by James R. Barrante, Ph.D.
Have you ever wondered why carbon dioxide and water vapor are called “greenhouse gases?” They have nothing to do with greenhouses. The major gases operating in a greenhouse are oxygen gas and nitrogen gas (air). In physical chemistry, we refer to gases like carbon dioxide, water vapor, methane, etc. as infrared active gases. An infrared active gas is, as it suggests, a gas that is capable of absorbing infrared light (I’m using the term “light” here rather than the more correct “radiation,” just to remind the reader that these gases are absorbing light and not heat). All gases absorb heat, but certain gases, such as N2 and O2, are not able to absorb infrared light. Also, as a reminder, the infrared light that CO2 absorbs at 15 microns cannot pass through glass, so its behavior in a greenhouse, aside from the fact that its concentration is 0.04%, is irrelevant. One might argue that the surfaces in the greenhouse are radiating infrared light at 15 microns and CO2 would absorb that radiation. That certainly may be true, but so what! That energy is going nowhere. The surfaces would in turn be cooling. The radiation at 15 microns is not going to pass out of the glass windows of the greenhouse, except to be absorbed by the glass and eventually make it out as heat along with any other heat energy, but that process will be slow.
The major confusion here is a gross misunderstanding of the concept of temperature. There are at least two ways to raise the temperature of a system, and one has nothing to do with heat. The temperature of a system is a measure of specific types of energy associated with the wiggling, spinning, and movement through space of the atoms and molecules making up a system. It is a property of matter. This energy is transferred between quantities of matter, when they come into physical contact with each other (collide), and this type of transfer is what we define as “heat.” It requires the presence of matter to take place and cannot take place in a vacuum. We get no heat from the sun; nor does the Earth radiate heat to space. The energy we receive from the sun comes to us in the form of light. When you are standing out in sunlight, you are not feeling the sun’s heat, you are feeling its light.
Now the temperature of a system also can be raised by doing work on the system. There is no heating unit in a microwave oven. The light radiation in a microwave oven causes water molecules in food to spin faster. This rotational energy is transferred to the food as heat. Likewise, when infrared light strikes an infrared active gas, its temperature increases, not because it has absorbed heat, but because the electromagnetic energy of light interacts with the electromagnetic properties of matter causing, in this case, the atoms making up the gas to jiggle faster. If a gas, like O2 or N2, has no way for the electromagnetic energy of light to affect it, it is infrared inactive and basically is transparent to this type of light.
The idea that a greenhouse effect operates in our atmosphere came about from the observation that a planet with an atmosphere is warmer than a planet without an atmosphere. But is this really an infrared active gas effect? Would a planet with an atmosphere of pure O2 and N2 or pure argon, for example, be warmer than a planet with no atmosphere? It seems logical that is would.
Let’s look at the operation of a real greenhouse. What makes a greenhouse work has nothing to do with the atmosphere inside the greenhouse. Any gas or mixture of gases would do the job. The most important features of a greenhouse are the glass (or suitable substitute) windows. Sunlight, mainly in the visible-uv region of the spectrum passes through the glass windows and strikes the solid surfaces in the greenhouse raising their temperature (remember, no heat is involved here). The sunlight is doing work on the atoms and molecules making up those surfaces, just like in a microwave oven. Nitrogen and oxygen gas molecules collide with the solid surfaces and absorb thermal energy. This is a process involving heat. If you are in doubt as to how much thermal energy is absorbed by these gases, sit in your car with the windows up in July in direct sunlight. That hot air you feel is not CO2. Because these gases cannot pass through the glass, the greenhouse warms. As we described above, the warmed solid surfaces of the greenhouse do radiate a large amount of infrared light, some wavelengths of which will pass through glass, exiting the greenhouse. If you believe that it’s the water vapor doing the warming, remember that over a desert, there basically is no water in the atmosphere. It is hard to believe that the collision of nitrogen and oxygen gases with the hot desert sand is not the major contributor of the hot, desert winds. Nitrogen and oxygen gases are truly the major greenhouse gases in Earth’s atmosphere.
Does the planet operate this way? Of course it does. We all have experienced it. During daylight hours, sunlight strikes the planet’s surface and raises its temperature. Again, no heat is involved in the process. Touch the pavement on a hot, summer’s day. Nitrogen and oxygen gases collide with the warmed surfaces and absorb some of this energy. This is a heat process. Since these gases are not infrared active, they cannot radiate this energy as light. This heat energy is truly “trapped” in primarily the translational motion of the molecules in the atmosphere. All nitrogen and oxygen can do is to spread the energy around to other air molecules as they rise from the surface by colliding with their neighbors. During the night, the gases near the surface pass the energy back to the surface by colliding with it, which, in turn, can radiate it to space. But keep in mind that the Second Law of Thermodynamics operates here. The atmosphere can pass the energy back to the surface as heat only if the surface is cooler than the air. Generally, in summer, this might not be the case, since solids hold on to heat energy more efficiently than do gases.
Does the “infrared active gas effect” involving CO2 and water vapor (what climate scientists refer to as the greenhouse effect) come into play here? Absolutely, but since these gases can radiate heat energy as light, it would seem logical that these do not hold this energy but either pass it on to N2 and O2 or radiate it to surroundings Interestingly, the Second Law does not operate here, as it does with N2 and O2. The irradiated photons do not go looking for a receiver that is cooler than the emitter. That light energy can be absorbed by substances that may be at a higher temperature than the emitter. Moreover, unlike thermal energy transfer which is agonizingly slow, light energy transfer (radiation) takes place at the speed of light. How about that?