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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Heat and mass balances are tools used by chemical engineers in heat and mass transfer analyses;
Energy in – energy out = accumulation
Material in – material out = accumulation
At equilibrium, accumulation is zero
TomVonk says:
August 9, 2010 at 4:27 am
“My point was that LTE (condition that I am using) is independent of the questions whether there is or is not radiative equilibrium .”
And that is where you are fundamentally wrong. You cannot have LTE without radiative equilibrium too. The most you can have is an approximation to LTE. The reason you cannot have full LTE is that the molecules interacting with the non-equilibrium radiation are pumped into non-equilibrium populations. Absorption of “hotter” radiation puts more molecules into excited states; re-radiation depletes the excited states. If the radiation is anything other than isotropic black body radiation at the temperature of the gas, the steady-state population of the excited states cannot be thermal. It must depart from LTE to some degree. This is how a laser works, and although the radiative pumping in Earth’s atmosphere is very very weak by comparison, the same processes are occurring there.
TomVonk says:
August 10, 2010 at 1:53 am
“I observe that after 300 comments , not a single one contested either the time symmetry or the energy equipartition law . One commenter contests the existence of LTE (“it never exists”) and Phil. contests the existence of IR emission by CO2 . Both are inconsistent with observational evidence .”
Perhaps if you read the comments more carefully your observations would be less inaccurate. Several commenters (including myself) have pointed out that time symmetry does not apply to conditions in the Earth’s atmosphere, or to any situation in which there is a macroscopic entropy change. Several commenters (including myself) have pointed out that equipartition is not an invariable law and is not valid under conditions of radiative pumping for example. Furthermore, I do not claim that there is no such thing as LTE, but rather that LTE is a limiting case never quite achieved but only approximated; it was you who denied the existence of approximate LTE and pedantically asserted that LTE is a binary either-or; for so pedantic a definition, then, one has to say that it does not exist. I would also invite you to think about how perfect LTE could possibly be observed if it did exist; any device you use to measure the thermal radiation or the distribution of velocities or the population of excited states must itself be at a different effective temperature from the gas in question, and must absorb energy from it, disturbing the very equilibrium you are trying to observe.
TomVonk says:
August 10, 2010 at 1:53 am
Absolute agreement . What Phil. doesn’t understand is that his thesis not only unexplicably violates the time symmetry of the processes which is at the basis of the statistical equilibrium but it would prevent any equilibrium to form . In order to have an equilibrium it is simply necessary that the rates of the V/T processes be equal . And yes this is text book stuff . Looking at the reference I linked for CBA helps too .
No I understand it very well but when you apply these laws to a process which doesn’t happen, such as Vonk’s fictitious V-T exchange, you get garbage out.
I observe that after 300 comments , not a single one contested either the time symmetry or the energy equipartition law . One commenter contests the existence of LTE (“it never exists”) and Phil. contests the existence of IR emission by CO2 . Both are inconsistent with observational evidence .
No I don’t, I apply the Stern-Volmer equation which indicates that, for vibronically excited CO2 (bending mode) at atmospheric conditions near the surface, emission will be a very unlikely mode of energy loss. This is in fact consistent with observation, emission from excited CO2 will increase with decreasing pressure (Stern-Volmer plot). It is Vonk who is so wedded to his beliefs that he is blind to the actual physics involved.
I leave it to theoreticians and specialists in molecular and laser physics to fight it out amongst themselves on the question of what sort of energy transfers may or may not occcur on the microscopic level between absorbent and non-absorbent gas species in the atmosphere. From the macro perspective of geophysics, that question is largely mooted by the fact that radiative transfer does not operate as the sole means of thermal energy transfer from surface to space. In fact, scores of careful total energy-flux experiments repeatedly show the Bowen ratio (sensible to latent heat transfer) almost invariably well below unity in the marine environment and whenever surface moisture (dew) is present. Yes, it may rise to values 5 or more in the ultra dry environment of Antarctica and some deserts, but these constitute a minor fraction of the surface.
What we have as a consequence is a totally different system than the one usually depicted by AGW proponents. Instead of a paradigm that leads people to think that any impedance to OLR necessarily must raise surface temperatures to maintain TOA radiative balance, Earth has a parallel non-radiative path of thermal energy flow, with the endothermic process of evaporation playing a central role. Neither this nor the paradigm system can be realistically modeled by a circuit with the bulk of the atmosphere representing an inductor or capacitor and the GHGs representing a resistor. Resistance is a dissipative mechanism, which in any isentropic (energy budgetr) analysis is disallowed. In fact, the atmosphere does thermodynamic work in in sustaining macro-scale pressure gradients that drive the geostropic winds and in overcoming vapor pressure during evaporation. This component of enthalpy is what almost everyone neglects.
It may surprise many that Earth would still have a “greenhouse effect” even if the atmosphere were totally transparent to IR. The lowest levels of the atmosphere would still be warmed by (dry) convection, and backradiation from the atmosphere would allow the surface to retain more energy in storage than it would adjacent to the heat sink of space.
sky says:
August 10, 2010 at 4:54 pm
“It may surprise many that Earth would still have a “greenhouse effect” even if the atmosphere were totally transparent to IR. The lowest levels of the atmosphere would still be warmed by (dry) convection, and backradiation from the atmosphere would allow the surface to retain more energy in storage than it would adjacent to the heat sink of space.”
I would agree with you concerning the importance of convection as a heat transfer mechanism within the atmosphere. It does magnify the night-time greenhouse effect by warming the clouds or the higher levels of the atmosphere, thus increasing the amount of heat radiated back to the surface; though the overall effect is to reduce net planetary greenhouse warming by limiting the temperature gradient. However, in the quoted sentence you contradict yourself: if the atmosphere were “totally transparent to IR” there could be no backradiation at all.
Vertical convection would not arise in a transparent atmosphere under constant insolation. Convection due to a diurnal cycle can cool the surface during the day and warm it at night; that is, it permits a larger fraction of the atmosphere to buffer the diurnal temperature changes. Because of the fourth power law of radiation, the average temperature is then higher than in the absence of this buffering. (Fully buffered, with heat capacity -> infinity, T~(average insolation)**1/4; unbuffered, for a simple model in which the sun switches full on for 12 hours then off for the other 12 hours, T~0 at night and ~(2*average insolation)**1/4 during day, so Tav~(0+2**1/4)/2=0.59, much lower than the buffered case). Whether this should be called greenhouse warming is debatable. I’d say no. It’s a different phenomenon (though one that happens in greenhouses too!) and only helps to bring the temperature up to the black body temperature calculated on the basis of the total insolation; it doesn’t warm it above that level.
Paul Birch says:
August 11, 2010 at 4:16 am
“you contradict yourself: if the atmosphere were “totally transparent to IR” there could be no backradiation at all.”
An IR transparent atmosphere means there’s no IR absorption by its constituents. It doesn’t mean no IR emissions from them. Every substance with a temperature above 0K emits in the EM spectrum regardless of the process by which it got thermalized. With conduction and convection being such processes at terrestrial temperatures, there is no requirement that the constituents be IR absorbers (i.e., GHGs) to obtain backradiation.
No doubt, with an IR transparent atmosphere and a dry planet, the lapse rate would stay close to the stable dry adiabatic. But the surface being fractal and composed of materials with different specific heats, it would be subject to differential heating and thus convection, at least in the boundary layer. The atmosphere would thus still be heated from below, but not as effectively as with deep moist convection. Its isotropic emissions would produce backradiation, which is the essence of the “greenhouse effect.”
Your simple model with T = 0 at night is an excercise in self-deception, which I will not take the time to dissect. Suffice it to say that backradiation operates steadily 24/7, without a diurnal cycle. This is a clear indication that that it does not come principally from GHGs, whose re-emissions should coherently follow the surface temperature, but from the non-absorbent bulk constituents thermalized by the chaotic process of convection.
sky says:
August 11, 2010 at 3:06 pm
Paul Birch says:“you contradict yourself: if the atmosphere were “totally transparent to IR” there could be no backradiation at all.”
“An IR transparent atmosphere means there’s no IR absorption by its constituents. It doesn’t mean no IR emissions from them. Every substance with a temperature above 0K emits in the EM spectrum regardless of the process by which it got thermalized. With conduction and convection being such processes at terrestrial temperatures, there is no requirement that the constituents be IR absorbers (i.e., GHGs) to obtain backradiation. ”
Sorry, but this is one thing Vonk has got right. Any molecular species or assemblage that can radiate IR can also absorb it. It’s a fundamentally symmetrical process. Yes, every substance does emit IR to some degree; but then it absorbs IR to the same degree. Under non-equilibrium conditions, the actual rates of absorption and emission can differ, but both processes must still occur together. If you have any backradiation at all the atmosphere cannot be perfectly transparent.
“No doubt, with an IR transparent atmosphere and a dry planet, the lapse rate would stay close to the stable dry adiabatic.”
No, it would be isothermal, so long as the insolation were constant. Otherwise it would be isothermal above a convective buffer layer. The depth of that layer would depend on the ratio of the insolation times the sol period (day length) to the volumetric heat capacity of the atmosphere.
“Your simple model with T = 0 at night is an excercise in self-deception, which I will not take the time to dissect. Suffice it to say that backradiation operates steadily 24/7, without a diurnal cycle. ”
The model as given was for a transparent atmosphere with no backradiation. Your discourteous rejection of it displays your own ignorance. The mechanism I described is quite well known in planetary science. The lack of buffering is why airless bodies and the skins of spacecraft display much greater temperature variations than the surface of planets like the Earth. Planets like Mars, with very low atmospheric pressures, are interesting intermediate cases. Even on Earth, higher altitude sites, with less atmosphere to buffer them, display wider diurnal temperature ranges. Obviously, night time temperatures do not in practice fall as far as the near zero temperature of deep space (~2.7K); heat is conducted into the ground during the day, and released again during the night, buffering the temperature extremes; the same basic mechanism, but employing the heat capacity of the regolith instead of the atmosphere. On the Moon, the effective buffering depth is ~1m, which for comparison gives about a tenth of the buffering capacity of the Earth’s atmosphere, while the sol is of course ~29 times as long, so the temperature swing of the lunar surface is much greater, reaching ~120C during the lunar day.
RE: cba: (August 7, 2010 at 4:00 am) “Spector, rather than 100/1, I’d suggest it’s closer to 2/1, excluding those that are simply not going very far. That’s based on there being a factor of two in absorption, roughly co2 total contribution 30W/m^2 versus h2o at 64w/m^2 in the approximately current clear sky atmosphere calculations.”
Update: My 100/1 ratio came a result of a seemingly general statement from a document at the DOE NETL website that implied the H2O concentration in the atmosphere was 3 to 4 percent and even went so far as to compare this with the concentration of CO2 expressed as a percentage. I now find that this probably applies only near the surface in very humid air. So far, I have not yet run across any good accepted numbers for tropopause-level, typical mono-molecular H2O concentrations.
The isothermal atmosphere exists only in academic textbooks. I was making a practical and not a theoretical point. Instead of saying “totally transparent to IR,” my intent would have been clearer had I said “without water vapor or any of the GHGs identified as AGW culprits.” And my reference to the more realistic dry adiabatic in my comment to you was a nod to the usual meteorological criterion for convective stability.
You can justify T=0K at night perhaps in your mind, but not mine. The whole point of my original comment directed to no one in particular was to get lay people to think in more in terms of “thermal mass and inertia” and all the means of thermal transfer, rather than relying upon simplistic radiative vector algebra. Your accusation of “ignorance” on my part about the “buffering” effects of a thermalized air mass is as amusingly misplaced as your ideas about what convection does throughout the diurnal cycle.
I see no prospect of fruitful discussion. Good night!
RE: sky says: (August 10, 2010 at 4:54 pm) “From the macro perspective of geophysics, that question is largely mooted by the fact that radiative transfer does not operate as the sole means of thermal energy transfer from surface to space.”
From above the tropopause level, radiation is the *only* ticket out. I am not sure that we really understand the process by which infra-red emitting gases at top of the atmosphere exhaust the heat convected up to that level and contribute to what I call the tropopause ‘ice-locker’ effect.
sky says:
August 11, 2010 at 7:53 pm
“The isothermal atmosphere exists only in academic textbooks.”
For the simple reason that no atmosphere is totally transparent to IR. However, most atmospheres do have an approximately isothermal upper region. They can also have approximately isothermal (or at any rate, nonconvective) lower regions, when layers considerably more opaque to sunlight than to thermal radiation shield them from further insolation.
“I was making a practical and not a theoretical point. Instead of saying “totally transparent to IR,” my intent would have been clearer had I said “without water vapor or any of the GHGs identified as AGW culprits.” ”
OK, but that wasn’t what you said. You could easily have replied, “Sorry, I didn’t mean totally transparent, just one without any of the usual suspects, like CO2 or H2O”. Instead, you tried to defend the obvious contradiction of combining backradiation with total transparency, arguing that you can get emission from substances that don’t absorb. Which you can’t.
However, even your “practical point” isn’t really true. If there were none of the strong absorbers present, like H2O, CO2 and aerosols, the backradiation would be far too weak to notice. There would be no significant warming from this cause. It is the strong absorbers that are also the strong emitters; the backradiation comes overwhelmingly from those strong absorbers (which are in very close to local thermodynamic equilibrium with the bulk gas).
“And my reference to the more realistic dry adiabatic in my comment to you was a nod to the usual meteorological criterion for convective stability.”
Fine, but it still wasn’t correct. We only get any sort of continuous vertical lapse rate when energy is being continually lost from the top of the atmosphere, by radiation. Convection only stops the lapse rate significantly exceeding the adiabatic rate, it doesn’t stop it being less than that. If the surface can radiate happily into space through a transparent atmosphere, it will be less than that. This is what we see on Mars, for instance.
“You can justify T=0K at night perhaps in your mind, but not mine. The whole point of my original comment directed to no one in particular was to get lay people to think in more in terms of “thermal mass and inertia” and all the means of thermal transfer, rather than relying upon simplistic radiative vector algebra. Your accusation of “ignorance” on my part about the “buffering” effects of a thermalized air mass is as amusingly misplaced as your ideas about what convection does throughout the diurnal cycle.”
So again you would rather sneer than attempt to address the science. Do you deny that the temperature of the night sky into which planets radiate is close to absolute zero? Do you deny that a non-illuminated surface radiating into space would (in the absence of warming from above or below) cool to that very low temperature? Do you deny that the rate at which the temperature falls would depend upon the heat capacity of the cooling mass? Do you deny that increasing the mass, by coupling to sub-surface material by conduction, or a (transparent) atmosphere by convection, would reduce the rate of change of temperature, both during the cooling phase, and in the warming phase when the illumination is switched on again? Do you deny that this is buffering the temperature extremes? Do you deny that the daytime vertical convection processes I describe are real? (Hint, they’re called thermals).
Note that, at night, vertical convection in a (mostly) transparent atmosphere will cease, because the required temperature gradient is the wrong way round; the ground is cooling the atmosphere, no longer warming it; this relies upon conduction and radiation. So buffering the cooling then comes mainly from the heat capacity of the ground. Which is why continental climates have much colder nights than maritime climates (heat capacity of water versus soil, plus land-sea convection which allows the atmosphere to continue buffering during the night).
Spector says:
August 11, 2010 at 8:45 pm
Of course radiation is the only energy ticket out to space. But it’s by no means the only ticket by which thermal energy is transfered from the surface to a level that is radiatively “seen” from space. IMO our understanding of the dynamics of moist convection and its consequences is even less than that of the “ice-locker” effect. In particular, our understanding of thermal capacitance and heat exchange in the realistic, cloud-filled troposphere is minimal.
The role of the bulk constituents in that heat exchange is usually minimized by trundling out backradiation spectra from the Antarctic or the Sahara and showing the prominent peak of 15 micron radiation over a narrow range of wavenumbers. But the thermal range of radiation extends from a fraction of a micron out to ~100microns, and even in those ultra dry environments the total (integrated) power associated with water-vapor is considerably more. The far infrared, where the bulk constituents (along with overlapping H2O) radiate in a continuum is almost never shown.
GHGs being strong IR radiators makes them excellent dispersers of thermal energy and very poor capacitors. The 24/7 nature of backradiation and the general absence of a diurnal cycle in-phase and coherent with the surface temperature (in marine environments) points to an atmospheric capacitance that GHGs suis generis cannot provide. Thus clouds and the bulk constituents are likely what provide the atmosphere the little heat retentivity that it posseses.
Your clarification that you had T=0K in mind for the sink of space, rather than the surface, is helpful. It’s ironic, however, that I should be presented with a long list of elementary facts put up as strawmen for me to “deny” about the “buffer” effects of the atmosphere and the role of the diurnal cyle. Perhaps you’re unaware that it was I who first argued the absorption/capacitance/discharge viewpoint of the “greenhouse effect” here at WUWT. This ploy is what makes me think that a fruitful discussion is unlikely. I do have one question for you: do you deny that a poor absorber of IR can be heated mechanically (conduction, convection, eddy diffusion) to a temperature that is asymmetric with its IR absorption aloft?
sky says:
August 12, 2010 at 5:07 pm
“Your clarification that you had T=0K in mind for the sink of space, rather than the surface, is helpful. It’s ironic, however, that I should be presented with a long list of elementary facts put up as strawmen for me to “deny” about the “buffer” effects of the atmosphere and the role of the diurnal cyle. Perhaps you’re unaware that it was I who first argued the absorption/capacitance/discharge viewpoint of the “greenhouse effect” here at WUWT. This ploy is what makes me think that a fruitful discussion is unlikely. I do have one question for you: do you deny that a poor absorber of IR can be heated mechanically (conduction, convection, eddy diffusion) to a temperature that is asymmetric with its IR absorption aloft?”
First, you seem to have forgotten what I said at the beginning of my first comment to you:
“I would agree with you concerning the importance of convection as a heat transfer mechanism within the atmosphere.”
I have never disputed that.
Second, the T~0 (I never wrote T=0) was for the temperature of a completely unbuffered unilluminated surface, contrasted with the even black body temperature of a completely buffered surface. Those are the two extremes. There are real physical examples approaching both of them. Heavenly bodies such as the Earth and the Moon lie between those extremes. Different parts of the Earth are buffered more than others.
Third, I have put up no “strawmen”. I presented a list of simple statements for you to accept or deny because you had failed to make any rational argument to back up your sneers. You have still not answered those questions; you have still failed to put forward any rational argument concerning them. There is no “ploy” here on my part. I seriously do not know whether you accept this basic physics or are fostering some crank notion of your own. Your unwillingness to answer inclines me to the latter view. If this seems unfair, you can help me by answering the questions directly.
Fourth, I do not know what you mean by “absorption/capacitance/discharge viewpoint”. I would guess that it means that you don’t think CO2 can be a major player because there isn’t enough of it to hold much energy. If so, then I have already answered that point: “It is the strong absorbers that are also the strong emitters; the backradiation comes overwhelmingly from those strong absorbers (which are in very close to local thermodynamic equilibrium with the bulk gas)” (emphasis added). There is no requirement for the thermal energy and latent heat to be stored in the same molecular species that provide the IR absorption and emission. If this isn’t what you meant, please explain.
Fifth, in answer to your question, it is certainly the case that poor IR absorbers can be heated by other means. I don’t know what you mean by “a temperature that is asymmetric to its IR absorption aloft”. If its temperature is the same as the surface below, then it will radiate as much to that surface as it absorbs from that surface. If its temperature is less, then it will in general radiate less to the surface than it absorbs from the surface. Since it radiates both up and down, but is absorbing only from below (assuming no IR from above), then in the absence of additional heat transfer (by eg., conduction or convection), its absolute temperature would fall to ~0.84 Tsurface. This is true for both poor and good absorbers, so long as the optical depth is less than unity. When the optical depth is greater than unity, the topmost layer no longer clearly “sees” the surface, only the layer immediately below it, so the overall temperature ratio would be correspondingly greater. However, to maintain such a temperature difference, the pressure ratio would have to be 1.85:1 or greater (in a dry atmosphere). Unless the gas is a very poor radiator or is very sparse (ie, very transparent), conduction alone will not be sufficient and convection will occur in the lower levels. Note that this is ignoring the effect of any absorption of energy from external sources (eg., sunlight) in the upper atmosphere.
Paul Birch says:
August 14, 2010 at 7:08 am
As someone who types very slowly and enjoys his weekends, I will not spend more time on a low-level discussion that grows increasingly more confused and OT relative to Tom Vonk’s clearly thought-provoking, high-level post.
sky says:
August 14, 2010 at 2:24 pm
“As someone who types very slowly and enjoys his weekends, I will not spend more time on a low-level discussion that grows increasingly more confused and OT relative to Tom Vonk’s clearly thought-provoking, high-level post.”
You have typed far more words wriggling out of answering my questions than you would have done simply answering them. The confusion is obviously on your side, not least in foolishly considering the debate off topic, but if there was anything you did not understand, you only had to ask. [snip]
RE: sky says: (August 12, 2010 at 4:24 pm )
“Of course radiation is the only energy ticket out to space. But it’s by no means the only ticket by which thermal energy is transfered from the surface to a level that is radiatively “seen” from space. IMO our understanding of the dynamics of moist convection and its consequences is even less than that of the “ice-locker” effect. In particular, our understanding of thermal capacitance and heat exchange in the realistic, cloud-filled troposphere is minimal.”
When I said “radiation is the only ticket out,” I was speaking *exclusively* of the tropopause altitude and above. This is the altitude where convection ceases to be an effective heat-transport factor. Also, there are very few clouds above this altitude. If the tropopause could not be cooled to a typical temperature of 220 deg K, the adiabatic lapse rate would translate the increase to equivalently warmer temperatures at the surface.
Heat transferred from the surface by radiation is already on that train unless it gets off at some intermediate stop.