Albedo Changes

Changes in the earth's albedo have also been implicated in climate changes. Although large-scale cultivation, irrigation, damming of rivers to form lakes, and cutting down of forests all result in changes in the earth's albedo, these causes have heretofore been considered minor. The possible results of volcanic and other dusts in the atmosphere have, however, been investigated fairly extensively.

Figure 3-15 shows an index of volcanic activity on earth between the years 1500 and 1970. Some of this volcanic activity was followed by worldwide cooling. For example, the "Little Ice Age" (1550-1850) occurred during a period of fairly high volcanic activity as well as low sunspot activity. Some of the highest peaks on Figure 3-15 are actually tremendous single volcanic explosions that were followed by noticeably cool years. The eruption of Asama in 1783 in Japan (the largest activity peak between 1700 and 1800 in Figure 3-15) was followed by three cool years, 1783-1785. The year 1816, called "the year without summer" (in New England, snow fell in June and there was frost in July), followed the eruption of Tambora, Sumbawa, in the Dutch East Indies in 1815. This eruption killed 5600 people and darkened the sky for days as far as 300 miles away. The large peak just before the year 1900 on Figure 3-15 is probably connected with the eruption of Krakatao in Indonesia in 1883. The eruption of Mount Agung in Bali in 1963, although not as spectacular as that of Tambora, occurred after accurate measurements of tropospheric temperature and aerosol measurements were being obtained, and is known to have resulted in a drop of 0.4 °C in the troposphere between latitudes 30°S and 30°N about one year later and lasting for about two years. It turns out that it is not the volcanic dust that contributes to tropospheric cooling, but aerosols consisting of submicrometer droplets of sulfuric acid derived from sulfur-containing volcanic gases. Thus, the eruption of Mount St. Helens in the northwestern part of the continental United States in 1980 did not cause noticeable cooling because the gases released by this volcano contained only a small amount of sulfur. Volcanic ash falls out of the atmosphere within a few

1500 1600 1700 1800 1900 1980

Year

FIGURE 3-15 Index of volcanic activity, in arbitrary units, that is proportional to the relative amount of material injected into the stratosphere in various 10-year periods. Redrawn from Inadvertent Climate Modification Report of the Study of Man's Impact on Climate (SMIC). Copyright © 1971. Used by permission of MIT Press.

weeks or months, but the sulfuric acid aerosol droplets can remain in the stratosphere for years. The 1982 eruption of El Chichon in Mexico apparently cooled the surface of the earth for several years because of the large amount of sulfuric acid aerosol formed afterward from sulfur-containing gases. The aerosols from the eruption of Mount Pinatubo in the Philippines and of Mount Hudson in Chile in 1991 apparently also caused global cooling for close to a year.

It has been postulated that a large amount of dust and soot would be released into the stratosphere during a major nuclear war. For various reasons, this dust and soot would probably remain aloft for about one year. This would then increase the earth's albedo to the extent that considerable cooling of the earth would occur over several years. Calculations indicate that the earth could cool up to 1°C for several years (recall that the difference between an ice age and an interglacial period is about 5°C). This scenario has been called "nuclear winter,'' and it includes major changes in precipitation and a probable depletion of the ozone in the stratosphere. Thus, this scenario includes a major disturbance of agriculture all over the earth in addition to depletion of the ozone layer that protects the surface of the earth from high-energy ultraviolet radiation.

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