The Bogus Greenhouse Gas Demo

by James R. Barrante, Ph.D.

I am sure many of you out there have seen the famous experiment that attempts to show the greenhouse gas behavior of CO2.  It’s a relatively simple and inexpensive experiment, and so it is quite popular as a demonstration in elementary and secondary schools.  The results are convincing, but erroneous.  The experiment involves two identical glass jars with glass stoppers, similar to large cookie jars.  Each jar contains a very precise thermometer.  One jar is filled with air and the other jar is filled with high-purity CO2, which can be purchased from most gas-supply houses.  Two exactly the same infrared heat lamps are placed in the exactly same positions, so that the same amount of infrared light will strike each jar.  For the sake of argument, we will assume here that both setups are identical.

Before the lamps are turned on, the temperature in each jar is carefully recorded.  It is important to note that they do not have to be exactly the same, since the focus of the experiment is to measure a temperature change.  The lamps are turned on at the same time and infrared radiation strikes both lamps equally.  At some point, the lamps are turned off, and the temperature in each jar is recorded (a good approach would be to have two individuals recording the temperatures at exactly the same time.)

The demonstrations that I’ve seen have always shown that the temperature of the gas in the CO2 jar always goes up faster and to a higher temperature than the temperature of the gas in the air jar.  The experimenter then announces that the results are evidence that CO2 is a greenhouse gas.

Here are the problems with this not-so-well-thought-out experiment.  First, we know that CO2 is a greenhouse gas, so it is always easy to prove something we know is true.  The ability of CO2 to be a greenhouse gas is that it absorbs a band of infrared light at a wavelength around 15 microns (1 micron = 0.000000001 meters.  It turns out that this band of infrared light cannot pass through glass.  A better way to look at it is to say that the glass absorbs all the radiation from this 15 micron band.  Consequently, we know that we cannot be looking at the greenhouse gas effect.  So what is changing the temperature in the jars?  The simple answer is that when you heat up a container, any gas inside the container also will heat up by simple convection.  It’s a transfer of heat to the CO2, not light.  The major question is why did the CO2 gas heat up faster and to a higher temperature than the air?  It should not have. Since both jars had the same volume, each contained the same number of moles of gas.  But CO2 has a higher heat capacity than air (see Thermal Behavior of CO2).  The appropriate equation describing the absorption of heat by a substance is

q = n CvΔT

where n is the number of moles of gas, Cv is its heat capacity at constant volume and ΔT is the temperature change.  Assuming both jars received the same amount of heat

( Cv T)­­CO2  =  (Cv T)air

Since Cv for CO2 is greater than Cv for air, ∆T for air must be bigger than ∆T for CO2.  Any experiment showing just the opposite effect has either been rigged or was not performed carefully.  Note that even if sunlight is used in place of heat lamps (visible light from sunlight will pass through the glass and directly heat the interior of the jar), the results still would be questionable due to the higher density of the CO2 gas.  Before drawing any conclusions, it would be useful to replace the CO2 with argon gas, which we know is not a greenhouse gas, but is heavy like CO2.  My guess is that one would get the same results.

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9 Comments

Filed under Global Warming

9 responses to “The Bogus Greenhouse Gas Demo

  1. m

    what material could be used (i’m assuming quartz would work but…)

    • m,

      Generally, in an infrared spectrometer, the windows of gas cells are made of highly polished windows of sodium chloride or potassium bromide. These also are used in liquid cells, if you are not stupid enough to put an aqueous solution in them.

      JRB

  2. Phil

    The Cv of air is higher than the Cv of CO2 (see http://www.engineeringtoolbox.com/specific-heat-capacity-gases-d_159.html and other sources)

    • Be careful with units! I defined the heat capacity in joules per mole-K. Heat capacity is an extensive property. The molar mass makes a difference. The average molar mass of air is about 30. The molar mass of CO2 is 44. Per mole CO2 has a Cv of about 28 and air has a Cv of about 20. You need to take a physical chemistry course!

    • They don’t need a P-chem class (well, ok, everyone does… I’m a big fan) just an intro chem class — a mol is just a unit, like a dozen, so a dozen CO2’s have a different mass than a dozen “air” units…

      so, basically, we need to run this in sodium chloride containers… more fun would be if we could isolate some CO2 in a large evacuated area to show the IR photon will find the CO2 molecule…

      i.e., one denier line of thought is that IR will “miss” the CO2 molecules at 400ppm, i.e., the one CO2 “hiding” in 2500 atmosphere molecules…

      well, using PV = nRT, we can see the width of those 2500 molecules is very small… so how wide is an IR photon? I.e., what is the magnitude of a photon’s electric field? well, from what I’ve been told, one has to solve Maxwell’s equations to get an exact answer, but it is certainly much much wider than 2500 molecules in the atmosphere… a friend of mine noted that if you had only one IR photon leaving earth from chicago, there is a chance it could be absorbed by a lone CO2 molecule hovering over Paris, France (the assumption is there is only one IR photon leaving earth and only one CO2 molecule above the earth)

      • I don’t know. A photon can leave a star, travel a million light years and hit a target the size of the pupil of your eye. That’s pretty good shooting. Anyway the uncertainty in the momentum of a photon puts the uncertainly in position at about the size of a bathtub. I don’t think a CO2 molecule can hide from it.

  3. So, if one IR photon will at least find CO2s in a something like a bathtub area, could we compare this to the width of the 2500 molecules of the atmosphere to illustrate roughly, how easy it is for IR photons to find CO2 molecules?

    I.e.:

    V = nRT/P, using top of troposphere at 200 millibars, and the temperature a little above that at about 218 K we have:

    V = (2500/6.022*10^23 mol)(8.31*10^-2*L*bar*K^-1*mol^-1)(218 K)(1/.2 bar) = (.42*8.31*218*5)/10^22 L = (3760*10^3 mL)/10^22 = 3.8/10^16 cc; taking the cube root to find the width facing the earth where the bathtub sized IR photon is approaching gives 7.2*10^-6 cm

    so assuming I’ve done my PV = nRT correctly, the photons are a lot wider that the 2500 molecules the CO2 molecule is “hiding” in.

    Does this analysis make any sense?

    thanks for your input

  4. I think it is safe to say that a bathtub-sized cross section is larger than 2500 molecules. The more important point is what happens to the light energy after it is absorbed. Remember IR radiation is light not heat. The only way CO2 can transfer the light energy as heat is to collide with air (N2 and O2) molecules around it . . . or . . . it can radiate the energy as light, which means no heat transfer takes place. Which comes first???

  5. my understanding is the CO2s run into other molecules which then release IR back to earth… the data agrees with this (satellite and measuring how much IR is coming down to earth from the atmosphere)

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