“[T]he contribution of CO2 to the greenhouse effect is 4% – 5%. Human CO2 emissions represent 4% of the total, which means that the total human contribution to the enhancement of the greenhouse effect is 0.16% to 0.20% – a negligible effect.” – Dr. Demetris Koutsoyiannis (2024)
New research exposes the vacuousness of the “imaginary world” models proclaiming CO2 the dominant regulator of the Earth’s climate.
A CO2-less or CO2-only atmosphere…an imaginary-world thought experiment
An oft-heard claim is that, due to the prominence of CO2 as the Earth’s climate “control knob” (see Lacis et al., 2010), the greenhouse effect could not exist – indeed, it would collapse – if there was no CO2 in the atmosphere.
However, it should go without saying that this CO2-less atmospheric condition itself is an imaginary-world conceptualization. Thus, fantasizing about what would happen if the atmosphere was comprised 0 ppm CO2, 1,000,000 ppm CO2, 0 ppm water vapor…are all just untestable, never-observable thought experiments. They cannot be subjected to the scientific method. Thus, they are unfalsifiable.
CO2’s climate effects are undetectable
Of course, this very unfalsifiable thought experiment is what believers in the CO2-is-the-climate-control-knob narrative rest their case on. But even if we do use this imaginary-world premise, the existing models (MODTRAN, HITRAN) that allegedly support the CO2-controls-climate orthodoxy actually undermine it.
For example, as Dr. Koutsoyiannis points out in his extensively-referenced paper, the MODTRAN data show that the Earth’s temperature remains at the default greenhouse-effect value if the water vapor scale is adjusted upwards slightly, by just 30%. The greenhouse effect does not “collapse” as claimed by Lacis et al. disciples – the control-knob believers.
The HITRAN database also indicates the imaginary-world condition of doubling CO2 from 400 to 800 ppm would only alter the radiation flux at the top of the atmosphere by -1.1%, a hypothetical realization that could not even be detected in future macroscopic measurements (if the atmosphere ever were to reach a CO2 concentration of 800 ppm).
MODTRAN models further affirm that (1) there is only a 1% temperature difference between either doubling (800 ppm) or halving (200 ppm) the atmospheric CO2 concentration, and (2) the change in the downwelling radiation resulting from an increase from 300 ppm (1900) to 420 ppm (2023) is only 0.5%. These tiny changes “could not be discerned by observations.”
CO2 has not been detectably reducing the outgoing longwave radiation
As a “control knob” for the climate, CO2 is believed to be the principal reason why the Earth’s outgoing longwave radiation (OLR) should be reducing (Dewitte and Clerbaux, 2018).
“…the increase of Greenhouse Gasses (GHG) primarily CO2 reduces the OLR”
However, despite the increase in CO2 concentration in recent decades, OLR has been increasing. Any reduction in OLR from the CO2 effect is not detectable.
Likewise, the MODTRAN clear-sky results (i.e., an atmosphere where no clouds exist and CO2 and water vapor are the main greenhouse effect agents) show OLR has increased by +0.38 to +0.60 W/m² from 2001 to 2022, consistent with the increase in temperature.
According to the data reviewed by Dr. Koutsoyiannis, any record of decrease in OLR “can hardly be attributed to increased CO2, but it can be related to water vapor and cloud profiles,” as the “effect of CO2 is trumped by the effect of clouds, which is consistent with the major role of water on climate and the minor one of CO2.”
95% of the greenhouse effect (downwelling radiation) is from water vapor, clouds
Per the MODTRAN calculations, CO2’s contribution to the planetary greenhouse effect is only 4% to 5%, and the contribution from water vapor and clouds is 87% (outgoing radiation) to 95% (downwelling radiation). The CO2 contribution to greenhouse effect changes over the last century, as CO2 increased from 300 ppm to 420 ppm, is only about 0.5%. This quantified percentage is “below any threshold to make it observable.”
Worse, when we consider that human CO2 emissions are only 4% of Earth’s total CO2 emissions, and that the CO2 contribution to the greenhouse effect is just 4% to 5%, “the total human contribution to the enhancement of the greenhouse effect is 0.16% to 0.20%.”
Supporting evidence for CO2’s negligible role
Another set of sources supporting the contention that CO2’s greenhouse effect contribution is only 4% to 5% comes from the notorious Trenberth radiation budget charts. First, notice the 333 W/m² value identified for downwelling radiation from greenhouse gases (lower right).
Now consider the estimated greenhouse effect (downwelling radiation) contribution from CO2 alone as shown in Schmithüsen et al., 2015.
Notice it ranges from about 0 W/m² (or less) near the poles, about 10-15 W/m² at mid-high latitudes, and about 25 W/m² around the tropics. Averaged over the globe, it would appear the total mean CO2 greenhouse effect contribution is about 13 to 15 W/m². If CO2 indeed accounts for ~14 W/m² of the 333 W/m² downwelling radiation from greenhouse gases, its contribution is 4.2%. The other ~95%, or ~320 W/m², is necessarily from water vapor and cloud.
As shown in the Koutsoyiannis review of results from HITRAN database, a doubling of CO2 from 400 to 800 ppm only produces a ~1.1% perturbation to the radiative flux at the top of the atmosphere (3 W/m²), which is not discernible. This is consistent with Chen et al. (2024), who assess that a doubling of CO2 produces a negative top-of-atmosphere (TOA) forcing (-1 W/m²), and a positive TOA forcing of just +2-3 W/m² in the mid-high latitudes and tropics.
These papers further establish the negligible role of CO2 in the Earth’s greenhouse effect.
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