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Circular reasoning

In the discussion about the CO₂ rise, it is common to use observed changes in nature as an argument to blame human emissions, for example changes in the relative concentration of oxygen, carbon isotopes or changes in the acidity of the oceans. This often involves circular reasoning: it is assumed that human CO₂ is responsible for the increased CO₂ concentration, the observed change is related to the increased CO₂ concentration, so the change is "evidence" of the human cause.

Ocean acidification

Ocean acidification is the ongoing decrease in the pH of Earth's oceans, primarily caused by the absorption of carbon dioxide from the atmosphere. Human emissions are blamed for that. The reasoning is as follows. From the human emitted CO₂ around 45% remains in the atmosphere. A large part of the rest is absorbed by the oceans. 99% of this CO₂ reacts with water to carbonic acid (H2CO3) which almost immediately dissociates to form bicarbonates, carbonates and hydrogen ions (H+). As a result, since the start of the Industrial Revolution, the average pH of the ocean surface has probably decreased from 8.19 to 8.05. See: Brittanica.

This is an brilliant example of circular reasoning. It is assumed that only human emissions are responsible for the extra absorption in the oceans, to conclude subsequently that the lower pH proofs that human emissions are the only cause of the increasing atmospheric concentration.

The reality is that the pH of the water is directly related to the atmospheric CO₂ concentration, independent of the cause of the change in concentration. CO₂ behaves like an ideal gas and dissolves from air into water until it reaches an equilibrium with dissolved CO₂, in line with Henry’s Law. Due to the described carbonate system, dissolved CO₂ results in a lower pH. So, in all cases an increasing CO₂ concentration (or pCO₂, partial pressure) leads to a decreasing ocean pH. See the relationship in Figure 4 (QES ).

The relationship between pCO₂ and pH
Figure 1: The relationship between the pH of the oceans and the atmospheric CO₂ concentration. Source: QES.

The decline of the oceanic pH is fully in line with the increased CO₂ level, but gives us no information on the cause of the increase.

Declining oxygen level

Another example of this reasoning is the decline of the oxygen concentration in the atmosphere. The idea is simple: in order to burn fossil fuels, you need oxygen, so this burning is responsible for the decline. In Figure 1 we see the declining trend combined with the increasing CO₂ concentration. Left is from Alert, Canada and right from Antarctica. The oxygen decline is relatively small, less than 0.002% per year.

the changes in the oxygen and carbon dioxide concentration
Figure 2: The decline of the O₂ concentration represented in the O₂/N₂ ratio compared with the increase of CO₂. Left: Alert, Canada, right: Antarctica. Source: Scripps Institution of Oceanography

The charts give a correct representation and there is no dispute about the amount of fossil fuel that we have used so far. It is however not at all a proof that human emissions are the cause of the oxygen changes. Oxygen isn't just used by the burning of fossil fuels, but is needed in all cellular respiration. Almost all the upward carbon flux to the atmosphere is the result of respiration, a biological process where carbohydrates and oxygen are converted to carbon dioxide and water. As we have seen in Figure 1 of The impact of greening, the natural respiration is much larger and has increased much more than human emissions.

The global oxygen cycle is too complex to draw simple conclusions about the human contribution. But given that CO₂ concentration in the atmosphere has increased and that O₂ is needed to produce CO₂, it is not surprising that the O₂ levels have slightly declined. But this is true regardless of the cause of the CO₂ rise.

Declining 13C ratio

13C is a stable isotope of carbon that differs the more common 12C isotope by having one extra neutron in its atomic structure. While 12C is far more abundant, 13C accounts for about 1.1% of naturally occurring carbon.

The ratio of 13C to 12C in atmospheric CO₂ has been decreasing over time, which is often regarded to be consistent with the burning of fossil fuels. The reasoning is as follows.

  1. Fossil fuels are derived from ancient plant matter.
  2. Plants preferentially absorb 12C during photosynthesis, making them depleted in 13C.
  3. When fossil fuels are burned, they release CO₂ that is enriched in 12C and depleted in 13C relative to the atmosphere.

As a result, the increasing proportion of 12C in atmospheric CO₂ creates a distinct isotopic signature that points to the combustion of fossil fuels as the source. This illustrated in Figure 2. δ13C (delta thirteen C) is metric of the relative ratio of 13C and 12C compared to a standard ratio. δ13C is an isotopic signature that measures the ratio of two stable carbon isotopes, 13C and 12C, in a sample. It is expressed in parts per thousand (per mil, ‰) relative to a standard. The δ13C value is calculated using the following formula: δ13C equation

[figure|Diagram of 13C in the global carbon cycle|/img/13C-ratio.png|c80|Diagram of 13C in the global carbon cycle showing the pools interacting with atmospheric CO₂ on the timescale of the Industrial Period. Typical ranges of δ13C are shown for each of the pools Global average δ13CO₂ was −8.4‰ in 2015 and −6.6‰ in 1850. Processes involving significant fractionation are shown in italics; processes without significant fractionation are shown in normal text. Source: Graven, H., Keeling, R.F. and Rogelj, J., 2020.

It is undeniable that δ13C levels have been decreasing while human emissions have been increasing since the Industrial Revolution. As illustrated in Figure 2, the combustion of fossil fuels, which are relatively depleted in 13C, can contribute to the reduction of δ13C levels. However, this is not the only contribution to the measured change. The biosphere emissions are more than 20 times larger than fossil fuel emissions and have increased much more the human emissions. As we can see in Figure 2, the isotopic signature δ13C of the biosphere is also much lower than that of the atmosphere.

By examining the isotopic data in four different observation sites, it has been demonstrated that the standard metric δ13C is consistent with an input isotopic signature that is stable over the entire period of observations (>40 years). This means that the changes in the 13C/12C ratio are not affected by increases in human CO₂ emissions (Koutsoyiannis, D. (2024a). A model reproduction under the hypothesis that changes seen in the isotopic composition of the atmospheric CO₂ are dominated by biosphere processes, show very good performance compared to the monthly δ13C observations, as can be seen in the example of Figure 3 of Barrow, Alaska.

Model reproduction of the monthly observations of evolution of δ13C at Barrow
Figure 3: Model reproduction of the monthly observations of evolution of δ13C at Barrow. Source: Koutsoyiannis, D. (2024a).

Concluding we can say that from modern instrumental carbon isotopic data of the last 40 years, there are no signs of human (fossil fuel) CO₂ emissions. The decline of the 13C/12C ratio confirms the major role of the biosphere in the carbon cycle and through this, in climate.




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