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Is the CO2 effect saturated?

What the science says...

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The notion that the CO2 effect is 'saturated' is based on a misunderstanding of how the greenhouse effect works.

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)


This myth relies on the use (or in fact misuse) of a particular word – 'saturated'. When someone comes in from a prolonged downpour, they may well exclaim that they are saturated. They cannot imagine being any wetter. That's casual usage, though.

In science, 'saturated' is a strictly-defined term. For example, in a saturated salt solution, no more salt will dissolve, period. But what's that got to do with heat transfer in Earth's atmosphere? Let's take a look.

Heat-trapping by CO2 in the atmosphere happens because it has the ability to absorb and pass on infra-red radiation – it is a 'greenhouse gas'. Infra-red is just one part of the electromagnetic spectrum, divided by physicists into a series of bands. From the low-frequency end of the spectrum upwards, the bands are as follows: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Gamma rays thus have a very high-frequency. They are the highest-energy form of radiation.

As our understanding of the electromagnetic spectrum developed, it was realised that the radiation consists of particles called 'photons', travelling in waves. The term was coined in 1926 by the celebrated physicist Gilbert Lewis (1875-1946). A photon's energy is related to its wavelength. The shorter the wavelength, the higher the energy, so that the very high-energy gamma-rays have the shortest wavelength of the lot.

Sunshine consists mostly of ultraviolet, visible light and infra-red photons. Objects warmed by the sun then re-emit energy photons at infra-red wavelengths. Like other greenhouse gases, CO2 has the ability to absorb infra-red photons. But CO2 is unlike a mop, which has to be wrung out regularly in order for it to continue working. CO2 molecules do not get filled up with infra-red photons. Not only do they emit their own infra-red photons, but also they are constantly colliding with neighbouring molecules in the air. The constant collisions are important. Every time they happen, energy is shared out between the colliding molecules.

Through those emissions and collisions, CO2 molecules constantly warm their surroundings. This goes on all the time and at all levels in the atmosphere. You cannot say, “CO2 is saturated because the surface-emitted IR is rapidly absorbed”, because you need to take into account the whole atmosphere and its constant, ongoing energy-exchange processes. That means taking into account all absorption, all re-emission, all collisions, all heating and cooling and all eventual loss to space, at all levels.

If the amount of radiation lost to space is equal to the amount coming in from the Sun, Earth is said to be in energy balance. But if the strength of the greenhouse effect is increased, the amount of energy escaping falls behind the amount that is incoming. Earth is then said to be in an energy imbalance and the climate heats up. Double the CO2 concentration and you get a few degrees of warming: double it again and you get a few more and on and on it goes. There is no room for complacency here. By the time just one doubling has occurred, the planet would already be unrecognisable. The insulation analogy in the myth is misleading because it over-simplifies what happens in the atmosphere.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

This myth relies on the use of a word – saturated. When we think of saturated in everyday use, the term 'soggy' comes to mind. This is a good example of a word that has one meaning in common parlance but another very specific one when thinking about atmospheric physics. Other such words come to mind too. Absorb and emit are two good examples relevant to this topic and we’ll discuss how they relate to atmospheric processes below.

First things first. The effect of CO2 in the atmosphere is due to its influence on the transport of 'electromagnetic radiation' (EMR). EMR is energy that is moving as x-rays, ultraviolet (UV) light, visible light, infrared (IR) radiation and so on (fig. 1). Radiation is unusual in the sense that it contains energy but it is also always moving, at the speed of light, so it is also a form of transport. Radiation is also unusual in that it has properties of particles but also travels with the properties of waves, so we talk about its wavelength.

The particles making up radiation are known as photons. Each photon contains a specific amount of energy, and that is related to its wavelength. High energy photons have short wavelengths, and low energy photons have longer wavelengths. In climate, we are interested in two main radiation categories - firstly the visible light plus UV and minor IR that together make up sunshine, and secondly the IR from the earth-atmosphere system.

The Electromagnetic Spectrum

Fig. 1: diagram showing the full electromagnetic spectrum and its properties of the different bands. Image: CC BY-SA 3.0 from Wikimedia.

CO2 has the ability to absorb IR photons – it is a 'greenhouse gas'.So what does “absorb” mean, when talking about radiation? We are all familiar with using a sponge to mop up a water spill. The sponge will only absorb so much and will not absorb any more unless it's wrung out. In everyday language it may be described, without measurements, as 'saturated'. In this household example, 'absorb' basically means 'soak up' and 'saturated' simply means 'full to capacity'. Scientific terms are, in contrast, strictly defined.

Now let's look at the atmosphere. The greenhouse effect works like this: energy arrives from the sun in the form of visible light and ultraviolet radiation. A proportion reaches and warms Earth's surface. Earth then emits the energy in the form of photons of IR radiation.

Greenhouse gases in the atmosphere, such as CO2 molecules, absorb some of this IR radiation, then re-emit it in all directions - including back to Earth's surface. The CO2 molecule does not fill up with IR photons, running out of space for any more. Instead, the CO2 molecule absorbs the energy from the IR photon and the photon ceases to be. The CO2 molecule now contains more energy, but that is transient since the molecule emits its own IR photons. Not only that: it's constantly colliding with other molecules such as N2 and O2 in the surrounding air. In those collisions, that excess energy is shared with them. This energy-sharing causes the nearby air to heat up (fig. 2).

CO2 heat transfer

Fig. 2: The greenhouse effect in action, showing the interactions between molecules. The interactions happen at all levels of the atmosphere and are constantly ongoing. Graphic: jg.

The capacity for CO2 to absorb photons is almost limitless. The CO2 molecule can also receive energy from collisions with other molecules, and it can lose energy by emitting IR radiation. When a photon is emitted, we’re not bringing a photon out of storage - we are bringing energy out of storage and turning it into a photon, travelling away at the speed of light. So CO2 is constantly absorbing IR radiation, constantly emitting IR radiation and constantly sharing energy with the surrounding air molecules. To understand the role of CO2, we need to consider all these forms of energy storage and transport.

So, where does 'saturation' get used in climate change contrarianism? The most common way they try to frame things is to claim that IR emitted from the surface, in the wavelengths where CO2 absorbs, is all absorbed fairly close to the surface. Therefore, the story continues, adding more CO2 can’t make any more difference. This is inaccurate through omission, because either innocently or deliberately, it ignores the rest of the picture, where energy is constantly being exchanged with other molecules by collisions and CO2 is constantly emitting IR radiation. This means that there is always IR radiation being emitted upwards by CO2 at all levels in the atmosphere. It might not have originated from the surface, but IR radiation is still present in the wavelengths that CO2 absorbs and emits. When emitted in the upper atmosphere, it can and will be lost to space.

When you include all the energy transfers related to the CO2 absorption of IR radiation – the transfer to other molecules, the emission, and both the upward and downward energy fluxes at all altitudes - then we find that adding CO2 to our current atmosphere acts to inhibit the transfer of radiative energy throughout that atmosphere and, ultimately, into space. This will lead to additional warming until the amount of energy being lost to space matches what is being received. This is precisely what is happening.

The myth reproduced at the top – incorrectly stating an analogy with roof insulation in that each unit has less of an effect - is misleading. Doubling CO2 from 280 ppm to 560 ppm will cause a few degrees of warming. Doubling again (560 to 1130 ppm) will cause a similar amount of additional warming, and so on. Many doublings later there may be a point where adding more CO2 has little effect, but recent work has cast serious doubt on that (He et al. 2023). But we are a long, long way from reaching that point and in any case we do not want to go anywhere near it! One doubling will be serious enough.

Finally, directly observing the specific, global radiative forcing caused by well-mixed greenhouse gases has - to date - proven elusive. This is because of irregular, uncalibrated or limited areal measurements. But very recently, results have been published regarding the deep reinterrogation of years of data (2003-2021) from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua Satellite (Raghuraman et al. 2023). The work may well have finally cracked the long-standing issue of how to make finely detailed, consistent wavelength-specific measurements of outgoing long-wave radiation from Earth into space. As such, it has opened the way to direct monitoring of the radiative impact (i.e. forcing + feedback) of greenhouse gas concentration changes, thereby complimenting the Keeling Curve - the longstanding dataset of measured CO2 concentrations, down at the planet's surface.

Note: Several people in addition to John Mason were involved with updating this basic level rebuttal, namely Bob LoblawKen Rice and John Garrett (jg).

Last updated on 31 December 2023 by John Mason. View Archives

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Further reading

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

Further viewing

Video by Rosh Salgado on his "All about Climate" YouTube channel in which he debunks Will Happer's claim that the CO2 effect is saturated in the atmosphere:


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Comments 1 to 25 out of 698:

  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? 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. . 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; 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 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.. 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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