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Has the greenhouse effect been falsified?

Posted on 19 May 2010 by sylas

Guest post by Chris Ho-Stuart

Most participants in climate debates can agree that the atmosphere's capacity to interact with thermal radiation helps maintain the Earth's surface temperature at a livable level. The Earth's surface is about 33 degrees Celsius warmer than required to radiate back all the absorbed energy from the Sun. This is possible only because most of this radiation is absorbed in the atmosphere, and what actually escapes out into space is mostly emitted from colder atmosphere.

This absorption is due to trace gases which make up only a very small part of the atmosphere. Such gases are opaque to thermal radiation, and are called "greenhouse gases". The most important greenhouse gases on Earth are water vapor and carbon dioxide, with additional contributions from methane, nitrous oxide, ozone, and others. If the atmosphere was simply a dry mix of its major constituents, Oxygen and Nitrogen, the Earth would freeze over completely.

Observing the greenhouse effect in action

The simplest direct observation of the greenhouse effect at work is atmospheric backradiation. Any substance that absorbs thermal radiation will also emit thermal radiation; this is a consequence of Kirchoff's law. The atmosphere absorbs thermal radiation because of the trace greenhouse gases, and also emits thermal radiation, in all directions. This thermal emission can be measured from the surface and also from space. The surface of the Earth actually receives in total more radiation from the atmosphere than it does from the Sun.

The net flow of radiant heat is still upwards from the surface to the atmosphere, because the upwards thermal emission is greater than the downwards atmospheric backradiation. This is a simple consequence of the second law of thermodynamics. The magnitude of the net flow of heat is the difference between the radiant energy flowing in each direction. Because of the backradiation, the surface temperature and the upwards thermal radiation is much larger than if there was no greenhouse effect.

Atmospheric backradiation has been directly measured for over fifty years. The effects of greenhouse gases stand out clearly in modern measurements, which are able to show a complete spectrum.

IR spectrum at the North  Pole
Figure 1. Coincident measurements of the infrared emission spectrum of the cloudfree atmosphere at (a) 20km looking downward over the Arctic ice sheet and (b) at the surface looking upwards. (Data courtesy of David Tobin, Space Science and Engineering Center, University of Wisconsin-Madison. Diagram courtesy of Grant Petty, from Petty 2006).

When you look down from aircraft at 20km altitude (Fig 1a), what is "seen" is the thermal radiation from Earth that gets out to that height. Some of that radiation comes from the surface. This is the parts of the spectrum that follow a line corresponding in the diagram to about 268K. Some of that radiation comes from high in the atmosphere, where it is much colder. This is the parts of the spectrum that follow a line of something like 225K. The bites taken out of the spectrum are in those bands where greenhouse gases absorb radiation from the surface, and so the radiation that eventually escapes to space is actually emitted high in the atmosphere.

When you look up from the surface (Fig 1b), what is "seen" is thermal backradiation from the atmosphere. In some frequencies, thermal radiation is blocked very efficiently, and the backradiation shows the temperature of the warm air right near the surface. In the "infrared window" of the atmosphere, the atmosphere is transparent. In these frequencies, no radiation is absorbed, no radiation is emitted, and here is where IR telescopes and microwave sounding satellites can look out to space, and down to the surface, respectively.

The smooth dotted lines in the diagram labeled with temperatures are the curves for a simple blackbody radiating at that temperature. Water vapor has complex absorption spectrum, and it is not well mixed in the atmosphere. The emissions seen below 600 cm-1 are due to water vapor appearing at various altitudes. Carbon dioxide is the major contributor for emission seen between between about 600 and 750 cm-1. The patch of emission just above 1000 cm-1 is due to ozone.

The term "greenhouse"

The term "greenhouse" was coined for this atmospheric effect in the nineteenth century. A glass greenhouse and an atmospheric greenhouse both involve a physical barrier that blocks the flow of heat, leading to a warmer temperature below the barrier. The underlying physics is different, however. A glass greenhouse works primarily by blocking convection, and an atmospheric greenhouse works primarily by blocking thermal radiation, and so the comparison is not exact. This difference is well understood and explained in most introductions to the subject. Where confusion arises, it is usually the glasshouse that is improperly described, rather than the atmospheric greenhouse effect.

The enhanced greenhouse effect

The greenhouse effect itself has always been an important effect on Earth's climate, and it is essential for maintaining a livable environment. Without it, the surface would rapidly freeze.

The existence of a greenhouse effect itself should not be confused with changes to the greenhouse effect. Global warming in the modern era is being driven by increasing concentrations of greenhouse gases in the atmosphere, which leads to an enhanced greenhouse effect. This is covered in more detail as a separate argument: How do we know more CO2 is causing warming?

Many thanks to Chris Ho-Stuart for this guest post. Chris is co-author of the recently published paper Comment on "Falsification of the atmospheric CO2 Greenhouse Effects within the frame of physics" (Halpern et al 2010). which is a peer-reviewed response to the paper by Gerlich and Tscheuschner which claims to falsify the greenhouse effect. Chris also runs Climate Physics Forums which is a very high quality forum featuring substantive and courteous discussions of climate science. There is also a discussion thread on the Halpern et al paper.

Further reading:
  • Most textbooks on climate or atmospheric physics describe the greenhouse effect, and you can easily find these in a university library. Some examples include:
  • The Greenhouse Effect, part of a module on "Cycles of the Earth and Atmosphere" provided for teachers by the University Corporation for Atmospheric Research (UCAR).
  • What is the greenhouse effect?, part of a FAQ provided by the European Environment Agency.

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Comments 1 to 50 out of 78:

  1. "The surface of the Earth actually receives in total more radiation from the atmosphere than it does from the Sun."

    I think there is a typo needing correction here, but if not, (as in the last article), taken on face value, the implication is another runaway scenario.
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  2. RSVP: I think this makes sense, if you divide the Earth's temperature^4 by the blackbody temperature^4 (i.e. temperature minus 33C) then you get a factor of 1.75, i.e. yes, more downwards longwave than solar longwave!

    Sounds nuts. I did the same calculation for Venus a while back and the factor is obscene; over 90% of the heat flux to Venus' surface can't be solar in origin.
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  3. Surprisingly, this is correct as given. Here's a rather more detailed account.

    The energy balance for the Earth is described in:
    Trenberth, K.E., Fasullo, J.T., and Kiehl, J. (2009) Earth’s Global Energy Budget, in Bulletin of the Amer. Meteor. Soc., Vol 90, pp 311-323. (open access link)

    The following diagram from that paper summarizes the situation:


    Basically, we are comparing the 184 W/m2 from the Sun which gets to the surface, and the 333 W/m2 which comes to the surface from the atmosphere.

    It may also help to do a quick accounting with these numbers:

    Total input to the surface (in W/m2):
    184 Solar radiation
    333 Atmospheric backradiation
    ---
    517 Total

    Total out from the surface:
    23 Reflected solar radiation at the surface
    17 Upwards atmospheric convection
    80 Upwards latent heat of evaporation
    396 Thermal radiation emitted by the surface
    ---
    516 Total

    Imbalance: 1 W/m2.

    These numbers are not perfect. In fact, the lead author of the paper has been particularly strident in calling for better measuring systems to nail down the balance much better. There are several good pages here on that matter. See, for instance, Trenberth can't account for the lack of warming, and John's recent blog Tracking the energy from global warming, which explains some of the more direct attempts to measure the imbalance, and obtain values around about 0.6 ± 0.2

    But to understand the point you have quoted, a crude estimate will suffice. The Earth's surface is quite warm (fortunately for us) and so it radiates a LOT of heat. On top of that, heat is carried away by sensible heat flows: convection and latent heat.

    By conservation of energy, that has to balance what is being received, with any small imbalance being because the planet warming or cooling by absorbing extra energy or shedding it. At present, we have global warming, and the ocean is sucking up some of the available energy as it slowly increases in temperature. But even when there is no warming or cooling, you still have all that energy leaving the surface, which must balance with the energy coming in.

    Most of the energy coming to the surface -- about 65% or so -- is atmospheric backradiation. Note that this has the advantage of coming in both night and day. Solar input in the day is larger, but at night it is zero. Without the natural greenhouse effect, nighttime temperature on Earth would plummet as the surface radiated away its energy straight into space.
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  4. Forgive me interjecting; I puzzled over the same sentence but I'm not a scientist. Should not the sentence therefore read, "The surface of the Earth actually receives in total more longwave radiation from the atmosphere than it does from the Sun." Or have I misunderstood?
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  5. "The net flow of radiant heat is still upwards from the surface to the atmosphere, because the upwards thermal emission is greater than the downwards atmospheric backradiation. This is a simple consequence of the second law of thermodynamics"

    I think this extract is at best confusing. If we ignore the heat source in the earth itself and direct anthropogenic heat which are insignificant, surely the net heat flow must balance at any point under steady state conditions, that is in the absence of a greenhouse effect. Radiative differences may occur of course due to convection within the atmosphere. There will be a small net radiative input with a greenhouse effect of course.

    Neither is it clear what this has got to do with the 2nd law.
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  6. 17 Upwards atmospheric convection
    80 Upwards latent heat of evaporation

    Yes any radiation imbalance under a steady state scenerio has to be made up from these other heat transfer movements, really this should be made clear.
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  7. Sorry, I now get it. Thanks for a very clear explanation, sylas. I wrote my comment and then yours appeared before mine after I'd posted.

    I think what is being said is that the Earth is like a bucket with a tap running into it. The inflowing water from the tap is matched exactly by the water flowing over the rim of the bucket. It's in equilibrium. What's happening within the bucket (the planet and its atmosphere) is irrelevant. Of course, what humans are doing at the moment is raising the sides of the bucket... and increasing the pressure at the bottom -- or is that taking the analogy too far?.

    Sorry if I'm being simplistic but it's the only way I get my head round things.
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  8. perseus, in the paragraph you have quoted, I am speaking of a continuous flow of heat between the surface and the atmosphere. Since the atmosphere is (and remains) cooler than the surface, there is a continuous flow of heat from the surface to the atmosphere.

    The backradiation is large; but it is not as large as the thermal radiation up from the surface. The actual heat flow, in the proper sense of a flow of energy between two reservoirs at different temperatures, is difference between thermal radiation up and thermal radiation back down. This is 63 W/m2 of radiant heat flow, using the numbers from the energy balance diagram. To this we add 97 W/m2 of sensible heat flows (convection and latent heat).

    This never comes to equilibrium; or rather, it is a dynamic equilibrium, because the Earth is continually receiving energy from the Sun. This means it maintains its temperature, and there is an unending flow of heat from the Earth to the Atmosphere.

    You are quite right that the heat sources within the Earth itself are negligible. The temperature of the Earth and the atmosphere are maintained by a continuous flow of energy from the Sun, to Earth, and then back out to space.
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  9. John, the tap analogy is a very good one, and can be used to good effect.

    It is just an analogy of course, but such things can be excellent stepping stones to understanding of the actual system itself.
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  10. perseus, on radiation balance, we never speak of radiation balance at the surface itself. We speak there of energy balance.

    Radiation balance applies at the top of the atmosphere, because this is the only way energy arrives at Earth or is taken off Earth out into space.

    Radiation balance also applies in the stratosphere (or very close to it) because there is no convection, no weather, no precipitation. In fact, this is a defining quality of the stratosphere for any planet. It is that part of the atmosphere which is in a radiative equilibrium, with negligible vertical energy flows by convection.

    But note that the title of the diagram I provided is "Global energy flows". At the surface and in the troposphere it is all about energy balance, not radiation balance.
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  11. "The surface of the Earth actually receives in total more radiation from the atmosphere than it does from the Sun."

    The confusion about the meaning of this sentence might be clarified for some by noting that the origin of all the energy is from the sun, just like the energy from the gas in your car is from the sun, it has just been transformed to another form (wavelength) by processes in the atmosphere, just like the energy in the gas in your car has been transformed from solar energy by biological and geological processes.
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  12. Has anyone seen graphs similar to those above, but covering a wider range of the EM spectrum? If we look at the wavelength factors along the top of each graph we see that the lowest value (on the right) is 6 micrometers... while the visible spectrum would be all the way down around 0.4 thru 0.7 micrometers. Ultraviolet light would go all the way down to about 0.01 micrometers. On the other hand the full range of infrared would go off the left side of the chart up to 1,000 micrometers.

    My understanding is that microwaves and x-rays (the ranges beyond those described above) represent a minuscule portion of the Earth's energy balance and can thus safely be ignored, but I think it would be very interesting to see the incoming and outgoing visible light range and how the atmosphere is impacting the rest of the infrared and ultraviolet light coming in from the Sun.
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  13. "This is possible only because most of this radiation is absorbed in the atmosphere"

    I thought most of the energy is absorbed by the ocean?
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  14. I would have thought that energy absorbed by the atmosphere can then, in turn, migrate to the sea, HumanityRules. Surely the whole nature of energy is that it's not in a steady state, it moves around and gets converted from one state to another but is never destroyed? I guess my terminology might be wrong but in principle that's what's happening.
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  15. Chris Ho-Stuart at Wednesday, 19 May, 2010 wrote:
    In some frequencies, thermal radiation is blocked very efficiently, and the "optical depth" of the atmosphere is very small.

    Come on. If radiation is blocked efficiently, optical depth is not very small, but huge.

    If I0 is the intensity of radiation at the source and I is the observed intensity after a given path, then optical depth τ is defined by the following equation:



    You can easily verify for yourself that whenever I gets tiny compared to I0 ("radiation is blocked very efficiently"), τ should be large.
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  16. Thank you Berényi, you are quite correct! I have emailed John to get that fixed.

    Much appreciated! -- sylas
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  17. Followup to Berényi's correction.

    How's this. I propose to get rid of some technical terms not used in the rest of the essay anyway. I have suggested the incorrect phrase be replaced to read:

    In some frequencies, thermal radiation is blocked very efficiently, and the backradiation shows the temperature of the warm air right near the surface.


    Thanks again -- sylas
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  18. Humanity rules, the sentence you quoted speaks of "this" radiation, which in the context of the preceding sentence, means the thermal radiation emitted from the surface. Most of this radiation is indeed absorbed in the atmosphere.
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  19. 18.sylas
    Thanks
    I sort of guessed that somebody was going to tell me that all that energy in the ocean was irrelevant to this. I'm not sure those two sentances go well together. I think something is wrong with the wording in the first, it doesn't seem to make sense.

    Do you know if there has been a change over time for the past couple of decades to the results seen in Fig 1?

    Also you make the following point "Water vapor has complex absorption spectrum". I thought water as a vapour has a discrete absorption spectrum. Isn't it condensed water droplets that's more complex? (So says apsmith on your forum)
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  20. Hey! The tap/bucket analogy was mine! LOL...

    I'm certain that I'm far from the only person who has come up with this comparison - I first used it years ago when discussing evolution with some people who thought that it violated entropy/thermodynamics.

    I think it's a useful mental image for GHG warming, and I've found it helpful in explaining these concepts to a number of people. One nice thing about it is that you can directly see in it that energy is flowing in the correct direction. But as sylas said, it's much better to actually understand the system itself, rather than analogies.
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  21. HumanityRules, I am open to clearer ways to express this! On the other hand, there will always be a need to explain it further, and that's ok.

    The current two sentences to which you refer are:
    The Earth's surface is about 33 degrees Celsius warmer than required to radiate back all the absorbed energy from the Sun. This is possible only because most of this radiation is absorbed in the atmosphere, and what actually escapes out into space is mostly emitted from colder atmosphere.

    The surface of the Earth is around about 15C, or 288K. If you take a uniform temperature and high emissivity you get about 390 W/m2 emitted. The real value is a few W/m2 higher mainly because temperatures are not uniform, which gives slightly higher total emission thanks to a 4th power relation from temperature to emission. But the energy we actually absorb from the Sun, in total, is about 240 W/m2, which is what you get from a sphere at a temperature of about 255K, or -18C. What the Earth radiates into space is about 240 W/m2, in total.

    This is huge difference between what is radiated at the surface and what gets out to space. This is only possible because surface radiation mostly never gets out to space, but is absorbed in the atmosphere. What eventually gets out to space is mostly emitted within the atmosphere, where it is colder.

    You also ask:
    Do you know if there has been a change over time for the past couple of decades to the results seen in Fig 1?

    I just noticed the other day, as I was browsing this site, a nice discussion of the changes from 1970 to 1996. It's described, with some good illustrations, at Empirical evidence that humans are causing global warming. Note that you get different emission spectra at different seasons, times of day, and locations. So changes actually refer to a global mean, and not the specific observations on a particular day, and a certain place, which I am using here to show direct observations of the greenhouse effect at work.

    I think we are best to continue to focus on the existence of the greenhouse effect in this page, rather than how it may be changing.

    As for the absorption spectrum of water; it is intrinsically complex in any case, with a large number of distinct absorption bands. On top of that it is not well mixed in the atmosphere. And to finish things off, you get the differences associated with phase changes, which apsmith is speaking of.
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  22. "Has the greenhouse effect been falsified?"

    No it has not. IMO, G&T should have never gone to press and represents yet another attempt by the contrarians to create the impression of debate based on sub par science (and some might argue that that description is too generous) and sow doubt amongst lay people.

    Steve Carson also does an excellent (and thorough) debunking of G&T.

    That all said, the terminology "greenhouse effect" is clearly a misnomer and as such remains problematic and confusing to some.

    Anyhow, congratulations (and a big thanks) to Halpern et al. for making the effort and taking the time to soundly refute G&T.
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  23. In a Geocanada 2010 talk last week, the presenter claims that the greenhouse effect is 34.5C and that CO2 is responsible for 10% of it, which is 3.45C. From this he concludes that a doubling of the current CO2 concentration in the atmosphere adds only 0.345C to global temperatures. Can anybody explain, how someone can arrive at these numbers?

    The talk was recorded as an mp3 and is found here.
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  24. I don't think it's too off-topic to contrast G&T's kamikaze attack on climate science w/the National Academies of Science release today of three reports regarding climate change. This NAS effort excellently illustrates just how far off track G&T have wandered with their thought experiment.

    Follow this link to get to the NAS materials:

    Strong Evidence on Climate Change Underscores Need For Actions
    to Reduce Emissions and Begin Adapting to Impacts


    For us amateurs and bystanders including G&T, here's the significant nut of the entire rather overwhelming set of three reports:

    Some scientific conclusions or theories have been so thoroughly examined and tested, and supported by so many independent observations and results, that their likelihood of subsequently being found to be wrong is vanishingly small. Such conclusions and theories are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities.

    That's the NAS speaking, not known for its rhetorical liberality.

    In this arena G&T are no better or worse or more importantly useful to the advancement of understanding than most of us other odd ducks who natter away on climate. We offer our best interpretations and guesses regarding a topic that we can't actually attack in a serious way because we're innocently too ignorant of the specialized information and practice needed. Like most of us, G&T's perspective, level of information and specific skills on the topic of climate change are not sufficient to produce useful contributions.
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  25. Monckhausen at #23

    Besides the fact that they start by understating the direct contribution of CO2 to the GH effect (more likely 15-25% depending on whether you include clouds), they are ignoring the feedback that increased temperatures due to higher CO2 have on water vapor in the troposphere. That water vapor amplifies the effect of CO2 on the GH effect substantially.

    Water vapor is the most powerful greenhouse gas

    This seems to be a common "mistake."
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  26. KR: I can't remember reading yours before but it looks like we both came up with exactly the same analogy independently, right down to humans raising the sides; so I guess it must be a good one but I'm happy to acknowledge you were first.

    I arrived at it as a re-think of the 'bucket with the hole analogy' where naturally-produced CO2 (the tap) is balanced by the same amount of CO2 being locked-up by natural processes (leaking through the hole), thus keeping atmospheric CO2 in equilibrium -- before a small but steady amount of human-produced CO2 causes the level to slowly rise and the bucket to overflow. But that's off topic and I guess will probably hit the cutting room floor.

    I agree; analogies are very useful.
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  27. Interesting!
    My introduction to Kirchoff was as a teenager in electrical engineering!
    http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws

    Now he turns up in climate science.

    OK. I understand Kirchoffs law, applied to large bodies.

    But I don't really understand how at the quantum level a CO2 molecule stops vibrating and suddenly re-emits the IR electromagnetic wave. What makes this happen.
    I can understand the absorption, but not so clear on the emission.
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  28. The Ville, the CO2 molecule doesn't stop vibrating. A CO2 molecule is constantly undergoing translation, rotational and bond vibrational motions according to the thermal energy (temperature) of its surrounds. Absorption of a photon of LWIR of appropriate energy excites a specific bond vibrational mode promoting the molecule to an excited "high energy" state. The equilibrium state appropriate to the local temperature is a lower energy (ground state) and the relaxation back to this ground state is achieved either by re-emitting a photon, or by transmitting the energy to local molecules (which need not be IR absorbing; e.g. O2 or N2) by molecular collisions (thus raising the temperature of the gas a tiny amount).
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  29. Stephen Baines # 25:

    Thanks! I knew it must have been something really simple that had been overlooked.
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  30. @Monckhausen... I think the best response (I can think of as a layperson) to people who say that water vapor is the cause for most of the greenhouse warming is this: Well, how does water vapor get there? What is the mechanism? The answer is temperature. What drives the temperature? Well, obviously GHG's. If the other GHG's were not there all the water vapor in the atmosphere would freeze out and we'd have a very cold planet. So, it's the other GHG's that act as the "control knobs" (to borrow from Richard Alley) for temperature.
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  31. I just finished watching this Stephen Schneider lecture where he addresses the very same issue of climate change being falsified. He brings up several papers that claim to falsify climate change in one way or another but says all these papers do is, at best, "move the needle" of understanding ever so slightly.

    This is an excellent lecture that is well worth the time to watch.
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  32. The Ville, a vibrating/moving molecule is a group of atoms held together by an electron cloud. Electron energies can change in several ways - collision, molecular changes (ionization, oxidation), and through thermal radiation absorption and emission (EM). The probabilities of various pathways vary between molecules.

    CO2 is an excellent absorber of IR (a good antenna configuration for receiving, if you will), which gives it an equal chance of radiating excess energy away (transmission). This makes IR energy exchange a fairly probable pathway for CO2. O2 and N2, not so much - their molecular configuration makes them lousy antenna in the thermal IR range.
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  33. HumanityRules at 00:46 AM on 20 May, 2010

    re your comments on a phrase and sentence from the top post:
    "The Earth's surface is about 33 degrees Celsius warmer than required to radiate back all the absorbed energy from the Sun. This is possible only because most of this radiation is absorbed in the atmosphere, and what actually escapes out into space is mostly emitted from colder atmosphere."


    This makes perfect sense HR, if you read the half sentence you referred to in the context of Chris’s two sentences above. The Earth absorbs around 240 W.m-2 of radiation from the sun. Obviously in order to maintain radiative equilibrium it must radiate this energy back to space.

    If this was achieved by direct emission to space from the surface then the Earth's surface need be only 255K (around -18 oC). This is easily calculated from the Stefan Boltzmann equation [240 W.m-2 = (255 K)^4 . 5.6704 x 10^-8 W.m-2.K^-4]

    However since the Earth's atmosphere is strongly absorbing of long wave IR of these "temperatures" (energies) due to greenhouse gases, radiative equilibrium can't be achieved by direct emission to space from the Earth's surface. Radiative balance is achieved by emission to space from the atmosphere. On average the emission will occur at an altitude where the temperature is around 255K. Clearly if a region (layer) of the atmosphere has a temperature of 255K, then the layers below, right down to the surface, will be warmer than 255K. In other words the greenhouse effect can be thought of as retaining energy in the system until the temperature of layers of the atmosphere where LWIR is radiated to space is “pushed up” to 255K. All the layers below this warm up (including the surface).

    As you increase the greenhouse gas concentration the efficiency of LWIR emission to space from any layer of the atmosphere decreases further, and so the average height of long wave IR emission to space increases. Since the atmosphere gets colder as one goes higher, emission becomes less efficient. Thus energy tends to build up in the system (positive radiative imbalance) further warming the atmospheric layers (right down to the surface) until the regions of the atmosphere where LWIR are emitted to space reach 255K.
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  34. #1 RSVP

    "I think there is a typo needing correction here, but if not, (as in the last article), taken on face value, the implication is another runaway scenario."

    In the last article, chris and others addressed your concerns about a runaway scenario.
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  35. #30 robhon

    But then, the skeptic could and will argue water vapor is the most abundant greenhouse gas - and it alone causes the T increase and thus the feedback.

    Really difficult to argue with someone who does not accept facts. I just checked Wattsupwiththat...they come up with all sorts of stuff for anything: e.g. that science is controlled by 50 'believers' and that the current Heartland conference is attended by 700 of the world's best scientists...

    I collected more CO2 'skepticism' here:

    http://friendsofginandtonic.org/page4/page7/page7.html

    The problem is that you have to do a lot of reading to debunk this...and then you are still arguing against someone who refers to you as a believer. You cannot win against someone who argues from a position of ignorance.
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  36. @monckhausen... I'm definitely not a physicist but I'd have to think that's impossible. It's a good question for some of the better trained folks here. What would happen if you pulled ALL the other GHG's out of the atmosphere? What would happen to water vapor?

    For that notion to hold true CO2, I believe, would have to have virtually NO IR absorption properties, and we know that's not true. You can prove that with a very very simple lab experiment.
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  37. monckhausen

    Well, technically if yer look at it extremely simply, if all the water vapor was stripped outta the atmosphere the average temp would be below freezing... but then again yah would have a reduction in clouds... and you would also have a massive albedo change of the globe... simple put, its not a simple question, with a simple answer.
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  38. Actually this has been both discussed and calculated. See the detail with links to calculation and code at: Calculating the greenhouse effect

    Short answer - it would be very cold without water. Around 20 deg colder.
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  39. 33.chris

    Thanks Chris very clear, I think it was the word "required" that's throwing me. I'm going to say I get it now.
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  40. Thanks Chris @ 28.
    A nice and clear explanation.
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  41. 35 Monkhausen, re water vapour.

    There is a significant difference between water and CO2 though in the range of temperatures that exist on earth.

    That is water can rapidly change between gas, liquid and solid. CO2 doesn't.

    Hence it is more likely that CO2 will influence water, eg. more or less water vapour, depending on how much CO2 is in the atmosphere. Making water a feedback mechanism.

    Although water vapour may be a greenhouse gas, I think water in general and its relation to CO2 at earth temperatures gives a bigger and better 'picture'.
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  42. .....but if it was so cold that the water vapor froze out of the atmosphere, then life might not have evolved, so the earth might still have a reducing atmosphere with lots of ammonia and methane which might warm the earth up so that life could evolve which....... !!!!!


    Is LOL the word?
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  43. Chris Ho-Stuart at Wednesday, 19 May, 2010 wrote:
    When you look down from aircraft at 20km altitude (Fig 1a), what is "seen" is the thermal radiation from Earth that gets out to that height. Some of that radiation comes from the surface. This is the parts of the spectrum that follow a line corresponding in the diagram to about 275K.

    No. The part of the spectrum in the atmospheric IR window (between 8 and 13 μm, i.e. wavenumber 770 and 1250 cm-1) has an approximate temperature of 268 K (-5°C), not 275 K (+2°C), provided the dashed lines do stand for blackbody radiation of temperatures indicated by labels in the figure. A 7 centigrade difference is not negligible.

    However, not even all of this radiation is coming from the surface. As you can see, clear sky atmospheric transmittance in window is below 80% at all frequencies, variable.



    Therefore some of the IR window radiation is absorbed on its way up, thermalized on a different local temperature and re-emitted, quite possibly on frequencies outside the window.

    This is why determining actual surface temperature of the ice sheet below (near Barrow, Alaska) from IR window radiation can get quite tricky, even if emissivity spectra of ice and snow makes them almost perfect blackbodies in this frequency range.

    Anyway, an ice sheet surface temperature of 275 K as claimed is not possible.
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  44. Berényi, you are right again. You currently my new best friend, and I really appreciate the editing work you are doing for me.

    D'oh. I was looking at the lines as being 20K separated, but of course it is 10K, and so the numbers in the post need to be fixed. I put this together in a bit of a rush before going on an overseas trip... and in fact I am posting this now from an airport hotel.

    As for the 80% transmittance, I think we may need to be a bit careful. There's a difference between the general mean over the whole planet and a specific spectrum observed at a certain time and place, which I am told was under clear sky conditions. But for the time being the worst error is with the numbers and I'll get that fixed first.

    What I am trying for here is something that is as straightforward as I can make it, suitable for a wide range of readers. I do appreciate there are all kinds of subtleties involved in obtaining surface temperatures with microwave brightness, but I did think these particular two spectra, which were taken simultaneously, gave one of the clearest direct observations of the greenhouse effect I have seen. I know that Tobin's team did take measurements over Barrow, but I am not 100% sure that this is from there. The diagram itself is not from a published paper, but from data that David Tobin provided to Grant Petty, and which he plotted for his text book.

    If you have concrete suggestions for better wording in any part of the essay, please go ahead and made the suggestions. You've been a great help! Sorry if I am a bit rushed over the next week or two, but I'll try and pop in here periodically while I am on the road.
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  45. sylas,
    we definitely need to be carefull before claiming something based on a graph which we do not know what's showing, as Berényi Péter did. Indeed, although the overall behaviour is similar in all the spectra one can find over the internet, typically they are just to illustrate the general behaviour, not intended for quantitative analysis. Here's an exaple of two spectra from the same site, one shows 90% trasmittance, the other 100%.

    But there's one more fundamental error that Berényi Péter did. He apparently thinks that there's a direct relation between transmittance level and temperature. Even without other light extintion mechanisms (e.g. scattering), there's no such relation except in the saturated part of the spectrum. Indeed in the intermediate cases scientists talk about brightness temperature, not temperature alone.
    Then, i find your claims in the context of the general description perfectly valid.
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  46. #44 sylas at 21:20 PM on 20 May, 2010
    You currently my new best friend

    You are welcome :)

    As for the 80% transmittance, I think we may need to be a bit careful

    I think the atmospheric transmittance spectrum is for clear sky conditions. It can't be a global average, since about half the skies are covered by clouds at any moment and they are opaque to IR (zero transmittance).

    However, it is far from being opaque in the mid IR window (optical depth ~0.22). Still, Fig. 1 (a) has a problem. Using Planck's formula for blackbody radiation



    with ν = 100cκ (c is the speed of light, κ is wavenumber in cm-1) at 260 K peak of spectral radiance is found at 510 cm-1 to be 100 mW m-2 sr-1 cm indeed as shown by the second dashed line from above in Fig. 1 (a).

    On the other hand, since IR transmittance in window is ~80%, spectral radiance coming from the ice sheet surface and measured at an altitude of 20 km can not be proper blackbody radiation, it must be an attenuated version of it. In other words the surface is seen as a grey body from there with emittance around 0.8 even if the surface itself is a close approximation of a black body in this frequency range. This must be so as in the stopband atmospheric absorptivity is orders of magnitude higher than in the window, therefore according to Kirchoff's law, almost all of the absorbed radiation after having got thermalized, is re-emitted outside the window.

    It means the radiance curve in Fig. 1 can not be produced by actual measurement or if it was, what is shown is a scaled up version of values measured. Undocumented tricks like this are not helpful.

    BTW, there are some actual IR transmission data for two observation sites (Cerro Pachon, 2700 m and Mauna Kea, 4200 m). It looks like at high altitudes with low atmospheric moisture IR windows get quite transparent. However, Barrow, Alaska is at sea level (elevation 3 m).
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  47. Berényi Péter,
    "Fig. 1 can not be produced by actual measurement or if it was, what is shown is a scaled up version of values measured. Undocumented tricks like this are not helpful."

    Well, you started with a false premise and arrived to the wrong conclusion, no surprise. And to cast unsupported doubts is not helpful.

    Didn't you notice any difference between the data shown here and the one you linked to? The transmittance is zero above 14 μm and below 8 μm. Is this an undocumented trick as well?
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  48. #47 Riccardo at 06:54 AM on 21 May, 2010
    you started with a false premise

    Which one do you mean?
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  49. Berényi Péter,
    the one I evidenced before, to begin with.
    Beyond that, what we know is that the measurements in fig. 1 behave like basic absorption/emission in the atmosphere dictates, the spectra nicely follow what's expected they should do, they also match what can be calculated from MODTRANS code. You, instead, throw in the discussion strange theories on the thermalization based on a descriptive graph found on Wikipedia to conclude that either those cannot be real measurements or that some "undocumented trick" (fake data?) has been used. Astonishingly.
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  50. I am very grateful for comments which have picked up a couple of errors in my original essay, now fixed. I will continue to accept corrections with thanks, and update the essay as appropriate to get rid of errors.

    I may not agree with all the criticisms or corrections suggested, however. I do not see any problem at present with the wording being used, which at the time of writing reads:

    When you look down from aircraft at 20km altitude (Fig 1a), what is "seen" is the thermal radiation from Earth that gets out to that height. Some of that radiation comes from the surface. This is the parts of the spectrum that follow a line corresponding in the diagram to about 268K. Some of that radiation comes from high in the atmosphere, where it is much colder. This is the parts of the spectrum that follow a line of something like 225K. The bites taken out of the spectrum are in those bands where greenhouse gases absorb radiation from the surface, and so the radiation that eventually escapes to space is actually emitted high in the atmosphere.

    The atmosphere's main IR window (the region of IR transparency) is around 800 to 1000 cm-1 in wavenumber, or about 10 to 12.5 microns in wavelength.

    Some of the radiation to space DOES come from the surface, and the wavelengths to look for surface radiation are those in this IR window. Wording can always be tweaked a bit, but I don't see any problem with what is there at present. The IR window in this specific case is almost all from the surface.

    The main point here is that we can observe the greenhouse effect at work. It is not falsified; it is a notion known from basic physics for well over a hundred years and it is now amply confirmed in measurements such as those I have shown and also such as those shown in the site linked by Berényi: IR Transmission Spectra. For example, take the midrange-IR spectrum shown for Mauna Kea, which matches most closely the range of Figure 1 in the blog. I show the image on the page, and below it a reversed image with wavenumbers written into the horizontal scale for a slightly easier comparison with the figure in the blog. I have also added a horizontal line at transmission level 1 (fully transparent).
    Mauna Kea Mid-IR transmission

    This also shows what is wrong with the specific criticisms made against Figure 1 in the blog. Berényi says:
    On the other hand, since IR transmittance in window is ~80%, [...snip...]

    It means the radiance curve in Fig. 1 can not be produced by actual measurement or if it was, what is shown is a scaled up version of values measured. Undocumented tricks like this are not helpful.


    Figure 1 is produced by actual, unscaled measurement. There are no undocumented tricks. Your problem here is assuming the ~80% IR transmittance. That is simply not valid. You can see clearly that it is not the case in Mauna Kea data either. And while you are right that Mauna Kea is high altitude... why not also note that the Artic is high latitude? It makes a comparable difference.

    Mauna Kea is at an altitude of 4200m, which is enough to get about 90% of the water vapour in the atmosphere; quite so! But it is also in a humid climate, where you have a high tropopause and lots of water vapour at low altitude. So sure; at low altitude in Hawaii you would have less IR transmittance (though you cannot assume a globally based mean figure will be a good specific estimate for this or any other location). But the Arctic regions, where the Fig 1 data was collected, have much lower specific humidity than Hawaii.

    Even if you don't trust the data I have supplied, you need a much better basis for presuming it is inaccurate and presuming to scold on that basis. Be skeptical by all means. I will try to get hold of the actual data myself if I can and follow up. But please be assured that I am not deliberately distorting things.

    You remain the most helpful contributor here for cleaning up errors in the blog post. I'd like to continue a constructive engagement. You are welcome to be skeptical, but without being overly sensitive... I'd really appreciate a bit more credit myself for not trying to be deliberately deceptive. Can we do that?

    Cheers -- sylas
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