I'm still not fully grasping some aspects of RealClimate's explanation (1 and 2) of how CO2 overcomes the band saturation of IR radiation. It seems they have two general explanations of it (correct me if I got it wrong):
1. Adding more CO2 increases the height in the atmosphere at which IR radiation can escape, thus "filling up the sink" and keeping the waves of IR in the atmosphere for a longer time. This also has the effect of increasing CO2 concentrations even higher than the height of this emission point, where it is still less saturated and thus much more effective as a GHG.
2. Increasing partial pressure of CO2 widens the wavelength that can be absorbed by CO2 (see the first two graphs in article 2).
I think I understand the first argument but I want to make sure...basically it is saying that since CO2 from the surface quickly mixes itself with all levels of the atmosphere, and since CO2 tapers off as you get higher and higher, that increasing the concentration at the ground level essentially increases concentrations at all altitudes equally, including these areas of lighter concentration high up. This in turn has the effect of raising the level of its greenhouse effect to a higher altitude--right? (By the way how high up are we talking about here? They just talk about the 'cold, dry' areas but they don't say how high...)
Where I really have trouble is the second part. You can see in the graphs in the second part of the explanation that adding CO2 increases the IR range where it has an effect as a GHG. But how does this happen?
It seems the only explanation they give is this:
CO2 colliding with itself in a tube of pure CO2 broadens the lines about 30% more than does CO2 colliding with N2 or O2 in air, which results in an additional slight overestimate of the absorption in the laboratory experiment.
...so adding more CO2 in effect causes itself to broaden the range, because collisions with itself (and I'd imagine other trace gasses) increase the range compared to collisions with nitrogen and oxygen. But...why? Is there a theory behind this or is it just an observation?
They also mention 'pressure broadening' which states that as you decrease pressure by increasing altitude, you increase the range of IR absorbed (by the way is there a theory behind this?) but that shouldn't be affected by increasing concentration right?