Although at first sight the effect which Pettersson sets out to explain seems out of all proportion to the smallness of his cause, the coincidences after 2000 B.C. are extremely interesting, and suggest that after the land and sea distribution reached its present form the astronomical cause adduced by Pettersson was possibly effective, but before that date the astronomical cause, if it existed, was masked by the much greater climatic variations due to changes in the land and sea distribution.
The opinion has frequently been expressed that the “Classical” and “Mediæval” rainfall maxima were phenomena similar to the Glacial period, but less intensive. This view is often carried to its logical conclusion, that the thirty-five-year cycle, the eleven-year, and still smaller cycles of climate, are also part of the same series, and that the Glacial period and, let us say, the three-year periodicity of rainfall are therefore due to variations of the same agent, in this case the sun. This logical extension of the theory is, however, completely untenable. The eleven-year periodicity is admittedly connected with variations in the solar activity, but there are other cycles which are completely independent of such variations, such as, for instance, the annual variation undergone by all meteorological elements, which depends entirely on the inclination of the earth’s axis. There is a well-marked 4.8 year period in the amount of ice off Iceland, the half-cycle of which is exactly equal to the distance travelled by the water taking part in the North Atlantic circulation, divided by the velocity with which it travels. There is, therefore, no a priori reason for assuming that the cause of the Glacial period was identical with the cause of the Classical and Mediæval rainfall maxima. Further, in the latter case, the chief phenomenon was the increase of rainfall; the decrease of temperature was merely incidental, but in the Glacial period the outstanding feature was a great lowering of temperature in the polar and temperate regions, and in this case it was the increase of rainfall which was incidental.
BIBLIOGRAPHY
Lewis, F. J. “The history of the Scottish peat-mosses and their relation to the Glacial period.” Edinburgh, Scot. Geogr. Mag., 22, 1916, p. 241.
Brooks, C. E. P. “The correlation of the Quaternary deposits of Great Britain with those of the Continent of Europe.” Ann. Rep. Smithsonian Inst., 1917, p. 277.
Huntington, Ellsworth. “The pulse of Asia.” Boston and New York, 1907.
——. “The climatic factor as illustrated in arid America.” Washington, Carnegie Institution, 1914.
——. “World power and evolution.” New Haven, 1919, pp. 186-207.
Pettersson, O. “Climatic variations in historic and prehistoric time.” Svenska Hydrogr.-Biol. Komm. Skrifter, Heft 5.