Vv

m2 I

b Incoming solar radiation

Absorption at surface (168)

Absorption in atmosphere by C02, 03, H20 and 02 (67)

Absorption at surface (168)

C Outgoing radiation

Heat transfer by thermals (24)

Release of latent heat of evaporation (78)

Release of latent heat of evaporation (78)

Heat transfer by thermals (24)

Absorption by H20, C02, 03 etc and clouds (350)

Absorption by H20, C02, 03 etc and clouds (350)

Radiation from surface (blackbody at 288 K)

Upward emission from HaO, C02, 03, clouds etc (195) Direct surface radiation (40)

Downward emission from H20, C02, 03, etc (324)

Absorption by surface

Radiation from surface (blackbody at 288 K)

FIGURE 14.2 Global average mean radiation and energy balance per unit area of earth's surface. The numbers in parentheses are the energy in units of W m~2 typically involved in each path (adapted with permission from IPCC (1996), with numbers from Kiehl and Trenberth (1997)).

Figure 14.2b summarizes typical fates of this radiation. Of the incoming solar radiation, about 31% is reflected back to space either at the surface (30 W m~2) or by the atmosphere itself (77 W m~2). The remaining 235 W m"2 is absorbed, about 168 W m"2 by the earth's surface and 67 W m~2 by 03, C02, H20, and 02 (see Fig. 14.1) and by particles and clouds in the atmosphere.

The net absorption of 235 W m~2 by the earth's surface and atmosphere leads to heating and hence to the thermal emission of radiation. The Stefan-Boltz-mann law can be applied to the combination of surface and atmosphere as a system (Ramanathan et al., 1987) approximated by a blackbody. Recall that this law says that the energy radiated by a blackbody at temperature T per unit time is given by E = aTA, where a is the Stefan-Boltzmann constant, equal to 5.67 X 10 ~x W m-2 j^-4 jf tjjjs a5Sorbed solar energy is radiated in accordance with the Stefan-Boltzmann law, the effective temperature, Tc, of the surface-atmosphere system can be estimated using E = 235 W m 2 = (5.67 X 10 8 W m~2 K~4)rc4, giving Tc = 254 K, or -19°C. This assumes no interaction with the atmosphere, which, we shall see, is certainly not the case.

Figure 14.3 is a schematic illustration of the wavelength distribution of the direct, incoming solar radiation and the outgoing, lower-energy, terrestrial radiation emitted by the earth's surface for a temperature of ~254 K. The global climate issues discussed in this chapter focus on the interactions of both the longer wavelength, terrestrial radiation and the shortwave, solar radiation with atmospheric gases and particles.

Clearly, 254 K is much colder than the typical temperatures around 288 K (15°C) found at the earth's surface. This difference between the calculated effective temperature and the true surface temperature is dramatically illustrated in Fig. 14.4, which shows the spectra of infrared radiation from earth measured from the Nimbus 4 satellite in three different locations, North Africa, Greenland, and Antarctica (Hanel et al., f972). Also shown by the dotted lines are the calculated emissions from blackbodies at various temperatures. Over North Africa (Fig. 14.3a), in the window between 850 and 950 cm-1, where C02, 03, H20, and other gases are not absorbing significantly, the temperature corresponds to blackbody emission at 320 K due to the infrared emissions from hot soil and vegetation.

The negative peaks that appear to be absorption bands superimposed on the continuous emission curve from the earth's surface are actually due to a combination of two processes involving the atmospheric greenhouse gases: (1) absorption of outgoing terrestrial infrared radiation by the gases, causing vibrationrotation transitions (and in the case of H20, pure rotational transitions), and (2) emission of infrared radiation by the greenhouse gases due to excited states populated by collisions. The population of these excited states is determined by the Boltzmann distribution (see Section A.2b) and hence the emission intensity is char-

FIGURE 14-3 Schematic of wavelength dependence of energy emitted by the sun and hence entering the earth's atmosphere and the energy emitted by the earth's surface at a temperature of ~254 K. The absorptions of various atmospheric constituents have been omitted for clarity.

FIGURE 14-3 Schematic of wavelength dependence of energy emitted by the sun and hence entering the earth's atmosphere and the energy emitted by the earth's surface at a temperature of ~254 K. The absorptions of various atmospheric constituents have been omitted for clarity.

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