This is the third (and last) post in our primer of the science of green house gases (see the first two here and here). Our objective is to explain what makes something a greenhouse gas. Why is CO2 a greenhouse gas and not O2 (oxygen) or N2 (nitrogen)? In the first two posts we set the table by explaining electromagnetic (EM) radiation, how we describe it and how it interacts (or doesn’t interact) with matter. We pointed out that all physical bodies act like little transmitters of EM radiation and that they can also absorb EM radiation — but only at the same wavelengths they emit it. Some physical bodies, like the earth and the sun emit EM at all wavelengths, although they emit most of it (peak emission) around a wavelength determined by the substance’s temperature. The hotter the body the more EM radiation it emits and the shorter the wavelength at its peak.
The wavelength at peak emission is given by Wien’s Displacement Law (see last post):
?max = a/T
where ?max is the wavelength (in microns) of maximum EM intensity for a body of absolute temperature, T, in Kelvins. a is a constant with a value 2898. We discuss wavelength and absolute temperature in the first two posts.
What are the values of ?max for the sun and the earth? The sun’s effective temperature is about 5800 Kelvin, so ?max = .5 microns = 500 nanometers. This is right in the middle of the “visible” range. This is quite nice because it allows us to see by the sun’s light. Coincidence? Obviously not. Our visual system evolved to take advantage of the sun’s peak EM radiation.
Let’s do the same calculation for the earth. The temperature at the surface of the earth is about 300 Kelvins, so we have peak EM emission of the earth, ?max = 2898/300 = about 10 microns. We can’t see this EM radiation. It is in the far infrared. But the earth is still radiating energy away at this wavelength.
As explained in the earlier posts, the earth and the sun are called black bodies because they emit and absorb EM radiation at all wavelengths. Some substances, however, especially gases at atmospheric pressure, only emit and absorb at very precise, discrete frequencies. Most atmospheric gases let EM radiation in the visible range pass without any interaction (they are transparent to these wavelengths). Some gases, like methane, CO2, N2O (nitrous oxide) and a few others (but not the most abundant gases in the atmosphere, nitrogen and oxygen) will absorb radiation in the infrared. That’s what makes them green house gases.
What happens now is something like those Chinese finger trap toys, the little tubes of crisscrossing fibers where you stick a finger from each hand in the ends easily enough but when you try to pull them out they get stuck as the tube closes down. EM energy from the sun comes streaming through the atmosphere with a peak at 0.5 microns. It easily reaches the surface of the earth because the atmosphere is transparent to visible light. About 30% of the sun’s light is reflected and the other 70% is absorbed and heats up the earth’s surface to a temperature of about 300 Kelvins. The earth then re-radiates the EM energy at 10 microns. But now instead of an unimpeded trip back to space, some of the gases in the atmosphere, the greenhouse gases, absorb in the 10 micron EM peak energy range. When a CO2 molecule absorbs an EM wave it gets hotter. When it gets hotter it also radiates EM, some of it back down in the direction of earth, some of it “sideways” and some of it back out to space. Whatever portion gets radiated back to earth is a portion of the incoming EM energy that doesn’t balance out the incoming stuff.
That’s how the greenhouse effect works. It’s not theoretical. Without the greenhouse effect the earth would be over 30 degrees Celsius cooler. Other planets also have pronounced greenhouse effects, notably Venus whose atmosphere of CO2 keeps it at around 500 degrees Celsius.
This much everyone agrees on. The purpose of these posts was to make clear exactly what a greenhouse gas was. CO2 is the most prominent but not the only one. Methane is an even stronger infrared absorber but there is much less of it in the atmosphere so its effects are overshadowed by the more prevalent CO2. CFCs and ozone are also greenhouse gases, but also of lesser importance.
Almost all climatologists also believe that the marked increase in greenhouse gases since the dawn of the industrial era (18th century onward) is contributing to an accelerating warming of the earth as well. The simple, purely radiative greenhouse mechanism we have outlined is too simple. It doesn’t take account of other important factors everyone agrees affect surface temperatures, most notably heat transfer by convection and the heat released or absorbed when water changes phase (the latent heat effects). Once we start considering these inter-related factors, things start to get complicated with feedback loops that can only be understood via computer simulations.
The computer simulations are absolutely necessary. It is not possible to rely solely on the historical record without some idea of the underlying mechanisms that affect global climate. The problem is not interpolation of the climatic record (estimating temperatures between measured points) but extrapolation — estimating temperatures outside the range of recorded points. To do this it is necessary to posit a model, even if the model is the simple one “no effect” one that says the future will be like the past. It is tremendously important to take care, since the climate system is like a huge supertanker that cannot be turned around without huge effort. If it is sailing for an iceberg we won’t be able to stop it.
An enormous amount of effort has been expended in trying to understand the consequences of allowing greenhouse gases to increase at the current rate. In our view, the problem is less in the science than in the inconvenient consequences of the science, but we won’t settle this here. Our goal was much more modest. It was clear from the comment threads that not everyone understood what a greenhouse gas was. We’ve tried to remedy that small point.