I had a preliminary remarks somewhere, but I can't locate it either.

My personal notes on this topic: a lot of this needs further thought.

CO2 is 392 ppmv (0.039%), H20 about 1%:
- About 99% of the atmosphere is greenhouse-neutral: O2 (21%) and N2  (78%)
- IR absorption spectrum for CO2, H20, and others.
- CO2 is present at up to 100 km in altitude; H20 only up to 10 km.
- Measurements of downward streaming IR at the CO2 lines: Evans, Philipona, Harries...
- The result of calculations is that RF = 3.7 W/m2 for a 2X in CO2. Since the amount of CO2 is so small, it doesn't take that much to get to 2X; we are already up to 35%.
In analogy: A small town has 2500 people. But all the elections are decided by 26 voters. How is that possible? Well, those 26 voters are the only folks in town that are registered to vote: the rest of them cannot vote, so it actually doesn’t matter what they think about the issues and the candidates. If you add more non-voters, to bring the town to a total of 26 million people, but still with only 26 voters, then elections will be decided by 1 person in 1 million. That’s the rules.

Basic: 1st layer
- What matters is the 1% of gases that are GHGs: the rest don't vote
- But CO2 is only 4% of GHGs: How can it produce 26% of the impact? 2x => 3.7 W/m2
- CO2 molecules have certain advantages over H20

Intermediate: 2nd layer
- Spectral differences: CO2 has 4-micron and 15-micron
- Distribution of CO2 goes much higher than H20, which freezes out
- High location gives CO2 greater leverage

Advanced: core of the onion
- Random walk of photons
- OPL and photosphere
-
http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/
http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument-part-ii/
http://chriscolose.wordpress.com/2010/08/27/adding-up-the-greenhouse-effect-attributing-the-contributions/

 

An additional question  on this theme of gas concentration significance .

I  saw a forum append comment that the atmosphere of Mars contains more molecules of  CO2 than Earth .Is this correct ?

The tenure of the argument was that  since Mars was a cold planet  CO2  was not significant  in warming . I'm assuming the dominant effect on Mars is   the density of the atmosphere . Again is this correct ?

Although Mars has 95% CO2, its atmosphere is about 100 times less thick than Earth’s, and it is also 1.5 times as far from the Sun so it receives only 1/2.25 as much radiation.