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.