[Footnote 10: James Croll: Climate and Time, 1876.]
[Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis; Jour. Geol., Vol. VII, 1899, pp. 545-584, 667-685, 757-787.
T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp.
93-106, 655-677, and Vol. III, pp. 432-446.
S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some investigations on carbon dioxide which have had a p.r.o.nounced effect on later conclusions.
F. Frech adopted Arrhenius" idea and developed it in a paper ent.i.tled Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.
The exact origin of the carbon dioxide theory has been stated so variously that it seems worth while to give the exact facts. Prompted by the suggestion, of Tyndall that glaciation might be due to depletion of atmospheric carbon dioxide, Chamberlin worked up the essentials of his early views before he saw any publication from Arrhenius, to whom the idea has often been attributed. In 1895 or earlier Chamberlin began to give the carbon dioxide hypothesis to his students and to discuss it before local scientific bodies. In 1897 he prepared a paper on "A Group of Hypotheses Bearing on Climatic Changes," Jour. Geol., Vol. V (1897), to be read at the meeting of the British a.s.sociation at Toronto, basing his conclusions on Tyndall"s determination of the competency of carbon dioxide as an absorber of heat radiated from the earth. He had essentially completed this when a paper by Arrhenius, "On the influence of carbonic acid in the air upon the temperature of the ground," Phil.
Mag., 1896, pp. 237-276, first came to his attention. Chamberlin then changed his conservative, tentative statement of the functions of carbon dioxide to a more sweeping one based on Arrhenius" very definite quant.i.tative deductions from Langley"s experiments. Both Langley and Arrhenius were then in the ascendancy of their reputations and seemingly higher authorities could scarcely have been chosen, nor a finer combination than experiment and physico-mathematical development.
Arrhenius" deductions were later proved to have been overstrained, while Langley"s interpretation and even his observations were challenged.
Chamberlin"s latest views are more like his earlier and more conservative statement.]
[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate; Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.
W. J. Humphreys: Volcanic dust and other factors in the production of climatic and their possible relation to ice ages; Bull. Mount Weather Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air, 1920.]
[Footnote 13: H. Arctowski: The Pleonian Cycle of Climatic Fluctuations; Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New York Academy of Sciences, Vol. 24, 1914.]
[Footnote 14: W. Koppen: uber mehrjahrige Perioden der Witterung ins besondere uzer die II-jahrige Periode der Temperatur. Also, Lufttemperaturen Sonnenflecke und Vulcanausbruche; Meteorologische Zeitschrift, Vol. 7, 1914, pp. 305-328.]
CHAPTER IV
THE SOLAR CYCLONIC HYPOTHESIS
The progress of science is made up of a vast succession of hypotheses.
The majority die in early infancy. A few live and are for a time widely accepted. Then some new hypothesis either destroys them completely or shows that, while they contain elements of truth, they are not the whole truth. In the previous chapter we have discussed a group of hypotheses of this kind, and have tried to point out fairly their degree of truth so far as it can yet be determined. In this chapter we shall outline still another hypothesis, the relation of which to present climatic conditions has been fully developed in _Earth and Sun_; while its relation to the past will be explained in the present volume. This hypothesis is not supposed to supersede the others, for so far as they are true they cannot be superseded. It merely seems to explain some of the many conditions which the other hypotheses apparently fail to explain. To suppose that it will suffer a fate more glorious than its predecessors would be presumptuous. The best that can be hoped is that after it has been pruned, enriched, and modified, it may take its place among the steps which finally lead to the goal of truth.
In this chapter the new hypothesis will be sketched in broad outline in order that in the rest of this book the reader may appreciate the bearing of all that is said. Details of proof and methods of work will be omitted, since they are given in _Earth and Sun_. For the sake of brevity and clearness the main conclusions will be stated without the qualifications and exceptions which are fully explained in that volume.
Here it will be necessary to pa.s.s quickly over points which depart radically from accepted ideas, and which therefore must arouse serious question in the minds of thoughtful readers. That, however, is a necessary consequence of the attempt which this book makes to put the problem of climate in such form that the argument can be followed by thoughtful students in any branch of knowledge and not merely by specialists. Therefore, the specialist can merely be asked to withhold judgment until he has read all the evidence as given in _Earth and Sun_, and then to condemn only those parts that are wrong and not the whole argument.
Without further explanation let us turn to our main problem. In the realm of climatology the most important discovery of the last generation is that variations in the weather depend on variations in the activity of the sun"s atmosphere. The work of the great astronomer, Newcomb, and that of the great climatologist, Koppen, have shown beyond question that the temperature of the earth"s surface varies in harmony with variations in the number and area of sunspots.[15] The work of Abbot has shown that the amount of heat radiated from the sun also varies, and that in general the variations correspond with those of the sunspots, although there are exceptions, especially when the spots are fewest. Here, however, there at once arises a puzzling paradox. The earth certainly owes its warmth to the sun. Yet when the sun emits the most energy, that is, when sunspots are most numerous, the earth"s surface is coolest.
Doubtless the earth receives more heat than usual at such times, and the upper air may be warmer than usual. Here we refer only to the air at the earth"s surface.
Another large group of investigators have shown that atmospheric pressure also varies in harmony with the number of sunspots. Some parts of the earth"s surface have one kind of variation at times of many sunspots and other parts the reverse. These differences are systematic and depend largely on whether the region in question happens to have high atmospheric pressure or low. The net result is that when sunspots are numerous the earth"s storminess increases, and the atmosphere is thrown into commotion. This interferes with the stable planetary winds, such as the trades of low lat.i.tudes and the prevailing westerlies of higher lat.i.tudes. Instead of these regular winds and the fair weather which they bring, there is a tendency toward frequent tropical hurricanes in the lower lat.i.tudes and toward more frequent and severe storms of the ordinary type in the lat.i.tudes where the world"s most progressive nations now live. With the change in storminess there naturally goes a change in rainfall. Not all parts of the world, however, have increased storminess and more abundant rainfall when sunspots are numerous. Some parts change in the opposite way. Thus when the sun"s atmosphere is particularly disturbed, the contrasts between different parts of the earth"s surface are increased. For example, the northern United States and southern Canada become more stormy and rainy, as appears in Fig. 2, and the same is true of the Southwest and along the south Atlantic coast. In a crescent-shaped central area, however, extending from Wyoming through Missouri to Nova Scotia, the number of storms and the amount of rainfall decrease.
[Ill.u.s.tration: _Fig. 2. Storminess at sunspot maxima vs. minima._ (_After Kullmer._)
Based on nine years" nearest sunspot minima and nine years" nearest sunspot maxima in the three sunspot cycles from 1888 to 1918. Heavy shading indicates excess of storminess when sunspots are numerous.
Figures indicate average yearly number of storms by which years of maximum sunspots exceed those of minimum sunspots.]
The two controlling factors of any climate are the temperature and the atmospheric pressure, for they determine the winds, the storms, and thus the rainfall. A study of the temperature seems to show that the peculiar paradox of a hot sun and a cool earth is due largely to the increased storminess during times of many sunspots. The earth"s surface is heated by the rays of the sun, but most of the rays do not in themselves heat the air as they pa.s.s through it. The air gets its heat largely from the heat absorbed by the water vapor which is intimately mingled with its lower portions, or from the long heat waves sent out by the earth after it has been warmed by the sun. The faster the air moves along the earth"s surface the less it becomes heated, and the more heat it takes away. This sounds like a contradiction, but not to anyone who has tried to heat a stove in the open air. If the air is still, the stove rapidly becomes warm and so does the air around it. If the wind is blowing, the cool air delays the heating of the stove and prevents the surface from ever becoming as hot as it would otherwise. That seems to be what happens on a large scale when sunspots are numerous. The sun actually sends to the earth more energy than usual, but the air moves with such unusual rapidity that it actually cools the earth"s surface a trifle by carrying the extra heat to high levels where it is lost into s.p.a.ce.
There has been much discussion as to why storms are numerous when the sun"s atmosphere is disturbed. Many investigators have supposed it was due entirely and directly to the heating of the earth"s surface by the sun. This, however, needs modification for several reasons. In the first place, recent investigations show that in a great many cases changes in barometric pressure precede changes in temperature and apparently cause them by altering the winds and producing storms. This is the opposite of what would happen if the effect of solar heat upon the earth"s surface were the only agency. In the second place, if storms were due exclusively to variations in the ordinary solar radiation which comes to the earth as light and is converted into heat, the solar effect ought to be most p.r.o.nounced when the center of the sun"s visible disk is most disturbed. As a matter of fact the storminess is notably greatest when the edges of the solar disk are most disturbed. These facts and others lead to the conclusion that some agency other than heat must also play some part in producing storminess.
The search for this auxiliary agency raises many difficult questions which cannot yet be answered. On the whole the weight of evidence suggests that electrical phenomena of some kind are involved, although variations in the amount of ultra-violet light may also be important.
Many investigators have shown that the sun emits electrons. Hale has proved that the sun, like the earth, is magnetized. Sunspots also have magnetic fields the strength of which is often fifty times as great as that of the sun as a whole. If electrons are sent to the earth, they must move in curved paths, for they are deflected by the sun"s magnetic field and again by the earth"s magnetic field. The solar deflection may cause their effects to be greatest when the spots are near the sun"s margin; the terrestrial deflection may cause concentration in bands roughly concentric with the magnetic poles of the earth. These conditions correspond with the known facts.
Farther than this we cannot yet go. The calculations of Humphreys seem to indicate that the direct electrical effect of the sun"s electrons upon atmospheric pressure is too small to be of appreciable significance in intensifying storms. On the other hand the peculiar way in which activity upon the margins of the sun appears to be correlated not only with atmospheric electricity, but with barometric pressure, seems to be equally strong evidence in the other direction. Possibly the sun"s electrons and its electrical waves produce indirect effects by being converted into heat, or by causing the formation of ozone and the condensation of water vapor in the upper air. Any one of these processes would raise the temperature of the upper air, for the ozone and the water vapor would be formed there and would tend to act as a blanket to hold in the earth"s heat. But any such change in the temperature of the upper air would influence the lower air through changes in barometric pressure. These considerations are given here because the thoughtful reader is likely to inquire how solar activity can influence storminess.
Moreover, at the end of this book we shall take up certain speculative questions in which an electrical hypothesis will be employed. For the main portions of this book it makes no difference how the sun"s variations influence the earth"s atmosphere. The only essential point is that when the solar atmosphere is active the storminess of the earth increases, and that is a matter of direct observation.
Let us now inquire into the relation between the small cyclonic vacillations of the weather and the types of climatic changes known as historic pulsations and glacial fluctuations. One of the most interesting results of recent investigations is the evidence that sunspot cycles on a small scale present almost the same phenomena as do historic pulsations and glacial fluctuations. For instance, when sunspots are numerous, storminess increases markedly in a belt near the northern border of the area of greatest storminess, that is, in southern Canada and thence across the Atlantic to the North Sea and Scandinavia.
(See Figs. 2 and 3.) Corresponding with this is the fact that the evidence as to climatic pulsations in historic times indicates that regions along this path, for instance Greenland, the North Sea region, and southern Scandinavia, were visited by especially frequent and severe storms at the climax of each pulsation. Moreover, the greatest acc.u.mulations of ice in the glacial period were on the poleward border of the general regions where now the storms appear to increase most at times of solar activity.
[Ill.u.s.tration: _Fig. 3A. Relative rainfall at times of increasing and decreasing sunspots._
Heavy shading, more rain with increasing spots. Light shading, more rain with decreasing spots. No data for unshaded areas.
Figures indicate percentages of the average rainfall by which the rainfall during periods of increasing spots exceeds or falls short of rainfall during periods of decreasing spots. The excess or deficiency is stated in percentages of the average. Rainfall data from Walker: Sunspots and Rainfall.]
[Ill.u.s.tration: _Fig. 3B. Relative rainfall at times of increasing and decreasing sunspots._
Heavy shading, more rain with increasing spots. Light shading, more rain with decreasing spots. No data for unshaded areas. Figures indicate percentages of the average rainfall by which the rainfall during periods of increasing spots exceeds or falls short of rainfall during periods of decreasing spots. The excess or deficiency is stated in percentages of the average. Rainfall data from Walker: Sunspots and Rainfall.]
Even more clear is the evidence from other regions where storms increase at times of many sunspots. One such region includes the southwestern United States, while another is the Mediterranean region and the semi-arid or desert parts of Asia farther east. In these regions innumerable ruins and other lines of evidence show that at the climax of each climatic pulsation there was more storminess and rainfall than at present, just as there now is when the sun is most active. In still earlier times, while ice was acc.u.mulating farther north, the basins of these semi-arid regions were filled with lakes whose strands still remain to tell the tale of much-increased rainfall and presumable storminess. If we go back still further in geological times to the Permian glaciation, the areas where ice acc.u.mulated most abundantly appear to be the regions where tropical hurricanes produce the greatest rainfall and the greatest lowering of temperature at times of many sunspots. From these and many other lines of evidence it seems probable that historic pulsations and glacial fluctuations are nothing more than sunspot cycles on a large scale. It is one of the fundamental rules of science to reason from the known to the unknown, from the near to the far, from the present to the past. Hence it seems advisable to investigate whether any of the climatic phenomena of the past may have arisen from an intensification of the solar conditions which now appear to give rise to similar phenomena on a small scale.
The rest of this chapter will be devoted to a _resume_ of certain tentative conclusions which have no bearing on the main part of this book, but which apply to the closing chapters. There we shall inquire into the periodicity of the climatic phenomena of geological times, and shall ask whether there is any reason to suppose that the sun"s activity has exhibited similar periodicity. This leads to an investigation of the possible causes of disturbances in the sun"s atmosphere. It is generally a.s.sumed that sunspots, solar prominences, the bright clouds known as faculae, and other phenomena denoting a perturbed state of the solar atmosphere, are due to some cause within the sun. Yet the limitation of these phenomena, especially the sunspots, to restricted lat.i.tudes, as has been shown in _Earth and Sun_, does not seem to be in harmony with an internal solar origin, even though a banded arrangement may be normal for a rotating globe. The fairly regular periodicity of the sunspots seems equally out of harmony with an internal origin. Again, the solar atmosphere has two kinds of circulation, one the so-called "rice grains," and the other the spots and their attendant phenomena. Now the rice grains present the appearance that would be expected in an atmospheric circulation arising from the loss of heat by the outer part of a gaseous body like the sun. For these reasons and others numerous good thinkers from Wolf to Schuster have held that sunspots owe their periodicity to causes outside the sun. The only possible cause seems to be the planets, acting either through gravitation, through forces of an electrical origin, or through some other agency. Various new investigations which are described in _Earth and Sun_ support this conclusion. The chief difficulty in accepting it hitherto has been that although Jupiter, because of its size, would be expected to dominate the sunspot cycle, its period of 11.86 years has not been detected. The sunspot cycle has appeared to average 11.2 years in length, and has been called the 11-year cycle. Nevertheless, a new a.n.a.lysis of the sunspot data shows that when attention is concentrated upon the major maxima, which are least subject to r.e.t.a.r.dation or acceleration by other causes, a periodicity closely approaching that of Jupiter is evident. Moreover, when the effects of Jupiter, Saturn, and the other planets are combined, they produce a highly variable curve which has an extraordinary resemblance to the sunspot curve. The method by which the planets influence the sun"s atmosphere is still open to question. It may be through tides, through the direct effect of gravitation, through electro-magnetic forces, or in some other way. Whichever it may be, the result may perhaps be slight differences of atmospheric pressure upon the sun. Such differences may set in motion slight whirling movements a.n.a.logous to terrestrial storms, and these presumably gather momentum from the sun"s own energy. Since the planetary influences vary in strength because of the continuous change in the relative distances and positions of the planets, the sun"s atmosphere appears to be swayed by cyclonic disturbances of varying degrees of severity. The cyclonic disturbances known as sunspots have been proved by Hale to become more highly electrified as they increase in intensity. At the same time hot gases presumably well up from the lower parts of the solar atmosphere and thereby cause the sun to emit more heat. Thus by one means or another, the earth"s atmosphere appears to be set in commotion and cycles of climate are inaugurated.
If the preceding reasoning is correct, any disturbance of the solar atmosphere must have an effect upon the earth"s climate. If the disturbance were great enough and of the right nature it might produce a glacial epoch. The planets are by no means the only bodies which act upon the sun, for that body sustains a constantly changing relation to millions of other celestial bodies of all sizes up to vast universes, and at all sorts of distances. If the sun and another star should approach near enough to one another, it is certain that the solar atmosphere would be disturbed much more than at present.
Here we must leave the cyclonic hypothesis of climate and must refer the reader once more to _Earth and Sun_ for fuller details. In the rest of this book we shall discuss the nature of the climatic changes of past times and shall inquire into their relation to the various climatic hypotheses mentioned in the last two chapters. Then we shall inquire into the possibility that the solar system has ever been near enough to any of the stars to cause appreciable disturbances of the solar atmosphere. We shall complete our study by investigating the vexed question of why movements of the earth"s crust, such as the uplifting of continents and mountain chains, have generally occurred at the same time as great climatic fluctuations. This would not be so surprising were it not that the climatic phenomena appear to have consisted of highly complex cycles while the uplift has been a relatively steady movement in one direction. We shall find some evidence that the solar disturbances which seem to cause climatic changes also have a relation to movements of the crust.
FOOTNOTES:
[Footnote 15: The so-called sunspot numbers to which reference is made again and again in this book are based on a system devised by Wolf and revised by A. Wolfer. The number and size of the spots are both taken into account. The numbers from 1749 to 1900 may be found in the Monthly Weather Review for April, 1902, and from 1901 to 1918 in the same journal for 1920.]
CHAPTER V
THE CLIMATE OF HISTORY[16]
We are now prepared to consider the climate of the past. The first period to claim attention is the few thousand years covered by written history. Strangely enough, the conditions during this time are known with less accuracy than are those of geological periods hundreds of times more remote. Yet if p.r.o.nounced changes have occurred since the days of the ancient Babylonians and since the last of the post-glacial stages, they are of great importance not only because of their possible historic effects, but because they bridge the gap between the little variations of climate which are observable during a single lifetime and the great changes known as glacial epochs. Only by bridging the gap can we determine whether there is any genetic relation between the great changes and the small. A full discussion of the climate of historic times is not here advisable, for it has been considered in detail in numerous other publications.[17] Our most profitable course would seem to be to consider first the general trend of opinion and then to take up the chief objections to each of the main hypotheses.
In the hot debate over this problem during recent decades the ideas of geographers seem to have gone through much the same metamorphosis as have those of geologists in regard to the climate of far earlier times.
As every geologist well knows, at the dawn of geology people believed in climatic uniformity--that is, it was supposed that since the completion of an original creative act there had been no important changes. This view quickly disappeared and was superseded by the hypothesis of progressive cooling and drying, an hypothesis which had much to do with the development of the nebular hypothesis, and which has in turn been greatly strengthened by that hypothesis. The discovery of evidence of widespread continental glaciation, however, necessitated a modification of this view, and succeeding years have brought to light a constantly increasing number of glacial, or at least cool, periods distributed throughout almost the whole of geological time. Moreover, each year, almost, brings new evidence of the great complexity of glacial periods, epochs, and stages. Thus, for many decades, geologists have more and more been led to believe that in spite of surprising uniformity, when viewed in comparison with the cosmic possibilities, the climate of the past has been highly unstable from the viewpoint of organic evolution, and its changes have been of all degrees of intensity.
Geographers have lately been debating the reality of historic changes of climate in the same way in which geologists debated the reality of glacial epochs and stages. Several hypotheses present themselves but these may all be grouped under three headings; namely, the hypotheses of (1) progressive desiccation, (2) climatic uniformity, and (3) pulsations. The hypothesis of progressive desiccation has been widely advocated. In many of the drier portions of the world, especially between 30 and 40 from the equator, and preeminently in western and central Asia and in the southwestern United States, almost innumerable facts seem to indicate that two or three thousand years ago the climate was distinctly moister than at present. The evidence includes old lake strands, the traces of desiccated springs, roads in places now too dry for caravans, other roads which make detours around dry lake beds where no lakes now exist, and fragments of dead forests extending over hundreds of square miles where trees cannot now grow for lack of water.
Still stronger evidence is furnished by ancient ruins, hundreds of which are located in places which are now so dry that only the merest fraction of the former inhabitants could find water. The ruins of Palmyra, in the Syrian Desert, show that it must once have been a city like modern Damascus, with one or two hundred thousand inhabitants, but its water supply now suffices for only one or two thousand. All attempts to increase the water supply have had only a slight effect and the water is notoriously sulphurous, whereas in the former days, when it was abundant, it was renowned for its excellence. Hundreds of pages might be devoted to describing similar ruins. Some of them are even more remarkable for their dryness than is Niya, a site in the Tarim Desert of Chinese Turkestan. Yet there the evidence of desiccation within 2000 years is so strong that even so careful and conservative a man as Hann,[18] p.r.o.nounces it "uberzeugend."