If CO2 IR absorption is "saturated" why is the earth's IR radiance at 690 cm-1 approx 205 K?

The latter model is correct. However, CO2 absorbance of upwelling IR radiations at the center of the major bands is saturated, that is why the temperature is 205 K (which is the temperature at the tropopause). Water vapor isn't radiating at a higher temperature, a higher temperature indicates that upwelling surface IR radiation is making it through to space (or, put another way, the water vapor absorbance isn't saturated at that wavelength). Note that the minimum temperature for water vapor is the same as for CO2, in other words the radiating temperature for both H2O and CO2 is 205 K, or the temperature of the tropopause.

The true skeptic physics argument is that because the atmosphere is saturated, adding more CO2 doesn't affect the net radiative forcing. However, this is incorrect for two reasons, one is that adding CO2 pushes the radiative layer upwards, where it is colder, so it radiates less and the surface has to warm. The second is that increasing the concentration broadens the absorption line a little, and this increases the GHG longwave surface radiative forcing.

The interaction between water vapor and CO2 is best described by the link below.

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I have never heard any credible source suggest it was saturated. Are you suggesting that existing CO2 doesn't absorb some radiation in those frequencies and reduce the effect with increasing concentration? If it isn't absorbing the spectra, then how exactly does it warm the atmosphere? It may be true that you can get additional absorption in the less dense air in the upper atmosphere but it seems to me that the less dense air in the upper atmosphere also holds less energy. The whole argument ignores other potential effects on climate and therefore cannot by itself explain variations in climate. Those that think they can accurately model climate in a lab experiment are mistaken IMO.

Really, the fact that deniers make this argument frequently is just a good sign that it's physically wrong.

"So, if a skeptical friend hits you with the "saturation argument" against global warming, here’s all you need to say:

(a) You’d still get an increase in greenhouse warming even if the atmosphere were saturated, because it’s the absorption in the thin upper atmosphere (which is unsaturated) that counts

(b) It’s not even true that the atmosphere is actually saturated with respect to absorption by CO2,

(c) Water vapor doesn’t overwhelm the effects of CO2 because there’s little water vapor in the high, cold regions from which infrared escapes, and at the low pressures there water vapor absorption is like a leaky sieve, which would let a lot more radiation through were it not for CO2, and

(d) These issues were satisfactorily addressed by physicists 50 years ago, and the necessary physics is included in all climate models."

For once I agree with jim - the many deniers who make the saturated gassy argument are not credible.

"use a diffusive radiative transport model in which radiation is absorbed and re-emitted multiple times ultimately being radiated to space at the temperature of the last effective layer. "

It absorbs about 8 percent of the IR. The air warms up and that emits IR. Then you get less than one percent of the original or 8 percent of 8 percent absorbed by the next layer. Obviously you don't get anything significant. If you are talking about it just re-emitting the photon, then it doesn't warm the air. You still only get one warming and 1 percent get absorbed by the second layer. It sounds like some sort of stupid shell game where alarmists hope people are just idiots.

Question is were the samples used in their experiments free of water vapor so the experiment actually measured co2 rather than water vapor effect. This question has been constantly raised, but never answered since Knut Angstrom questioned the validity of Svante Arrhenius research on co2 and its potential contribution to warming. After much review it was scientifically determined that the Arrhenius samples were heavily contaminated by water vapor because of the way he obtained his co2 samples virtually guaranteed it. Angstrom on the other hand obtained his co2 samples that showed co2 to be an extremely marginal contributor to warming compared to water vapor from sources like dry ice which had no water vapor and thus did not show a high warming effect.

So Angstroms experiments illustrate why the scientific method is preferred over peer review for real scientists while those with a commercial or political agenda prefer the easier to influence peer review system.

http://en.wikipedia.org/wiki/Scientific_method

http://en.wikipedia.org/wiki/Peer_review

http://www.geocraft.com/WVFossils/greenhouse_data....

Good question, and good information about AIRS. Peter J felt compelled to "answer" even though he said nothing substantive, and James E gives his usual wrong and irrelevant about water vapor contamination. Let me say this one more time James E: not only has the infrared absorption spectrum of CO2 been measured without water vapor contamination, it can be calculated numerically from quantum mechanics--leaving NO possibility at all of contamination. That argument is wrong, time to move on.

However, trying to contrast peer review with the scientific method is also wrong, since they are not in conflict and peer review and the scientific method are both used in modern science.

To Peter J:

A simple "huh?" would have sufficed.

As for James E:

A detailed explanation of where Angstrom went wrong can be found at: http://www.realclimate.org/index.php/archives/2007...

Folks should remember that James E is the guy who thought that dissolving more CO2 in water would *raise* its pH. With his latest post, we see that he knows as little about physics as he does about chemistry. This is not the guy you should ask to help you with your physics/chemistry homework.

Edited to add: CO2 and H20 absorb at different IR frequencies -- if that were not the case, then the most popular CO2 detectors would not work. See http://www.braschmfg.com/Products/Detectors/BGS-CD... for more info.

Peter's "answer" contained the following gem: "It has holes in it anyone could drive a truck through.. love how they say it can "validate" models instead of saying it can "test" them." Well, the first term is more precise than the 2nd, for starters. At any rate, I'm no scientist, but I scanned through all 120 pages of the document and found it both interesting and thorough.

A lot of CO2 saturation questions floating around lately, I guess I'm going to have to investigate this more thoroughly. I just wanted to say that I could drive a 747 through Peter's reply. If I had a commerical pilots license, that is.

IANAP (I Am Not A Physicist) but as I understand it there are two main reasons why the "CO2 is saturated" argument fails -

1: CO2, unlike water vapour, is fairly evenly distributed throughout the atmosphere. Regardless of the situation at ground level (even if CO2 *was* saturated), at a high enough altitude the atmosphere is so thin that most of the infrared radiation from warmer lower layers escapes to space (i.e. it's not saturated). This *must* be true *whatever* the absorption properties of the atmosphere. Add more CO2 to this thin upper atmosphere and more of the infrared radiation will be absorbed, causing all the lower levels of the atmosphere to get warmer and infrared radiation to increase accordingly until Earth's radiation budget once again reaches equilibrium - i.e. the Earth warms up.

2: Absorption of infrared by CO2 isn't saturated anyway, even in the lower atmosphere, as modern instruments clearly show.