Effects of Adding Specific Amounts of Greenhouse Gases

We will now evaluate the effect of adding specific amounts of additional greenhouse gases to the earth's atmosphere. Figure 5 gives a quantitative description of the impact of each greenhouse gas in terms of the change in the heating rate per unit time per unit area (Watts per square meter per decade, Wm^-2decade^-1) at the earth's surface.

Recall that concentrations of greenhouse gases are about 370 ppmv for carbon dioxide, 1.7 ppmv for methane, and parts per trillion for nitrous oxide and for CFCs. Each formula given in the second column describes the surface heating rate in terms of gas concentration. Note the different functional forms for different gases. For carbon dioxide, the radiative forcing (atmospheric heating rate) is proportional to the natural logarithm of the concentration divided by some base concentration. For methane, nitrous oxide, and stratospheric water vapor, the leading term of the radiative forcing increases as the difference of square roots, and for gases that are less abundant, the forcing is directly proportional to the concentration.

The impact of adding one additional molecule of a greenhouse gas is much larger if the relative abundance of the gas is low. For example, the impact of adding one molecule of methane is 21 times as large as adding one molecule of carbon dioxide (see Figure 6).

A molecule of nitrous oxide has the equivalent greenhouse effect of 206 carbon dioxide molecules, and the CFC molecules have an impact 12,000 to 18,000 times that of carbon dioxide. The absorption bands (wavelength regions) for carbon dioxide are nearly saturated, but those for other gases are not, so one additional molecule makes a larger impact. However, we should keep in mind that each person in the US on average puts about 20 metric tons (20,000 kg or 44,000 pounds) of carbon dioxide into the atmosphere from burning fossil fuels each year, compared to 100 kg of methane, 2 kg of nitrous oxide, and 2 kg of CFCs. Multiplying the CFC emission rate by 12,000 gives a CFC greenhouse impact comparable to that for carbon dioxide.

At the bottom of the last table are listed the possible replacements for the CFCs. Their impact, in some cases, is considerably reduced, but by no means do they eliminate radiative forcing.

Figure 7 gives a comparison of the radiative forcing of the CFCs and their possible substitutes.

This information can be used to evaluate the relative advantages of CFC substitutes for reducing global warming. Quantitative calculations such as these help policy makers appreciate and understand how science can assist in shaping new legislation. It also demonstrates to the private sector (e.g., power-generating industry and chemical manufacturers in this case) the relative impact of various greenhouse gases. The detrimental effects of CFCs are clearly evident, and these calculations, in addition to their calculated effect on ozone, led to the creation of the international agreement to eliminate CFCs (Montreal Protocol) and led DuPont Corporation to stop manufacturing CFCs before the international mandate to do so was implemented.

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