Climates of Hunger: Mankind and the World's Changing Weather

12. Pollutants in the Air

CHAPTER 12 Pollutants • In the Air ANYTHINGIN THE atmosphe" whim can ch"'ge the net amount of sun energy the earth receives, or which can change where sun energy is concentrated, will affect the westerlies and will therefore change climate. Dust and other substances can alter the atmosphere rapidly: within a human generation or less they can change climates significantly (Bryson and Dittberner, 1976). In a world where a change in our climate can bring mass starvation, we nee~d to consider the ways pollution can change climate. Air pollutants can change the westerlies by changing the equator-to-pole temperature difference, or by changing the bottom-to-top temperature difference. An increased temperature difference in either direction, you will recall, expands the westerlies. We'll consider one substance that can change the equator·pole difference and one that can change the vertical difference. First, bottom·to-top. 143 144 IV. A Perspective on Climatic Change People who live in a greenhouse Within the past decade, the "greenhouse effect" of carbon dioxide in the atmosphere has become famous. In a greenhouse, sun energy comes in through the windows. When it strikes the surfaces inside, it warms them. The wavelength of the radiation is changed-"light," you might say, is changed to "heat." The greenhouse windows pass light more readily than heat. Some of the energy that enters is trapped, and the greenhouse gets warm. Certain substances in the atmosphere do the same thing, letting sunlight in but preventing some heat from radiating back out to space. Water vapor is one such substance, and this is why humid nights cool off more slowly than very dry ones. Carbon dioxide (C02 ) has a similar effect (Manabe and Wetherald , 1967). Some CO2 comes from natural processes, including breathing. It is not poisonous like carbon monoxide; green plants need it to live. But CO2 in the atmosphere has increased about 10 percent in the last century, and most of this increase seems to be due to fuels we have been burning in our age of industry and the automobile. Carbon dioxide, by itself, like a greenhouse window, should warm up the earth. Some people have feared a global heat wave and the melting of ice caps. But before jumping to that conclusion, we need to consider just what carbon dioxide can do in the atmosphere. The temperature of the whole system, earth and atmosphere, depends on the balance between the energy coming from the sun and that energy reflected back away. Carbon dioxide does not change the amount of energy coming from the sun, and it does not reflect the sun's energy back to space. Therefore it can't change the average temperature of the whole system-can't make it warmer. It does, however, warm the lower parts of the atmosphere; so the upper atmosphere must get colder (Bryson, 1973). This, in combination with our earlier argument about an increased bottom-to-top temperature difference in the atmosphere, has an important implication. We discussed how an increase in this temperature difference could expand the westerlies. The implication, then, is that increases in carbon dioxide can work to expand the westerlies (Bryson, 1973, p. 370; Bryson, 1974a, p. 756). Pollutants in the Air 145 A veil of dust Carbon dioxide does not change, but redi8tributes, the total energy received by the, atmosphere and earth. If we could turn the sun's intensity up or down, that of course would change the total energyand so would a change in how much sunlight is reflected back. Some things do change reflectivity. Photographs from space show that clouds are reflecting sunlight back-they are white. If something changed average cloudiness significantly, the earth's overall temperature would change. But there is some feedback, some self-regulation, in the process of cloud formation, as explained earlier. Greater cloudiness cools the earth somewhat, les!; water evaporates from oceans and land, and cloudiness diminishes. Recent research indicates that this self-regulation prevents changes in average cloudiness from seriously affecting overall temperature (Cess, 1976). There is another substance in the atmosphere that reflects back sunlight, and that does not regulate itself: dust. Dust can cool the earth by reflecting away a significant portion of the sun energy that would otherwise be received. Dust from volcanoes has brought times of cold climate. In a volcanic eruption, molten rock finds its way up through the earth's surface. It may...