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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

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Related Arguments

Further reading

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


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Comments 426 to 450 out of 627:

  1. I understand you're saturation point now. Just be aware of your phrasing because this is often used by climate deniers to suggest there is no greenhouse effect. Relative to your questions about other bands outside of the 667cm-1 range, I think most of the other bands are overwhelmed by WV. It's that 667 window which has the primary scattering effect. 

  2. David Thorn @425, the specific absorption frequencies for different isotopologues of CO2 are available at Hitran (requires registration to download data).  The difference in isotopes will make a difference, if only because the different isotopes will have a different velocity distribution at different temperatures due to differences in mass, and hence different doppler broadening patterns.  This can be illustrated by this listing of relevant frequencies of a particular transition for different isotopologues:

    Note that the transition in question is at too high a frequency to be relevant to the atmospheric greenhouse effect, but the small differences shown are likely to be typical at lower frequencies as well, although I have not specifically checked.

    In any event, Hitran allows the downloading of a composite absorption spectrum for all isotopologues of CO2 at relative concentrations as found in the atmosphere; and it is likely that this is the data used in LBL models of radiation.  Broad band models of radiation, and Global Circulation Models typically use lower resolution data in which the differences are not likely to be significant.

  3. Bogfetto,

    When you make posts with many questions if in impossible to answer them all in detail.  Please ask one question at a time so that they can be answered.  After the first misconception is cleared up we can then move on to the next misconception.

    I will take a single example form your post to Tom Curtis.  You claim:

    "The only way to increase temperature of a radiating body, is to increase the temperature of the heat source heating it. And an increase in temperature always have to be initiated by a rising temperature of the radiating body, never by a decreasing temperature (and decreasing intensity) in a colder body heated by the hotter radiating body."

    When I got out of bed this morning it was cold.  I put on a cold shirt.  My surface temeprature increased even though my rate of heat production remained the same.  This is experimental evidence that your clain that " an increase in temperature always have to be initiated by a rising temperature of the radiating body" is incorrect.  

    It appears that you have a basic misunderstanding of how heat is transferred.  I suggest you reread the posts you have questioned, and the OP, and ask questions about what seems off to you.  Ask about one or two imortant items first.  As those misconceptions are cleared up you will start to understand the basics.  When you have the basics incorrect it is impossible to understand how the atosphere works.

  4. Bogfetto @429:


    "If the altitude of emission is increased, it means that a larger volume absorbs heat from the surface. Your argument is based on that this increase in volume is caused by an increase of a powerful heat absorber."

    First, while the second sentence is entirely ungrammatical and difficult to interpret, if I interpret it as saying, "Your argument is based on [the fact] that this increase in volume is caused by an increase of a powerful heat absorber" your claim is false, and you have not understood my argument, or its basis at all.  It is clear that it misrepresents my argument in that I limit the discussion to situations in which the mean altitude of radiation to space is lower than the tropopause.  If it rises to the tropopause (and assuming the temperature structure with altitude remains constant), further increases will initially have no effect in temperature, and then reduce the Global Mean Surface Temperature.  That despite the fact that the volume of gas below the mean altitude of radiation to space will continue to increase.  In practise, the same increases in CO2 that increases the mean altitude in radiation to space will also cool the stratosphere, and sufficient CO 2 increase to raise the mean altitude of radiation to space into the current stratosphere may well cool the stratosphere sufficiently that it has a declining temperature with altitude, in which case that will result in further surface warming.

    This discussion should make it very clear that my argument is based on the temperature of the mean altitude of radiation to space, something laid out very specifically in my article on the topic.  I suggest you move further discussion of that particular point to that article, and make sure you read it before you do.

    I will note that on rereading my comment from which you took the quotation, it was very clear and very specific.  Your misinterpretation has no justification in the text, and is based entirely on your attempt to shoe horn that discussion into your very rudimentary grasp of physics.  If you are going to persist in that behaviour, save us all some time and end the discussion now.


    "The only way to increase temperature of a radiating body, is to increase the temperature of the heat source heating it."

    If you have a radiant filament (such as in a light bulb) fed with a constant power source, and with a constant resistance, but place that filament in a gas that gradually corrodes its surface, thereby reducing emissivity, the filament will increase in temperature over time even though the amount of energy radiated will remain constant.  That is a common place experience in cold climates where low emissivity films are placed over windows to increase the internal temperature while decreasing the power used in heating.

    This is an analog the the greenhouse effect in which the increase in CO2 reduces the effective emissivity of the Earth over time in the IR bandwidth while not reducing the emissivity in visible light bandwidths (and therefore not decreasing the incoming energy).  The result will be that:

    When we compare the quasi-eqilibrium states before and after the introduction of the increased CO2:

    1)  The IR radiation to space will be the same in both cases (ignoring albedo feedbacks);

    2)  A reduced proportion of the IR radiation to space will come from CO2 because the CO2 will be radiating to space from a higher, and hence (typically colder) altitude;

    3)  Therefore the IR radiation from other components of the system, particularly the surface, will have to increase to compensate for that shortfall;

    4)  The only way for them to increase will be through a rise in temperature.

    In the interval between when the first quasi-equilibrium state is perturbed by the addition of CO2 and reaching the second quasi-equilibrium state:

    a)  Total IR radiation will be decreased (ignoring albedo feedbacks);

    b)  The resulting energy imbalance will result in an increase in temperature over time; and

    c)  The time between the two quasi-equilibrium states will be a function of the heat capacity of the system (most of which is in the ocean) and the cumulative energy imbalance in the interval todate.

  5. bogfetto @433:

    "Give me a source supporting a claim that heat is transferred from the freezing fluid to the hotter surface. In what other situations do you experience heat coming from low temperature fluids?"

    In the theory of the greenhouse effect, all heat (ie, net energy flow) is from:

    1)  The Sun to the Earth's atmosphere and surface, with the vast majority going to the surface;

    2)  From the Earth's surface to the atmosphere and to space, with the majority going to the atmosphere; and

    3)  From the atmosphere to space.

    In each case, the net energy flow is from a warmer to a colder object.  If you do not think this is the case, you simply do not understand the theory.  Period.  This is so important that I would put it in all caps were that not forbidden by the comments policy.

    And if you do not understand the theory, learn to understand it before you pretend to criticize it!

    You will certainly doubt my claim about the direction of energy flow in the greenhouse theory.  Very well.  Here is the Earth's energy budget as determined by NASA:

    It shows energy flows from the Sun, to and from the surface, to and from the atmosphere, and to space.  Sum the energy flow across any boundary between the Earth and the Atmosphere, and the Atmosphere and Space and you will find the incoming and outgoing energy are approximately equal, with only a 0.6 W/m^2 net imbalance.  Sum the energy flows between any warmer and colder object, and you ill find more energy flows from the warmer to the colder object than the reverse.

    Attn moderators:  I have no strong interest in playing whack-a-mole on these issues again with somebody who cannot bother actually learning the theory they purport to criticize.  Would you please restrict bogfetto's responses to just this issue until such time that he acknowledges the simple truth, or proves he is incapable of doing so by excessive repetition.  At your discretion, I will shift the discussion to the 2nd Law of Thermodynamics thread, where it is far more on topic.


    [JH] Bugfotto has relinquishd his privilege of posting comments on this site.

  6. bogfetto, you claimed "The atmosphere is air that moves freely."

    But the atmosphere does not move freely into outer space. Only trivial amounts of atmosphere escape to space. So outer space is the equivalent of the shirt. The atmosphere is the equivalent of the air inside the shirt. The atmosphere insulates the surface from the cold of space. The air trapped inside the shirt insulates the skin from the cold of the air outside the shirt.

  7. Tom Dayton @440, that is not an improvement of the analogy.  Specifically, the air between the shirt and the skin quickly obtains the same temperature as the skin, which does not occur in the atmosphere.  A better defense of the analogy is to point out that there is a temperture gradient within the fabric of the shirt, with the inner most layer of the shirt being skin temperature, and the outermost layer being room temperature.  That is, as bogfetto points out, because of the restricted circulation of air within the shirt - but such restricted circulation is not necessary to set up a temperature gradient, and thereby an insulating effect.  Specifically, the troposphere also has a distinct energy gradient which is a product of the inefficiency of energy transfer within the atmosphere.  Absent convection, it would be even more inefficient and the surface temperature would be higher (as shown by Manabe).  If the atmosphere were perfectly efficient at transfering energy, as appears to be assumed by bogfetto, then the temperature of the atmosphere would be constant with altitude.

  8. This picture shows at least three layers of the atmosphere that do not mix.  This disproves the wild claim that the atmsosphere "moves freely"  Cloud picture


    Heat is ony transferred slowly by wind and thermals through the troposphere.  Uninformed people think it is fast because they do not consider how big the Earth is.  Wind takes days to cross even the distance across a single ocean. It takes weeks and months to transfer energy to the Arctic from the equator.  The Atmosphere has many layers that do not freely mix.  IR transfer of heat is much faster than thermal transfer.  IR heat is blocked by CO2, causing greenhouse warming.


    Please provide data to support your wild claim that the atmosphere "moves freely" at a rate that is significant to release of energy from the top of the atmosphere.  I have provided data showing that the atmosphere does not freely mix to a significant extent. 


    [JH] Bugfotto has relinquishd his privilege of posting comments on this site.

  9. Interesting picture of a cloud showing at least five atmospheric layers that are not mixing.


  10. Tom Curtis at 10:19 AM on 12 March, 2017


    [DB] The user is a sock puppet of a previously banned user, which was also a sock puppet of another previously banned user.  Posting rights rescinded.

    [JH] The person behind the bogfetto screen has retuurned as beardface. All of his/her posts will be summarily deleted.

    As time permits, I will also delete all of bogfetto's prior posts. 

  11. I am having trouble following an argument that doesn't take into account the mass of the individual gases. CO2 weighs almost three times what oxygen does and takes up two thirds the space. Direct tests have shown a sharp drop in CO2 with altitude. At the tropopause there is a sharp drop of CO2 presence into the stratosphere demonstrating that while mixing may occur in the heavier, lower, atmosphere the fact that CO2 is a heavier component is still there and CO2 is still much higher near the ground than higher. In the heavier troposphere most of the transmission of heat is due to conduction and not radiation. This leaves one wondering why we are even considering 100 ppm change in a minority gas, that has a lower heat content than other gases, being considered as any sort of problem.

  12. Wake... "Direct tests have shown a sharp drop in CO2 with altitude."

    I don't think so...


  13. Here's another graph showing the vertical profile of 4 major greenhouse gases.

    Source: Earth and Planetary Sciences » "Global Warming - Causes, Impacts and Remedies", book edited by Bharat Raj Singh , ISBN 978-953-51-2043-8, Published: April 22, 2015

  14. If you look at you will see that through direct measurements there is a 3-5 ppm step from the tropopause to the stratosphere. The stratosphere starts at about 15 km altitude and that step is not shown on that graph of yours. I would therefore question that source.

    And in any case isn't the more important point that the lower atmosphere transfers heat more in conduction mode that radiation? If this is so we can assume that heat is reaching the upper atmosphere via the entire atmosphere and not any specific gas. Once above the cloud levels it would appear that the lower latent heat content of CO2 and Tom Curtis' explantion of heat radiation would be more applicable.

    I would also question Michael Sweet's idea that the atmosphere doesn't have large scale mixing by showing unusual conditions of atmospheric layering.


    [PS] Fixed link

  15. Wake,

    This graph, originally posted above at 430 (and many other locations) by Tom Curtis,

    energy graph


    shows that only 20% of energy transfer in the Troposphere is from conduction and latent energy according to measured data.  Please provide a reference to support your claim that " the lower atmosphere transfers heat more in conduction mode that radiation".  Since measured data indicates that the majority of energy is transferred by radiation, CO2 is indeed more important than conduction.

    Mixing is very slow compared to radiation transfer of energy.  The atmosphere is always layered.  Look at the clouds. Many days there are two or three (or more) layers of clouds.  The first picture is visible on any long airplane trip.

    A change of 4 ppm at the Tropopause is too small to be visible on the graphs Rob Honeycutt posted.  It is an insignificant change.  It was an interesting factoid for me to learn.  The scientists who study the atmosphere undoubtedly already know this factoid.

  16. Wake @436, I would certainly be interested in a citation of the scientific article from which you conclude  that at"... the tropopause there is a sharp drop of CO2 presence into the stratosphere ...".  I am aware of Georgii and Jost (1969) who find a "quite sudden change of about 3–5 p.p.m. CO2 is usually observed at the transition from tropospheric into stratospheric air and vice versa".  I am also aware of Bischof et al (1980) and there conclusion that "...the CO2 mixing ratio is not constant with altitude but rather decreases in the stratosphere, by about 7 p.p.m.v., between the tropopause and 33 km", along with their speculation that the variation is because "...recently increased concentrations of CO2 in the troposphere have not propagated far into the stratosphere".  I am further aware that up to the mesopause, CO2 concentrations stay within 16 ppmv of contemporary surface values:


    {Source: Emmert et al (2012) ; 2004-2012 mean of upper atmosphere CO2 concentrations.  Contemporary surface value (Mauna Loa): 385.7 ppmv}

    My problem is that none of these seem qualified as "a sharp drop".  Indeed, even the 16 ppmv difference between the tropopause and the mesopause represents only a difference in radiative forcing of 0.2 W/m^2 if it were applied across the whole atmosphere.  It applies, however, not across the whole atmosphere but over that part which has an optical depth of less than one, ie, were IR radiation typically escapes straight to space with the consequence of minimal further impact on the greenhouse effect.

    Regardless of impact on the greenhouse effect, the fact that CO2 concentrations at 80 Km altitude are a close approximation of those at 10 Km altitude refutes the idea that there is significant gravitational sorting of concentration below the thermosphere.  That is further refuted by Aoki et al (2003) who show from d13C concentrations that:

    "This quantitative agreement of Δδ13C/ΔCO2 indicates that the vertical profiles of CO2 concentration and δ13C observed in the lower stratosphere over Japan, Scandinavia and Antarctica were thought to be formed by the height-dependent poleward transport of tropospheric air intruded into the stratosphere in the tropical region."

  17. In terms of how far CO2 measurement has improved since Wake's 1969 paper, see Fouchier 2011. And for spatial mixing, it is hard to beat NASA's 1 year video of CO2 from NASA

  18. Michael Sweet @440, the graph doesn't show that.  

    Rather, it shows energy transfers across the surface/atmosphere and atmosphere/space boundaries.  It does not show the relative proportions of energy transport at any particular altitude (or averaged across all altitudes) within the troposphere.  The former because that will change with altitude, and the later because of its design.

    Further, even if it did show the proportion of the energy transfer mix within the troposphere, the relevant values would be net radiant energy transfer (57.9 W/m^2) vs energy transfer by convection (86.4 W/m^2) and latent heat transfer (18.4 W/m^2) giving respective percentages of 35.6%, 53.1% and 11.3%. 

  19. Thanks Tom, I misread the graph.  I will have to read your posts about this graph more closely.

  20. Tom Curtis - Thought you may not have seen this and since actual measurements are a lot more indicative of the real world than computer modeling it could be of interest.


    [PS] Fixed link please learn to do this yourself using the link tool in the comment editor. Discussions here could be more fruitful if you tried applying some of your skepticism to your own sources.

  21. Wake, your link points to a non-peer reviewed article with questionable conclusions, not to raw data as implied by your description of it as "actual measurements".

    I will let more knowledgeable people comment on the alleged science in the article, but the article has not even been edited by someone who speaks English as their first language. If an article is to be taken seriously, proof-reading it is a minimum first step, followed by peer review.

  22. Wake @445:

    1)  If you are going to say "actual measurements are a lot more indicative of the real world than computer modeling" you should at least point me to actual measurements rather than just a crude mathematical model (which is what Nahle's calculations amount to).

    2)  Nahle uses surface values for CO2 and H2O concentration.  Therefore, even if calculated correctly, he would only be calculating relative emissivity for back radiation at the surface - not the effective emissivity for radiation to space which is the basis of the greenhouse effect.  Therefore, the best that can be said for Nahle's calculations is that he simply does not understand the theory of the greenhouse effect.

    3)  Nahle uses the formula for the emissivity of a 128.2:1 H2O/CO2 isothermal gas mixture.  Therefore, at best all he shows is that if the an isothermal gas in a container was 100% water vapour, adding 0.78% CO2 would reduce the emissivity of the gas mixture relative to the 100% water vapour.  He goes on to demonstrate that adding oxygen to the H2O/CO2 mixture would further reduce emissivity.  The obvious point is that if adding oxygen to a H2O/CO2 mix decreases emissivity, adding a H2O/CO2 mix to an oxygen atmosphere will increase emissivity.  That is, it will induce a greenhouse effect.  For a more detailed exposition of this point, go here.

    To summarize, Nahle's calculations do not apply to the atmosphere as a whole because it is not isothermal; do not calculate the strength of the greenhouse effect because they apply to the surface layer only, and cannot calculate the effect of the change in outgoing radiation due to the presence or absense of a gas; and are incorrectly applied in any event.  Worse, they are not observations and do not produce an observationally testable hypothesis with regard to atmospheric radiation.  In contrast, the theory he wishes to rebut is based on models that apply the type of formula he is using radiation band by radiation band, layer by layer across a large number of layers in the atmosphere, each of which is small enough to be approximately isothermal.  Those models produce observationally testable predictions which have been compared to actual observations as with this example from 1969:


    Those detailed, band by band, layer by layer models also predict a greenhouse effect from CO2.

  23. Wake @445,

    The denialist paper you link to is actually five years old now, one of a number of crazy climatological ideas promulgated by Nasif Nahle. Concerning that particular paper, Walter Hannah of the Lawrence Livermore National Lab addressed Nasif Nahle’s Shaky Math last year.

  24. Considering the rebuttals by Tom Curtis @447 and by Walter Hannah at MA Rodger's link @448, the paper linked by Wake appears to be incredibly poorly conceived...  But even if its flaws were more difficult to spot, it should be clear that any valid paper disproving the greenhouse effect would make its author both rich and famous. Not only would the author become the new darling of the Koch Bros, touring the world giving triumphant defences of BAU fossil-fuel policy, the author would collect the Nobel Prize and all the other accolades befitting a genuine Galileo who had successfully overturned decades of understanding. Instead, the paper remains deservedly unpublished and largely obscure, and not even the denialists have bothered funding a round of much-needed editing for this paper. Thus, even for those unable or unwilling to follow the scientific rebuttal, there are ample clues that this is nonsense. Wake might want to consider what it is about his own world views that made him miss those clues.

  25. Nahle's incompetence isn't confined to how the greenhouse effect actually works.
    He has claimed (nearly halfway down this page) that this sequence of pictures proves that Mars was warming between 1995 and 2005 because the polar caps were shrinking, implying that it must be the sun.
    He apparently didn't understand (or forgot to tell his readers!) that each picture was taken a little more than one Martian year after the previous one, and that the changes in the polar caps are almost entirely due to seasons! (more on that in the thread Mars is warming)
    A similar sequence at 13 months intervals of the Arctic from September 2005 to March 2012 would seemingly show a nearly 3-fold increase in sea ice extent, but we know what really happened in September 2012, don't we?

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