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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...

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)

The mistaken idea that the Greenhouse Effect is 'saturated', that adding more CO2 will have virtually no effect, is based on a simple misunderstanding of how the Greenhouse Effect works.

The myth goes something like this:

  • CO2 absorbs nearly all the Infrared (heat) radiation leaving the Earth's surface that it can absorb. True!
  • Therefore adding more CO2 won't absorb much more IR radiation at the surface. True!
  • Therefore adding more CO2 can't cause more warming. FALSE!!!

Here's why; it ignores the very simplest arithmetic.

If the air is only absorbing heat from the surface then the air should just keep getting hotter and hotter. By now the Earth should be a cinder from all that absorbed heat. But not too surprisingly, it isn't! What are we missing?

The air doesn't just absorb heat, it also loses it as well! The atmosphere isn't just absorbing IR Radiation (heat) from the surface. It is also radiating IR Radiation (heat) to Space. If these two heat flows are in balance, the atmosphere doesn't warm or cool - it stays the same.

Lets think about a simple analogy:

We have a water tank. A pump is adding water to the tank at, perhaps, 100 litres per minute. And an outlet pipe is letting water drain out of the tank at 100 litres per minute. What is happening to the water level in the tank? It is remaining steady because the flows into and out of the tank are the same. In our analogy the pump adding water is the absorption of heat by the atmosphere; the water flowing from the outlet pipe is the heat being radiated out to space. And the volume of water inside the tank is the amount of heat in the atmosphere.

What might we do to increase the water level in the tank?

We might increase the speed of the pump that is adding water to the tank. That would raise the water level. But if the pump is already running at nearly its top speed, I can't add water any faster. That would fit the 'It's Saturated' claim: the pump can't run much faster just as the atmosphere can't absorb the Sun's heat any faster

But what if we restricted the outlet, so that it was harder for water to get out of the tank? The same amount of water is flowing in but less is flowing out. So the water level in the tank will rise. We can change the water level in our tank without changing how much water is flowing in, by changing how much water is flowing out.

water tank

Similarly we can change how much heat there is in the atmosphere by restricting how much heat leaves the atmosphere rather than by increasing how much is being absorbed by the atmosphere.

This is how the Greenhouse Effect works. The Greenhouse gases such as carbon dioxide and water vapour absorb most of the heat radiation leaving the Earth's surface. Then their concentration determines how much heat escapes from the top of the atmosphere to space. It is the change in what happens at the top of the atmosphere that matters, not what happens down here near the surface.

So how does changing the concentration of a Greenhouse gas change how much heat escapes from the upper atmosphere? As we climb higher in the atmosphere the air gets thinner. There is less of all gases, including the greenhouse gases. Eventually the air becomes thin enough that any heat radiated by the air can escape all the way to Space. How much heat escapes to space from this altitude then depends on how cold the air is at that height. The colder the air, the less heat it radiates.

(OK, I'm Australian so this image appeals to me)

So if we add more greenhouse gases the air needs to be thinner before heat radiation is able to escape to space. So this can only happen higher in the atmosphere. Where it is colder. So the amount of heat escaping is reduced.

By adding greenhouse gases, we force the radiation to space to come from higher, colder air, reducing the flow of radiation to space. And there is still a lot of scope for more greenhouse gases to push 'the action' higher and higher, into colder and colder air, restricting the rate of radiation to space even further.

The Greenhouse Effect isn't even remotely Saturated. Myth Busted!

Basic rebuttal written by dana1981

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 7 July 2015 by pattimer. View Archives

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Related Arguments

Further reading

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


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Comments 51 to 75 out of 135:

  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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