It is important to understand that the changes of, climate during the cycles of historic times do not appear to have caused great changes in temperature. Variations of temperature from day to day, how ever, were apparently more numerous than now because of more fre quent cyclonic storms which at certain times followed paths nearer the equator than at present.
Effect of Climatic Cycles upon Man.—(1) How Economic Pros perity Varies.—Climatic changes appear to have produced three kinds of effects, economic, political, and physical. In the early days of Rome, about 400 s.c., five acres of cultivated land together with space for pasturage was considered enough to support a family. About 200 me., however, when the trees of California were growing particularly slowly and the lakes of Asia were dwindling, a great change became evident. The careful agriculture of the past with its abundant crops and careful fertilization disappeared. The farmers became so poor that many of them gave up their land, which was sold at low prices and fell into the hands of large owners. The new owners stocked it with cattle and sheep since ordinary farming did not pay. The people who remained on the farms fell so deeply into debt that they were almost slaves. Many were so eager to escape from this condition that they flocked to the cities in search of work, until laws were passed which forbade them to leave their homes. This, it will be seen, is like what happens in our own day during dry periods in western Kansas, only far worse. It happened not only in Italy, but in all the countries around the Mediterranean Sea as far east as Persia and central Asia. Other causes such as exhaustion of the soil and mis government helped to produce these bad conditions, but they would not have done so much harm, had they not happened during the dry part of a climatic cycle.
At the time of Christ the return of favorable climatic conditions did much to help Rome recover her prosperity, but two centuries later there began a decline in rainfall which was one of the main causes of Rome's final collapse.
(2) How Political Life Varies.—The drought and poverty of the dry parts of climatic cycles are apt to exaggerate political troubles.
For example, in the second century B.C. the taxes which the Roman farmers had previously paid with ease from full grain bins became very hard to pay now that the bins were half empty. Hence Rome suffered from what is called the Slaves' Revolt, and from other agrarian troubles. The famous Gracchi brothers lost their lives in a vain attempt to remedy the trouble, but when the rainfall increased the remedy was easy.
Another political effect of the dry part of climatic cycles is bar barian invasions. Nomads such as those of central Asia are the first to feel the effect of increased aridity. The springs in the desert dry up, grass for pasturage is unusually scanty, and the nomads are forced to seek new pastures. Many of the barbarian invasions of Europe during the early part of the Christian era appear to have been due to this cause. Finally in the seventh century the greatest of all outpourings from the desert occurred just at the time when the trees, the lakes, and the ruins indicate that the climate of subtropical and desert regions was driest. Under the influence of the Mohammedan religion the Arabs surged out and overwhelmed all the surrounding regions. Their new religion kept them together and guided them, but the dryness of the desert apparently made them ready for a migration.
Thus large parts of the Roman Empire changed their political allegiance.
(3) How Human Energy Varies.—We have already seen that variations in the weather from day to day are one of the greatest stimulants to human activity. Apparently the part of climatic cycles which has abundant rain in subtropical climates has many more storms and much more variable weather than the dry part of the cycle. Therefore the wet part is stimulating and people have much energy. This seems to be one of the great reasons why the people of coun tries like Greece, Palestine, Egypt, Mesopotamia, and Persia were much more energetic in the past than at present. In those days they apparently Ifad somewhat the same kind of climatic stimulus which the people of the United States and northwestern Europe now have.