Temporary droughts, such as the one that took place during the 1930s, occur when the high-pressure belt is closer to the pole than normal. The conditions that produce a temporary drought are the same as those that cause a heat wave: a persistent ridge in the jet stream over a continent. Then, in the northern hemisphere, the desert belts shift or expand northward. Summer rainfall decreases while the temperature increases.
There were other droughts during the 20th century besides the "dust bowl" of the 1930s. The periods of 1912-1914, the middle 1950s, the middle and late 1970s, and the late 1990s were characterized by above-normal temperatures and below-normal rainfall over much of the United States. These drought periods were spaced at intervals of a little more than 20 years. Is this a coincidence, or does it indicate a cycle? If droughts are periodic in nature, what factors are responsible for the cycle?
Sunspots have been suggested as a cause of climate cycles (and plenty of other phenomena, too!). The number of sunspots varies from year to year, reaching a maximum approximately once every 11 years. Each succeeding sunspot maximum takes place during a period of opposite solar magnetic polarity. Thus, the full sunspot cycle has a period of about 22 years. Some climatologists think sunspot numbers influence the climate, and recent studies have shown that the radiation output from the sun actually fluctuates along with the sunspot cycle. It would not take much of a change in solar radiation to cause a significant climate change on the earth.
Some scientists are skeptical of the cyclic theory of drought, and especially the sunspot theory, pointing out that humans have a tendency (almost an obsession) to seek cycles in nature. They suggest that people may think a cycle exists 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.
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