The Greenhouse Effect

A partial breakdown for the composition of the Earth's atmosphere is given in Table 5.2. The major components, as already seen in Chapter 4, are nitrogen and oxygen. Although these two molecules account for 99.03% of the atmosphere's total volume, they are mostly irrelevant with respect to the greenhouse-heating phenomenon. It is, in fact, the minor constituents, such as water vapor,

Table 5.2. Composition and abundance of selected elements in the Earth's atmosphere.

Component

Symbol

Volume percentage

Nitrogen

N2

78.08

Oxygen

O2

20.95

Argon

Ar

0.93

Water vapor

H2O

< 4.0

Carbon dioxide

CO2

353 ppm'1'

Methane

CH4

1.7 ppm

Ozone

O3

< 50 ppb'2'

Hydrogen

h2

0.4-1 ppm

Helium

He

5.2 ppm

(1)The abbreviation ppm stands for parts per million. '2)The abbreviation ppb stands for parts per billion.

(1)The abbreviation ppm stands for parts per million. '2)The abbreviation ppb stands for parts per billion.

carbon dioxide, and methane, in the Earth's atmosphere that drive the all-important greenhouse forcing effect.

The atmospheric greenhouse effect acts in the same fashion as a fisherman's lobster pot: the lobsters can get in easily enough, but they have great difficulty in getting out. For the atmospheric greenhouse heating effect, the key issue is that the incoming radiation from the Sun has a much shorter wavelength than the outgoing radiation from the Earth. Most of the energy received from the Sun (see Figure 5.2) is in the form of visible light (with wavelengths l ~ 10-7 m) because the Sun has a surface temperature of 5,780 K. The Earth, on the other hand, radiates energy back into space, mostly in the form of far infrared radiation (at wavelengths l ~ 10-5m) because it is a relatively cool object of 291 K.

All this is a consequence of what is known as Wien's law, which states that the wavelength at which the greatest amount of energy is radiated into space per second per unit area is related to the temperature. (See the Appendix in this book for more details.) The greenhouse effect comes about because of a change in the predominant wavelength of the incoming and outgoing radiation (Figure 5.13) and, to use our fishing analogy, the Earth's atmosphere is the lobster pot. The short wavelength visible light from the Sun can penetrate through the atmosphere to heat the ground with little absorption. The longer wavelength radiation emitted by the warmed Earth, however, can and indeed is absorbed by molecules such as carbon dioxide, water vapor, and methane (to mention just three), and this warms the atmosphere. The warmed atmospheric gas then radiates long wavelength radiation both into space and back toward the Earth's surface. It is this latter backheating component that is called the greenhouse effect. (It is perhaps worth pointing out, just for the record, that the way in which the atmospheric greenhouse effect works is not the same process that causes an actual greenhouse to become heated.)

There is no simple formula to describe the greenhouse heating effect, but put simply, if the greenhouse gas abundance is decreased, then the surface temperature will fall. Alternatively, the temperature will rise if the greenhouse gas abundance is increased. Modulating the greenhouse gas abundances of an atmosphere, therefore, provides a whole new suite of possibilities for future terraforming engineers.

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