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.
 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.