I have found what appears to be an explanation of the difference in the percentage of the so-called "greenhouse gas" effect attributed to CO2 between the "believers" and the "skeptics" of the global warming theory. (The "believers" cite 20% while the "skeptics" cite 3 or 4 %; this difference matters a lot in the anticipated effect on average surface temperature due to higher CO2 concentrations in the atmosphere.)

A paper by J.T. Kiehl and K. E. Trenberth titled "Earth's Annual Global Mean Energy Budget" concludes that the CO2 effect is 20% of the total "in clear air." The point in the paper is that water vapor only contributes 60% when there are no clouds, the clouds containing liquid water rather than water vapor. This explains the 20% for CO2, but is a bit dodgy because the water in the clouds is always in the atmosphere. In fact, oceans are cloudy about 65% of the time while land areas are cloudy about 50% of the time according to my missile design handbook. It is a bit misleading to ignore the clouds. It might also be noted that the Global Circulation Models used for IPCC projections of future climate do a poor job of predicting cloud action.

The Kiehl and Trenberth paper has CO2 absorption centered at 15 microns, but has a CO2 absorption band of 12 to 18 microns. That is much wider than my missile design handbook, which only shows about a 1 micron band. The larger band seems to be predicated on the assumption that higher concentrations of CO2 will cause the molecular spin to change, causing the wider bandwidth. I've got to think about that one for a while. It has been 40 years since I did any work on the thermodynamics of matter. (I can accept that argument for the Venus atmosphere because it is much hotter than the Earth's atmosphere, so the CO2 molecules would have more spin action.)

Of course the "skeptics" case rests on more than just the level of CO2 absorption of infrared radiation. Basically, the argument is that radiation to the atmosphere is not the dominant mechanism for transfer of heat from the surface to the atmosphere. Convection heat transfer to the air due to wind, and latent heat (ie, evaporation of water on the surface and condensation at higher altitudes) are also major heat transfer mechanisms. There are some other major issues that I have not seen addressed, such as why the average temperature has not been going up with increased CO2 concentration in the atmosphere as required by the CO2 warming theory.