Revisiting Wood’s 1909 Greenhouse Box Experiment: Part I

August 23rd, 2013 by Roy W. Spencer, Ph. D.

Much is made in some circles of R.W. Wood’s 1909 experiment which supposedly “disproved” the “greenhouse effect”. As we shall see (below) the experiment reported on in the literature has only cursory detail. It also raises questions over the ability of the setup to demonstrate anything of use to the issue of whether downward IR emission from the sky raises the average surface temperature of the Earth.

I’m finally putting together my own experimental setup, which could be easily replicated by others. We now have widely available materials which are better suited to performing the experiment, and it should be an ideal candidate for High School science experiments.

Part of my interest is the fact that at least two attempts at replicating Wood’s experiment (Pratt’s and Nahle’s) came to totally opposite conclusions(!) Vaughan Pratt has told me he is interested in revisiting the experiment he did as well.

But first, here is R.W. Wood’s original note from Philosophical Magazine (1909 Vol. 17, pp. 319-320), which most have probably not bothered to read, and which I believe reveals some serious shortcomings in his experimental setup:

XXIV. Note on the Theory of the Greenhouse
By Professor R. W. Wood (Communicated by the Author)

THERE appears to be a widespread belief that the comparatively high temperature produced within a closed space covered with glass, and exposed to solar radiation, results from a transformation of wave-length, that is, that the heat waves from the sun, which are able to penetrate the glass, fall upon the walls of the enclosure and raise its temperature: the heat energy is re-emitted by the walls in the form of much longer waves, which are unable to penetrate the glass, the greenhouse acting as a radiation trap.

I have always felt some doubt as to whether this action played any very large part in the elevation of temperature. It appeared much more probable that the part played by the glass was the prevention of the escape of the warm air heated by the ground within the enclosure. If we open the doors of a greenhouse on a cold and windy day, the trapping of radiation appears to lose much of its efficacy. As a matter of fact I am of the opinion that a greenhouse made of a glass transparent to waves of every possible length would show a temperature nearly, if not quite, as high as that observed in a glass house. The transparent screen allows the solar radiation to warm the ground, and the ground in turn warms the air, but only the limited amount within the enclosure. In the “open,” the ground is continually brought into contact with cold air by convection currents.

To test the matter I constructed two enclosures of dead black cardboard, one covered with a glass plate, the other with a plate of rock-salt of equal thickness. The bulb of a thermometer was inserted in each enclosure and the whole packed in cotton, with the exception of the transparent plates which were exposed. When exposed to sunlight the temperature rose gradually to 65oC., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate.

There was now scarcely a difference of one degree between the temperatures of the two enclosures. The maximum temperature reached was about 55oC. From what we know about the distribution of energy in the spectrum of the radiation emitted by a body at 55oC., it is clear that the rock-salt plate is capable of transmitting practically all of it, while the glass plate stops it entirely. This shows us that the loss of temperature of the ground by radiation is very small in comparison to the loss by convection, in other words that we gain very little from the circumstance that the radiation is trapped.

Is it therefore necessary to pay attention to trapped radiation in deducing the temperature of a planet as affected by its atmosphere? The solar rays penetrate the atmosphere, warm the ground which in turn warms the atmosphere by contact and by convection currents. The heat received is thus stored up in the atmosphere, remaining there on account of the very low radiating power of a gas. It seems to me very doubtful if the atmosphere is warmed to any great extent by absorbing the radiation from the ground, even under the most favourable conditions.

I do not pretend to have gone very deeply into the matter, and publish this note merely to draw attention to the fact that trapped radiation appears to play but a very small part in the actual cases with which we are familiar.

Regarding Wood’s setup, the first question I have is with his use of a rock salt plate, which is indeed mostly transparent to infrared…if it is kept very dry. He said nothing regarding his efforts to keep the plate from absorbing humidity, which will affect its IR transparency. Today, we can use thin polyethylene sheets (e.g. Saran Wrap), which are about 90% transparent to IR.

The second question I have comes from this passage (emphasis added):
“When exposed to sunlight the temperature rose gradually to 65oC., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate.”

Say what? He put a glass plate in front of the rock salt plate? Well, that would invalidate the experiment altogether! The point was to see whether the IR opaqueness of the glass caused warmer temperatures in the box than did the IR-transparent salt plate. If he put glass over the salt plate, then we no longer have IR transparency, do we?

Another question I have is whether the salt plate and the glass were transmitting the same levels of visible sunlight. Using Saran Wrap (for IR transparency) and Plexiglass (for IR opaqueness) should each pass around 99% over 90% of visible sunlight, from what I have read. Glass is seldom as much as 90% transparent, so it is not the best choice, in my view. The issue is important because, assuming direct sunlight, you might have 800 W/m2 of solar heating available, but a 10% difference in transparency will result in 80 W/m2 difference in how much sunlight is entering the box. This is the same as the difference between downwelling sky radiation (say, 350 W/m2) versus the downward IR emission from a plexiglass plate at 73 deg. F (assuming an IR emissivity close to 1.0).

In other words, two boxes might produce the same interior temperatures (seemingly contradicting greenhouse theory) if a glass covered box is “trapping” 80 W/m2 more IR, but the Saran Wrap covered box is letting in 80 W/m2 more visible sunlight. The covering materials need to be passing very close to the same levels of solar energy. (The IR portion of solar energy flux should be very small since the sun’s emitting temperature is so high, and most of that IR is absorbed by the atmosphere anyway before it ever reaches the ground.) You want an experimental setup where everything is close to identical, except the IR transmission characteristics of the cover material.

Anyway, this post is meant as an introduction to the experiment I will be carrying out. (I’ve talked to Anthony Watts, who might perform his own experiment at some point.) What I will be using is nested Styrofoam coolers, lined with poster board painted with Krylon #1502 flat white spray paint (0.99 IR emissivity). One cooler will be covered with two layers of Saran Wrap (or equivalent), with an air space for insulation. The other cooler will be covered with Plexiglass. Both coolers will be sealed to be relatively air-tight. I’ve done some calculations which suggest that the similarity in thermal conductivity of the covering materials will be the biggest source of uncertainty….possibly 10 W/m2 or more. Conduction through the doubled Styrofoam containers will be essentially the same, and limited to not much more than 1 W/m2.

I will be monitoring temperatures with an Extech SD200 3-probe thermometer data logger, which continuously stores 3 temperatures at regular intervals.

A few of you might recall my backyard box experiment from a few years ago, where I chilled air in a Styrofoam box. This part of the problem is also of interest to me…to see how much cooler at night the air gets in the Saran Wrap covered box than in the Plexiglass covered box. Stay tuned.

61 Responses to “Revisiting Wood’s 1909 Greenhouse Box Experiment: Part I”

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  1. Tim Folkerts says:

    Roy says: “If he put glass over the salt plate, then we no longer have IR transparency, do we?”

    As you say, the details are minimal in the original paper. I suspect that the additional sheet of glass was well above the experiments — perhaps 0.3 – 3 meters away.

    This would only “sort of” invalidate the experiment. The box with the rocksalt cover would “see” IR from the “relatively cool” glass rather than the much cooler atmosphere. So he would be comparing “warm glass vs cool glass” rather than “warm glass vs cold atmosphere”.

    • Maybe. But even if that’s the case, then we have less solar going into the salt covered box, which will reduce its temperature, since the salt plate + glass will be less transparent to solar than glass alone. In any case, there are so few details given we will probably never know exactly what he did.

      • steveta_uk says:

        I had always assumed that he meant that a much large glass sheet was used to block IR from both boxes, and that it was positioned some distance away as Tim suggests.

        Any other arrangement would seem daft, and I don’t think Wood would have been that poor an experimentalist.

    • Massimo PORZIO says:

      Wood wrote:
      “When exposed to sunlight the temperature rose gradually to 65oC., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate.”

      I don’t believe he was ingenuous, he just wanted to get the same energy radiate from the bottom of the boxes, this to evaluate if the blocking of the outgoing LWIR radiation was significant to the whole effect.
      He did almost as he placed two electric heather of the same power inside the boxes and analyzed the efficiency in retaining the heat of the two boxes.

      So, if Dr. Spencer want to evaluate what is more effective in a real greenhouse, between the heat conduction of the walls and roof and the radiative insulation of them, I suggest to do two boxes with the walls made of glass and salt too, not only the roof. That because IMHO the real error of Wood in evaluating the greenhouses work is ignoring that the outgoing LWIR gate is much wider than the incoming Vis/SWIR one because of the spreading effect of the bottom which irradiate the inner of the boxes as a Lambertian surface radiator.
      Constraining the output energy gate being the same roof window of the incoming one, means that the air inside of the salt windowed box is heated by the walls which absorbs the LWIR from the bottom LWIR radiator. In a real salt greenhouse the bottom LWIR radiated energy in part escapes from the walls too, so they cool the inside air instead of heating it.

      Have a nice day.


  2. Massimo PORZIO says:

    I have some doubts about the use of so thin polyethylene.

    Anyways, it seems that NaCl (if pure) isn’t so hygroscopic:

    Excerpts form the link:
    “Furthermore, sodium chloride itself is not highly hygroscopic. The absorption of humidity results from traces of other minerals contained in the salt, such as magnesium chloride. The myth of salt being highly hygroscopic dates from those times when people were using mostly sea salt and rock salt.”
    “A pure salt with a high sodium chloride content, such as a food-grade vacuum salt, will work as a dehumidifier only from 75 per cent of relative humidity on, due to its very low hygroscopicity.”

    I wait for your experiment, thank you for doing real science.


  3. Massimo PORZIO says:

    By the way, NaCl is not so better than glass for Vis/SWIR:

    It’s just a little better in the UV where the Sun at ground is not so active.

    Have a nice day.


  4. I’m wondering if maybe a salt window could be created by dissolving some Rock Salt in hot water to get it supersaturated, then pouring the mixture into a square pan with saran wrap in it, and letting it cool and evaporate over days.

    Then you could replicate more accurately.

    • My sister used to grow salt crystals, which is what tends to form through this process…not a solid sheet.

      Anyway, I’m more interested in doing the experiment the right way than in trying to replicate an experiment where so many details are missing, and we have better methods available anyway.

  5. Should have said – use the saran wrap to lift out of the pan.

  6. bw says:

    Low cost glass panes were machine made in 1909 and may have had enough metal contamination to affect the color. I’ve salvaged window glass from that era. It looks green now.

    Glass panes in 1909 were commonly 1/16 inch thick (standard strength) or 1/8 inch thick (double strength).
    Commercial glass plates of 1/4 inch were also available.

    Wood may have had access to hand made plate glass.

  7. bw says:

    Also, what were the exact IR characteristics of “Rock salt” Wood used in 1909. Different salts have very different IR transmission curves.

  8. Hops says:

    Alas, this experiment won’t convince anyone of anything. There’s no CO2 involved, nor any feedback. An increase in temperature won’t raise the reflection of input radiation via clouds, not will it cause the release of additional greenhouse effect in the way in which melting permafrost will on our planet.

    No, there is only one experiment that would follow the scientific method:

    Set up 1000 solar systems identical to that of ours with an Earth identical to ours.

    Into 500 of the earth planets, inject CO2 at the rate at which we are currently injecting it into ours.

    Over 1000 years, measure the temperature differences between the controls and the experiments.

    Since this is impossible, we are experimenting with the planet we live on, and even if the result is warming, certain people will deny it has anything to do with CO2, either because they make a living emitting CO2 or they are just plain contrarian.

    • Scott Scarborough says:

      Nop, that wouldn’t work either. Cosmic rays effect the earths climate and 1000 solar systems would have to be in different positions in the galaxy therefore receive different cosmic rays.

    • KLang says:

      HOPS – “and even if the result is warming”

      Propaganda-resistant people will deny this statement when our current measuring devices indicate a steady temperature or one that has cooled slightly for the last 15+ years, despite a steady increase in CO2.

      • Hops says:

        KLang, you might as well say that if today isn’t warmer than yesterday, warming has paused. Even a casual glance at the historical temperature variations shows not only 15+ year pauses, but even large declines.

        In 1940 if you had predicted warming, you would have seemed crazy because it actually cooled for a decade or so. Yet in the long run, the temperature increase reverted to trend.

        I’m so looking forward to the next major El Nino, which will set a new record. Then you’ll have to start over with “warming has paused for a day…”

  9. Hops says:

    I forgot about cosmic rays, to which some attribute the recent warming. (Or have the cosmic rays paused?)

    I suppose we would need identical galaxies, or even universes, to have adequate controls on the experiment.

    • DirkH says:

      you are probably just joking; but in case you are not: the Svensmark theory assumes the average cloudiness to be modulated by the intensity of cosmic rays. He does not assume that cosmic rays are directly warming or cooling anything.

    • Aaron shunk says:

      We just went from a long term solar max to the lowest solar activity since the dalton minimum. You don’t think this should at least be considered as a hypothesis for climate change? By the way cosmic rays increase when solar activity decreases. So the smoothed cosmic ray trend has increased. It’s alarming to me how the co2 people have simplified the climate to driven by co2 and it’s feedbacks but won’t consider feedbacks from the sun. Fortunately there is a great natural experiment and the sun and co2 are out of phase (sun minimum co2 max). We will know more in a decade’! But you can see solar periodicities in many ancient records like tree rings or annual sediments so it is difficult for me not to anticipate changes based on empirical data.

  10. Peter Chapman says:

    I hope you plan to replicate this experiment.

    With a bit of thought you could make it much more interesting. Such as a replicated 2 by 2 factorial.

    I thought Wood’s comment about “ground” warming the air was odd. My car gets very hot inside on hot summer days, but there is no “ground” in my car. Some people put reflective material in their windscreens to cool their cars. I have never done this so I don’t know whether it works.

    • Mike Flynn says:

      Peter Chapman,

      The “ground” in the case of your car are the portions which receive direct sunlight. Steering wheel, dashboard top, seats etc.

      Depending on construction materials, these can get very hot.

      Reflective material either behind or in front of the windscreen reduces the amount of radiation available to the interior surfaces. Try it. You’ll like it!

      Live well and prosper,

      Mike Flynn.

      • Peter Chapman says:

        Thanks Mike. So, ground means stuff under the glass that has a capacity to absorb heat and then release it later. I should have thought of that.

        Back to experimental design, which was my main point. Woods was working in 1909 before Fisher invented statistical design. So he could have easily introduced bias and the scientific community could have spent the last 100 years trying to interpret noise. So, I would advocate use of statistical design for this experiment.

  11. Nullius in Verba says:

    I interpreted the bit about the glass sheet as one positioned some distance away so as to obscure the box’s view of the sun but not the sky.

    I would also note that it would be a good idea for the windows to have the same thermal resistance to conduction as one another. Part of the effect may be mediated by the difference in temperature between the upper and lower surfaces.

  12. dallas says:

    “Say what? He put a glass plate in front of the rock salt plate? Well, that would invalidate the experiment altogether! ”

    Not really. CO2 and H2O absorb in both directions. Atmospheric absorption of SW is ~75Wm-2 which is about 1/2 of the atmospheric/greenhouse effect with a large portion in the near infrared. Without CO2 and H2O the “surface” would be a lot warmer with larger variations. With H2O and CO2, it is cooler with less variation.

  13. pochas says:

    Dr Spencer,
    Be sure to substitute a blackened thermally conductive opaque plate (copper or aluminum) for the transparent one to see what
    effect that has.

  14. KevinK says:

    Dr. Spencer, with respect;

    A number of points to consider;

    1) To remove the LW portion of the incoming light I suggest you use a “cold mirror” (reflects visible, transmits IR)pointed at the sky to reflect ONLY the visible portion of the light parallel to the ground into the side of the boxes. Of course you will want to do that for each box. This will ensure that each box receives the same incoming spectrum. And will also not impede the outgoing light (it either reflects back to the sky or passes through the mirror and travels along the ground) regardless of the spectral content.

    2) If point 1 is impractical (large cold mirrors are not stocked at Lowe’s(tm) the last time I checked) you may consider a LW blocking filter above the boxes (Woods used a glass plate for this, My reading of his report is that he covered both boxes with this plate). Covering both boxes is necessary to account for the transmission loss introduced in both the visible and LW portions of the spectrum.

    3) You will want the “glass/plastic” window materials to be as similar as possible WRT thermal properties (thermal capacity, thermal conductance, mass and thickness). Each of these properties WILL affect your results. I would recommend two plastic sheets of similar (very small) thickness, one transmitting visible and one transmitting visible and IR.

    4) One way to reduce errors would be to make two boxes with a repeatable way to attach a frame holding the “glass” window. This will need to be air tight of course. Then run the experiment: (box 1/window 1, box 2/window 2) followed by (box 1/window 2, box 2/window 1). This will cancel out any fabrication errors in the boxes (dimensions, thermometer placement, air leaks, etc.) All of these WILL affect your results. For example a simple 1/r^2 loss (bottom of box to window) will occur if the boxes are different in height by as little as 1/8 inch.

    5) Ideally you would want to “swap” thermometers between the boxes, but this may be difficult to do with sufficient accuracy in placement. Probably easier to get several high precision thermometer probes that have all been calibrated at the same time. They may all be off by 1/2 degree but they will all agree within 0.1 degree C.

    All of this ultimately comes down to precision control of the absolute amount of optical radiation striking a series of surfaces (box inside walls, window (inside and outside) box outside walls, box floor). Precision control of optical radiation and measuring absolute optical radiation is one of the most difficult calibration challenges there is.

    Any result showing temperature differences less than 1 degree C should be considered as inconclusive when all the error sources are properly considered.

    In the end I think you may discover that any talk of “trapping” radiation or energy is just “silly” (not a scientific term). Energy flows all the time from warmer to colder locations, the speed at which it flows is an important consideration. Optical radiation (indeed all EMR) flows so quickly through a system that the other energy flow mechanisms(conduction/convection) can hardly “get their shoes on” before the radiation has left town.

    Good luck, FYI, I have reviewed both experiments and cannot see any glaring errors in the Nahle or Woods versions. But sealing a cardboard box with tape (the Pratt version) seems to be a bit “sloppy” (no disrespect intended towards Dr. Pratt).

    Cheers, Kevin.

    • Mike Flynn says:


      Well said.

      Live well and prosper,

      Mike Flynn.

    • DeWitt Payne says:

      No glaring errors? You didn’t read very carefully.

      For Wood, we have no idea whether his boxes were equally well insulated. He should have switched covers and repeated the experiment. Based on some work I’ve done, the insulation was poor for both boxes. A well insulated box would have reached a much higher temperature above ambient.

      For Nahle, did you not notice that the box with the IR transparent cover was insulated and the other boxes weren’t?

  15. Mike Flynn says:

    Dr Spencer,

    I congratulate you! A climatologist carrying out a real, actual, physical experiment!

    Others have already pointed out some areas where your proposed experiments could possibly be altered to ensure scientific rigour, and avoid criticism if the results displease some people.

    I await the results.

    Live well and prosper,

    Mike Flynn.

  16. Dr. Spencer, thanks for Part 1. Looking forward to Part 2.

  17. Bri says:

    The problem you mentioned of the incoming power could be solved by not using the sun. Put an IR light bulb in the box and put the box inside. Use the same bulb for each box then you know the incoming power is the same for both. The insulating properties of the different materials would be a factor but you could put the box in a room that is nearly the same temperature as the inside temperature of the box.


  18. Andrew McRae says:

    I agree more with Dr Spencer’s goal of redesigning a new experiment to test warming by backradiation, rather than dogmatically recreating Wood’s experiment exactly. This is science, not ancestor worshipping.

    But I must also agree that such an experiment still does not give you the final answer about the effect of CO2 on the climate, it just tests one step in the “greenhouse” mechanism. There is still merit in testing all the required steps of the theory individually.

  19. Kasuha says:

    “Say what? He put a glass plate in front of the rock salt plate? Well, that would invalidate the experiment altogether! The point was to see whether the IR opaqueness of the glass caused warmer temperatures in the box than did the IR-transparent salt plate. If he put glass over the salt plate, then we no longer have IR transparency, do we?”

    I believe there was some space between the salt plate and the glass plate with intent being to block incoming IR from sun, not block incoming visible light, and not block IR coming out of the box.

    I believe adding this side experiment to your own experiment might be valuable, too. After finishing measurements on the primary experiment, take a large Plexiglass plate and put both boxes under it, with the plate being far enough from both boxes to allow air to freely circulate. Then measure the difference between the two boxes and compare it with results from the primary experiment.

  20. Milton Hathaway says:

    It does seem like the experiment would be simpler (and therefore more convincing, maybe?) if the sun were removed from the equation. The easiest way to do that would be to do the experiment at night, with a cloudless sky, and an internal heat source in the ‘test chambers’.

    Another simplifying change would be to keep everything as isothermal as possible. In other words, instead of using temperature rise as a result, measure the heat input required to maintain the isothermal conditions. If the heat input comes from an electric current, it can be measured very accurately.

    One approach that seems promising would be to use two small Styrofoam coolers and a large Styrofoam cooler. A thick aluminum or copper plate would be installed in the bottom of each of the small coolers, with an electronic heater installed on the bottom side of the plate and the top side painted with the high emissivity paint. The electronic heaters would closely regulate the plates to a constant temperature.

    Both small coolers would be placed in the large cooler, one small cooler covered with the Saran Wrap and the other covered with the Plexiglas. The large cooler would then be covered with Saran Wrap. A third (large surface area) electronic heater in the large cooler would maintain the temperature of the air around the small coolers at the same temperature as the plates in the small coolers.

    If the whole arrangement were instrumented up with a number of temperature sensors, one could determine how successful the setup is in generating the isothermal conditions.

  21. John Marshall says:

    You seem to be confusing the emission spectra of CO2/H2O with the so called back radiation due to the false GHE. One is expected due to insolation reacting with the IR reactant gasses O2 and H2O which is part of the total solar radiation ensemble the other is back radiation from the surface which from a colder atmosphere cannot warm a hotter surface. It is questionable whether the surface radiated energy would be adsorbed due to the IR gasses already being saturated with energy from the sun.

  22. Bert Walker says:

    Woods design appears to be an attempt to isolate the effect of transparency to only upwelling IR. The additional glass pane covering both greenhouses was to limit downwelling IR from contaminating the experiment, while allowing visible spectra to heat the ground of both greenhouses.

    Of course Dr. Spenser is correct in that if Wood placed glass directly in contact with or even near the NaCl plate, the upwelling IR would simply be directly reflected downward back through the NaCl plate into the Greenhouse.

    I suspect this occurred to Woods as well, and as others have mentioned above, the additional glass pane was likely some distance away form the glass and NaCl plates at the top of the greenhouses. Never-the-less, if the additional glass plate was parallel to the ground, some upwelling IR could still be reflected directly downward interfering with the NaCl greenhouse results.

    Perhaps a way to minimize this back contamination would be to conduct the experiment with the additional glass pane at a 45 degree angle above each of the green houses. Any upwelling IR would then be likely to be reflected mostly in a parallel orientation to the ground surface, away from each greenhouse.

    How big do the greenhouses have to be? NaCl plates approximately 25 mm diameter are used in IR spectroscopy and are available for about $50 each. These would make for a small Greenhouses, but size may not matter as long as a temperature probe could be placed inside. Even so NaCl has only 87-90% transmittance to from 300 to 10,000 nm wavelength. If SaranWrap (Tm) is equivalent in IR transmittance, it would be much easier and economical to use.

    If you use NaCl plates, only touch the rims. Contrary to comments above, these pure NaCl plates made from a single crystal of NaCl are VERY sensitive to moisture. Even the moisture from an unprotected finger will cloud their surface, interfering with IR transmittance.

    • Massimo PORZIO says:

      Hi Bert,

      “If you use NaCl plates, only touch the rims. Contrary to comments above, these pure NaCl plates made from a single crystal of NaCl are VERY sensitive to moisture. Even the moisture from an unprotected finger will cloud their surface, interfering with IR transmittance.”

      If you referred to my comment, maybe you misread it.
      Because the link I highlighted well explained that only pure NaCl is not so hygroscopic as it’s believed to be.
      Absorption of humidity in NaCl results from traces of other minerals, so if you touch with your finger the surfaces of the window you do exactly that. Your contamination make taht surfaces locally more hygroscopic and maybe useless for the experiment, it depends on how much wet the surfaces became of course.

      Have a nice day


    • Nullius in Verba says:

      The additional glass pane is more easily arranged by placing it at a distance from the apparatus. All that is required is that it’s shadow is cast on the boxes. If it is placed several metres away, it will obscure only a very small part of the sky.

      Incidentally, the same consideration applies to any the buildings or trees (or observing scientists) around the apparatus. You’re trying to compare open sky against the IR-opaque box cover. An IR-opaque box cover compared to a nearby IR-opaque brick wall would give much less contrast.

  23. Tim Folkerts says:

    Here is my thought for an experiment — simplify. So rather than seeing how much role back-radiation plays vs convection, by freely allowing convection to see if the back-radiation makes any difference. And get rid of the sun as a heater and used a controllable electric heater.


    Get 2-4 cheap electric heating pads. Perhaps paint them to make sure the emissivity is close to 1. Remove the original controls and plug them into some sort of Variac to allow unifrom, adjustable power for all of the heating pads at once.

    Place the heating pads on sytrofoam sheets (for uniformity underneath them). The whole experiment could be done both indoors or outdoors at night to get different ambient thermal IR.

    Positions various flat covers ~ 10 cm above the heating pads: glass, plexiglass, saran wrap, aluminum foil, rock salt, etc. These would be a few cm bigger than the heating pad.

    Place cheap thermistors on the heating pad, the underside of the covers, and the top side of the covers. Maybe place a few of them at different locations just for completeness.

    As an added precaution, I might put an EXTRA layer of saran wrap between the heating pads and the covers, to prevent direct conduction to/from the heating pad & cover. In this case, I would add a few thermistors to measure air temperature above and below the “conduction barrier”. I could do like Prof Pratt and blow air between a pair of conduction barriers just to be doubly sure that conduction is not involved.

    I’d use a factorial design as Peter Chapman suggested — probably a fractional factorial to more quickly explore the myriad combinations.

    AND … just because numerous people say it is impossible, I might use an unpowered heating pad as the cover, to see if the unpowered heating pad truly does warm the heated heating pad. 🙂

    I have (or can easily get) most of the supplies required, so I might have to try this 🙂


    There are innumerable follow-up possibilities. Feel free to critique or to offer your own favorite variations.

  24. Bill Hunter says:

    Roy you might consider some of the high tech films for the greenhouse argibusiness market instead of plexiglass and saran wrap.

    Greenhouse operators now have inexpensive thin films available with IR blocking added. In this way you might be able to eliminate a lot of uncertainty in your conductive/convective heat loss by using identical materials with and without small amounts of additives and be able to stay away from complexities and uncertainties with dual glazing one to compensate for single glazing differences.

    Double glazing in itself introduces a great deal of uncertainty from “home manufactured” double glazed windows as humidity is poorly controlled. Other factors play roles also. Far more uncertain than a handbook of engineering data would suggest.

    From the Engineering Toobox:
    Convective Heat Transfer Coefficients

    The convective heat transfer coefficient – hc – is dependent on the type of media, gas or liquid, the flow properties such as velocity, viscosity and other flow and temperature dependent properties.

    In general the convective heat transfer coefficient for some common fluids is within the ranges:

    Free Convection – Air : 5 – 25 (W/m2K)
    Free Convection – Water: 20 – 100 (W/m2K)

    I could be incorrect but it does not appear that humidity is the cause of that wide range of results. And thats only the beginning of dual glaze issue. Reflection is another issue. You can’t get films flat so its hard to control for reflective issues.

    I also somewhat disagree that Woods did not control for moisture absorption in the rocksalt. After all he did obtain much faster warming in the rocksalt box because of a lack of IR blocking.

    But if you got similar films with different additives you could dump the dual glazing and maybe get close enough on the reflectivity problems if you take care in attaching the films carefully without wrinkles and similar corner to corner tension.

    The problem of incoming heat blockage by the IR opaque glass prompted woods to add the glass filter. Now I agree that could be a problem.

    But the fact the rocksalt box did allow more energy in is a problem that has to be considered.

    I have felt uneasy with the concept of computing outgoing blockage and converting that to surface warming when nobody has accounted for the lack of surface warming from incoming IR blockage created by the addition of more IR blocking substance.

    The incoming combined with outgoing blockage is ostensibly why you have a warm sky that supposedly “slows cooling” in the first place. It may not entirely rip to shreds the “insulation” argument for the greenhouse effect but its at least a huge dent in it quantitatively.

    Having a data logger should solve for that problem where total heat is going to be computed, but if the boxes are going to heat unevenly in the first place enthalpy of water vapor becomes an issue.

    I think greenhouse gases operate in complex ways. For example, I think you need to set the light source at something around 30 degrees inclination from the ground to get an average incoming absorption effect through a 3d object. But thats a big problem with dual glazing.

    Not angling the box is kind of a “flat earth/uniform radiation” approach. It will probably give tainted results as we all observe the effects of a declining sun. Absorption goes way up even in the “transparent” frequencies so that we get wildly colored sunsets.

  25. Dave says:

    This all, including the original experiment, rather seems to miss the point. We want to know if it’s warmer under a sheet of glass than not. So put a thermometer under a sheet of glass, and another not under it. Don’t box them in, because we want to rule out trapped airflow, as Wood intended to do.

    It is immediately obvious that this does in fact cause the thermometer under the glass to register a higher temperature, whether you analyse it theoretically or test it empirically. We mostly know it to be true from experience: if you’ve ever walked past a free-standing pane of glass with the sun shining through it, you’ll notice the temperature differential.

  26. KevinK says:


    “if youve ever walked past a free-standing pane of glass with the sun shining through it, youll notice the temperature differential.”

    Likewise, “if you’ve ever walked past an (opening where a) free-standing pane of glass (should be) with the sun shining through it, you’ll notice the temperature differential.”

    The question is about radiation being “trapped” by materials (glass,”GHG’s”, plastic films) that are transparent(in some way; visible only or visible and IR), not about clearly opaque materials like walls.

    If you walk from an area with direct sunshine into an area behind a free-standing pane of glass which polarity does the noticeable temperature differential follow ?

    Cheers, Kevin.

  27. Allen Eltor says:

    Magic Gais boils down to the claim that a frigid gas bath, nitrogen and oxygen,

    phase change refrigerated by a one percent shot of water,

    warms the rock warming the bath, as the bath cools the rock,

    and that the refrigerant is the primary source of the heating.

    Every word which drizzles from you peoples’ befuddled brains,

    has been proven untrue so many times and by so many people

    your own scientist Peilke Jr told the Senate TO their FACE:

    “[t]here are a lot of dishonest people in this field.”

    Do any of you admit you’ve never showed up with the book from the refrigeration field showing how you calculate the heating done by the refrigerant?

    Or are you all still claiming there’s no instrument which can measure the giant infrared light on in the sky 24/7/365.25, which is influencing climate by making it warmer, as it emits from the refrigerant?

    “As the heat emits from the refrigerant, global temperature is expected to grow warmer and warmer” is your “message”.

    “The more refrigerant in the air, the warmer we as Magic GAiSSers expect it to get.”

    Isn’t this the place where the guy got caught pointing a thermometer at the sky claiming he was getting water/CO2 readings and the company had to issue an urgent “explanation of the technology”

    because people believed their thermometer was measuring the temperature of gaseous co2 and water,

    and they explained to the guy who made the claim, that he COULDN’T be measuring that, because they intentionally took their measurements from a different band of infrared light precisely so that couldn’t happen?

    Yes that’s this place, and that’s the man who runs this blog who got caught doing that.

    You people really do need to give up the deception of anyone who’ll come by and listen to your falsehoods.

    Even a man who studies refrigeration knows there’s no way your claim can be true.

    The earth’s oceanic basins are awash ten thousand feet deep in phase change refrigerant so cold it’s liquid. In some places like the poles and mountain tops it’s solidified: frozen solid.

    While the temperature of the big warm rock just six feet below the surface, average, is sixty five degrees F.

    You people are clowns. You need to grasp just because there’s an internet to tell lies to strangers on,
    you don’t have to get on and do it.

  28. James Goodone says:


  29. James Goodone says:

    Dr.Spencer says: Glass is seldom as much as 90% transparent, so it is not the best choice, in my view. The issue is important because, assuming direct sunlight, you might have 800 W/m2 of solar heating available, but a 10% difference in transparency will result in 80 W/m2 difference in how much sunlight is entering the box. In other words, two boxes might produce the same interior temperatures (seemingly contradicting greenhouse theory) if a glass covered box is trapping 80 W/m2 more IR, but the Saran Wrap covered box is letting in 80 W/m2 more visible sunlight.

    Dr.Spencer, this would seriously damage the greenhouse effect.

    Assuming there is one, 80 W/m2 of trapped IR by glass would be responsible only for 10C difference in temperature and not for 33C..

    Besides, greenhouse gases are less opaque to IR than glass, I guess, so the difference would be even less than 10C. Isnt it too little for a greenhouse effect?

    It seems you will need 350 W/m2 of trapped IR for the 33K difference and 44% difference in transparency.

  30. Bri says:

    I think only one cooler should be used and the glass should be changed . This eliminates the differences in heating and temperature measuring devices.
    I still think a light should be used to heat it to simulate the sun more closely . Some of the visible light from the sun is reflected away and some is absorbed. The IR comes from the heating caused by all wavelengths and a heating element would just heat the air in the box. I would change the light to a daylight bulb and shield it from directly shining on the window with some aluminum foil.


  31. Fulco says:


    can you comment on this ?
    I am curious how you see this in relation to your own measurements.

    A new peer-reviewed paper published in Energy & Environment analyzes 24 years of data from the European Meteosat weather satellite and finds global temperatures decreased over the period 1982-2006. According to the authors, Our observations point to a decrease in planetary temperature over almost the entire hemisphere, most likely due to an increase of cloudiness.

    The cloud filtered temperature change patterns, in figure 2c, indicate that the largest decrease occurs in the more cloudy regions of the hemisphere: the tropics and the temperate zones, while in the desert belt the temperature decrease is much smaller. This suggests that cloudiness changes could be the mechanism behind the observed global cooling since 1982: an increase in cloudiness would decrease global radiation and increase rainfall and evapotranspiration. Both effects tend to decrease the surface temperature.

    Full paper available here

    Meteosat Derived Planetary Temperature Trend 1982-2006

    Andries Rosema1, Steven Foppes1, Joost van der Woerd1

    1EARS Earth Environment Monitoring Ltd., Delft, the Netherlands Kanaalweg 1, 2628 EB Delft, the Netherlands


    24 year of Meteosat hourly thermal infrared data have been used to study planetary surface temperature change. Thermal infrared radiation in the 10.5-12.5mm spectral window is not affected by CO2 and only slightly by atmospheric water vapor. Satellite thermal infrared data have been converted to brightness temperatures as prescribed by Eumetsat. Hourly brightness temperature images were then composed to corresponding noon and midnight temperature data fields. The resulting data fields were cloud filtered using 10, 20 and 30 day maximum temperature substitution. Filtered data were subsequently averaged for two 10 yearly periods: 1986-1995 and 1996-2005. Finally the change in brightness temperature was determined by subtraction. In addition nine locations were selected and data series were extracted and studied for the period 1982-2006. Our observations point to a decrease in planetary temperature over almost the entire hemisphere, most likely due to an increase of cloudiness. Two small areas are found where a considerable temperature increase has occurred. They are explained in terms of major human interventions in the hydrological balance at the earth surface.

  32. Hi Dr Spencer.
    sorry to use this comment post to try to contact you but I see you are monitoring these comments so hopefully you will get this message.
    I have twice tried emailing you at the address given on your website: [email protected] but have had no joy. Could you please contact me at the above email address? ([email protected] ) I have some questions I would like you to answer for me if at all possible.
    Many thanks,

  33. Wayne Job says:

    Take three identical boxes, cover one in rock salt one in glass and one with a sheet of black iron. They will all get hot and prove nothing about green house gasses, the one covered in iron is at least practical as you can cook eggs on it. The rest is a waste of time as the earth has no roof.

  34. Mac says:

    Dr. Spencer,

    While there is a lot of theoretical advice here from many sources, it would appear that you are the only scientist of the group willing to put their theories to physical tests.

    That, to me, is the strong point of your arguments.

    Thank you for your efforts.

  35. M. Adeno says:

    I like those simple and convincing presentations. Is there also a simple evaluation why additional CO2 increases the greenhouse effect? Why increases additional CO2 the downwelling sky radiation? Is there a saturation limit, after further increasing CO2 is irrelevant and only the temperature determinates the downwelling sky radiation ?

  36. Lars Silen says:

    If the idea is to demonstrate back radiation from CO2 I think the best way would be to invert the experiment.

    Take an insulated vertical pipe plugged at the upper end and add for example LEDs that produce visible light as energy input (or simpler heat the top with a ohmic heater) … IR will be produced due to the heat.

    Put a sensitive temperature sensor some ten cm from the heater with heavy insulation upwards towards the heater. The sensor thus measures the local air temperature plus back radiation.

    The bottom of the pipe is plugged by a cooled fully absorbing surface that simulates space. This could be for example a black aluminum plate. If so desired the bottom plate could be cooled by liquid Nitrogen to provide a more realistic “space” simulation.

    The design means there is no convection in the system with the cold part at the bottom and hot gas at the top. We have a perfect temperature inversion and thus no convection should develop.

    Two runs consisting of a large number of heater on/off cycles needs to be done:

    Use ordinary air, it is ok to have 400 ppmv CO2 in the air.
    Start with heater off, wait for stable temperature. Switch heater on and wait for new higher stable temperature measure dT=Tmax-Tmin. Repeat as long as needed to get reliable data.

    Next fill the pipe with one hundred percent CO2 at ambient pressure. Do the same experiment cycling between heater on/off and check dT. If the back radiation works as advertized the run in plain CO2 should produce a higher dT than running in air.

    How long should the pipe be to contain a CO2 amount similar to the CO2 from sea level to the top of the atmosphere? It turns out, if my back of the envelope calculation is correct, that a chamber with a length of 2.24 m at one atm should have the same “IR optical” thickness as the full atmosphere.

    It is easy to see that the effect of the 400 ppmv CO2 in plain air when enclosed in the measurement system produces an back radiation that should be roughly 1/5600 th of the plain CO2 situation …

    Has somebody done something like this? Are there any hidden problems?

    Notice that the system as described completely eliminates the need for IR absorbing and IR transparent plates. The plates are replaced by simulated “cold space”. The system thus simulates two atmospheres one with the same thickness as todays 400 ppmv but compressed into 2.24 meters (100% CO2). The reference run with plain air is very close to an atmosphere without CO2 because ve are using only 2/10000 of the atmosphere which means that CO2 backradiation fr air probably isn’t detectable when the length of the system is 2.24 meters. The system also eliminates the probably much bigger problem of convection in a non inverted design.


    /Lars Silen

    • Tim Folkerts says:

      The biggest problem I initially see is the IR from the walls of the pipe. The walls of the pipe will cover most of the ‘solid angle’ that the heater ‘sees’. Since the walls are also much warmer then the cold plate at the bottom, most of the ‘back radiation’ will come from the pipe’s wall — not from the bottom and/or the gas inside. This will greatly reduce the sensitivity of the experiment and make it tough to distinguish the signal from the CO2.

      Oh — and a second big problem — cryopumping! If the bottom plate is chilled to 77 K with liquid nitrogen, then all the CO2 will freeze solid onto the plate at the bottom.

      • Lars Silen says:

        Good answer 😉 thanks. This means, I think, that the thermo sensor should be designed with a filter restricting the IR view sideways … of course the sensitivity will also suffer. The other option is to increase the total volume by increasing the diameter of the “pipe” creating somethin timilar to a thick barrel roughly 2 m high.

        The comment about cooling the bottom to a really low temperature is also valid. I don’t think the actual absolute temperature of the bottom plate is very important, it should be kept low compared to other parts of the system but at a constant temperature.

        Looks like I have to build a system to see if anything can be detected 😉 .

        I think the interesting point is that it should be possible to simulate the totality of CO2 in the atmosphere in a device of reasonable dimensions. Because only differences between power off equilibrium and power on background plus greenhouse gas backradiation is measured I think a fairly sensitive instrument should be the result.

  37. Jerry L Krause says:

    Roy and others:

    I just read about Woods experiment and your comments about it, but hopefully at least Dr. Spencer will read this. The clear purpose of Woods experiment was to test the idea of the greenhouse effect. For he and most reason that if there was (is), the temperature of the glass covered box should rise to a greater temperature than the rock salt covered box. When he observed just the opposite, he made a hypothesis as to the cause of this surprising observation. Which was that the rock salt was passing a greater fraction of the solar radiation than the glass. To test this he placed a sheet of glass on the rock salt and observed that the temperature dropped to that of the glass covered box. Fully supporting his hypothesis and confirming the everything else about the two boxes was basically identical.

    Based upon what Wood observed. Might we conclude that if the concentration of greenhouse gases increase, that while the temperature of the atmosphere might increase but that the temperature of the earths surface should decrease? Another question to confuse the issue is: How is the atmosphere primarily heated, by solar radiation or by conduction-convection, evaporation of water from the surface and condensed in the atmosphere and longwave radiation from earths surface heated by solar radiation?

    • David Wieland says:

      This is the first comment I’ve seen that clearly notes the purpose of Wood’s experiment was to test the theory of greenhouse function. Unfortunately, it seems that there’s precious little understanding that the dominant (and likely the only) principle is the enclosing of the warmed space.

      The comment from Bill Hunter about IR film now being available is true, although I found only supplier claims that it can reduce heating cost. What is definitely valuable is UV protection to slow degradation.

      It would be interesting to perform the experiment using a black film, even a heat-conducting one like sheet metal, versus glass. That would be useless as a greenhouse covering of course, but it would show how much effect the supposed IR-reflecting property of glass actually has.

  38. torontoann says:

    “Another question to confuse the issue is: How is
    the atmosphere primarily heated…?”

    That does not confuse the issue – it is the essence
    of any practical result, of greenhouse gas theory, for
    the temperatures in the atmosphere.

    Of course, there is “back-radiation” to the solid/liquid
    skin of the Earth. But this tells you little about
    whether the skin is net-warmed by the phenomenon.
    THAT depends on the existence or not, of an efficient
    non-radiative system of transferring heat from the skin to
    the atmosphere.

    A 100% effective conduction system of that sort would
    automatically return ALL of the energy in the down-welling radiation, back to where it came from. Through measuring such a return system we would notice: a hundred Joules down, a hundred Joules up; a trillion Joules down, a trillion Joules up. The Greenhouse gas effect would “exist” but it would call into play its own nemesis.

    Conversely, a LACK of conduction-communication between
    the skin and the atmosphere would imply the skin must heat,
    until the increased temperature sufficiently enhances its radiation loss.

    Conduction, as such, from the Earth’s skin to the atmosphere
    is not rapid. But there is a good substitute; for the removal of heat from the one place to the other, by evaporation of water and subsequent condensation in the atmosphere, is huge. One must certainly say; the “return system” is essentially vigorous.

    If you took all the greenhouse gases* out of the planet
    (and imagined the Seas dry) you could easily calculate that the effective black-body temperature of the Earth would fall (only) about 10 Deg. C. This is partly because water in the atmosphere shields us at present from much of the energy of the sun – in one way and another. And partly because greenhouse gases are the main EMITTERS of radiation to space, pari passu with being absorbers of radiation from Earth’s skin.

    *including water vapour – the most active by far.

  39. Rick Spielman says:

    For scientific accuracy Saran is NOT polyethylene. It is polyvinylidene chloride. I don’t have the IR or visible transmission of Saran but it is not that of polyethylene.

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