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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 1 to 50 out of 247:

  1. Nice article here. I have been looking at this issue as well recently. One thing I would argue is that the tropopause would be much less defined (or possibly not exist) in this situation. The tropopause on Earth is impacted by water vapor and in this case water vapor would not be an issue.

    As the density of the atmosphere dropped, less energy would be needed to cause a comparable amount of temperature increase. I really don't see how the tropopause would develop in this situation unless full saturation was reached.

    It is an interesting situation. Also. Curious if you have run a model with just nitrogen. The atmosphere would warm from convective transfer, but after that the model gets odd and I am still sorting through it.

    This is fun science.
    John Kehr
    The Inconvenient Skeptic
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  2. Thanks John,

    It is a completely artificial and impossible situation. However, my objective was to make it understandable and to do that I had to keep it very simple.

    Also, the nitrogen is not necessary. People understand parts per million but the concept of an equivalent CO2 vapor pressure to that of 100 ppm and 1000 ppm would have made it more complicated.

    I read other explanations on the Internet explaining this but I could not understand any of them. It drove me crazy and then I had one of those Eureka moments when I realized what was going on. It is really not explainable without considering the absorption spectrum.

    I suppose for simplicity, I cannot allow convective transfer:):)

    Bob
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  3. Bob,

    I agree in keeping it simple for this. It would be interesting to see what would happen with the temperature profile with the model. I tinkered with the one in the link and didn't see anything that would generate a temp profile as a response.

    I am curious about the convection only model. Nitrogen would have to dissipate heat at the top, but I am curious as the method. Say a planet with nitrogen density comparable to Venus. What would happen there? I don't know, but I am thinking about that model. That is why I ask.

    Hope to see more like this one.
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  4. This is a nice simple model, explains the absorption for us non-experts.
    I have reviewed some old posts and found one under "climate's changed before" with a question that I have also wondered, from jebjones42. It did not seem to have been addressed in any subsequent posts.

    "I'm curious. Do we know what caused the reversal in past warm periods in the Earth's history? What made it get cool again? Clearly, despite CO2 having a positive feedback loop, we didn't get runaway warming. We're not living on Venus.

    Even if we're headed for higher temps, rising sea levels, drought, mass extinctions, catastrophic loss of human life, etc. At some point won't it top out an head back to another ice age?

    What's prevented a runaway greenhouse effect in the past?"

    I am also interested in this question. Is it sun cycles and precession/sun obliquity, and is the prevailing thought that we will overwhelm these historical cyclical temp. changes ?

    I apologize if I am posting this in the wrong place. I am new to blogging.
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  5. This is a tough problem that I have long struggled to grasp and I thank you for trying to explain it and for provoking me to think about it some more. However, I can't help thinking that your model is a bit too simple to allow you to conclude that increasing CO2 is the only possible mechanism to explain stratospheric cooling.

    For one thing, I'm not sure that there even would be a stratosphere (ie with temperatures increasing with height) if there was no oxygen/ozone in the atmosphere as there is in your model. So it may not make sense to talk about a warmer lower atmosphere causing an even cooler upper atmosphere in such a simplified case.

    Also, I understand that other variations, in water vapour, volcanic aerosols, chlorofluorocarbons and methane concentrations, can cause temperature changes in the stratosphere.

    Having said that, I don't actually doubt that rising CO2 does result in a cooling stratosphere, I'm just struggling to understand how exactly and by how much.

    There's a helpful article by some German scientists here: They conclude: We now know that stratospheric cooling and tropospheric warming are intimately connected and that carbon dioxide plays a part in both processes. At present, however, our understanding of stratospheric cooling is not complete and further research has to be done. ....
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  6. It's late at night and so much has been mentioned above but other things do cause cooling. The ozone layer has thinned out and ozone absorbs incoming solar energy which causes the stratosphere to warm up. As I understand it, ozone is responsible for the stratosphere and for the temperature inversion of the stratosphere. Since there is less ozone, less incoming solar radiation is being absorbed. Less solar energy being absorbed means that it is cooler.

    As I understand Venus, the runaway greenhouse effect on Venus was caused by evaporation of water. The sun then broke the water molecules apart and the hydrogen escaped into space. So the water is gone forever. I believe that CO2 was a feedback as it has been on earth for the past million or so years. Of course the contrarians use this to imply that CO2 is not a problem.

    This cannot happen on earth because earth's atmosphere is sufficiently cool such that water vapor condenses and rains back to earth.

    I'm saying a lot here but I must stress that I am very much an amateur at this and may not be totally correct.

    I'm also a bit tired and may not be writing very clearly.

    Bob
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  7. Bob
    Thermal energy flows are determined by temperature gradients. In the atmosphere there are two sinks that collect the aforesaid flows to radiate them to space: the tropopause and the mesopause. If the atmosphere has to dispose of more energy it will have to increase the temperature of its sinks. Then no cooling of the stratosphere but warming of tropopause/mesopause and drop of the lapses rates in the whole atmosphere.
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  8. Re: reluctant skeptic (4)
    "Do we know what caused the reversal in past warm periods in the Earth's history? What made it get cool again?"
    Off-topic, but deserving of an answer. Just ran across this brief, but apt, summary from Bob (Sphaerica) over on RC. Check it out.

    As far as the runaway effect, see here.

    As far as heading back to another ice age:
    "Our research shows why atmospheric CO2 will not return to pre-industrial levels after we stop burning fossil fuels. It shows that it if we use up all known fossil fuels it doesn't matter at what rate we burn them. The result would be the same if we burned them at present rates or at more moderate rates; we would still get the same eventual ice-age-prevention result.

    Ice ages occur around every 100,000 years as the pattern of Earth's orbit alters over time. Changes in the way the sun strikes the Earth allows for the growth of ice caps, plunging the Earth into an ice age. But it is not only variations in received sunlight that determine the descent into an ice age; levels of atmospheric CO2 are also important.

    Humanity has to date burnt about 300 Gt C of fossil fuels. This work suggests that even if only 1000 Gt C (gigatonnes of carbon) are eventually burnt (out of total reserves of about 4000 Gt C) then it is likely that the next ice age will be skipped. Burning all recoverable fossil fuels could lead to avoidance of the next five ice ages."
    Source here

    [Sorry, Bob, for going so far OT]

    The Yooper
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  9. Interestingly, the article by the German scientist is what drove me crazy. He correctly explains why cooling of the stratosphere takes place and explains that heat is trapped in the troposphere. This is correct but suppose the CO2 level were to miraculously stabilize at 390 ppm. The earth would continue to heat up until the IR leaving the troposphere into the stratosphere was the same as before all this started. This has to be because of conservation of energy. The total solar energy coming into the troposphere from the sun must equal the total IR energy leaving the troposphere in the steady state. I asked the question "Why wouldn't the temperature of the stratosphere go back to what it was originally since the same amount of IR energy is leaving. The German scientist doesn't address this. The reason that it doesn't go back to the original temperature is that there is now more IR energy in the atmospheric window and less in the absorption band and only the energy in the absorption band can can react with the CO2 in the stratosphere and the absorption of IR by CO2 is what heats up the stratosphere. Less absorption means a lower temperature permanently.

    By the way, I wasn't concerned about the cooling due to thinning of the ozone layer because that is easy to understand. It's much more difficult to understand how CO2 causes cooling of the stratosphere.

    Is this making sense? I'm already thinking about revising this post into two very distinct parts. First, the steady state solution which my post addresses and second, getting to that steady state solution which I'm talking about here.

    Bob
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  10. (4) reluctant skeptic: it depends on a number of things. Very long term changes can be understood pretty well in terms of things like the carbonate silicate cycle.

    In summary, 'short term' feedbacks seem to be net positive - if you warm Earth you get some melting ice and more water vapour and more heating etc, although it is diminishing returns so you don't get a runaway effect. But longer term changes can swing us back the other way: more CO2 in the air means it dissolves more quickly in the oceans and silicate rocks weather more quickly at higher temperatures too, so they suck down CO2 as well.

    As CO2 drops away, the positive feedbacks amplify this cooling: cooler air holds less water vapour and cooling lets ice expand again.

    It's a very complex picture and it's different at different timescales, but the carbonate-silicate cycle is a great explanation for lots of geological observations so it's a good place to start! (even if it's not particularly relevant on 'short' timescales like the past few hundred thousand years)
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  11. Regarding the change in spectra from 100 to 1000 ppm, the center bands in the CO2 spectrum appear to be saturated, with all of the change coming in the peripheral bands. If the central bands are saturated, what is going on with the scale on the y axis - shouldn't it drop to zero in the center of the spectrum?
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  12. Bob, one thing which this doesn't explain (and I'm not entirely clear on) is why the extra energy in the atmospheric window doesn't result in the stratosphere maintaining the same temperature. That is, even though CO2 is blocking the passage of a band of IR energy the total energy in and out must be equal. So the same amount of energy is passing through the stratosphere (in each direction) with or without the CO2 there. So why is the stratosphere cooler? Because the bandwidth distribution has changed?
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  13. CBDunkerson

    The frequencies of the IR in the atmospheric window do not react at all with CO2. It's like light going through glass.

    Yes. In one case you have extra energy in the atmospheric window (higher tropospheric temperature/lower stratospheric temperature) and in the other case you have extra energy in the absorption band (lower tropospheric temperature/higher stratospheric temperature).

    If you had IR energy only in the atmospheric window, it would not heat up the CO2 at all.

    Bob
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  14. oamoe:

    I assume you mean the trough should be at zero??

    No. The trough bottoms out at an imaginary curve (not shown) that represents the temperature at the top of the atmosphere.
    Effectively you would have two upper and lower limit curves, the higher one represents T at ground level, the other T at the top of atmosphere. The spectrum curve (for someone observing the planet from space and looking down) will be something in between the two, depending on what gases you include, with chunks taken out at different wave lengths.

    It would have been useful if Bob had shown the upper (ground) and lower (TOA) limits.
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  15. Oamoe,

    My understanding is that the bandwidth, if I could use this word, is not precisely defined. At 1000 ppm the width is wider than at 100 ppm and the center of these two bands with different bandwidths is saturated in both cases.

    Bob
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  16. Everything we are talking about except "Why the stratosphere is cooler" is explained in the videotaped course given by David Archer at http://geoflop.uchicago.edu/forecast/docs/lectures.html

    The two blackbody levels, band saturation, CO2 weathering is all there.

    I recommend it highly. I also recommend the book although it has quite few typos.

    Bob
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  17. Actually oamoe, if you think about it. If the trough did go down to zero, it would mean no IR was escaping the planet.

    The graphs are of emissions, so if a part of the spectrum dropped to zero, then it would mean that part of the spectrum was being absorbed by the planet but never emitted, which wouldn't be very nice!
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  18. Bob Guercio

    This post provides one deserves a true science because it does not translate or approaches the climatic reality of our planet's atmosphere to exclude of the study, the most important component, "steam".
    The water vapor is part of the mechanism of convective transport of heat from the sun.
    He is able to pierce the "blanket" of greenhouse gases, leading over 50% of heat to the top of the troposphere, above this "blanket called CO2." Change the heat from troposphere to the stratosphere

    See:
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  19. Thanks Bob, this is a brave attempt to explain a surprisingly complex subject.

    As I understood it, the post is coherent with the simplified version of the explanation that can be found in the NOAA Global Warming FAQ:

    An enhanced greenhouse effect is expected to cause cooling in higher parts of the atmosphere because the increased "blanketing" effect in the lower atmosphere holds in more heat, allowing less to reach the upper atmosphere.

    I've seen other competent guys try to explain the concept before, with limited success: Gavin Schmidt at RealClimate and Science of Doom.
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  20. Ville,

    This could be the case if the temperature was at absolute zero where the spectrum is being measured and total saturation occurred. However, were that to happen, hypothetically of course, the temperature of the planet would increase allowing more IR energy to be in the atmospheric window. The area under that curve would still be the same or that of the energy of the incoming solar radiation.

    Just a minor point for precision. The graphs we are talking about represent energy but the y axis is actually power/meter squared wavenumber.

    Bob
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  21. Tarcisio,

    I kept it ridiculously simple in order to explain a concept. I thought about bringing ozone into it which is also responsible for stratospheric cooling but for a different reason which is easy to understand.

    Incoming solar radiation interacts with the ozone causing the stratosphere to heat up. The ozone has thinned recently so less solar energy is reacting with ozone, thus cooling the stratosphere.

    However, by bringing ozone into the picture, it gets more complicated.

    Bob
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  22. Alexandre,

    That's because, in my opinion, they try to make it too simple. They made it so simple that all they did was "handwaving".

    I sent my thoughts to RealClimate for confirmation and here is the email that I received from Gavin.

    Gavin's comment would have made my simple model more complex so I stayed with the simple model.

    Bob


    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.
    >
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  23. Let me add something to all this. The following passage came from:

    http://www.atmosphere.mpg.de/enid/20c.html
    _________________________________________________________________
    Cooling due to the greenhouse effect

    The second effect is more complicated. Greenhouse gases (CO2, O3, CFC) absorb infra-red radiation from the surface of the Earth and trap the heat in the troposphere. If this absorption is really strong, the greenhouse gas blocks most of the outgoing infra-red radiation close to the Earth's surface. This means that only a small amount of outgoing infra-red radiation reaches carbon dioxide in the upper troposphere and the lower stratosphere. On the other hand, carbon dioxide emits heat radiation, which is lost from the stratosphere into space. In the stratosphere, this emission of heat becomes larger than the energy received from below by absorption and, as a result, there is a net energy loss from the stratosphere and a resulting cooling. Other greenhouse gases, such as ozone and chlorofluorocarbons (CFC's), have a weaker impact because their concentrations in the troposphere are smaller. They do not entirely block the whole radiation in their wavelength regime so some reaches the stratosphere where it can be absorbed and, as a consequence, heat this region of the atmosphere.
    _________________________________________________________________



    This passage absolutely drove me crazy because I didn't understand it. I now understand it because I know that it is only part of the story.

    What bothered me was with regard to what happens after the earth heats up and reaches a steady state with a higher tropospheric temperature. Of course, this presumes stabilization of greenhouse gases but nevertheless, in the steady state the same amount of IR energy is going to be entering the stratosphere as before the CO2 concentration increased which caused the temperature to increase. This is necessary because conservation of energy requires that the solar energy entering the troposphere from the sun must equal the IR energy leaving the troposphere since the solar energy entering did not change. If the same amount of IR energy is entering the stratosphere from the troposphere, why doesn't the temperature of the stratosphere return to what it was before all this started.

    The reason is that more of this energy is in the atmospheric window which does not react with CO2 and less is in the absorption band which does react with CO2. So less in the absorption band than before means less energy reacting with the CO2 and a permanent lowered temperature.

    The stratosphere is also cooler because of the thinning of the ozone layer but this is relatively easy to understand and is explained in the referenced website.

    Bob
    __________________
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  24. Bob #23

    That part also helps me understand. More energy is now outside the absorption band of the stratosphere, and goes right through it.

    But that's that other part, about CO2 emiting more than absorbing, that I do not understand. I've seen this already somewhere else.

    In a similar version of the explanation, they say the CO2 absorption/re-emission helps "conduct" the heat upwards in the stratosphere, instead of trapping it.

    This part is frankly beyond my reach. Maybe it's something that's only fully understandable if you put together all the relevant physics and run the model... :-)

    In this case, we could only partially understand it with intuitive conceptual explanations.
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  25. As Gavin Schmidt mentioned above, by increasing the CO2 content of the atmosphere (in the stratosphere as well as the troposphere), the emissivity of the stratosphere (which is rather low due its low column density of gases) increases. The heating rate of the stratosphere depends on what energy it is able to absorb from below (the troposphere) and above (the Sun) in wavelength bands that its gases (mainly O3 and CO2) can efficiently absorb.

    From the point of view of the stratospheric gases that can absorb radiation from below, they see less radiation emerging from below due to the enhanced tropospheric opacity there (the effective emitting layer moves to higher altitudes where the T and thus thermal emission are lower). At the same time, the stratosphere can emit more efficiently with the enhanced CO2 content (the C02 column density there is small enough that very few wavelength bands are "saturated"). Thus the stratosphere T drops to re-establish heating(photo-absorption rate)--cooling(photo-emission rate) balance.

    I might be missing some of the details, but I think the above captures the gist of what happens.
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  26. Alexandre - 24

    In keeping with my ridiculously simple model:

    In the steady state the amount of IR entering the stratosphere equals the amount leaving it. Now CO2 molecules are constantly gaining and losing energy. Some gain, some lose, this goes back and forth etc. In the steady state, the number gaining at any instant equals the number losing.

    Now suddenly there is less IR energy coming into the stratosphere from the troposphere. At this time, energy at the initial rate is still leaving the stratosphere going into space but less is coming in. Not all the molecules that have lost energy can get replenished because the necessary energy is no longer there. The energy leaving must diminish to equal that coming in for the steady state.

    Does this help?

    Bob
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  27. Spaceman Spiff - 25

    I'm not 100% sure but I think you are saying only what other explanations on the web say. You must consider the absorption spectrum which includes the absorption band and the atmospheric window.

    Bob
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  28. Spaceman Spiff - 25

    With respect to Gavin's comment on emissivity and absorptivity, I think Gavin is saying that until the steady state is reached, the emissivity of the stratosphere is greater than the absorptivity because it is emitting more than it is absorbing. When the steady state is reached, the emissivity equals the absorptivity.
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  29. Guys,

    I'm doing a lot of talking here but let it be known that although I am educated in Physics, I am not an expert at what we are talking about. In fact, I have never been a scientist.

    I'm hoping that someone jumps in to corroborate all of this because I don't want to mislead.

    This is what I actually hate about the Internet. Often it's hard to tell fact from fiction and I find myself somewhat doing now what I always rail against.

    Bob
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  30. Hi Bob,

    You mentioned Gavin Schmidt, and that reminded me of this informative post over at RealClimate. Some of the content is relevant here.
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  31. Bob Guercio @27:

    Of course, it wasn't my intent not to add something substantial to understanding. However, the concept of an increasing emissivity of the stratosphere hadn't been mentioned in the above discussion.

    My post did not ignore the role of the absorption bands (or the complementary atmospheric window in your example) -- in fact they are key in describing the spectrum of radiation emerging from the troposphere and thereby the means by which the heating rate within the stratosphere changes. However, I should have clarified my point that the tropospheric emission diminishes within the C02 absorption band region center on 667 cm^{-1}.

    And, apparently, the increased emission arising from below the stratosphere over wavelengths corresponding to the ozone absorption band (centered on 1050 cm^{-1}) doesn't add enough to the heating rate budget of the stratosphere to offset the decreased heating rate there due to the diminished intensity of radiation within the main C02 band emerging from below. But it looks as though your example didn't want to consider this complication (?).

    In any case, I do appreciate your efforts!
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  32. Albatross - 30

    That's a fantastic writeup from RealClimate which is the cadillac of Climatology sites. If it's there, you can believe it.

    In any case, it's going to take me at least another reading to digest that. Did you look at Part II! It gets very heavy.

    Thanks for that recommendation.

    Bob
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  33. Bob Guercio @28:

    It is possible that the following may be an important consideration here: most of the stratosphere, unlike most of the troposphere, is not in local thermodynamic equilibrium (LTE). i.e., the rates of collisional processes do not exceed greatly spontaneous decay rates of energetically excited molecules, as they do within gases for which LTE is an excellent approximation.

    The heating rate within the stratosphere should be dominated by ozone dissociation via solar photons from above. However, by increasing the CO2 content of the stratosphere, you mostly increase the rate of that gas' ability to radiate energy away via collisional excitation followed by spontaneous photon emission. This tips the heating-cooling balance to lower temperatures.

    I know I've a few technical papers on the subject sitting on my laptop at home, and I'll try to remember to read them tonight.
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  34. Bob #26

    Yes, it does help. Thanks. I think I can put the pieces together a bit better, now, putting you post in the context, btw. It was a good basic explanation.

    Thanks Spaceman Spiff too.
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  35. Some further RealClimate articles worth reading

    Why does the stratosphere cool when the troposphere warms? (In which Gavin Schmidt shows that explaining this effect is not simple, even for him.)

    The sky IS falling (Which includes a link to the ESPERE article that I referenced in post #5 above)

    The wisdom of Solomon. (Discussing, among other things, the role of the small quantities of water vapour in the stratosphere.)
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  36. This 1950s video is fascinating, from the days when people knew how to explain things using animation:

    http://lasp.colorado.edu/igy_nas/flash_videos/theInconstantAir.html

    I think I got the link from Grumbines blog a few months ago.
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  37. Andy - 35

    It wouldn't be difficult for Gavin to explain if he was explaining it to a college sophmore Physics major. He would talk about the absorption spectrum as I have done.

    In my opinion, you cannot explain it without talking about the absorption spectrum which is what Gavin is trying to do in the referenced link.
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  38. Re OP It says:-

    "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"

    The stratosphere is caused by the warming effect of UV energy absorbed by O2 & O3; this couldn't happen in an atmosphere free of O2; no other atmospheric gas absorbs UV in the same way.

    This warming due to UV absorption produces a classical temperature inversion, suppressing convection; the result is the well known absence of storms etc. in the stratosphere. The absence of storms is one of the reasons why airliners fly there.
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  39. Bob,

    I remember seeing a completely different explanation for stratospheric cooling, and one which is easier to understand and makes more sense to me (as long as one understands what heat really is, and how energy is transferred between molecules in a gas).

    The gist of it was that in the more dense troposphere, a CO2 molecule is likely to absorb IR (in the appropriate band), and it is particularly likely to then collide with the more numerous O2 and N2 molecules (before simply re-radiating the energy away in the same band). In the collision, it transfers the energy (gained through IR) to the O2/N2 as translational kinetic energy, and in so doing heats the atmosphere.

    In the more rarefied stratosphere, CO2 is more likely to do the opposite, colliding with an O2 or N2 molecule, becoming excited by the collision (gaining vibrational energy), and then emitting the energy gained away as IR.

    Thus, increased amounts of CO2 warm the troposphere while cooling the stratosphere; they cause IR to primarily be an absorption mechanism in the troposphere, and an emission mechanism in the stratosphere.

    In this explanation, it's not merely the blocking of the CO2 IR band in the troposphere which causes the cooling, but rather an active effect of CO2 within the stratosphere itself.

    Can you reconcile the differences in the explanations? Is this description wrong? Or are they both accurate and true?
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  40. Sphaerica 39

    I have added a footnote explaining that the fictitious planet that I am using is a physical impossibility. I stated further that it would be difficult to explain this basic concept with a more realistic and complex model.

    Bob
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  41. In the above post, I responded to the wrong person. I meant to respond to damorbel
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  42. Sphaerica - 39

    I can't say that I understand the mechanism that you describe. Could you provide the source.

    It does seem odd that in a situation where you have two types of molecules all at the same temperature, one type of molecule is going to lose energy while the other gains energy. It seems to defy some fundamental law such as heat energy can only be transferred from a hot to a cold body and not from a cold to a hot body.

    Also, Gavin basically told me that I was correct in my explanation.

    Bob
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  43. I'm looking for it, but in the interim, the key to the molecules behaving differently comes from two basic factors. The first is that energy can be gained or lost through one of two mechanisms (in this model); absorption/emission of radiation, or through a collision. The second is the fact that the density of the atmosphere dramatically changes the relative probabilities and likely timings of the two events.

    In the more dense troposphere, collisions are more likely, and so likely that they will probably occur before a CO2 molecule gets a chance to emit its vibrational energy as radiation (but not always, and the reverse can happen, too).

    In the less dense stratosphere, collisions are highly unlikely, and so a CO2 molecule is more likely to emit its vibrational energy as radiation before colliding with another molecule.

    But I'll keep looking for the source. All I remember right now was that it included a fairly effective flash animation of CO2 and O2/N2 molecules (close up, so to speak). I think it was put together by a major MSM news corporation, but that part is fuzzy in my addled mind.
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  44. I think this explanation is incorrect... Ok so why is the tropopause, cooler than in the stratosphere? It shouldnt be by this reasoning, we should have an adiabatic lapse rate all the way to the outer atmosphere.

    Now, the average height the LW escapes the atmosphere from the troposphere is around 6km, in the co2 band its around 10km(On average) Most is not reabsorbed in the stratosphere, 90% of the atmosphere is in the troposphere. In fact co2 is a net emmitter at those pressures(much greater distances between molecules), at 2/1.
    Ramanthan & Dickenson 79
    So increasing co2, is only going to lead to a slight reduction in radiative absorbtion from CO2 in the stratosphere during transition, but increase the rate at which it radiates energy away, cooling it... O3 is the main greenhouse gas in the stratosphere, its primarily warmed by O3 absorbtion of UV, thats why it has a stratified temperature profile, its heated from the top down, but cooled through C02 being thermalized, then radiating the energy away in longer wave lengths.
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  45. Spaceman spiff and Sphaerica have hit the nail on the head.

    Bob, you really do have to ask yourself why the tropopause is cooler than the stratosphere. Its a contradiction to this article. The measured LW spectrum often shown, are showing that in the co2 band, its escaping to space at around 220k, and given the fact that the tropopause is cooler than the stratosphere, we can safely assume its below this in the troposphere that the average LW escapes to space from co2. Some will be absorbed by co2 and O3, but thats not the predominant reason for the T profiles, or why elevating co2 should cause the stratosphere to cool.
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  46. Bob,when I was first trying to understand the greenhouse effect, I figured out that the statosphere should be cooling before I learnt that, in fact, it had been (which is always pleasant); but the mechanism I thought of was different to the one you describe. Specifically, it occured to me that the temperature in the stratosphere is determined by the balance between the energy from ulatraviolet light absorbed by ozone, and energy emited as IR light by CO2. If the CO2 is increased, then the amount of energy emitted by the CO2 at a given temperature will also increase. Therefore, the CO2, and surrounding gas, will cool until equilibrium is reached again. Doubling the energy radiated by the CO2 will result in an approximate 15% reduction in temperature, all else being equal.

    Of course, the mechanism you describe will also cool the stratosphere. If IR absorbed from the troposphere were the only energy input of the stratosphere, halving the energy recieved would again, I believe, reduce stratospheric temperatures by about 15%.

    Which of these two mechanisms is most important would depend on the ratio of the energy absorbed from UV light to that absorbed by IR light. I believe that makes the mechanism I have described (as have Spaceman spiff, Sphaerica and Joe Blog above) more important.
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  47. Joe Blog, Bob,

    I understand what you are saying, but I think the mere fact that the stratosphere is warmed from the top down by UV could be enough to justify the temperature profile there. I don't think that is necessarily at odds with Bob's discussion (i.e. that the tropopause is cooler than the stratosphere). The two are compatible.

    At the same time, however, I'm heartened by the fact that four other people (yourself, Spiff, Tom Curtis, and Gavin), in varying ways, have reiterated my understanding.

    Gavin, however, is (to me, at least) the final authority on all things climate. That he confirms Bob's description is good enough for me, at least as far as saying that it is a relevant part of the explanation.

    Gavin's own response to Bob, however, also confirms what we've said (in different words -- he talks about the ratio of change in absorption/emittence, which is equivalent to what I said about the chance of emitting versus not):

    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.
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  48. Sphaerica - 47

    I've actually emailed Gavin asking him to take a look at this blog. All I could do is hope that he responds.

    I'll keep you guys posted.

    Bob
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  49. Sphaerica at 11:51 AM

    I would interpret the tropopause as a result of pressure/transparency(in LW), reaching a level where the balance of energy is lost..ie its no longer effectivly opaque to the passing of terrestrial LW, more energy is lost from here than what is received both above and below it from SW and LW absorption.

    So adding more co2, should increase opacity vrs altitude, raising the level of the tropopause, simply put, because there are more opaque molecules in a given area than prior to the rise. But the change in incoming and out going energy, is only going to be small, it will require a rise in T at all levels below the tropopause to enable the transport of energy up, but the difference in quantity of LW emitted will be inside the margin of error for measurements at the now, but an accumulation of energy should occur.

    Its lengthening the path length for the escape of energy, not stopping it until it reaches an overflow point. But the tropopause, marks the altitude, where the atmosphere is no longer effectively opaque to the passing of LW.
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  50. You guys have left me behind on a subject I thought I've kind of understood...

    ...so you made me start looking around.

    Fortunately, the Rabett has a timely post on the subject that may hopefully shed some light on the issue. Or obscure it (you tell me).

    Also ran across this video linked by Timothy Chase (from his discussion of his avatar page - TIP: has some neat GIFs of the energy states of CO2).

    HTH,

    The Yooper
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