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The paper, Dependence of Earth’s Thermal Radiation on Five Most Abundant Greenhouse Gases1, has been reportedly rejected from several major journals. By analysing absorption spectra of greenhouse gases in more detail than has been done previously, it claims to show that CO2 is now effectively saturated (with respect to radiative forcing) and any further increase in atmospheric concentrations will lead to negligible temperature change at the surface. Their numerical results agree with previous literature on the subject. The authors are known climate emergency sceptics. While noting that it is obvious that if the result is correct there would be enormous political ramifications, my question is not concerned with those.

Presuming the article is not being published because it's so incorrect it doesn't merit peer review, I would like to know precisely and technically why the paper is wrong. The authors are physicists so I would like to understand its flaws from the perspective of climate science if possible.

I am specifically not interested in hearing arguments along the lines of "the IPCC is right and this is incompatible with that so it must be wrong" - again I am looking for a precise and technical rebuttal.

Thanks!

[1] W. A. van Wijngaarden and W. Happer, “Dependence of Earth’s Thermal Radiation on Five Most Abundant Greenhouse Gases.” arXiv, 2020. doi: 10.48550/ARXIV.2006.03098.

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    $\begingroup$ I've seen the basic argument before, and the model would seem to fail to explain the climate of Venus. $\endgroup$
    – gerrit ♦
    May 31 at 9:15
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    $\begingroup$ The paper does not say that further increase in CO2 will cause 'negligible' temperature rises. In fact its conclusions about the direct forcing of a doubling in CO2 are in line with values commonly quoted as you note. Adding in feedbacks you arrive at the range of forcing commonly accepted and backed up by observation. Only expert reviewers will know why the paper is (if it has been?) rejected., but it certainly doesn't overturn basic climate science. $\endgroup$
    – Andy M
    May 31 at 9:17
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    $\begingroup$ From the conclusions on Page 34, Doubling the CO₂ concentration (...) surface warming (...) result of 2.3 K, that is not consistent with "any further increase in atmospheric concentrations will lead to negligible temperature change at the surface". Their estimate is well below the range of estimates by other models, but 2.3 K is not negligible. $\endgroup$
    – gerrit ♦
    May 31 at 9:22
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    $\begingroup$ @gerrit The sensitivity to CO2 doubling (including feedbacks) has been debated for quite some time, resulting in a wide range of 1.5 to 4.5 K. The three key problems with this paper are (a) The authors think they've done something new, a line-by-line analysis. This is not new. (b) The authors repeatedly use the word "saturated" so as to downplay the effects. All atmosphere scientists worth their salt know this. It's why the effect of adding CO2 is logarithmic rather than linear. (c) The authors appear to have ignored feedbacks. They lament the lack of water vapor data. These data do exist. $\endgroup$
    – David Hammen
    Jun 1 at 10:49
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    $\begingroup$ Outgoing IR (from ground in one direction) being absorbed within the atmosphere, then reradiated in all directions (half up, half down) before it reaches space - halving how much (of the portion absorbed is going spacewards - is how the greenhouse effect works, If 'saturation" means all is absorbed and none makes it all the way to space in one go, it just means it is absorbed and re-radiated lower in the atmosphere, which retains more heat - and that is the very definition of enhanced greenhouse effect. $\endgroup$
    – Ken Fabian
    Aug 15 at 23:59

2 Answers 2

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To answer the original question about CO2 saturation, I'll describe spectral absorption. Each gas in the atmosphere absorbs light at specific wavelengths, and some wavelengths are absorbed more than others. The spectral absorption of CO2 is shaped like a V. With no CO2, the radiative spectrum of Earth would resemble a black-body. When CO2 is added, this 'V' is pushed into the black-body spectrum, making a notch. At present concentrations, the bottom of this notch is 'saturated', meaning it hits the bottom. As more CO2 is added, the V is pushed further down into the radiative spectrum. The bottom certainly remains saturated, but the edges of the notch continue to erode.

There are other problems with this paper.

Figure 6 in the paper shows that doubling the atmospheric concentrations of CO2, N2O, and CH4 would cause increases in radiative forcing. They claim these increases are negligible, but they are not. Their own data show that doubling CO2 would cause an additional increase of 3.74 W/m2. To put this in perspective, all radiative forcing to date, referenced to the start of industrial times in 1750, is 3.4 W/m2. Doubling N2O would cause an additional 1.99 W/m2. Doubling CH4 would cause an additional 1.12 W/m2. These numbers are not "negligible".

Figure 15 in the paper presents their model's results, side-by-side with satellite measurements. To the naked eye, they look good. However, when they are plotted against each other, they do not match. As their model is meant to study spectral absorption, you might think their model would match the satellite data in the "spectral window" where no absorption occurs. It does not. Their model must be scaled by +5.6%, -3.9%, and -12.5% for the Sahara, Mediterranean, and Antarctic datasets, respectively. Even after adjusting the model to match the satellite data, the model overestimates the amount of outgoing radiation. This infers that the Earth is staying cool, when in fact it is retaining heat. The discrepancy is on the order of 3-5% globally, which is quite large.

In answering this question, I have used the data presented in the paper itself. As the authors did not provide the raw data, I used software to digitize the graphical data. My results may be off by a decimal place, but they are in the ballpark.

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  • $\begingroup$ It would be helpful if the answer could include a sentence on how close models generally are expected to match reality in this field. In my personal work experience in other fields (which is admittedly dated by now), a particular predictive model matching to within 10% of the current reality on a particular metric can be acceptable, and matching to within 5% would be excellent. As a consequence, one may look towards the use of ensemble modelling. $\endgroup$
    – njuffa
    Sep 9 at 22:14
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The abstract of the paper says it is based on the assumption absorption bands are not saturated. This is true only at low levels of CO2, much lower than at present. See NASA Technical Memorandum 103957, Appendix E, which shows that zero IR energy gets to space in the 14-16micron band. That is, the band is salready saturated

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  • $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$
    – Peter Jansson
    Sep 9 at 16:13
  • $\begingroup$ Where's this abstract at? $\endgroup$
    – JeopardyTempest
    Oct 25 at 6:40
  • $\begingroup$ Being saturated just means all the outgoing IR is caught within the atmosphere, where it re-radiates equally in all directions, ie half going back down. More CO2, the lower down it gets absorbed and re-radiated as well as the higher up it must re-radiate to escape to space. More energy is retained within the atmosphere. Almost the textbook definition of global warming. $\endgroup$
    – Ken Fabian
    Oct 28 at 0:31

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