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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 351 to 400 out of 403:

  1. Satoh @347:

    "The mean path length at sea level for photons between CO2 molecules was 33 mm years ago but I'm saying 30 mm now which is .03 m. The emissivity of CO2 at sea level goes along the curve of .0004 x .03 which is the .000021 I mentioned, which is not on the graph but you can visualize it."

    Ah!  So because partial pressure has increased from 360-400, pathlength has decreased from 0.033 to 0.03, or in other words (on your interpretation) atmosphere meters is a constant for any given gas at a constant temperature.  Do you perhaps want to rethink that?

  2. Satoh @349/350.

    That paper you cite manages to conclude saying:-

    "By considering also that the carbon dioxide has by far a lower total emissivity than the water vapor I conclude that the carbon dioxide has not an effect on climate changes or warming periods on the Earth."

    This is not the only argument from the author Nasif S. Nahle that purports to have  disproved AGW. He has even shown that CO2 is cooling the planet, rather than warming it. This, of course, is 'men-in-white-coats' territory and not the stuff that should be presented here at SkS.

  3. Nahle claims to disprove AGW by pointing out that the earth is not an actual physical greenhouse, which suggests the concept of metaphor is completely lost on him.  

  4. Tom@351

    Yes, of course, if the partial pressure goes up, the path length goes down. Naturally.

    I still need to know why you said 10,000 feet and why you used the top curve on graph 1.

  5. Satoh,

    Citing unpeer reviewed internet blogs that claim everyone else is wrong is not very useful in a scientific discussion, although it is better than nothing.    Please raise your game.  Nahles claim that his "paper" was "peer reviewed" by his friends in the Physics department is not a  suitable scientific citation.  Think: if this is the best you can find, is it really comparable to the recent AR5 report?  That report was written by several thousand specialists and peer reviewed by tens of thousands of scientists.  You must provide stronger evidence to get informed people to believe what you say.

    You will look better if you think longer before you post.  Tom's graphs A and B were clear to me.

    Tom: thank you for defining emissivity and emission.

  6. MA Rodger, thanks for the link to Nasif Nahle's claim that CO2 is cooling the planet.  On that thread, a commenter named Neutrino heroically corrected Nahle's "logic."  After revealing many of Nahle's astonishingly sloppy transcriptions of equations, misplacing of parentheses in equations, inconsistencies in units, and more, Neutrino summarized the crux of Nahle's error as treating the emissivity of one meter of atmosphere as if it is of the entire atmospheric column.  Nahle then vanished from the discussion.

  7. Satoh

    Most people try to figure out extinction, absorption or emissivity over an arbitrary length scale of interest — say, the depth of the atmosphere or 10,000m.  That is the pathlength TC is using.  That length scale is not simply a random choice to get a result, but central  to the entire point of the conversation, so it's a mystery why you should be surprised that he uses it.  You appear to be saying that the amount of CO2, or water vapor, in that 10,000 meter column of air has no effect on the opacity of the atmosphere to absorb IR emitted from earth.  I doubt that is what you mean, because we would have to throw spectroscopy right out the window...just to start.

    You seem to be using "pathlength" to mean something completely different, maybe the average distance a photon emitted by a CO2 molecule must travel before (possibly) running into another CO2 molecule? or an extinction length scale?  I can't tell for sure.  If I as cynical I would say that you seem to change depending on which definition gives you the number for emissivity that you want.

  8. Stephen, you are quite wrong. The definition of mean free path is well known and nothing new. You can't call a column of air the path length. The path length for CO2 is clearly defined as the mean length a photon will go before bumping into a CO2 molecule. It can't be scattered or re-radiated, and still be called the same path. Your comment is in grave error. The concept of mean free path length applies to moving molecules, atoms, electrons, photons, cosmic rays, etc. Emissivity of a solid or liquid is a surface phenomenon, emissivity of a gas uses the path length for obvious reasons...the photons that originate behind the path length don't originate at the surface....which is the path length.

    Michael, I did not cite that Nahle paper. I brought it up because it's the only paper online that estimates the path length for CO2 at sea level, and is cited several times around the web. In fact, I said it was wrong as part of my argument.

    Tom Dayton, atmospheric column? Read what I said to Stephen.

    Stephen again,  TC did not say 10,000 meters as the column length, he said 10,000 FEET so he could use the top curve on the graph, because it gave the higher emissivity of 0.3 for his argument.

    Everybody, if the path length of CO2 was 10,000 feet, photons would go 10,000 feet on average before hitting a CO2 molecule, there would be no greenhouse effect from CO2, and we would not be here.

  9. Satoh,

    I read some of your cite.  It contains chemistry calculations which I am familiar with.  The first equation is:

    "The density of the gas carbon dioxide in the atmosphere is obtained by the following formula:

    ρCO2 = (12.187 * Molar mass of CO2 * volumetric fraction of CO2) / (276.69 K) = 756 mg/m^3. (Ref. 7)

    Where 12.187 is the molar mass of elemental carbon, 44.01 is the molar mass of carbon dioxide, 390 ppmV is the volumetric fraction of CO2 and 276.69 K is its temperature."

    I went to his reference 7 and got 756 mg/m3 for 390 ppmV. I found several problems with this calculation.

    1) The units of Nahle's calculation are g/mol carbon*g/mol atmosphere/K.  He incorrectly uses the units from reference 7 of mg/m3.

    2) The textbook I teach chemistry from lists  the molar mass of carbon as 12.0107 not 12.187.  Nahle's number appears to have been made up to get the correct result.

    3) Reference 7 uses 273.15K as the temperature not 276.69.  Once again Nahle appears to have made up his number.

    4) Additonal errors are smaller and not worth the text space.

    Tom Dayton above links many additional basic errors.

    How can you rely on a citation which has so many basic errors?  Why do you think such junk is worth sharing?  What websites are you reading to pick up this junk?  Why do you believe what those websites say?

    If you continue to rely on websites that think pseudoscience rife with basic errors can counter the IPCC report you will never understand the greenhouse effect and AGW.  Perhaps you should start asking questions to become more informed, rather than citing obvious junk to support your mistaken notions.

  10. "The definition of mean free path is well known and nothing new. You can't call a column of air the path length. "

    Satoh, this is getting tiresome.  You can't simply refer to mean free path and then call it path length and expect anyone to understand what you are saying.  Those are two different concepts.  If you conflate them (and others) noone can have a sensible conversation with you.  

  11. And yes, I accidentally said 10,000 m instead of feet. Mea culpa, but you are completely missing the main point.

    TC picked that line because on that graph it best reflects the combination of the real ppmv of CO2 in the atmosphere (~400) and the real thickness of the atmosphere (which is actually thicker than 10,000', but you take what you can get.) Thus, 0.0004atm*10,000' = 4 ft.atmospheres.  

    The concept is need to consider the distance over which light traverses as well as the concentration of substances that may absorb light to determine the fraction of IR that is absorbed as it passes along that path. It's the basis of Beer-Lambert law in spectrometry and underlies a lot of quantitative chemistry.  

  12. Michael, I am not interested in discussing that Nahle paper. I did not cite it and don't care about it. I didn't even read it. Attacking that paper is a straw man.

    Stephen, the mean free path length is the path length. The photon leaves the surface and hits a CO2 molecule. That's the path. After that, the energy converts to kinetic heat when the CO2 molecule bumps into an O2, N2, H2O, Argon, another CO2 molecule, a tree, a drop of rain, or anything else. It another photon gets emitted by the H2O, tree, raindrop or anything else, it's not a continuation of the "path." It's a new path.

    Please end that line of argument because your line of argument has already bumped into something and was absorbed.



    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

  13. Stephen, the curve for 4.0 atm ft doesn't tell us anything about the combination. They could be 4 atmosphere with a path of 1 foot, .04 atmospheres with a path of 100 feet, or .000000000004 atmospheres with a path of 100000000000 feet. Like I said, if you increase the partial pressure you decrease the path, and are still on the same curve of the graph and have the same emissivity.

  14. Satoh,

    You cite the paper I referenced at 249 and 250.

    Since you now deny what you previously cited I will no longer post responses to you.  It is impossible to resolve diferences when one side denys what they previously claimed.


    [Dikran Marsupial] The comment numbers should be 349 and 350 respectively, in both of which Satoh cited this article by Nasif Nahle (note it appears that it is not actually a peer reviewed scientific article).

  15. "Stephen, the mean free path length is the path length."

    Satoh, saying so does not make it so. I linked to the wikipedia definitions for both terms that make the difference in common usage clear. You refused to acknowledge those links.  Hottel et al did not mean mean free path when they refer to pathlength, nor does anyone who uses the graphs.  Simply saying they mean something different does not change that.

    "Stephen, the curve for 4.0 atm ft doesn't tell us anything about the combination. They could be 4 atmosphere with a path of 1 foot, .04 atmospheres with a path of 100 feet, or .000000000004 atmospheres with a path of 100000000000 feet. Like I said, if you increase the partial pressure you decrease the path, and are still on the same curve of the graph and have the same emissivity."

    So, why are their multiple lines on the Hottel emissivity graph if the meaning of pathlength, as used by the authors, corresponds to what you say?  By your definition, there should only be one line, as concentration and pathlength would be perfectly anticorrelated for a given gas, although varying with temperature.  

    (BTW..I must apologize to everyone for getting issue involving emmisivity and absorptivity mixed up above in discussion of pathlength. Unnecessarily confusing that.)

  16. Satoh @347, 348, 349, 350, and 354 criticized my understanding of path length as used as a measure of pressure-length (pL) in Hottel diagrams.  This is despite the fact that he responded to my post @340 in which I describe my understanding by saying "excellent work" with no expressed quibble about my understanding of pL or emissivity, both of which he now disputes.  In particular, he wrote @349:

    "You randomly picked the top line and said "it's 4 foot atmospheres which is .0004 X 10,000 feet" so it would point to the 0.2 mark. That's pretty arbitrary. The mean free path length of 15 micron photons at sea level is definitely not 10,000 feet so you can't use that line.

    This paper has been floating around the web which says the mean path length for photons in CO2 at sea level is 32 meters. I don't buy it. They first measured it many years ago and it was in millimeters. I'd like to know what the latest calculations are for that.

    Mean path length applies to absorption, and it also applies to emission. They are inverse of the same process."

    The first and most crucial point is that L is not defined as "mean free path length", and that "mean free path length" is not the same thing as "mean path length".  Indeed, Mehrota et al (1995) (download PDF), define L in pL as "mean Beam width".  Further, in a worked example, they calculate estimates of emissivity for a Claus plant (described in Nasato et al, 1994), saying, "A mean beam length of L = 0.9 x diameter = 0.04 m can be used."  (The inside diameter of the tube in question was 43.99 mm.)

    Further, that is consistent with the lecture slides by Dr Prabal Talukdar on Gas Radiation in which he defines "Mean Beam Length":

    "• The simple expression for the hemisphere of gas is not
    applicable for other geometries
    • A concept of mean beam length is introduced for
    engineering calculations

    • This is an equivalent path length L which represents the

    average contributions of different beam lengths from the
    gas body to the striking surface

    •  In the absence of information available, mean beam

    length is approximately calculated as

    L =~= 3.5*V/A
    Where A=total surface area of the enclosure
    and V = total volume of the gas"

    He then shows a slide of a table of formula for different shapes including a "Circular cylinder of semi-infinite height" radiating to "an element at the center of the base" for which the formula  is 0.9 *D.  As these formula are not restricted as to the actual volumes enclosed, and as the worked example by Mehrota et al has a Beam Length significantly greater than your estimated Mean Free Path Length, I take this to show how experts in the field interpret L for the pL contours in Hottel diagrams (as opposed to the interpretation of biologists working outside of their field that even Anthony  Watts considers to be a pseudo-scientist).

    Worse, however, the mean free path length is given by the general formula l=1/(nσ), where l is the mean free path length, n is the number of particles involved, and σ is the effective cross sectional area of collision.  (In Nahle's varian he uses  "l=m/(nσ)" where m is the mass of the gas, and n is the number of molecules per unit mass which is equivalent.)  However, by the ideal gas law, 

    P=nRT/V, where n is the number of molecules in moles, P is pressure, T is temperature, V volume and R a constant.  Ergo, for constant temperature and volume, P is proportional to n.  But n is inversely proportional to l (free path length), so that if L in pL is mean free path length, pL is constant for a constant temperature and volume.  Ergo, if L were mean free path length, contours of constant pL in Hottel diagrams (which assume constant volume) would by necessity be vertical, ie, have a constant value in the x-axis (Temperature).  Therefore it is mathematically impossible that L from pL = mean free path length.  (Put another way that may be less obscure, because p is the inverse of l, of L=l then Hottel diagrams should revert to a mapping of Temperature directly onto emissivity, and shoud require no pL contours for that mapping.)

    Turning to emissivity, we have the statement of Byun and Chen (2013) that Hottel diagrams model total emissivity, not spectral emissivity.  The latter is the emission at a given wavelength or frequency relative to that predicted by the appropriate form of Planck's law for a black body at that wavelength or frequency.  The former is the integral of the spectral emissivions as a ratio to the emission predicted by the Steffan-Boltzmann law for the total emission of a black body.  Both, or course, are relative to a particular temperature.

    Because the emissivity plotted in Hottel diagrams is total emissivity, it is irrelevant that the emission at 15 microns is absorbed within a very short distance.  Emissions just above or just below 15 microns may not be absorbed for meter, or even kilometers and hence make a substantial contribution to mean Beam Length (L).  Therefore, in determining the total emissivity of the atmosphere, you cannot assume very short mean beam lengths.  (Nor should you assume mean beam lengths equivalent to the total height of the atmosphere because of decreasing pressure with altitude.)  But looking horizontally, mean beam lengths of multiple kilometers are possible with near constant pressure.  Hence my example of 10 horizontal kilometers, which with CO2 and amtospheric concentrations gives a total emissivity of approximately 0.2 (which I know independently to be the approximate total emissivity of CO2 looking vertically in clear sky conditions).

  17. Here is a thought experiment. If 15-micron photons are streaming up from the surface, being absorbed, re-emitted, absorbed, re-emitted, all the way from the surface for 10,000 feet, how can you consider that 10,000 feet would ever come into any sort of calculation for total emissivity? Do you think anything gets bigger and bigger for 10,000 feet? If photons are absorbed and re-emitted 100 times from the surface to the TOA, that somehow that means there are 100 times more photons than if you only considered the uppermost photon paths? When a photon is absorbed and re-emitted, it's still just one photon? The amount of emissivity at the surface depends ONLY on the number of photons coming from the uppermost path length. That is the reason they say path length, and not total distance that all these have traveled. Only the top photons even REACH the surface, the others were all absorbed!

    In a relay race, where 4 runners pass the baton, the emissivity at the end of the race is one baton, not 4.

  18. Satoh @367, I thought you would take that line.  That is why I pre-rebutted it, writing:

    "Because the emissivity plotted in Hottel diagrams is total emissivity, it is irrelevant that the emission at 15 microns is absorbed within a very short distance. Emissions just above or just below 15 microns may not be absorbed for meter, or even kilometers and hence make a substantial contribution to mean Beam Length (L). Therefore, in determining the total emissivity of the atmosphere, you cannot assume very short mean beam lengths."

    (Emphasis added)

    I will illustrate what I mean with an example from Modtran, a Broad Band Model of atmospheric radiation.  The example shows the downward IR radiation from CO2 only for a tropical atmosphere with no clouds at 0 and 5 kms:

    Because the IR spectra are show the net downward flux from CO2 in the absence of other radiative molecules, it shows the net emission from CO2 (ie, total downward emission by CO2 less total absorption of downward emission by CO2).  This is not observed data, for which we cannot obtain H2O (and O3, and CH4 etc) free conditions.  The model is of the type that has been shown to be reasonably accurate at predicting emissions from CO2 in furnaces and flames (ie, reasonably approximate to Hottel diagrams), especially near room temperatures.  Ergo they will show the correct basic principles.

    In atmospheric conditions pL is ill defined because, as shown in the side graph pressure is not constant with altitude, and nor is temperature.  For this reason, Hottel diagrams are useless in determining atmospheric emissivities.  However, pL in Hottel diagrams is proportional to the number of molecules in the mean beam length.  Obviously, there are fewer molecules from 5 km to the top of the atmosphere (TOA) than there are from the ground to the TOA, so that the 5 km look up corresponds to having a smaller pL in a Hottel diagram.

    The crucial difference between the two as seen above is that the emission in weak bands is stronger for 0 km looking up than it is for 5 km looking up.  This is partially obscured by the lower base temperature for the 5 km looking up, but is very clear in the 10 micron emission range which is moderately strong for 0 km looking up, but near non-existent for 5 km looking up.  That difference results in a reduced total emissivity for the 5 km looking up example than for the 0 km looking up example.

    In fact, the model output gives the emission over the wavelengths shown in the graph, and the temperatures for the different levels.  From that it can be calculated that the total emissivity of CO2 for the tropical atmosphere from 0 km up is 0.264 and for 5 km looking up it is 0.18.  Because the calculation does not include the full spectrum, these will be slight over estimates of the emissivity.  However, the total emissivity from 0 km looking up will not drop appreciably below 0.2.  

    SOD shows that it cannot be below 0.13 from observational evidence by SOD using measurements of back radiation spectra:

    But that, is known to be an underestimate, not least because the example used is from an Arctic autumn (a factor not mentioned by SOD).  It is possible to constrain the lower limit of total emissivity of CO2 in the atmosphere like this for a given surface temperature because CO2 absorbs so strongly in the 15 micron band, making it easy to distinguish.

    So, while your reasoning is approximately valid the 15 micron peak of CO2 emissions, it is not valid for total emissivity.  That mean beam length at 15 microns is so constrained is one of the reasons why emissivity does not increase linearly with pL.

  19. A slight correction and clarrification for my preceding post.  The emissivities calculated with modtran are for the tropical atmosphere only.  Further, as I took the ratio of the integrated emission across the wave lengths shown to the total emissions for a black body with the surface temperature the emissivities are underestimates, within the limits of accuracy of the model, not over estimates as I indicated.

  20. Yes, but emissions just above and below the 15 micron band, that go for kilometers, have nothing to do with CO2. They come from water vapor, cloud, and terrestrial. The ones from right next to 15, along the sides of the band that do come from CO2, are negligible in amplitude compared to the emissions from 15, or more exactly, 14.9, and even they don't go for kilometers.  

    Your attempt to rationalize a path length of 10,000 feet, in an attempt to rationalize a curve on the graph that shows a CO2 emissivity of 0.3, need to come to an end. Especially since you just said the Hottel charts don't work for atmospheric emissivities. (Correct, they don't. The Hottel charts are industrial charts for short distances in factory settings, under high pressure and very high heat.)

  21. Satoh @370, you are evidently determined in your decision to be absolutely resistant to evidence.  In this case, the modtran graphs shown above the only emissions shown are from CO2.  The show emissions from an altitude of 80 km, and a difference of 5 km makes a difference in the emissivity.  A difference of 10 km would also make a difference.  Ergo, your introduction of Hottel graphs into the discussion, and your insistence that only values for the free path length of photons with a wave length of 14.9 microns with respect to a CO2 gas are shown (respectively) to be a red herring, and a blunder.  Further, your insistence that you know more on this subject than the scientists who have spent their career pursuing the topic and wrote textbooks on the subject is simple arrogance of a breath taking quantity.

  22. TC...lest your get frustrated, I just wanted to say that the only reason I followed this discussion was because I am less familiar with this aspect of atmospheric physics thanI should be (it's off my specialty by quite a ways) and I wanted to learn some more.  And I have certainly learned a lot in this exchange, perhaps more than if you had simply been trying to explain things de novo.

  23. [JH] Moderator's Comment

    Satoh: Your "Artful Dodger" schtick has run its course. Any future posts by you will be summarily deleted. 

  24. Stephen Baines @372, firstly, thanks.  Second, the reason I have persisted so long in the discussion is because of a hope that it would prove instructive to interested readers.  I judge that by now, any questions that need to be answered have already been answered (several of them several times).  However, if somebody other than Satoh has questions, I would be happy to address them if I am able.

  25. I have a question. This debunking depends on the fact that stratospheric temperatures decrease with increasing elevation, so the CO2 gets colder as you gp higher, and radiates less. But I was always told that the stratosphere does not decrease in temperature with increased height. Please explain.

  26. Anne Hyzer @375, you are correct about the stratosphere increasing in temperature with altitude.  Most IR radiation from the atmosphere comes from the tropospheresphere, however, so the above explanation is a reasonable simplification.

    For a more detailed explanation, considering the following spectrum of outgoing IR radiation from an unknown (by me) location:


    The spectrum has conveniently placed black body curves for particular temperatures (dashed lines).  From that you can see that the chief absorption band for CO2 (centered at 650 cm^-1) has a broad flat base at approximately 220 K.  That flat base represents the tropopause, where temperatures are unchanging with altitude for several kilometers (see temperature profile below).  At the center of that flat base are two peaks, one much larger than the other.  That represents radiation from the stratosphere.  Also, and very importantly, on either side of the broad absorption band, at about 240 K, and againg at 250 K temperature, you will see notches in the wall, with the former being deeper and the latter broader.  These are notches are from local peaks in absorption (emission) which are located within the troposphere.

    As CO2 concentration increases, emissions at all wavenumbers in the CO2 brand will come from higher altitudes.  That will have several effects.  First, the broad plateau at 220 K will become wider emissions from the upper troposphere move into the tropopause.  Second, the notches at 240 and 250 K will become wider and deeper as the emissions causing them move higher in the stratosphere.  Also, the small notch at 800 cm^-1 will also deepen and widen (that notch also being due to CO2).  Against this, the two peaks at the center will grow higher and widen.  The combination of the other effects, however, will result in a greater reduction in IR radiation than will the increase due to increased emission from the stratosphere.  As an added nuance, the increased CO2 will cool the stratosphere, which will tend to limit the increase in height and bredth of the central peak.

    So, overall, outgoing IR emissions will decrease with increasing CO2, and that decrease will be entirely due to higher emission in the troposphere as indicated in the original post.  The increase will be only partly offset by an increase in radiation from the stratosphere.  All of this is included in determinations of CO2 forcing from standard radation models and Global Circulation Models, although arguably it was neglected prior to the correction to the basic formular for CO2 forcing by Myrhe et al, 1998.

  27. Anne Hyzer - In the troposphere convective overturning dominates, with the atmosphere warmed from below, under a state of constant inversion. Once past the GHG effective radiating altitude (where more than ~50% of upward emission for the wavelength in question is escaping to space without further absorption) and into the stratosphere, the dominant influence is UV warming from the sun, which is strongest at higher altitudes. And since the stratosphere is thermally stable, warmer at the top, convective overturning doesn't occur there.

  28. I understand that most radiation comes from the troposphere as you say, outside the CO2 band, but this source says almost all of the CO2 band's radiation comes from the upper stratosphere.

    This source 

  29. radiation by pascal

  30. Anne, I'm not sure that you're correctly interpreting the Iacono and Clough graph. It is essentially a heat loss diagram and shows the role of various GHG in stratospheric cooling. I'm sure Tom Curtis can help more.

  31. Anne Hyzer @379, that is a very complicated graph that has been launched like an iceberg on an unsuspecting public.  It is unsurprising that you have misinterpreted it (as I have on a previous occasion).  

    Teasing out the complexities, the first thing to note is that it is a graph of spectral cooling rates by pressure altitude and wave number.  Because it shows cooling rates, positive values show a net cooling effect at that wave number.  It is important to recognize that it is by wavenumber, as integrated across all wavenumbers (ie, into the ultraviolet), ozone has a net warming effect rather than the net cooling effect shown which appears to be shown here.

    The second complexity is that the spectral cooling rate is the total emission less total absorption at that altitude and wave number divided by density times the heat capacity of the atmosphere at that level.  The heat capacity is in fact not constant with altitude because atmospheric composition is not constant with altitude.  In particular, the water vapour concentrations falls rapidly as you ascend in the troposphere.  Further, density falls rapidly with altitude as well.  Density scales very approximately with pressure in the atmosphere, so that at a pressure of 100 mb, the energy emitted minus energy absorbed is very approximately one tenth of what it would be for an equivalent spectral cooling rate at sea level.  At 1 mb pressure, it is 1000th.  That is a significant underestimate of the actual scaling due to the greatly reduced heat capacity at altitude as a result of the greatly reduced water vapour content.

    Combining these two factors, and the title of the graph seen when you run your mouse over it here (which I assume you put in) and your interpretation of the graph are both completely wrong and misleading.

    Adding further complexity is that the scale is not uniform (being linear below six, and logarithmic above it), and the colour scale was chosen to emphasize features in the stratosphere and hence are not terribly informative in the troposphere.  Further, this graph is for Mid Latitude Summer conditions, and do not represent a global average.

    As it happens, the original paper has a host of interesting figures which would obviate the confusion caused by the above graph without detailed discussion.  Most helpful in this case is Table1:

    As can be seen both upward and downward flux are highest at the surface, where the difference between them (net flux) is also smallest.  At the surface, upward flux is 423.5 W/m^2 but downward flux is 346.9 W/m^2 leaving a net upward flux of 76.6 W/m^2. At the tropopause, upward is 287.6 W/m^2, downward is 22.3 W/m^2 and net is 265.3 W/m^2.  At the TOA, there is no downward, so upward= net = 283.3 W/m^2.  Further, because at any location, upward emissions from that location equals downward emissions from that location, we know that upward emissions from the stratosphere never exceed the downward emissions at the bottom of the stratosphere, ie, the 22.3 W/m^2 downward emissions at the Tropopause.

    With that in mind, consider the chart of net upward IR flux below:

    As you can clearly see, the net upward IR flux is smallest at the surface, and rises rapidly with altitude up to the tropopause.  You will also see that doubling CO2 reduces the net upward IR flux.  Table 1 from the original paper specifies the reduction in net upward IR flux due to doubling CO2 to be 2.8 W/m^2 at the TOA, and 5.6 W/m^2 at the tropopause.  Given that the normal definition of radiative forcing is the change in net upward IR flux at the tropopause after equilibrium adjustment for the stratosphere, and that this is a Mid Latitude Summer atmosphere, these figures are consistent with the accepted forcing of 3.7 W/m^2.

    Finally, one more figure from Clough and Iacono 1995:

    This figure shows the change in spectral cooling rate for an increase in CO2 concentration from 335 to 350 ppmv (approx) among other changes.  As you can clearly see, the effect is a warming effect in the troposphere (100 mb and lower) with most of the warming being in the wings of the CO2 band.  There is a cooling effect in the stratosphere.  (Note, this graph uses a more detailed resolution, allowing more detail of differences in wave number to emerge than in the more commonly shown graph.)

    So, as figure 5 and plate 9 of Clough and Iacono 1995 clearly show, the CO2 effect is not saturated, and increasing CO2 warms the troposphere.


    Note:  In the original version of this comment, I made a blunder in interpreting Fig 5.  The original text is preserved below in the interests of transparency.  I have, however, struck it through to make it clear that (except where it agrees with the text above) it no longer represents my opinion.

    Most helpful in this case is their plot of net upward IR flux by altitude integrated across the spectrum:


    As you can clearly see, the net upward IR flux is greatest at the surface, and falls rapidly with altitude up to the tropopause. You will also see that doubling CO2 reduces the net upward IR flux. Table 1 from the original paper specifies the reduction in net upward IR flux due to doubling CO2 to be 2.8 W/m^2 at the TOA, and 5.6 W/m^2 at the tropopause. Given that the normal definition of radiative forcing is the change in net upward IR flux at the tropopause after equilibrium adjustment for the stratosphere, and that this is a Mid Latitude Summer atmosphere, these figures are consistent with the accepted forcing of 3.7 W/m^2.


  32. WRT the effective radiating altitude, it's worth comparing the spectra of emitted IR to the atmospheric temperature profile.

    Emitted IR, single point, US Standard Atmosphere:

    MODTRAN Emitted IR


    Atmospheric temperature profile:

    Vertical Temperature Profile


    What's interesting is that for any GHG wavelength you can go from the amount of IR radiating to the temperature to the effective radiating altitude.

    In the CO2 trough you can see emissions bottoming out around 220K, or -53C. At the tropopause temperatures are around -50 to as low as -70C, ~223 to 203K, indicating that valley and those wavelengths represent an effective CO2 emission altitude at the tropopause.

    In the center of the CO2 trough where absorbance is particularly high there is a smaller peak - that comes from a small band of IR emitted by CO2 in the warming stratosphere. For most of the IR spectra the tropopause represents the upper limit on the effective radiating altitude. And for that very reason the upper end of convective overturning, as the energy necessary for the tropospheric inversion and convection radiates to space at that point. 

  33. KR, you say the 220 K value in your first graphic corresponds with the -50 to -70 C temps of the tropopause, but your second graphic shows that the temperature of the atmosphere does not change at all between the tropopause, at 10 km, and the lower stratospphere up to around 20 km. This is seen more clearly in the graph in post #376 in Tom's post. It means, judging by temperature, the CO2 radiates anywhere from the tropopause to the lower stratosphere, or, anywhere from 10 km up to 20 km.

  34. Digby @383, if you look at the right hand panel of the second figure in KR's post you will see three "typical" temperature profiles.  The temperature profile in my post @376 corresponds to the green profile in KR's post, ie, middle latitude.  As you can see, the profile varies based on latitude, but also on season and local conditions (including local humidity).  The profile over desert, for example, would be different to that over ocean.

    KR's refference to a temperature range, therefore, does not represent a range of temperatures in the tropopause.  It represents a range of temperatures of the tropopause at different latitudes (as shown in the right hand panel of his second figure).  While it would be possible with a sufficiently distant instrument to get a whole hemisphere IR spectrum for the Earth, the actual instruments used are in low Earth orbit and so can only profile a limited area at a time so the brightness temperature of the base of the CO2 trough will vary depending on where and when the profile was taken.

  35. Digby - And as Tom Curtis and I have noted, this means that the upper stratosphere is not the location of the effective radiating altitude, nor where the majority of the CO2 radiated energy comes from. 

  36. Digby, I think you are furiously agreeing with KR.  The only thing you are not noting is that KR's post was a response to "Anne Hyzer" who claimed the majority of CO2 radiation came from the upper stratosphere (see 378).

  37. I'd venture to guess that Digby is M. Wright -> Satoh -> Anne Hyster -> Anne HysterII. Each of these folks has been banned for excessive repetition, sock puppetry and sloganeering.

    I would propose to allow continued commenting if Mr Wright can abide by the rules. But that will require the capacity to actually move the discussion forward when shown where he is in error.

    Note that Mr Wright has an extensive blog post where he contradicts nearly every aspect of established scientific research related to climate change (which, if any of it were actually correct, would earn him a Nobel Prize).


    [DB] Yes, sock puppetry is indeed confirmed.  Reprehensible activities such as this are subject to automatic forfeiture of posting rights, permanently.

  38. I think what Digby fails to realize is that the radiation along the side wings of the CO2 band do come from the troposphere, yet are radiated and/or absorbed by CO2. If you study Toms graph in post 376, you see that along the sides of the CO2 band, around 650/cm and on the other side at around 750/cm., the radiation matches up with 240 K or even 250 K, which correspond with temperatures in the troposphere. Adding CO2 to the atmosphere would push those jagged edges downward on the graph and closer to 220K, which is colder.

    Digby is partially correct by saying the bulk of the CO2 band radiates from 220 K which is the lower stratosphere, which does not get colder with altitude, and he is also corect that the bulk of the radiation from the troposhere is well outside the region of the spectrum which is affected by CO2, but he fails to note that the wings of the CO2 band would be pushed slightly downward on the graph, and into colder regions on the graph (because, in the real world outside the graph, radiation of those 650 and 750 wave numbers would have to travel higher up in the atmosphere to get around all the extra CO2 molecules).

  39. Mike Hills

    That spreading oof the CO2 'notch' due to the sides dropping can be seen in this image. I have used Modtran to show the OLR spectrum for 400 and 4000 ppm of CO2, everything else kept the same.

  40. There is a concern about saturation of CO2 absorption that I haven't seen addressed. Most of the action - like increased IR aborption - takes place at 16+ km above ground level.  How can this affect ground level temperature?
    Take into account that the greenhouse effect will not let radiation through.

  41. commonsense - Two words: lapse rate. Rising air decreases in pressure and cools, descending air rises in pressure and warms (by the ideal gas law, PV - nRT). Surface temperatures will be directly related to tropospheric emission temperatures by altitude difference and the lapse rate. See Fig. 4 at this Realclimate article.

    At the surface IR in GHG wavelengths is absorbed within meters - but the real action takes place around the tropopause where pressures and absolute amounts of GHGS decrease to the point that ~50% of IR escapes to space. And the more GHGs, the higher that altitude is, and the larger the difference between the emission altitude temperature and that of the surface. 

  42. commonsense @390, in the troposphere, convection induces a lapse rate (fall of temperature with altitude) of about 6.5 C per kilometer.  The exact value depends on the specific heat of the atmosphere, the local graviational acceleration, and the mixing ratio of water to dry air in the atmosphere.  Because of this, any change in temperature at any level of the troposphere will in general be reflected across all altitude levels in the troposphere.  Specifically, if a 1.2 C increase in temperature at the effective altitude of radiation to space is required to balance the energy effect of doubling CO2, because of the effect of the lapse rate that same temperature change will also be felt at all lower altitudes.

    This is not a magical effect.  Suppose the effective altitude of radiation to space increases in height, ie, lifts to a cooler altitude.  It follows that less radiation to space will occur, so that the temperatures will warm.  The warmer temperature at higher altitudes will then slow convection, reducing the rate at which energy leaves the surface, which will in turn warm the surface.  This process will continue until the lapse rate is restored to its former value, ie, the temperature increase at the surface equals that at altitude.

    All this ignores feedbacks.  As it happens, one of the feedbacks, the lapse rate feedback slightly decreases the lapse rate due to increased water vapour in the atmosphere with increased temperatures (the cause of the tropospheric hotspot).  This is more than balanced by the increased greenhouse effect from the water vapour (the water vapour feedback) so that the net feedback is positive.  Consequently the final change in temperature is much more than the 1.2 C found in the no feedback case (and mentioned above).

  43. A lot the trouble here is around trying to make a plain english explanation of physics as opposed to the "shut up and calculate approach".

    So commonsense, you have identified an issue with the explanation but not with the physics. At the heart of this is the Radiative Transfer equations. Solve these and the hard-to-explain stuff like saturation, stratagraphic cooling etc fall out of the solution. The solutions to the RTEs can be verified with equisite accuracy by measurement. (eg Harries 2001, Evans 2006, and most recently directly see here).

  44. commonsense:

    To add another analogy to the concept, if you had a person suffering from hypothermia, witha low body core temperature, and all you had on hand was a blanket, do you think it would be worth wrapping the person with the blanket? After all, that only adds insulation on the outside, not the body core. The fact is that the body core will continue to lose heat to the body surface, and insulating the surface (reducing the heat loss to the air) will help maintain core temperature. A heavier blanket --> higher core temperature.

    The earth-atmosphere system is similar: all portions of the system are linked together, and changing one component of heat transfer will cascade through the entire system. To know exactly how, you need to apply the known physics and "do the math". The math says "increasing atmospheric CO2 leads to increased surface temperatures".

  45. With 8 pages of comments, I am not sure that I covered all of the conversations. However, the original premise of the article seems to contradict the study and paper by Hermann Harde. It is a 45+ page paper so it is impossible to shorten in this comment however it is clear that the study disagrees with the analysis of the original post and points out that the IPCC estimate of 1.5C to 4.5C is likely to be in error with a more realistic sensitivity of 0.6C including a declining increase of CO2 impact due to absorption and emission spectra of greenhouse gases.

    We present an advanced two-layer climate model, especially appropriate to calculate the influence of an increasing CO2-concentration and a varying solar activity on global warming. The model describes the atmosphere and the ground as two layers acting simultaneously as absorbers and Planck radiators, and it includes additional heat transfer between these layers due to convection and evaporation. The model considers all relevant feedback processes caused by changes of water vapour, lapse-rate, surface albedo or convection and evaporation. In particular, the influence of clouds with a thermally or solar induced feedback is investigated in some detail. The short- and long-wave absorptivities of the most important greenhouse gases water vapour, carbon dioxide, methane and ozone are derived from line-by-line calculations based on the HITRAN08-databasis and are integrated in the model. Simulations including an increased solar activity over the last century give a CO2 initiated warming of 0.2 ˚ C and a solar influence of 0.54 ˚ C over this period, corresponding to a CO2 climate sensitivity of 0.6 ˚ C (doubling of CO2) and a solar sensitivity of 0.5 ˚ C (0.1 % increase of the solar constant).


    [PS] You might want to look at discussion that begins here. You might want to sharpen your critical skills by seeing if can spot some reasons why there might be an issue.

  46. SeanO, HotWhopper dealt with that paper last year:


    [PS] Fixed link

    [PS] Since the input is the hitran database, this isnt really about CO2 saturation. Any further discussion should take place on "Climate sensitivity is low"

  47. SeanO - Harde is a very good spectroscopist with an absolutely lousy climate model, see the discussion on Rabett Run:

    The major problem is the use of a 'toy' model with only two atmospheric levels; as I understand it MODTRAN results don't start to stabilize until you use at least 20 levels - too few levels result in a poor approximation. It's like attempting numerical integration of a complex curve with only two intervals - use too few samples, and your answer will be wrong. 

    Two-level models are fine for demonstrating basic principles in intro classes, but insufficient for obtaining realistic answers. 

  48. A better discussion on this runs in a Comments section on the EIKE website (German climate denial group) where Harde's paper was discussed, in particular the comments by Dr. Gerhard Kramm - an expert on climate modeling. From his first comment (translated, emphasis added):

    Apparently not know Harde what he does. [...] The rivers of sensible and latent heat may be charged only according to local temperature and humidity distributions. Everything else is Blooming nonsense. [...] ...Harde temperatures...have nothing to do with reality.

    Enough said.

  49. Since CO2 is only present in our atmosphere at 0.04%, I've always thought it strange that it could have such a large impact. I've looked into "Radiative Forcing" and found out that as a model, it's not fitting, so there's another model that's more accurate that's being used instead for most climate models.

    At the center of it all tho, as this article describes, is the effective "greenhouse" effect of CO2. If CO2 makes up 20% of our greenhouse effect, light from stars at this wavelength should be diminished by 20%. 

    According to this article, it isn't even a concern in IR astronomy. 

    (I apologize for the tone of this article, I don't think it should be as inflamatory as it is, yet the points he makes seem valid to me)

    If IR at CO2's wavelengths aren't affecting light coming from stars (almost undetectable amount) then IR at CO2 wavelenghts is free to radiate to space even from the surface. That should be easily measurable using a light source at that frequency pointed out to hit a sattelite, or even one of the mirrors we have on the moon. 

    If the article I listed or the premise I've asserted is false, please let me know.


    Thank you...

  50. Quick FYI... Stevengoddard (not his real name) is probably one of the worst sources of information on climate change available on the internet. 

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