" Actually doubling the concentration increments absorption of CO2 by a fixed increment due to the fact that CO2 spectal lines are saturated (i.e. not enough IR at the right wavelengths to go around for the density of CO2 molecules in the atmosphere). The only additional absorption that can occur by increasing CO2 concentration is in those areas of the spectrum at the tails of strong line responses and in the weak spectral lines of CO2 thus the absorption of IR is logarithmic as opposed to linear increasing at something less than 5.35*ln(C/Co) w/m2, how much less depending on amount of water vapor in the atmosphere overlapping CO2 spectral lines reducing the amount of IR available to CO2." The CO2 is contained in a bulk of other gasses. It transfers kinetic energy to those other gases, so that equation doesn't apply. The equation you posted is a version of Beer's law. That equation only applies if there is no other interactions. In this case there is. So, CO2 is not near saturation and has considerable absorbing power. It absorbs about 6.5 W/M2 and doubling the concentration, ~doubles the energy absorbed, because it transfers the energy to the atmosphere. edsheppa wrote out the idea in #103. The transfer is probably not complete though, because there's an equilibrium established here. The effective "Co and the "5.xx", change with C, so it's pointless to use Beer's law. Here's a plot that shows the Earth's black body profile from space, showing the CO2 is still strongly absobing.
So, CO2 is not near saturation and has considerable absorbing power.
Actually CO2 is near total saturation, that is why it's response it logarithmic.
The graphic you are displaying shows outgoing surface emissions (outer envelope), probably mid-latitude mediterranean or thereabouts, limited on the low side by blackbody emissions from tropopause at 217K (i.e. -56oC), The spectrum include water vapor, and all other ghg emissions as well, that is why it is so ragged.
The actual strong band response of CO2 around wavenumber 666 is very saturated, and is why the absorption by CO2 is so wide in the graph. If it weren't saturated it would be a vary narrow dip in the graphic not showing skirt response.
The primary absorption spectrum of CO2 at current concentrations compared with H20 response looks like this:
http://earthobservatory.nasa.gov/Study/Iris/Images/greenhouse_gas_absorb_rt.gif
Note: Water vapor bounds the CO2 strong absorption line on either side as well as overlapping a dominant portion of CO2 response. The red jagged absorption lines between 5 & 10mm are the weak line responses of CO2 which are not saturated but are overwhelmed by H2O absorption.
The flat at the top is 100% absorption. When concentration is increased, all that happens is the skirts of the strong lines are raised alittle broadening the line, that is a logarithmic response, not linear. This is true of all strong line responses including water vapor.
Doubling CO2 only provides an additional 3.71 absorption of IR from the atmosphere, downwelling radiation from atmospheric CO2 heating the surface is actually limited by water vapor absorption to something less than 1.2w/m2 heating reaching the surface due to CO2.
See my response to jwalsh07 in comment #109 above for more detail on that subject;
I highly recommend that you read Jack Barret's paper on the subject of CO2 absorption in the atmosphere.
PDF-> Greenhouse molecules, their spectra and function in the atmosphere
He has a very good explanation of how IR absorption works as well as a range of graphics demonstrating the effects on IR emissions from the equator through the polar regions.