400 600 800 1000 1200 1400 1600 Wavenumber (cm1)
FIGURE 14-4 Infrared emission from earth measured from the Nimbus 4 satellite (a) over the Niger Valley, North Africa (14.8°N, 4.7°W) at 12:00 GMT; (b) over Greenland (72.9°LN, 41.1°W) at 12:18 GMT, and (c) over Antarctica (74.6°S, 44.4°E) at 11:32 GMT. Emissions from blackbodies at various temperatures are shown by the dotted lines for comparison (adapted from Hanel et al, 1972).
1040-cm-' ozone band is very weak in the Greenland spectrum (Fig. 14.4b) not because 03 is absent, but because the average temperature at which the atmospheric ozone is emitting at this location is about the same as the surface temperature, so that absorption and emission balance out. In Antarctica (Fig. 14.4c), the atmosphere is warmer than the surface so that infrared emissions due to C02 and 03 more than counterbalance the absorption of terrestrial radiation and their bands actually appear as "positive" peaks on top of the colder surface emissions.
While there are obviously extreme variations in surface temperature, the measured emission profile in the "window" between the strong gas absorptions corresponds to a temperature of 288 K as an average over the earth's surface. From the Stefan-Boltzmann relationship, the measured average temperature of 288 K corresponds to an energy of emission of E = crT4 = 390 W m 2. This temperature and the corresponding energy are clearly much greater than the effective temperature of 254 K calculated earlier assuming the earth is a blackbody emitting the absorbed solar radiation of 235 W m~2 with no interactions with the atmosphere above it.
Figure f4.2c schematically summarizes the transfer of thermal radiation and heat in the troposphere. Since there is a net absorption of 235 W m~2 of incoming solar radiation (Fig. 14.2b), a net 235 W m~2 in outgoing radiation is needed to balance this. Of the 390 W m~2 emitted as thermal infrared radiation by the earth (corresponding to the satellite-derived temperature of 288 K), approximately 40 W m~2 is radiated directly to space in the atmospheric "window" region from 7 to 13 /jum where absorptions by C02, HzO, and 03 are relatively weak. It is this radiation that is detected as the "background" in Fig. 14.4 upon which the greenhouse gas bands are superimposed. The remaining 350 W m~2 is absorbed by the greenhouse gases and clouds. Water vapor is by far the most important greenhouse gas (e.g., Wang et al., 1976). For example, Kiehl and Trenberth (1997) calculate that in a standard atmosphere containing 353 ppm C02, 1.72 ppm CH4, and 0.31 ppm N20 as well as ozone and water vapor, water vapor contributes ~ 60% of the total radiative forcing (defined later). C02 is the next larger contributor, at ~26%, followed by 03, at ~8%. Water vapor in the stratosphere, although present in small concentrations (see Chapter 12), is particularly important (Wang et al., 1976).
While we shall focus on the global view in this chapter, it is important to recognize that the processes shown in Fig. 14.2 are not homogeneous on a global scale. Thus, Fig. 14.5 shows the absorbed short-wavelength energy and the emitted long-wavelength energy
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