Yes No Pundithere, I can see what you are saying, but there's a big difference between Mars where the CO2 has almost certainly reached a saturation point but the atmosphere is extremely rarified, and the earth where we are at a very low concentration of 390 ppm CO2. Also, to say each extra molecule of CO2 has a lesser and lesser effect, even if true, is a long way from saying any extra CO2 will have no effect. Since it is such a trace gas, as Birdman has pointed out, I struggle to see that a saturation level will be reached any time soon. It seems like over-enthusiastic hopefulness rather than any sort of scientific rigour. Why on earth would such a saturation point (and I don't believe there is any scientific agreement that such a point exists at low concentrations) be at such a low concentration of 0.039% of the atmosphere? That is counter-intuitive and also not supported by observation.
Also you need to remember it is not all about CO2. There is also CH4 and other greenhouse gases which need to be accounted for. I see a MODTRAN chart has a curve for CH4, although much shallower. That's the other big criticism I have of Birdman's post - its all about CO2 as if that's the only gas people need to be worried about.
Phil K and I have discussed in the past if and when the concentration of CO2 in the earth's atmosphere will reach greenhouse saturation, but since he thinks Birdman's post is a "top post", I can only assume his critical faculties have left him temporarily, since most of it is such an obvious load of unscientific codswallop.
Note there is a dispute over the way MODTRAN is used - it seesm the climate sensitivity parameter value used by someone to discredit AGW was way too low:
"My model, used for deception
* Climate Science
-- david @ 4 October 2007
Well, not my model exactly. I developed and host a web interface to the modtran model of atmospheric infrared radiation, an early example of a line-by-line code which I downloaded and use to teach and as part of a textbook. Now David C. Archibald from Summa Development Limited, Perth, WA, Australia claims that my ?University of Chicago modtran facility? proves that global warming won?t happen.
Archibald begins by discovering that the IR light flux at the top of the atmosphere is more sensitive to changes in atmosphere CO2 when the concentration of CO2 is lower. This will come as no surprise to regular readers of realclimate who will know that the energy flux scales with the logarithm of CO2. The log dependence is why the climate sensitivity parameter is often posed as a temperature change for doubled CO2 concentration; to first order, a change from 10 ppm to 20 would have about as much climate impact as a change from 1000 to 2000 ppm. So Archibald is right on this score, clearly climate is more sensitive to CO2 when levels are lower. However, I think most climate models are aware that atmospheric CO2 is 380 ppm rather than 10 ppm, and they predict global warming anyway. If we were starting out from 10 ppm, the warming would be even worse.
Archibald then takes an atmospheric increase of 40 ppm which he thinks will happen by the year 2030. I?d have guessed 60 ppm by then at least, the way things are going, but whatever, we?ll see. He uses my setup of modtran to calculate that the IR flux to space would drop by 0.4 Watts / m2 as a result of this 40 ppm. Try it yourself. Run the model once with 375 ppm CO2 and another time with 415 ppm, and compare the Iout values in Watts / m2. The exact number you get depends on humidity, setting, clouds, etc. Formulas given in IPCC would say 0.5 Watts / m2; zeroing out water vapor in modtran gets the IR response up to 0.6 Watts / m2 for the default tropical atmosphere case. At any rate Archibald isn?t wildly off here either.
But then Archibald multiplies the radiative forcing by an absurdly low value of the climate sensitivity parameter. In this case he is using the parameter in units of degrees C per Watt / m2. The two forms of the climate sensitivity parameter that we have discussed here are related by a factor of about 4 Watts / m2 for a doubling of CO2. The value Archibald uses is 0.1 degree C per Watt / m2 which was ?demonstrated? in a paper entitled ?CO2-induced global warming: a skeptic?s view of potential climate change? by Idso, 1998. Translated, Idso?s climate sensitivity winds up to be 0.4 degrees for doubling CO2. IPCC finds it essentially impossible (yeah, I know, highly unlikely or whatever) that the climate sensitivity could be less than 1.5 degrees C for doubling CO2, and 3 degrees C is a best-guess value.
In the end, Archibald concludes that the warming from the next 40 ppm of CO2 rise (never mind the rest of it) will only be 0.04 degrees C. Archibald?s low-ball estimate of climate change comes not from the modtran model my server ran for him, but from his own low-ball value of the climate sensitivity."
The link to Archibald's claim was here but it seems the domain climatepolice is now defunct:
http://climatepolice.com/Climate_Outlook_2030.pdf
Maybe HotCopper posters should buy the domain?
There were a number of interesting comments following the RealClimate article above, but I thought I would quote this one as it rings true with me:
"6 Joel Norton says: 4 Oct 2007 at 1:19 PM
Archibald's garbage is not meant to fool science minded people. His article is meant for the lay public to create a sense of doubt and hesitation. This is one of the key principles of propaganda. It does not need to be true, it just needs to sound true, and then repeat it again and again and again.
I am sure someone is rewarding Archibald for his efforts. Watch for the shrill citations and repititions from the henchmen. I can only hope the organizations behind this get exposed sooner rather than later."
