Temperature as the main driver

The global warming since the Little Ice Age is the most likely cause for the higher CO₂ concentration in the atmosphere. Higher temperatures lead to more emissions from soil and oceans and to more greening.

Global greening can explain the level of the CO₂ concentration in the atmosphere. The larger fluxes due to more photosynthesis and respiration, and the longer residence time are consistent with more CO₂ in the atmosphere. In a stable situation photosynthesis and respiration can be balanced, but new vegetation is a net sink as respiration lags photosynthesis. So there must be another reason for the observed rise.

The two most discussed options for this rise are human emissions and temperature. Human emissions are less than 5% of the natural emissions, but it is assumed that they disturb the natural balance and accumulate in the atmosphere. In this article we will show however that accumulation is not possible and that there is no physical justification for an exceptional (ad-hoc) behavior of human CO₂.

Several studies have indicated that global warming since the Little Ice Age was the main initial driver of increased CO₂ levels (Humlum, O., Stordahl, K. and Solheim, J.-E. (2013); Harde, H. (2019); Koutsoyiannis, D. (2024b)). Since 1750 the average global temperature has increased by 1.1 °C (IPCC (2023) to 1.5 °C (Copernicus), which has led to more emissions from the ocean and, more importantly, from the soil (Lee, J.-S. (2011); Palmer, P.I. et al., 2019).

Increased emissions from ocean and soil

Temperature is an important factor in Henry's Law. The solubility of CO₂ in sea water decreases at higher temperatures, see Figure 1. For the oceans and other waters higher temperatures lead to a decreasing solubility, which result in more emission and/or less absorption. The relative change per degree Celsius is however small (in the order of 3% at a temperature of 20 °C).

The temperature dependence of Henry's Law — Figure 1: The solubility of CO₂ in water decreases at higher temperatures. Source: The Engineering Toolbox.

Figure 2: Soil respiration is exponentially related to temperature. Source: Lee, J.-S. (2011).

A second, more important factor, is the temperature dependence of the biological processes that are responsible for the decomposition of organic matter in the soil and oceans. All cellular respiration releases energy, water, and CO₂ from organic compounds. From soil respiration, which includes microbial activity and root respiration, it is well known that it is exponentially related to temperature, see Figure 2 and Wikipedia.

The temperature dependence of biological processes is crucial in the understanding of the imbalance between the natural flows to and from the atmosphere. It is well summarized in this quote from Koutsoyiannis, D., 2024c:

❝..the biosphere processes offer the key mechanisms to this causal relationship. CO₂ is produced during degradation of biological matter, by bacterial and thermophilic processes. During cool periods, degradation slows more than photosynthesis, and this traps CO₂ into soil. During warm periods, carbon trapped in soils is released faster than photosynthesis can absorb it, and atmospheric CO₂ increases.❞

In this regard it is important that the total amount of ❛carbon trapped in soils❜ is very large compared to the annual difference between photosynthesis and respiration that was responsible for the atmospheric increase. According to the IPCC the total amount of organic material in the soils is around 1700 PgC (the total Dissolved Organic Carbon in the oceans 700 PgC).

Temperature induced greening

Higher temperatures also have a direct positive impact on global greening. Generally, plants tend to grow more quickly in warmer conditions, and elevated temperatures can extend the growing season in moderate to high latitude regions. Multiple studies have identified climate warming, after the higher atmospheric CO₂ concentration, as an important driver of this greening (Haverd, V. et al. (2020), Lai, J. et al. (2024), Zhu, Z. et al. (2016)).

The combined effects are illustrated in Figure 3. Higher temperatures increase emissions from soil and oceans, reduce the solubility of CO₂ in the oceans and contribute to the greening of the Earth. Combined with the increased respiration due to the greening of the Earth, the global warming since the Little Ice Age can explain why the yearly total up flux was larger than the down flux in the past decades and still is.

The present level in the atmosphere is a natural level — Figure 3: Higher temperatures since the Little Ice Age result in more emission from the oceans and soil and contribute to the greening of the Earth.