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Is the CO2 effect saturated?

What the science says...

Select a level... Basic Intermediate Advanced

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 51 to 75 out of 658:

  1. Riccardo - a better link I think is Dessler & Davis 2010 ?
  2. Thank you scaddenp :)
  3. #48 Albatross at 05:50 AM on 11 November, 2010 Berényi Péter, Please clarify. Do you believe that the CO2 effect is saturated? Of course it is, in most of the 14 μm - 16 μm (wavenumber 625 cm-1 - 710 cm-1) absorption band. In this frequency range effective height of the photosphere (the region from where photons have a reasonable chance to escape to space) is above 20 km, well in the stratosphere. As there is a thermal inversion there (the higher one goes the hotter it gets), with increasing CO2 levels outgoing thermal radiation increases (this is why it is not shown in Harris 2001). There are two narrow bands on both sides of this range which belong to the wings of multiple absorption lines there. In these bands CO2 IR optical depth is close to unity and this is where effective height of photosphere is still below the tropopause. In the troposphere temperature usually decreases with increasing height, so at a specific wavelength more CO2 means less outgoing radiation. On the low wavenumber (long wave) side there are strong H2O absorption lines as well, so the effect only works in an extremely dry troposphere (mostly in the polar regions where low level dry-freezing occurs). Therefore stuff usually happens only at the upper edge of the 8-14 μm main atmospheric thermal IR window (lower edge of wavenumber 710 - 1250 cm-1). In this frequency region there are no major absorption lines (except O3 lines around 1040 cm-1), just the somewhat mysterious water vapor continuum. Partial pressure of water vapor decreases more rapidly with increasing height than that of carbon dioxide, so at frequency bands dominated by H2O absorption effective thickness of photosphere is much smaller. Therefore outgoing thermal IR radiation in these regions is extremely sensitive to minor variations of water vapor distribution. As atmospheric H2O distribution is fractal-like on a scale spanning many orders of magnitude, this effect is neither modeled nor measured sufficiently.
  4. #50 scaddenp The actual wording in the IPCC report is "Note that for shorter recent periods, the slope is greater,indicating accelerated warming." Link to source: IPCC page used in the Climate4you section. Quote from the Climate4you web page: "From the text above the period 1981-2005 is identified by IPCC as being unique, representing a new trend characterised by an accelerated temperature rise. The accelerated temperature increase is suggested to be caused by atmospheric increases in carbon dioxide and other greenhouse gases, assumed to dominate the observed warming after the mid-1970s.
  5. #37 Daniel Bailey at 07:28 AM on 10 November, 2010 Re: Climate4you stuff Went to Norman's website source for his graph & poked around a bit. On this page I noted that: 1. All data is in absolute temps, not anomalies 2. They establish the post-industrial runup in the temperature trend and use that trend to de-trend the signal in the data. I.e., they "hide the incline" in the 20th Century temperature data. 3. They attribute 100% of CO2's effects on temperatures when comparing the CO2 rise to temps, showing that since temps don't rise in lockstep with CO2 levels it can't be the CO2 affecting temps 4. They use a paper by Dr. Syun-Ichi Akasofu as a basis to say that any warming since the LIA is just a reflection of the Earth returning to "normal" and that it's a natural cycle. Trenberth demolished Akasofu here. The whole site is a bait-trap for the unwary. The Yooper Not sure what page you looked at. They have several graphs using anomalies. He did not claim CO2 did not effect temp...Direct quote from the page. "Consequently, the complex nature of the relation between global temperature and atmospheric CO2 since at least 1958 therefore represents an example of empirical falsification of the hypothesis ascribing dominance on the global temperature by the amount of atmospheric CO2. Clearly, the potential influence of CO2 must be subordinate to one or several other phenomena influencing global temperature. Presumably, it is more correct to characterize CO2 as a contributing factor for global temperature changes, rather than a dominant factor."
  6. #44 Tom Dayton Thanks for the link to the posts concerning Outgoing Longwave Radiation. I am reading through the posts working to understand the content. I do love learning.
  7. Norman, thank for that. Now to why I wanted it. Firstly, the claim that 1981-2005 represents a unique new trend is Climate4you, not IPCC. The text never uses the word unique. There is no argument easier to demolish than a strawman. Note how John does it here? He quotes the skeptic claim verbatum with pointer to source of origin. What DOES the IPCC paragraph claim. 1/ The earth is warming, stratosphere cooling in accordance with models. 2/ It details the nature of the warming. 3/ It outlines the basis of the measurement 4/ It notes the consilience of measurements with sea-level rise, glacial melt etc. It also claims that the temperature rise is consistant with modelling of known forcings (GHG, aerosol). As to climate4you's claim that he/she has shown CO2 is not the dominant factor since 1975, as noted above in posts, he/she has only shown a misunderstanding of the actual climate predictions - trying to demolish what the physics doesnt claim.
  8. Re: Norman (55) This page. By taking the underlying signal (Figure C) out of the data, you homogenize (Figure D) the data. When you take the pits out of the cherries, is what is left really a cherry? Let me ask you this, Norman: Post-1976, what forcings other than CO2 have had any significance on global temperatures? Simple question, right? The Yooper
  9. #57 scaddenp The person who runs Climate4you, has a lot of peer-reviewed publications.
  10. #58 Daniel Bailey, "Let me ask you this, Norman: Post-1976, what forcings other than CO2 have had any significance on global temperatures? Simple question, right?" Yes and Climate4you does have an alternative forcing. If I am successful I will Post the graph. Alternate forcing that can affect Global temps. Quote from the page on Climate4you that had this graph (from the Homepage click the Climate & Clouds tab): "Within the still short period of satellite cloud cover observations, the total global cloud cover reached a maximum of about 69 percent in 1987 and a minimum of about 64 percent in 2000 (see diagram above), a decrease of about 5 percent. This decrease roughly corresponds to a radiative net change of about 0.9 W/m2 within a period of only 13 years, which may be compared with the total net change from 1750 to 2006 of 1.6 W/m2 of all climatic drivers as estimated in the IPCC 2007 report, including release of greenhouse gasses from the burning of fossil fuels. These observations leave little doubt that cloud cover variations may have a profound effect on global climate and meteorology on almost any time scale considered"
  11. Norman - so? So do various fossil fuel shills. Does he have any expertise in climate science, radiative physics etc? Has he published his critique in peer-reviewed journal? If you don't have enough expertise in an area to be able to evaluate competing claims, (I'm assuming you havent been able to follow the critiques above) then unfortunately you have either got to acquire the expertise yourself, preferably from textbooks and papers, or you have to rely on expert opinion from people actively working in the field with appropriate domain knowledge. Now where in physical geography do they teach thermodynamics and radiative physics? Not my idea of expert opinion. In my opinion, if you are interested in climate science, then you start with IPCC WG1. This reflects the published science and gives you an almighty index into it. You can see what is actually claimed rather than the zillions of strawmen that denialists like to doubt. You dont have to agree with the assessment but at least you get it one place. If you are looking a "skeptic" claims, then only bother with what's been published for reasons I stated earlier.
  12. Norman somewhat offtopic here. Disproving the real science is not as easy as you (and many others) apper to think. Look here for some problems related to ISCCP cloud data. Be carefull and, if in doubt, rely on the published litterature.
  13. Berényi Péter @53, "Well of course it is", referring to my question as to whether or not the CO2 effect is saturated. Let us put aside the rest of your post for now, because your statement raises some relevant questions. 1) Could you please tell us at what CO2 concentration you believe the CO2 effect became saturated in all bands and at all altitudes. 2) Perhaps a moot question in view of your belief that the CO2 effect is saturated, but what is your understanding of expected warming arising from doubling CO2 (without feedbacks). 3) What is your understanding of warming arising from doubling CO2 (with Charney and slow feedbacks).
  14. #57 scaddenp "The text never uses the word unique. There is no argument easier to demolish than a strawman." From the IPCC page: "Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years. The last time the polar regions were signifi cantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 m of sea level rise. {6.4, 6.6}" IPCC source of above quote. Look at the Thesaurus for the word you see unique in that list?
  15. #62 Riccardo I read through your link. It does not make the claim that the cloud cover percentage has not changed. They just warn against using this data to form any conclusion about climate because of problems with how the cloud cover is measured. As far as this article goes, Cloud cover could still be the dominant driver. There argument is don't use the cloud cover data at this time.
  16. Norman, in general the IPCC tend to be very cautious in their use of words. I would not recommend reading "unusual" and substituting the word "unique".
  17. I need help from someone more knowledgeable, but it seems to me that BP is incorrect in assuming a 2 degrees increase in brightness temperature equivalent to blackbody temperature in the narrow band of 750 cm-1 and 900 cm-1 requires a layer of atmosphere having increased its measured "temperature" by 2 degrees.
  18. Norman, Ned is understating his case. "Unique" really means one of a kind, not merely unusual. In everyday conversation, "unique" is used casually as a synonym for "unusual," but the IPCC has gone out of its way to be careful in its choice of words, due to certain people jumping to conclusions based on the IPCC's more casual usage in the past.
  19. Norman - so we are having "unusual" weather at the moment. Does that make it unique? A unique event would by unusual but not the other way round. But instead of playing word games, look at the sentence that you quoted from the IPCC. Has climate4You refuted this statement? No.
  20. Norman indeed. It's exactly what wanted to say, we have no evidence of a significant change in cloud cover, let alone take it as an alternative explanation of current warming trend. This is one more reason why the site you link is unreliable.
  21. #70 Riccardo Climate4you is a work in progress as are most climate sites. There is a vast amount of information concerning Global Warming and new information growing daily. I believe he leaves an email address on his site. I can send him the link you posted and see if he does anything with it.
  22. Tom Dayton #67 there's no need to dig into the well known difference between brightness temperature and temperature. I can't tell if Berényi Péter didn't actually read the paper, deliberately ignored what the authors say or has well founded reasons to believe they're wrong. A few relevant quotes from the paper. "the difference in the 800-1,000 cm-1 region is positive, and lies between about 1 and 2K. It is important not to over-interpret the observations to an accuracy that is not justifed by the errors (see below), nor to lose sight of our principal result, which is the observation of the sharp spectral features discussed in the preceding paragraph." "we do conclude that the observed window difference spectra strongly indicate an effect involving residual small ice crystal effects, incompletely cleared from the data." "we must also take into account inter-annual variability as a possible cause of the observed difference spectra. In the window region, the brightness temperature difference is strongly modulated by short-term fluctuations, such as inter-annual variability (specific concern involves the 1997 warm El Nino/Southern Oscillation, ENSO, event)."
  23. I see the skeptics argument differently. What I see they are preaching is not that GHGs are saturated with energy, but are at or near peak absorption for the energy available. This is quite different from saying the molecules themselves are saturated. They are not. Why increases in GHGs do little to add to atmospheric warming is due to the fact that there is no additional energy available, it has already been used up. With this argument, the skeptics say any increase in any or all GHGs will cause little meaningful warming by citing the example when the Earth had 10x to 15x the amount of CO2. No runaway warming! Since CO2 is a GHG, increases in any GHG will not increase global warming. Humph! Because the GHG are at maximum warmth already. Finally, they state that the energy in the GHGs are in a sort of equilibrium with the atmospheric humidity. So, if more CO2 is added, the atmosphere rains out the moisture and re-equilibrates by noting an observed drop in upper atmospheric humidity.
  24. In an argument about this article a friend sent me this. Can someone help me with a layman's language rebuttal? **************************************************** "Water, a 3-atom dipolar molecule has several ways of rotating and several ways of vibrating, so it interacts with and absorbs electromagnetic radiation in many parts of the spectrum. It is a strong "greenhouse gas". Carbon dioxide is linear and symmetrical, so it has no resultant dipole and it therefore cannot absorb in the rotational frequency region. Its symmetrical stretching vibration is also infrared inactive. Its asymmetrical stretching vibration however produces a constantly reversing dipole, so it absorbs in a narrow band of frequencies around 2350 wavenumbers. Likewise the bending vibrations, around 670 wavenumbers. So in most of the infrared and microwave spectrum the molecule behaves just like N2 and O2. It doesn't absorb at all. It does absorb in two narrow frequency bands, and absorbs so strongly there that the present concentration of CO2 (about 340 parts per million by volume) achieves almost complete absorption. Increasing the concentration can cause only a little more absorption. The high extinction coefficients are known, the concentration is known, the calculation is not difficult abstruse or speculative, and I think you will agree that it is relevant to anyone who wants to write informatively about the greenhouse effect." ****************************************************
  25. Re: Colin Bridge (74) Hey, welcome to Skeptical Science! Thanks for posting your first question on an appropriate thread! Your friend's article is incorrect in many respects: 1. Even if the absorption band of carbon dioxide would be fully saturated in the lower parts of atmosphere, it is not saturated in higher atmosphere and the addition of carbon dioxide (CO2) will cause more absorption of thermal radiation (extra CO2 has extra effect for a multitude of reasons). Nice-to-know: CO2 exerts its effects primarily though bending mode (you can see visualizations of the various modes of CO2 here). As shown there, CO2 is infrared (IR) active due to a transient dipole: bending results in charge being asymmetrically distributed with net positive near the carbon atom and negative near the two oxygen atoms. 2. Water vapor is a condensible GHG. Short-term increases in concentrations of it condense out and equalize in about 9 days time. CO2 stays aloft for centuries-to-millennia. Like the Energizer Bunny, it keeps on doing its thing. 3. The Earth currently takes in more energy than it emits. We can physically measure this. This emission takes place at the Top Of Atmosphere (TOA), well above where water vapor has anything but a minimal effect (it is found in only trace amounts there). Only the increasing concentrations of CO2, which we can isotopically tell come from fossil-fuel emissions, explain the imbalance. And they explain it quite well. Turning up the CO2 control knob is like cranking up the thermostat to max...and breaking it: The NOTES section at the bottom of this page contain much useful material. Also see here for background on the greenhouse effect and the 2nd Law of Thermodynamics, which usually comes in the next contrarian article. :) BTW: The introduction of N2 and O2 is a complete red herring, as they are unaffected by infrared. Hope that helps. The Yooper

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