During the 20th century, the 1930s drought was much worse than any of the others. The reason is not certain, but perhaps improper farming techniques had something to do with it. People found out that New England farming methods did not work in the Great Plains. When the soil "out west" was plowed, it dried out during the summer months, and the almost constant winds blew it away. After the 1930s, trees were planted as windbreaks, and much of the land was irrigated.
Droughts are correlated with above-normal temperatures. Are there causative factors at work? Do higher temperatures cause droughts, or do droughts cause higher temperatures? Or are both the heat and the drought caused by some other factor, such as above-normal atmospheric pressure over a region, or increased or decreased solar radiation? The exact cause-and-effect relationship is not known, although theories abound.
In recent years, global warming (a general upward trend in the average temperature of the earth's atmosphere) has become an issue. If drought is caused by temporary global warming, such as would be produced by an upward spike in solar radiation, there is little we humans can do about it. But evidence has been mounting that human activities are making the earth warmer. Pollutants, especially carbon dioxide (CO2), are believed to be the main culprits.
When visible light and shortwave IR strike the surface, especially a land mass not covered by snow or ice, the earth is warmed and emits longwave IR. Some of the longwave IR is radiated back into space, but some is also trapped by the atmosphere, in a manner similar to that shown in Fig. 8-7, in which solar energy warms up a room with a glass window. This is the same effect that keeps a greenhouse warm in winter. For this reason, the phenomenon is called the greenhouse effect. The CO2 in our atmosphere contributes to this effect. This colorless, odorless gas is present in trace amounts (less than 1% of the air at the surface, by weight). If there were less CO2, the earth would be cooler, and if there were more, the earth would be warmer, all other factors remaining constant.
The burning of conventional fuels such as wood, coal, and methane (natural gas) produces CO2 as a byproduct. As more and more people have begun to use these fuels, particularly in developing countries and emerging economies such as China, the amount of CO2 in the earth's atmosphere has been increasing. The average temperature in the atmosphere has, over the same period, grown warmer. Most scientists today believe that these phenomena are correlated, and that there is a causative effect. The consensus is that the use of conventional fuels is making the earth warmer than it would be if these fuels were not used. Whether the consequences of human-induced global warming will ultimately prove to be good or bad can be debated, but few scientists continue to call it fiction.
As things work out, a small increase in atmospheric CO2 can make a big difference in the extent to which the earth radiates longwave IR energy back into space. The burning of fossil fuels in particular is raising the concentration of CO2 in our air. Some climatologists have warned that, if fossil fuels continue to be consumed in increasing amounts, the average temperature of the earth 100 years from now might be several degrees Celsius warmer than it would be if alternative fuels were used.
An increase of 5°C (9°F) in the average global temperature would have a profound impact on our climate. A global warming trend could cause the annual precipitation to change in many regions. The zones of the trade winds, prevailing westerlies, and polar easterlies might be redistributed. Suppose a new Hadley cell developed near the equator, producing unbelievably hot deserts? Then previously fertile farmland would turn into desert. Some desert areas, in contrast, would become arable. The level of the oceans would rise because some of the ice now locked up in glaciers and polar ice caps would melt. Many of the world's major coastal cities would be submerged. Drinking water in these places would be contaminated with sea salts.
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