Popular Science Monthly/Volume 69/November 1906/Changes of Climate
| CHANGES OF CLIMATE |
By ROBERT DeC. WARD
HARVARD UNIVERSITY
Popular Belief in Climatic Change.—Belief in a change in the climate of one's place of residence, within a few generations, and even within the memory of living men, is widespread. It is confined to no special region or people. It finds support among the most intelligent as well as among the uneducated. Here it may be the view that the climate is growing milder; there, that the winters are becoming more severe; here, that there is increasing aridity; there, that the rainfall is greater. Whenever a season attracts attention because of weather conditions which seem in any way unusual, this belief is strengthened. This popular impression has often found support in the facts of distribution, or the dates of flowering or ripening of certain cereals or fruits. It is asserted that because grapes, or corn, or olives, for example, are now no longer grown in parts of Europe where their cultivation was once an important occupation, we must conclude that the climate has changed from a favorable to an unfavorable one.
Evidences of Climatic Changes within Historical Times.—Evidence is constantly being brought forward of apparent climatic variations of greater or less amount which are now going on. Such reports, largely those of travelers or explorers in little-known regions, are usually based on fluctuations in the extent of inland lakes; on the discovery of abandoned dwelling sites, the ruins of aqueducts and irrigating canals, and the like. Thus we have accounts of a gradual desiccation which seems to have been going on over a large region in Central Asia, during historical times. In eastern Turkestan the lakes have been reported as drying up, Lake Balkash falling one meter in about fifteen years, and Lake Alakul gradually becoming a salt deposit. In his work on Turkestan, Muschketoff gives numerous examples of progressive desiccation, and Bossikoff speaks of the drying up of the lakes on the northern side of the Caucasus. The same thing is reported of lakes in the Pamir. Prince Kropotkin believes that the desiccation of Central Asia in the past drove the inhabitants out on to the lowlands, producing a migration of the lowland peoples and thus bringing on the invasions of Europe during the first centuries of our era. In his recent work on the basin of eastern Persia and Sistan, Huntington believes that, so far as it can be made out, the history of Sistan also indicates a gradual desiccation from early historical times down to the present day. His study of climatic changes in that region is one of the most thorough ever made, for the evidence of archeology, of tradition, of history and of physiography has been carefully matched and found to accord in a very striking manner. Huntington has found evidence of the abandonment of successive village sites as the inhabitants moved further upstream in search of more water. Patches of dead jungle show that vegetation once flourished where aridity now renders plant growth impossible.
In northern Africa certain ancient historical records have been taken by different writers to indicate a general decrease of rainfall during the last 3,000 or more years, the remains of cities and the ruins of irrigating works pointing to a larger population and a greater water supply formerly than at present. The presence of certain animals, now no longer found there, is implied by ancient records, and from this fact also a change of climate is inferred. In his crossing of the Sahara between Algeria and the Niger, Gautier found evidence of a former large population. A gradual desiccation of the region is, therefore, believed to have taken place, but to-day the equatorial rain belt seems to be again advancing farther north, giving an increased rainfall. Gautier divides the history here into three periods, (1) dense population, (2) aridity, and (3) the present change to a steppe character.
Farther south, several lakes have been reported as decreasing in size, e. g., Chad, Ngami and Victoria; and wells and springs as running dry. In the Lake Chad district, Chevalier reports the discovery of vegetable and animal remains which indicate an invasion of the Sudan by a Saharan climate. Neolithic relics indicate the former presence there of prosperous communities. Again, to note another instance, it is often held that a steady decrease in rainfall has taken place over Greece, Syria and other eastern Mediterranean lands, resulting in a gradual and inevitable deterioration and decay of their people. These examples might be multiplied, for reports of climatic changes of one kind or another are numerous from many parts of the globe.
What Meteorological Records show.—As concerns the popular impression regarding change of climate, it is clear at the start that no definite answer can be given on the basis of tradition, or of general impression, or even of the memory of the 'oldest inhabitant.' Human memories are very unreliable things, and there are many reasons for their being particularly untrustworthy in matters of this kind. The only answer of real value must be based on what the instrumental records of temperature, and of rain and snowfall show. Accurate instruments, properly exposed, and carefully read, do not lie; do not forget; are not prejudiced. When such instrumental records, scattered though they are, and difficult as it is to draw general conclusions from them, are carefully examined, from the time when they were first kept, which in a few cases goes back about one hundred and fifty years, there is found no evidence of any progressive change in temperature, or in the amount of rain and snow. Apparent signs of a permanent increase or decrease in one or another element have been fairly easy to explain as due to the method of exposing the thermometer or of setting up the rain gauge. Little care was formerly taken in the construction and location of meteorological instruments. They were usually in cities, and as these cities grew, the temperature of the air was somewhat affected. The rain-gauges were poorly exposed on roofs or in court-yards. The building of a fence or a wall near the thermometer, or the growth of a tree over a rain-gauge, was enough, in many cases, to explain any observed change in the mean temperature or rainfall. Even when the most accurate instrumental records are available, care must be taken to interpret them correctly. Thus, if a rainfall or snowfall record of several years at some station indicates an apparent increase or decrease in the amount of precipitation, it does not necessarily follow that this means a permanent, progressive change in climate, which is to continue indefinitely. It may simply mean that there have been a few years of somewhat more precipitation, and that a period of somewhat less precipitation is to follow.
For the United States, Schott, some twenty years ago, made a careful study of all the older records of temperature and rainfall, including snow, from Maine to California, and found nothing which led to the view of a progressive change in any one direction. There was evidence of slight variations of temperature, occurring with the same characteristics and with considerable uniformity over large areas. These variations have the characteristics of irregular waves, representing slightly warmer and slightly cooler periods, but during the fluctuations the temperature differed by only a degree or two on one side or the other of the mean. Obviously, this is too slight a range to be of any general or practical interest, and in any case, these oscillations give no evidence of a continuous change toward a warmer or a cooler climate. Schott found that these waves of higher and lower temperature followed one another at intervals of about twenty-two years on the Atlantic coast. In the interior the intervals were about seven years. The records of the closing of rivers, the Hudson, for example, to navigation, show no permanent change in the dates for the last hundred years or so.
It has been well pointed out that if a list were carefully compiled of heavy snowstorms, of droughts, of floods, of severe cold, of mild winters, of heavy rains, and of other similar meteorologic phenomena for one of the early-settled sections of the United States, beginning with the date of the first white settlements and extending down to the present day, we should have the following situation: Dividing this list into halves, each division containing an equal number of years, it would be found, speaking in general terms, that for every mild winter in the first half there would be a mild winter in the second; for every long-continued drought in the first division there would be a similar drought in the second; for every 'old-fashioned' winter in the first group there would be an 'old-fashioned' winter in the second. And so on through the list. In other words, weather and climate have not changed from the time of the landing of the earliest pilgrims on the inhospitable shores of New England down to the present day.
Why the Popular Belief in Climatic Changes is Untrustworthy.—Why is the popular belief in a change of climate so widespread and so firmly fixed, when instrumental records all go to show that this belief is erroneous? It is not easy to answer this question satisfactorily, but several possible explanations may be given. The trouble arises chiefly from the fact that we place absolute trust in our memories, and attempt to judge such subtle things as climatic changes on the basis of these memories, which are at best short, defective, and in the highest degree untrustworthy. We are likely to exaggerate past events; to remember a few exceptional seasons which, for one reason or another, made a deep impression on us, and we thus very much overrate some special event. To make use of an illustration given by another, individual severe winters which, as they occur, may be some years apart, seem, when looked back upon from a distance of several years later, to have been close together. It is much as in the case of the telegraph poles along a railroad track. When we are near the individual poles they seem fairly far apart, but when we look down the track, the poles seem to stand close together. The difference in the impressions made upon youthful and adult minds may account for part of this misconception regarding changes of climate. To a youthful mind a heavy snowstorm is a memorable thing. It makes a deep impression, which lasts long and which in later years, when snowstorms are just as heavy, seems to dwarf the recent storms in comparison with the older. The same is true regarding heavy rains, or floods, or droughts.
Changes of residence may account for some of the prevailing ideas about climate. One who was brought up as a child in the country, where snow drifts deeply and where roads are not quickly broken out, and who later removes to a city, where the temperatures are slightly higher, where the houses are warmer, and where the snow is quickly removed from the streets, naturally thinks that the winters are milder and less snowy than when he was a boy. Similarly, a change of residence from a hill to a valley, or vice versa, or from the coast to the interior, may easily give the impression of a changing climate. Even in cases where individuals have kept a record of thermometer readings during a long series of years, and are sure that the temperatures are not as low or as high as they used to be, or who are convinced that the rainfall is lighter or heavier than it was some years before, the chances are that the location of the thermometer or the exposure of the rain gauge has been changed sufficiently to account for any observed difference in the readings.
Value of Evidence concerning Changes of Climate.—The body of facts which has been adduced as evidence of progressive changes of climate within historical times is not yet sufficiently large and complete to warrant any general correlation and study of these facts as a whole, especially from the point of view of possible causation. But there are certain considerations which should be borne in mind in dealing with this evidence, certain corrections, so to speak, which should be made for possible controls other than climatic, before conclusions are reached in favor of climatic changes. In the first place, it has been noted above that changes in the distribution of certain fruits and cereals, and in the dates of the harvest, have often been accepted as undoubted evidence of changes in climate. Such a conclusion is by no means inevitable, for it can easily be shown that many changes in the districts of cultivation of various crops naturally result from the fact that grapes, or corn, or olives are in time found to be more profitably grown, or more easily prepared for market in another locality. Thus the area covered by vineyards in northern Europe has been very much restricted in the last few hundred years, because grapes can be grown better and cheaper farther south. Cultivation in one district is abandoned when it is more profitable to import the product from another. It is easy, but not right, to conclude that the climate of the districts first used has changed. Wheat was formerly more generally cultivated far north in the British Isles than is the case at present, because it paid. Later, after a readjustment of the taxes on breadstuff's, it was no longer profitable to grow cereals in that region, and the area thus cultivated diminished. Changes in the facility or in the cost of importation of certain articles of food from a distance are speedily followed by changes in the districts over which these same crops are grown. Similarly, the introduction of some new plant, better suited to the local soil and climate, will result in the replacement of the older product by the newer. In France, Angot has made a careful compilation of the dates of the vintage from the fourteenth century down to the present time, and finds no support for the view so commonly held there that the climate has changed for the worse. The dates of the vintage do, however, indicate some oscillation of the climatic elements. In the period 1775-1875, the average date of the grape harvest in Aubonne was about ten days earlier than during the preceding century, but three days later than during the second century preceding. At the present time, the average date of the grape harvest in Aubonne is exactly the same as at the close of the sixteenth century. After a careful study of the conditions of the date tree, from the fourth century, B. C., Eginitis concludes that the climate of the eastern portion of the Mediterranean basin has not changed appreciably during twenty-three centuries. In China, a comparison of the ancient and present-day conditions of cultivation, of silk production, and of bird migrations, has led Biot to a similar conclusion. In some cases, the reported cultivation of cereals, or other soil products, in certain at present unfavorable climates has been shown to be purely a myth; as in the case of a supposed extended cereal cultivation in Iceland in former times.
Secondly, a good many of the reports by explorers from little-known regions are contradictory. Thus Lake Aral, which was diminishing in area for many years, is recently reported by Berg as increasing. Lake Balkash, which was rapidly drying up, has also begun to fill again. Partly submerged trees are noted as having been seen by Berg, who in June, 1902, found the lake waters quite fresh. As the lake has no outlet, this is an interesting fact. In Africa, Lake Victoria which, it was generally agreed, was sinking in the period 1878-1892, has since shown a tendency to rise. Lake Bukwa, east of Tanganyika, has risen within the last few years. Reports that the Sea of Azov is drying up have been explained as due to a silting up of a lake. Lake Chad is very probably subject to oscillations, sometimes spreading beyond its usual limits as the result of several years of heavy rainfall. Such diverse reports show the need of caution in jumping at conclusions of climatic change. An increased use of water for irrigation may cause the level of water in a lake to fall, as has been the case to some extent in Great Salt Lake. Periodic oscillations, giving higher and then lower water, do not indicate progressive change in one direction. Many writers have thus seen a law in what was really a chance coincidence. Partsch believes that the ancient settlements on the interior lakes of northern Africa show that these lakes contained no more water formerly than they do now. Some have claimed that the supposed desiccation of the climate of northern Africa resulted from deforestation, but no certain evidence exists of the presence or destruction of such forests, and if deforestation did take place, no considerable change of climate could have resulted.
Thirdly, where a progressive desiccation seems to have taken place, the question should be asked, is less rain actually falling, or have the inhabitants less capacity, less energy, less ability than formerly? Is the change from a once cultivated area to a barren expanse the result of decreasing rainfall, or of the emigration of the former inhabitants to other lands? The difference between a country formerly well irrigated and fertile, and a present-day sandy, inhospitable waste may be the result of a former compulsion of the people, by a strong governing power, to till the soil and to irrigate, while now, without that compulsion, no attempt is made to keep up the work. The incapacity of the present inhabitants, or of their rulers, is often responsible for effects which have been interpreted as due to climatic change. It has been shown that where irrigation is resorted to in parts of the districts about the Mediterranean which have been reported to be drying up, there the former fruitfulness returns. In many cases the reports of increasing dryness really concern only the decrease in the water supply from rivers and springs, and it is well known that a change in the cultivation of the soil, or in the extent of the forests, may bring about marked changes in the flow of springs and rivers without any essential change in the actual amount of rainfall. These conditions are particularly likely to occur in regions where there is no snow covering, and where the rain falls in a few months only. In Tripoli, the Vicomte de Mathuisieulx finds that the Latin texts and monuments seem to establish the fact that, so far as atmospheric conditions and soil are concerned, everything is just as it was in ancient times. The present condition of the country is ascribed to the idleness of the Arabs, who have allowed wells to become choked and vegetation to perish. "In a country so little favored by nature, the first requisite is a diligent and hard-working population. The Romans took several centuries to make the land productive by damming rivers and sinking wells in the wady beds." In an arid region, man has a hard task if he is to overcome the climatic difficulties of his situation. Irrigation; the choice of suitable crops adapted to arid conditions; steady, thoughtful work, are absolutely essential. To a large extent, an intelligent man may thus overcome many of the obstacles which nature has put in his way. On the other hand, a region of deficient rainfall, once thickly settled and prosperous, may readily become an apparently hopeless desert, even without the intervention of war and pestilence, if man allows the climate to master him.
Lastly, a region whose normal rainfall is at best barely sufficient for man's needs, may be abandoned by its inhabitants during a few years of deficient precipitation, and not again occupied even when, a few years later, normal or excessive rainfall occurs. It is a very striking fact that the districts from which comes most of the evidence of changes of climate within historical times are subtropical or subequatorial, i. e., they are in just those latitudes in which a slightly greater or a slightly less migration of the rain-bringing conditions easily produces a very considerable increase or decrease in the annual rainfall.
It is apparent, on examining the evidence thus far at hand, that the fact of permanent, progressive changes in climate during historical times has not yet been definitely established.
Periodic Oscillations of Climate: Sunspot Period.—The discovery of a distinct eleven-year periodicity in the magnetic phenomena of the earth, naturally led to investigations of similar periods in meteorology. Numerous and varied studies along this line, extending back even into the seventeenth century, but beginning actively about 1870, have been and are still being prosecuted by a considerable number of persons, and the literature on the subject has assumed large proportions. The results, however, have not been satisfactory. The problem is difficult and obscure. It is natural to expect a relation of this sort, and some relation certainly exists. But the results have not come up to expectations. Fluctuations in temperature and rainfall, occurring in an eleven-year period, have been made out for certain stations, but the variations are slight, and it is not yet clear that they are sufficiently marked, uniform and persistent over large areas to make practical application of the periodicity in forecasting possible. In some cases, the relation to sunspot periodicity is open to debate; in others, the results are contradictory.
Köppen has brought forward evidence of a sunspot period in the mean annual temperature, especially in the tropics, the maximum temperatures coming in the years of sunspot minima. The whole amplitude of the variation in the mean annual temperatures, from sunspot minimum to sunspot maximum, is, however, only 1.3° in the tropics and a little less than 1° in the extra-tropics. There are, however, long periods during which there appears to be no influence, or at least, an obscure one, and the relation before 1816 seems to have been opposite to that since then. More recently Nordmann (for the years 1870-1900) has continued Köppen's investigation, using the mean annual temperatures of certain tropical stations, and finds that the mean temperatures run parallel with the sunspot curve, but that the minimum temperatures occur with the sunspot maxima (amplitude 0.7°). This seems to contradict Köppen's conclusion, and also the fact that the sun is hotter at a time of maximum sunspots. The latter difficulty has been explained on the ground that the rainfall and cloudiness, both of which are at a maximum with the sunspot curve, lower the temperature, especially in the tropics. It is obvious that the situation in this matter is rather confusing just at the present time, and that the relation of sunspots and terrestrial temperatures is not wholly clear. The sunspots themselves are probably not the immediate or sole control. "There seems little doubt," says Sir Norman Lockyer, 'that we must look to the study of the solar prominences not only as the primary factors in the magnetic and atmospheric changes in our sun, but as the instigators of the terrestrial variations.' These investigations, however interesting and important they may be to astronomers and physical meteorologists, are really outside the field of climatology.
In 1872 Meldrum, then director of the meteorological observatory at Mauritius, first called attention to a sunspot periodicity in rainfall and in the frequency of tropical cyclones in the South Indian Ocean. Tbe latter are most numerous in years of sunspot maxima, and decrease in frequency with the approach of sunspot minima. Poëy later found a similar relation in the case of the West Indian hurricanes. Meldrum's conclusions regarding rainfall were that, with few exceptions, there is more rain in years of sunspot maxima. This is to be taken only for means, and for a majority of stations, and is not to be expected at all stations or in every period. Hill found it to be true of the Indian summer monsoon rains that there seems to be an excess in the first half of the cycle, after the sunspot maximum. The winter rains of northern India, however, show the opposite relation; the minimum following, or coinciding with, the sunspot maximum. Many studies have been made of a possible relation between rainfall and the sunspot period, but the conclusions are not very definite, are sometimes contradictory, and do not yet warrant any general practical application for purposes of forecasting the wet or dry character of a coming year. Particular attention has been paid to the sunspot cycle of rainfall in India, because of the close relation between famines and the summer monsoon rainfall in that country. In 1889 Blanford admitted that the rainfall of India as a whole did not give evidence of the sunspot cycle in the records of the twenty-two years preceding. More recently the Lockyers have studied the variations of rainfall in the region surrounding the Indian Ocean in the light of solar changes in temperature. They find that India has two pulses of rainfall, one near the maximum and the other near the minimum of the sunspot period. The famines of the last fifty years have occurred in the intervals between these two pulses, and these writers believe that if as much had been known in 1836 as is now known, the probability of famines at all the subsequent dates might have been foreseen.
Relations between the sunspot period and various meteorological phenomena other than temperature, rainfall and tropical cyclones have been made the subject of numerous investigations, but on the whole the results are still too uncertain to be of any but a theoretical value. Some promising conclusions seem, however, to have been reached in regard to pressure variations, and their control over other climatic elements.
Brückner's Thirty-five-year Cycle.—Of more importance than the results thus far reached for the sunspot period are those which clearly establish a somewhat longer period of slight fluctuations or oscillations of {hws|cli|climate}} climate, known as the Brückner cycle, after Professor Brückner, of Berne, who has made a careful investigation of the whole subject of climatic changes and finds evidence of a thirty-five-year periodicity in temperature and rainfall. Brückner began with the long-period oscillations in the level of the Caspian Sea. He then investigated the levels of the rivers flowing into the Caspian, and next the dates of the opening and closing of the rivers of the Russian empire, and finally extended his study over a considerable part of the world, including data concerning mean temperatures, rainfall, grape harvest, severe winters, and the like. The dates of opening and closing of Russian rivers go back in one case to 1559; the dates of vintage to the end of the fourteenth century, and the records of severe winters to about 1000 A.D. In a cycle whose average length is thirty-five years there comes a series of years which are somewhat cooler and also more rainy, and then a series of years which are somewhat warmer and drier. Brückner has found that the price of grain averages 13 per cent, higher in the wetter lustrum than in the drier. This thirty-five-year period is not to be thought of as being a perfectly systematic recurrence, in exactly that term of years. The interval in some cases is twenty years; in others it is fifty. The average interval between two cool and moist, or warm and dry periods, is about thirty-five years. Moreover, not only the intervals, but the intensities of the individual periods vary. The mean amplitude of the temperature fluctuation, based on large numbers of data, is a little less than 2°, which makes it greater than that obtained by Köppen for the sunspot period, and it is natural to expect it at a maximum in continental climates. The fluctuations in rainfall, also, are more marked in interiors than on coasts. The general mean amplitude is 12 per cent., or, excluding exceptional districts, 24 per cent. In western Siberia more than twice as much rain may fall in wet as in dry periods. Regions whose normal rainfall is small are thus most affected. In years of minimum precipitation they may become uninhabitable, and the population may be forced to move away, perhaps never returning, and allowing towns and irrigating works to fall to decay. Slight fluctuations in rainfall are most critical in regions having a normal precipitation barely sufficient for agriculture. The extent of land cultivated, and the returns of agriculture here fluctuate directly with the temporary increase or decrease of rainfall. A supplementary study of the newer rainfall observations for Russia and for the United States, as well as for certain stations in central Europe and eastern Siberia, has given Brückner satisfactory confirmation of his earlier conclusions in the fact that he finds a decrease of rainfall over these districts as a whole, beginning about the middle of the decade 1880-90. The time of the 'boom' in western Kansas and Nebraska, and in eastern Colorado, in the decade 1880-90, followed one of Brückner's wet periods, and the collapse of the 'boom' came when the drier period advanced. Farmers who went out on to the high plains in the years of slightly greater rainfall preceding the boom, and who lost all their capital, and more too, in the vain attempt to raise their grain in the years which followed, could with difficulty be convinced that the climate of the plains had not permanently changed for the worse. The impression left upon their minds, and upon the mind of anyone who saw the country later, was one of decreasing rainfall, unsuccessful agriculture and financial ruin. Within more recent years, in this same region of Kansas, with a somewhat increased rainfall during a wetter cycle, but without any permanent change to a wetter climate, the intelligent choice of cereals better adapted to the soil and climate, and the rational use of the available water supply, have wrought a wonderful change in the aspect and economic value of the state.
The following table shows the dates and characters of Brückner's periods:
| Warm | 1740-1755 | 1791-1805 | 1821-1835 | 1851-1870 | |
| Dry | 1756-1770 | 1781-1805 | 1826-1840 | 1856-1870 | |
| Cold | 1731-1745 | 1756-1790 | 1806-1820 | 1836-1850 | 1871-1885 |
| Wet | 1736-1755 | 1771-1780 | 1806-1825 | 1841-1855 | 1871-1885 |
Interesting confirmation of Brückner's thirty-five-year period has been found by Richter in the variations of the Swiss glaciers, but as these glaciers differ in length, they do not all advance and retreat at the same time. The advance is seen during the cold and damp periods. Supan has pointed out that the Brückner periods appear to hold good in the south polar regions. And Hann's study of the monthly and annual means of rainfall at Padua (1725-1900), Klagenfurt (1813-1900) and Milan (1764-1900) brings to light an alternation of wet and dry periods in harmony with the thirty-five year cycle. It should be noted that Brückner has found certain districts in which the phases and epochs of the climatic cycle are exactly reversed. These exceptional districts are almost altogether limited to marine climates. There is thus a sort of compensation between oceans and continents. The rainier periods on the continents are accompanied by relatively low pressures, while the pressures are high and the period dry over the oceans, and vice versa. The cold and rainy periods are also marked by a decrease in all pressure differences. It is obvious that changes in the general distribution of atmospheric pressures over extended areas, of the great centers of high and low pressure, are closely associated with fluctuations in temperature and rainfall. An oscillation of a few hundred miles one way or another may mean the difference between drought or plentiful rainfall over extended areas. These changes in pressure distribution must in some way be associated with changes in the general circulation of the atmosphere, and these again must depend upon some external controlling cause or ca W. J. S. Lockyer has called attention to the fact that there seems to be a periodicity of about thirty-five years in solar activity, and that this corresponds with the Brückner period. This longer cycle, underlying the sunspot period, alters the time of occurrence of the sunspot maxima in relation to the preceding sunspot minima. He makes out three periods in solar activity, of between three and four years, about eleven, and about thirty-five year respectively. These are related as 1:3:9.
It is clear that the existence of a thirty-five-year period will account for many of the views that have been advanced in favor of a progressive change of climate. A succession of a few years wetter or drier than the normal is likely to lead to the conclusion that the change is permanent. Accurate observations extending over as many years as possible, and discussed without prejudice, are necessary before any conclusions are drawn. Observations for one station during the wetter part of a cycle should not be compared with observations for another station during the drier part of the same, or of another cycle.
Climatic Cycles of Longer Period.—There are evidences of longer climatic cycles than eleven or thirty-five years. Brückner calls attention to the fact that sometimes two of his period; seem to merge into one. Richter shows much the same thing for the Alpine glaciers. James Geikie, in Scotland, has brought forward evidence of several climatic changes in postglacial times. Blytt, in Norway and Sweden, finds some botanical evidence of four great climatic waves since the last glacial period. Brögger estimates that a mean annual temperature between 3° and 4° higher than the present was found in the Christiania Fjord in postglacial time. Lorié, in Holland, finds confirmation of Blytt's views. Gradmann, on botanical evidence, beliein a warmer climate in central Europe after the last ice age, and then a cooler one. dough concludes that a three hundred-years cycle exists in solar and terrestrial phenomena, the thirty-six year cycle being as it were superimposed upon the longer one. Kingsmill reports a periodicity of three hundred years in droughts and famines in northern China. And so on. As yet. nothing sufficiently definite to warrant discussion here ha; been brought forward.
Geological Changes in Climate.—Changes of climate in the geological past are known with absolute certainty to have taken place: periods of glacial invasions, as well as periods of more genial conditions. The evidence, and the causes of these changes have been discussed and re-discussed, by writers almost without number, and from all points of view. Changes in the intensity of insolation; in the sun itself; in the conditions of the earth's atmosphere; in the astronomical relations of earth and sun; in the distribution of land and water, in the position of the earth's axis; in the altitude of the land; in the presence of volcanic dust—changes now in cosmic, now in terrestrial conditions—have been suggested, combated, put forward again. None of these hypotheses has prevailed in preference to others. No actual proof of the correctness of this or that theory has been brought forward. No general agreement has been reached. Under these conditions, and in view of the fact that practical climatology is concerned with climatic changes, not of the geological past but of the historical present, this portion of our subject may be dismissed with this brief mention.
Conclusion.—There is a wide-spread popular belief in permanent, progressive changes of climate during a generation or two. This belief is not supported by the facts of meteorological record. Abundant evidence has been adduced in favor of secular changes-of climate in historical times. Much of this is unreliable, contradictory, and has been interpreted without sufficient regard to possible controls other than climatic change. Without denying the possibility, or even the probability, of the establishment of the fact of secular changes, there is as yet no sufficient warrant for believing in considerable permanent changes over large areas. Dufour, after a thorough study of all available evidence, has concluded that a change of climate has not been proved. There are periodic oscillations of slight amount. An elevenyear period has been made out, with more or less certainty, for some of the meteorological elements, but it has been of no practical importance as yet. A thirty-five year.period is less uncertain, but is nevertheless of considerable irregularity, and can not as yet be practically applied in forecasting. Longer periods are suggested, but not made out. As to causes, variations in solar activity are naturally receiving attention, and the results thus far are promising. But climate is a great complex, and complete and satisfactory explanations of all the facts will be difficult, perhaps impossible, to reach. At present, indeed, the facts which call for explanation are still in most cases but poorly determined, and the processes at work are insufficiently understood. Climate is not absolutely a constant. The pendulum swings to the right, and to the left. And its swing is as far to the right as to the left. Each generation lives through a part of one, or two, or even three, oscillations. A snap-shot view of these oscillations makes them seem permanent. As Supan has well said, it was formerly believed that climate changes locally, but progressively and permanently. It is now believed that oscillations of climate are limited in time, but occur over wide areas. Finally, it is clear that man, whether by reforestation or deforestation, by flooding a desert or by draining a swamp, can produce no important or extended modifications of natural climate, which is governed by factors beyond human control.