" 3.2. Separate Treatment of CO2 Depending on Its Origin
The ambiguity is accompanied by inappropriate assumptions and speculations, the weirdest of which is that the behavior of the CO2 in the atmosphere depends on its origin and that CO2 emitted by anthropogenic fossil fuel combustion has higher residence time than when naturally emitted. This is clear in the IPCC AR5:

    Simulations with climate–carbon cycle models show multi-millennial lifetime of the anthropogenic CO2 in the atmosphere.
    [31] (p. 435)

It is also repeated in IPCC AR6:

    This delay between a peak in emissions and a decrease in concentration is a manifestation of the very long lifetime of CO2 in the atmosphere; part of the CO2 emitted by humans remains in the atmosphere for centuries to millennia."

...

"More recently, in their study entitled “The millennial atmospheric lifetime of anthropogenic CO2”, Archer and Brovkin [44] stated,

    The largest fraction of the CO2 recovery will take place on time scales of centuries, as CO2 invades the ocean, but a significant fraction of the fossil fuel CO2, ranging in published models in the literature from 20–60%, remains airborne for a thousand years or longer.

In addition, Archer et al. [45] stated,

    The models agree that 20–35% of the CO2 remains in the atmosphere after equilibration with the ocean (2–20 centuries)."

...

" Data
Systematic measurements of the atmospheric CO2 have been made since 1958 [52] by the Scripps CO2 Program of the Scripps Institution of Oceanography, University of California, and are available online [53,54,55]. The data include observations of CO2 concentration (in micro-moles CO2 per mole, or parts per million—ppm), and are processed to extract monthly values, filled in in case of missing data. Here, the monthly time series have been retrieved and processed for two stations, namely, Mauna Loa Observatory, Hawaii (19.5° N, 155.6° W, 3397 m a.s.l., 1958–present), and Barrow (recently renamed to Utqiagvik), Alaska (71.3° N, 156.6° W, 11 m a.s.l., 1961–present).
Data on global human carbon emissions are also available online for the years 1850–2022 and have been retrieved from [56,57]. The value of 2023 was taken as that of 2022 increased by 1.01, according to the International Energy Agency’s report [58] (p. 3)].
For conversion of different units, we use the following coefficients:

    From mass of C to mass of CO2, we multiply by 44/12 = 3.67 kg CO2/kg C (where 44 and 12 are the molecular masses of CO2 and C).
    From atmospheric CO2 concentration in ppm to total atmospheric mass in Gt CO2, we multiply by 7.8 Gt CO2/ppm CO2.

4.2. Premises of the Application
Based on the IPCC AR6 estimates of the global carbon balance [32] (Figure 5.12), Koutsoyiannis et al. [11] compiled a summary graph of total carbon emissions and sinks, distinguishing the preindustrial quantities (before 1750) and modern additions. This graph is reproduced here as Figure 9, after conversion from Gt C to ppm CO2.
Water 16 02402 g009
Figure 9. Annual carbon balance in the Earth’s atmosphere, in ppm CO2/year, based on the IPCC estimates [32] (Figure 5.12). The balance of 2.4 ppm CO2/year is the annual CO2 accumulation in the atmosphere. The total of the modern natural additions (64.2 + 36.5 − (52.2 + 25.6)) = 22.9 ppm is 4.4 times larger than the human emissions (4.4 + 0.8 = 5.2 ppm). (Adapted from [11]).
Based on this graph, we make the following observations, which are important for the modeling of the CO2 exchanges that follow:

    Human activities are responsible for only 4% of carbon emissions.
    The vast majority of changes in the atmosphere since 1750 (red bars in the graph) are due to natural processes, respiration and photosynthesis.
    The increases in both CO2 emissions and sinks are due to the temperature increase, which expands the biosphere and makes it more productive.
    The terrestrial biosphere processes are much stronger than the maritime ones in terms of both production and absorption of CO2.
    The CO2 emissions by merely the ocean biosphere are much larger than human emissions.
    The modern (post 1750) CO2 additions to pre-industrial quantities (red bars in the right half of the graph, corresponding to positive values) exceed the human emissions by a factor of ~4.5. In the most recent 65 years, covered by measurements, the rate of natural emissions is ~3.5 times greater than the CO2 emissions from fossil fuels.

Point 3 above implies a causality direction between temperature and CO2 concentration that is opposite to the popularly assumed one, which is also the one assumed and embedded in climate models. Indeed, according to conventional wisdom, it is the increased atmospheric carbon dioxide concentration ([CO2]) that caused the increase in temperature (T). However, this was questioned by Koutsoyiannis and Kundzewicz [59], while later Koutsoyiannis et al. [11,33,34] provided evidence, based on analyses of instrumental measurements of the last seven decades, for a unidirectional, potentially causal link between T as the cause and [CO2] as the effect. The same causality direction was confirmed for the entire Phanerozoic by using several proxy data series [35]."

https://www.mdpi.com/2073-4441/16/17/2402



"These values can hardly be reconciled with the fact that the residence time of CO2 is no more than 4 years, as admitted even by IPCC [32] (p. 2237)