Posted by Objection to the scientific basis of greenhouse warming seems to be the gift that keeps on giving. That is, if you like getting the same gift over and over again and returning it because it’s defective never works. Still, hope springs eternal that understanding something about it will make the disagreements clearer. So this will be the first post about the underlying science. There will be more. It’s a primer, so if you know the science it’s not for you. But understanding what’s under the hood can be explained without requiring agreement on global warming. On the grounds that learning about science is an end in itself, we will plunge ahead. That’s called idealism, folks.
First we have to do some elementary physics. High school level. The central theme is the interaction of electromagnetic (EM) radiation and matter. EM radiation is a mysterious thing that can pass through empty space and also through matter without interacting with it. That’s how the sun’s radiation gets to us and how its visible portion lights up our world. To get here it has to pass through the atmosphere without being stopped by it. But EM radiation can also interact with matter. Whether it interacts or not is what we have to talk about. It depends on the nature of the EM and the matter it encounters. Let’s talk about the EM part first.
An electromagnetic field has two components. As its name suggests, one is the magnetic field and one the electric field. Electric fields are produced by separated or stationary electric charges. Magnetic fields are produced by moving charges. You can’t see the fields but you can detect their presence. The electric field can be seen by putting an electric charge in it and seeing if it experiences a force. You can do the same by putting a magnet in a magnetic field or seeing what happens to a moving electric charge. The two components are different but intimately connected. Changing one causes changes in the other and vice versa.
The field can also transfer energy from one place to another. Here’s one way to visualize this abstract idea. Energy is the ability to do work. Take a magnet in your right hand and hold a compass in your left hand. The magnet produces a magnetic field. Now wave the magnet around. By moving the muscles of your arm you are making the magnet move and with it the field. You are putting work into the field. Now watch as the compass needle wiggles back and forth as you wave the magnet. You are transferring the work from your arm to the magnet to the compass needle — via the magnetic field. So the field can transfer energy.
Charges and moving charges produce electromagnetic fields that move. One way to do this is to send electrons (negative charges) racing up and down a long wire many thousands of times a second. The long wire is called an antenna, and the changing electromagnetic field it produces is called a radio wave. Think of a very tiny test or indicator charge at a particular point in space, like the magnet in our example. When the radiowave passes what you would see is the charge wiggling back and forth. The back and forth wiggling corresponds to the passage of an electromagnetic wave. The “width” of the wiggle is called the amplitude of the wave and how fast it wiggles per second is the frequency of the wave. The wave is usually pictured something like this:
Source: NASA
Think of this like a water wave (except you don’t need the water). the test charge is a boat sitting on the water bobbing up and down as the wave “passes” through. The boat doesn’t travel in the direction of the wave, which is really a disturbance on the surface of the water. The boat bobs up and down, moved by the energy in the wave. The test charge does the same thing at a point in space (no water needed) as the EM wave “passes through.”
All EM radiation is like this (for our purposes we don’t need to talk about the particle version of EM radiation). The different “kinds” of EM are related to differences in the frequency of field wiggling. The EM produced by power lines produces a wiggling back and forth pretty slowly, 60 times a second. Radiowaves produce a wigglng thousands of times a second.
Since the waves all travel at the same speed no matter their frequency (that’s an empirical fact), we can also talk about them in terms of the crest to crest distance (the “wavelength”), which is an alternative to frequency as a way to classify them. Some of these distances are very long (miles) and some extremely short (billionths of a meter or less). Here’s how that works. Think of standing alongside a railroad track. Consider two trains traveling at the same speed, say 80 miles an hour. The number of box cars that pass you a second is the frequency. If one train has much shorter box cars more will pass you in a second. The shorter the wavelength (the shorter the boxcar) the higher the frequency. Since we know the speed the train goes (which is always the same, the speed of light), knowing the length of the box car will also tell us how many will pass us in a second. And vice versa. So we could characterize the train either by the number of boxcars per second (the boxcar frequency) or the length of the boxcar (the wavelength). Confusingly, EM radiation can be characterized either in terms of its frequency (cycles per second or hertz) or by its wavelength (meters or some subdivision of meters like nanometers). They are equivalent in the sense that if you know one you also know the other.
We have familiar names for EM of various frequencies (alternatively, wavelengths). Here’s another pic (using wavelength) from our friends at NASA, where you can also learn more about the various forms of EM (see links at bottom):
Source: NASA
What does all this have to do with greenhouse gases? It turns out that all matter both emits and absorbs EM radiation and the frequencies (or wavelengths) at which a gas does the emitting and absorbing is what makes something a greenhouse gas or not. We also need to discuss the frequencies (or wavelenghts) of the EM radiation emitted by the earth and the sun. That’s coming up. Stay tuned.
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