Inquiring minds, in the comments section, wanted a simple explanation for why. I've had a crack at this before, and Eli Rabett took me to task for my explanation. Instead, he offered this version from Professor Uherik, which he, Gavin, and most of the RC crowd seem to like. I don't buy it.
The picture, Uherik's figure 3, depicts the atmospheric cooling versus height and wavenumber. The dotted line is the tropopause. The light blue color represents regions of the height-wavenumber space where no net radiative cooling or heating is taking place. The colors to the right show how much or how little radiative cooling takes place, with green representing a lot and gray meaning that net radiative heating is taking place.
At 250 cm^(-1) for example, the region near the surface is light blue because radiation absorbed and emitted are in balance. Higher up, in the same spectral region the radiation decouples from the matter for lack of water molecules, and is lost to space. At 670, it's light blue from surface to the tropopause, because at the center of the CO2 band we are in thermal equilibrium all the way up.
The following sentence from Uherik seems to be at least seriously misleading if not outright bogus: If this [CO2 in the troposphere] absorption is really strong, the greenhouse gas blocks most of the outgoing infra-red radiation close to the Earth's surface.
HUMILIATING CONCESSION UPDATE (with quibble): The CO2 in the troposphere does a lot of blocking once the concentration reaches about 1/4 of the present level. Additional blockage due to additional CO2 at present concentration levels is purely on the wings of the 670 cm^(-1) band.
No frequency makes much of a *net* contribution until the sky above it is largely transparent, which doesn't happen for CO2 band center until you reach the stratosphere.
670 cm^(-1) is near the center of the strong CO2 absorption band. Notice that for that region, the atmosphere is light blue to the tropopause and beyond. That is emphatically not because it isn't radiating - it's radiating very strongly - but the downward flux of radiation from above is in balance with the upward flux at each level until well up into the stratosphere. Eventually the CO2 thins out enough so that it can start losing radiation to space (the colored regions above 18 km and on up well into the mesosphere.
It's wrong to say that more CO2 in the atmosphere limits the amount of thermal radiative transport into the troposphere - there really isn't any such net transport - not at any CO2 levels comparable to the present. The cooling effect of CO2 on the stratosphere is due almost entirely to the additional CO2 in the stratosphere making it a better radiator.
UPDATE: After study, meditation, and a couple of furry
PS - About the beat downs. I would have dropped that
PPS - In compensation, he did give me this pretty MODTRAN pony. It's really cool. Now I want ones for Venus, Mars, and Titan.