IR light is light with wavelengths a little longer than red light, and just as different wavelengths of light are absorbed by various materials, some things absorb IR light and some things don't. For example, water absorbs IR light, but air doesn't. That's why it feels suddenly cool when a cloud passes in front of the sun -- the water in the cloud absorbs the IR light (=heat) before it can reach us; the clear air lets the IR light pass.

If you've taken organic chemistry, you might remember doing IR spectroscopy, and how the bonds between some pairs of atoms absorb IR light. Some of the more well-known examples of these pairs of atoms are shown in the spectrogram below, which shows the IR absorbance of formaldehyde. Absorbance is recorded as downward pointing peaks at different wavelengths.

The blue labels on the peaks sound like exercises we might do in aerobics class, but they describe what the light does to the atoms. For example, IR light at 1750 cm^-1 causes the double bond between carbon and oxygen (C=O) to lengthen and shorten, as if the atoms were connected by a spring. The energy from the IR light is absorbed and converted to kinetic energy, which is what heat is: kinetic energy. When this happens in the atmosphere, the atmosphere gets warmer.

Notice that the molecules below have lots of C-H bonds (in methane) and C=O bonds (in carbon dioxide). The O-H bonds in water also pick up kinetic energy when struck by IR light. There are other greenhouse gases, including NO_x, and they all work the same way.

So now we can understand how greenhouse gases cause global warming: the molecules absorb IR light from the sun, which increases the kinetic energy -- and so the temperature -- of the molecules.

Find out more on:
Global Warming
Fossil Carbon Dioxide