for a certain phenomenon when in fact there is none. Even in a chaotic system,
occasional repetitive events, similar to cycles, take place. People also sometimes
read their own conclusions into observed data. It has been said, only half in jest,
that no one should let reality disprove a good theory.
It takes more than a few consecutive events to make a solid case for the
sunspot-cycle theory of drought. Unfortunately, the only way to be sure about
this is to continue observations for another couple of centuries, at least. Even
then, the data will have to be evaluated objectively.
THE EFFECTS OF HUMAN ACTIVITY
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 tem-
perature 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, espe-
cially carbon dioxide (CO
2
), 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 green-
house warm in winter. For this reason, the phenomenon is called the greenhouse
effect. The CO
2
in our atmosphere contributes to this effect. This colorless, odor-
less gas is present in trace amounts (less than 1% of the air at the surface, by
weight). If there were less CO
2
, the earth would be cooler, and if there were more,
the earth would be warmer, all other factors remaining constant.
CHAPTER 8 Abnormal Weather
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