Can someone please exapain the "CO2 doesn't saturate" argument?

To be honest, I've never understood the "CO2 saturates" argument. Even if the bottom layers of the atmosphere were saturated (and I'm not saying they are), then those layers will still radiate to layers above them that are NOT saturated. Since CO2 is well-mixed in the atmosphere, those layers will warm and radiate also. Also, I don't see radiation as "trying to escape the planet." Blackbodies radiate following Stefan's Law. I don't really follow your logic about being warmed by some "constant amount." If more energy keeps coming in than is going out, the planet keeps warming; it is only when equilibrium is achieved that the warming stops.

Hm, since the planet will continually move toward radiative equilibrium, the assumption that we'll reach a point at which all radiation will be blocked and thus no more CO2 ever can cause further warming seems specious - wouldn't pressure broadening (and such) continue to take effect and CO2's bands widen? I think there are others here that are more qualified to answer this, though.

However, the article does briefly mention this, near the end:

"In any event, modern measurements show that there is not nearly enough CO2 in the atmosphere to block most of the infrared radiation in the bands of the spectrum where the gas absorbs."

They link to another post, Part II of the one you're reading:

http://www.realclimate.org/index.php/archives/2007...

From a practical standpoint, it appears that the absorption bands of CO2 will not broaden at a rate fast enough to matter in terms of saturation, considering projected emission scenarios, for instance.

From your link

<<<What happens if we add more carbon dioxide? In the layers so high and thin that much of the heat radiation from lower down slips through, adding more greenhouse gas molecules means the layer will absorb more of the rays. So the place from which most of the heat energy finally leaves the Earth will shift to higher layers. Those are colder layers, so they do not radiate heat as well.>>>

I have heard this argument before. It seems to me that they suggest that warming the upper atmosphere is significant. I don't know the exact number but something like 90 percent of the atmosphere is below 20 thousand feet. It seems to me that warming that part of the atmosphere far above that isn't nearly as significant as warming the much more massive lower atmosphere. They don't deny that there is less effect to adding more CO2. They simply say it is far from saturated. I don't like terms like "far". Their idea of far may differ from mine. They seem to go out of their way to dance around that fact. When you see people behave that way, you have to wonder what they are hiding. It is also obvious that the less dense atmosphere would tend to absorb less infrared energy. In other words, it is less likely that a photon would hit the CO2 molecule.

We have at most increased the CO2 in atmosphere by a hundred ppmV. Much of our emitted CO2 gets dissolved in the ocean. Alarmists like to pretend that the ocean can only absorb so much but they ignore geologic history IMO. The ocean may be able to absorb all our CO2 and then some. It has something like 50 to 80 times as much dissovled carbon compounds as does the atmosphere. I don't know what the CO2 levels would be in three decades if we doubled our emissions and no one else does either.

CO2 radiates at different wavelengthses, A CO2 molicule over another CO2 molecule will not get the same wavelength of radiation.

The argument may not matter, because CO2 is a toxic gas. So if the concentration gets too high we would all suffocate. So we need to limit it for than reason alone. However it will take many hundreds of years to get that high at the rate we are going.