We come now to Bruckner periods and sunspot cycles. The Bruckner 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 _Klimaschw.a.n.kungen seit 1700_, Bruckner has collected an uncommonly interesting a.s.sortment 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, Dougla.s.s, 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.
Pa.s.sing 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 ill.u.s.trated 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, j.a.pan, 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 a.s.sumed 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 until after the third group of sequences has been discussed. This group includes glacial fluctuations, historic pulsations, Bruckner periods, sunspot cycles, pleionian migrations, and cyclonic vacillations. The outstanding fact in regard to all of these is that while they are greatly modified by purely terrestrial conditions, they seem to owe their origin to variations in the sun. They form the chief subject of _Earth and Sun_ and in their larger phases are the most important topic of this book also. The last group of sequences includes...o...b..tal precessions, seasonal alternations, and daily variations. These may be regarded as purely solar in origin. Yet their influence, like that of each of the other groups, is much modified by the earth"s own conditions. Our main problem is to separate and explain the two great elements in climatic changes,--the effects of the sun, on the one hand, and of the earth on the other.
CHAPTER III
HYPOTHESES OF CLIMATIC CHANGE
The next step in our study of climate is to review the main hypotheses as to the causes of glaciation. These hypotheses apply also to other types of climatic changes. We shall concentrate on glacial periods, however, not only because they are the most dramatic and well-known types of change, but because they have been more discussed than any other and have also had great influence on evolution. Moreover, they stand near the middle of the types of climatic sequences, and an understanding of them does much to explain the others. In reviewing the various theories we shall not attempt to cover all the ground, but shall merely state the main ideas of the few theories which have had an important influence upon scientific thought.
The conditions which any satisfactory climatic hypothesis must satisfy are briefly as follows:
(1) Due weight must be given to the fact that changes of climate are almost certainly due to the combined effect of a variety of causes, both terrestrial and solar or cosmic.
(2) Attention must also be paid to both sides in the long controversy as to whether glaciation is due primarily to a diminution in the earth"s supply of heat or to a _redistribution_ of the heat through changes in atmospheric and oceanic circulation. At present the great majority of authorities are on the side of a diminution of heat, but the other view also deserves study.
(3) A satisfactory hypothesis must explain the frequent synchronism between two great types of phenomena; first, movements of the earth"s crust whereby continents are uplifted and mountains upheaved; and, second, great changes of climate which are usually marked by relatively rapid oscillations from one extreme to another.
(4) No hypothesis can find acceptance unless it satisfies the somewhat exacting requirements of the geological record, with its frequent but irregular repet.i.tion of long, mild periods, relatively cool or intermediate periods like the present, and glacial periods of more or less severity and perhaps accompanying the more or less widespread uplifting of continents. At least during the later glacial periods the hypothesis must explain numerous climatic epochs and stages superposed upon a single general period of continental upheaval. Moreover, although historical geology demands cycles of varied duration and magnitude, it does not furnish evidence of any rigid periodicity causing the cycles to be uniform in length or intensity.
(5) Most important of all, a satisfactory explanation of climatic changes and crustal deformation must take account of all the agencies which are now causing similar phenomena. Whether any other agencies should be considered is open to question, although the relative importance of existing agencies may have varied.
I. _Croll"s Eccentricity Theory._ One of the most ingenious and most carefully elaborated scientific hypotheses is Croll"s[10] precessional hypothesis as to the effect of the earth"s own motions. So well was this worked out that it was widely accepted for a time and still finds a place in popular but unscientific books, such as Wells" _Outline of History_, and even in scientific works like Wright"s _Quaternary Ice Age_. The gist of the hypothesis has already been given in connection with the type of climatic sequence known as...o...b..tal precessions. The earth is 93 million miles away from the sun in January and 97 million in July. The earth"s axis "precesses," however, just as does that of a spinning top. Hence arises what is known as the precession of the equinoxes, that is, a steady change in the season at which the earth is in perihelion, or nearest to the sun. In the course of 21,000 years the time of perihelion varies from early in January through the entire twelve months and back to January. Moreover, the earth"s...o...b..t is slightly more elliptical at certain periods than at others, for the planets sometimes become bunched so that they all pull the earth in one direction. Hence, once in about one hundred thousand years the effect of the elliptical shape of the earth"s...o...b..t is at a maximum.
Croll argued that these astronomical changes must alter the earth"s climate, especially by their effect on winds and ocean currents. His elaborate argument contains a vast amount of valuable material. Later investigation, however, seems to have proven the inadequacy of his hypothesis. In the first place, the supposed cause does not seem nearly sufficient to produce the observed results. Second, Croll"s hypothesis demands that glaciation in the northern and southern hemisphere take place alternately. A constantly growing collection of facts, however, indicates that glaciation does not occur in the two hemispheres alternately, but at the same time. Third, the hypothesis calls for the constant and frequent repet.i.tion of glaciation at absolutely regular intervals. The geological record shows no such regularity, for sometimes several glacial epochs follow in relatively close succession at irregular intervals of perhaps fifty to two hundred thousand years, and thus form a glacial period; and then for millions of years there are none. Fourth, the eccentricity hypothesis provides no adequate explanation for the glacial stages or subepochs, the historic pulsations, and the other smaller climatic variations which are superposed upon glacial epochs and upon one another in bewildering confusion. In spite of these objections, there can be little question that the eccentricity of the earth"s...o...b..t and the precession of the equinoxes with the resulting change in the season of perihelion must have some climatic effect. Hence Croll"s theory deserves a permanent though minor place in any full discussion of the causes of climatic changes.
II. _The Carbon Dioxide Theory._ At about the time that the eccentricity theory was being relegated to a minor niche, a new theory was being developed which soon exerted a profound influence upon geological thought. Chamberlin,[11] adopting an idea suggested by Tyndall, fired the imagination of geologists by his skillful exposition of the part played by carbon dioxide in causing climatic changes. Today this theory is probably more widely accepted than any other. We have already seen that the amount of carbon dioxide gas in the atmosphere has a decided climatic importance. Moreover, there can be little doubt that the amount of that gas in the atmosphere varies from age to age in response to the extent to which it is set free by volcanoes, consumed by plants, combined with rocks in the process of weathering, dissolved in the ocean or locked up in the form of coal and limestone. The main question is whether such variations can produce changes so rapid as glacial epochs and historical pulsations.
Abundant evidence seems to show that the degree to which the air can be warmed by carbon dioxide is sharply limited. Humphreys, in his excellent book on the _Physics of the Air_, calculates that a layer of carbon dioxide forty centimeters thick has practically as much blanketing effect as a layer indefinitely thicker. In other words, forty centimeters of carbon dioxide, while having no appreciable effect on sunlight coming toward the earth, would filter out and thus retain in the atmosphere all the outgoing terrestrial heat that carbon dioxide is capable of absorbing. Adding more would be like adding another filter when the one in operation has already done all that that particular kind of filter is capable of doing. According to Humphreys" calculations, a doubling of the carbon dioxide in the air would in itself raise the average temperature about 1.3C. and further carbon dioxide would have practically no effect. Reducing the present supply by half would reduce the temperature by essentially the same amount.
The effect must be greater, however, than would appear from the figures given above, for any change in temperature has an effect on the amount of water vapor, which in turn causes further changes of temperature.
Moreover, as Chamberlin points out, it is not clear whether Humphreys allows for the fact that when the 40 centimeters of CO_{2} nearest the earth has been heated by terrestrial radiation, it in turn radiates half its heat outward and half inward. The outward half is all absorbed in the next layer of carbon dioxide, and so on. The process is much more complex than this, but the end result is that even the last increment of CO_{2}, that is, the outermost portions in the upper atmosphere, must apparently absorb an infinitesimally small amount of heat. This fact, plus the effect of water vapor, would seem to indicate that a doubling or halving of the amount of CO_{2}, would have an effect of more than 1.3C. A change of even 2C. above or below the present level of the earth"s mean temperature would be of very appreciable climatic significance, for it is commonly believed that during the height of the glacial period the mean temperature was only 5 to 8C. lower than now.
Nevertheless, variations in atmospheric carbon dioxide do not necessarily seem competent to produce the relatively rapid climatic fluctuations of glacial epochs and historic pulsations as distinguished from the longer swings of glacial periods and geological eras. In Chamberlin"s view, as in ours, the elevation of the land, the modification of the currents of the air and of the ocean, and all that goes with elevation as a topographic agency const.i.tute a primary cause of climatic changes. A special effect of this is the removal of carbon dioxide from the air by the enhanced processes of weathering. This, as he carefully states, is a very slow process, and cannot of itself lead to anything so sudden as the oncoming of glaciation. But here comes Chamberlin"s most distinctive contribution to the subject, namely, the hypothesis that changes in atmospheric temperature arising from variations in atmospheric carbon dioxide are able to cause a reversal of the deep-sea oceanic circulation.
According to Chamberlin"s view, the ordinary oceanic circulation of the greater part of geological time was the reverse of the present circulation. Warm water descended to the ocean depths in low lat.i.tudes, kept its heat while creeping slowly poleward, and rose in high lat.i.tudes producing the warm climate which enabled corals, for example, to grow in high lat.i.tudes. Chamberlin holds this opinion largely because there seems to him to be no other reasonable way to account for the enormously long warm periods when heat-loving forms of life lived in what are now polar regions of ice and snow. He explains this reversed circulation by supposing that an abundance of atmospheric carbon dioxide, together with a broad distribution of the oceans, made the atmosphere so warm that the evaporation in low lat.i.tudes was far more rapid than now. Hence the surface water of the ocean became a relatively concentrated brine. Such a brine is heavy and tends to sink, thereby setting up an oceanic circulation the reverse of that which now prevails. At present the polar waters sink because they are cold and hence contract. Moreover, when they freeze a certain amount of salt leaves the ice and thereby increases the salinity of the surrounding water. Thus the polar water sinks to the depths of the ocean, its place is taken by warmer and lighter water from low lat.i.tudes which moves poleward along the surface, and at the same time the cold water of the ocean depths is forced equatorward below the surface. But if the equatorial waters were so concentrated that a steady supply of highly saline water kept descending to low levels, the direction of the circulation would have to be reversed. The time when this would occur would depend upon the delicate balance between the downward tendencies of the cold polar water and of the warm saline equatorial water.
Suppose that while such a reversed circulation prevailed, the atmospheric CO_{2} should be depleted, and the air cooled so much that the concentration of the equatorial waters by evaporation was no longer sufficient to cause them to sink. A reversal would take place, the present type of circulation would be inaugurated, and the whole earth would suffer a chill because the surface of the ocean would become cool.
The cool surface-water would absorb carbon dioxide faster than the previous warm water had done, for heat drives off gases from water. This would hasten the cooling of the atmosphere still more, not only directly but by diminishing the supply of atmospheric moisture. The result would be glaciation. But ultimately the cold waters of the higher lat.i.tudes would absorb all the carbon dioxide they could hold, the slow equatorward creep would at length permit the cold water to rise to the surface in low lat.i.tudes. There the warmth of the equatorial sun and the depleted supply of carbon dioxide in the air would combine to cause the water to give up its carbon dioxide once more. If the atmosphere had been sufficiently depleted by that time, the rising waters in low lat.i.tudes might give up more carbon dioxide than the cold polar waters absorbed. Thus the atmospheric supply would increase, the air would again grow warm, and a tendency toward deglaciation, or toward an inter-glacial condition would arise. At such times the oceanic circulation is not supposed to have been reversed, but merely to have been checked and made slower by the increasing warmth. Thus inter-glacial conditions like those of today, or even considerably warmer, are supposed to have been produced with the present type of circulation.
The emission of carbon dioxide in low lat.i.tudes could not permanently exceed the absorption in high lat.i.tudes. After the present type of circulation was finally established, which might take tens of thousands of years, the two would gradually become equal. Then the conditions which originally caused the oceanic circulation to be reversed would again destroy the balance; the atmospheric carbon dioxide would be depleted; the air would grow cooler; and the cycle of glaciation would be repeated. Each cycle would be shorter than the last, for not only would the swings diminish like those of a pendulum, but the agencies that were causing the main depletion of the atmospheric carbon dioxide would diminish in intensity. Finally as the lands became lower through erosion and submergence, and as the processes of weathering became correspondingly slow, the air would gradually be able to acc.u.mulate carbon dioxide; the temperature would increase; and at length the oceanic circulation would be reversed again. When the warm saline waters of low lat.i.tudes finally began to sink and to set up a flow of warm water poleward in the depths of the ocean, a glacial period would definitely come to an end.
This hypothesis has been so skillfully elaborated, and contains so many important elements that one can scarcely study it without profound admiration. We believe that it is of the utmost value as a step toward the truth, and especially because it emphasizes the great function of oceanic circulation. Nevertheless, we are unable to accept it in full for several reasons, which may here be stated very briefly. Most of them will be discussed fully in later pages.
(1) While a reversal of the deep-sea circulation would undoubtedly be of great climatic importance and would produce a warm climate in high lat.i.tudes, we see no direct evidence of such a reversal. It is equally true that there is no conclusive evidence against it, and the possibility of a reversal must not be overlooked. There seem, however, to be other modifications of atmospheric and oceanic circulation which are able to produce the observed results.
(2) There is much, and we believe conclusive, evidence that a mere lowering of temperature would not produce glaciation. What seems to be needed is changes in atmospheric circulation and in precipitation. The carbon dioxide hypothesis has not been nearly so fully developed on the meteorological side as in other respects.
(3) The carbon dioxide hypothesis seems to demand that the oceans should have been almost as saline as now in the Proterozoic era at the time of the first known glaciation. Chamberlin holds that such was the case, but the constant supply of saline material brought to the ocean by rivers and the relatively small deposition of such material on the sea floor seem to indicate that the early oceans must have been much fresher than those of today.
(4) The carbon dioxide hypothesis does not attempt to explain minor climatic fluctuations such as post-glacial stages and historic pulsations, but these appear to be of the same nature as glacial epochs, differing only in degree.
(5) Another reason for hesitation in accepting the carbon dioxide hypothesis as a full explanation of glacial fluctuations is the highly complex and non-observational character of the explanation of the alternation of glacial and inter-glacial epochs and of their constantly decreasing length.
(6) Most important of all, a study of the variations of weather and of climate as they are disclosed by present records and by the historic past suggests that there are now in action certain other causes which are competent to explain glaciation without recourse to a process whose action is beyond the realm of observation.
These considerations lead to the conclusion that the carbon dioxide hypothesis and the reversal of the oceanic circulation should be regarded as a tentative rather than a final explanation of glaciation.
Nevertheless, the action of carbon dioxide seems to be an important factor in producing the longer oscillations of climate from one geological era to another. It probably plays a considerable part in preparing the way for glacial periods and in making it possible for other factors to produce the more rapid changes which have so deeply influenced organic evolution.
III. _The Form of the Land._ Another great cause of climatic change consists of a group of connected phenomena dependent upon movements of the earth"s crust. As to the climatic potency of changes in the lands there is practical agreement among students of climatology and glaciation. That the height and extent of the continents, the location, size, and orientation of mountain ranges, and the opening and closing of oceanic gateways at places like Panama, and the consequent diversion of oceanic currents, exert a profound effect upon climate can scarcely be questioned. Such changes may be introduced rapidly, but their disappearance is usually slow compared with the rapid pulsations to which climate has been subject during historic times and during stages of glacial retreat and advance, or even in comparison with the epochs into which the Pleistocene, Permian, and perhaps earlier glacial periods have been divided. Hence, while crustal movements appear to be more important than the eccentricity of the earth"s...o...b..t or the amount of carbon dioxide in the air, they do not satisfactorily explain glacial fluctuations, historic pulsations, and especially the present little cycles of climatic change. All these changes involve a relatively rapid swing from one extreme to another, while an upheaval of a continent, which is at best a slow geologic process, apparently cannot be undone for a long, long time. Hence such an upheaval, if acting alone, would lead to a relatively long-lived climate of a somewhat extreme type. It would help to explain the long swings, or geologic oscillations between a mild and uniform climate at one extreme, and a complex and varied climate at the other, but it would not explain the rapid climatic pulsations which are closely a.s.sociated with great movements of the earth"s crust. It might prepare the way for them, but could not cause them. That this conclusion is true is borne out by the fact that vast mountain ranges, like those at the close of the Jura.s.sic and Cretaceous, are upheaved without bringing on glacial climates. Moreover, the marked Permian ice age follows long after the birth of the Hercynian Mountains and before the rise of others of later Permian origin.
IV. _The Volcanic Hypothesis._ In the search for some cause of climatic change which is highly efficient and yet able to vary rapidly and independently, Abbot, Fowle, Humphreys, and others,[12] have concluded that volcanic eruptions are the missing agency. In _Physics of the Air_, Humphreys gives a careful study of the effect of volcanic dust upon terrestrial temperature. He begins with a mathematical investigation of the size of dust particles, and their quant.i.ty after certain eruptions.
He demonstrates that the power of such particles to deflect light of short wave-lengths coming from the sun is perhaps thirty times more than their power to retain the heat radiated in long waves from the earth.
Hence it is estimated that if a Krakatoa were to belch forth dust every year or two, the dust veil might cause a reduction of about 6C. in the earth"s surface temperature. As in every such complicated problem, some of the author"s a.s.sumptions are open to question, but this touches their quant.i.tative and not their qualitative value. It seems certain that if volcanic explosions were frequent enough and violent enough, the temperature of the earth"s surface would be considerably lowered.
Actual observation supports this theoretical conclusion. Humphreys gathers together and amplifies all that he and Abbot and Fowle have previously said as to observations of the sun"s thermal radiation by means of the pyrheliometer. This summing up of the relations between the heat received from the sun, and the occurrence of explosive volcanic eruptions leaves little room for doubt that at frequent intervals during the last century and a half a slight lowering of terrestrial temperature has actually occurred after great eruptions. Nevertheless, it does not justify Humphreys" final conclusion that "phenomena within the earth itself suffice to modify its own climate, ... that these and these alone have actually caused great changes time and again in the geologic past."
Humphreys sees so clearly the importance of the purely terrestrial point of view that he unconsciously slights the cosmic standpoint and ignores the important solar facts which he himself adduces elsewhere at considerable length.
In addition to this the _degree_ to which the temperature of the earth as a whole is influenced by volcanic eruptions is by no means so clear as is the fact that there is some influence. Arctowski,[13] for example, has prepared numerous curves showing the march of temperature month after month for many years. During the period from 1909 to 1913, which includes the great eruption of Katmai in Alaska, low temperature is found to have prevailed at the time of the eruption, but, as Arctowski puts it, on the basis of the curves for 150 stations in all parts of the world: "The supposition that these abnormally low temperatures were due to the veil of volcanic dust produced by the Katmai eruption of June 6, 1912, is completely out of the question. If that had been the case, temperature would have decreased from that date on, whereas it was decreasing for more than a year before that date."
Koppen,[14] in his comprehensive study of temperature for a hundred years, also presents a strong argument against the idea that volcanic eruptions have an important place in determining the present temperature of the earth. A volcanic eruption is a sudden occurrence. Whatever effect is produced by dust thrown into the air must occur within a few months, or as soon as the dust has had an opportunity to be wafted to the region in question. When the dust arrives, there will be a rapid drop through the few degrees of temperature which the dust is supposed to be able to account for, and thereafter a slow rise of temperature. If volcanic eruptions actually caused a frequent lowering of terrestrial temperature in the hundred years studied by Koppen, there should be more cases where the annual temperature is decidedly below the normal than where it shows a large departure in the opposite direction. The contrary is actually the case.
A still more important argument is the fact that the earth is now in an intermediate condition of climate. Throughout most of geologic time, as we shall see again and again, the climate of the earth has been milder than now. Regions like Greenland have not been the seat of glaciers, but have been the home of types of plants which now thrive in relatively low lat.i.tudes. In other words, the earth is today only part way from a glacial epoch to what may be called the normal, mild climate of the earth--a climate in which the contrast from zone to zone was much less than now, and the lower air averaged warmer. Hence it seems impossible to avoid the conclusion that the cause of glaciation is still operating with considerable although diminished efficiency. But volcanic dust is obviously not operating to any appreciable extent at present, for the upper air is almost free from dust a large part of the time.
Again, as Chamberlin suggests, let it be supposed that a Krakatoan eruption every two years would produce a glacial period. Unless the most experienced field workers on the glacial formations are quite in error, the various glacial epochs of the Pleistocene glacial period had a joint duration of at least 150,000 years and perhaps twice as much. That would require 75,000 Krakatoan eruptions. But where are the pits and cones of such eruptions? There has not been time to erode them away since the Pleistocene glaciation. Their beds of volcanic ash would presumably be as voluminous as the glacial beds, but there do not seem to be acc.u.mulations of any such size. Even though the same volcano suffered repeated explosions, it seems impossible to find sufficient fresh volcanic debris. Moreover, the volcanic hypothesis has not yet offered any mechanism for systematic glacial variations. Hence, while the hypothesis is important, we must search further for the full explanation of glacial fluctuations, historic pulsations, and the earth"s present quasi-glacial climate.
V. _The Hypothesis of Polar Wandering._ Another hypothesis, which has some adherents, especially among geologists, holds that the position of the earth"s axis has shifted repeatedly during geological times, thus causing glaciation in regions which are not now polar. Astrophysicists, however, are quite sure that no agency could radically change the relation between the earth and its axis without likewise altering the orbits of the planets to a degree that would be easily recognized.
Moreover, the distribution of the centers of glaciation both in the Permian and Pleistocene periods does not seem to conform to this hypothesis.
VI. _The Thermal Solar Hypothesis._ The only other explanations of the climatic changes of glacial and historic times which now seem to have much standing are two distinct and almost antagonistic solar hypotheses.
One is the idea that changes in the earth"s climate are due to variations in the heat emitted by the sun and hence in the temperature of the earth. The other is the entirely different idea that climatic changes arise from solar conditions which cause a _redistribution of the earth"s atmospheric pressure_ and hence produce changes in winds, ocean currents, and especially storms. This second, or "cyclonic," hypothesis is the subject of a book ent.i.tled _Earth and Sun_, which is to be published as a companion to the present volume. It will be outlined in the next chapter. The other, or thermal, hypothesis may be dismissed briefly. Unquestionably a permanent change in the amount of heat emitted by the sun would permanently alter the earth"s climate. There is absolutely no evidence, however, of any such change during geologic time. The evidence as to the earth"s cosmic uniformity and as to secular progression is all against it. Suppose that for thirty or forty thousand years the sun cooled off enough so that the earth was as cool as during a glacial epoch. As glaciation is soon succeeded by a mild climate, some agency would then be needed to raise the sun"s temperature. The impact of a shower of meteorites might accomplish this, but that would mean a very sudden heating, such as there is no evidence of in geological history. In fact, there is far more evidence of sudden cooling than of sudden heating. Moreover, it is far beyond the bounds of probability that such an impact should be repeated again and again with just such force as to bring the climate back almost to where it started and yet to allow for the slight changes which cause secular progression. Another and equally cogent objection to the thermal form of solar hypothesis is stated by Humphreys as follows: "A change of the solar constant obviously alters all surface temperatures by a roughly constant percentage. Hence a decrease of the heat from the sun would in general cause a decrease of the interzonal temperature gradients; and this in turn a less vigorous atmospheric circulation, and a less copious rain or snowfall--exactly the reverse of the condition, namely, abundant precipitation, most favorable to extensive glaciation."
This brings us to the end of the main hypotheses as to climatic changes, aside from the solar cyclonic hypothesis which will be discussed in the next chapter. It appears that variations in the position of the earth at perihelion have a real though slight influence in causing cycles with a length of about 21,000 years. Changes in the carbon dioxide of the air probably have a more important but extremely slow influence upon geologic oscillations. Variations in the size, shape, and height of the continents are constantly causing all manner of climatic complications, but do not cause rapid fluctuations and pulsations. The eruption of volcanic dust appears occasionally to lower the temperature, but its potency to explain the complex climatic changes recorded in the rocks has probably been exaggerated. Finally, although minor changes in the amount of heat given out by the sun occur constantly and have been demonstrated to have a climatic effect, there is no evidence that such changes are the main cause of the climatic phenomena which we are trying to explain. Nevertheless, in connection with other solar changes they may be of high importance.
FOOTNOTES: