chief hypotheses are outlined in the next chapter, and are frequently referred to throughout the volume.
Between glacial fluctuations and historical pulsations in duration, but probably less severe than either, come orbital precessions. These stand in a group by themselves and are more akin to seasonal alternations than to any other type of climatic sequence. They must have occurred with absolute regularity ever since the earth began to revolve around the sun in its present elliptical orbit. Since the orbit is elliptical and since the sun is in one of the two foci of the ellipse, the earth's distance from the sun varies. At present the earth is nearest the sun in the northern winter. Hence the rigor of winter in the northern hemisphere is mitigated, while that of the southern hemisphere is increased. In about ten thousand years this condition will be reversed, and in another ten thousand the present conditions will return once more. Such climatic precessions, as we may here call them, must have occurred unnumbered times in the past, but they do not appear to have been large enough to leave in the fossils of the rocks any traces that can be distinguished from those of other climatic sequences.
We come now to Brückner periods and sunspot cycles. The Brückner periods have a length of about thirty-three years. Their existence was suggested at least as long ago as the days of Sir Francis Bacon, whose statement about them is quoted on the flyleaf of this book. They have since been detected by a careful study of the records of the time of harvest, vintage, the opening of rivers to navigation, and the rise or fall of lakes like the Caspian Sea. In his book on Klimaschwankungen seit 1700, Brückner has collected an uncommonly interesting assortment of facts as to the climate of Europe for more than two centuries. More recently, by a study of the rate of
growth of trees, Douglass, in his book on Climatic Cycles and Tree Growth, has carried the subject still further. In general the nature of the 33-year periods seems to be identical with that of the 11- or 12- year sunspot cycle, on the one hand, and of historic pulsations on the other. For a century observers have noted that the variations in the weather which everyone notices from year to year seem to have some relation to sunspots. For generations, however, the relationship was discussed without leading to any definite conclusion. The trouble was that the same change was supposed to take place in all parts of the world. Hence, when every sort of change was found somewhere at any given sunspot stage, it seemed as though there could not be a relationship. Of late years, however, the matter has become fairly clear. The chief conclusions are, first, that when sunspots are numerous the average temperature of the earth's surface is lower than normal. This does not mean that all parts are cooler, for while certain large areas grow cool, others of less extent become warm at times of many sunspots. Second, at times of many sunspots storms are more abundant than usual, but are also confined somewhat closely to certain limited tracks so that elsewhere a diminution of storminess may be noted. This whole question is discussed so fully in Earth and Sun that it need not detain us further in this preliminary view of the whole problem of climate. Suffice it to say that a study of the sunspot cycle leads to the conclusion that it furnishes a clue to many of the unsolved problems of the climate of the past, as well as a key to prediction of the future.
Passing by the seasonal alternations which are fully explained as the result of the revolution of the earth around the sun, we may merely point out that, like the daily vibrations which bring Table 2 to a close, they
emphasize the outstanding fact that the main control of terrestrial climate is the amount of energy received from the sun. This same principle is illustrated by pleionian migrations. The term "pleion" comes from a Greek word meaning "more." It was taken by Arctowski to designate areas or periods where there is an excess of some climatic element, such as atmospheric pressure, rainfall, or temperature. Even if the effect of the seasons is eliminated, it appears that the course of these various elements does not run smoothly. As everyone knows, a period like the autumn of 1920 in the eastern United States may be unusually warm, while a succeeding period may be unseasonably cool. These departures from the normal show a certain rough periodicity. For example, there is evidence of a period of about twenty-seven days, corresponding to the sun's rotation and formerly supposed to be due to the moon's revolution which occupies almost the same length of time. Still other periods appear to have an average duration of about three months and of between two and three years. Two remarkable discoveries have recently been made in respect to such pleions. One is that a given type of change usually occurs simultaneously in a number of well-defined but widely separated centers, while a change of an opposite character arises in another equally well-defined, but quite different, set of centers. In general, areas of high pressure have one type of change and areas of low pressure the other type. So systematic are these relationships and so completely do they harmonize in widely separated parts of the earth, that it seems certain that they must be due to some outside cause, which in all probability can be only the sun. The second discovery is that pleions, when once formed, travel irregularly along the earth's surface. Their paths have not yet been worked out in detail, but a general
migration seems well established. Because of this, it is probable that if unusually warm weather prevails in one part of a continent at a given time, the "thermo-pleion," or excess of heat, will not vanish but will gradually move away in some particular direction. If we knew the path that it would follow we might predict the general temperature along its course for some months in advance. The paths are often irregular, and the pleions frequently show a tendency to break up or suddenly revive. Probably this tendency is due to variations in the sun. When the sun is highly variable, the pleions are numerous and strong, and extremes of weather are frequent. Taken as a whole the pleions offer one of the most interesting and hopeful fields not only for the student of the causes of climatic variations, but for the man who is interested in the practical question of long-range weather forecasts. Like many other climatic phenomena they seem to represent the combined effect of conditions in the sun and upon the earth itself.
The last of the climatic sequences which require explanation is the cyclonic vacillations. These are familiar to everyone, for they are the changes of weather which occur at intervals of a few days, or a week or two, at all seasons, in large parts of the United States, Europe, Japan, and some of the other progressive parts of the earth. They do not, however, occur with great frequency in equatorial regions, deserts, and many other regions. Up to the end of the last century, it was generally supposed that cyclonic storms were purely terrestrial in origin. Without any adequate investigation it was assumed that all irregularities in the planetary circulation of the winds arise from an irregular distribution of heat due to conditions within or upon the earth itself. These irregularities were supposed to produce cyclonic storms
in certain limited belts, but not in most parts of the world. Today this view is being rapidly modified. Undoubtedly, the irregularities due to purely terrestrial conditions are one of the chief contributory causes of storms, but it begins to appear that solar variations also play a part. It has been found, for example, that not only the mean temperature of the earth's surface varies in harmony with the sunspot cycle, but that the frequency and severity of storms vary in the same way. Moreover, it has been demonstrated that the sun's radiation is not constant, but is subject to innumerable variations. This does not mean that the sun's general temperature varies, but merely that at some times heated gases are ejected rapidly to high levels so that a sudden wave of energy strikes the earth. Thus, the present tendency is to believe that the cyclonic variations, the changes of weather which come and go in such a haphazard, irresponsible way, are partly due to causes pertaining to the earth itself and partly to the sun.
From this rapid survey of the types of climatic sequences, it is evident that they may be divided into four great groups. First comes cosmic uniformity, one of the most marvelous and incomprehensible of all known facts. We simply have no explanation which is in any respect adequate. Next come secular progression and geologic oscillations, two types of change which seem to be due mainly to purely terrestrial causes, that is, to changes in the lands, the oceans, and the air. The general tendency of these changes is toward complexity and diversity, thus producing progression, but they are subject to frequent reversals which give rise to oscillations lasting millions of years. The processes by which the oscillations take place are fully discussed in this book. Nevertheless, because they are fairly well understood, they are deferred