A Global Warming Primer

This Global Warming Primer was prepared for "Global Thinking," Government 294/Philosophy 294, a course at Cornell University. It
covers the scientific basis for global warming. The rest of the course (in the section dealing with this topic) considers the ethical and
economic consequences of global warming.

How Molecules Absorb Light

First, we need to know about properties of light. Light, or radiation, can be viewed as an oscillating electromagnetic field. For every
color of light, the oscillating field has a wavelength and a frequency. They are related by the speed of light, which is the product of the
two. The energy of the light or radiation is proportional to its frequency. Sometimes this energy is expressed by stating the
wavenumber of the light, which is just the reciprocal of the wavelength. A summary of these definitions provides the mathematical
relationships.

What wavelengths correspond to different colors?

A chart may be helpful. Most of what we see, the visible region, is actually a rather small region of wavelength from about 0.6 to 0.4 microns (A micron is a millionth of a meter). Since the radiation emitted by a body at
the temperature of the Earth is mostly at longer wavelengths than the visible, we will be most interested in the infrared region of the
spectrum, from about 0.7 to 100 microns in wavelength. It is interesting to look at the intensity of emission as a function of wavelength
and temperature as well as some of the gases that absorb in this region [1].

Why don't we see any absorption from molecules like nitrogen and oxygen that are much more abundant in the Earth's atmosphere than those that do absorb (like methane, water, and carbon dioxide)? We need to consider that the absorption in this region corresponds to vibrations of a molecule, and that these vibrations can be excited by the oscillating field of the light only if the dipole moment of the molecule changes during the vibration. Because both oxygen and nitrogen are symmetric molecules (composed of two atoms each), there cannot be any net dipole moment on these molecules; there is thus no dipole moment to change as the molecules vibrate. On the other hand, the dipole moment of carbon dioxide changes if it vibrates in its asymmetric stretching or bending modes.

The reason can be seen by considering how the oscillating field might "shake" the dipole moment.