CHANGES OF CLIMATE

Popular Belief in Climatic Change—Evidence of Climatic Changes Within Historic Times—What Meteorological Records Show—Why the Popular Belief in Climatic Changes is Untrustworthy—Value of Evidence Concerning Changes of Climate—Periodic Oscillations of Climate: The Sunspot Period—Brückner’s 35-Year Cycle—Climatic Cycles of Longer Period—Geological Changes in Climate—Conclusion.

_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 favourable to an unfavourable one.

_Evidences of Climatic Changes within Historic Times._ Evidence is constantly being brought forward of apparent climatic variations of greater or less amount which have occurred during historic times, and some of which seem now to be going on. Such reports, largely those of travellers or explorers in comparatively little-known regions, are based on fluctuations in the extent of inland lakes; on the discovery of abandoned dwelling sites, the ruins of aqueducts and irrigating canals, and of human utensils and implements; as well as on the study of strand lines, dunes, river terraces, abandoned river channels, vegetation, the salinity of lakes, desert roads now too dry for caravans, and the like. In several cases descriptions have been given of extended ruins showing human occupation by a former large population in districts where there now seems to be an absolutely insufficient water supply, or where the waters are too saline for use. Thus we have many accounts, based on various lines of evidence similar to that just referred to, of a gradual desiccation which seems to have been going on over a large region in central Asia during historical times.

In northern Africa, certain ancient historical records have been taken by different writers to indicate a general decrease of rainfall during the last 3000 years or more, 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 pictographs, and from this fact, as well as from a study of the past and present flora of the region, and of its river history also, a change from a moister to a drier climate is inferred. From the Sahara, evidence of a former large population has been adduced and a gradual desiccation of that region is, therefore, believed to have taken place. Recently, however, the equatorial rain-belt seems to have been advancing farther north, giving an increased rainfall. Gautier has divided the history here into three periods: (1) dense population; (2) aridity; and (3) the present change to steppe character.

Farther south, several lakes have been reported as decreasing in size, and wells and springs as running dry. In the Lake Chad district, Chevalier has reported 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, many writers hold that a decrease in rainfall has taken place within historic times over Greece, Syria, and other eastern Mediterranean lands, as well as in Italy and other countries over the western portion of the Mediterranean. These examples might be multiplied, for reports of climatic changes of one kind or another are numerous from many parts of the globe. The literature on the subject is already very extended and relates to many different parts of the world, including southern as well as northern Africa, Australia, south-western North America, Central America, and several of the more arid districts of South America, where the recent investigations of Bowman seem to confirm the views of progressive desiccation previously expressed by several others. As a whole, the general impression which the reader gains from a study of much of this literature is that of a widespread tendency toward increasing aridity. There are, however many excellent authorities who do not unqualifiedly accept the evidence thus far brought forward, and who take the opposite view, or at least regard the matter as still far from being definitely settled.

Dr. Ellsworth Huntington has contributed largely to the discussion regarding variations of climate within historic times. Beginning his investigations in Asia,—in Turkestan, Transcaspia, and Persia,—he has extended his studies to include Palestine and the Libyan Desert, and, more recently, the more arid portions of North America, Central and South America. His evidence is derived from the archæology, physiography, botany, history, and tradition of the areas under discussion. The various kinds of evidence are matched, and are found by him to accord in a striking manner. Huntington’s latest work, in western North America, includes an examination of the rings of the big trees of California, which are believed, to furnish accurate gauges of the climatic variations which have taken place during their long period of life. The general conclusion drawn from a study of this great body of evidence is that the main climatic variations were synchronous in the Mediterranean regions, in Central Asia, and in North and Central America; that from the dawn of human history there has been a general change from moister, and presumably cooler, to drier and warmer climates. This change has not gone on uniformly, but has taken place more or less irregularly, in a pulsatory fashion, drier and moister periods alternating with one another.

These climatic “pulsations” are correlated with certain important historic events, such as the decline of Persia, agrarian troubles in Rome, barbarian invasions, and the decline and subsequent revival of civilization in Central America. A “remarkable agreement” has been found between these historic events and the “apparent climatic changes.” The latter “seem competent to give rise to the observed results.” Dr. Huntington has plotted curves showing variations in the rapidity of growth of the “big trees” of California since 1300 B.C., and these curves, in his judgment, are in substantial agreement with his climatic curves for central and western Asia. Hence the conclusion that, in similar latitudes of western Asia and of western North America, the pulsations of climate were essentially synchronous and of the same character.

_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 untrustworthy, 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, is 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 mean simply 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 thirty 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, but it did appear that 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 earliest known measurements of rainfall were made in Palestine, in the first century A.D., and the results were preserved in the Mishnah, a collection of Jewish religious books. A comparison of these early observations with those of the present shows that there has been no change in the climate of Palestine.

Non-instrumental records, both regular and occasional, of the opening and closing of rivers to navigation, of severe winters, etc., in Europe, in some cases go back several centuries, but give no evidence of any progressive change in climate there. The records of the closing of the Hudson 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 meteorological 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 down to the present day.

_Popular Belief in Climatic Changes 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 deep 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._ There are certain considerations which should be borne in mind in dealing with the evidence of changes of climate within historic times; certain corrections, so to speak, which should be made for possible controls other than climatic, before definite conclusions are reached in favour of climatic changes.

In the first place, there is a large body of facts concerning the past and present conditions of cultivated crops, and of vegetation in general. 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. In other words, economic, and not climatic, controls, may often be the determining factor. Thus the area covered by vineyards in northern Europe has been very much restricted in the last few hundred years, because grapes can be raised 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 was profitable. Later, after a readjustment of the taxes on breadstuffs, 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. A careful examination of the writings of Homer, Hesiod, Aristotle, Theophrastus, Strabo, and other ancient classic authors, has led several recent students of this question to the definite conclusion that no appreciable climatic change has taken place in the Mediterranean region within the last thirty centuries. Lately, Hildebrandsson has again reviewed the botanical evidence from Palestine, and finds that the culture of the vine and the date indicates the same climatic conditions now as formerly. In Italy, too, the laurel, the myrtle, and the fir are growing now where they grew in the days of Pliny and Virgil. Arago, in his time, concluded that there had been no change in Palestine in three thousand years. 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. To this evidence, derived from the past and present distribution of vegetation, those who believe in climatic change object on the ground that at the height of the last glacial epoch the mean temperature of the earth was probably only about 10° or at most 20°, cooler than now. On that basis, an historic change a tenth as great as the enormous change since the height of glaciation would mean a temperature change too small to detect by means of vegetation.

Again, in some cases, the reported cultivation of cereals, or other soil products, in certain climates at present unfavourable, has been shown to be purely a myth; as in the case of Greenland and of Iceland. Thoroddsen, the leading authority on Iceland, has shown that political and economic changes amply suffice to account for all changes in crop cultivation in that country.

The evidence derived from a study of the “big tree” rings in California has not been unchallenged. Seasonal, rather than annual, rainfall, and other factors such as temperature, insect pests, the height of the surrounding trees, etc., are controls which enter in to complicate the problem, and make it difficult to draw definite conclusions regarding the climates of the past. On the other hand, those who accept the evidence of the tree rings reply that in our own day exact records show an unmistakable harmony between spring rainfall and tree growth in California, on the one hand, and Mediterranean rainfall on the other. Hence, they maintain, it seems legitimate to infer that the main fluctuations in tree growth represent climatic pulsations of almost world-wide extent.

Secondly, many of the reports by explorers from little-known regions are extraordinarily contradictory. Thus Lake Aral, which was diminishing in area for many years, has recently been reported by Berg to be increasing. A railroad built along the shores of this lake has been moved because of the encroachment of the waters upon the road bed. 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. Woeikof, in his last study of the Turkestan region, reported increasing rainfall and expanding and deepening lakes, and opposed the view of a general desiccation. Stein (1907) reported a distinct extension of the cultivated area as having taken place in Khotan between the times of his last two visits to that region. In Africa, Lake Victoria, which, it was generally agreed, was sinking in the period 1878-1892, has since shown a tendency to rise. Lake Rukwa, east of Tanganyika, has risen within the last few years. 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. Where irrigation is now being again resorted to in parts of the districts about the Mediterranean which have been reported to be drying up, there the former fruitfulness is returning. In Asia Minor, for example, the rule of the Turk brought a change from a settled and civilised to a semi-nomadic state of society; industries died out, the land to a great extent passed out of cultivation; irrigation works were destroyed. Recently the building of railroads and of roads has been followed by a revival of industry and of agriculture, and by the reclamation of waste land. 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 favoured 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. A change in man doubtless explains many of the facts which have been brought forward as evidences of a change in climate.

Other factors also enter into the problem. Many primitive tribes readily abandon their tilled fields and their dwelling places without the spur of necessity due to decreasing water supply. The scanty supply of firewood, soon exhausted, may enforce migration. Wars, by withdrawing labour, result in the neglect of agriculture, so that the land in an arid region reverts to its original desert condition. Irrigation systems are vulnerable, and are easily silted up by blowing sand, or are destroyed. In Mesopotamia, the neglect of dykes which had been built to check the Great Swamp resulted in a change in the course of the Tigris, and land which the river had rendered fertile became a desert. Former caravan routes are abandoned because altered economic conditions have rendered them useless. Continued irrigation usually produces increasing salinity of the soil, and may necessitate the abandonment of cultivated fields. This cause, and also the invasion by nomadic tribes in prehistoric times, are believed by Fewkes to have been more important than possible climatic variations in causing the abandonment of many ancient Indian settlements in the south-western United States.

Various purely human controls, like those here enumerated, are believed by many competent authorities to explain the facts which have been brought forward from Asia, from the Mediterranean area, from Africa and elsewhere, as evidence of climatic changes within historic times. There is by no means unanimity of opinion on this matter.

Fourthly, 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 striking fact that the districts from which comes much of the evidence of changes of climate within historical times are in latitudes in which a slight temporary increase or decrease in rainfall will have far-reaching consequences for human settlement and human activity.

Lastly, geologic or physiographic processes, such as elevation or depression of the land, cutting down of the outlets, or the silting up of lakes, the lowering of river beds below the level of irrigating canals, and the like, in certain cases doubtless explain some of the facts which have been interpreted as indications of climatic changes.

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 the fact that the sun is hotter at a time of maximum sunspots. The latter difficulty has been explained on the ground that convection is most active at a time of sunspot maxima and that rainfall and cloudiness, both of which are also at a maximum with the sunspot curve, lower the temperature, especially in the tropics. It is obvious that the condition of this matter is rather confusing just at the present time, and that the relation of sunspots and terrestrial temperatures is not wholly clear. Köppen’s latest results (1914) show that the increase of temperature noted within the tropics at times of sunspot minima is about 1° Fahr. higher than in years of sunspot maxima, and becomes less and less apparent outside the tropics, disappearing in the far north. 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. The 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 indicated 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 pulses. 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. The latest study of the relation of rainfall and sunspots, by Gilbert T. Walker, shows that the coefficient is, in general, not much larger than would result from chance.

Relations between the sunspot period and various meteorological phenomena 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 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 onto 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 characters and dates of Brückner’s periods:

Warm 1746-1755 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 centres 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 and 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 causes. 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 years, 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 favour 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 periods 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 post-glacial 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 Christiana Fjord in post-glacial time. Lorié, in Holland, finds confirmation of Blytt’s views. Gradmann, on botanical evidence, believes in a warmer climate in central Europe after the last ice age, and then a cooler one. Clough concludes that a three-hundred-year 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 has 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, combatted, 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._ The subject of changes of climate is highly complex. The exact nature of the problem is not yet perfectly clear. Hence it is not surprising that there is much confusion regarding it. No clear-cut, definite, decisive answers can yet be given to many of the questions which naturally arise. There is even much confusion in the use of the word _changes_ of climate. What is often spoken of as a _change_ may simply be an oscillation, of longer or shorter period. A difference obviously exists between a permanent, progressive _change_ and an oscillation, or periodicity.

The occurrence, in geologic times, of varying climatic conditions is established, although the actual differences between the climates which prevailed at various epochs were doubtless much smaller than was formerly believed to have been the case. There is, therefore, no prejudice on the part of climatologists against the view that climate may also have varied during historic times. On the other hand, changes of climate seem to offer a very simple explanation of many otherwise puzzling facts. There is, therefore, need of great caution before appealing to this cause alone. Every other possible explanation should also be carefully tested, and rejected only if proved to be incompetent and unsatisfactory.

There is a widespread popular belief in permanent progressive changes during a generation or two. This belief is not supported by the facts of meteorological record. Abundant evidence has been adduced in favour of “changes” of climate within historic times. Some of this is untrustworthy; some is contradictory; much has been interpreted without sufficient regard to possible controls other than climatic changes. A great deal of the evidence is of such a nature that it requires critical examination by specialists in physiography, in geology, in archæology, in botany, in history. In cases where such experts have looked into the facts, there is at present a decided tendency, if not to explain the facts in other ways, at least to hesitate before ascribing them solely to climatic changes. Further, historians as a group are not ready to accept a general relation of cause and effect between curves of possible climatic variations and the great accepted facts of historical migrations and adjustments.

The whole question is still being actively debated. In some cases, a complete dead-lock seems to have been reached. While accepting the possibility, or even the probability, of climatic variations, it is clearly the general consensus of expert meteorological opinion that there is no sufficient unimpeachable evidence for believing in permanent progressive changes of climate within historic times. “Not proven” is the verdict of the majority of climatologists who have thus far studied the evidence.

There are clearly periodic climatic oscillations, of relatively slight amount. An eleven-year period has been made out, with more or less certainty, for some of the meteorological elements, but it has proved of no practical importance as yet. A period of about thirty-five years has been widely recognized, but it is of considerable irregularity, and cannot yet be practically applied in long-range forecasting. Longer periods, also, have been suggested, but are not surely established.

As to the possible cause, or causes, of climatic variations within historic times, there is no agreement. Fluctuations in the amounts of solar radiation, the effect of volcanic dust veils, oscillations of the climatic belts, changes in the internal mechanism of the earth’s atmosphere—these and other controls have been suggested. The hypothesis that climatic variations, both in geological and historical times, were probably the result of an alternate equatorward and poleward shifting of the climatic belts has been urged. Many diverse types of change may, in this way, occur synchronously in different parts of the world. According to Huntington, the chief basis for this conclusion, so far as historic times are concerned, is the fact that the Maya ruins in the dense forests of Central America seem to indicate periods of relative dryness when the trees of California indicate moist periods, and _vice versa_. There is, however, disagreement as to the ultimate cause behind this hypothetical shifting of the zones.

In our own day, short-period variations in the character of the seasons are known to depend upon changes in the position and the development of the great permanent areas of low and high pressure, the so-called “centres of action” of the earth’s atmosphere. Hence it is natural that some should look close at hand for the most probable cause of climatic variations. This view was well expressed by the late Professor Cleveland Abbe as follows:

We may, in general, conclude that in the present state of the globe and the atmosphere, and without any change in latitude or altitudes, moisture or sunshine, it is perfectly possible for such combinations of winds to occur as to give us in one century conditions favourable for rain, snow, and glaciers, but in another century drought, sand, and desert. These alterations depend essentially on extreme variations in what is called the general circulation of the atmosphere; they are perturbations produced by its own internal mechanism.

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 many cases 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 probably 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 may easily make them seem permanent.

Much remains to be done in the study of climatic variations, not only in the further collection, correlation, and critical analysis of the evidence, but also in the investigation of the human relations of such variations. Such differences in rainfall as are associated with the thirty-five-year period of Brückner, for example, may easily affect the size, character, and price of crops; control the level and supply of water in lakes, rivers, and underground; and have important relations to industry and transportation. Brückner has himself shown that his climatic periods, by determining crop conditions, have a marked influence upon emigration from Europe, and upon immigration to the United States. H. H. Clayton and Professor H. L. Moore have pointed out that cycles of rainfall in the United States are also economic cycles. For north-western Europe, Petterson has traced relationships between climatic variations in historic times and certain important historical and economic events there. Arctowski has studied short-cycle variations of temperature, and has traced the movements of areas of plus and minus temperature departures. The agreement between conditions over broad geographic areas leads him to a belief in extra-terrestrial control.

It is clear that great interest attaches to the whole question of variations in climatic conditions. When the facts of such oscillations, and the laws which govern them, are accurately determined, long-range forecasts will become possible which will be of incalculable benefit to man.

INDEX

A

Abbe (C.), on changes of climate, 369 Abercromby (R.), on Mohammedanism and rainfall, 258-259 Abscess, tropical, of the liver, 195-196 Acclimatisation, 203-205 Africa, interior, temperature, 89-91 Agriculture in tropics, 239 Alice Springs, Australia, temperature, 90, 98 Amundsen (R.), on Antarctic temperature, 163 Angot (A.), on dates of vintage, 349 ————, on transmitted insolation, 15 Antarctic, _see_ Polar Arctic, _see_ Polar Arrhenius (S.), on cloudiness by latitude, 116 Arts in polar zones, 333-335 ———— tropics, 239-241 Atmosphere, effect on insolation, 13-16 Auckland, temperature, 129, 131 Australia, rainfall of western, 125, 126

B

Bagdad, temperature, 39-40, 129, 131 Batavia, rainfall, 105 ————, temperature, 89-91 _Belgica_ expedition, 174 Beri-Beri, 201 Berlin, rainfall, 115 Bermuda, temperature, 129, 131 Berne, pressure, 47-48 Blagoweschtschensk, temperature, 135, 136 Blizzards, 141, 176 Bora, 53, 132 Bowman (I.), on climatic changes, 341 Bronchitis, 211 Brückner (E.), 35-year cycle, 360-364, 370 Bryce (J.), on climate and government, 231 Buchan (A.), on north polar winds, 169 Buckle (T. H.), on climate and man, 222, 231, 274 Buran, 141

C

Calcutta, rainfall, 85 _Challenger_ expedition, 37, 79 Chamsin, 132 Changes of climate, 338-371 Characteristics of the zones: polar, 151-177 temperate, 108-150 tropics, 76-107 Charcow, temperature, 135, 136 Chinook, 310 Cholera, 196-197 ———— infantum, 213 Civilisation in temperate zones, 272-276 ————, primitive, in the tropics, 232-235 Classification of climates, 35-75 ———— zones, 19-34 Clayton (H. H.), on mental effects of weather, 309 ———— rainfall, commerce, and politics, 319, 370 Climate and crops in temperate zones, 295-298 ———— government, 230-232 ———— habitability, 224-226 ———— health, 178-219 ———— man, general, 220-226 ———— man, in temperate zones, general, 272-274 ———— weather and military operations, 310-312 ———— mental effects of, 309-310 ———— of tropics, 76, 77 Climate, definition of, 2 ————, changes of, 338-371 ————, geological, 365-366 ————, monsoon, 44, 46 ————, physical, 16, 18 ————, relation to weather, 2 ————, solar, 7, 16 Climates, classification of, 35-75 ————, coast or littoral, 35, 43-44 ————, continental, 35, 38-42 ————, desert, 35, 42-43 ————, marine, 35, 37-38 ————, Mediterranean, 58, 62, 72, 122-134 ————, mountain and plateau, 35, 46-53, 106-107, 149-150, 265-271 Climatic changes in historical times, evidences of, 339-343 ———— divides, mountains as, 53-54 ———— elements and their treatment, 5-7 ———— provinces, Supan’s, 55-60, 73-74 ———— zones and subdivisions, 19-34 Climatology, literature of, 3-5 ————, meaning and scope, 1 ————, relation to meteorology, 2-3 Clothing in polar zones, 330-331, 333 ———— tropics, 236-237 Cloudiness by latitude, 116 ———— in polar zones, 172-174 ———— temperate zones, 116-117 ———— tropics, 87-88 Coast or littoral climates, 35, 43-44 Cold waves, 141 Continental climates, 35, 38-42 Continents and temperate zone, 278-280 Cordoba, temperature, 129, 131 Crops and climate in temperate zones, 295-298 Culture in polar zones, 324-325 Customs in polar zones, 335 Cyclones in polar zones, 174-177 ———— temperate zones, 112, 117-120 ———— tropical, 263-264

D

Darwin (C.), on climate and man, 242, 243, 246, 271, 279-280 Davis (W. M.), on Bosnian farm-houses, 290 ————, wind zones, 30-32 Death-rates, tropical, 183-185 De Candolle (A.), on plant zones, 61, 63 Dengue, 200-201 Desert climates, 35, 42, 43 Deserts, life of man in temperate zone, 298-303 ———— of sand and of snow, 335, 337 ————, trade wind, life of man in, 253-260 Dexter (E. G.), on mental effects of weather, 309 Diarrhœal disorders, 194-195 Diphtheria, 210 _Discovery_ expedition, 174, 176 Disease, distribution of, 182-183 Diseases of temperate zones, 205-214 ———— tropical, 186-201 Doldrums, 82-83 Drygalski (E. von), on rain in polar zones, 171 Dwellings in polar zones, 329-330, 331-333 ———— tropics, 235-236 Dysentery, 194-195

E

Eiffel Tower, temperature, 49, 50 Elements of climate, 5-7 Equatorial belt of tropics, 89-98 ———— forests, life of man in, 243-251 Eskimos, 331-335

F

Foehn, 53, 54, 167-168, 310 Fog in polar zones, 172-173 Food in polar zones, 330, 333 ———— in tropics, 238-239 Forest clearings, life of man in, 287-289 Forests, equatorial, life of man in, 243-251 ————, temperate, life of man in, 283, 287 Franz Joseph’s Land, temperature, 164 Funchal, temperature, 39-40

G

Gebelin’s climatic zones, 30 Geikie (A.), on climate and Scotch character, 281-282 Geneva, pressure, 47-48 Geographical regions, Herbertson’s, 72-73 Geological changes of climate, 365-366 Government, climatic control of, 230-232 ———— of tropics, 230-232 Grinnell Land, temperature, 164 Guyot (A.), on climate and man, 228, 273, 324

H

Habitability and climate, 224-226 Hann (J.), _Klimatologie_, 3, 5 ————, on Antarctic temperatures, 163 ————, lightning at Maracaibo, 250-251 ————, periodicity in rainfall, 358-359 ————, temperature of tropics, 79-80 ————, climatic changes, 363 Hay fever, 213-214 Health and climate, 178-219 Herbertson (A. J.), natural geographical regions, 72-75 Hildebrandsson (H. H.), on climatic changes, 349-350 Hilgard (E. W.), on civilisation in irrigated regions, 233 Hilo, rainfall, 92, 94, 101, 106 Hirsch (A.), on malaria, 190 ————, plague, 198 ————, yellow fever, 193 Honolulu, temperature, 90, 97-98 Humidity and cloudiness in temperate zones, 116-117 ———— in polar zones, 172-174 Hungarian plain, cloudiness, 146 Huntington (E.), on changes of climate, 341-343 ————, on Kashmiris, 307-308 Hyetal regions, 71 Hygiene in the tropics, 185-186 ———— of the zones, 178-219 Hygrothermal types, Ravenstein’s 68, 70-71, 75

I

Iceland, 331 Industries in the tropics, 239-241 Influenza, 210-211 Insolation, distribution and amounts of, 8-16 Ireland (A.), on labour problem in tropics, 229-230 Isotherms, polar, 155-165 Italy, rainfall, 126-127

J

Jaluit (Marshall Islands), temperature, 89-91 Jamestown (St. Helena), temperature, 90, 97-98

K

Kiakhta, temperature, 135, 136 Kidd (B.), on civilisation in temperate zones, 275 Koch (R.), on cholera, 196 ————, malaria, 189, 190, 192 Köppen (W.), on classification of climates, 60-68, 73-74 ————, hyetal regions, 71 ————, sunspots and climate, 357, 358 ————, temperature zones, 28-30, 33

L

Labour problem in the tropics, 229-230 Land and sea breezes in the tropics, 86 Land, relation of, to temperature, 36-37 Latitude, classification of zones by, 19-23 Leste, 132 Leveche, 132 Life of man in polar zones, 322-337 ———— temperate zones, 272-321 ———— tropics, 220-271 Literature of climatology, 3-5 Littoral climates, 35, 43-44 Liver, tropical abscess of, 195-196 Livingstone (D.), on climate and man, 222, 247-248, 257 Lockyer (W. J. S.), on climatic oscillations, 364

M

Mackinder (H. J.), on climatic changes, 301 Madeira, temperature, 39-40, 131 Malaria, 188-192 Malta, rainfall, 125-126, 127 Man, life of, in polar zones, 322-337 ———— temperate zones, 272-321 ———— tropics, 220-271 Manson (Sir P.), on tropical diseases, 186-188, 189, 193, 212 Marine climates, 35, 37-38 Mawson expedition, 176 McGee (W. J.), on Papago Indians, 299-300 Measles, 212 Mediterranean climates, 58, 62, 72, 122-134 Meinardus (W.), on polar climates, 161-162, 167-168 Meldrum (C.), on sunspots and cyclones, 358 Mental effects of climate and weather, 309-310 Meteorology, relation to climatology, 2-3 Mexico, rainfall, 92, 94-95 Micro-organisms, climate and disease, 181-182 Migrations in temperate zones, 276-278 Military operations, 310-312 Mill (H. R.), on Uganda Railway, 250 Mistral, 53, 132 Mohn (H.), on Arctic climate, 152, 157, 159 Monsoon belts, 72, 102-106 ————, life of man in the, 264-265 Monsoon climate, 44-46 Mont Blanc, pressure, 47-48 Mossman (R. C.), on polar temperatures, 162 Mountain and plateau climates, 35, 46-53, 106-107, 149-150, 265-271 Mountains as climatic divides, 53, 54 Mountains, life of man on, in temperate zone, 303-309 Mountain sickness, 46-47 Mühry (A.), rainfall types, 71

N

Nagpur, temperature, 90, 97, 98, 103 Nansen expedition, 152, 157, 165, 168, 171, 173, 174, 175 Nerchinsk, temperature, 39-40 New York, rainfall, 115 Nordenskjöld (O.), on Antarctic, 176 Norte, 310 Northerners and Southerners, and climate, 280-282 North polar isotherms, 155-160 North pole, temperatures, 155-160 North temperate zone, characteristics of, 134-150 Novaya Zemlya, temperature, 165

O

Olympia, rainfall, 137-138 Optical phenomena in polar zones, 177

P

Pampero, 132 Paris, temperature, 49-50 Peary (R. E.), on Arctic, 169-170 Physical climate, 16-18 Physiological effects in tropics, 81, 183, 241-243 ———— polar zones, 214-217 Plague, 197-198 Plants in polar zones, 151-155 Plateau climates, 35, 46-53, 106-107, 149-150, 265-271 Pneumonia, 210 Polar zones, arts in the, 333-335 ————, characteristics of, 151-177 ————, life of man in, 322-337 ————, physiological effects, 214-217 Population in polar zones, 328-329 Port Darwin, rainfall, 92, 94, 102, 105 Prague, temperature, 135-136 Pressure in polar zones, 165-166, 168-169 ———— temperate zones, 111 ———— tropics, 81-82 Ptolemy, climatic zones, 2, 21 Purga, 141

Q

Quito, rainfall, 92-94

R

Railroads in temperate zones, 312-315 Rain and snow in polar zones, 170-172 Rainfall and Mohammedanism, 258-259 ————, equatorial type, 90-93 ———— in temperate zones, 113-116 ———— in tropics, 82-86 ———— systems, classification of, 71 ————, trade type, 101-102 ————, tropical type, 94, 96 Ratzel (F.), on climate and man 222, 225, 233-234, 262, 278 Ravenstein (E. G.), hygrothermal types, 68, 70-71, 75 Reindeer, 327-328 Rheumatism, 211-212 Ross (R.), on malaria, 189

S

Sakhalin, temperature, 135, 136 Säntis, pressure, 47-48 Sâo Paulo, rainfall, 92, 94-95 Savannas, life of man in, 251-253 Scarlet fever, 212 Schott (G.), on climatic changes, 344 Scilly Isles, temperature, 135, 136 Scurvy, 217 Sea breezes in tropics, 86 Seasons, effect on man, 117 ———— in tropics, 80 Semipalatinsk, temperature, 135, 136 Sirocco, 132, 310 Sleeping-sickness, 201 Smith (G.), on British empire in India, 232 Snow in polar zones, 170-172 Solano, 310 Solar climate, 7-16 Southerly burster, 132 Southerners and Northerners, 280-282 South polar temperatures, 160-163 ———— temperate zone, 121-122 Steppes, life of man in the, 289-295 Subdivisions of temperate zone, 120-121 Sub-tropical belts, 113, 122-134 Summer diseases of temperate zones, 206-207 Sunspot period in climatic oscillations, 356-360 Sunstroke, 199-200 Supan (A.), on Arctic pressures, 169 Supan, climatic provinces, 55-60, 73-74 ————, rainfall of eastern Atlantic, 127, 128 ————, rainfall of Europe, 126-127 ————, temperature zones, 24-28, 32-33

T

Temperate forests, life of man in, 283-287 ———— zones, characteristics of, 108, 150 ————, diseases of, 205-214 ————, life of man in the, 272-321 Temperature in polar zones, 152-165 ————, equatorial type, 89-90 ————, land and water and, 36-37 ————, tropical type, 96-98 ————, zones, 24-30, 32-33 Thorshavn, temperature, 135, 136, 140 Thunderstorms in tropics, 86-87 Trade wind belts, 98-102, 253-264 ———— at sea, 260-264 ————, life of man in, 253-260 ———— winds, 83-84 Transportation by water in temperate zones, 315-318 Travel and transportation in polar zones, 333-335 Tropical abscess of the liver, 195-196 ———— death-rates, 183-185 ———— diseases, 186-201 Tropics, agriculture in the, 239 ————, arts in the, 239-241 ————, characteristics of the, 76-107 ————, development of, 226-229 ————, government of the, 230-232 ————, hygiene of the, 185-186 ————, labour problem in the, 229-230 ————, life of man in the, 220-271 ————, physiological effects, 81, 183, 241-243 ————, seasons in the, 80 Tuberculosis, 207-210 Tundra, characteristics of the, 326-328 Twilight in polar zones, 177 ———— tropics, 88-89 Typhoid fever, 212-213

V

Valentia, rainfall, 116 ————, temperature, 39-40

W

Wadi Halfa, temperature, 90, 97, 98 Walker (G. T.), on climatic changes, 359 Water, relation to temperature, 36-37 Weather and climate in tropics, 76-77 ————, mental effects of, 309-310 ————, military operations and, 310-312 ———— of polar zones, 174-177 ———— temperate zones, 117-120 ————, relation to climate, 2 ————, various effects of, 318-321 Westerly winds, 111-113 Whooping cough, 213 Winds and rainfall in tropics, 82-86 ———— polar zones, 175 ———— temperate zones, 111, 113 Wind zones, 30-32 Winter diseases of temperate zones, 206-213 Woeikof (A.), _Klimate der Erde_, 4 ————, on climatic changes, 351 ————, on tropical rainfalls, 85-86 ————, wind zones, 32 Woodruff (C. E.), on light in tropics, 242-243

Y

Yakutsk, temperature, 135, 136, 140 Yaws, 201 Yellow fever, 192-194

Z

Zonda, 310 Zones, characteristics, polar, 151-177 ————, temperate, 108-150 ————, tropics, 76-170 ————, climatic, and subdivisions, 19-34 ————, hygiene of the, 178-219 ————, polar, life of man in the, 322-337 ————, temperate, life of man in the, 272-321 ————, temperature, 24-30, 32-33 ————, tropics, life of man in the, 220-271 ————, wind, 30, 32