Popular Science Monthly/Volume 72/June 1908/Coincident Activities of the Earth and the Sun
|COINCIDENT ACTIVITIES OF THE EARTH AND THE SUN|
By Dr. ELLSWORTH HUNTINGTON
AMONG the inorganic factors by which the fitness of the earth for human occupation is most profoundly influenced, two of the chief are climate, on the one hand, and telluric activity on the other. To climate we owe much of the nature and depth of the soil upon which all life depends; it determines the character of vegetation, and makes possible the vast commerce which consists of the exchange of the vegetable products of one land for the commodities of another; it causes men to engage in different occupations, some, for instance, raising rice in the warm plain of Egypt, and others leading the life of the lumberman or trapper in the cold woods of Canada; and finally climate exerts a profound influence upon human temperament, the inhabitants of the torrid zone, as is generally recognized, being notably less energetic than those of temperate regions. Telluric activity, manifested in movements of the earth's crust, past and present, is equally important, though its effects are not so immediately visible. The ravages of earthquakes and volcanoes, great as they are, fade into insignificance when compared with the stupendous results which have followed from the upheaval of continents and folding of thick strata of solid rock. If telluric processes had not throughout the ages again and again upheaved the crust of the earth, the climatic forces of weathering and erosion would long ago have reduced the original continents to featureless plains of small extent compared with the present great areas of land. There would be no mountains full of minerals, and the use of metals would probably be unknown because none would have been discovered by reason of the enormous depth of soil which would cover the country.
In the study of climate and of telluric activity, attention has till recently been concentrated upon the earth. Within a few years, however, scientists have begun to turn to the sun to see if its changes are in any way connected with changes of climate or with the occurrence of earthquakes and volcanic eruptions. At first the results were negative. Of late, however, students of solar physics have shown that there are now in progress fluctuations of climate which appear to be coincident with variations in the activity of the sun as evinced by the occurrence of sunspots; and the investigations of Jensen, as I shall presently show, seem to indicate an even closer coincidence between solar activity and the occurrence of earthquakes and volcanic eruptions.
Lockyer, Bigelow, Brückner, Clough and others have described climatic cycles averaging 3, 11, 36 and 300 years in length, all closely coincident with variations in solar activity. The 36-year cycle of Brückner is the best established and most easily recognizable. It is now accepted by the majority of meteorologists. It appears to pertain to the continental regions of the earth as a whole, although there are certain parts of the land close to the sea where the changes are the reverse of those in the interior. As to the seas data are not available. In continental regions the temperature is comparatively low at one extreme of the cycle; storms, clouds and precipitation are relatively abundant; storm-tracks of temperate regions approach the equator; snow lies long in winter; glaciers and rivers increase in size; lakes, especially those having no outlets, stand at a high level; and vegetation and animal life are appropriately influenced, as is evident from the time of the ripening of crops, and the expansion of irrigated areas in arid regions. These conditions prevail regularly at an interval of a few years after periods of exceptional activity in the sun. During the period of activity the 11-year sunspot cycle is reduced to 9 or 10 years, and there are other signs of unusual movement in the solar atmosphere. The other extreme of thecycle follows a period of comparative inactivity in the sun, and is characterized by climatic phenomena the opposite of those just described. Meteorologists are not yet agreed as to the cause of the climatic cycles, but it seems to be well established that they are somehow connected with the sun.
In regard to the relation of solar activity to earthquakes and volcanoes, there is at present no agreement among students. On the whole, the evidence has seemed to most investigators to indicate that there is no relation. This appears to be largely due to the use of individual cases instead of averages, and to an attempt to find a coincidence between telluric activity, manifested in earthquakes and volcanoes, and maximum epochs of solar spottedness. Jensen, however, who has taken up the subject in a comprehensive fashion in volume thirty-six of the "Proceedings of the Royal Society of New South Wales," has come to a different conclusion. He has compiled a list of notable earthquakes and volcanic eruptions from 1783 to 1902. Each occurrence has been assigned a value of one, two, three or four, according to its severity, and all the earthquakes and eruptions for the whole series of years have been plotted as shown in Fig. 1. Having in this way obtained a graphic representation of the intensity of telluric activity in each year, Jensen added a curve showing the occurrence of sunspot maxima and minima. An inspection of the diagram thus obtained shows that earthquakes and volcanic eruptions are most frequent and most intense during the years shortly before and after sunspot minima.
In order to estimate the reliability of Jensen's conclusion, it is necessary, first, to eliminate the personal equation by comparing his data with another set compiled independently; and, second, to eliminate accidental or sporadic occurrences and faulty observations by an appeal to averages. It has been possible to accomplish the first result by means of data which Mr. Robert W. Sayles, of Harvard University, has kindly put at my disposal. In the pursuit of certain researches having no immediate connection with the problem in hand, he had prepared a table showing the years of occurrence of notable earthquakes and eruptions from 1755 to 1902. He had divided the years into three classes according to the severity and number of the phenomena of both sorts in each year. He has kindly prepared the accompanying diagram (Fig. 1), showing, on the one hand, the years of telluric activity by means of the row of dots at the bottom, and, on the other, the number of sunspots by means of the wavy line. The open circles indicate years when notable earthquakes or eruptions occurred, although not in large numbers, nor of exceptional severity. They are reckoned as unity. The solid round dots represent years of greater severity than the preceding, and are reckoned as having a value of two in computation. The solid squares indicate extreme severity, and are reckoned at three. To the diagram as prepared by Sayles, I have added Jensen's data, as appears in the small rectangles above the sunspot curve. Jensen, unlike Sayles, has separated earthquakes and volcanic eruptions. Earthquakes are shown above the heavy horizontal line, and eruptions below. The size of the rectangles indicates Jensen's estimate of severity and frequency combined. For convenience of reference I have added the appropriate numerals.
The data of Sayles and Jensen supplement each other admirably. Neither investigator lays claim to absolute completeness in his data; but, on the contrary, both express regret that they have not been able to obtain fuller information. Nevertheless neither appears to have omitted any phenomenon of first-class importance. The method of compilation was quite different in the two cases. Sayles lays special stress upon the severity of individual earthquakes or eruptions; while Jensen emphasizes the total number of occurrences in a given year. The two sets of data were prepared without reference to each other; and different sources of information were evidently used, as appears from the relatively large importance which Jensen, an Australian, naturally assigns to the phenomena, of Oceania. For instance, to illustrate the difference in the point of view, Jensen gives to 1835 the value of 6; while Sayles makes it one of the severest years, which on Jensen's scale would give it a value of 20. Again Jensen reckons 1855 at 17, while Sayles put it in his lowest class which would give it a value of only 7. The next year, on the other hand, is reckoned by Jensen at only 4, and by Sayles at 14. Other years, such as 1873–4–5–6 and 22 others are given values of from 1 to 16 by Jensen, but are not mentioned by Sayles; while the latter gives an open circle to 1803, 1805, 1853, 1861, 1863 and 1885, although Jensen does not assign them a value of even one. Other discrepancies might be mentioned. They are natural, indeed unavoidable, in a subject where there is so much opportunity for the personal equation, as well as for diverse authorities. A man's estimate of the severity of an earthquake or eruption is sure to depend largely upon the vividness of the account which he happens to read. Hence the great value of having two independent sets of data compiled for different purposes by men living at the antipodes, New England and New South Wales. The discrepancy in the two sets of data is an advantage because the one supplements the other, and because where the original sources of information are so diverse, the harmonious result derived from averages is highly remarkable. It can not be the result of chance.
Inspection of Fig. 1 shows that according to both Sayles and Jensen periods of minimum sunspots are times of maximum seismic and volcanic activity; whereas at periods of maximum sunspots, telluric activity almost ceases. There are certain glaring exceptions, such as 1883, or 1906 which does not appear in Fig. 1; but it should be noted that in 1883 the sunspot maximum was only about two thirds as high as the average. In order to estimate the true importance of such exceptions, I have plotted the curves shown in Figs. 2–5, showing the relative frequency and intensity of telluric activity in years of sunspot minima as compared with other years. Figs. 2 and 3 show the frequency of years in which one or more notable earthquakes or eruptions—combined in the case of Sayles, separate in that of Jensen—have occurred at the sun-spot minima and during the intervening years. To illustrate concretely, it appears in Fig. 1 that out of fourteen years of minima included in the period covered by the investigations of Sayles, 11, or 79 per cent., have, according to him, been characterized by notable earthquakes or eruptions. Out of the thirteen years immediately preceding a minimum only 5, or 38 per cent., have had noteworthy seismo-volcanic phenomena; out of those preceding a minimum by two years, 6, or 46 per cent., and so on. Of course, the curve soon comes to zero at either end, because, on an average, five or six years before or after a minimum we come to a maximum separating one wave of the sunspot curve from another. To illustrate again, 1867 was a minimum year, and has a solid circle below the sun-spot curve and two rectangles marked 2 and 6 above it. Therefore in calculating the data for the three curves of Figs. 2 and 3 it counts one in every case. So too does 1868, although it was a year of greater severity as is indicated by the solid square below the curve and the rectangles marked 12 and 8 above it. The year 1869, however, having only a single rectangle with a value of two, counts only in the computation of the data for the solid line of Fig. 3. 1870, on the other hand, is reckoned as one in computing the curve of Fig. 2 and the dotted line of Fig. 3. With 1870, which was a maximum year, we cease to count the years as being after the preceding minimum. 1871 is reckoned not as four years after 1867, but as seven years before the minimum of 1878, and so forth. By adding the figures for all the sunspot waves, and plotting the results, we get the simple frequency curves of Figs. 2 and 3. Figs. 4 and 5 are derived in the same way, except for one thing. Instead of reckoning each year of the occurrence of earthquakes or eruptions as having a value of only one, each is reckoned according to the value given it by Sayles or Jensen, respectively, as shown by the character or size of the spots and rectangles of Fig. 1. An inspection of the four curves of Figs. 2 to 5 shows that they agree in essential points. Each of the six curves, two for Sayles, and four for Jensen, has a pronounced maximum at or within a year of the time of sun-spot minimum. That is, when sunspots are fewest, earthquakes and volcanic eruptions are most numerous and most severe.
The four curves of Figs. 6 to 9 on the right-hand side of page—were drawn in exactly the same way as the four which lie beside them (Figs. 2–5), except that the sun-spot maxima were used as the reference points instead of the minima. They are introduced by way of contrast. It is evident that telluric activity is weak at times of sun-spot maxima. All the curves of Figs. 2 to 9 show the lack of symmetry characteristic of sun-spot variations. The lapse of time from maximum to minimum is greater than from minimum to maximum.
Having seen that there is a coincidence of some sort between sun-spot minima and seismo-volcanic maxima, the next step is to compare the mean sun-spot curve from maximum to maximum with the mean seismo-volcanic curve for the same period. The mean sun-spot curve is, of course, easy to obtain. Figs. 10 to 13 show the first stages in the construction of the mean seismo-volcanic curve. The time from one sun-spot maximum to the next is divided into eight periods as follows:
1. The year of maximum spots.
2. The year succeeding that of maximum spots.
3. An intermediate period of decreasing number of spots,—average length about 3½ years.
Fig. 1. The Relation of Sunspots to Earthquakes and Volcanoes, (After Sayles.)
4. The year preceding that of minimum spots.
5. The year of minimum spots.
6. The year succeeding that of minimum spots.
7. An intermediate period of increasing number of spots,—average length about 11⁄2 years.
Figs. 2–9. The relative Frequency and Intensity of Seismic and volcanic Phenomena at Periods of Sunspot Minima (2–5) and Maxima (6–9). The numerals above the diagrams indicate years, those on the sides, percentages. In the diagrams compiled from Jensen's data the full lines indicate seismic phenomena, and the dotted lines volcanic phenomena.
8. The year preceding that of maximum spots.The irregular periods numbered 3 and 7 are necessary because of the variation of the length of the sun-spot cycle. Number 3 may vary in length from 1 to 7 years, and number 7 from 0 to 3.
For each of the eight periods of the sun-spot cycle, the percentage of years of seismic and volcanic activity has been calculated from the data of Fig. 1, as shown in the table at the end of this article, and has been plotted in the frequency curves of Figs. 10 and 11. For Sayles, 13 complete sunspot cycles are available, and for Jensen, 10. In the same way the average intensity of the phenomena of the years of each period has been calculated and plotted in Figs. 12 and 13. In order to obtain the mean curve it is necessary to combine the six shown in Figs. 10 to 13 into one. The first process is the combination of Jensen's four curves into two which shall be comparable to the two of Sayles. This is done in Figs. 11 and 15. In Fig. 14 the solid line reproduces Sayles's line of Fig. 10 directly, while the dotted line gives the mean between Jensen's two curves of Fig. 11. Thus we have two mean seismo-volcanic frequency curves. In the same way, in Fig. 15 the solid line reproduces Fig. 12, and the dotted line gives the mean of the two lines of Fig. 13; and we have two mean seismo-volcanic intensity curves. In these curves, as in the others, percentages are used, so that when diverse phenomena such as frequency and intensity come to be compared and averages computed, each receives the same weight. In Fig. 16 the frequency curves of Sayles and Jensen shown in Fig. 14 are combined into the solid line and the intensity curves of Fig. 15 into the dotted line. In order to make the two curves comparable the maximum in each case has been reckoned as a hundred. As a result of the combination of the data of our two authorities, the personal equation is largely eliminated. It will be noticed that the curves of Fig. 16 are much smoother than those of preceding figures. This is especially true of the frequency curve where there is least liability to errors of judgment. Finally, in Fig. 17 the two curves of Fig. 16 are combined into one, shown by the solid line. This represents the net result of Sayles's data as to the combined seismic and volcanic frequency and intensity of 13 complete sun-spot cycles, and of Jensen's independent data as to the uncombined seismic and volcanic frequency and intensity of 10 complete sun-spot cycles. The dotted line in Fig. 17 is the mean sun-spot curve derived from Fig. 1, and calculated and plotted in precisely the same manner as the six curves of Figs. 10 to 13 from which the mean seismo-volcanic curve is derived. For the sake of convenience in comparison, the sun-spot curve has been plotted with the minimum at the top.
The resemblance between the mean sun-spot and mean seismo-volcanic curves is extraordinary. The maximum of the one occurs at the same time as the minimum of the other, and in both cases there is a steady progress from maximum to minimum and back. If our terrestrial data of earthquakes and volcanoes were as complete as our solar data of sun-spots, it is probable that the resemblance between the two curves would he still closer. It may be that the occurience of earthquakes and eruptions lags somewhat behind the change in the number of sun-spots, but the lag is so slight that it does not appear where the unit of measurement is a year, although it might if the unit were a month. It seems to be impossible to avoid the conclusion that the marked coincidence between telluric and solar activity indicates a relation of some sort between the internal phenomena of the earth and the sun.
As to what that relation may be we have as yet no clue. The best that we can do is to speculate. It may be, perchance, that there is some cosmic source of energy as yet unknown, which pulsates through the universe causing both the earth and the sun to respond, each according to its kind. Possibly changes in the amount or in the nature of the energy emitted by the sun engender corresponding changes in the earth in some manner as yet beyond our ken.
At the present time, as we have seen, changes in the sun appear to be coincident with climatic and telluric changes in the earth. So far as we can judge, the climatic changes, though on a very small scale, seem to be of the same nature as the great climatic changes of the various glacial periods of earlier geological times. The telluric changes, also on a very small scale, are apparently of the same nature as the great movements of the past by which mountains have been formed and continents uplifted. It is notable that according to the general opinion of geologists the three best known and most severe climatic changes through which the earth has passed have been closely associated with profound modifications of the earth's crust. The glacial period which occurred just before the Cambrian period, far back near the beginning of legible geological records, was followed by a great change in the distribution of land and sea. Again after the prolonged period of comparative stability known as the Paleozoic era there ensued the severe Permian glaciation composed of many glacial epochs separated by warm epochs. At approximately the same time, or shortly afterward, there was a great uplifting of the continents and the formation of mountain ranges such as the Appalachians. Finally the last great glacial period, that of the Pleistocene and Pliocene was also a time of great mountain-building, when the Alps, the Sierra Nevadas, and the Himalayas received a marked uplift giving them their present altitude.
It thus appears that in geologic history the greatest known climatic changes have been closely associated with remarkable telluric changes. It appears that at present climatic and telluric changes on a small scale are coincident with or follow closely upon changes in the sun. The question at once arises whether there may not have been a similar coincidence in the past. No attempt can be made to answer the question as yet, but it opens a most fascinating field of speculation and of investigation. If the activities of the earth and of the sun are related to one another in any such manner as is suggested above, the study of the one will add vastly to our knowledge of the other. An examination of solar changes, on the one hand, may enable us to foretell something of what is about to occur upon the earth. A careful reading of the geological history of the earth, on the other hand, may disclose the history of the sun for millions of years past, and may shed light upon the fascinating problem of the thermal history and ultimate destiny of the body which, as knowledge increases, appears more and more to be the arbiter of terrestrial life.