Over at ScienceBlogs, Paul Revere has written a three part primer (one, two, three) about the physics of climate change. It begins with the nature of electromagnetism and moves on to discuss the energy relationship between the Earth, the sun, and outer space. It is the sort of thing that feels very basic, but which is nonetheless important to understand through-and-through. In particular, the explanation of black bodies in the second portion is clear and informative.
The discussion of Wien’s Displacement Law is also quite informative. The law holds that every object in the universe emits electromagnetic radiation, and that the most common frequency exists in relation to that object’s temperature in degrees Kelvin. To go from one to the other, divide 2898 by the temperature in degrees Kelvin. The quotient is the peak wavelength, expressed in microns. Human body temperature is about 310 degrees Kelvin, so our peak electromagnetic wavelength is about 9.35 microns long – in the infrared portion of the electromagnetic (EM) spectrum. Since we are pretty similar in temperature to the surface of the Earth, the wavelengths radiated by the planet are in a nearby portion of the spectrum.
It is is ability of greenhouse gases to absorb this infrared energy that lets them prevent energy from returning to space. They are transparent to the dominant wavelengths emitted by the sun, but opaque to those radiating from the Earth. Increasing their concentrations in the atmosphere (through fossil fuel burning, deforestation, etc), causes more of the energy that comes to the Earth from the sun to remain in the atmosphere. As a result of the extra energy, the temperature rises. Incidentally, this is also why people sometimes mention using ground-based mirrors to fight climate change. They reflect light at the same peak wavelength as that of the sun (which passes relatively unimpeded through the atmosphere). By re-radiating at that visible wavelength, rather than the infrared one favoured by greenhouse gases, the energy can be made to escape again. Of course, it would take a massive number of mirrors to balance out the effect of increased greenhouse gas concentrations on the EM emissions from all non-mirrored areas.
One upshot of understanding the nature of these gases is the ability to appreciate how their increased concentration simply must add more energy to our planetary system. The scientific questions that remain are about precisely what changes that energy will generate, and at what rate. The three posts are well worth reading in their entirety.
[Update: 17 December 2009] See also: Greenhouse gases other than CO2