Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.

Settings

Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup

Settings


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Donate

Twitter Facebook YouTube Pinterest

RSS Posts RSS Comments Email Subscribe


Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...



Username
Password
Keep me logged in
New? Register here
Forgot your password?

Latest Posts

Archives

Climate Hustle

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.

0 0

Bookmark and Share Printable Version  |  Link to this page

Comments

Prev  1  2  3  4  5  Next

Comments 151 to 200 out of 247:

  1. Daniel,

    This is the other cause of cooling of the stratosphere. However, this is relatively easy to understand and I do not want bring it into my writeup because it will only confuse the very complex mechanisms that cause greenhouse gases to cool the stratosphere.

    But yes. Thinning of the ozone layer also causes the stratosphere to cool.

    Bob
    0 0
  2. Hi All,

    I've finished a rough cut of my writeup and offer it to you for comment. I apologize for the roughness of it but you will not have any trouble understanding what I'm saying.

    Stratospheric Cooling and Tropospheric Warming - Revised

    Again, nobody has emailed me to tell me who they are and if they would like to be aknowledged. I do feel that I should give credit where credit is due.

    Bob
    0 0
  3. It's interesting to note but I did a search on a forum that I am a member of and now realize that I have been fighting this problem for almost four months.

    The thread is here:

    Original Thread

    But don't bother reading it. It's not necessary.

    Bob
    0 0
  4. Bob @152:

    One mechanism involves the conversion of translational energy of motion or translational kinetic energy (KE) into Infrared radiation (IR) and the other method involves the absorption of IR energy by CO2 in the troposphere such that it is no longer available to the stratosphere.


    I don't think the first mechanism should be discusses without mentioning the source of the heat the CO2 is radiating away, ie, UV absorbed by ozone.

    Therefore, these excited CO2 molecules will emit IR radiation which in the rarefied stratosphere may simply sail off into space with the associated energy lost forever.


    In fact, approximately 50% of the energy is radiated downward and absorbed by CO2 in the troposphere. It still leaves the stratosphere, which is therefore cooled.
    0 0
  5. Bob Guercio at 01:23

    Id be interested in the exact figures for the reduction in LW emitted (around 15micron) with the increase in CO2... because you are increasing the CO2 in the stratosphere by 260 odd percent... and i really doubt the reduction in LW would offset this... and with a quick over lay of the graphs, it seems to be increasing emission around the 9-10 micron mark(O3 band)

    Just with a quick play with modtran, it just seems that the radiative sign from the troposphere @ startosphere may be positive... but its just offset, by the increasing radiative losses in the stratosphere with increasing CO2...

    I dont have time at the moment to go hunting through the input files, this is just from overlaying graphs, so i could well be wrong...
    0 0
  6. Joe,

    You're talking about mechanism 2 I think and you are saying that although there is less energy in the IR radiation entering the stratosphere, there is more CO2 to absorb it so the energy content may very well be the same. Am I correct?

    I don't think it works that way. You could triple the number of molecules but if the energy per molecule doesn't change, the temperature would be the same.

    I remember something like 1/2kT of energy per molecule for each degree of freedom. So if you have a situation with three times the molecules and you have the same temperature, you have three times the energy.

    After energy is absorbed by the troposphere, the absorption band of the energy is coming from a cooler black body so each molecule would take up less energy.

    I still don't have a good understanding how the energy absorbed by a molecule, which causes it to vibrate, gets converted Kinetic Energy.

    Bob
    0 0
  7. Tom,

    I'm changing it to reflect your second comment about energy being radiated both up and down.

    I'm thinking about the first comment. It would explain something that is not pertinent to the mechanism I'm talking about and would make my discussion more complicated.

    I'm inclined to say that it is at whatever temperature it is and we need not be concerned with how that happened.

    Thank you,
    Bob
    0 0
  8. Tom,

    I changed the paragraph that you commented on as follows:

    "Consider now the atmosphere of our fictitious model. Nitrogen and CO2 molecules are in motion and the average speed of these molecules is related to the temperature of the stratosphere. Now imagine that CO2 molecules are injected into the atmosphere causing the concentration of CO2 to increase. These molecules will then collide with other molecules of either nitrogen or CO2 and some of the K.E. of these particles will be transferred to the CO2 resulting in excited CO2 molecules and a lowered stratospheric temperature. All entities, including atoms and molecules, prefer the unexcited state to the excited state. Therefore, these excited CO2 molecules will emit IR radiation which, in the rarefied stratosphere, will simply be radiated out of the stratosphere. The net result is a lower stratospheric temperature. This does not happen in the troposphere because, due to higher pressures and shorter distances between particles, any emitted radiation gets absorbed by another nearby CO2 molecule."

    Bob
    0 0
  9. Hi All,

    I think that somewhere in this thread there was talk about the stratosphere being more rarefied than the troposphere. I also think that this was used to justify why the troposphere doesn't cool. The logic was that any emitted radiation in the troposphere would get reabsorbed because of the proximity of the particles.

    I don't think this is correct. Radiation goes from the stratosphere to the troposphere because the stratosphere is warmer than the upper troposphere.

    Or did I misinterpret those remarks concerning pressure?

    Bob
    0 0
  10. Bob Guercio at 15:30

    Temperature reduces with altitude in the troposphere, because of the reducing pressure with altitude in the troposphere. And thus the shortening of the optical pathway. This is correct. (Truth is convection moves the bulk of the energy at low/mid altitudes in the troposphere, the optical path is too long for radiation to effectively move it, but this energies escape is limited by the path length at higher altitudes in the troposphere. By adding CO2 it is raising the height that radiation can effectively escape. The warming of the lower atmosphere is a necessary result, to enable the T differential between layers to allow the transfer of energy up to this higher altitude, through convection, conduction and radiation.)

    The net transfer of energy from the stratosphere to the troposphere, is because the stratosphere is warmer than the upper troposphere, because it absorbs UV, but has a short optical pathway for transferring this energy once in the co2 bands... So LW energy can pass straight through it without interacting from the troposphere up, and also from higher up back down, until it is absorbed by the thicker atmosphere of the upper troposphere.

    And this is why i am a lil curious on the figures for the change in radiation, because by increasing CO2, not only will it radiate energy out of the stratosphere more efficiently, but it will absorb more of the radiation in transit up, that would have previously passed through. But i dont think i was correct on the increase in the 9-10 micron area. But would be curious to the exact figures. radiation dosnt care whether its being absorbed by a warmer or colder body, it will be absorbed by what it is incident on pending its "properties". Although intensity is an important consideration.
    0 0
  11. Bob,

    two suggestions,

    1) If increased emissivity of the stratosphere rather than decreaased transmission of surface emissions through the tropsphere is the main cause, can this be shown graphically?

    eg by showing the emissions spectrum just for the stratosphere for different CO2 concentrations, at equal total power. This would show the higher emission in the CO2 bands and the corresponding lower temperature necessary to get the same total power out(ie area under the graph). It would, though, require someone to do the calcs which is way beyond me.

    It might be good enough to do this as a single slab for the stratosphere, just to show the principle?

    2) From your link "Since power is energy per unit time, the energy content of IR radiation can be indicated by its IR spectrum which is a graph of power density as a function of frequency." I'd suggest the unit time bit here is unnecessary and confusing - we are not looking at dynamics.

    I think I'm starting to understand and have enjoyed the thread, but I'd suggest the final version needs a proper academic critique as we've shown just how easy it is to get it wrong.
    0 0
  12. Bob Guercio:
    "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."


    Molecular vibration is kinetic energy Bob, because the molecule is moving.
    eg. if you have a lump of steel that is red hot, the molecules vibrate a lot, they don't fly about, except at the surface.

    http://en.wikipedia.org/wiki/Temperature
    0 0
  13. #159

    It doesn't matter if troposphere is colder - radiation goes both ways regardless of temperature gradient. Net heat transfer must be from hotter to colder, but not necessarily radiative.
    0 0
  14. VeryTallGuy - 161

    I think I'm starting to understand and have enjoyed the thread, but I'd suggest the final version needs a proper academic critique as we've shown just how easy it is to get it wrong.




    I couldn't agree more.

    I'm not exactly sure how I'm going to get that critique because, in general, professionals don't bother with amateurs. This is understandable because there is such a vast difference in the scope of their knowledge. It could be different here because I am not debating with a professional but only asking that they check it for accuracy.

    I too am understanding this more and more every day. There is really nothing astounding in all of this. It is just a combination of elementary Physics concepts; however, when you put so many elementary Physics concepts together, it does get a bit confusing!

    Bob
    0 0
  15. Joe Blog - 160

    I think that I see what you are saying. I have explained two mechanisms for cooling but there is a mechanism for warming the stratosphere because of the extra CO2 in it. More CO2, more warming due to absorption of radiation passing through from the troposphere.

    I think that this is an interesting thought but I don't think it would have any effect on my writeup and would only add an unnecessary level of complexity to it.

    Maybe someone can address this point.

    Bob
    0 0
  16. The Ville - 162

    I agree. That's why I referred to temperature being caused by Translational kinetic energy rather than just kinetic energy.

    Do you still think I need to make this clearer? Or are you referring to something else I said in my blog where I may have said something wrong?

    Thanks,
    Bob
    0 0
  17. Hi All,

    Regarding a professional critique, I shot off an email to John Cook. He may have some clout with the professionals at Real Climate.

    Bob
    0 0
  18. The Ville @162, Bob's description is correct. The vibrational modes of atoms within a molecule enter into the equation as the heat capacity of the molecule, and not as temperature. This is discussed at the Wikipedia article on heat capacity

    As the caption of a nifty graphic puts it:

    "Molecules undergo many characteristic internal vibrations. Potential energy stored in these internal degrees of freedom contributes to a sample’s energy content, but not to its temperature. More internal degrees of freedom tend to increase a substance's specific heat capacity, so long as temperatures are high enough to overcome quantum effects".
    0 0
  19. Bob 165

    That was exactly the point I tried to make earlier up the thread - that at very low CO2, where tropospheric absorbtion isn't significant, you'd expect increased absorbtion in the stratosphere, hence increased temperatures

    ie for x Co2 concn radiation coming through from surface in the CO2 window = (1-x)
    absorption in the stratosphere = x

    Total warming effect = x(1-x) ie heating at low conc, net cooling at higher concs
    0 0
  20. Tom:
    "The Ville @162, Bob's description is correct. The vibrational modes of atoms within a molecule enter into the equation as the heat capacity of the molecule, and not as temperature. This is discussed at the Wikipedia article on heat capacity"



    Thanks for that.

    I can see I have more work to do on a certain project.
    0 0
  21. VeryTallGuy - 161

    2) From your link "Since power is energy per unit time, the energy content of IR radiation can be indicated by its IR spectrum which is a graph of power density as a function of frequency." I'd suggest the unit time bit here is unnecessary and confusing - we are not looking at dynamics.





    The ordinate of the graph is "watts/meter square wavenumber".
    I think that you are suggesting that I leave the phrase "Since power is energy per unit time". Am I correct?
    If so, I'm tending to agree with you.

    Thanks,
    Bob
    0 0
  22. VeryTallGuy - 161

    I went too fast with post 171.

    This is what I meant to say:

    I think that you are suggesting that I leave the phrase "Since power is energy per unit time" out of my writeup. Am I correct?
    If so, I'm tending to agree with you.

    Bob
    0 0
  23. Hi All,

    Here is my latest iteration.

    John Cook is going to help me with some artwork in order to help explain the first mechanism.

    Bob



    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 which is caused by two mechanisms. One mechanism involves the conversion of translational energy of motion or translational kinetic energy (KE) into Infrared radiation (IR) and the other method involves the absorption of IR energy by CO2 in the troposphere such that it is no longer available to the stratosphere. The former dominates and will be discussed first. For simplicity, both methods will be explained by considering a model of a fictitious planet with an atmosphere consisting of CO2 and an inert gas such as nitrogen (N2) 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 CO2 will be 100 parts per million (ppm) and will be increased to 1000 ppm. These parameters were chosen in order to generate graphs which enable the reader to easily understand the mechanisms discussed herein. A short digression into the nature of radiation and its interaction with CO2 in the gaseous state follows.

    Temperature is a measure of the energy content of matter and is indicated by the translational K.E. of the particles. A gas of fast particles is at a higher temperature than one of slow particles. Energy also causes CO2 molecules to vibrate but although this vibration is related to the energy content of CO2, it is not related to the temperature of the gaseous mixture. Molecules undergoing this vibration are in an excited state.

    IR radiation contains energy and in the absence of matter, this radiation will continue to travel indefinitely. In this situation, there is no temperature because there is no matter.

    The energy content of IR radiation can be indicated by its IR spectrum which is a graph of power density as a function of frequency. Climatologists use wavenumbers instead of frequencies for convenience and a wavenumber is defined as the number of cycles per centimeter. Figure 1 is such a graph where the x axis indicates the wavenumber and the y axis indicates the power per square meter per wavenumber. The area under the curve represents the total power per square meter in the radiation.

    The interaction of IR radiation with CO2 is a two way street in that IR radiation can interact with an unexcited CO2 molecule and cause it to vibrate and become excited and an excited CO2 can become unexcited by releasing IR radiation.

    Consider now the atmosphere of our fictitious model. N2 and CO2 molecules are in motion and the average speed of these molecules is related to the temperature of the stratosphere. Now imagine that CO2 molecules are injected into the atmosphere causing the concentration of CO2 to increase. These molecules will then collide with other molecules of either N2 or CO2 and some of the K.E. of these particles will be transferred to the CO2 resulting in excited CO2 molecules and a lowered stratospheric temperature. All entities, including atoms and molecules, prefer the unexcited state to the excite state. Therefore, these excited CO2 molecules will emit IR radiation which, in the rarefied stratosphere, will simply be radiated out of the stratosphere. The net result is a lower stratospheric temperature. This does not happen in the troposphere because, due to higher pressures and shorter distances between particles, any emitted radiation gets absorbed by another nearby CO2 molecule.

    In order to discuss the second and less dominant mechanism, consider Figure 1 which shows the IR spectrum from a planet with no atmosphere and Figures 2 which shows the IR spectrums from the same planet with CO2 levels of 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. As previously stated, these parameters were chosen in order to generate graphs which enable the reader to easily understand the mechanism discussed herein.



    Figure 1. IR Spectrum - No Atmosphere



    The curves of Figures 2 approximately follow the intensity curve 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 Figure 1. These wavenumbers represent the atmospheric window which is so named because the IR energy radiates through the atmosphere unaffected by the CO2.



    Figure 2. CO2 IR Spectrum - 100/1000 ppm



    A comparison of the curves in Figure 2 shows that the absorption band at 1000 ppm is wider than that at 100 ppm 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 than at a level of 1000 ppm. Therefore, the stratosphere is cooler because of 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.

    In concluding, this paper has explained the mechanisms which cause the troposphere to warm and the stratosphere to cool when the atmospheric levels of CO2 increase. The dominant mechanism involves the conversion of the energy of motion of the particles in the atmosphere to IR radiation which escapes to space and the second method involves the absorption of IR energy by CO2 in the troposphere such that it is no longer available to the stratosphere. Both methods act to reduce the temperature of the stratosphere.

    *It is recognized that a fictitious planet as described herein is a physical impossiblity. The simplicity of this model serves to explain a concept that would otherwise be more difficult using a more complex and realistic model.
    Copyright 2010 - Robert J. Guercio
    0 0
  24. Bob

    thats a good article... one nit pick ;-) Where you say

    "K.E. of these particles will be transferred to the CO2 resulting in excited CO2 molecules and a lowered stratospheric temperature"

    "I" would say, "K.E. of these particles will be transferred to the CO2 resulting in excited CO2 molecule, which will radiate this energy away, resulting in a cooler stratosphere etc etc."

    Overlaying the graphs was a good idea. There are also one or two places when you say "atmosphere" when you are obviously talking about the stratosphere.

    Are you going to get a pro to have a nosey? Science of Doom may be worth asking to give it a quick look over if you are looking, he knows his stuff.
    0 0
  25. Joe Blog - 174

    We discussed the big issues and only nitpicking is left. I want nitpicking and your wording is much better than mine was. Thank you.

    I want very much to get a pro. I corresonded with Rasmus Benestad who is one of the pros on Real Climate. He said he hadn't thought about all of this since graduate school but gave me some ideas including the need to combine those graphs. He also recommend two scientists from Real Climate to send it to.

    So I'm not going to just send a new email. I'm going to forward his mail to me recommending the scientist. That will pretty much force him to critique it.

    I combined the graphs by painting pixel by pixel. Laborious is a word that comes to mind.

    John is going to do some artwork for me. Something to clarify the first process.

    Thanks again.

    Bob

    Bob
    0 0
  26. Bob, I have two points, and I will start with your second explanation first.

    Below is an image of the outgoing emmissions spectra over the Sahara, the Mediterainian, and over Antarctica:



    You will notice the small spike at the center of the CO2 absorption/emmision pattern in each case. That spike represents emissions by stratospheric CO2, which being warmer than stropospheric CO2, has a higher brightness temperature. The important point to notice is that the spike is confined to the center of the CO2 absorption/emmision band.

    Looking at your figure 2 brought my attention to the fact that the majority of excess absorption in the troposphere with increased CO2 takes place on the wings of the band, not at the center. It appears from your figure 2 that there is no reduction in CO2 absorption at the center of the band. But because it is at the center of the band where stratospheric CO2 emits and absorbs, it follows that the reduction in IR on the wings of the band will have no tendency to cool the stratosphere.

    Clearly, in the non-equilibrium state, adding extra CO2 will reduce the brightness temperature at the center of the CO2 band as well as at the wings, and will consequently reduce stratospheric temperatures. But as the troposphere achieves radiative equilibrium, it may be that the loss of IR radiation on the wings of the band undercompensates for, exactly compensates for, or over compensates for the increased emmisions outside that band due to increased surface temperature. In the first case, the equilibrium brightness temperature at the center of the CO2 band will be less than it was before introducing more CO2, thus cooling the stratosphere. In the second, it will have no effect; and in the third it will slightly warm the stratosphere. As to which case will actually apply, you will have to ask an expert; and it may be the models insufficiently clarify the situation. For practical purposes though, it appears that the cooling of the stratosphere due to the second method is a temporary effect, which declines to close to zero as the atmosphere achieves radiative equilibrium.

    (As an aside,the emission spectrum for Antarctica is especially interesting; showing, as it does, that the tropospheric CO2 was warmer than the surface. In the situation at the time of this observation, the effect of CO2 in the atmosphere would have been to cool the surface of Antarctica, rather than to warm it. ;) )

    On to the first method:

    Where you say, "... this vibration is related to the energy content of CO2, it is not related to the temperature of the gaseous mixture", this is not strictly correct. The energy stored as vibration is not measured by the temperature, but there is an equilibrium relationship between the heat stored as molecular vibrations and the temperature of the gass. The actual relationship varies from gas to gas, and depends of the degrees of freedom of their vibrational modes.

    Because the relationship between heat stored as vibration, and heat stored as translation energy, adding more CO2 at the same temperature will not cool a gass (ignoring considerations of pressure and volume), for the added CO2 will have the same proportion of energy stored as internal vibrations. Adding a cooler or warmer amount of CO2 will, of course, temporarilly cool or warm the stratosphere, but the stratosphere will quickly return to equilibrium.

    What is happening in any gass is that the energy stored as vibration interacts with, and seeks to achieve equilibrium with two sources of energy. The first is the energy from collisions within the gass, which is a function of temperature. The second is the radiant energy it emits (which is a function of its temperature) and recieves (which is a function of the temperature of the source of the radiation it captures). If the temperature of the gas is less than the temperature of source of its radiant energy, its the energy it radiates will be less than that which it recieves, increasing its vibrational energy. This excess will then be passed onto the ambient gass, increasing its temperature. If the radiant energy it recieves has a lower "temperature" than the ambient gass, its will emit more energy than it recieves, draining its pool of vibrational energy. This shortfall will then be made up by collisions with other gass molecules, cooling the ambient gass.

    Applying this to your model, and assuming all energy transfers are radiant, the effect is that the stratospheric gass will reach an equilibrium temperature equal to the brigtness temperature of the tropospheric CO2. If the stratosphere were cooler than that, than the stratospheric CO2 would be a net absorber of radiant energy, thus warming the stratosphere. If it were warmer, the CO2 would be a net emitter, thus cooling the stratosphere. (In reality, the temperature would be determined by convection and the adiabatic lapse rate, which would dominate at stratospheric altitudes were it not for a major source of radiant energy to those levels.)

    So, once again, I come back to Ozone. where it not for Ozone being a net absorber of energy in the stratosphere, CO2 would not be a net emitter of energy in the stratosphere. And it is only by being a net emitter that CO2 can cool.
    0 0
  27. Bob @173,

    you asked for nit-picking so here goes.

    "The former dominates"
    Are you sure - I'd like to see a figure put on this to judge it personally.

    "Energy also causes CO2 molecules to vibrate but although this vibration is related to the energy content of CO2, it is not related to the temperature of the gaseous mixture."
    If (big if) I have understood correctly, then this isn't quite right. As temperature rises, a larger proportion of CO2 molecules will enter the higher vibrational states and preferentially emit IR in those frequencies. And this is the mechanism by which stratospheric cooling happens - conversion of thermal energy to IR by CO2. So whilst temperature does not measure vibrational energy, increased temperature does cause more CO2 molecules to enter excited states and hence emit IR. (I think)(Tom makes a similar point @176)


    "IR radiation contains energy and in the absence of matter, this radiation will continue to travel indefinitely. In this situation, there is no temperature because there is no matter."
    I think this is just confusing - all it amounts to is saying that vacuum has no temperature, and light can travel through it. Can you just cut this para?

    "These molecules will then collide with other molecules of either N2 or CO2 and some of the K.E. of these particles will be transferred to the CO2 resulting in excited CO2 molecules and a lowered stratospheric temperature. All entities, including atoms and molecules, prefer the unexcited state to the excite state. Therefore, these excited CO2 molecules will emit IR radiation which, in the rarefied stratosphere, will simply be radiated out of the stratosphere. The net result is a lower stratospheric temperature. This does not happen in the troposphere because, due to higher pressures and shorter distances between particles, any emitted radiation gets absorbed by another nearby CO2 molecule."
    A suggestion to simplify: "Due to the lower density of the stratosphere, IR is less likely to be reabsorbed, and more likely to escape either to space or back to the troposphere. With increased CO2 concentration, IR emission increases in the part of the spectrum where CO2 dominates. Thermal emissions in the rest of the spectrum must reduce to maintain overall equilibrium, which results in a lower temperature to maintain the heat balance."

    Tom Curtis @176
    "where it not for Ozone being a net absorber of energy in the stratosphere, CO2 would not be a net emitter of energy in the stratosphere. And it is only by being a net emitter that CO2 can cool."
    I think this may be incorrect. I think that CO2 will increase the emissivity of the stratosphere regardless of ozone and therefore decrease the temperature. However, without Ozone, the temperature of the stratosphere would be much lower, as the UV energy absorbed by Ozone would then be transmitted. The relative effect of extra CO2 would, though, I think reduce the temperature still further.

    And I'd just like to emphasise I'm no expert in this and could be very much mistaken in my analysis. Interesting to try and understand though.
    0 0
  28. VeryTallGuy @177, I have thought something similar in the past. Your suggestion is not correct, though, because absorptivity increases with emissivity, so that if you double stratospheric CO2, you double the amount of IR from tropospheric CO2 absorbed. You also double the the amount emitted. Obviously, if the stratospheric CO2 is warmer than the tropospheric, doubling both absorption and emmission will result in a larger increase of emission, thus cooling the stratospheric CO2. If the stratospheric CO2 is cooler, doubling both will result in a greater increase in absorption, thus cooling the stratospheric CO2. In the respective cases, the CO2 was warming (cooling) the stratosphere already, and just does so at an increased rate.
    0 0
  29. OK, I think I've got a good way to show this, although I've also found some more of my own limitations.

    I've had a play with the MODTRAN model interface available online: http://geoflop.uchicago.edu/forecast/docs/Projects/modtran.html
    and tried the following.

    I used the defaults (tropical, no clouds, zero ground offset temperature.

    I ran the model with 100ppm CO2, altitude 20km (ie at the tropopause), looking UP (ie at stratospheric emissions coming back) then again with 1000ppm.

    The results are striking. The increase in emissivity is clearly visible, and total IR from the stratosphere increases from 6.6W/m2 to 11.0W/m2. What I don't understand is that the model produces the same temperature profile at both concs, but this clearly shows that for increased CO2 and constant temperature, IR emission increases, ergo in order to maintain heat balance you'd expect the temperature to reduce.

    Someone who understands MODTRAN better can probably get the temperature profile to work properly too.

    Bob - this sort of graphical output, once correct, could be used to very effectively demonstrate the effect of CO2 in the stratosphere, I think.

    Apologies, my html isn't up to pasting the graphics straight in here.
    0 0
    Moderator Response: [Daniel Bailey] 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. Should work for you now
  30. Very Tall Guy 179

    Most of your comments are about my writing and I'm going to digest them and change my writeup if necessary.

    Regarding this comment:

    "Energy also causes CO2 molecules to vibrate but although this vibration is related to the energy content of CO2, it is not related to the temperature of the gaseous mixture."

    I think it would have been more accurate to say ""Energy also causes CO2 molecules to vibrate but although this vibration is related to the energy content of CO2, the temperature is not a function of this vibrational energy. The temperature is only a function of the translational K.E."

    Does this take care of your issue?

    Bob
    0 0
  31. Tom Curtis

    Ah, yes, I realise now that the total power output of the CO2 is limited by it's temperature ie it can't emit more than the equivalent blackbody in its emission bands. So your argument goes that unless there is another factor heating the stratosphere, to raise the CO2 temperature, then CO2 will actually absorb more CO2 from the troposphere than it will emit.

    In which case, how about this for a really simple explanation:

    1) The stratosphere is heated by UV absorption in the ozone layer.

    2) Due to low density, radiative heat transfer rather than convective heat transfer dominates.

    3) At a given temperature, emissivity of the stratosphere increases as CO2 concentration increases, and at given CO2 conc, total IR emission from the stratosphere increases with temperature.

    4) Therefore the temperature must fall to compensate if CO2 rises to keep the overall heat balance satisfied.

    The issue of the spectrum of IR coming up from the troposphere is then pretty much irrelevant I think?
    0 0
  32. Bob @180

    Yes, I think that's more accurate.

    Tom @ 180 has put forward what seems to me like a pretty solid case that you need an external heat source (ie UV absorption) to effect cooling with higher CO2.
    0 0
  33. VeryTallGuy @179, Modtran is a radiative transfer model only. That means it calculates the emission and transmission for various atmospheric conditions, but does not adjust those conditions to establish equilibrium. In the version at David Archer's site, you can make a partial equilibrium adjustment for outgoing radiation by adjusting the ground temperature offset until the Iout matches the original value. However, it only adjusts ground temperature so it is not entirely accurate, and cannot (by that means) adjust for equilibrium in the stratosphere.
    0 0
  34. VeryTallGuy @181, that is the first explanation in a nutshell, except ...

    My quibble is with (2). Convection doesn't work in the stratosphere because the temperature increases with altitude in the Stratosphere, not because of the low density.

    Also, Bob's second explanation is also a significant, if possibly transient effect.
    0 0
  35. Tom @183

    On Modtran, that's what I assumed - I tried adjusting the ground temperature in exactly the way you suggest, but it doesn't affect the atmospheric profile.

    If not by thermal equilibrium, how does it calculate the temperature gradient through the atmosphere and why doesn't CO2 concentration change the temperature profile at all?

    On radiative vs convective heat transfer - I'd thought about the inversion limiting convection as well but I'd also assumed that the reduction in density would be sufficient to significantly reduce heat transfer by turbulent mixing.

    Thank you for your time btw, I've learned a lot.

    Now, those pics, the red line is effectively the total stratospheric emission I think:
    20km altitude, looking up, 100ppm CO2, total emission 6.6 W/m2


    And again with 1000ppm, total emission 11.0 W/m2



    You can clearly see the increase in stratospheric emissions as CO2 rises. Ergo in equilibrium you'd expect T to decrease.
    0 0
  36. VTG @185, from trial and error, I have figured out that it only adjusts the ground temperature. It uses several standard temperature profiles, which you choose by selecting "tropical" etc. It doesn't calculate the temperature at all. Rather, it determines the radiation out given a specific temperature profile and atmospheric composition. If you read the paperwork, you will see that the version available at David Archer's site is one from the early 1990's. I don't know the capabilities of more recent models.

    If you want to calculate equilibrium temperatures, you should probably use the NCAR model at the same site. I have not played with it very much, so I cannot advise you except for two caveats: it also is an obsolete model, and it does not handle cloud albedo well, giving non-physical results for significant change in cloud cover.

    Regarding convection, as convection operates in the mesosphere, presumably it would also operate in the stratosphere without the inversion. You may find these lecture notes a usefull summary of basic facts about the upper levels of the atmosphere.
    0 0
  37. Tom Curtis at 02:05

    "Convection doesn't work in the stratosphere because the temperature increases with altitude in the Stratosphere, not because of the low density."

    As i understand it, convection stops working in the upper troposphere, because radiative transfer moves energy to quickly, preventing a build up/heating of a layer above that above it, the inversion between the troposphere and the stratosphere would prevent convection. But it is the result of the opacity reaching a level where radiation is the dominant means of energy transport... and the opacity is the result of the decreasing pressure. obviously higher up in the stratosphere convection once again plays a role, from the O3 absorption.

    Interesting points. Particularly about number two.
    0 0
  38. Joe Blog 187

    Convection occurs occurs when the air above is cooler than the air below. In the stratosphere the temperature increases with height. Therefore convection can not occur there. The cooling due to increased levels of CO2 nevertheless continues into the next layer (Mesosphere) becoming even more pronounced.
    I Don't buy Bobs 2nd explanation as being relevant. I am convinced that the effect is too small and is overwhelmed by the warming that occurs at the top of the troposphere due heat being dumped there from the condensation of water vapor. See the troposphere hot spot on this site for more info on how this works.
    0 0
  39. mars at 11:20 AM

    I dont doubt what you say. What i wrote was a lil messy... this

    "preventing a build up/heating of a layer above that above it"

    Was meant to be, that reducing opacity, means that more energy is moved through radiation, resulting in less and less energy being trapped in the levels as you rise through the troposphere, which would mean radiation becomes more and more dominant, and convection less and less with reducing pressure as you rise through the atmosphere... but in the upper stratosphere, due to O3 absorption, convection once more plays a role. Because as you say its due to the T differential.. a thread at Science o Doom is discussing this very issue at the moment.

    But what i was getting at, was that the inversion between the upper troposphere and lower stratosphere, is due to the opacity, which is a result o the pressure & concentration of GHG's. The inversion itself is proof of this, if it was opaque, you would expect an adiabatic T profile up until the energy could escape... in the upper stratosphere.
    0 0
  40. Joe Blog at 189

    Quote
    "But what i was getting at, was that the inversion between the upper troposphere and lower stratosphere, is due to the opacity, which is a result o the pressure & concentration of GHG's. The inversion itself is proof of this, if it was opaque, you would expect an adiabatic T profile up until the energy could escape... in the upper stratosphere."
    End quote

    The inversion is simply caused by the fact that the formation of ozone causes atmospheric heating. Thus preventing further convection. Inversions are common in the atmosphere and are the result of pools of warmer air forming above. The mechanism is usually due to air descending and heating adiabatically.
    0 0
  41. mars at 13:11

    Yes and no.... if the atmosphere had a layer that absorbed all the SW high up, and we had an optically thick atmosphere, the T profile should be isothermal... We have inversions here at the surface, frosts for example, they are the result of increased transparency, through low humidity/ no clouds, and still air, to prevent the mixing of the layers. So it opens the atmospheric window, and radiation passes up from the surface, cooling the surface faster than the higher layers... this is how inversions happen, the energy must pass up through the higher layers without interacting with it.

    Because energy cannot be created or destroyed... and radiation is not directional. So if half of the energy from the stratosphere is going down, but cannot escape, why isnt its temperature moving toward equilibrium with its source?
    0 0
  42. Re VeryTallGuy
    185
    Re the temperature profile can not be changed in modtran
    Modtran assumes that the atmosphere contains oxygen and therefore calculates its effects on the stratosphere via the UV spectrum. So even if you put Zero tropospheric ozone into the model you will still get get a result calculated for the stratospheric ozone. The give away is in the graph for a planet with supposedly no atmosphere where we have dip in emissions around wave number 1300 due to ozone. In fact this is graph for a planet with an atmosphere the same as earths but with no CO2 or water vapor. Whether this invalidates Bobs whole argument or not I don't know but it does explain the temperature profile.
    0 0
  43. Joe Blog @187, my opinion on the tropopause is based more on my personal reading of the admitedly limited evidence that I have rather than reading any expert on the matter. Consequently, of course, I could be entirely wrong, and if you (or anyone else) could reffer me to an expert disussion of the issue, I would be very appreciative.

    However, consider the following representation of the thermal profile of the atmosphere:



    If the tropopause was the location where thermal radiation moved energy faster than convective processes could, then we would expect the effects of that transition to be even greater in the mesosphere, where collisions are fewer and radiation less likely to be absorbed. Therefore, in the mesosphere, we would expect a larger heat differential between layers would be required to drive convection. We would also expect convection to cease being effective at a higher temperature than is the case in the troposphere.

    Neither of these is the case. Looking at the diagram, we see that there is a steeper (less temperature difference per unit altitude) lapse rate than in the troposphere, contrary to our expectations. We also see the temperature at which the lapse rate inverts and convection ceases is 30 degrees C less than that at the tropopause. In both cases, we see less energy available to drive convection than exists at the tropopause still managing to drive convection, and this despite the fact that radiation is even more important as a heat transfer mechanism at this level than at the tropopause.

    We also see a general pattern of increased surface temperature raising the tropopause, which we would expect if the inversion is the cause of the cessation of convection, rather than its consequence. This even applies to the consequences of global warming. Increasing CO2 concentrations increases the importance of radiation as a heat transfer mechanism. Therefore, if you are correct, we should expect increased CO2 to result in a fall in the altitude of the tropopause. In contrast, if the tropopause is a consequence of differences in heat with altitude, then increasing surface temperature should raise the tropopause, which is what we see.

    I do believe the increased relative importance of radiaton as a heat transfer mechanism is relevant. Specifically, by allowing heat to be transfered over long distances, it allows a thick tropopause because air at that altitude can be heated by warmer air several kilometers above and below it. But without the warmer air above, it seems to me that no tropopause would develop.
    0 0
  44. Tom Curtis at 17:16 PM

    There is nothing you have written i really disagree with, the inversion is the reason for the cessation of convection, yes. Why is there an inversion? This is what i was getting at, if the atmosphere was optically thick at those altitudes, the downward radiation from the warmer higher layer, would heat the layers below, until they were in equilibrium. And with the continuing up ward flux from the top of the troposphere, it would revert to an adiabatic profile.

    If you add co2, it increases opacity, decreases radiative efficiency, meaning that you have to go higher, to a lower pressure, where the opacity will enable the transfer of energy...raising the tropopause... we agree on that as well.

    We are talking past each other, its a question of cause and effect, if there was no O3 absorption of UV, we wouldnt have an inversion, if the air above the tropopause was optically thick, but we had O3 absorption, we also would not have an inversion.

    There was a good conversation on one of the venus threads over at science of doom, that loosely covers this.
    0 0
  45. Joe Blog @191, I think I have completely misunderstood your point. Consequently, much of my discussion in 193 is irrelevant, so feel free to ignore it.

    If I have correctly understood your point now, it is this:

    A parcel of warm air in the lower atmosphere is surrounded by other parcels of air of similar temperature. Because the atmosphere is opaque to 15 micron radiation at that level, heat it radiates away is matched by heat absorbed from the neighbouring parcels of air. Consequently cooling by radiation is very inefficient at best, and the only way for the parcel of air to cool is to rise through the cooler air above it, thus driving convection.

    In constrast, once the parcel of air reaches about 8 km altitude, approximately half of the heat it radiates away radiates to outwards. In return, it receives very little energy from that direction, so radiation now effectively cools the parcel of air. This cooling deprives it of the ability to continue to rise by convection.

    Is this a fair statement of your view? And what corrections would you make, if any?
    0 0
  46. Tom Curtis at 17:41

    exactly ;-)
    0 0
  47. As a glider pilot I spent many hours studying the weather and using convection as a source of lift. I am fairly sure Tom is right as above.
    0 0
  48. Tom the parcel of air in your example cools adiabatically (if non saturated) at a rate of 9.8 deg c per 1km and can rise at rates 300 meters per min. The cooling due to radiation is too slow and is normally ignored in weather forecasting of thermal heights.
    0 0
  49. mars at 18:05 PM

    The point is, if the atmosphere was totally transparent, there wouldnt be convection... because radiation would simply pass through the atmosphere without interacting... So at low altitudes, where the atmosphere is opaque, convection is the dominant means of energy transfer, but as you rise through the troposphere, radiation, moves more and more of this energy, and less is moved through convection. Because the path length for the LW shortens. Put very simply, because there are less and less radiatively opaque molecules per volume.

    in the stratosphere, the atmosphere is opaque to UV, but less so to LW, so it does heat, and the upper levels do convect, but LW cooling from below causes the inversion, preventing convection.
    0 0
  50. Joe Blog 199 18:27 PM
    The atmosphere at low levels is transparent to nearly all of the incoming radiation. The atmosphere is heated by contact with the surface. As a result a parcel of air close to the ground forms which is warmer than the surrounding air. This parcel then expands making it less dense than the surrounding air and it starts to ascend cooling adiabatically. Simple put in the case of dry air it will cool at the rate 9.8 C per KM but the environmental lapse rate will be less than this so at some height the parcel will reach equilibrium with the local air mass and the parcel will stop rising. Thus heat from the surface is moved into the troposphere. This process is only marginally impacted on by radiation transfers. I hope we are not straying to far from the main topic here.
    0 0

Prev  1  2  3  4  5  Next

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.



The Consensus Project Website

THE ESCALATOR

(free to republish)


© Copyright 2019 John Cook
Home | Links | Translations | About Us | Privacy | Contact Us