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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. -56^oC), 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.

"The graphic you are displaying shows outgoing surface emissions (outer envelope), probably mid-latitude mediterranean or thereabouts..."

The Plot is tropical, so it's ~equitorial. The dips are due to absorption.

The spectrum given in your post is unlabeled. Where did it come from? It means nothing to me w/o conditions.

"If it weren't saturated it would be a vary narrow dip in the graphic not showing skirt response."

The p, r and q branches are obscured.

It looks like the 5.xxln(C/C,sub>o) is a fitted eq. above a baseline of 278ppmv in 1750. I'll have to look into that.

I think the two of you have a terminology issue here. When AG says "saturation," S is taking that to mean the CO2 could not absorb more IR and any additional IR would therefore radiate into space. But instead AG is using the term to mean only that all the IR at many wavelengths the Earth *would* radiate currently if there were no atmosphere would be blocked 100% by the atmosphere's CO2.

What is the model for composing the absorption amounts in that chart I wonder? Obviously they can't simply be added because, e.g., at 15µm the CO2 is at 100% and the H2O is at 50% so together they'd be blocking 150% of the radiation which isn't very physical. Seems the right way is f₁+f₂-f₁f₂.