Greenhouse Gas Absorption Spectrum

Figure 4 gives the amount of energy absorbed by greenhouse gases in various wavelength regions, from ultraviolet radiation on the left, to visible light in the middle, to infrared radiation on the right.

The CFCs are not plotted here but will be considered separately. For each gas is given a plot of the absorptance of the gas, ranging from 0 to 1, for each wavelength.

As an example, if we look at the plot for oxygen and ozone, we see that the absorption is very high in the ultraviolet region but essentially zero in the visible and infrared regions, except for isolated peaks. We interpret this to mean that this gas absorbs essentially all radiation in the ultraviolet but is transparent in the visible and mostly transparent in infrared portions of the spectrum. This gas then is responsible for shielding earth-based biological systems from lethal ultraviolet radiation, radiation with wavelengths less than 0.3 micrometers (or 300 nanometers), but allows visible light and infrared radiation to pass through without much absorption.

Other gases have much different absorption properties. Methane (CH₄), for example, has a couple of very small wavelength regions in which it absorbs strongly and these occur at about 3.5 and 8 microns, which are in the infrared region. Nitrous oxide, N₂O, having peaks at about 5 and 8 microns, absorbs in fairly narrow wavelength ranges.

Carbon dioxide has a more complex absorption spectrum with isolated peaks at about 2.6 and 4 microns and a shoulder, or complete blockout, of infrared radiation beyond about 13 microns. From this we see that carbon dioxide is a very strong absorber of infrared radiation. The plot for water vapor shows an absorption spectrum more complex even than carbon dioxide, with numerous broad peaks in the infrared region between 0.8 and 10 microns.

The total spectrum of all atmospheric gases is given in the bottom plot. This shows a "window" between 0.3 and 0.8 microns (the visible window), which allows solar radiation (without the lethal UV component) to reach the earth's surface. "Earth radiation", the upwelling infrared radiation emitted by the earth's surface, has a maximum near 10 microns. The total atmosphere plot shows that a narrow window (except for an oxygen spike) exists in the range of wavelengths near 10 microns.

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