Guest post By Tom Vonk (Tom is a physicist and long time poster at many climate blogs. Note also I’ll have another essay coming soon supporting the role of CO2 – For a another view on the CO2 issue, please see also this guest post by Ferdinand Engelbeen Anthony)
If you search for “greenhouse effect” in Google and get 1 cent for statements like…
“CO2 absorbs the outgoing infrared energy and warms the atmosphere” – or – “CO2 traps part of the infrared radiation between ground and the upper part of the atmosphere”
…you will be millionaire .
Even Internet sites that are said to have a good scientific level like “Science of doom” publish statements similar to those quoted above . These statements are all wrong yet happen so often that I submitted this guest post to Anthony to clear this issue once for all.
In the case that somebody asks why there is no peer reviewed paper about this issue , it is because everything what follows is textbook material . We will use results from statistical thermodynamics and quantum mechanics that have been known for some 100 years or more . More specifically the statement that we will prove is :
“A volume of gas in Local Thermodynamic Equilibrium (LTE) cannot be heated by CO2.”
There are 3 concepts that we will introduce below and that are necessary to the understanding .
- The Local Thermodynamic Equilibrium (LTE)
This concept plays a central part so some words of definition . First what LTE is not . LTE is not Thermodynamic Equilibrium (TE) , it is a much weaker assumption . LTE requires only that the equilibrium exists in some neighborhood of every point . For example the temperature may vary with time and space within a volume so that this volume is not in a Thermodynamic Equilibrium . However if there is an equilibrium within every small subvolume of this volume , we will have LTE .
Intuitively the notion of LTE is linked to the speed with which the particles move and to their density . If the particle stays long enough in a small volume to interact with other particles in this small volume , for example by collisions , then the particle will equilibrate with others . If it doesn’t stay long enough then it can’t equilibrate with others and there is no LTE .
There are 2 reasons why the importance of LTE is paramount .
First is that a temperature cannot be defined for a volume which is not in LTE . That is easy to understand . The temperature is an average energy of a small volume in equilibrium . Since there is no equilibrium in any small volume if we have not LTE , the temperature cannot be defined in this case.
Second is that the energy distribution in a volume in LTE follows known laws and can be computed .
The energy equipartition law
Kinetic energy is present in several forms . A monoatomic gas has only the translational kinetic energy , the well known ½.m.V² . A polyatomic gas can also vibrate and rotate and therefore has in addition to the translational kinetic energy also the vibrational and the rotational kinetic energy . When we want to specify the total kinetic energy of a molecule , we need to account for all 3 forms of it .
Thus the immediate question we ask is : “If we add energy to a molecule , what will it do ? Increase its velocity ? Increase its vibration ? Increase its rotation ? Some mixture of all 3 ?”
The answer is given by the energy equipartition law . It says : “In LTE the energy is shared equally among its different forms .”
As we have seen that the temperature is an average energy ,and that it is defined only under LTE conditions , it is possible to link the average kinetic energy <E> to the temperature . For instance in a monoatomic gas like Helium we have <E>= 3/2.k.T . The factor 3/2 comes because there are 3 translational degrees of freedom (3 space dimensions) and it can be reformulated by saying that the kinetic energy per translational degree of freedom is ½.k.T . From there can be derived ideal gas laws , specific heat capacities and much more . For polyatomic molecules exhibiting vibration and rotation the calculations are more complicated . The important point in this statistical law is that if we add some energy to a great number of molecules , this energy will be shared equally among their translational , rotational and vibrational degrees of freedom .
Quantum mechanical interactions of molecules with infrared radiation
Everything that happens in the interaction between a molecule and the infrared radiation is governed by quantum mechanics . Therefore the processes cannot be understood without at least the basics of the QM theory .
The most important point is that only the vibration and rotation modes of a molecule can interact with the infrared radiation . In addition this interaction will take place only if the molecule presents a non zero dipolar momentum . As a non zero dipolar momentum implies some asymmetry in the distribution of the electrical charges , it is specially important in non symmetric molecules . For instance the nitrogen N-N molecule is symmetrical and has no permanent dipolar momentum .
O=C=O is also symmetrical and has no permanent dipolar momentum . C=O is non symmetrical and has a permanent dipolar momentum . However to interact with IR it is not necessary that the dipolar momentum be permanent . While O=C=O has no permanent dipolar momentum , it has vibrational modes where an asymmetry appears and it is those modes that will absorb and emit IR . Also nitrogen N-N colliding with another molecule will be deformed and acquire a transient dipolar momentum which will allow it to absorb and emit IR .
In the picture left you see the 4 possible vibration modes of CO2 . The first one is symmetrical and therefore displays no dipolar momentum and doesn’t interact with IR . The second and the third look similar and have a dipolar momentum . It is these both that represent the famous 15µ band . The fourth is highly asymmetrical and also has a dipolar momentum .
What does interaction between a vibration mode and IR mean ?
The vibrational energies are quantified , that means that they can only take some discrete values . In the picture above is shown what happens when a molecule meets a photon whose energy (h.ν or ђ.ω) is exactly equal to the difference between 2 energy levels E2-E1 . The molecule absorbs the photon and “jumps up” from E1 to E2 . Of course the opposite process exists too – a molecule in the energy level E2 can “jump down” from E2 to E1 and emit a photon of energy E2-E1 .
But that is not everything that happens . What also happens are collisions and during collisions all following processes are possible .
- Translation-translation interaction . This is your usual billiard ball collision .
- Translation-vibration interaction . Here energy is exchanged between the vibration modes and the translation modes .
- Translation-rotation interaction . Here energy is is exchanged between the rotation modes and the translation modes .
- Rotation-vibration interaction … etc .
In the matter that concerns us here , namely a mixture of CO2 and N2 under infrared radiation only 2 processes are important : translation-translation and translation-vibration . We will therefore neglect all other processes without loosing generality .
The proof of our statement
The translation-translation process (sphere collision) has been well understood since more than 100 years . It can be studied by semi-classical statistical mechanics and the result is that the velocities of molecules (translational kinetic energy) within a volume of gas in equilibrium are distributed according to the Maxwell-Boltzmann distribution . As this distribution is invariant for a constant temperature , there are no net energy transfers and we do not need to further analyze this process .
The 2 processes of interest are the following :
CO2 + γ → CO2* (1)
This reads “a CO2 molecule absorbs an infrared photon γ and goes to a vibrationally excited state CO2*”
CO2* + N2 → CO2 + N2⁺ (2)
This reads “a vibrationally excited CO2 molecule CO2* collides with an N2 molecule and relaxes to a lower vibrational energy state CO2 while the N2 molecule increases its velocity to N2⁺ “. We use a different symbol * and ⁺ for the excited states to differentiate the energy modes – vibrational (*) for CO2 and translational (⁺) for N2 . In other words , there is transfer between vibrational and translational degrees of freedom in the process (2) . This process in non equilibrium conditions is sometimes called thermalization .
The microscopical process (2) is described by time symmetrical equations . All mechanical and electromagnetical interactions are governed by equations invariant under time reversal . This is not true for electroweak interactions but they play no role in the process (2) .
Again in simple words , it means that if the process (2) happens then the time symmetrical process , namely CO2 + N2⁺ → CO2* + N2 , happens too . Indeed this time reversed process where fast (e.g hot) N2 molecules slow down and excite vibrationally CO2 molecules is what makes an N2/CO2 laser work. Therefore the right way to write the process (2) is the following .
CO2* + N2 ↔ CO2 + N2⁺ (3)
Where the use of the double arrow ↔ instad of the simple arrow → is telling us that this process goes in both directions . Now the most important question is “What are the rates of the → and the ← processes ?”
The LTE conditions with the energy equipartition law give immediately the answer : “These rates are exactly equal .” This means that for every collision where a vibrationally excited CO2* transfers energy to N2 , there is a collision where N2⁺ transfers the same energy to CO2 and excites it vibrationally . There is no net energy transfer from CO2 to N2 through the vibration-translation interaction .
As we have seen that CO2 cannot transfer energy to N2 through the translation-translation process either , there is no net energy transfer (e.g “heating”) from CO2 to N2 what proves our statement .
This has an interesting corollary for the process (1) , IR absorption by CO2 molecules . We know that in equilibrium the distribution of the vibrational quantum states (e.g how many molecules are in a state with energy Ei) is invariant and depends only on temperature . For example only about 5 % of CO2 molecules are in a vibrationally excited state at room temperatures , 95 % are in the ground state .
Therefore in order to maintain the number of vibrationally excited molecules constant , every time a CO2 molecule absorbs an infrared photon and excites vibrationally , it is necessary that another CO2 molecule relaxes by going to a lower energy state . As we have seen above that this relaxation cannot happen through collisions with N2 because no net energy transfer is permitted , only the process (1) is available . Indeed the right way to write the process (1) is also :
CO2 + γ ↔ CO2* (1)
Where the use of the double arrow shows that the absorption process (→) happens at the same time as the emission process (←) . Because the number of excited molecules in a small volume in LTE must stay constant , follows that both processes emission/absorption must balance . In other words CO2 which absorbs strongly the 15µ IR , will emit strongly almost exactly as much 15 µ radiation as it absorbs . This is independent of the CO2 concentrations and of the intensity of IR radiation .
For those who prefer experimental proofs to theoretical arguments , here is a simple experiment demonstrating the above statements . Let us consider a hollow sphere at 15°C filled with air . You install an IR detector on the surface of the cavity . This is equivalent to the atmosphere during the night . The cavity will emit IR according to a black body law . Some frequencies of this BB radiation will be absorbed by the vibration modes of the CO2 molecules present in the air . What you will observe is :
- The detector shows that the cavity absorbs the same power on 15µ as it emits
- The temperature of the air stays at 15°C and more specifically the N2 and O2 do not heat
These observations demonstrate as expected that CO2 emits the same power as it absorbs and that there is no net energy transfer between the vibrational modes of CO2 and the translational modes of N2 and O2 . If you double the CO2 concentration or make the temperature vary , the observations stay identical showing that the conclusions we made are independent of temperatures and CO2 concentrations .
Conclusion and caveats
The main point is that every time you hear or read that “CO2 heats the atmosphere” , that “energy is trapped by CO2” , that “energy is stored by green house gases” and similar statements , you may be sure that this source is not to be trusted for information about radiation questions .
Caveat 1
The statement we proved cannot be interpreted as “CO2 has no impact on the dynamics of the Earth-atmosphere system” . What we have proven is that the CO2 cannot heat the atmosphere in the bulk but the whole system cannot be reduced to the bulk of the atmosphere . Indeed there are 2 interfaces – the void on one side and the surface of the Earth on the other side . Neither the former nor the latter is in LTE and the arguments we used are not valid . The dynamics of the system are governed by the lapse rate which is “anchored” to the ground and whose variations are dependent not only on convection , latent heat changes and conduction but also radiative transfer . The concentrations of CO2 (and H2O) play a role in this dynamics but it is not the purpose of this post to examine these much more complex and not well understood aspects .
Caveat 2
You will sometimes read or hear that “the CO2 has not the time to emit IR because the relaxation time is much longer than the mean time between collisions .” We know now that this conclusion is clearly wrong but looks like common sense if one accepts the premises which are true . Where is the problem ?
Well as the collisions are dominating , the CO2 will indeed often relax by a collision process . But with the same token it will also often excite by a collision process . And both processes will happen with an equal rate in LTE as we have seen . As for the emission , we are talking typically about 10ⁿ molecules with n of the order of 20 . Even if the average emission time is longer than the time between collisions , there is still a huge number of excited molecules who had not the opportunity to relax collisionally and who will emit . Not surprisingly this is also what experience shows .
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Dave Springer,
Antarctica receives more solar insolation than any place else on the planet during December, yet temperatures there remain far below zero.
http://earthobservatory.nasa.gov/Features/EnergyBalance/images/annual_solar_insolation.png
The lack of a greenhouse effect keeps temperatures very cold there.
If there is a particular word in what I post that you don’t like, why not just delete that word, instead of junking paragraphs that I’ve put time and effort into writing?
—-
LTE is a poor assumption in much of the Earth’s atmosphere. In any case, the logic does not work. There is energy being absorbed by CO2 molecules which is not absorbed directly by N2 or O2 molecules, and it is transferred collisionally to these molecules. It’s as simple as that. Even if the rate of CO2->N2 energy transfer is exactly equal to the rate of N2->CO2 energy transfer, the fact that there is energy being put into the CO2 side means that N2 is heated up.
To claim otherwise is simply anti-science.
The supposed experiment does not work, because it is obviously in thermal contact with the rest of the atmosphere. Doubling the CO2 concentration inside a tiny packet of air will make a negligible difference to the temperature of the whole atmosphere.
Pamela Gray says:
August 5, 2010 at 10:06 am
Gail, you point out again that much of chemistry is engineering design and physics, not chemical properties. Which is why I prefer to teach chemistry from a visual building blocks perspective. I wish Legos would sell a chemistry set. That would be so cool.
This is where I’m compelled to say that much of physics and engineering design is physical chemistry, the physicists and engineers just don’t know it.
It reminds me of the days when one of my post-doctoral prefessors (a physicist) used to call gaseous calcium a “molecule” because it has two valence electrons and therefore electron correlation. “That’s what defines a molecule, right?”
Well I consider myself as an AGW skeptic, but I’m not a skeptic to the existence of a natural greenhouse effect. Tom Vonk says in essence that no greenhouse effect can exist.
He defines the system to be in equilibrium and then proof that the system indeed is in equilibrium.
If you substitute CO2 with glass you may use the same arguments to proof that conventional greenhouses don’t work.
Nylo says:
August 5, 2010 at 5:51 am
By the way, CO2 doesn’t heat, that’s true, and that makes your small experiment work. What it does is slow down the cooling of the surface of the Earth. It reduces the net radiation at 15um. As a result, you get higher temperatures than you would have without CO2. But all the initial heat came from the Sun, of course.
I am uncomfortable with the statement CO2 slows down the cooling of the surface of the Earth.
To me, a “slowing down” of the cooling of an object means less energy per unit time leaves the object. Assume a fixed rate of energy per unit time is being absorbed by and/or generated within an object; and at temperature T the object is in “heat transfer” equilbrium–i.e., the amount of outgoing energy per unit time is equal to the incoming radiation per unit time. If you “slow down the cooling”, aren’t you decreasing the rate at which energy leaves the object? But if you slow down the cooling without decreasing the rate of energy input, as a function of time the object must retain more and more heat. The object will stop retaining heat only when the outgoing rate of energy equals the input rate of energy. A rise in object temperature may be the mechanism that re-establishs input rate/output rate equality; but once reached, the outgoing rate of energy will be equal to the input rate of energy (which hasn’t changed) and hence cooling (or the rate of cooling) will also unchanged. As such, it may be correct to say that CO2 may temporarily affect the rate of cooling, but without a change in the rate of energy input, eventually the rate of cooling will return to its original value, at which point the “cooling” has not been “slowed down”. Bottom line, CO2 has been added but for a fixed rate of energy input, the “cooling or rate of cooling” is unchanged.
This article is totally and grossly misleading. When you say
“These observations demonstrate as expected that CO2 emits the same power as it absorbs and that there is no net energy transfer between the vibrational modes of CO2 and the translational modes of N2 and O2.”
you must add the qualifier that this is “in an isolated system at thermodynamic equilibrium”. In the Earth’s atmosphere, there is a very real external IR source .. the surface of the planet. IR radiation absorbed from this source very definitely WILL heat the atmosphere (if convection from the surface is ignored). Energy WILL be transferred to the nitrogen. The way the article’s conclusions are stated implies that this does not happen.
Then your conclusions say “The main point is that every time you hear or read … you may be sure that this source is not to be trusted … ”
THAT is way beyond misleading. (The caveats help, but don’t go far enough.) To compare a closed system LTE analysis with open system real world results is simply wrong!
A more complete analysis that includes convection changes the heat balance so that adding CO2 actually causes increased atmospheric cooling. (The proof is trivial.)
Merrick says:
August 5, 2010 at 7:15 am
Sorry, just need to weigh in one more time.
That’s better ! I’m a physicist but was having great difficulty with this essay.
Dr. Vonk, (and I should include anna v)
I include you anna because I am going to bring back up a subject that you and I went around and around months ago and I was unable to have you see exactly what I was trying to portray.
Dr.Vonk, you mention in your article that CO2 (and I in silently add any GHG from here out) absorbs radiation from the ground but due to the LTE cannot hold that energy and I totally agree. However this gets into the intersection of topics with ‘back radiation’. That back radiation is photons emitted toward the nadir hemisphere since the remittance direction is randomized.
My point to both of you is a huge point that is being missed here. Huge. Since the LW radiation from the ground is always in the azimuth hemisphere’s direction and ½ of the reemitted photons are in the nadir hemisphere, a radiation pressure exists here which will cause and infinitesimal but real expansion of the atmosphere which will exactly cause a temperature decrease that exactly counteracts any warming from the downward radiation. anna, I stated this in January or February and never could get you to see.
Get it? Please, please give this a moment of thought this time through. I will keep bring this up every month or two until 1) someone says they clearly see what I am talking about or 2) someone clearly explains how this radiation pressure created by these LW radiations, up from the ground, ½ down from GHGs does not cause a pressure which will expand the atmosphere and by thermodynamics fundamental equations will cool the atmosphere in exactly an equal amount. Please give me a moment.
Wish I was better with words, you might have to read some proper terms in to make it clear to you. I give you permission to read between my lines.
Dr. Spencer, if you are reading, this is the one point you and I disagree with the common acceptance that CO2 (or any LW interacting GHG) will always warm. I still don’t see it because of the above explanation of increased radiative pressure.
Thanks for the moment.
– wayne
Jan K. Andersen says:
August 5, 2010 at 12:28 pm
Well I consider myself as an AGW skeptic, but I’m not a skeptic to the existence of a natural greenhouse effect. Tom Vonk says in essence that no greenhouse effect can exist.
There will be no ‘greenhouse effect” if the IR is ‘given up’ in sufficiently small timescales. So what I think I’m trying to say is that when a mole absorps radiation in the IR in translate to the first excited level and falls back to the ground state within a fraction of a second.
That is a very poor description but I’m tired and fed up.
Merrick says:
August 5, 2010 at 12:28 pm
Pamela Gray says:
August 5, 2010 at 10:06 am
Gail, you point out again that much of chemistry is engineering design and physics, not chemical properties. Which is why I prefer to teach chemistry from a visual building blocks perspective. I wish Legos would sell a chemistry set. That would be so cool.
I used to think that physical chemistry was a subset of physics. Was I arrogant or simply wrong?
Karel,
Yeah, I’ll take a whack at it.
Someone made an attempt to describe something, and a bunch of intelligent people decided it was darned near fraudulent, or something.
Because arguing in good faith is evidently not scientific enough?
Hey, you ruffled feather types: just deal with the argument and stop assuming some fraud is being perpetrated. Make your case and let the guy respond.
Thanks Tom. This provides an excellent explanation for my idea of a “CO2 fog” that just redistributes the infrared radiation. You stirred up a record number of concern trolls! (“Uh, this is such a disappointing article on this otherwise excellent blog”. Yeah right you people go back to RealClimate Hell where you belong.)
For the people who said: “But you disturb the LTE when you add more CO2” please consider the time scale! The re-radiation processes work in milliseconds; the rise in concentrations over years. It’s practically unrelated.
Max Hugoson says:
August 5, 2010 at 11:36 am
Dear Dr. Tom:
Please use this reference:
Dr. Elsasser’s classic, “Infrared Heat Transfer in the Atmosphere” is available at ScribeD.
Thanks Max. I have read this several times and find it fasinating. I will save the link this time.
A lot of people have stated that Tom Vonk’s explanations mean that there cannot be a greenhouse effect. This is false; it is NOT what Tom’s explanations result in.
Rather, it results in 50% of the LWIR radiation in the CO2 absorption band passing through to space and 50% being re-radiated back to Earth for a sufficiently dense CO2 fog. This is of course a greenhouse effect; without the LWIR-absorbing gas 100% would reach space.
Of course, this effect will not be affected by further CO2 concentration rises as the CO2 fog is already dense enough to catch every LWIR photon in its absorption band emitted by the surface.
LOL. I think y’all will find that Vonk really understands this physics. Can’t wait for his responses!
“stevengoddard says:
August 5, 2010 at 8:43 am
bushy,
There isn’t any question that the greenhouse effect is real and that CO2 is a contributor. Places that have very little GHG (i.e. South Pole) also have very cold temperatures.”
And places like Phoenix also have relatively low amounts of GHGs, yet have very hot temperatures (relative to Atlanta, which is at virtually the same latitude and altitude, but which has over 3 times as much GHG). Are you forgetting about solar insolation?
Why do so many folks keep trying to explain atmospheric temperatures with only infrared radiation? What about heat storage, convection, conduction? No matter what you believe about the greenhouse effect, it is only ONE effect!
wayne says:
August 5, 2010 at 1:04 pm
“Please, please give this a moment of thought this time through. I will keep bring this up every month or two until 1) someone says they clearly see what I am talking about or 2) someone clearly explains how this radiation pressure created by these LW radiations, up from the ground, ½ down from GHGs does not cause a pressure which will expand the atmosphere and by thermodynamics fundamental equations will cool the atmosphere in exactly an equal amount. ”
Within any gas in which radiation is being absorbed and re-radiated anisotropically, there is indeed a differential radiation pressure on the gas. So the partial trapping of solar energy near the Earth’s surface by clouds and greenhouse gases does cause the atmosphere to fill a volume greater than it otherwise would at that temperature. However, the magnitude of the effect is very small ~1E-6 N/m2 (radiative flux/speed of light), compared to the atmospheric pressure ~1E5 N/m2. Furthermore, it does not cause net ongoing cooling to offset the greenhouse warming because once the atmosphere has made that very slight adjustment it ceases to expand.
Although this mechanism has negligible effect on planetary atmospheres it does become significant in stellar atmospheres, especially in stars of very high luminosity. Radiation pressure limits the luminosity of stars of mass M solar masses to ~33,000M solar luminosities (the Eddington Limit) . Above that limit the radiation pressure would blow the star apart.
JAE
The absolute humidity in Phoenix is orders of magnitude higher than Antarctica.
I have been in Phoenix when the dew point was over 70F. Even on a dry day, there is plenty of water vapour.
Dave Springer says:
August 5, 2010 at 10:18 am
stevengoddard says:
August 5, 2010 at 8:43 am
There isn’t any question that the greenhouse effect is real and that CO2 is a contributor. Places that have very little GHG (i.e. South Pole) also have very cold temperatures.
“You don’t think it has anything to do with far lower insolation at the poles due to small angle of incidence compounded by an albedo of 0.85?
Not even a little bit? /sarc off
CO2 signature might be barely detectable at the south pole due to so little water vapor swamping the signal but the cold is due to the factors above”
The CO2 signature at the South Pole would seem to be greater than anywhere else on the planet. Spectral analysis of Downward Longwave Radiation done there suggests that CO2 provides a full third of the DLR signal virtually yearround.
http://journals.ametsoc.org/doi/abs/10.1175/JCLI3525.1
“Annual cycles of downwelling broadband infrared radiative flux and spectral downwelling infrared flux were determined using data collected at the South Pole during 2001. Clear-sky conditions are identified by comparing radiance ratios of observed and simulated spectra. Clear-sky fluxes are in the range of 110–125 W m−2 during summer (December–January) and 60–80 W m−2 during winter (April–September). The variability is due to day-to-day variations in temperature, strength of the surface-based temperature inversion, atmospheric humidity, and the presence of “diamond dust” (near-surface ice crystals). The persistent presence of diamond dust under clear skies during the winter is evident in monthly averages of clear-sky radiance.
About two-thirds of the clear-sky flux is due to water vapor, and one-third is due to CO2, both in summer and winter”
This percent of CO2’s contribution to DLR doesn’t appear to be exceeded anywhere else on the planet. To me this would suggest that if increasing CO2 concentrations in the atmosphere are the prime driver of temperature trends, it should be most evident at the South Pole, but it is fairly evident that temps there have been laying about like a dead dog for 50 years.
John W. says:
August 5, 2010 at 5:04 am
“II have some questions:….”
John W, why do you have these questions? Because you must rely on authority?
http://www.antarcticconnection.com/antarctic/weather/index.shtml
Tom V;
Thanks for that. I think however that many people will miss the point.
can you explain for folks what you mean by this? and how this caveat related to AGW.
Also, Just for the record, do you accept the fundamental physics of RTE’s
Or simply, to a first order, what is the impact of having more C02 or H20 or any GHG in the atmosphere?
The Caveat that needs some explantion:
Caveat 1
“The statement we proved cannot be interpreted as “CO2 has no impact on the dynamics of the Earth-atmosphere system” . What we have proven is that the CO2 cannot heat the atmosphere in the bulk but the whole system cannot be reduced to the bulk of the atmosphere . Indeed there are 2 interfaces – the void on one side and the surface of the Earth on the other side . Neither the former nor the latter is in LTE and the arguments we used are not valid . The dynamics of the system are governed by the lapse rate which is “anchored” to the ground and whose variations are dependent not only on convection , latent heat changes and conduction but also radiative transfer . The concentrations of CO2 (and H2O) play a role in this dynamics but it is not the purpose of this post to examine these much more complex and not well understood aspects .”
Anthony,
Nice to have these guest posts, but it would greatly improve the science if you would peer review these first. This is a classic unhelpful post with much that is correct (and could help the debate) yet the conclusion is all wrong as a result of some simple errors (e.g. not closed system due to IR); Ferdinand Engelbeen post on CO2 is very helpful; well written and referenced. More of this.
Well Tom, I understood most of what you wrote; but I am not sure I can agree with your conclusions.
CO2 consists of only one in about 2570 molecules of the atmosphere so CO2-CO2 collisions are highly unlikely, and CO2-N2 and CO2 O2 collisions ought to occur in about 4:1 ratio.
But N2-N2, and N2-O2, or O2-O2 collisions are far more likely.
So a CO2 molecule colliding with either a N2 or an O2 molecule is highly unlikely to encounter the same N2 molecule again and retrieve the exchanged energy.
Yes I know you said that LTE is assumed; but how real is that in an open atmosphere with convection currents as well as both a Temperature gradient, and a density gradient.
Which brings to mind another question.
A solid material; say ice for example can and does emit black body like thermal continuum radiation since it is above absolute zero. Heat it above zero deg C and it becomes a liquid and continues to emit a continuum thermal radiation with a roughly black body spectrum and Stefan Boltzmann output.
But perish the thought that you should heat the water to above 100 deg C whereupon it becomes a gas, and immediately stops radiating thermal radiation; because everybody knows; or seems to think that gases do not emit black body like thermal radiation.
Of course nobody explained this to the sun; or the argon or other gas in a gas filled incandescent light bulb. By some miracle, a very large fraction of the broadband radiation emitted from a gas filled lamp comes from that very gas filling, which of course can’t emit BB like continuum radiation.
A consequence of the model you have proposed would seem to be that the “back radiation” due to CO2 interception of surface emitted (from solid or liquid continuum thermal radiation can consist only of the specific wavelengths that the CO2 absorbed in the first place; since you say no net energy is exchanged between the CO2 and the Atmosphere.
I agree with your contention that symmetrical non polar molecules do not interract with the IR radiation; but in collisions, even symmetrical polyatomic molecules become polar, and therefore can absorb and radiate.
By the same token, in collisions even symmetrical molecules like N2 or O2 can exhibit an accelerating electric charge and therefore emit EM radiation in a continuum spectrum.
Do you have a rigorous QM proof that gases do not emit ordinary BB like thermal continuum spectra just like any other material above absolute zero does ?
No I am not asking for your proof here; just do you have or know of one ?
If I accept your premise that the trace GHG gases do not transport thermal energy to the ordinary atmospheric gases; then that of course would apply to H2O as well; so how come we are not that frozen ice ball.
In any case; the question seems to me to be moot, since there is general agreement that CO2 and H2O and other GHG molecules DO capture LWIR from the surface or other atmospheric layers; which must increase the net energy (and Temperature) of THAT layer.
So is it relevent that CO2 and N2 can’t unilaterally transport energy following your explanation; the atmospheric Temperature is nevertheless increased by the GHG captures.
I don’t see any assymmetry in your N2-CO2 exchange. On average the same applies to any two molecules so there isn’t any net transfer from one N2 molecule to another, or N2-O2 either.
But anyway’ I appreciate your explanation. Any insight is always better than none at all.
George
As to the world’s dries deserts; I recently watched a PBS show on the Andes; and they stated quite specifically that a particular high desert on the eastern side of the Andes (with real sand) was the driest desert on earth.
In any case could people please distinguish bewteen Absolute humidity and Relative humidity when using the term so we know what they are referring to.
I am under the impression that the water content; by molecular abundance of the driest desert air anywhere on earth is always higher than the molecular abundance of CO2 in the same location at the same time.
Nearly all of the details of the argument are correct but it fails because of a faulty assumption. The assumption is that all parts of the atmosphere are in perfect local thermal equilibrium. This is not the case.
In actual fact the atmosphere are in APPROXIMATE local thermal equilibrium.
There is a thermal gradient between the surface (25C) and outer space (-270C) and this cannot happen if there is LTE.