In the first post in this primer series we discussed the nature of electromagnetic radiation. It is via EM radiation that the sun’s energy reaches the earth and since it is the balance between the energy that reaches the earth and the energy that is radiated away from the earth that is at the center of the global warming problem, we need to get this part straight. So far we have only talked generally (and very superficially) about what EM radiation is and pointed out that it can carry energy from one point to another (e.g., from the sun to the earth and from the earth back out to surrounding space). If more energy comes in than goes out, we warm up, and vice versa. None of this says anything about whether any imbalance is caused by human activity. That’s another topic. What we want to do here is explain how “green house gases” get into the picture. None of what we will be saying is controversial or controverted. It’s high school physics.
For our purposes it is sufficient to talk about EM radiation in terms of its frequency (how fast it wiggles in space). We will often use an equivalent to frequency, wavelength (see previous post; longer wavelengths go with lower frequencies). We give different wavelengths (respectively, frequencies) of EM waves different names related to where they appear in nature or how they are used in applications, but physically they are all the same thing, wiggling electric-magnetic fields in space. Many familiar things are EM radiation: radio waves, “short waves”, microwaves, visible light, infrared, x-rays, etc. Visible light is just EM radiation of a narrow range of wavelengths (waves of 400 to 700 nanometers). The different colors of visible light correspond to different wavelengths, with red the longest, followed by the shorter and shorter wavelengths of orange, yellow, green, blue, indigo and violet (“ROY G. BIV” is the mnemonic for the colors of the rainbow). Thus the longest wavelengths (lowest frequencies) of visible light are the red ones, but there is EM radiation with even longer wavelengths (lower frequencies) we can’t see with our visual systems. The longer ones closest to the red frequencies are called infrared (“below the red” in frequency). The ones with frequencies above violet light are called ultraviolet (“above the violet” in frequency).
Recall from the first post that EM radiation is produced by moving charges. We gave an example of artificial production of radiowaves in the last post, where negative charges were sent racing back and forth on a long wire or set of metal rods. We call these wires or rod assemblies antennas. But EM radiation is produced whenever we have moving charges and that’s virtually everywhere. Matter is made up of molecules and atoms and they have charged subparts that are moving. The higher the temperature of something the faster its constituent molecules are vibrating back and forth. Thus each one is like a little antenna, sending out EM radiation.
This means that every piece of physical matter is an emitter of EM radiation. Some physical bodies emit radiation at almost every frequency. They are called “black bodies” (we’ll see why in a minute). Some physical matter, especially gases at low pressure, only radiate at very specific frequencies. Greenhouse gases fall into this category. It also turns out that every bit of living matter can absorb EM radiation, too, that is, when an EM wave passes by its vicinity some of the EM energy may be absorbed by the matter.
This is starting to get complicated, so we’ll summarize it in the form of the Rules of the Game for interaction of EM radiation with matter:
- All matter emits EM radiation, the amounts and nature depending on its temperature
- Some substances emit only at certain wavelenths. This is particularly true of gases at low pressure
- Some substances emit at all wavelengths; they are called black bodies.
- Now here’s a new fact: A substance can only absorb EM radiation at the same wavelengths at which it emits it
- Now another important fact: even black bodies don’t emit radiation in equal amounts at all wavelengths. There is a peak wavelength at which most energy is radiated, with lesser amounts for other wavelengths. The peak radiation depends upon the temperature of the substance. The higher its temperature the shorter the wavelength at the peak.
Now you know how black bodies got their name. If they can only absorb radiation at the same wavelengths that they emit it, then if they emit it at all wavelengths they also absorb at all wavelengths, i.e., they absorb and don’t reflect at any wavelength. So they look “black.” Most things are only approximately black bodies. The earth and the sun are examples. Both emit (and absorb) radiation over a huge range of wavelengths. But the earth also reflects some colors (ro we couldn’t see it), so it isn’t a perfect black body.
Here’s a pic of the last point, which is critical to our story:
What do these graphs mean? On the horizontal axis is the wavelength of the emitted EM radiation for black bodies of various temperatures (the different colored lines). (The symbol for wavelength is the Greek letter lambda, ?). The different hump-like graphs in the figure are labeled by their temperature, given in Kelvins. Kelvins are a measure of temperature above absolute zero and have the same unit size as degrees centigrade. You can get Kelvins from Celsius (centigrade) degrees by adding a little over 273 to them. The temperature at the surface of the earth is therefore around 300 Kelvins. Finally, the vertical axis is a measure of the energy at each wavelength.
You can see the black bodies in this graph are pretty hot. Since the surface of our sun is effectively a little less than 6000 Kelvins [corrected number], these curves are typical of many stars in the galaxy. But the same kind of curve would also be found for much cooler bodies, like our earth. The difference would be that the height of the curve at all wavelengths would be much lower than the ones you see here (since the power of the emitted EM radiation ratchets up with the fourth power of the temperature in Kelvins, the Stefan-Boltzmann Law), and, more importantly for our purposes, the location of the peak would be much farther to the right, that is, the most energy would be at a much longer wavelength. The relationship between the peak wavelength of EM radiation and temperature is called the Wien Displacement Law. You can see the general pattern quite easily in the picture. As the temperature decreases the curves get lower and the peaks move to the right.
This is the source of some familiar phenomena. When a blacksmith heats a metal bar in the forge, it first starts to glow a dull red, then orange, then yellow. These are the first colors of the rainbow, the colors of visible light arranged in order of wavelength (from longer to shorter). As the bar gets hotter the energy emitted has its predominant component at shorter and shorter wavelengths. If iron didn’t melt at higher temperatures and if the fire were hot enough, it would glow green, indigo and blue with increasing temperatures. But just because we can’t see EM radiation outside the wavelengths of visible light doesn’t mean it isn’t there. If we use a special infrared camera we can “see” the EM radiation being given off by our own bodies, with the relatively hotter parts giving off more infrared than the cooler parts. This is the principle behind the medical diagnostic technique called thermography, where an infrared camera is used to find areas with increased blood flow like a tumor in breast tissue.
The Wien Displacement Law is the key to the greenhouse gas story. We’ll explain that in the next post. For now, here’s a summary of where we are to this point. Electromagnetic radiation carries energy from one place to another and can dump off some of that energy by interacting with matter. Black bodies like the sun and the earth emit and absorb radiation at all wavelengths, although they do so at characteristic peak wavelengths that depend on the temperature of the surface of the sun and the surface of the earth. Gases at atmospheric pressures, on the other hand, only absorb EM radiation at certain discrete wavelengths. Other wavelengths don’t interact with the gas molecules, i.e., the gas is “transparent” to the EM radiation. Our atmosphere is transparent to EM radiation in the visible range. That’s why our world is illuminated by visible light.
Now we have all the ingredients we need to understand what makes a greenhouse gas a greenhouse gas. We’ll explain it in the next post.