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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Is the CO2 effect saturated?

What the science says...

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This argument originates from Angstrom's work in 1901. We now know that the planetary energy balance is determined by the upper levels of the troposphere and that the saturation of the absorption at the central frequency does not preclude the possibility to absorb more energy.

Climate Myth...

CO2 effect is saturated

"Each unit of CO2 you put into the atmosphere has less and less of a warming impact. Once the atmosphere reaches a saturation point, additional input of CO2 will not really have any major impact. It's like putting insulation in your attic. They give a recommended amount and after that you can stack the insulation up to the roof and it's going to have no impact." (Marc Morano, as quoted by Steve Eliot)

After the famous Arrhenius paper in 1896, where he did the first calculations of the CO2 greenhouse effect, his theory was dismissed by Angstrom with a simple experiment. He let an infrared beam pass through a tube filled with CO2 and measured the emerging light intensity. Upon reducing CO2 concentration in the tube, only a tiny difference could be found and he concluded that very few CO2 molecules are enough to completely absorb the IR beam. The conclusion was that a CO2 increase could not matter. This was the birth of the first skeptic of the then called "CO2 theory" and of the more recent "CO2 effect is saturated" skeptic argument.

Thirty years later, E. O. Hulburt (Phys. Rev. 38, 1876–1890 (1931)) added convection to the purely radiative equilibrium assumed by Arrhenius. He found that convective equilibrium holds in the lower part of the troposphere up to about 10 Km, while radiative holds equilibrium above. The important consequence is that the details of the absorption in the lower troposphere do not matter since heat "is spread around and transferred upward by convection". In other words, what govern the energy balance of the earth is the radiative balance in the upper troposphere and CO2 concentration there does matter.

Hulburt was very prudent in his conclusions:

"The agreement is no doubt better than is warranted by the accuracy of the data on which the calculations are based. Apparently the uncertainties and omissions have conspired to counteract each other to some extent."

Nevertheless, his work is definitely a milestone in the understanding of our atmosphere.

Hulburt's work should have put the controversy on the CO2 theory to an end, since "objections which have been raised against it by some physicists are not valid". Unfortunately, this paper passed almost unnoticed, I guess because meteorologists and geologists do not read Physical Review so often.

At the time of Hulburt the CO2 absorption coefficient was not known very accurately and even less its line shape, forcing Huburt to use a "box-like" shape. We may now build a simple model with a more realistic line shape and show that we get an increased absorption with increasing CO2 concentration anyways.

Consider the CO2 absorption band around 15 μm (about 650 cm-1), it is strong enough to not let any light go through after a few tens of meters at surface temperature and pressure. Did this energy disappear forever? Surely not, radiatively or convectively this energy "is spread around and transferred upward". But on the way up this light will find a decreasing pressure, i.e. less CO2 molecules. There will be a point where the light can escape to the outer space. The intensity of the emerging light will be appropriate for the temperature of this "last" layer layer.

We can crudely model this behavior using the Plank law and a gaussian-shaped absorption coefficient. We consider just two layers, the surface and the "last" layer, and the emissivity of this outer layer is modulated between 0 and 1 according to the absorption coefficient α. The result is shown in the figure below.

In the calculations I used an absorption wavenumber of 650 cm-1 and tuned the optical depth to reach saturation. The two dashed lines correspond to the Plank law for T=300 K and T=220 K. The red curve is the calculated emission; it follows the 300 K curve but deviates from it near the absorption band. This dip represents the energy prevented to reach the outer space, i.e. the greenhouse effect.

This graph can be qualitatively compared with real measurements to be sure we're not too far off.

We can now look at what happens when we increase α. Following Angstrom (and many others in his times) the energy absorbed should not change. On the contrary, if we recall that the absorption coefficient is gaussian we would expect an increase in the energy retained by our layer along the wings. The effect is shown in the figure below.

We can see that although the absorption dip cannot fall below the 220 K curve, it becomes wider and the absorbed energy increases accordingly. This is as far as we can get with this simple model. Needless to say that there's much more than what can be done with the very crude model presented here. We know, for example, that the line shape of the absorption coefficient changes with both pressure and temperature due to what are called pressure and Doppler broadening. In the upper layers of the atmosphere the band initially gets narrower and then splits into several narrow bands (the roto-vibrational spectrum) leaving more room for the increase in CO2 concentration being more effective. We also know that there are weaker absorption peaks other than the stronger one quoted above which are not saturated.

Gilbert Plass in 1956 used these words:

One further objection has been raised to the carbon dioxide theory: the atmosphere is completely opaque at the center of the carbon dioxide band and therefore there is no change in the absorption as the carbon dioxide amount varies. This is entirely true for a spectral interval about one micron wide on either side of the center of the carbon dioxide band. However, the argument neglects the hundreds of spectral lines from carbon dioxide that are outside this interval of complete absorption. The change in absorption for a given variation in carbon dioxide amount is greatest for a spectral interval that is only partially opaque; the temperature variation at the surface of the Earth is determined by the change in absorption of such intervals.

There's one more subtle effect related to increased absorption. Upon increasing CO2 concentration, the layer at which the absorption coefficient at each wavelength is low enough to let the IR light escape will be found higher in the atmosphere. The emitting layer will then have a lower temperature, at least until the tropopause is reached, and hence a lower emitting power.

Clearly there's a world behind the absorption of IR light by CO2 in the atmosphere which I omitted. The physics behind it is now solid thanks to the decades of work of many different scientists, and despite the first highly respected skeptic ever who put the CO2 theory on hold for half a century. But you know, this is how science works.

Note: I cannot conclude without acknowledging the fundamental role of Spencer Weart "The Discovery of Global Warming" from which I borrowed (and learned) a lot. His book and the supporting website are a treasure cove for anyone interested in how our current knowledge has been built step by step over time.

Last updated on 18 April 2017 by Riccardo. View Archives

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Further reading

V. Ramanthan has written a comprehensive article Trace-Gas Greenhouse Effect and Global Warming.

Comments

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Comments 1 to 25 out of 637:

  1. My apologies for what is in effect a repost, but I asked this question at the bottom of a second page of comments where I first saw this diagram. Here I can be first :-) (and I think it's a relatively important question) There is one thing that bothers me about figure 1 (the differential spectrum). The decreased emission in certain absorption bands makes perfect sense. But it's a fact of the (surface) instrumental record that the planet was (a little) warmer in 1996 than in 1970 (somewhere in the 0.1 to 0.2C range). So shouldn't the flat parts of the differential spectrum be just a bit above zero? I suppose it's possible due to different instruments (i.e. different satellites) that they had to normalize, but it would have been so much better if they didn't. Does anyone have the answer?
  2. I suppose it's also possible that the satellites were in more-or-less low equatorial orbits, were thus only measuring the tropics, and the tropics have seen the least surface warming. Ok, I'm out of ideas for the moment.
  3. What I don't understand about this argument is that we have a clear demonstration of what high levels of CO2 can accomplish in respect to greenhouse effect with Venus. From what I understand Venu's atmosphere is over 95% CO2 and it's surface tempuratures are almost 500 degrees celcius. That makes it hotter than Mercury the closest planet to the Sun. If CO2 had a saturation point wouldn't Venus have reached it or am I totally misunderstanding the premise?
  4. GFW, as I understand it, Figure 1 shows outgoing radiation. Therefore, if the Earth has warmed, more radiation remains inside, i.e., less radiation goes out. Therefore, the net outgoing radiation must be below zero (less radiation going out, i.e. radiation that remains inside the atmosphere, in a transient radiative imbalance, until the earth warms enough as to "expel" again the same energy that comes in, going back to equilibrium (at zero) but with a higher temperature given that the greenhouse blanket is now thicker). In this graphic the different wavelengths are differenciated, so we can see which exact wavelengths are being trapped inside (those that are below zero), i.e. the exact wavelengths that are responsible for the warming inside. The rest of wavelengths escape to space the same as before (the components for those wavelengths have not changed, so the same energy goes in and out), and that's why they are at zero level. rlasker3, I think you are perfectly right. If the CO2 saturated, we woudn't see that runaway greenhouse effect in Venus.
  5. Your half correct PeterPan The figure in this article is out going radiance but is only part of the actual figure from the Harris 2001 paper. It has been manipulated to highlight the drops in radiance CO2 and CH4. I'm not trying to suggest dishonesty it's just a way to present the data. In the 'real' data the 0 point is actually lower, you can see this in the publication. So what you have comparing 1970 and 1996 is reductions in radiance at the C02 and CH4 positions but increases in radiance in the areas in between. I've never seen any good explanations for the changes at these other wavelengths. Or seen any reasoning behind highlighting the spectra in the CO2 and CH4 regions and ignoring the rest of the spectra. My own explanation would be that while there is increased energy retention at the wavelengths absorbed by CO2 and CH4 there are also other changes which are allowing more energy to escape at different wavelengths. I have no problem with suggesting that this paper identifies a signature in the radiance data for an increase in CO2 in the atmosphere but without complate analysis of the spectra (which the publication doesn't do) you can't conclude that this is leading to warming
  6. i thought venus's atmospheric pressure has a lot to do with its temperature?
  7. Wouldn't a much more useful test of a saturation point be, testing for a saturation point? This only seems to directly indicate that if there is such a saturation point, we haven't hit it yet, not that there is no saturation point to hit. I haven't been able to find anything to this effect, but a test in a laboratory using a similar method to observe the absorption rate of the same wavelengths passing through a chamber with various concentrations of the gasses we would expect to see in the upper atmosphere could show whether it does have an asymptotic behavior and thus 'saturates'? Just seems somewhat fishy that the logic goes "It could reach a saturation point!" "But it's still increasing as we increase the CO2, so there is no saturation point!", analogous to "If you eat too much candy you get sick!" "But I've been eating candy for the last 20 minutes and I continue to feel fine!"
  8. NkThrasher, that has been done. A long time ago, and repeated with increasing precision and thoroughness right up to currently. See the post How do we know CO2 is causing warming?; in that post, look in the green "Acknowledgements" box just above the start of the Comments section. Click on the link "laboratory measurements...".
  9. What do you make of this claim? http://jimpeden.blogspot.com/2009/11/norm-kalmanovich-on-global-warming-hoax.html I haven't read it thoroughly, but it seems like they are missing the fact that CO2 is absorbed at several different wavelengths. The post devolves into the usual questioning of motives, but I was curious to hear thoughts on the main claim about the 14.77 micron band being used up. Thanks, M
  10. mazibuko, the problems with those guys is that their reasoning looks straightforward and anyone can understand it. The mistake he makes becomes apparent only if you know the physics. So common people tend to trust those bogus falsifications. The problem with that "falsification" is not just related to the single band he considers, he arbitrarly put a limit to the amount of radiation CO2 can absorb. This is not true both experimentally (as shown in this post) and theoretically even for the 15 micron band alone; and it would not matter anyway.
  11. Hi Riccardo, Thanks for the answer and links. I have trouble enough keeping on top of the literature in my own field, so it is very useful to come to a place like this and have someone with a good knowledge of the particular issue provide/point me to a summary. I saw a good clip on Richard Dawkin's TV today about how to spot baloney, which is perhaps apropos to this matter. One common form of baloney occurs when someone claims that his/her theory shoots down an existing and better understood theory, based on falsifying only one particular aspect of the better understood theory. The new theory, however, can't explain all the other phenomena that are explained by the better understood theory. Thus, the notion that all the multiple lines of evidence for a human forcing on climate can be falsified because all those silly (and agenda-driven!) scientists overlooked CO2 saturation seems to qualify as baloney. Thanks again, M
  12. NkThrasher #7: the whole concept of saturation is based on a misconception of how radiation is passed up, level by level, through the atmosphere. Spencer Weart has some good non-technical descriptions of this, which I've adapted here.
  13. hey guys, i think you should all look at Dr. Ferenc Miskolczi's theory on the saturated effect of greenhouse gases. you can download his paper for free on scribd. http://www.scribd.com/doc/25071132/The-Saturated-Greenhouse-Effect-Theory-of-Ferenc-Miskolczi . i would love it if you guys have a look at it and get back to me.. because it looks very very convincing.. so if someone can have a look at the math involved and see if its all legit that would be excellent.. here is the link to the summary report of his findings aswell; http://www.scribd.com/doc/25071473/Saturated-Greenhouse-Effect-Theory He says that the climate models used by the ipcc are based on old math from the 1920's which make an assumption that the atmosphere is infinitly large.. he redoes the math with proper boundary conditions and comes up with very interesting results.. i'll leave it up to you guys to look further into it email me on coatesy91@hotmail.com if you'd like to send me your thoughts if youd prefer that than this comment cheers
  14. qball17 #13 That's not a proper scientific paper, it's a paper from the partisan Science and Public Policy Institute (home of among others the rather badly discredited "Lord" Christopher Monkton, and the australian geologist Bob Carter). If there was any merit in the presentation/summary that you printed, then it would be easy for him to publish in a reputable scientific journal. So you have to ask the question why hasn't he? The papers cited at the top of this page are a much better source for you to form your opinions from.
  15. qball17, Miskolczi's "paper" is well known and badly flawed. As Eli at Rabett Run ironically puts it, his theory could be summarized in just two sentences: "The greenhouse gas theory that has been used for the last century is TOTALLY WRONG! The proof is left as an exercise for the reader.".
  16. so the atmosphere is infinitely large is it? so what is the better model? can you show me some solid evidence that debunks the saturated greenhouse effect and proves that CO2 drives climate?
  17. the reason why he couldn't publish it was because NASA refused to let him, so then he went back to hungary and made his paper.. just like all the other scientists who say how hard it is to get a scientific journal published that is against or is skeptical about anthropogenic climate change. Don't you find it funny how all the readily available journals are CO2 CO2 CO2.. how can science come up with such a definite conclusion with all the other factors that would contribute.. this issue has turned into a political one, not scientific... how do these papers justify the CO2 lag behind temperature, and solar activity etc etc
  18. and by the way.. the title of this topic is ridiculous.. its not the "saturated Co2 effect", its the "saturated GREENHOUSE effect".. we all know that we can increase the concentration of CO2.. what about the work from Dr. Nicola Scafetta?
  19. and another thing.. he did publish it in hungary. Dr. Miskolczi first published his work in the Quarterly Journal of the Hungarian Meteorological Services in 2004, Volume 108, No 4. He published further statistical proof in the same Journal in 2007, Volume 111, No. 1. In the 5 years since he first published his results, not one peer review has come back disproving his theory, or his Constant. To date, not one scientist has come forward to disprove Miskolczi’s theory that the Earth’s climate is at equilibrium, and that Carbon Dioxide cannot be released in amounts great enough to upset that equilibrium.
  20. you should have a look at the following link and see what you think.. http://miskolczi.webs.com/Answers_to_some_criticism.htm it also has a link to his 2004 paper which people don't look at, that is why they get confused and see his paper as making too many assumptions etc.. its because 2007 paper is the sequel to his 2004 paper where he defines many of his terms and comes up with the science behind the theory
  21. qball17, please refrain from nitpicking, and refrain from using over-the-top terms such as "ridiculous." You might also read more carefully, and when something doesn't make sense, consider the hypothesis that you might not be parsing it correctly; the title is "Is the CO2 Effect Saturated?," which to most readers parses correctly as "Is the CO2-Effect Saturated?" As for Scafetta, scroll down to the link to his paper in this post: It’s the sun.
  22. Hi How can these guys be right “Falsification of the Atmospheric CO2 greenhouse effect within the frame of physics” by Gerhard Gerlich and Ralf D. Tscheuschner , Ive beeen looking for rebuttals to there paper but now luck so far can anyone help ? thanks Dave
  23. Hi ok ive realised from google searching that someone is posting this in just about any blog they can and by the body of the post it seems to be the same person . thanks Dave
  24. Hi, Daved. You're right, people do post links to that paper a lot. Fortunately, this site is here to help people quickly assess those kinds of claims. The short answer is that the "falsification" paper by G&T is just nonsense. You can find a rebuttal to it on this site at The greenhouse effect and the 2nd law of thermodynamics Or, if you want more depth, there's a lot of discussion of this at the website "Science of Doom": On Having a Laugh – by Gerlich and Tscheuschner (2009) On the Miseducation of the Uninformed by Gerlich and Tscheuschner (2009) and subsequent posts at Science of Doom.
  25. Thanks Ned After another hour of searching I found Science of Dooms response in another blog with a link to his ? blog .

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