This logarithmic fall-off in the effect of atmospheric CO2 is also discussed by Gerlich and Tscheuschner in their paper Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics.
From the Physics Conclusion,
“The wavelength of the part of radiation, which is absorbed by carbon dioxide is only a small part of the full infrared spectrum and does not change considerably by raising its partial pressure.”
In other words, more CO2 has little effect, since what CO2 is in the atmosphere is already absorbing most of the energy in the fraction of the infrared spectrum that CO2 can affect. More CO2 just increases absorption of an increasingly smaller amount of radiant energy, hence the logarithmic fall off.
That is a very worthwhile paper on this subject, by the way, though very technical.
Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics
http://arxiv.org/abs/0707.1161
Right, and available as a pdf with just a click. Thanks!