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Climate Hustle

Is the CO2 effect saturated?

What the science says...

Select a level... Basic Intermediate Advanced

The notion that the CO2 effect is 'saturated' is based on a misunderstanding of how the greenhouse effect works.

Climate Myth...

CO2 effect is saturated
"Each unit of CO2 you put into the atmosphere has less and less of a warming impact. Once the atmosphere reaches a saturation point, additional input of CO2 will not really have any major impact. It's like putting insulation in your attic. They give a recommended amount and after that you can stack the insulation up to the roof and it's going to have no impact." (Marc Morano, as quoted by Steve Eliot)

The mistaken idea that the Greenhouse Effect is 'saturated', that adding more CO2 will have virtually no effect, is based on a simple misunderstanding of how the Greenhouse Effect works.

The myth goes something like this:

  • CO2 absorbs nearly all the Infrared (heat) radiation leaving the Earth's surface that it can absorb. True!
  • Therefore adding more CO2 won't absorb much more IR radiation at the surface. True!
  • Therefore adding more CO2 can't cause more warming. FALSE!!!

Here's why; it ignores the very simplest arithmetic.

If the air is only absorbing heat from the surface then the air should just keep getting hotter and hotter. By now the Earth should be a cinder from all that absorbed heat. But not too surprisingly, it isn't! What are we missing?

The air doesn't just absorb heat, it also loses it as well! The atmosphere isn't just absorbing IR Radiation (heat) from the surface. It is also radiating IR Radiation (heat) to Space. If these two heat flows are in balance, the atmosphere doesn't warm or cool - it stays the same.

Lets think about a simple analogy:

We have a water tank. A pump is adding water to the tank at, perhaps, 100 litres per minute. And an outlet pipe is letting water drain out of the tank at 100 litres per minute. What is happening to the water level in the tank? It is remaining steady because the flows into and out of the tank are the same. In our analogy the pump adding water is the absorption of heat by the atmosphere; the water flowing from the outlet pipe is the heat being radiated out to space. And the volume of water inside the tank is the amount of heat in the atmosphere.

What might we do to increase the water level in the tank?

We might increase the speed of the pump that is adding water to the tank. That would raise the water level. But if the pump is already running at nearly its top speed, I can't add water any faster. That would fit the 'It's Saturated' claim: the pump can't run much faster just as the atmosphere can't absorb the Sun's heat any faster

But what if we restricted the outlet, so that it was harder for water to get out of the tank? The same amount of water is flowing in but less is flowing out. So the water level in the tank will rise. We can change the water level in our tank without changing how much water is flowing in, by changing how much water is flowing out.

water tank

Similarly we can change how much heat there is in the atmosphere by restricting how much heat leaves the atmosphere rather than by increasing how much is being absorbed by the atmosphere.

This is how the Greenhouse Effect works. The Greenhouse gases such as carbon dioxide and water vapour absorb most of the heat radiation leaving the Earth's surface. Then their concentration determines how much heat escapes from the top of the atmosphere to space. It is the change in what happens at the top of the atmosphere that matters, not what happens down here near the surface.

So how does changing the concentration of a Greenhouse gas change how much heat escapes from the upper atmosphere? As we climb higher in the atmosphere the air gets thinner. There is less of all gases, including the greenhouse gases. Eventually the air becomes thin enough that any heat radiated by the air can escape all the way to Space. How much heat escapes to space from this altitude then depends on how cold the air is at that height. The colder the air, the less heat it radiates.

atmosphere
(OK, I'm Australian so this image appeals to me)

So if we add more greenhouse gases the air needs to be thinner before heat radiation is able to escape to space. So this can only happen higher in the atmosphere. Where it is colder. So the amount of heat escaping is reduced.

By adding greenhouse gases, we force the radiation to space to come from higher, colder air, reducing the flow of radiation to space. And there is still a lot of scope for more greenhouse gases to push 'the action' higher and higher, into colder and colder air, restricting the rate of radiation to space even further.

The Greenhouse Effect isn't even remotely Saturated. Myth Busted!

Basic rebuttal written by dana1981


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 7 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Related Arguments

Further reading

V. Ramanthan has written a comprehensive article Trace-Gas Greenhouse Effect and Global Warming.

Comments

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Comments 451 to 500 out of 509:

  1. Hope this is the right place to point out that Figs 1 & 2 have disappeared from the 'Advanced' article, apparently after Altervista suspended their hosting. They're still available in the PDF and via the Wayback Machine.

    Response:

    [DB]  Updated.  Thanks for the heads-up!

  2. I fail to find convincing evidence as to how CO2 can be the cause of global warming with a occurence of only 400 ppm. This would entail that 1 CO2 molecule would need to heat up 2,500 other molecules in the atmosphere to cause any increase in overall temperature. How is this possibly explainable! It is impossible.

  3. Barcino:

    You are making an argument from incredulity. An equivalent counterargument would be for me to say "I can't believe you don't understand how this works". Very easy to say, but carries no weight.

    The place you want to look is on the "CO2 is a trace gas" page:

    https://skepticalscience.com/CO2-trace-gas.htm

    Please read it before you comment again, and place comments on that thread, not this one.

    Response:

    [PS] I am reasonably sure that nothing said by anyone will convince a person that doesnt want to be convinced but lets see. Barcino's statement suggests he hasnt actually read a proper explanation of how the greenhouse effect really works.

  4. @Barcino2017

    Are you trolling? Or can you genuinely not come up with an idea of how molecules of CO2 can heat up other molecules in the atmosphere? This idea around 2500 molecules seems to be quite common in denier circles and reminds me of a "discussion" I had with someone who was pushing this idea:

    Denier: There are 2500 molecules for each molecule of CO2. For an increase of 1C the CO2 molecule would have to be heated to 2500C.

    Me: Why would the CO2 molecule have to be heated to 2500C?

    Denier: Because er, 2500, you dishonest green rent grant seeker.

    Me: What? Why?

    Denier: Your smears and lack of empirical data and science are obvious. You are a dishonest liar. I win!!!!!!!!!!?

    Now, you don't want to be that person, do you? Your idea is slightly less extreme than his as you do not propose a CO2 molecule has to be heated all in one flash and heat 2500 other molecules at the same time. But you are suggesting a CO2 molecule cannot heat more than one molecule. Ever. Like, once it has heated another molecule it can never do it again? A one-off never to be repeated event?

    Perhaps you need to rethink that misconception.

  5. The facts are obscured by the above analysis. The facts are that the effects of CO2 on the atmospheric absorption of IR by the atmosphere diminish rapidly and logartithmically with increasing concentration, so most of the so called greenhouse effect, that absorbs and re radiates IR, which isn't how greenhouses work, occurs in concentrations below 150ppm, which is so low trees and other vegetation die. Lower than the lowest and ice age minimums.

    The effect is approximately the same for each doubling, so has expoentially LESS effect per ppm, so that at 200ppm another 200ppm, to get to where we are now, only produces the same effect as 20-40ppm.

    So the statement that its never satuarted is deceitfully true, because it is partial by omitting the dominant factor, that while it's never quire saturated, it may as well be for all the effect it has, versus the natural variation in the 300 times greater effect of Water vapour, for example.

    Reference? There is a useful course on this by the University of Chicago you can take for free w/o a certificate - that costs - which gives this initial fact in itshttps://skepticalscience.com/CO2-is-not-the-only-driver-of-climate.htm promo video.  These are the facts on IR absorption of CO2 in the atmosphere. Not as advertised. The reverse of a tipping point in fact. We need to be looking elsewhere than CO2 for the rue cause of the tiny variation we are currently observing with the interglacial peak tempertaure range of an ice age interglacial. Soon be over, but at a rate humans cannot detect in a lifetime, as a quick study of the data shows anyone numerate. Real planetary climate hange of an interglacial is significant, but again any effect takes seberal lifetimes to be on bservable, even at the geologically rapid end of the current ice ages, over only 1,000 years...................

    https://www.coursera.org/learn/global-warming/lecture/CnAIV/the-band-saturation-effect

    Response:

    [JH] url link activated. Please learn how to do this using the edit function provided.

    For future reference, you may want to spell-check your draft text prior to posting a comment.

    [TD] SkepticalScience is organized into a large number of narrow topics. You have commented on a topic that is only slightly relevant to your point about the direct IR effect of CO2 increase being logarithmic. Please click the View All Arguments link at the bottom of the list of Most Used Climate Myths, in the left margin, for more relevant posts, such as How Do We Know More CO2 Is Causing Warming?, and CO2 Is Main Driver of Climate Change. Note that many posts have Basic, Intermediate, and Advanced tabbed panes. The log relationship of IR absorption to CO2 level has been known for over 150 years, and always has been explicitly accounted for in all calculations of temperature effects of increased CO2.

    [TD] To find posts specifically addressing the logarithmic relationship, enter the word logarithmic in the Search field at the top right of the page.

  6. Brian Catt @455 , from what I've seen of the Uni. Chicago lecture, it does not support the thrust of the statements you have made.  Please explain better, what you are intending to say — since your comments are coming across as "confused about the science".

    Then there are the simple errors in your statements, such as (A) the current "tiny variation" [unquote] in surface temperature.  [Alas, not so.  The temperature is shooting upwards like a rocket.  There's been nothing like this in the 200,000+ years of human history.  And scientifically, the cause is obvious and undisputed. ]

    .... And (B) water vapour causing "300 time greater effect" [unquote] in warming compared with CO2.  Is that what you think?  Or did you mean to state 3 times greater?  Because that is around the upper limit of the multiplying "feedback" response by H2O to the driving force of CO2.

  7. BC embarrassed himself, and forever established himself as a denier, on this NOAA thread on Facebook. 

    Put your coffee down before reading.

  8. Daniel, "This page isn't available

    The link you followed may be broken, or the page may have been removed."

  9. As a brief background to this comment, I had an encounter with a denialist last week who stated that Arrhenius' observations regarding CO2 as a greenhouse gas had been debunked. In hindsight,  I should have asked precisely what he meant by that. Instead, I retorted that this was news to me, at which he remarked that my kind was unlikely to look up the facts (I have a masters degree in Physics, and found this an interesting example of projection rhetoric...). Anyway, here I am.

    I'm trying to follow the argument here, so that I can express it in layman's terms. It seems the idea of greenhouse warming isn't as straightforward as I thought. I get Angstrom's counter-argument to Arrhenius about current CO2 levels absorbing all IR radiation long before it has a chance to escape the atmosphere and that, therefore, adding more CO2 won't alter things. I'm having a problem grasping the explanation in response the Angstrom being put forward here. If I may paraphrase what I understand of it:

    1. IR photon from surface is repeatedly absorbed and re-emitted (by CO2) in random directions (losing a little energy each time). Check.

    2. Photon is finally able to escape into space at a high enough altitude. Check.

    3. Adding more CO2 increases the density, and hence the altitude this  escape occurs. Check.

    4. Being higher means lower temperature, and hence less energy is emitted. Errm... as a general consequence of a blackbody spectrum, sure. As a discrete energy photon emission, of what significance is the temperature?

    I'm not saying the explanation's wrong, but it is very hard to follow, and is even counterintuitive (we're talking about rising temperatures, and then lowered temperatures? It's hard enough for me to follow, let alone someone without a scientific background)

    My own quick explanation would be that increasing CO2 concentrations reduces the mean free path of IR photons, and that increases the number of scattering (heating) events before the photons can escape. It seems a lot more concise and intuitive than the explanation put here, but is it right? What am I missing?

  10. Arf @459 , the usual explanations follow the course of IR photons (of the bandwidth absorbed/emitted by CO2 molecules) as they are radiated out from the warm surface of the land/ocean.   Layer after layer of atmosphere absorbs and re-emits IR photons in all directions (as you are already aware, of course).

    Rising through air, as the layers become less dense, the individual photon "journeys" (between CO2 atoms) become longer — yet each same-depth layer is still emitting the same previous total of upwards and downwards amount of IR radiation (of course).   Only in the most tenuous uppermost layers, does this "stacking" of upward/downward emissions begin to break down, as an increasing percentage of upward IR photons evade reabsorption and make a straightline escape to outer space.  In effect, we can think of the upper atmosphere as producing only back-radiation (at this particular bandwidth we are interested in) as far as the Earth is concerned.

    I am sure I am telling you nothing new, in all this.   I will point out that at an individual level, each IR photon maintains its same energy level, as it is "reincarnated" — the individual CO2 molecule recipient of photonic energy "cools itself" by imparting kinetic energy to a neighbouring N2 or O2 . . . and at a later time regains energy kinetically from a neighbouring N2 molecule, and "reincarnates" & emits an IR photon of the same energy level as previously received but in a random direction.   Of course, as intermolecular distances increase, and the kinetic temperature of N2 molecules reduces, then these "deaths & reincarnations" of IR photons (per second per cubic mm) must reduce.   But the final total product is back-radiation towards the Earth's surface plus upwards "lost" radiation (and of course the totality of all "lost" radiation over the whole spectrum must equal what's originally entered the planetary system from the sun, at equilibrium — or at least extremely close to that total while the system is in transition to equilibrium). 

    Myself, I find it easier to mentally picture these events if you rotate the Earth surface 90 degrees.   Instead of a horizontal surface emitting upwards, choose to picture the surface as the y-axis and the atmosphere layers stacked outwards along the x-axis.   The cool outermost layers of air are losing radiation outwards to space, and are emitting "back-radiation" inwards.   Through the bulk of the atmosphere, each layer is transmitting fractionally more energy inwards than outwards, and these fractional differences integrate to produce a gradient of temperature, highest at the surface and "sloping down" to the outermost air.   Hence the surface is warmer than the outermost air.

    When the atmospheric CO2 concentration becomes raised, the x-axis is extended further (so to speak) . . . and the same gradient produces a higher cumulative back-radiation at the planetary surface: in other words, the surface becomes warmer than under the previous conditions.

  11. Arf,

    My understanding is a little different from yours, just a little.  To review your points:

    1) I think the energy of photons is fixed so energy is not lost each time.  I am not sure what you mean by lost energy.  This point is not important.

    2) Check.

    3) Check.

    4) This is the important step for the greenhouse effect.  We agree that the escape altitude increases. 

    As you know the amount of energy radiated is related to the temperature.  The temperature at the escape altitude must be high enough for all the energy that comes from the sun to escape to conserve energy.  This fixes the temperature at the escape altitude.  When more greenhouse gas is added the temperature at the new altitude increases so that energy is conserved.  

    The temperature in the troposphere (the lower atmosphere) increases as you decrease in altitude.  The rate of increase is called the lapse rate.  The lapse rate is about 6.5C per kilometer.  The lapse rate is a physical property of the atmosphere and is fixed by basic physics and chemistry laws and properties.

    The temperature at the altitude of escape is fixed by the law of conservation of energy and the temperature varies in the atmosphere according to the lapse rate.  When the altitude of escape is increased that results in an increase in temperature at that altitude.  This is then passed down to the surface according to the lapse rate.

    It is interesting to note that an increase in the altitude of only 100 meters results in an increase in the surface temperature of 0.65C.  2C is only 300 meters increase in escape altitude.

    On Venus the CO2 concentration is much higher so the altitude of energy escape is much higher.  That results is the surface of Venus being about 462C.  This temperature can be calculated using the equations that describe the greenhouse effect on Earth.

    Your explaination seems OK to me but does not well describe the changes in the atmosphere.

  12. Hi all, i'd like to follow on from Arf @459 above. It doesn't seem his query on why the temperature of the upper atmosphere matters when we're talking about discrete emission of a photon has been adequately addressed. The comment at 461 seems contradictory, as if there is an increase in temperature at the altitude of escape then this will increase black body radiation to space...

    My other query relates to how the co2-absorbed photons actually contribute to warming, if the re-emitted photons are of the same energy as the absorbed ones. Is it that through a kinetic effect where a proportion of high energy absorbed co2 molecules vibrate surrounding air molecules, losing the absorbed energy and therefore never re-emitting the photon? Or is it because a proportion of the re-emitted photons are absorbed by something else (water, the ground etc). Or both?

    I assume the photons we're talking about make their way out pf the atmosphere through a random walk type process

  13. LTO: Contrary to your second sentence, the temperature at the altitude of escape decreases, not increases. That is not because the temperature at a given altitude (of the troposphere) decreases. Instead, it is because the altitude of escape increases, and higher altitudes have lower temperatures. Increase in temperature of a given altitude due to greater CO2 absorbing IR from below, is insufficient to counteract the lower temperature due to the higher altitude of escape. The altitude of escape increases because of the larger number of CO2 molecules between a given altitude and outer space.

    Regarding your second query: A CO2 molecule collides with other molecules--CO2 or other--about 100,000 times more often than that CO2 molecule emits a photon. So the vast majority of the time, the energy a CO2 molecule acquires from absorbing a photon is transferred to other molecules. That is part of the reason why Mars is so cold despite having so much more CO2 than the Earth does. Mars has so few molecules of anything in its atmosphere, there are few transfers of energy to other molecules, so much more of the energy remains in the CO2 molecule until it is emitted as a photon.

    Yes, photons are emitted from CO2 molecules in random directions.

  14. Goodbye man-made global warming? As an independent (i.e. impartial) consulting geologist (doctorate in sedimentary geology) with 35 years of experience, having conducted an unpaid (impartial) full-time 3-year (since Nov 2015; continuing) review of the literature from ALL scientific disciplines relevant to climate- and sea-level change (geology, archaeology, physics, astrophysics, oceanography, meterorology, etc, etc), here are my main conclusions:

    (1) There's obviously no doubt that Earth has warmed since thermometer measurements began in the 1800s (HadCRUT data; and online NASA/GISS online charts [yearly, monthly, and others], updated every few weeks). However, Earth began COOLING in February 2016 (NASA/GISS monthly chart). This cooling already exceeds all other measured coolings since 1995, in both duration (nearly 3 years so far) and magnitude (0.5 degrees C, fully one-third of IPCCs dreaded '1.5 degrees C by 2100', but in the wrong direction) ...

    (2) Warming was driven by increasing solar-MAGNETIC output (controlling cosmic rays, therefore cloudiness; Svensmark's breathtakingly elegant theory), nothing to do with mankind's CO2 emissions which just happened, by pure (bad) luck, to grow during a solar upswing (rather than downswing), a ghastly coincidence; the reverse was about equally likely, 50:50.

    (3) Changes in temperature are lagging about 25 years behind changes in solar-magnetic output, due to ocean thermal inertia (google it), dismissed by IPCC.

    (4) Sea level is about to rise about 3 metres (sic), before 2100, driven by the increase in solar-magnetic output (up until its 1996 peak), its effect on sea level delayed a further 20 years (approx.; i.e. total sea-level lag is about 45 years) due to ocean 'conveyor-belt' circulation (also ignored by IPCC) delaying the arrival at Antarctica of 'solar-overwarmed' Atlantic surface water, via downwelling and southward mid-depth flow (AMOC). The floating ice shelves buttressing Antarctic on-land glaciers are NOW disintegrating at an accelerating rate (led by Pine Island, Thwaites and Totten), so catastrophic glacier failure by MISI and/or MICI is likely to begin within a decade, raising sea level by at least 3m within about 50 years. It's unstoppable.

    Am I right? We'll know very soon. Regarding solar control of global temperature, the next two years will tell: I predict continued cooling, so keep a close eye on that NASA temperature chart. Regarding sea level, we'll know within 10 years, possibly much sooner: I predict the rate of sea-level rise, currently a trivial 3mm/year, but already increasing exponentially, will be at least ten times higher (3cm/year) by 2030, if not 2025. Watch NASA's online sea-level chart, updated every few months.

    See my 20 ResearchGate contributions, mostly one-page items or single figures, fully self explanatory ... https://www.researchgate.net/project/Imminent-metre-scale-non-anthropogenic-sea-level-rise

    Response:

    [DB] "Earth began COOLING in February 2016"

    Statistical significance testing shows that, for climate related changes, 17 years (Santer et al) are the bare minimum, with 30 years or more being typically used.

    For ANY of the instrumental series, over ANY time span ending in the present:

    • There is NO period where warming is invalidated, against a null hypothesis of no warming. NONE.
    • Against a null hypothesis of the long term warming trend, there is NO period where a "no warming" hypothesis is validated. NONE.
    • Over ANY period with enough data to show statistical significance, that data shows a statistically significant warming trend. ALWAYS

    Ergo, the warming continues, unabated.

    Note that commenting here at Skeptical Science adheres to the nature of the OP of the thread upon which you comment.  Please follow that rule.  Thousands of comment threads exist here upon virtually every topic related to climate change and the denial of it.  Use the search function to find the most appropriate thread for your expansion of your knowledge of the science.

    Off-topic snipped.

  15. Geologist-for-a-change [of pseudonym? ]  @464 , 

    Attention !

    # You have posted on the wrong date.  It is not yet April 1st.

    # You have posted in the wrong thread.  This thread is for "CO2 Saturated"-related comments.

    # You have posted on the wrong website.  You should be on WattsUpWithThat ~ the home website for commenters who have the deluded belief that the scientists are all wrong about everything.

    # And you appear to have posted 27 years prematurely.  If you have (as you say) studied climate science full-time for 3 years, and have not yet disentangled yourself from (almost) every piece of climate crackpottery known to man . . . then it sounds like you have stepped out no more than 10% of your journey from ignorance to knowledge.

    Please return in 2046, and let us know how your education succeeded.

  16. Eclectic @465,

    I think your response to the trolling is appropriate. I would perhaps add that the fool cannot even provide the numbers in his first point correctly. The date since which peak monthly surface temperature anomalies exceeded later lower anomalies in duration and in temperature difference was 2008 not 1995. This should be no surprise given the 2010 El Nino wasn't as big as the 2016 or 1998 versoins.

    Response:

    [DB] Inflammatory snipped.

  17. LTO @462,

    Regarding the altitude issue, I think the explanations you see as being at variance is due to them being part-explanations.
    To explain:- if atmospheric CO2 levels increase, the altitude at which CO2 can emit photons dirctly into space increases. This results in the temperature of the space-emitting CO2 being lower and this lower temperature reduces the number of photons emitted and thus the global energy being lost to space.
    These CO2-emitted photons are all in a small part of the IR spectrum with ~15 microns wavelength.
    As the global energy is now out-of-balance, global temperature will rise, this temperature rise increasing the photons lost to space over all of the IR spectrum. When the energy balance is restored through this warming, the CO2 emission altitude will still be cooler than the emissions altitude prior to the increase in atmospheric CO2 levels.

    And I feel your "other query" hasn't been fully addressed.
    A photon at the right energy (ie wavelength) can be absorbed by a CO2 molecule and set it into a bendy wobble. In almost all these occurances, the CO2 molecule will then be involved in a collision with another air molecule and the photon's energy will be absorbed within the gas, it being transferred to other modes of gas energy. You ask what then happens to this energy. It will be passed around the gas, this constituting a temperature increase. But a temperature increase will also mean more CO2 molecules are being walloped by the air molecules about them and this will result in more of them being in that bendy wobble which allows them to emit a photon. So more temperature also means more photons emitted by CO2, this cooling the gas, this providing an energy balncing mechanism. And note that if the number of absorbed photons increases because of more CO2, there is also more CO2 to go into a bendy wobble and then to emit photons, which also balances out the energy equation.

    Of course, those are still much-simplified descriptions.

  18. MA, Tom: really appreciate your replies, explains some points but also raise a few things I don't quite follow.

    1. How significant is the effect of temperature on the likelihood of excited CO2 photon release? At higher altitude the atmospheric pressure is also decreased, which means that the length of time between molecule collisions is also increased. Similarly lower temperatures will decrease the rates of collision. Part of the issue here is the reporting of CO2 in ppm only - given the changes in pressure and temperature I'd have thought you needed accompanying concentration /pressure / temp data to really makes sense of how the competing phenomena interact.

    2. Do you know what altitude range the CO2 photon to space release currently happens, how its changed and how sensitive this is to CO2 concentration? As I understand it the temperature actually increases between the tropopause (~11 km) and stratopause (~50 km). It decreases again to the mesopause (~85 km) and then stays pretty constant. The pressure by contrast drops 10 fold every 15 km or so. (see attached)

    3. Is it only the altitude of escape atmospheric layer that is the relevant metric here in determining warming, or is it the depth of the envelope from ground to the altitude throughout which warming occurs. I'm trying to understand where exactly in the atmosphere the warming actually happens, and how this is then reflected back at ground level.

    Thanks!

  19. Graph of temperature and pressure by atmospheric height here: https://imgur.com/a/juS7yVf

  20. LTO: Sorry, i’m Not knowledgeable enough to answer those questions.

  21. LTO: ScienceOfDoom has excellent explanations of the greenhouse gas effect.

  22. Thanks Tom - that site looks good, but will take some time to work through before can divine anawrrs to those questions. This is clearly very complicated physics - complicated to the extent that I'm wondering how many people really understand it well enough to be completely confident they have taken account of all complexities given the unknowns in the system, as opposed to just taking someone else's word for it.  MA may have the answers!

  23. LTO: Oh, so no one should ever believe or act on any science in any field unless one completely understands everything about it? I’m sorry I wasted the time to answer your questions, because I strongly suspect that you never had any genuine interest in the answers.

  24. LTO,

     I thought I could chime in since the original question  was about my post.

    I think you want a lot of detail and Science of DOom and And Then Theres Physics are good sources of detailed information.

    For your questions:

    1) The number of collisions is so great that the temperature and pressure do not make much difference. (I found an on line calculator once and was amazed at how many collisions there were and how small the temperature and pressure affected the colision rate). 

    It is probably too difficult to specify the temperature and pressure since it varies so much  in different places.  The ppm of CO2 is relatively constant through most of the atmosphere. 

    2) 10 km is commonly used as the escape altitude.  This is a simplification for a basic explaination.  The actual escape altitude would be different for different wavelengths,  different in the tropics and the Arctic, different in deserts than over water and different over storms versus calm weather.  Think of how much warmer it is at night when  it is cloudy.   Only the temperatures in the troposphere matter to the surface temperature.  Increasing CO2 causes the stratosphere and the mesosphere to cool at the same time the troposphere warms!  source 

    3) I think the altitude of escape is the key figure but you should check at Science of Doom.  Come back here and post if you find out exactly what the explaination is.

  25. Tom: I'm not sure what I did to attract your ire, but I apologize. The point I was trying to express is that the actual theory behind co2-induced global warming is significantly more complicated than I'd thought for a long time. To the extent that I genuinely wonder how many people would really claim to fully understand it, scientists (of which I'm one, albeit not a physicist) included. Is there anyone on this site who would put themselves in that category?

    The impression I get (which may not be accurate) from that science of doom website is that the theory is based on modelling of complex interrelated agonistic and antagonostic phenomena, which makes it somewhat different class of scientific theory. Models are necessarily simplifications, and in highly complex systems hidden variables can often lead to very unexpected effects in the real world, which therefore leaves room for reasonable skepticism and uncertainty. Anyway, I'm on a journey of discovery here and have already learnt a lot, in part thanks to you, so once again thank you for your help and I'm sorry you feel you wasted your time.

  26. LTO @472 ,

    I must say I am puzzled by your assessment of the Science of Doom website.   The Greenhouse Effect is understood through observational studies combined with well-established basic physics.   Any usage of models came much later, and is certainly not foundational to the science of it all.

    Please do not be discouraged.  The CO2/Greenhouse Effect is actually quite simple & straightforward ~ once you have gotten your head around it.   But it is not immediately intuitive.

    Just like the Galileo/Tower-of-Pisa/falling-weights tale . . . and like the concept of Gravity . . . and Newton's Laws of Motion.   All these things can be "unsimple" to explain in a few paragraphs ~ but are quite simple and obvious, after you have grasped the concepts.   But for previous thousands of years, they were not intuitive at all !   And still are not ~ until you take a scientific approach and think things through.

    As Tom mentioned in #463 : in air, the molecular collisions occur at a rate many orders of magnitude above the "relaxation times" of a CO2 molecule (where a CO2 molecule accepts the energy of a 15um InfraRed photon, and later "relaxes" to emit an equivalent IR photon in a random direction).    Even where you reduce that collision rate by a hundred-fold (by reducing density & temperature, e.g. in the upper troposphere), you still get the situation where the collision rate is still vastly greater than the IR relaxation rate.   When you think it through, you see that the end result gives a negligible difference in the actual effect [ e.g. comparing the bulk difference between 99.99% and 99.9999% ].

     

    ** And LTO ~ a word in your ear.  While I myself am sweetly naive and unsuspicious that you might be uttering some phrasing of words which is alas too often heard coming from the mouths/keyboards of trollish science-deniers . . . nevertheless you have managed to cause Tom's ears to vibrate, by your using terms of the type: "incredulity / hoax-like / too-complex-to-be-an-honest-description / etcetera [obviously I am harshly paraphrasing your comments]."

    Those sorts of phrasings are common among science-deniers [= faux-skeptics] who subconsciously wish to reject reality ~ and who summon all their powers of distraction & rhetoric, in order to deceive themselves.

    ( I do read the WattsUpWithThat website, for entertainment.  Half the posters commenting there, are angry-crazies & political-extremists who are still in complete denial that CO2 & other Greenhouse gases have any global warming effect at all . . . and many of the other half are intelligent but so deeply affected by their Motivated Reasoning, that they distract themselves by using rhetorical smoke & mirrors ~ basically for deceiving themselves into a viewpoint that "there's nothing really unusual going on, and there's little or no global warming happening . . . and even if it is happening, then it's gonna be good for us, and with no major downsides". )

  27. Michael: fantastic thank you

    1. I find this a little difficult to believe in its face (which isn't to say it isn't true!) for two reasons.  First, while the number of collisions is high, the relaxation rate of an excited co2 molecule is presumably quite fast, and so the net effect could be considerably more than it appears from the raw collision rate itself. Second, and more importantly, if temperature and pressure don't make much of a difference then why is the increase in escape altitude so important? I'm still not clear on why the relatively small decrease in temperature as the escapeealtitude increases affects the emission of a discrete photon so significantly.

    On the ppm point, difficulty isn't a great excuse in my view! The constancy of co2 in ppm is a bit of a red herring, if i'm understanding things correctly, because really it's the molar concentration combined with pressure that gives effect to the phenomena. For example, both venus and mars have comparable (very high) co2 ppm levels, but clearly completely different effects, at least in psrt because of the difference in atmospheric pressure (as I understand it).

    2. That's really useful re the troposphere thanks. The complexity in escape altitude mustmmake things very difficult to model.

    That link is really interesting for  two reasons. First, it implies that actually the hole in the ozone layer should uave been responsible for a significant portion of troposphere warming, as opposed to CO2. Presumably the UV light that would otherwise have warmed the stratosphere warmed the troposphere instead.  Do we know how much troposphere warming is attributable to this?

    Second, the point on CO2 cooling the upper atmosphere really didn't make sense to me. The argument appeared based on an assertion that the earth is always radiating the same amount of heat so an increase in troposphere temp must lead to a decrease in upper atmosphere temp, but I thought the whole point was that the earth was retaining more heat. Why isn't the increasing CO2 conc in the stratosphere leading to increased heat retention of photons released by co2 in the troposphere and concomitant warming? Could be related to the pressure point above, but in that case why isn't it a relevant consideration for increasing escape altitude?

    3. I will do! May take a while - hopefully will get answered here first.

  28. LTO @477 ,

    we seem to have cross-posted at 21:54 PM.

    Your first paragraph shows that you are still a long, long way from understanding the Greenhouse Effect.

    The "complexity in escape altitude" does not require a "model" of mathematical ingenuity & tour-de-force.  You can do a good approximation on the back of the proverbial envelope, using a blunt pencil.   Basically, use the temperature lapse rate of around 6.5 degrees per 1000m altitude.   (Of course, the escape altitude is not a razor-thin layer, but a fuzzy zone . . .but your can treat it as one particular altitude (as described in some of the comments upthread) . . . while always remembering that other Greenhouse gases have different escape altitudes.

    The stratospheric cooling is interesting, in that it demonstrates that modern global warming is not of solar initiation.   But in practical terms, the stratosphere is so low density, as to have minimal effect (and similarly with the thermosphere).

    Better for now, to focus your thoughts on understanding the GHE.   Think about the transient condition, where (as right now) there is a nett inflow of heat into our planet . . . compared with future condition, where the Greenhouse influence has stabilized at a higher surface temperature.

  29. LTO @472 &475,
    I think I should add to the message from Tom Dayton @473.
    You talk of "unknowns in the system" and "taking someone else's word for it."
    I appreciate that getting a grasp of AGW can be frustrating. I remember when I first encountered the Greenhouse Effect and the idea that there could be some equivalent to a sheet of glass allowing light in but preventing IR escaping seemed a bit much to accept. What you tend not to find, even now, is convincing explanations as to why the Earth's atmosphere is more hermitically sealed than any greenhouse or blanket. So I'll say it here. The atmosphere is incredibly well balanced vertically. Outside hurricanes, volcanic eruptions and other relatively rare events, the day-to-day reality is the motion of the Hadley Cells, They are responsible for most of the up-down movement in the troposphere and they take about two weeks to rise from surface to tropopause. Yet this is not something you will readily learn if you start asking folk. I share with you here my own frustrations from a few decades ago.
    Yet there are some (probably very many) apparent incongruities that can be expressed about AGW that are not readily answerable in a simple way using non-scientific argument. This SkS site addresses many but there are always different flavours of incongruity to consider in such a complex system.

    Yet such incongruities do in no way support the notion of potential "unknowns in the system" where we have to "tak(e) someone else's word for it." And in particular here we are discussing an aspect of AGW that is in no way in dispute as, despite the complexity, there are absolutely no "unknowns in the system."

  30. LTO,

    Unfortunately I do not have much time.

    1) The number of collisions at the surface is about 1 million times faster than the relaxation time.  The change in temeprature to 10 kmn is about 40C.  That is about a 20% change in speed and collision rate.  The pressure change is about 40 kPa.  About a 40% change which changes the collision rate a little more than 40%.  Combined they change the collision rate less than a factor of 5.  At the escape altitude there are 200,000 times as many collisions as emissions.

    The atmosphere is a black body.  At a lower temperature it emits less energy.  Science of Doom will have a graph of energy emitted compared to temperature.  Black body radiation changes relative to T to the fourth power so small changes are a much larger effect.

    Complex models can exactly calculate the emission spectrum of the entire atmposphere at any level or all combined but are not needed to explain the greenhouse effect.  They demonstrate that scientists know what they are talking about. (I do not have time to find a reference, sorry)

    2) Most of the UV light is still absorbed.  There is not that much energy in the UV remaining (it can be calculated and is considered by climate scientists).

    The Stratosphere is a completely different situation than the Troposphere.  The Troposphere is heated by energy coming up from the surface.  CO2 blocks this energy from escaping (until it reaches the escape altitude) so the Troposphere warms.

    The Stratosphere is warmed by UV light from the Sun.  Increased CO2 causes increased emission of IR energy.  Since the Stratosphere is above the escape altitude (as discussed above) the increased IR emission results in increased loss of energy and cools the Stratosphere.  The key understanding is the escape altitude (which is very complicated but we simplify to 10 km for these discussions)

    Scientists predicted this effect in advance.  It is a key signature of the greenhouse effect.  Post hoc explainations about unknown "global oscellations" do not hold the same weight as predictions made in advance.  I know of no alternate explainations for how the Troposphere could warm as the Stratosphere cools.

  31. Michael: thanks for taking the time. Apologies on the collision point, i see it's a bit of a red herring. I'm still not getting the importance of temperature on the escape altitude. I understand black body radiation (kind of) but not how that relates to the discrete emission of a photon from an excited molecule. In other words, the collision between bulk property thermodynamics and a discrete quantum event. This may just be my own deficiencies.

    What i don't follow is your point on UV and stratosphere. The article you linked to https://www.wunderground.com/resources/climate/strato_cooling.asp was clesr that tbe decrease in ozone layer was the main cause in a large decrease in stratosphere temperature through decreased UV absorption. It *must* therefoelre be the case that this energy was transferred to the troposphere instead. Given ozone depletion very simply predicts a decrease in stratosphere temps, the greenhouse effect prediction would seem to carry less weight: the two effects would need to be disambiguated, increasing uncertainty, particularly as according to that article the ozone effect is dominant. 

     

    I'd suggest its unfair to call explanations based on temperature cycles post hoc - this idea is obviously very well supported by the pre-industrial historical record. The weakness in that argument is of course that it predicts anything, amd therefore predicts nothing .NNevertheless, the challenge is to show something unusual is happening, which is difficult to do persuasively when we've only been measuring certain metrics for a short period. That's a very different topic to this one though!

  32. To clarify my query: I understand well enough the principles behind the greenhouse effect. However, what I recently only became aware of was that there is already a vast overabundance of co2 to absorb all the IR emitted at 15 um, so the effect of adding more co2 at current levels must have a vastly smaller effect than adding thse same amount of co2 at much lower levels. This seems to be common ground amongst those in the know, but I'd say unknown to 99% of people.

    The explanation for why it matters nevertheless seems to be twofold, and here is where I'm struggling to understand quantitatively how significsnt the effects are.

    1. CO2 also has other minor bands of absorption, which may depend on concentration amongst other factors, that arent saturated. My question here is just how much additional energy this actuslly captures and re-radiates back to the ground. Is it really significant in the grand scheme.

    2. Increasing CO2 increases the altitude of emission (perfectly happy here), and because 15 um photons are being released at this higher altitude therefore global warming (this is where I'm getting lost).

  33. LTO @482,

    An unsaturated GHG (like methane) does provide for stronger warming effects with rising concentrations, these being roughly linear increases in warming with rise concentrations, rather than the logarithmic relarionship found with CO2.

    (1) What you mean by "bands of absorption" is not clear. The bendy wobble absorption band at 15 microns is made up of a set of wavelengths which are weaker the further away from the central part of the band. Thus the CO2 absorption appears as a wide dip at 15 microns as per this graph below.

    EarthIR spectrum

    Note the small spike in the centre of the dip. This is the strongest part of the CO2 bendy wobble absorption. Here at this precise wavelkength you would be up into the stratosphere before a photon has a clear shot at space. (It is an upward spike because the stratosphere is warmer at that altitude than the upper troposphere.

    One of the effects of adding CO2 to the atmosphere is to widen the broad CO2 dip as there are weaker wavelengths at the edges that are not saturated and in dry air would allow a photon to be emitted by CO2 and have a clear shot at space from ground level.

    If you mean by "band" an energy of photon that imparts a different wobble into CO2, there are none of consequence operating in the IR range, the closest being 4.3 microns.

    (2) The impact of altitude-increase is that (and here your question up-thread was hidden by your additional comments on 'rate of collision') through the troposphere temperature drops with altitude and so the Stephan-Boltzmann relationship applies. A colder gas is unable to emit as much IR. See the contours on the graph above. Less photons emitted to space, more energy accumulating on the planet, a warming planet until the energy fluxes are balanced

  34. MA Rodgers:  Thanks for posting that graph.

    LTO:

    Black body radiation is the net of all the photon emitting events.  When the temperature is higher more photons are emitted.  According to the Boltzman equation, the number of photons emitted is proportional to T raised to the fourth power.  A small change in T means a large change in photons emitted.

    UV radiation not absorbed in the stratosphere passes through the Troposphere and is absorbed at the surface.  There has to be an absorbing molecule, like ozone in the stratosphere, for the energy to be absorbed.

    It appears to me that you are applying a double standard.  Unknown "Global cycles" do not need evidence while scientific explainations require every T crossed and I dotted.  Fortuantely, the T's and I's have all been done.  Keep  reading scientific sites and you will find out what you seek.  Be careful of reading "Skeptic" sites as they traffic in nonsense which has to be unlearned.

    1) Exact numbers are beyond my pay grade.  Look at MARodgers graph.

    2) The temperature at the escape altitude is essentially fixed because it must be high enough to allow all the energy incoming from the sun to be emitted.  The lapse rate of the atmosphere (the decrease in temperature with increasing altitude) is a physical constant and is also fixed.  When the escape altitude increases, the temperature at the new escape altitude also increases to ensure conservation of energy.  When the temperature increases at the new escape altitude the increase propagates down to the surface to comply with the lapse rate.

    While at the surface the 15 micron absorbtion is saturated at the escape altitude it is not.  Therefore increasing CO2 increases temperature.  People who do not understand the greenhouse effect think because absorbtion at the surface is saturated temperature cannot increase.  The escape altitude is where the action is .

    Someone else suggested that the temperature at the escape altitude did not change as much as I think it does.  I think there is an issue of different simplifications of a complex subject.

  35. LTO @482 ,

    forgive my bluntness, but your recent questions show that you are still floundering rather than "understanding the principles [behind the GHE]".

    The GHEffect is multi-faceted, but straightforward.   Take your time, think things through and put the pieces together in your mind.   There is no trickery, no hidden or undiscovered "unknown unknown" factors . . . it is all simply very basic physics [high school level physics will be quite adequate].

    Picture the Earth of 300 years ago, when things were very close to equilibrium [though to be more accurate, the Earth has been cooling very gradually for around 5 thousand years].   The air CO2 level was about 280ppm, and the "escape altitude" was at the appropriate level.   Now look at today : CO2 level 410ppm, and the escape altitude has risen 100 or 200m higher and colder ~ fewer and slower molecular collisions.   And therefore marginally less excitation and emission of 15um photons to space.   And yet we still have about the same incoming heat energy from solar radiation.   Result : imbalance.

    Now jump 100 years into the future.   Wise political leaders have (of course!) long ago brought "zero nett carbon emission" into being, and have fostered projects which incorporate carbon (dioxide) into the soil . . . bringing air CO2 levels down to the low 400's.   Planetary surface temperature is 1 degreeC above 2019 levels [i.e. 2 degrees above pre-industrial levels] and is steady.   The escape altitude is at (say) +300m, but the air at that point has [stabilized] become slightly warmer . . . enough for the 15um IR loss-to-space to have increased back to the pre-industrial amount ~ so the Earth is in thermal equilibrium again (solar radiational input and terrestrial radiational output are matched).   But we on the surface here are 2 degrees warmer than pre-industrial.

    The alternatives are rather worse, if we allow the GHE to push things up 3 or 4 or 5 degrees.

  36. MA, Michael - thank you for this; very helpful, and it's tht first time Ive heard about the lapse rate as an explanation for how global warming works. There are a few things I'm still not following:

    1. How much of a difference does the effective widening lf the bands actually makw at the concentrations werew talking about? Eg if the effective drop in radiance (ie area under thr curve) at 400 ppm co2 was 100, what would it be at 800 ppm? 

    2. I'm not following the black body radiation argument, because co2 excitation and subsequent emission of 15 um isn't a black body phenomena (correct me if I'm wrong here). A single co2 molecule in an excited state will release a 15 um photon, and this is separate to black body radiation.

    3. I'm not sure I'm following why the temperature of the escape altitude increases. As indicated by MA earlier, there isn't necessarily energy transfer to the surrounding gas as excited co2  molecules increasing the temp  of surrounding air through collision then in principle increases the number of co2 molecules excited through collisions with surrounding air, hence minimising net energy transfer. I'd thought thr main mechanism behind the greenhouse effect was re-radiation of 15 um photons back to the surface (or water vapour),  not through heating of surrounding air. Is that wrong? Or are these minor secondary effects that are only really relevant once you get past the primary saturation point?

    4. Even if the temperature of the escape altitude is increased, it's not clear to me why that is necessarily transmitted to the ground through the lapse rate. What is the mechanism? Presumably not convection. Given the existence of eg temperature inversions in the troposphere and the day-night temperature cycling, it isn't clear to me why that is necessarily the case.

    Thanks!

  37. LTO,

    SkS is always happy to help those who want to learn the science.

    1) My understanding is that line broadening is a very small effect on Earth.  It is important on Venus.  It is not necessary to understand line broadening to get the basic greenhouse effect.

    2)  CO2 molecules emit a variety of radiation lines with15 micron being the most important.  The number of photons emitted by a section of the atmosphere (with a great many CO2 molecules in it) is determined by the black body equation.  Most of the CO2 molecules that absorb a photon coming up from below transfer the energy of the photon to other molecules in the air through collisions.  They do not re-emit the photon they absorbed.

    There is always a population of excited CO2 molecules that can emit a photon.  These molecules are excited by collisions with other molecules.  The size of this population is determined by the black body equation.  When it is hotter there are more molecules that are excited and more photons emitted.  When cooler less excited molecules, less photons.  The number of photons increases with Temperature to the fourth power.  The population of excited molecules is the important idea, not individual molecules.

    3) Let us imagine the escape altitude is 10.00 km and the Earth is at equilibrium.  Exactly the same amount of energy is emitted from the molecules at the escape altitude as is absorbed by the Earth (the energy comes from the Sun and is primarily absorbed on the surface).  The Earth receives 240 W/m2 and emits 240 W/m2.  The Earth is at a stable temperature.   The temperature at the escape altitude is 255.0K.  

    Someone adds 1,000 gigatons of CO2 to the atmosphere.  This causes the CO2 concentration to double.  This causes the escape altitude to increase to 10.50 km (500 meters). 

    The temperature of the atmosphere decreases with height according to the lapse rate (6C per km).  The temperature at the new altitude is only 252.0K (255 - [0.5km x 6C/km]).  Because it is colder less energy is emitted from the Earth (the amount can be calculated using the Boltzman equation.  It takes me a long time to calculate with this equation.).  For the purpose of discussion let us say at the new altitude only 239 W/m2 is emitted.

    The Earth is no longer at equilibrium.  It is absorbing 1 W/m2.  It starts to heat up.  The temperature at the escape altitude must increase to 255.0K in order for the Earth to emit 240W/m2 again. (There are some complications like a small increase in surface area that do not matter). 

    The atmosphere always has a lapse rate of 6C/km.  Since the temperature at 10.30 km has increased 3.0C the rest of the atmosphere also increases.  The lapse rate is a measured physical property so it must be applied.

    I do not understand your question about energy.  Most of the absorbed energy is transferred to the surrounding atmosphere.  That is how energy reaches the escape altitude and is emitted to space.

    The main effect is sometimes in the eye of the beholder.  I think the main effect is to  increase the temperature of the atmosphere.  That occurs because CO2 (and other greenhouse gases) absorb upwelling IR radiation and slow the emission of energy into space.  Both re-radiating energy back to the surface and heating the surrounding air are important.  The most important effect is increasing the escape altitude.

    4) The maintenance of the lapse rate in the atmosphere is complex (scientists who study the lapse rate understand how it works).  See this article for background information (found using Google).  Convection is involved but there are other factors.

    When we say the escape altitude is 10 km that is an average over the entire Earth: Tropics to Arctic, night and day (a few wavelengths escape from the surface).  The escape altitude is not the same everywhere on Earth.  In the Tropics it is higher than in the Arctic.  The lapse rate is an average property of the entire atmosphere, individual storms or other phenomena can violate the lapse rate (and the escape altitude) for periods of time.

    I recommend you accept the lapse rate and escape altitude on faith while you learn how the greenhouse effect works.  After you understand the basics you can add other effects that you are interested in.  Line broadening, convection, heat transfer by phase changes, clouds and other effects all occur in the atmosphere and alter the greenhouse effect.  Climate models have to deal with all these effects but they do not alter the basics.

  38. LTO:

    I found a Boltzmann Equation calculator on line (Google)

    It finds 239.8 W/m2 at 255 K and only 228.7W/m2 at 252K.  That means the Earth heats up faster than if the difference was only 1W/m2 but in the end the temperature at the escape altitude must increase to 255K so all the energy is emitted.

  39. LTO @486,

    I'm conscious that directly answering some of your queries would lead toward some rather incongruous implications with complex explanations required to sort them out. So I'm torn between simply answering #486, going back to first principles as an explanation or introducing a mathematical model into the mix. Haven't decided which yet.

    But I will pitch in with (1).

    michael sweet @487 mixes up the broadening of the CO2 dip in the IR spectrum (most important) and pressure broadening (not important). These are two different phenomena.

    The 15 micron wave band absorbed by CO2 is flanked by weaker bands which result from spinning CO2 molecules. Spin being a quantum process, there are only certain speeds of spin that can happen, resulting in the graph below (I assume it is for 1 atm).

    CO2 absorption probabilities

    It is the strenghtening in these flanking bands that broadens the CO2 IR dip.

    But you will also note there is a small probability of absorption at wavelengths between the seperate bands. This is the pressure broadening which is a big effect on Venus with its 90bar atmosphere.

    As for your actual question, the effect of this broadening of the CO2 dip with an increase 400-to-800ppm relative to a 280-to-400ppm increase (=100). I think, as a component of a logorithmic ratio of 194/100, it would possibly be something like 400/100. By 800ppm, the emissions height for the central part of the band is increasingly in the stratosphere and so acts as a cooling mechanism counteracting much of the warming through the strengthened absorption at the edges of the CO2 dip. You may find Zhong & Haigh (2013) 'The greenhouse effect and carbon dioxide' Figure 5b a useful reference.

  40. I recall Gavin Schmidt going at length over this at RC and saying basically the same thing: the absorbtion in the "wings" is where the additional watts/sq.m happen as concentration goes up. It adds up significantly. This may even figure still in the "Saturated gassy argument" posts linked below the thread.

  41. Chris Colose is a scientist who studies Climate Change who used to write sometimes for SkS.  He wrote a description of the greenhouse effect here.  His sumary states:

    "So…review: Because of energy balance, the planet must get rid to space as much energy as it receives from the sun. Averaged over the Earth, taking into account the albedo and geometry, this is about 240 W m-2. In the absence of an atmosphere, this flux of radiation is lost by the surface by \sigma T^{4}_{s}. With an atmosphere, this flux of radiation is allowed to emanate from upper, colder layers of the atmosphere, say on average at some altitude H. Increasing greenhouse gases increases the altitude of H, a height in the atmosphere which depends on wavelength, and characterizes a level of mean emission to space. Because the atmosphere is now emitting from colder levels of the atmosphere, the OLR has decreased, and the result is that the planet must warm to re-establish radiative equilibrium."

    I think my description is similar to his.  His summary is more technical and those who want to increase their knowledge of the greenhouse might want to read it.

    Apparently I mistook line broadening and pressure broadening.  Line broadening is important for the greenhouse on Earth.  Both these effects, and many others, contribute to the magnitude of the greenhouse effect.  If we double the concentration of CO2, the CO2 will directly cause heating of about 1C.  Some of that will be due to line broadening.  Feedbacks from other causes like increased water vapor and changes in clouds will contribute additional heating.  The feedbacks are difficult to calculate exactly but if the climate sensitivity is 3.0C (about midrange in the estimates) they will contribute 2C.

    Philippe Chantreau: can you link Gavin Schmidt's comments, I could not find them at Realclimate.

    As I said above, there are many phenomena that contribute to the greenhouse effect.  Different scientists sometimes emphasize different phenomena as important.  All these effects together make the greenhouse effect.

  42. michael sweet @391,

    I cannot fault what you say. Particularly, that "there are many phenomena that contribute to the greenhouse effect" is one of the difficulties in setting out a succinct statement of how it operates. Yet the simple energy balance is an overriding principle - if energy-out is different from energy-in, there has to be consequences, in the present case a period of global warming. The point with this aspect of AGW is that it is more than "settled", it is cast in concrete! All that people are lacking is an explanation appropriate for their needs.

    I have been putting a bit of thought on a journey to introduce these GHG mechanisms in a way I've not seen before. I'm hoping it may be useful to folk like LTO. I've not quite routed out that journey yet, but it is looking useful.

  43. The links are in the notes below and take you to the RC threads, where Ray Pierrhumbert did most of the initial comments. In fact, I don't know that anything recently discussed above is not adequately addressed in the notes right here below the comments, where there are many good links.

    http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/

    Part 2 is more interesting from the technical point of view, especially the extra absorbtion in the wings of the spectrum.

    http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument-part-ii/

    This is old news. The RC posts are almost 12 years old. It has all been worked out with the highest level of precision in HITRAN. The appropriate physics are in the models. It is not an area of very active research or debate. HITRAN was pretty much as far it was worth going with it, from any practical point of view.

  44. Thanks all, very useful. MA: that sounds like it would be extremely helpful for me and others.

    The point on many contributing phenomena is the key one - not just the existence of the phenomena, but how quantitatively significant it is. This no doubt varies with co2 concentration, and easily leads to confusion. The current understanding I have is that the most significant co2-induced warming mechanism (which is the one most people know about - re-radiation of IR photons to the surface) is indeed saturated at much lower co2 concs, and it is other effects that are purported to combine to trap significant amounts of energy in the troposphere. Once comfortable on this point, the point on energy balance is a given.

    One really interesting point that has come up is what happens when the altitude of emission is in the stratosphere. I understand that the tropopause varies between about 9 and 17 km, while co2 concs are relatively homogeneous across the globe, therefore the altitude of emission at ~10 km is already in the stratosphere around thr poles. it would seem that increasing co2 concs will, by raising the altitude of emission, progressively increase the proportion of the atmosphere in which this is happening. Will this offset troposphere warming effects to some extent?

  45. LTO @494 ,

    there are worrisome ambiguities in your comments.

    The "tropopause" is a very different concept from the "escape altitude i.e.  emission altitude".  For CO2 that altitude is dependent on the absolute density of CO2, while the tropopause is a temperature-related concept.  If I have correctly understood the figures quoted earlier, the [CO2] altitude you are interested in does always remain in the troposphere (not the stratosphere).

  46. Hi Eclectic, it doesn't seem to me like you've correctly understood the figures quoted earlier (or my post@494), but others may have a different view. Why do you think the altitude of emission must always remain in the troposphere?

  47. LTO,

    I am not an expert on the tropopause but I doubt that the escape altitude will move into the stratosphere to a significant amount (I do not have a reference for that opinion).

    In the poles where the stratosphere is lowest the troposphere is very cold.  That lowers the escape altitude so that the escape altitude is still below the stratosphere (we discussed that the escape altitude varies across the planet from the tropics to the poles).  Perhaps Science of Doom can answer this question.

    These detailed questions are secondary to how the basic greenhouse effect works. 

     Keep in mind that the description of the greenhouse we have discussed is a description of some of the most important basic features of a very complex phenomenon. Many additional complications exist. If it were simple to evaluate the error bars on climate sensitivity would not be so large.

  48. Philippe,

    Thank you for the references.

    I have noticed that it is becoming harder to find direct answers to some basic questions because they were answered so long ago.

  49. LTO @496 ,

    I commented (@495) because I was surprised that you (@494) were giving the impression that you thought the escape/emission altitude (for 15um IR) was in the stratosphere at some latitudes.

    The tropopause is a temperature- & weather-related concept.   OTOH, the 15um photon "escape" (for CO2 emission) is dependent on absolute CO2 density ~ and of course also dependent on temperature of "local" air which energizes the CO2 molecules to emit a sufficient energy flux to achieve the appropriate contribution to cooling the planet.   (The "altitude" you are interested in is not an ultra-thin single altitude [for any particular latitude] but is a weighted average).   The emission is from a fuzzy band (of altitude), so we mustn't oversimplify too far.

  50. Golly!! More complexity being considered.

    The tropopause is a temperature thing. The tropopause height drops at night, it drops through winter and it drops greatly with latitude towards the poles. The emissions height is a pressure thing (as well as a wavelength thing). Averaged across the globe, there are parts of the CO2 emissions band that have an emissions height up in the stratosphere, and an increase in CO2 concentrations (& thus the emissions height) will thus see wider wavelengths with emission heights up in the stratosphere. A rising stratosphere emissions height sees an increase in emissions temperature which would counteract part of the the otherwise full CO2 AGW effect. But with rising CO2, there will also be more wavelengths becoming significant to CO2 absorption, for instance the two compound bands at roughly 10 microns.

    As for the poles, these are small in area relative to the tropics and emit much less radiation from the surface (which can be absorbed by CO2) being colder. The tropopause at the poles drops to ~250mbar from ~100mbar in the tropics, so 2.5x the atmosphere above it with CO2 content. So at the poles the balance between above/below the tropopause emissions height will be greatly slanted towards stratosphere, at the poles relative to the tropics. But averaged over the globe, the poles are a small part of the equasion. It is all latitudes that are averaged out to give the logarithmic relationship between CO2 and CO2 forcing, a relationship which stands (from memory) up to 1,200ppm.

    Beyond that, the reference linked @489 is reporting that the CO2 forcing would be greater than logorithmic above 1,200ppm. Thus the rise of CO2 emissions heights for more wavelengths into the stratosphere would not see an end to CO2-powered AGW.

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