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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 51 to 59 out of 59:

  1. #50 sylas at 02:23 AM on 23 May, 2010 please be assured that I am not deliberately distorting things I have never told you would do such a thing, not even thought about it. If it gave an impression like that I am sorry. Figure 1 comes from a textbook (it is Fig 8.2 there). A First Course In Atmospheric Radiation (2nd Ed.) By Grant W. Petty 460 pp. (paperback) Sundog Publishing, Madison, Wisconsin Publication Date: March 2006 ISBN-10: 0-9729033-1-3 ISBN-13: 978-0-9729033-1-8 Unfortunately we have seen more than one error in textbooks. Of course the arctic air could be extremely dry then and there. We may never know since neither date & exact location of measurement is specified, nor arctic window (long wave, low wave number) frequencies are included in the upwelling IR graph. Otherwise the sheer mass of air above sea level also absorbs some window radiation. Anyway, it is always better to understand in detail what you see than not to. Fig 8.3 a-b-c-d of the same book are also enlightening.
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  2. Berényi Péter, could you be so kind to provide the figure captions or give the details yourself?
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  3. #52 Riccardo at 04:05 AM on 24 May, 2010 could you be so kind to provide the figure captions Unfortunately I don't have the book itself, just web references (publisher's site). There are some excerpts from first edition along with all the figures. But Fig 8.3 a-c is rather self explanatory. The effect of really low specific humidity can be seen at the low frequency end of Fig 8.3 b (Antarctica). I think Fig 8.2 (Barrow, Alaska) shown as Fig 1 in the post does not have extreme low humidity after all. Fig 1 a (20 km looking down) may lack vapor fingerprint below 600 cm-1, but Fig 1 b (surface looking up) has it. With really low vapor contents it would be a partial see-through in this frequency range to 2.7 K space above (as it is at the Antarctic ice sheet during winter). On top of that the rather high surface temperature (268 K) indicates the shot was not made in winter. Also, note how CO2 has a cooling effect in Antarctic winter and to a lesser extent for deep convective cloud tops at the Tropics (Fig 8.3 c, lower graph). It is worth considering the highly variable contribution from water vapor as well, even if none of the graphs shows anything below 400 cm-1 in spite of the significant (wet) action happening there.
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  4. Berényi Péter, while i can see logic of the sequence in the book, i still can not understand your point. Why did you show that sequence of graphs? The extreme -90 °C case in winter in Antarctica (presumably Vostok), what has to do with fig. 1 here taken at Barrow at a temperature of about 0 °C? Leave the "cooling effect" aside. Remember that 20 Km above the surface in Antarctica is well inside the stratosphere and temperature increases with altidude there. What you improperly call "cooling effect" is just emission from the stratosphere at a higher temperature than the ground. It's always there.
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  5. Hi Berényi Péter; thanks -- sorry if I was over sensitive. I am reassured and happy to continue to explain what is necessary for your concerns, and to keep fixing any errors I have allowed into the essay! The main thing I can do at this point is assure you that the plot in the Figure is one is of real measurements, unscaled; and there's nothing particularly unexceptional about them. The IR transmittance for those observations is clearly well over 80%, as you note, but this is not unusual. It is not a global average, but a nice clear sample for comparing up and down fluxes in the Arctic. Essays and textbooks can indeed have errors, as we've seen in my writing here as well! But the specifics of the figure here are not an error. You've got a nice open IR window there. Other spectra will show other characteristics of the conditions at other times and places, and this is indeed very interesting -- but I don't understand the point. They ALL show the greenhouse effect at work, and there's nothing to cast the slightest doubt on the figure I used. Dave Tobin is a busy working scientist and I appreciate his willingness to help out with this essay, but I don't want to overload him with questions when the basic fundamentals here are pretty straightforward. He has said to me unambiguously that there are real measurements, from real instruments, and that it is not uncommon to have > 80% transmission in the 10 micron window. He also points out that "we" (the various scientists involved in this work on remote sensing) have spent many careers worth of time on improving and verifying the accuracy of the measurements. The best reference for more about how this is done is a paper cited for this essay, Tobin et al 2006, which shows up in the copy under the "list of arguments" here. Follow this link: Has the greenhouse effect been falsified. Transcribing the reference to here for now, it is: Tobin, D. C., et al. (2006) Radiometric and spectral validation of Atmospheric Infrared Sounder observations with the aircraft-based Scanning High-Resolution Interferometer Sounder, in J. Geophys. Res., Vol 111, D09S02. This describes validation of atmospheric emission measurements from space, using high altitude aircraft measurements. Similar measurements obtained the upwards emission spectrum shown in figure 1. Unfortunately, the "footnote" sections on further reading and acknowledgments didn't appear here on the blog; I'll see about having them added to this blog post, maybe. I don't know what you mean about a cooling effect. There is a cooling effect in the upper atmosphere, certainly, but at the surface? I don't think so. If you really want to dig into fine details you are better to go to the scientific literature rather than this first level introduction essay. But nothing whatsoever in the details does anything to "falsify" the greenhouse effect! I recommend the references and further reading sections of the main entry as a starting point for people wanting more on the physical details.
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  6. Sylas, it is easy to see that radiation temperature in CO2 stopband (between 14 and 16 μm) is about as cold as it can get. It means that photosphere (the region from where photons have a reasonable chance to escape to space) in this frequency band is above the troposphere. Below that line atmosphere is opaque (optically thick) in stopband. Now. In that region (lower stratosphere) temperature does not decrease with height anymore. If you put more carbon dioxide into air, photosphere will ascend, but its temperature may even increase slightly. Therefore OLR (Outgoing Longwave Radiation) should not diminish in this range with increasing CO2. If you want to explain "greenhouse effect" anomaly due to changing carbon dioxide levels, you should provide some more details. Thanks.
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  7. Berényi Péter, could you please tell to which of the many figures posted in you comments you're referring to? Also, could you elaborate on the concept of the radiation temperature at the CO2 wavelength being "as cold as it can get"?
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  8. #57 Riccardo at 00:18 AM on 27 May, 2010 You can determine the temperature of the photosphere in the 14-16 μm range (CO2 stopband) comparing it to the blackbody radiation curves. As the atmosphere is absolutely opaque in this frequency range, it acts as a perfect black body here. As from photosphere height up it gets transparent fast, this black body radiation segment is seen above wit no attenuation. Approximate temperatures - Fig 8.3 a (Sahara Desert): 215 K (-58°C) Fig 8.3 b (Antarctic ice sheet): 197 K (-76°C) Fig 8.3 c (Tropical Western Pacific): 215 K (-58°C) Fig 8.3 d (Southern Iraq): 217 K (-56°C) Temperatures a & d are close to the respective tropopause temps. Temperature for b may be much colder than indicated for polar tropopause, but it must be during polar winter when tropopause drops to the surface. Temp c is warmer than expected, but troposphere temperature over the tropics is variable both in space and time (of day). Anyway, in that graph photosphere temperature in CO2 stopband is the same over clear sky and thunderstorm anvil. Therefore it should be above tropopause.
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  9. Berényi Péter, you did it again. You used a descriptive graph and said that it's sylas that should provide more details. Even a superficial look at your graph in comment #56 should evidence that it's not worth any quantitative comparison with actual data. And indeed, of the three environments (out of the four spectra shown) two do not much your expectations, and for good reasons.
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  10. #59 Riccardo at 17:22 PM on 27 May, 2010 for good reasons Specify those reasons, please.
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  11. Berényi Péter, because you used temperature profiles which are not the real ones, let alone the ones in that particular place and particular moment when the measurements were taken. Then even if you reasoning is right i do not expect it can be shown in this way.
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  12. Funny. It looks like absorptivity of CO2 in the 14-16 μm band is so high that in this frequency range air is opaque even at a 20 km altitude. Therefore measurements taken from aircraft around 15 μm only show the temperature of air nearby. This is why tropical Western Pacific (Fig 8.3 c) is warmer than expected there. However, proper high resolution molar absorptivity spectra for carbon dioxide mixed with dry air, measured in a controlled lab environment at different pressures from sea level down to perhaps 10 Pascal in tabular format (no pictures) would be appreciated. With these data at hand carbon dioxide photosphere altitude could be computed as a function of frequency. BTW, I can't believe it is not done already. A pointer, anyone?
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  13. Berenyi, the absorptivity spectra will be dependent on the partial pressure of CO2, not the pressure of other gasses - in other words, you can simply adjust partial pressure of CO2 based on total pressure at different altitudes to calculate total absorption/re-emission for CO2. There's an application here for calculating spectra for various gasses, in graphic and tabular forms - that might be of some help...
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  14. Note that even if the peak absorption is extremely high, the absorption spectra shows a set of approximately Gaussian peaks. Increasing amounts of GHG's will therefore raise the edge effects, increasing the bandwidth of blocked radiation, even if the center of the band is saturated - as discussed here to some extent.
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  15. For more on saturation effects, also see the wonderfully titled A Saturated Gassy Argument, at RealClimate.
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  16. Sorry I am a bit slow answering at present, I am not getting a lot of net time. Brényi Péter says:
    If you want to explain "greenhouse effect" anomaly due to changing carbon dioxide levels, you should provide some more details. Thanks.

    Actually, I do not want to explain the anomaly in the greenhouse effect, and I say so in the final paragraph of this blog post. This essay aims at a much more basic level: does the greenhouse effect exist at all? Most people do understand that there is a greenhouse effect. However, some people are skeptical even of this. The aim of this blog and this answer is to help people understand, at a very basic level and using direct observational data, simply that there really is an atmospheric greenhouse effect and that it really does give warmer temperatures at the surface than if the atmosphere did not have the greenhouse gases. All the diagrams shown, and your own comments, continue to confirm this basic point, so I presume we don't have a disagreement on this. The distinct question of the impact of changes in atmospheric composition on the magnitude of the greenhouse effect is dealt with as a separate issue in another page, which I have linked previously. However, I will just quickly respond to your comment about the effects of changing concentrations of carbon dioxide, since you may have missed the most important consequence, which is change to the width of the stopband. You say:

    It is easy to see that radiation temperature in CO2 stopband (between 14 and 16 μm) is about as cold as it can get. It means that photosphere (the region from where photons have a reasonable chance to escape to space) in this frequency band is above the troposphere. Below that line atmosphere is opaque (optically thick) in stopband. Now. In that region (lower stratosphere) temperature does not decrease with height anymore. If you put more carbon dioxide into air, photosphere will ascend, but its temperature may even increase slightly. Therefore OLR (Outgoing Longwave Radiation) should not diminish in this range with increasing CO2.

    The basic theory involved for calculating OLR is covered in some of the more technically detailed textbooks. In particular, Principles of Planetary Climate by Ray Pierrehumbert, due to be published by Cambridge Uni Press in Dec 2010 is excellent and designed to give the student all the tools to do the calculations themselves. This requires a computer to do a large numeric integration through all different frequencies and up a series of graduated steps in altitude of the atmosphere. But in the end you can calculate OLR for a given atmospheric profile. A major primary reference used for the effects of changing CO2 concentration is:

    This paper reports the original calculation of the approximately logarithmic effect of carbon dioxide, at about 5.35 W/m2 per natural log. This is the impact on OLR for a given temperature. Of course, the consequence is that temperatures will increase until OLR again matches the solar absorption. What you are likely to find of particular interest is that this calculation reduced earlier calculations of the effect of increasing CO2 by about 15%, because of more thorough consideration of all effects in particular in the stratosphere. The IPCC 2nd AR used about 6.3 W/m2 per natural log CO2. The IPPC 3rd AR and 4th AR used the improved value of 5.35, and this remains the best estimate for the approximately logarithmic relationship. I do not think there is any credible objection to this relation. If the optical depth at a given frequency is very small, or very large, then there is not much consequence for increasing concentrations for that frequency. Given a frequency in the stopband (with a large optical depth) emissions to space all come from high in the atmosphere, and doubling concentrations doesn't make much difference. Similarly, for a very low optical depth (transparency) the changes at that frequency are comparatively slight. The largest impact by far is for those frequencies where optical depth is close to unity. These are the frequencies for which additional concentrations move the effect of the atmosphere most strongly from being transparent to being opaque. Roughly speaking, higher concentrations mean the stopband is a little bit wider, stopping additional frequencies. This is the most important consequence of higher concentrations. The proof of that can be given in various ways. It can be done theoretically, as in calculations explained in Pierrehumber (2010) or reported in Myhre et al (1998). There are also observational confirmations of the enhanced greenhouse effect described in these pages at How do we know more CO2 is causing warming? I don't propose to go into a long further explanation here. There's ample description of the technical details in various references that have been given for people to chase up themselves if they have an interest, and the impact on OLR (the forcing) is taken for granted by major working scientists who happen to be skeptical of AGW, such as Lindzen or Christy. They tend to focus on the more reasonable question of response to forcings, or "climate sensitivity". Changes to atmospheric composition is, of course, a more technically complex question than I cover in this essay. That is deliberate. But I hope this brief account in the comments may go some way to answering your questions. Cheers -- sylas

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  17. #66 sylas at 00:44 AM on 29 May, 2010 The largest impact by far is for those frequencies where optical depth is close to unity Thanks, Sylas. I will do the calculations along the skirt of an absorption line. I'll let you know the result. But for now back to the topic. Neither nitrogen nor argon are greenhouse gases, that is, they have no significant absorption in thermal infrared. Now, imagine removing half the nitrogen from the atmosphere. Or doubling it. Or increasing argon contents a hundredfold. What happens to average surface temperature? Why? Think about it.
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  18. Also, consider this. Equilibrium temperature of a perfect black sphere in space 1 AU form the Sun is close to +6°C, provided it has high heat capacity and conductivity. On the other hand, average surface temperature of the same sphere with no heat capacity and conductivity whatsoever is 93 K (-180°C). Lunar Bond albedo is about 0.11. Therefore effective temperature of the Moon should be higher (-3°C) than that of Earth (-18°C, Bond albedo 0.3). In spite of this average temperature of lunar regolith at 1 m depth is -35°C. It is a bit overstretched to imagine Earth with no greenhouse gas in its atmosphere, i.e. with no water vapor whatsoever, but having the same albedo, heat capacity and distribution efficiency currently provided by icesheets, snow, clouds, oceans, currents and winds. That much about the alleged 33°C "greenhouse effect".
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  19. Berényi Péter, you may already have seen this, but Science of Doom has a couple of recent posts on the radiative balance of the Moon and what the Earth's temperature would be in the absence of a greenhouse effect. * Lunar Madness and Physics Basics * The Hoover Incident And in light of the overall topic of this thread, there are a whole series of recent posts about Gerlich and Tscheuschner (2009) and the "question" of whether the existence of the greenhouse effect has been falsified. Plus, our host has nice things to say about Science of Doom, too. Check it out if you haven't already done so.
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  20. What is the asymptotic wing shape of a single absorption line look like?
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  21. Berényi Péter, the absorption line shape is governed by collisional and Dopler broadening. The former is a lorentzian, the latter a gaussian. The resulting line shape is called Voigt line shape. It's the convolution of the two and depends on both temperature and pressure. Far from the central frequency it may be approximated by a lorentzian.
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  22. #71 Riccardo at 16:55 PM on 9 June, 2010 Far from the central frequency it may be approximated by a lorentzian Right. That's what I figured based on my rather old and dusty QM. Just one never knows for sure with these jumpy-bumpy molecules.
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  23. Nah. Spectral line shape of far wing in fact does not even come close to a lorentzian. Next guess?
    Lorentz line profile: dashed curve
    laboratory measurements: shown by +
    Reality is missed by up to three orders of magnitude.
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  24. Berényi Péter, one more try to disprove something with trivial high school level arguments? I rememebr I was 16 when I was thaught about the lorentzian and gaussian curves. Read the scientific litterature on line broadening if you think you've found something wrong. Anyways, first quote the origin of the data and how they were taken. Second, detail your calculations. Third, show the full spectrum. Otherwise your graph is meaningless. Finally, as I said before, lorentzian is just an aproximation; try the full calculations including all the sources of broadening at the relevant temperature and pressure.
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  25. #74 Riccardo at 23:45 PM on 11 June, 2010 Read the scientific litterature on line broadening I thought you understood what you were talking about. With a lorentzian-like spectral profile gaussian line broadening has negligible effect on far wing shape. That's pretty simple math. first quote the origin of the data and how they were taken I reckoned you could figure out you could have the source by clicking on the graph. Your browser should show a tiny hand or something while moving your cursor over a clickable image. lorentzian is just an aproximation I knew that. Just were not aware of the fact it was such a poor approximation at the far wing. Still looking for a better one. Help would be appreciated.
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  26. Berényi Péter, I see, it's really pointless to answer to your questions. Usually if someone asks a question is because he's intrested in an answer about something he doesn't know. If it's not the case, don't ask, don't play this nasty game.
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  27. #76 Riccardo at 00:49 AM on 14 June, 2010 Listen, it's not a game. Looks like far wing shape is not described well by a lorentzian and I really don't know what is the right approximation for that region. It is also important, because CO2 has a very strong absorption line at 15 μm. It is so strong that between 14-16 μm photosphere is high up in the stratosphere, where further CO2 increase has no effect on TOA, or if any, it is the opposite of what is generally believed. Because if you go high enough, temperature starts to rise again (due to O3 UV absorption). Therefore any difference can show up only in the wings. As the wings have a general negative slope and they converge to zero as you get farther from line center, at some point the atmosphere gets absolutely transparent to CO2 absorption. It is the transition zone where stuff happens, where photosphere slowly descends through downward warming troposphere until bumps to surface or H2 continuum. It depends quite sensitively on the asymptotic behavior of wing shape, that is, on how fast they converge to zero. With a lorentzian, it is proportional to Δν-2, but apparently the convergence is much faster. There is no way line broadening can do such a thing to wing shapes, therefore it sould be something neglected in the lorentzian approximation. I am looking for that something and I do it quite honestly.
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  28. Berényi Péter, radiative transfer codes generally do not use model line shapes, they use the measured absorption coefficient. The proper understanding of the behaviour of CO2 absorption in the atmosphere is then not linked to an exact theoretical line shape model. So, the best thing to do is to go and see what radiative trasnfer codes have to say. Whatever the shape might be, no doubt it will shrink at lower pressures and temperatures. This simple fact makes the atmosphere more transparent to a progressively wider range of wavelength going up in altitude. Also, although the mixing ratio is aproximately constant, CO2 density (mass per unit volume) decreases. The commonly used simplification of a well defined altitude from which IR radiation escapes to space should not be taken too litterally. Back to the radiative tranfer codes, your claim that the emitting layer is already above the tropopause is not supported. Also, should the CO2 rise to such high levels the thermal structure of the troposphere would change. For example, we can anticipate that the tropopause would rise, as apparently is already happening. I'm not able to give more details because a full radiative-convective equilibrium should be considered. But for sure we can not extrapolate a simplified behaviour that far.
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  29. This is my first post on SkepticalScience. I recognize that I'm over a decade late on this thread, and I don't know whether or not there might be a more appropriate place to post this. Yet, in case it's of interest, I want to share that I recently wrote a new critique of Gerlich and Tscheuschner's 2009 paper purporting to falsify the greenhouse effect. While my essay overlaps some of the insights offered by Halpern et. al. (2010), it offers its own perspective, and might be accessible to readers in a different way.

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    Moderator Response:


    Welcome to Skeptical Science.

    None of our threads here are ever closed. As long as your post is on-topic (as yours is) it is appropriate to add a comment. It is always useful to have more information.

  30. Thank you for the link Bob. I felt very validated to see that, as I understood it already at the time, the stupid G&T canard was little more than a play on words leading to a convoluted strawman argument...

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