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Stratospheric Cooling and Tropospheric Warming

Posted on 1 December 2010 by Bob Guercio

This post has been revised at Stratospheric Cooling and Tropospheric Warming - Revised

Increased levels of carbon dioxide (CO2) in the atmosphere have resulted in the warming of the troposphere and cooling of the stratosphere. This paper will explain the mechanism involved by considering a model of a fictitious planet with an atmosphere consisting of carbon dioxide and an inert gas such as nitrogen at pressures equivalent to those on earth. This atmosphere will have a troposphere and a stratosphere with the tropopause at 10 km. The initial concentration of carbon dioxide will be 100 parts per million (ppm) and will be increased instantaneously to 1000 ppm and the solar insolation will be 385.906 watts/meter2. Figure 1 is the IR spectrum from a planet with no atmosphere and Figures 2 and 3 represent the same planet with levels of CO2 at 100 ppm and 1000 ppm respectively. These graphs were generated from a model simulator at the website of Dr. David Archer, a professor in the Department of the Geophysical Sciences at the University of Chicago and edited to contain only the curves of interest to this discussion. The parameters were chosen in order to generate diagrams that enable the reader to more easily understand the mechanism discussed herein.

Prior to discussing the fictitious model, consider a planet with no atmosphere. In this situation light from the sun that is absorbed by the surface is reemitted from the surface. Figure 1 is the IR spectrum of this radiation which is known as Blackbody radiation.

Figure 1 

                  Figure 1. IR Spectrum - No Atmosphere

Consider now Figure 2 which shows the Infrared (IR) radiation spectrum looking down at the planet from an altitude of 10 km with a CO2 concentration of 100 ppm and Figure 3 which shows the IR spectrum with a CO2 concentration of 1000 ppm. Both figures represent the steady state and approximately follow the intensity curve for the blackbody of Figure 1 except for the missing band of energy centered at 667 cm-1. This band is called the absorption band and is so named because it represents the IR energy that is absorbed by CO2. IR radiation of all other wavenumbers do not react with CO2 and thus the IR intensity at these wavenumbers is the same as that of the ground. These wavenumbers represent the atmospheric window and is so named because the IR energy radiates through the atmosphere unaffected by the CO2. The absorption band and the atmospheric window is the key to stratospheric cooling.

Figure 2/3 

                    Figure 2. CO2 IR Spectrum - 100ppm                             Figure 3. CO2 IR Spectrum - 1000 ppm

The absorption band in Figure 3 is wider than that of Figure 2 because more energy has been absorbed from the IR radiation by the troposphere at a CO2 concentration of 1000 ppm than at a concentration of 100 ppm. The energy that remains in the absorption band after the IR radiation has traveled through the troposphere is the only energy that is available to interact with the CO2 of the stratosphere. At a CO2 level of 100 ppm there is more energy available for this purpose than at a level of 1000 ppm, thus the stratosphere is cooler for the higher level of CO2 in the troposphere. Additionally, the troposphere has warmed because it has absorbed the energy that is no longer available to the stratosphere.

One additional point should be noted. Notice that the IR radiation in the atmospheric window is slightly higher in Figure 3 than Figure 2. This is because the temperature of the troposphere has increased and in the steady state condition, the total amount of IR entering the stratosphere in both cases must be the same. That total amount of energy is the area under both of these curves. Thus, in Figure 2, there is more energy in the absorption band and less in the atmospheric window while in Figure 3, there is less energy in the absorption band and more in the atmospheric window.

In concluding, this paper has explained the mechanism which causes the troposphere to warm and the stratosphere to cool when the atmospheric levels of CO2 increase.

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Comments 101 to 150 out of 247:

  1. As I see it: Conservation of energy must be considered at all times. At equilibrium, the energy into a system must equal the energy out. Consider a box of CO2 with infrared coming in and leaving. CO2 molecules are constantly emitting and absorbing infrared; however, at equilibrium the number of absorbers equals the number of emitters. Now imagine that the incoming IR is cut in half. Suddenly there is less IR to be absorbed and for a while there will be more emitters than absorbers until equilibrium is reached. Again, emitters will equal absorbers with less energy coming in and less energy going out than before it was cut in half. My intuitive guess would be that equilibrium occurs very quickly and at any time the stratosphere is in equilibrium. The same holds true for sunlight coming in and interacting with the ozone. Ozone molecules are constantly absorbing and emitting radiation with the amount of energy coming in equaling the amount leaving. Fortunately for us, the UV energy absorbed is at a different frequency from that emitted but for the total energy spectrum, the amount coming in equals the amount leaving. At present, more energy is coming into the earth than is leaving. This energy is being absorbed by the earth and, were the CO2 in the air to miraculously stabilize today, the earth will continue warming up for another few decades. As the temperature increases, more heat energy will leave the earth until such time that the temperature is warm enough so that the IR energy leaving is equal to the sunlight energy entering. The troposphere will be warmer and at equilibrium so that the IR energy leaving the troposphere will be the same as the IR energy entering. The IR entering the stratosphere will be the same as that leaving but the stratosphere will forever be cooler than it was before CO2 levels increased. This is because the IR spectrum leaving the troposphere is different. Even though there is the same amount of IR energy leaving, less of this energy is able to react with the CO2 in the stratosphere which keeps it at whatever temperature it is. Bob
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  2. Bob Guercio at 06:21 Bob, i dont disagree with much in this post, except your understanding onb the mechanisms of stratospheric cooling, O3 is a net absorber, CO2 & H2O are net emitters in the stratosphere, and they do balance. In my first comment @ 44 i linked a paper by Ramanthan and Dickenson, it quantifies the radiative exchanges between troposphere and stratosphere... the stratosphere is a net emitter to the troposphere, not the other way around.
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  3. Joe Blog 102 At equilibrium, how can a group of molecules be net absorbers or net emitters. If CO2 is a net absorber, that would mean it is constantly absorbing more than it is emitting. That can't be. Now ozone is different. It absorbs and emits both IR and UV. So it could absorb UV and emit IR so I guess you could say it is a UV absorber and an IR emitter in the stratosphere. But I don't believe that CO2 reacts appreciably with visible light so at equilibrium, it cannot be a net absorber or a net emitter. Bob
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  4. Bob Guercio At equilibrium, how can a group of molecules be net absorbers or net emitters. Because the energy is brought into the stratosphere by O3, through UV and 9-10micron LW absorption, but emitted by co2 and h2o. CO2 does absorb some LW, but emits twice what it absorbs. I'll be the first to admit it seems counter intuitive. I did not get this until i came across this paper paper Its about variable O3 effects, but it quantifies all the radiative exchanges involving the stratosphere/troposphere very clearly.
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  5. Joe, That paper is heavy. Can you explain it in your own words or point to the salient portions of that paper. I'm not sure if this is going to work but I have an image on the CO2 spectrum here. If it doesn't appear, please go to the URL that you see. The spectrum is almost entirely in the IR region. So how can it be a net absorber or emitter of IR. Or is the little amount in the visible region enough to make it that. Bob
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    Moderator Response: [Daniel Bailey] - Fixed HTML
  6. Joe, There are two situations. One is what is going on now. The stratosphere is cooler and the earth is gaining more energy than it is emitting. This cannot go on forever. Let's say we stabilize the CO2 level to what it is now. The earth continues to heat and in a few decades the energy entering the earth will be the same as that leaving. That is the second situation. And there too, the stratosphere will be cooler than what it was before more co2 was put into the atmosphere. We have to be clear which situation we are talking about in order to understand each other. My blog that was posted by John Cook addresses only the steady state solution. Bob
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  7. The image above shows how the Ir emission from of CO2 and other GHG produces cooling in the stratosphere. This suggests that Bobs explanation is inadequate.
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  8. Hmm I don't seem to be able to post the image It comes from this site. The image I wish to refer to is the 4th from the top of the page Heat loses and gains in the atmoshere
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  9. Re: mars (108) This one?
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  10. It comes from this site. The image I wish to refer to is the 4th from the top of the page Heat loses and gains in the atmosphere This is the site if the link does not work http://lasp.colorado.edu/~bagenal/3720/CLASS14/14EVM-5.html
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    Moderator Response: [Daniel Bailey] You've been including an extra / at the end of the URL. Make sure you use the preview function before submitting; formatting errors in URLs and posting of images are by far the most common errors in posting. Most would have been caught by previewing.
  11. I understand the points Tom Curtis was making and I agree I made a mistake in 88. I'm thinking that in Toms scenario, because all the Suns energy would get to the surface, like the Moon, the surface would get hot (like the Moon) it would also emit a lot of radiation, which would go straight back out to space. However assuming the atmosphere was able to warm by conduction, the atmosphere would get warmed at the surface, and that warmed atmosphere would behave roughly as Tom described (lapse rate as described). The question would be how warm? I guess the warmed atmosphere would help to increase the temperature of the surface which would emit more, thus removing energy and keeping a balance between incoming and outgoing energy. Apologies that it took so long for me to get my brain around it.
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  12. Re Daniel 109 Thanks that is the one.
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  13. Bob Guercio @105 et al.: There are many mechanisms that populate the upper level of a radiative transition. One is by absorbing a photon having (virtually) that wavelength. But another possibility is collisional excitation. If the collision rates (gas density) aren't too large relative to the rates of radiative decay, then the collisional excitation has a chance to radiatively decay, rather than redistributing the energy back out into the thermal pool of gas due to a subsequent collisional de-excitation. So the thermal energy of the gas is converted into radiative energy -- and if that radiative energy escapes, this is a net cooling process. This is what is meant by a "net emitter". The point is that with exception of the CO2 transitions lying near the very strongest ones near 15 microns, the stratosphere is largely transparent to radiation from below (in particular, that which arises in wavebands corresponding to transitions in C02). Run the default model from David Archer's website (70 km, looking down), linked within the OP. See that sharp reversal in the spectrum within the center of the CO2 band? That light is emerging from an effective "photosphere" that lies within the stratosphere. Nearly all of the other light you see in that spectrum, including that within the C02 band is emerging from an effective photosphere somewhere within Earth's troposphere (but in the 800-1200 cm^{-1} spectral region it's emerging mainly from near Earth's surface, excepting the O3 trough near 1050 cm^{-1})-- and passes largely unscathed through the stratosphere. The stratosphere emits escaping radiation at a rate (that's the cooling rate) that is balanced by the energy deposited via absorption of short wavelength solar radiation by O3 and O2, resulting in molecular dissociation (that's the heating rate). One of the most radiatively active gases in the stratosphere is C02. Increase its abundance, and the cooling rate will exceed the heating rate until a new (lower) equilibrium temperature is reached.
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  14. A point of clarification to my last post (#113): Yes, light emitted by CO2 in the stratosphere is escaping upward, but the low gas density there makes it a weak emitter (except near the center of the band at 15 microns) compared to the light emerging from the troposphere via CO2. In some ways, Earth's stratospheric properties and spectrum are much like those of the Sun's chromosphere, that lies just above its photosphere.
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  15. Tom Curtis - 83 Please help me understand. I went only so far so once I understand this I'll continue and probably have more questions. Thank you, Bob After carefull consideration, I believe the explanation of stratospheric cooling given in the original post is simply wrong. To see this, consider a hypothetical planet whose atmosphere is completely transparent at all wavelengths of electromagnetic radiation. In this case, its surface temperature will be its temperature as measured from space, ie, its effective temperature. The temperature at any point in the atmosphere above the surface will be less than the effective temperature, and the temperature profile of the atmosphere will be defined by the adiabatic lapse rate up to the thermosphere. (Like Venus, see graphic in 80 above, it will have no stratosphere.) Question: I understand what lapse rate and adiabatic means but what is meant by the adiabatic lapse rate? Is there another kind of lapse rate? Now, as we introduce CO2 into the atmosphere, what happens is that the altitude of the effective temperature gradually increases in height. As the temperature profile is still defined by the lapse rate, the temperatures at every altitude up to the thermosphere will also increase. Question/Comment: Yes. CO2 absorbs IR in accordance with the greenhouse effect. Even if we exclude convection as a means of transfering energy in the atmosphere, and hence exclude the adiabatic lapse rate as a temperature profile, radiative transfer in an optically absorbing atmosphere will generate a lapse rate, indeed, typically a shallower (greater change in temperature for a given change in altitude) lapse rate than the adiabatic lapse rate. Therefore this reasoning should still hold. Question/Comment: To me, shallower means a more gradual or lower lapse rate. Please clarify. Also, less shallow than what? The lapse rate is generated by the radiative transfer of energy so it seems as if you are comparing something to itself. Looked at differently, and using Earth as our model, we need to consider that the effective altitude of radiation, ie, the average altitude from which radiation reaches space, lies several kilometers below the tropopause. Therefore most outgoing radiation at the tropopause reaches space, and the Beer-Lambert law is an appropriate approximation of the effect of changing CO2 concentrations in the stratosphere. Given that, then if we double the CO2 concentration we also double the amount of IR radiation absorbed by CO2 in the stratosphere. The amount of IR radiation outgoing from the tropopause will not itself double, but infact will slightly fall because the atmosphere is optically thick below the tropopause. Consequently, although the net radiation entering the stratosphere will fall in this scenario, the amount absorbed in the stratosphere will increase. Quesion/Comment: But I think that the portion of the IR spectrum that would be absorbed by the stratosphere has less IR energy in it after going through and being absorbed by the troposphere. Comment: To be continued. Of course, the amount of IR radiation emitted at a given temperature will also double with doubling of CO2 in the stratosphere. The result is that, if the temperature of the stratospheric CO2 is less than the brightness temperature radiation emitted by CO2 in the troposphere, it will warm. If it is greater it will cool. Of course, had the temperature in the stratosphere followed the adiabatic lapse rate, it would have been less than that of the troposphere; and increasing CO2 would warm the stratosphere, all else being equal. But all else is not equal - the stratosphere is much hotter than the upper levels of the tropospere because of the absorption of UV radiation by ozone. Therefore, I would have to conclude that stratospheric cooling with increased CO2 is primarilly due to increased efficiency at radiating away energy absorbed by ozone due to increased concentration of CO2. There would be a small additional boost due to reduced outgoing radiation of IR in the 15 micron (CO2) band; but that is ony a secondary cooling effect, and would have been a warming effect where it not for the presence of ozone in the stratosphere. Having said all that, I now hope some one will knock some holes in my reasoning.
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  16. Spaceman Spiff - 113 Please help me understand. There are many mechanisms that populate the upper level of a radiative transition. One is by absorbing a photon having (virtually) that wavelength. But another possibility is collisional excitation. If the collision rates (gas density) aren't too large relative to the rates of radiative decay, then the collisional excitation has a chance to radiatively decay, rather than redistributing the energy back out into the thermal pool of gas due to a subsequent collisional de-excitation. So the thermal energy of the gas is converted into radiative energy -- and if that radiative energy escapes, this is a net cooling process. This is what is meant by a "net emitter". Questions: I think what you are saying here is that sometimes during a collision, the kinetic energy of motion is lost and reappears as internal energy of a molecule. That molecule then emits radiation. Do I have this correct? The point is that with exception of the CO2 transitions lying near the very strongest ones near 15 microns, the stratosphere is largely transparent to radiation from below (in particular, that which arises in wavebands corresponding to transitions in C02). Run the default model from David Archer's website (70 km, looking down), linked within the OP. See that sharp reversal in the spectrum within the center of the CO2 band? That light is emerging from an effective "photosphere" that lies within the stratosphere. Nearly all of the other light you see in that spectrum, including that within the C02 band is emerging from an effective photosphere somewhere within Earth's troposphere (but in the 800-1200 cm^{-1} spectral region it's emerging mainly from near Earth's surface, excepting the O3 trough near 1050 cm^{-1})-- and passes largely unscathed through the stratosphere. Question: Isn't this kind of what I'm saying. The energy in the absorbtive portion of the band has been depleted after going through the troposphere. To be continued. The stratosphere emits escaping radiation at a rate (that's the cooling rate) that is balanced by the energy deposited via absorption of short wavelength solar radiation by O3 and O2, resulting in molecular dissociation (that's the heating rate). One of the most radiatively active gases in the stratosphere is C02. Increase its abundance, and the cooling rate will exceed the heating rate until a new (lower) equilibrium temperature is reached.
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  17. Bob Guercio at 10:52 AM The answer to your first question is "yes". The up and down transition rate equations contain many terms (spontaneous radiative decay, radiative absorption, induced emission, collisional excitation and de-excitation, ....). Collisional excitation followed by radiative decay via a photon that escapes is an important process in the stratosphere. Radiative absorption is not dominant (excepting in the ozone bands and the central CO2 transitions very near to 15 microns). I am not sure I know what you mean by, "The energy in the absorbtive portion of the band has been depleted after going through the troposphere." What you see in the main CO2 band isn't due to Beer-Lambert's law. What you are seeing is largely due to light emerging from the layer at which the optical depth (dimensionless measure of matter's ability to absorb or scatter light) has fallen to ~1 (the "photosphere"). Deeper down, the optical depth is too large (the photon mean free path too small), and most of the light is absorbed (and "re"-emitted) locally. The thermal emission is very sensitive to temperature, in that higher T gas emits more strongly. So where in wavelength the atmosphere is most opaque, the emitting layer (photosphere) lies high up in the troposphere where the T is low and so the thermal emission intensity is also low. Where it is less opaque, it arises from deeper layers in the troposphere where T is higher and thus is the thermal emission intensity. An exception to this is the set of very, very optically thick transitions of CO2 lying very near 15 microns. Here the atmosphere is so opaque (due to the strength/probability of the transition) that this light's effective photosphere lies up in the stratosphere, where T is substantially higher than in the troposphere below (due to O3 dissociation by UV sunlight). The escaping emission there is therefore brighter than in surrounding wavelengths within the surrounding CO2 transitions, and thus the sharp, narrow, reversal you see at 15 microns at the center of the main CO2 "trough". Does that help at all?
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  18. Bob Guercio @115: Question: I understand what lapse rate and adiabatic means but what is meant by the adiabatic lapse rate? Is there another kind of lapse rate? The Lapse rate is the change of temperature with change of altitude, and more specifically the lapse rate = -dT/dz. Because convection and latent energy dominate heat transfers in the troposphere, the lapse rate in the troposphere approximates to the the dry adiabatic lapse rate or the moist (or saturated) adiabatic lapse rate depending on relative humidity. At 0% humidity, the adiabatic lapse rate equals the dry adiabiatic lapse rate. At 100% humidity it equals the saturated adiabatic lapse rate. The adiabatic lapse rate is the rate of fall of temperature with increasing altitude predicted by the fall in pressure with altitude, as adjusted by release of latent heat as moisture condenses due to falling temperature. In practise, the environmental lapse rate rarely equals the adiabatic lapse rate because energy release from condensation and energy absorption of sunlight by clouds is rarely gruadual. However, the adiabatic lapse rate dominates only in the troposphere (and possibly mesosphere). At the tropopause, the lapse rate is zero; and in the stratosphere it is negative. It is again zero at the stratopause, positive in the mesosphere, zero at the mesopause, and negative again in the thermosphere. Even if we exclude convection as a means of transfering energy in the atmosphere, and hence exclude the adiabatic lapse rate as a temperature profile, radiative transfer in an optically absorbing atmosphere will generate a lapse rate, indeed, typically a shallower (greater change in temperature for a given change in altitude) lapse rate than the adiabatic lapse rate. Therefore this reasoning should still hold. Question/Comment: To me, shallower means a more gradual or lower lapse rate. Please clarify. Also, less shallow than what? The lapse rate is generated by the radiative transfer of energy so it seems as if you are comparing something to itself. A shallower slope is one with a greater change in the x axis for a given change in the z axis. In this case, the x axis is temperature and the z axis is altitude. In speaking of slope I am speaking of dZ/dX, whereas the lapse rate is measured as - dT/dZ; so I can see how this can cause confusion, for which I apologise. In a grey atmosphere, ie, one which is uniformly absorbing of IR at all wavelengths and has no convective heat transfers, the lapse rate is much greater than that determined by convection, ie, the adiabatic lapse rate. (As the lapse rate is the negative inverse of the slope of the plot of altitude against temperature, that means it has a much smaller slope.) As the IR absorption is restricted to fewer wavelengths, the radiative lapse rate will decrease; but in Earth's atmosphere it is still greater than the adiabatic lapse rate (I believe) although not much greater. I believe it is because of this that at the 60th parralel, the convectivion that drive the polar cell forms. Quesion/Comment: But I think that the portion of the IR spectrum that would be absorbed by the stratosphere has less IR energy in it after going through and being absorbed by the troposphere. It does, or more correctly that portion has less IR energy after being absorbed by CO2 and reradiated at a lower energy level because the CO2 is at a lower temperature. However, most of that energy escapes to space, so it is quite possible for there to be less energy coming from the troposphere in that band of the spectrum, but more energy being absorbed by the stratosphere in that band. It just means that less of the energy from the troposphere escapes to space in that band.
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  19. Bob, I don't know if this helps, but you seem to be thinking of the interactions as entirely a radiative story. In fact, it is a radiative/colissional story. Instead of: UVin + IRin = IRout, we have: UVin + IRin + Collisionalin = IRout + Collisionalout. In the stratosphere, for O3, UVin >> IRout > IRin; so O3 is a net absorber of radiation, with the excess energy being distributed to other components of the atmosphere by collission. With CO2, IRout >> IRin, with UVin being zero. The energy defecit is drawn from the surrounding atmosphere by collisions. If we simply doubled the CO2 in the stratosphere, leaving that in the troposphere untouched, then initially IRout and IRin would double. Because IRout is larger than IRin, that results in a net energy deficit which the CO2 draws from colisional energy, in the process cooling the surrounding atmosphere until a new steady state is reached. If we instead doubled the tropospheric CO2, leaving the stratospheric CO2 untouched, that would reduce IRin, again creating an imbalance restored by local cooling of the atmosphere. If we do the actually possible, and double CO2 at both levels of the atmosphere, both factors will come into play. However, I believe, the first has the larger effect. More importantly, the second is a cooling effect only because IRout larger than, or equal to IRin. If IRin >> IRout, as would be the case in the absence of O3, then doubling both IRin and IRout would have a significant warming effect, more than sufficient to compensate for the small reduction of IR radiation from the troposphere in the 15 micron band.
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  20. The answer to your first question is "yes". The up and down transition rate equations contain many terms (spontaneous radiative decay, radiative absorption, induced emission, collisional excitation and de-excitation, ....). Collisional excitation followed by radiative decay via a photon that escapes is an important process in the stratosphere. Radiative absorption is not dominant (excepting in the ozone bands and the central CO2 transitions very near to 15 microns). Answer: Understood. I am not sure I know what you mean by, "The energy in the absorbtive portion of the band has been depleted after going through the troposphere." Answer: In my blog the difference in the absorbtion band between figures 2 and 3. More of a chunk has been taken out of figure 3 because the CO2 level is higher. What you see in the main CO2 band isn't due to Beer-Lambert's law. What you are seeing is largely due to light emerging from the layer at which the optical depth (dimensionless measure of matter's ability to absorb or scatter light) has fallen to ~1 (the "photosphere"). Response: I think that you are talking about the chunk that I am talking about in my figures 2 and 3. Correct? We are talking about band saturation here. Aren't we? Also, just to clarify what you mean by "photosphere" If you are looking down at the IR spectrum from an altitude, the earth would be the photosphere for the portion of the spectrum correcsponding to the black body temperature of the earth. If part of the spectrum is due to blackbody radiation from an altidute of 3 miles, the photosphere for this would be that altitude of 3 miles. Correct? Deeper down, the optical depth is too large (the photon mean free path too small), and most of the light is absorbed (and "re"-emitted) locally. The thermal emission is very sensitive to temperature, in that higher T gas emits more strongly. So where in wavelength the atmosphere is most opaque, the emitting layer (photosphere) lies high up in the troposphere where the T is low and so the thermal emission intensity is also low. Where it is less opaque, it arises from deeper layers in the troposphere where T is higher and thus is the thermal emission intensity. Response: Understood. An exception to this is the set of very, very optically thick transitions of CO2 lying very near 15 microns. This is in the center of the band that I refer to as the absorbtion band in my figures 2 and 3. Isn't it? Here the atmosphere is so opaque (due to the strength/probability of the transition) that this light's effective photosphere lies up in the stratosphere, where T is substantially higher than in the troposphere below (due to O3 dissociation by UV sunlight). The escaping emission there is therefore brighter than in surrounding wavelengths within the surrounding CO2 transitions, and thus the sharp, narrow, reversal you see at 15 microns at the center of the main CO2 "trough". Response: Understood Does that help at all? Yes. But now let me try to put everything together. Also, are you saying that my blog is wrong or that it describes only one of many mechanisms and it is not even the main mechanism? My brain hurts!
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  21. Bob, I don't know if this helps, but you seem to be thinking of the interactions as entirely a radiative story. In fact, it is a radiative/colissional story. Instead of: UVin + IRin = IRout, we have: UVin + IRin + Collisionalin = IRout + Collisionalout. Understood. In the stratosphere, for O3, UVin >> IRout > IRin; so O3 is a net absorber of radiation, with the excess energy being distributed to other components of the atmosphere by collission. You say atmosphere but I think you mean stratosphere. Am I correct? If this is the case, I think I understand. With CO2, IRout >> IRin, with UVin being zero. The energy defecit is drawn from the surrounding atmosphere by collisions. Again, I think you mean stratosphere? If we simply doubled the CO2 in the stratosphere, leaving that in the troposphere untouched, then initially IRout and IRin would double. The same radiation enters the stratosphere so I guess this means that twice as much energy would be taken out of the IR radiation since there are twice as many CO2 molecules to absorb it. Correct? Because IRout is larger than IRin, that results in a net energy deficit which the CO2 draws from colisional energy, in the process cooling the surrounding atmosphere until a new steady state is reached. Understood. If we instead doubled the tropospheric CO2, leaving the stratospheric CO2 untouched, that would reduce IRin, again creating an imbalance restored by local cooling of the atmosphere. stratosphere? If we do the actually possible, and double CO2 at both levels of the atmosphere, both factors will come into play. However, I believe, the first has the larger effect. More importantly, the second is a cooling effect only because IRout larger than, or equal to IRin. If IRin >> IRout, as would be the case in the absence of O3, then doubling both IRin and IRout would have a significant warming effect, more than sufficient to compensate for the small reduction of IR radiation from the troposphere in the 15 micron band. I think I understand. I must ask you the same question that I asked Space Spiff. Are you saying that my mechanism is wrong or that it is one of several mechanisms and may not even be the dominant mechanism? I want to thank both of you guys for helping me understand a very complex process. As I said to Spaceman, my brain hurts but it hurts more now. Bob
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  22. As I'm putting the pieces together, I'm starting to think that this is a terribly difficult thing to explain simply!
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  23. Bob @122, everywhere where I say "atmosphere" in comment 120, you can substitute "stratosphere". It is not that atmosphere is wrong, so much, as that stratosphere is more specific. "The same radiation enters the stratosphere so I guess this means that twice as much energy would be taken out of the IR radiation since there are twice as many CO2 molecules to absorb it. Correct?" Yes. "I must ask you the same question that I asked Space Spiff. Are you saying that my mechanism is wrong or that it is one of several mechanisms and may not even be the dominant mechanism?" Essentially correct. Rather than saying it "... is not even the dominant mechanism", I would rather say it is probably not the dominant mechanism (from my understanding). Further, I think it is important to be aware that the mechanism you describe is only a cooling mechanism because the stratosphere is warmer than the upper troposphere. It is like playing boiling water (100 degrees C) through a fire hose onto some object. Is it a warming or a coolinig mechanism? That depends critically on whether the object you are drenching is a block of ice or several tonnes of red hot steel! Your mechanism is like reducing the temperature in the water in the hose by 10 degrees C. That will reduce the rate at which ice being drenched would warm, and increase the rate at which the red hot steel, being drenched, would cool. But I am unsure that calling it a cooling mechanism is informative.
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  24. Bob Guercio -- If there is anything more that you'd like me to clarify, please ask. You left a few questions for me, but I think Tom Curtis might have answered most of them. Similar to Tom, I would say that the initial explanation was significantly incomplete. At the same time I am still uncertain whether all the important pieces have yet been identified. In any case, my hat is off to you for wading through it all!
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  25. Bob Guercio -- Another point of clarification. The 15 micron spectral feature I have been referring to in previous posts is the *very central-most spike* shown in this figure of transition probability of CO2 (for a particular value in temperature and pressure). Most of the CO2 spectral feature that you see in Archer's upwelling radiation spectrum of Earth, and that has been under discussion throughout this post, lies in the 13.5-17 micron region of the transition probability figure.
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  26. In any case, my hat is off to you for wading through it all! Thank you but not just yet. I'll go through it tomorrow and I will probably understand it. It's still a little fuzzy but I'm pretty confident that I will get it. It's good that I posted the blog that I did. How else would I have fleshed this problem out. I think that now I understand Gavin's email to me. I'll repeat it here and comment: mostly right. You miss two key facts. First, all GHGs emit as well as absorb, and whether you will get warming or cooling in a region depends on the ratio of the change in absorption and the change in emittence. Second, the troposphere has many IR absorbers, the stratosphere only two (CO2 and O3 - everything else is minor). So the impact of CO2 above the tropopause is amplified. Otherwise you are spot on! Gavin See the first part. That is Gavin's short version of everything you guys have said. And then his comment that I was "spot on". I was "spot on" with that one mechanism but there is so much more to it. Thanks again but I really don't expect this thread to end just now. There's just too much to it and I'm sure that half the world is reading it and wants to understand it also. In the field of amateur climatolgy, my guess is that this is one of the most difficult concepts to grasp. I would like to have another go at writing a more comprehensive and accurate blog on this which is needed on the Internet. However, credit where credit is due and you guys have to be included. Please email me to discuss it further. robertguercio@optonline.net
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  27. I'm not sure whether Science of Doom's website has been referenced yet on this thread but there are two articles in particular there that people here may find useful. I certainly did. Stratospheric Cooling Tropospheric Basics
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  28. It occured to me to use Ramathan and Dickson, 1999 and the version of MODTRAN at David Archer's website to better characterize the relative importance of the two effects we are discussing. Using Modtran with a lookdown altitude of 10 km on the 1976 US Standard Atmosphere, I determined the difference in outgoing radiation between an atmosphere with 375 ppm and one with 750 ppm of CO2 at approximately 3.25 w/m^2. From Ramathan and Dickson, the IR absorption by CO2 + H2O in the stratosphere is 19 w/m^2, and IR emission in the stratosphere is roughly 35 w/m^2 (Table 3); a difference of 16 w/m^2. Doubling CO2 will double both absorption and emission, to a first approximation, giving values of 38w/m^2 and 70w/m^2, and a difference of 32 w/m^2; making the net difference if there were no change in radiation from the troposphere 16 w/m^2. Introducing that change, the reduction in IR absorption will not excede the reduction in outgoing IR from the troposphere, and so will not excede 3.5 w/m^2. Subtracting that value from the stratospheric absorption, and then doubling for the increased CO2 concentration yields a minimum absorption of 31 w/m^2, and a net difference for doubling CO2 of (at most) 23w/m^2. Therefore the maximum cooling introduced to the stratosphere by shielding the stratosphere at some frequencies of IR is about 7 w/m^2, compared to a 16 w/m^2 from simply doubling the CO2 in the stratosphere. For comparison, the cooling introduced by redusing stratospheric ozone by 30% is about 5 w/m^2, or about 3 w/m^2 once we allow for reduced IR emissions by O3. I must emphasise that these are ballpark figures. However, we can reasonably conclude that the cooling of the stratosphere because of reduced IR emissions in the 15 micron band has a similar cooling effect to that of ozone reduction, and that the cooling effect of doubling stratospheric CO2 is 2 to 4 times as strong as that. Two additional points. First, it is evident that the as CO2 concentrations increase, the relative importance to the thermodynamics of the stratosphere of Ozone will decrease. This is quite apart from any changes in ozone concentration induced by temperature changes (which may be quite significant). Second, an additional important effect on stratospheric temperatures is albedo variations, with higher albedo resulting in higher stratospheric temperatures. This is a dominating effect following major tropical volcanoes, but probably less significant than other factors discussed here at other times. Finally, thankyou for the offer, Bob, but no thanks. I would much rather you got Gavin to review your rewritten blog. An expert is somebody who knows how to avoid fundamental errors. In this field, Gavin is an expert and I am not. So no matter how sound my reasoning appears to me, it is always possible I am overlooking something completely obvious.
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  29. Tom, Just one more point. Do your discussions here cover the transient state, the steady state or both. We are in the transient state now because more energy is reaching the earth's surface than that which is leaving. Let's assume that we miraculously stabilized atmospheric CO2 concentration today. The temperature of the earth will continue to rise until the output energy from the earth equals the input energy. That would be the steady state. I'm not talking about energy coming in and out from beyond the thermosphere. I'm talking about the ground. I meant my blog to be strictly for the steady state. Bob
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  30. Bob Guercio at 07:45 I think Tom and Spaceman have covered your questions as well as i could have(truth is, most of my posts on this thread have been "trying" to convey the mechanisms alluded to by the radiative exchanges in that paper.) And the existence of the tropopause itself, is pretty convincing evidence that this is the main phenomena responsible for stratospheric cooling/energy loss.
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  31. Bob, my discussions have covered the steady state. However, I am also not sure how important a distinction that is in the stratosphere in that, not lying next to any oceans, and being a rarified gass, the time interval between perturbation and adjustment to steady state would be quite short.
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  32. Tom, But more energy is coming into the earth than leaving. This applies to both the ground and deep space. My prevous remark regarding a distinction here was gratuitous. If we stabilized CO2 levels, this would continue for a few decades. Bob
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  33. Bob, I actually think that once you sort it out, you can come up with a simple explanation. A lot of the complications can still be removed. The difficulty with anything like this lies primarily in removing relatively inconsequential complications. The reader probably does not need to know that the stratosphere is heated through UV and ozone creation, or what "optical thickness" means (that is a very confusing, obscure term that is meaningful to people that are familiar with it, but requires a paragraph just to explain, and so distracts and confuses the reader). Terms like "adiabatic lapse rate" and the 15 micron issue are similarly confusing details which do not really add anything to the heart of the explanation. A lot of things can be left out. Figuring what to leave out is the key to any simple model created for explanatory purposes, as you well know (and despite what the "it's not realistic" crowd moronically screams). The only factor that you really need to incorporate with what you have already written, IMO, is that energy is transferred at the molecular level via collision or emission/absorption, and in varying proportions depending on the density of the atmosphere. The main point is simply that energy can be absorbed one way but then surrendered in another, so that at equilibrium the total-radiation-in need not equal total-radiation-out in a particular band. As a result, the CO2 in the troposphere is more likely to absorb IR in that narrow CO2 band, but then passes it on through collisions to the abundant, non-emitting O2/N2, raising temperatures and somewhat "blocking" that band of radiation. Alternately, the CO2 in the more rarefied stratosphere is more often excited by collisions with the more abundant O2/N2, and emits the gained energy through radiation before it can pass it on through another collision. So CO2 prevents energy from escaping from the troposphere into the stratosphere in the CO2-IR bandwidth, and CO2 actively cools the stratosphere by emitting energy in the CO2-IR bandwidth. The only question left to clarify is relative amounts of these mechanisms (i.e. how much does stratospheric emission contribute to cooling, versus tropospheric "blocking"). I suspect that the latter is minor, at least as far as its influence on stratospheric temperatures (but I don't know).
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  34. Hi All, I've tried to post an image but again I am not successful. Could someone take care of this for me and then show me the exact code used for this image. I was going to ask the question "The ozone layer is not at the top of the stratosphere so how does heating of the ozone layer cause the temperature to be the highest at the top?" But then I saw this image which shows the temperature increasing and then decreasing as you go up through the statosphere. This is what I would expect. That said, how is the stratosphere defined. Apparently it is not simply simply defined by a negative lapse rate! Bob
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    Moderator Response: Fixed. See the help on posting images; you need an html tag: img src= "url" width=no more than 500 inside the usual lt and gt brackets.
  35. 134.Sphaerica at 03:12 AM on 5 December, 2010 Bob, I actually think that once you sort it out, you can come up with a simple explanation. . Figuring what to leave out is the key to any simple model created for explanatory purposes, as you well know (and despite what the "it's not realistic" crowd moronically screams). I think that I understand what you are saying. Regarding your comment about realism, I doubt if these people have ever had even a basic Physics or Chemistry course. Bob
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  36. Bob Moderator Response: Fixed. See the help on posting images; you need an html tag: img src= "url" width=no more than 500 inside the usual lt and gt brackets. I don't get it. Could you please put exactly what you typed inside quotes or brackets. Thank you, Bob
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  37. Re: Bob Guercio (137) 1. First type this symbol: < 2. Next, type: img width="500" src="http://image_url/" 3. Replace the URL in Double Quotes "" with the actual URL intended 4. A common error is to have an extra / at the end of the URL; this can be avoided by using the preview function Hope this helps! The Yooper
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  38. Dan, Success! Thank you, Bob
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  39. Sphaerica So CO2 prevents energy from escaping from the troposphere into the stratosphere in the CO2-IR bandwidth, My blog does this. and CO2 actively cools the stratosphere by emitting energy in the CO2-IR bandwidth. Once I fully understand this, I'll incorporate it into my blog and maybe it will be complete. This sentence that you wrote would be part of my opening statement. And then my blog, as it is now, would address the first method and I'll then add the second method. This has been like wrestling an alligator. The problem is that, as far as professionals are concerned, all this is so trivial that they wouldn't waste their time on it. It's probably in some climate textbook or maybe not; maybe it's a problem at the end of some chapter on stratospheric chemistry or some such name! Thank you, Bob
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  40. Folks, I corresponded via email with a distinguished professor at Rutgers and this is that correspondence. Dear Bob, You correctly wrote up what Gavin told you, but he is wrong. The stratosphere cools because its emissivity goes up with more CO2, and it still absorbs the same amount of energy being emitted from below. It is a balance of energy. You cannot just look at one term. Designing a graph with not enough room to do it correctly is not a good reason in my opinion to do it wrong. Alan Alan Robock, Professor II (Distinguished Professor) Editor, Reviews of Geophysics Director, Meteorology Undergraduate Program Department of Environmental Sciences Phone: +1-732-932-9800 x6222 Rutgers University Fax: +1-732-932-8644 14 College Farm Road E-mail: robock@envsci.rutgers.edu New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock On 12/2/2010 10:22 AM, Robert Guercio wrote: > Dear Alan, > > I'm sure you're correct; however, here is the email correspondence > that I had with Gavin. > > The ordinate should be (watts/meter square wavenumber) and I had > trouble making it all fit. I was actually working pixel by pixel and > I guess I was a bit lazy but that is easily corrected. > > The solar insolation that I used was very fictitious as everything > about my model is. I just kept on playing with different numbers > until the fictional atmosphere that I came up with made for "good" > graphs. > > Bob > > > > ----- Original Message ----- From: "RealClimate" > > To: "Robert Guercio" > Sent: Friday, November 19, 2010 8:03 PM > Subject: Re: Stratospheric Cooling > > >> mostly right. You miss two key facts. First, all GHGs emit as well as >> absorb, and whether you will get warming or cooling in a region >> depends on >> the ratio of the change in absorption and the change in emittence. >> >> Second, the troposphere has many IR absorbers, the stratosphere only two >> (CO2 and O3 - everything else is minor). So the impact of CO2 above the >> tropopause is amplified. >> >> Otherwise you are spot on! >> >> Gavin >> >>> Hi, >>> >>> I've searched for an explanation of the reason that the Stratosphere >>> cools >>> due to Global Warming and have not found a satisfactory answer. There >>> does seem to be quite a bit of hand waving though. >>> >>> I think that I now understand it but would like the confirmation of a >>> professional. If my understanding is correct, I would like to write a >>> blog on this most misunderstood subject. >>> >>> Please confirm if this is correct. >>> >>> Thank you, >>> >>> Robert Guercio >>> >>> The earth radiates Infrared Radiation in accordance with Black Body >>> theory. Most of the IR energy absorbed by CO2 has wave numbers of >>> approximately 650 and 1050. There is CO2 in both the troposphere and >>> the >>> stratosphere so frequencies associated with these wave numbers >>> emanating >>> from the heated earth heat up both the troposphere and the >>> stratosphere. >>> Frequencies of all other wave numbers simply sail on through without >>> effecting either layer. >>> >>> If there is more CO2 in the troposphere, more of a chunk of the >>> spectrum >>> is going to be taken out around these two wave numbers in heating up >>> the >>> troposphere. Therefore, there is less energy in these two IR bands >>> to heat >>> up the CO2 in the stratosphere and thus the stratosphere cools. >>> >> I don't think Gavin was wrong. I'm interpreting this email to mean that the mechanism of my blog is not significant in comparison to the second method that I am now learning. I think Gavin's email to me was, to use his words, "Spot On". Here it is again posted: mostly right. You miss two key facts. First, all GHGs emit as well as absorb, and whether you will get warming or cooling in a region depends on the ratio of the change in absorption and the change in emittence. Second, the troposphere has many IR absorbers, the stratosphere only two (CO2 and O3 - everything else is minor). So the impact of CO2 above the tropopause is amplified. Otherwise you are spot on! Gavin > Hi, > > I've searched for an explanation of the reason that the Stratosphere cools > due to Global Warming and have not found a satisfactory answer. There > does seem to be quite a bit of hand waving though. > > I think that I now understand it but would like the confirmation of a > professional. If my understanding is correct, I would like to write a > blog on this most misunderstood subject. > > Please confirm if this is correct. > > Thank you, > > Robert Guercio > > The earth radiates Infrared Radiation in accordance with Black Body > theory. Most of the IR energy absorbed by CO2 has wave numbers of > approximately 650 and 1050. There is CO2 in both the troposphere and the > stratosphere so frequencies associated with these wave numbers emanating > from the heated earth heat up both the troposphere and the stratosphere. > Frequencies of all other wave numbers simply sail on through without > effecting either layer. > > If there is more CO2 in the troposphere, more of a chunk of the spectrum > is going to be taken out around these two wave numbers in heating up the > troposphere. Therefore, there is less energy in these two IR bands to heat > up the CO2 in the stratosphere and thus the stratosphere cools. > His first comment, in my opinion, addresses what you guys are saying. His second comment addresses my blog. But he didn't say which one was more important. So I'm saying that Gavin addressed both methods and I picked up on only one. I'm interpreting my method as trivial because of Dr. Roboks comment that Gavin was wrong. Are you guys interpreting these two very important emails the same way that I am? Again thank much, Bob
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  41. Bob,
    Are you guys interpreting these two very important emails the same way that I am?
    Yes, I think that's exactly it, it all seems to add up to the fact that the primary mechanism by far is IR emission by CO2 in the stratosphere, as professor Robock explicitly says and Gavin implies when he says "First, all GHGs emit as well as absorb, and whether you will get warming or cooling in a region depends on the ratio of the change in absorption and the change in emittence." I think Gavin's spot-on comment probably meant that what you told him was correctly stated, but he didn't emphasize how much importance lay in the pieces missed.
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  42. Sphaerica, You said: As a result, the CO2 in the troposphere is more likely to absorb IR in that narrow CO2 band, but then passes it on through collisions to the abundant, non-emitting O2/N2, raising temperatures and somewhat "blocking" that band of radiation. Alternately, the CO2 in the more rarefied stratosphere is more often excited by collisions with the more abundant O2/N2, and emits the gained energy through radiation before it can pass it on through another collision. Me: I can't believe this but I understand this perfectly. I've had one of those Eureka moments. I'll add one fine point for total clarity or maybe I'm simply rephrasing what you guys have said. And I'm keeping with my very simple model of an atmosphere of two layers, the troposphere and stratosphere and composed only of nitrogen and carbon dioxide. Temperature depends only upon the kinetic energy of the molecules. Thus, after a collision, a molecule with no vibrational energy may now have vibrational energy and that molecule has less kinetic energy. So this diminution of kinetic energy from multiple molecules lowers the temperature. That molecule that has more vibrational energy deexcites and emits IR that may be absorbed by another deexcited molecule or it may simply fly off into space. This IR flying off into space is kinetic energy that is now lost forever from the stratosphere. I also now believe that, as Tom has stated, it doesn't matter whether we are talking about the steady state or the transient state with these states being as I have defined them. So I can now make my model simpler yet. I won't talk about whether or not we are at equilibrium! It's amazing how much you can do in Physics without the heavy mathematics. Just say "you've got it!" and I'll run with it. I also would like all you guys that helped me to send me email so that I can acknowledge you with your real names and, something tells me, titles. robertguercio@optonline.net Thank you, Bob
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  43. Sphaerica and everybody else who helped me. I'm sure that temperature differentials play a role in all of this but getting into that would just add complexity to a nice and simple model and make the essence of all of this more difficult to understand. Do you agree? Bob
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  44. Guys, I guess I'm thinking of one thing after another. The troposphere. More CO2 so more absorption of IR. This causes the vibrational energy of CO2 molecules to increase. Somehow this vibrationalal energy gets converted to K.E. to increase the temperature. In this case, a collision results in more k.e. of the particles. Right? My intuition here is not as solid even though this is probably what is happening. Bob
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  45. Bob Guercio @ 145 Yes... but the thing is with the denser atmosphere at lower altitudes, is that what is emitted by co2, is absorbed by co2, simply because there are more molecules per volume. As you rise in altitude, with the reducing pressure, the molecules absorb less and less of the emitted radiation of their neighbors. Just due to the distance/space between molecules. So at lower altitudes, even though a molecule at a warmer T is emitting more, its simply swapping energy with its neighbors. But once you reach the tropopause, the distance between molecules, means that more energy escapes than what is absorbed from its neighbors. I tried to explain the reason for this at 57. Collisional exchanges with a gas at radiative equilibrium, will mean that energy is being deposited into the n2, o2, when the co2 is a net radiator, the collisional exchange will work the other way, from the warmer n2 @ o2 to the cooler co2(because it is loosing energy through radiation.)
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  46. Joe, As I reread what you wrote, I realize that you were correct. I needed time to digest everything and put the pieces together. Thank you, Bob
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  47. Bob Guercio at 08:51 Yes, its one of those subjects that takes a bit o nutting out... but then it all becomes clear, and the T profiles vrs altitude suddenly make sense. It was something that took me many hours to fully "get". (thanks to science o doom, and Ramanthan and Dickinson)
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  48. Sphaerica at 03:12 AM on 5 December, 2010: You said: "So CO2 prevents energy from escaping from the troposphere into the stratosphere in the CO2-IR bandwidth, and CO2 actively cools the stratosphere by emitting energy in the CO2-IR bandwidth." This is not correct. CO2 within the troposphere is responsible for emitting most of the light within the 13.5-17 micron CO2 band. This light (except for the sharp spike right near 15 microns) emerges from somewhere within the mid and upper troposphere, where the gas is just becoming optically thin to those transitions. This is what is meant by a "photosphere". The light of stronger transitions (near the bottom of the CO2 spectral feature) will emerge higher up in the troposphere (where the density is lower and the remaining path length out is shorter), where the T is lower and so the thermal emission is weaker. The light of weaker transitions emerges from deeper within the troposphere, where T is higher and thus the thermal emission is greater (the walls of the CO2 spectral feature). Have a look at the figure (from RealClimate) I posted in #126: CO2 absorption strength vs. wavelength. Nearly all of this radiative energy passes through the stratosphere, with the exception of the very strongest transitions lying very near to 15 microns (the sharp spike in the figure I linked to, and visible by running a default radiative model from Archer's website). The photosphere for these transitions lies in the stratosphere, where T is higher and thus so is the emission intensity.
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  49. Bob @133, this is correct, and I must have misunderstood what you where asking. My claim was only about the stratosphere which will (I believe) reach a steady state very quickly given constant inputs. Whether "very quickly" is a few month or a couple of years I'm not sure. @144, I believe this is correct, with motion of the whole molecule contributing to temperature, while vibration contributes to heat capacity. I am, however, not sure. In passing, Joe Blogg's first paragraph @146 is a brilliantly succinct explanation of optical depth as related to this issue. Regarding your furture article, in a topic this subtle, I suspect it would be better to write an advanced version first, and only post a basic version once the advanced version is up. When you write a basic article, you may be able to do something along the lines of comment by Nullius in Verba at Science of Doom. Doing this, I would not treat the temperature profile as a rigid bar, but rather treat the stratosphere seperately from the troposphere. Essentially, you would be appealing to the not often commented upon fact that in the presence of a negative lapse rate, greenhouse gasses cool rather than warm.
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  50. From one following the discussion as best I can, thought I'd throw this out there:
    "The loss of ozone that has occurred in the Antarctic lower stratosphere during each spring since 1980 has led to a decrease in the lower stratospheric temperature that persists into the summer season." "Comparison of the summer temperatures in the NH and SH indicates a distinctive offset beginning around 1980. The increase in temperature near the SH summer mesopause has implications for the presence of polar mesospheric clouds." "The Antarctic ozone hole is perhaps the largest persistent perturbations to the atmosphere during recent decades. As shown here, the climate impacts of this anthropogenic change extend into the upper mesosphere. As the ozone recovers in upcoming decades, we expect to see shifts in the SH summer mesopause that bring it closer to that in the NH."
    From a science news article in Science daily; free copy of source study available here. A good chunk of the study goes over my head, like much of this thread. But these caught my eye (eye-candy, heh-heh): and If this was discussed already here, my apologies. The Yooper
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