[And the whole POINT here is that the greenhouse absorption of thermal radiation lower in the atmosphere means you get the cooler radiation getting out into space.
Cheers  sylas[/QUOTE]
I find the above statement badly irrelevant because of the actual values for absorption. The 15 micron band (1318) seems to me to have its primary absorption (the first absorption event for the photon) within 2 meters to the 99.99% probablility. I have found this via an interesting model and thought experiment. Specifically the height at which 63 % of the photons are absorbed is given by the formula
H = k * T / ( C*X) where T is the kelvin Atmospheric temp, C is the concentration of CO2, X is the crossection of absorption for CO2 at 15 microns.
k is a constant = (1.36*10^22 cc/deg K).
This turns out to be 21.7 cm at 303 deg K. By compounding 9 times, 99.99% absorption is reached.
Doubling the CO2 concentration clearly just lowers the "saturation" level to 1 meter. I have not added a correction factor for pressure, but it goes in the denominator. This same result would apply elsewhere in the atmosphere for secondary photons of atmospheric black body origin.
If the result was 10 km vs 5 km when the CO2 is doubled, I could see some possibility for the upper atmosphere machinations to be useful, but not if the values are 2 m and 1 m.
