties
The 29% increase of the down flux and 16% increase of the residence time are based on estimates as presented in Figure 1. The increase in down flux is consistent with the findings of the aforementioned studies on global greening. Particularly for the year 1750, the accuracy of the data is nevertheless questionable. The atmospheric concentration for that year is derived from ice core proxies from Antarctica, while the down flux is based on model calculations by the IPCC (2023). Both sources are subject to considerable uncertainty. It is quite possible that, for example, the down flux was smaller and the RT at a comparable level as today. However, if the CO₂ concentration is now 50% higher than in 1750, this increase would still be the result of the combined natural changes in down flux and residence time. The conclusions regarding the natural character of the present CO₂ level and the dependence of the Earth's greenness would not be fundamentally different.
New vegetation net sink?
It might appear paradoxical that the Earth's greening is an explanation for a higher atmospheric concentration, as new vegetation absorbs CO₂. New vegetation, such as young plants and trees, generally acts as a net sink of CO₂, meaning it absorbs more carbon dioxide from the atmosphere than it releases. So, during a period of greening the respiration is partly delayed compared to the photosynthesis. This means that only greening cannot be the cause of the CO₂ rise. An increase of the CO₂ concentration is only possible if more CO₂ is coming in (up flux) than going out (down flux).
From the numbers in Figure 1 we can see that this is indeed the case, so there must be another cause that respiration has increased more than photosynthesis. It is however not difficult to see that this is possible as greening is a result of environmental changes.
- The delayed respiration due to new vegetation on an annual basis is relatively small. The above mentioned greening of 30% over 120 years, corresponds to an average increase of only 0.22% per year
(1 + 0.0022)120 = 1.3 . - The total amount of organic material on and in the soil is according to the IPCC 1700 PgC. So, the relative impact of new vegetation on this stock is small.
- The respiration from the soil is not just influenced by the amount of organic material in the soil, but also by environmental factors, in particular temperature.
In the next article we will discuss in more detail the impact of increased global temperature on bacterial processes in soil and ocean as a potential cause of the increasing CO₂ concentration.
Larger fluxes or longer residence time means higher level
The CO₂ concentration in the atmosphere is proportional to the down flux and to the residence time. This means that the CO₂ level goes up when there is more down flux and/or a longer residence time. To illustrate this mechanism, we have made a simple animation with the CO₂ represented as water in a reservoir. If the up flux increases, the water level will rise. Due to the higher level also the down flux increases, until up and down are equal again. A longer residence time, in this case a smaller drain, also leads to a higher level in the reservoir.