Comment on Koutsoyiannis, D. Net Isotopic Signature of Atmospheric CO₂ Sources and Sinks: No Change Since the Little Ice Age. Sci 2024, 6, 17

In his paper [1], Koutsoyiannis tries to refute one of the main arguments against his idea that the undisputed accumulation of CO₂ in the atmosphere over the last >100 years is of natural and not of anthropogenic origin [2]. The point is that, according to the prevailing notion, the isotopic ratios of the carbon in the CO₂ molecules provide evidence that the additional CO₂ is due to human activities. However, the paper claims otherwise.

Before discussing his hypothesis, a few remarks on the basics: Isotopes are atoms of an element with the same number of electrons and protons but different numbers of neutrons. All naturally occurring elements occur as isotopes, with one isotope being by far the most common, often the isotope with the same number of protons and neutrons. In the case of carbon, ¹²C (6+6) is by far the most common, while ¹³C with one more neutron in the nucleus is much rarer. Koutsoyiannis does not consider the third of the essential isotopes of carbon, the radioactive ¹⁴C (although this isotope was the first indicator of carbon enrichment from fossils fuels even before CO₂ measurements from Mauna Loa were available [3]), in his work, nor does he go into the oxygen isotopes [1]. To keep this from getting out of hand, I will also limit myself to these two isotopes. The abundance ratio of ¹³C and ¹²C changes in different environmental systems; the ratio is given as δ¹³C in parts per thousand, where the value 0 is defined on the basis of an internationally accepted standard (also known as VPDB). It is undisputed that δ¹³C has been decreasing in recent decades, as also shown in Figure 2, left of [1].

Changes in isotope abundance ratios between different environmental systems or geographic regions are called fractionation. In the case of carbon, there is a dominant fractionation process: plants preferentially take up the lighter CO₂ molecules with the isotope ¹²C, so that living organisms (animals feed directly or indirectly on plants) contain less ¹³C, while the atmosphere and the oceans are relatively enriched in ¹³C because it remains behind. So, when organic matter, such as fossil fuels, is burned, its isotopes mix with those in the atmosphere and δ¹³C is modified [4].

Koutsoyiannis arrives at a completely diametrically opposed interpretation of the measured decrease of δ¹³C, which he summarizes as follows:

“These results confirm the major role of the biosphere in the carbon cycle and an undetectable human signature. … Immediately after [the Little Ice Age] (Addition by the present author), a warming period began that has lasted until now. The causes of the warming must be analogous to those that led to the Medieval Warm Period around 1000 A.D., the Roman Climate Optimum around the first centuries B.C. and A.D., the Minoan Climate Optimum around 1500 B.C., and other warming periods throughout the Holocene; As a result of recent warming, and as explained in Koutsoyiannis & Kundzewicz (2020), the biosphere has expanded and become more productive, leading to increased atmospheric CO₂ concentrations and the greening of the Earth; As a result of the increased CO₂ concentration, the isotopic signature δ¹³C in the atmosphere has decreased”.

Ref. [1]

He is, in this way, formulating a scientific hypothesis. A basic tenet of the philosophy of science is that hypotheses cannot be proven. They can, however, be falsified (this is how scientific knowledge is gained: false hypotheses are eliminated). One approach is to formulate predictions based on the hypothesis and test whether they are correct. Applying this here, we can formulate: If the hypothesis is correct that the isotope signature δ¹³C becomes negative as a result of natural warming, then a similar correlation must be found for warming phases in climate history.

This can be tested because, for example, ice cores from Antarctica have preserved air from the past in bubbles trapped in the ice, which can be measured in terms of δ¹³C. In particular, the phases of global warming following glacials (also known as terminations) and cold stadials should show the presumed effect even more clearly than the current warming of the last few decades. Carbon isotope ratios reconstructed over the last glacial cycle clearly indicate that all three major warming episodes were characterized by increasing δ¹³C values, whereas the hypothesis of [1] would predict exactly the opposite (Figure 1).

At a much smaller temporal scale, the hypothesis would also demand that δ¹³C values are lower in winter than in summer, i.e., that they are negatively correlated to temperature and positive to the interannual fluctuations of atmospheric CO₂, which is lower in winter than in summer. Ref. [1] provides as his Figure 7 a graph of this relationship, which I have slightly modified and supplemented in Figure 2.

The reciprocal presentation of CO₂ depicted in red suggests a positive correlation between δ¹³C and CO₂, at least at first glance, but the curve in blue shows that it is actually a negative correlation and that the seasonal pattern of δ¹³C is anticyclical. In spring and especially in summer in the northern hemisphere, when plants are actively photosynthesizing they remove CO₂ from the atmosphere and fixate it in biomass. This leads to a decrease in the CO₂ concentration at this time of year. At the same time, ¹²C is preferentially removed from the atmosphere, and the proportion of the heavier isotope accumulates relatively in the atmosphere. In the fall and winter, as plant activity decreases and organic matter decomposes, CO₂ is released back into the atmosphere mainly through cellular respiration. This leads to an increase in CO₂ concentration and an increase in δ¹³C.

In none of the precedents of the past, nor in the interannual cycles typically determined by the biosphere of the northern hemisphere, is there a correlation in which warming leads to a decrease in δ¹³C. According to current knowledge, the latter can only be explained by the fact that more light oxygen isotopes are released into the atmosphere each year than the biosphere can absorb—despite, not because of, the increased photosynthetic capacity of the biosphere. The current decrease of δ¹³C despite rising temperatures can only be derived from organic matter with a low δ¹³C, such as coal, oil, and natural gas. For the first time in the last glacial cycle, the trends in δ¹³C and in temperature are in opposite directions, not parallel. Koutsoyiannis’ [1] mistake is to assume or pretend that an increase in photosynthesis should lead to a decrease in δ¹³C, whereas the exact opposite is the case.

Concluding remark: Koutsoyiannis [1] argues that the decrease of δ¹³C in CO₂ molecules in recent decades is due to the expansion of the biosphere due to global warming and not to human activities. Herein I show that δ¹³C isotope ratios (1) from Antarctic ice and (2) during the northern hemispheric summer increase during periods of warming. Both findings clearly contradict Koutsoyiannis’ hypothesis, suggesting that the observed change in ratios is not due to global warming, but is related to its causes.