Not only does CO2 have no discernible effect on climate, but any alleged anthropogenic role within the hypothetical greenhouse effect is not detectable either.

In recent decades there has been a concerted effort to assert it is “settled” science to characterize variability in the atmospheric CO2 concentration – assumed to be modulated by human activity – as the predominant factor in both climate change and the so-called greenhouse effect.

Science, however, is never truly settled.

A new Frontiers study succinctly unsettles this prevailing paradigm with surgeon-like precision. In under 20 pages the authors deliver a cogent critique of the CO2-drives-climate presumption. A few of the key points include:

• CO2 only contributes about 4-5% to the greenhouse effect, whereas water vapor and clouds contribute 95%.

• Of that 4-5% greenhouse effect contribution from CO2, just 4% of that can be attributed to human activities (i.e., fossil fuel emissions). Thus, about 96% of the 4% contribution from CO2 can be attributed to natural processes.

“WV [water vapor] and clouds (for which WV is responsible) dominate the ARE [atmospheric radiative effect], while CO2 contributes only 4-5% to it. Also, anthropogenic CO2 emissions are only 4% of the total, with the vast majority (96%) being natural. Additionally, evidence suggests that changes in temperature precede those in CO2 concentration, thus challenging the assumption that CO2 drives temperature.”

• As Fig. 10 in the study indicates, observed changes in the atmospheric CO2 concentration cannot be demonstrated to have exerted any effect in altering longwave radiation measurements, much less the surface temperature. A hypothetical doubling the CO2 concentration [NC-RAGs, or non-condensing radiatively active gases] “results in a temperature increase of zero”.

“[W]hile the role of CO2 in photosynthesis is important in biochemical terms, it becomes negligible in terms of its contribution to the surface energy balance.”

“[T]he observed increase of the atmospheric CO2 [from 300 ppm to 420 ppm] has not altered the ARE [atmospheric radiative effect or greenhouse effect] in any discernible way.”

Image Source: Koutsoyiannis and Tsakalias, 2025

Greenhouse Effect and Greenhouse Gases = Atmospheric Radiative Effect and Radiatively Active Gases

Common-use terms like greenhouse effect and greenhouse gases are misrepresentations of what occurs in the real-world atmosphere. Heat transfer for both a greenhouse and in the real-world surface-troposphere is dominated by convection, not radiation.

Atmospheric mass is much denser near the surface, decreasing with altitude. This leads to a 6.5°C per km temperature gradient in the troposphere.

“[H]igher atmospheric mass increases the heat capacity of the atmosphere, and thus decreases the surface net radiative cooling [and] increases the global mean surface temperature” – Chemke and Kaspi, 2017

So, for example, while the temperatures at the base of equatorial Mount Kilimanjaro range around 24°C annually, the mean summit temperatures average about -18°C. This 42°C temperature differential is similar, physics-wise, to the 36 Kelvin (K) surface warming (252 K vs. 288 K) commonly attributed to the Earth’s so-called greenhouse effect (atmospheric radiative effect), or to greenhouse gases (radiatively active gases) like CO2 and water vapor.

But just as the summit-to-base 42°C Kilimanjaro temperature differential has to do with the lapse rate/temperature gradient, and not the radiative effect of variations in the concentration of gases like CO2, so too does the 36 K temperature differential for the surface-atmosphere. Thus, CO2, a non-condensing radiatively active gas (NC-RAG), can be said to have exactly zero effect on the 252 K vs. 288 K temperature gradient.

“Hence, it is the temperature gradient that makes the surface-level temperature increase from about 252 K … to about 288 K (i.e., by 36 K). This increase is usually attributed to the ‘greenhouse effect’, but it is mainly the result of the temperature gradient.”

“The effect of the NC-RAG [non-condensing radiatively active gases] is zero for an isothermal atmosphere.”

Image Source: Koutsoyiannis and Tsakalius, 2025

The paper – including the supplementary data compilation – is notable both for its concise simplicity and its wide-ranging coverage in critiquing the “settled” significance of the CO2 impact.