Other Factors

The mean temperature of the earth's surface is certainly affected by the magnitude of the greenhouse effect, but it is also affected by the percentage of incoming solar radiation that is reflected back out to space, that is, by the earth's albedo. It has already been stated that approximately 30% of the incoming radiation is immediately reflected. This is an average value;

Table 3-1 shows the albedo of various major features on earth. Major changes in cloud cover, snow and ice, field and forest, and so on would change the average albedo of the earth. For example, a decrease in snow and ice cover would decrease the earth's albedo, thus leading to absorption of more solar radiation and, in the absence of other effects, a higher surface temperature for the earth. As it turns out, however, clouds also contribute to the greenhouse effect—they reflect some of the earth's infrared radiation back to earth instead of reradiating it like the atmospheric greenhouse gases. In the simplest picture, however, like the one considered in this book, clouds are considered only with respect to their reflectivity.

These and other factors in the energy balance of the earth are also important as shown schematically in Figure 3-7. The left-hand side of Figure 3-7 shows that 30% of the 343 W/m2 that comes toward the earth from the sun is immediately reflected with no change, that water vapor and clouds in the troposphere and ozone and other ultraviolet absorbers remove an additional 20% of the total, and that the remaining 50% of the incoming radiation, about 172 W/m2, is absorbed by the earth's surface. The central portion of Figure 3-7 shows the fate of the radiation from the earth's surface (at 15°C), about 390 W/m2, which is 114% of the solar energy that enters the atmosphere (a result of the greenhouse effect, Section 3.1.2). Most of this energy is absorbed and reemitted (or reflected) by clouds and gases in the atmosphere. Other types of energy transfer also occur, as shown on the right-hand side of Figure 3-7. Latent heat transport refers to the heat of vaporization of water; water evaporates from the surface of the earth, thus cooling the surface, and then condenses in the atmosphere, thus heating the atmosphere. Sensible heat transfer refers to heat transfer by convection as heated air near the ground rises while cooler air higher in the troposphere sinks.

TABLE 3-1

Albedo of Various Atmospheric and Surface Features of the Earth

TABLE 3-1

Albedo of Various Atmospheric and Surface Features of the Earth

Albedo (% reflected)

Reflected from

30

Total (earth and atmosphere)

50-80

Clouds

40-95

Snow and ice

16-20

Grass

20-25

Dry, plowed fields

15-25

Green crops

5-20

Forests

18-28

Sand

14-18

Cities

7-20

Bare ground

7-23

Oceans

FIGURE 3-7 Energy-flux diagram showing radiative and nonradiative exchanges between the earth's surface, the atmosphere, and space. Units are percentages of global-average insolation (100% = 343W/m). The flux at the surface exceeds 100% of the solar energy flux reaching the earth because of the absorption-emission cycle between the surface and the atmosphere. Redrawn from G. J. MacDonald and L. Sertorio, eds., Global Climate and Ecosystem Change. Plenum Press, New York. Copyright © 1990. Used by permission of Kluwer Academic/Plenum Publishers.

FIGURE 3-7 Energy-flux diagram showing radiative and nonradiative exchanges between the earth's surface, the atmosphere, and space. Units are percentages of global-average insolation (100% = 343W/m). The flux at the surface exceeds 100% of the solar energy flux reaching the earth because of the absorption-emission cycle between the surface and the atmosphere. Redrawn from G. J. MacDonald and L. Sertorio, eds., Global Climate and Ecosystem Change. Plenum Press, New York. Copyright © 1990. Used by permission of Kluwer Academic/Plenum Publishers.

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