Popular Science Monthly/Volume 40/April 1892/Variations in Climate
By W. H. LARRABEE.
SPECULATIONS concerning changes of climate have an interest that never flags. It rarely happens in the succession of seasons that two of an identical character come in succession; and any more than usually marked variation easily prompts the fancy that some modification in the character of the climate is impending.
The subject of climatology is a difficult one. The data for the proper study of it have hardly begun to be collected. We are embarrassed when we undertake to define climate and what marks to accept as its characteristics. Hann and Humboldt define it as comprising the whole of the meteorological phenomena characterizing the state of the atmosphere at any place, particularly as they affect our organs or have an influence on animal or vegetable life. The general character of the conditions can not be determined by the observations of one year, for they are liable to be contradicted by those of the next year; nor by those of any short term of years, for a similar reason. A period must be taken long enough to furnish the data for composing a type; and the more the years vary, as between one another, the longer must the period be. Many factors enter into the composition of a climate and form complicated combinations, all of which must be unraveled so as to give each factor its true force and position; and then the determination of their relative importance affords another source of embarrassment. Temperature and moisture are accepted as the most important factors, and temperature as the dominant one; and the climate is deduced by considering the average mean temperature for a term of years. Equal yearly averages do not, however, signify identical climate. A place where the summer heat and the winter cold are extreme has not the same climate as one where the range is relatively narrow, though the yearly averages may be the same in both. Hence we need separate determinations of summer and winter averages. The combinations of conditions of temperature and moisture may be endless, while the averages of either may be hardly disturbed. These facts make it hard to compare climates even when they are steady for long periods. In the capricious climates of our temperate latitudes a just determination and comparison form a baffling task.
Observations, more or less systematic, with instruments, have been made of climatological features for about a hundred years, but on a general co-operative plan they have been carried on imperfectly for less than a third of that time, or about the period within which some observers suppose a round of meteorological changes is accomplished for a single locality. Popular opinions are founded most largely on hap-hazard recollections of vague impressions that can not be depended upon; and even if we had accurate records in place of these, they could not be used to determine the trend of climate on account of the short time they cover. It has happened more than once during that time that a series of seasons of a peculiarly marked character has been followed abruptly by a series of opposite character, nullifying the conclusions that may have been taking shape from the former series. The speculations concerning a decrease of rainfall in the United States in consequence of the removal of the forests have been disturbed by the recent prevalence, in part of the disforested area, of a succession of seasons of heavy and continuous rains.
Cosmical revolutions and changes taking place on the surface of the earth have been mentioned as causes by which climates may be permanently modified, and have been brought in to account for the changes which geology shows have taken place in the past.
Among the theories of cosmical causes, one, which supposes the solar system to be carried through parts of space having different constitutions or different temperatures, may be dismissed as being purely hypothetical. No fact has been adduced in support of it, and no valid reason has been presented for supposing that there are differences in the parts of space. Other theories, which refer climatic changes to astronomical cycles affecting the earth's orbit and its position therein, have a more substantial basis. They have been considered by sober authors and have a hold on the minds of intelligent students; and the cycles have a real existence and are capable of producing effects that can be calculated. They comprise a secular variation in the obliquity of the ecliptic; the precession of the equinoxes, with the attendant revolution of the apsides; and an oscillation in the eccentricity of the earth's orbit—all conforming to regular and well-defined periods.
The variation in the obliquity of the ecliptic affects the distance to which the sun departs from the equinoctial at midsummer and midwinter. Its action is to heighten or reduce the contrast between those seasons according as its measure is greater or less. By precession the equinoctial points shift their places backward along the ecliptic, accomplishing a revolution in 21,500 years. It entails the revolution of the apsides, which is equivalent to a displacement in relation to the seasons of the points of the earth's greatest and of its least distance from the sun. By the variation in the eccentricity of the earth's orbit these distances, called the aphelion and perihelion distances, are lengthened and shortened, the difference between them is increased and diminished. and the quantity of heat received from the sun by the earth in different parts of its orbit is supposed to be correspondingly modified. These differences are greatest when the eccentricity is greatest. If with this is combined such a position of the equinoxes that summer in one hemisphere shall correspond with the perihelion and winter with the aphelion, the contrast of the seasons in that hemisphere will be most marked, and we shall have the conditions, according to one theory, for a glacial period.
Such, according to M. Jean Reynaud, was the case in the northern hemisphere about 9500 b. c., when, he thinks, our last glacial period was at its height. From that time the differences were gradually reduced till about 1250 a. d., when they became least, and the northern seasons were mildest and most equable. The differences then began to enlarge again, and we are now advanced a little more than six hundred years toward another glacial period. According to this theory, the seasons were growing milder all through human history till 1250 a. d., and have been tending to become more severe since then.
A question of fact is here presented, evidence respecting which is sought, in the absence of exact observations, in such records as may happen to exist of the character of seasons in the past. M. Arago several years ago collected a considerable list of mentions in the literature and documents of former times of periods of unusual cold, of long or cold winters, unusually hard freezing of rivers, and remarkable heat, drought, or rain, which constitutes our principal source of information on the subject. Parts of this list have been used by M. Jules Peroche and M. Amadée Guillemin to establish opposite conclusions as to the validity of M. Reynaud's hypothesis.
Latin poets furnish some of these data, as when Ovid complains of the inclemency of his place of exile on the Black Sea, in what is now pleasant southern Russia; or Horace and his compeers describe terrible storms in Rome; or Juvenal tells of a Roman lady having to break the ice of the Tiber to wash her face. Cicero and some of the historians speak of the severe climates of Gaul and other outlying provinces, evidently contrasting them with the pleasures of life in Italy. The discomforts experienced by Hannibal in crossing the Alps were what an army from the south would suffer in any age in crossing those mountains in winter, if they were roadless and inhabited by barbarians. To a candid critic, these representations mean nothing on one side or the other, and such is the conclusion which M. Angot has reached after carefully examining the subject.
Of fifty-six instances of extreme winter severity cited by M. Peroche from M. Arago's list, fourteen occurred before the supposed "Great Summer" year, 1250. There seem to be more of them as we approach the nineteenth century, but we have a right to assume that that is because the records are fuller near our times, not necessarily because extremes are growing more frequent or marked. None of the instances, ancient or modern, betoken greater severity than the frosts of 1234—sixteen years before the "Great Summer" year—when "the Po and Rhône were frozen, and loaded wagons crossed the Adriatic on the ice opposite Venice"; 1236, when "the Danube was frozen to the bottom for a considerable time"; or 1305, fifty-five years after it, when "the Rhône and all the rivers of France were frozen." With all the greater completeness and systematic organization of modern observations, the records of the nineteenth century contain no mention of such seasons as those of 1323, 1333, 1349, 1402, and 1407, when the southern part of the Baltic was frozen so hard that men could ride on horseback from Copenhagen to Lubeck and Dantsic.
These occasional winters of exceptional severity can not be taken as typical of the general character of the seasons, any more than we can characterize a winter by an extreme day in January, or a summer by an unusually sultry July day winding up in a thunder-shower. A surer guide to the habitual climate would be afforded by regarding the development of plant growth and the maturing of crops. Of these the vine has been taken as a type. It is said that, cultivated in the time of Julius Cæsar only in the southern parts of Gaul, or France, it was gradually carried northward to the fullest expansion in the thirteenth century, when there were vineyards and wine was made as far north as Flanders and England. Since then it has retired from the most northern points it had reached, where the grape is now ripened under glass. So the cultivation of the olive is said to be falling back toward the south; the sugar-cane has disappeared from Provence, where it once grew; less tender plants are taking the place of the orange in some quarters; and a depression of the zone of forest vegetation is mentioned as taking place in the Alps and the Carpathians.
There are many other causes than climate, as the present operations of agriculture and horticulture amply demonstrate, by which the cultivation of a crop in any place is determined. It may be found after some years of experiment to be unprofitable or of poor quality there; or may be supplanted by new and better varieties growing in more favored localities, or superseded by the introduction of new and more profitable products, which the cultivator is always ready to take up. Such causes have more force now than they ever had before, because of the great increase in the facilities for exchange under which it is no longer necessary to cultivate anything except in the places where it will do best. M. Angot has, moreover, found, by consulting the official records of the times of the opening of the vintages as far back as to the fourteenth century, that there has been no real change. The times have varied in the same places, in different years or series of years, during all this period, as much as two months, but there has been no regular variation, or any of a character to support the hypothesis of a constant, secular movement.
M. Arago undertook, about fifty years ago, to measure the value of these cosmical influences on climate, and declared that they were not competent to produce an effect within the period of historical time worthy to be regarded. He found that the present effect upon the surface of the cooling of the earth's interior, which some were disposed to regard, could be comprised within a thirtieth of a degree. Sir William Thomson makes it still less, and limits it to one seventy-fifth of a degree. M. Arago saw no reason for supposing there were differences of temperature in the parts of space, while, if there were, they would affect all the earth alike and not one hemisphere more than another. The variation in the obliquity of the ecliptic, small in its total at the most, could not cause a change of more than a quarter of a degree in two thousand years.
M. Arago likewise depreciated the importance of the precession of the equinoxes and the variation in the eccentricity of the earth's orbit as climatological factors, because, as he showed, during a period of long eccentricity with summer at the perihelion, while the hemisphere may receive a more intense heat during the summer part of the revolution the excess is balanced by the season's being shorter; but the winter will under those conditions be both colder and longer. Sir John Herschel and M. Reynaud have answered him as to this point by saying that character IS given to the season, not by the absolute quantity of heat received, but by its distribution; not its mean temperature but its maxima and mimima of temperature are to be considered, and the greater or less rapidity of the ascent and descent of thermic movements. A difference of four and a half times in eccentricity, such as is possible, might work great changes in these properties; so that in the case considered by M. Arago "half the annual heat would be concentrated into a summer of very short duration, while the other half would be distributed through a long and gloomy winter, made intolerable by the intensity of the cold, increasing in proportion to the distance of the sun." M. Arago thinks that it would take ten thousand years for variation in eccentricity to effect a change of temperature in the earth measurable by the thermometer. No evidence is produced that it has had any effect within the historical period.
Thus, whatever may be the importance of these astronomical causes in determining the climatic features of geological periods, it is usually agreed that they may be disregarded in accounting for such changes as may have taken place within man's memory. Mr. James Croll, who has discussed this question with considerable fullness, and is inclined to allow them all the force they are entitled to, ascribes less importance to their direct operation than to the secondary effects they induce through their influence on the currents of the ocean and air and upon features of the earth's surface. M. Woeikoff allows them still less agency in the matter than Mr. Croll, and ascribes the greatest influence upon climate to the elevation and configuration of the land, as Mr. Lyell did in the earlier days of geology; and M. A. Blytt, of Christiania, has shown, by pertinent contemporaneous examples, how climate in Scandinavia may be influenced by slight differences in situation, soil, and exposure.
The whole subject has just been reviewed by Sir Robert S. Ball, Astronomer Royal of Ireland, one of the most eminent living mathematicians, in his book on The Cause of an Ice Age. Speaking particularly of Glacial periods, he shows that changes in the intensity of solar radiation, relatively unimportant to the sun, may produce enormous climatic effects on the earth. By an exact calculation he finds that, with the present obliquity of the ecliptic, while the earth as a whole receives equal amounts of heat from the sun during the two halves of the year, the distribution as to a single hemisphere is extremely unequal—a fact which previous writers seem to have overlooked—the exact distribution being sixty-three per cent of the whole amount of heat during the summer and thirty-seven per cent during the winter half. When the line of the equinoxes is perpendicular to the major axis of the earth's orbit and the eccentricity is at its maximum—the conditions establishing the greatest possible difference in the length of the seasons—the sixty-three per cent of heat is distributed over a very short and therefore intense summer, and the thirty-seven per cent over a long and therefore cold winter. The northern hemisphere, when placed in such a condition, will have a summer of one hundred and sixty-six days, during which the sun is at its least possible distance, and a winter of one hundred and ninety-nine days, with the sun at its greatest possible distance. This Prof. Ball regards as a condition favorable to glaciation. The ice and snow will accumulate during the rigors of the long winter, while the succeeding brief summer has not power enough to thaw as much water as has been solidified in the winter, and the ice will grow from year to year. All this time the southern hemisphere would be enjoying a widely different condition. Its summer would contain as great a number of days as it is possible for that season to possess, while the fierce heat of the sun would be abated from its average amount, because the sun would be at the greatest distance from the earth it is possible for it to attain. The winter would be short and warm. The present difference in the length of the seasons is seven days, and the position of the perihelion is such that it is near its maximum for the present eccentricity. The directions in which the precession of the equinoxes and the variation of the obliquity of the ecliptic are tending are for reduction of the inequality, and ice ages are not to be expected from vicissitudes such as are now possible.
It is not denied that climates have been, and are, changing; but the changes are believed to be special, local, temporary, and oscillatory, and most largely determined by causes that may be found on the surface of the earth. M. Arago thought they might all be attributed to agricultural works, to the clearing of woods from plains and mountains, to the drying up of marshes; and he doubted if it could be proved that the climate had become warmer or colder in any place the physical aspect of which had not been perceptibly changed during a series of ages.
The present drift of the opinion of many careful students of the subject seems to be that exaggerated ideas have been held of the extent of climatic variations, both in the present and the past. M. Woeikoif, whose opportunities for studying climatological phenomena over a large extent of territory have not been surpassed, believes that this is so, even when the application is made to the Glacial period; that not intense cold, but those conditions of temperature and moisture most conducive to the precipitation and accumulation of snow, formed the chief factors of its characteristic phenomena. Chief among these were proximity of the sea and a temperature of the surface-water rather below than above the freezing-point. The effect on glacial accumulation of the conditions commonly supposed to correspond with the combination of high eccentricity and an aphelion winter would, in his opinion, be the opposite to what is attributed to it; for the greater cold assumed to prevail in winter would not be conducive to the precipitation of snow, while the more intense heat of midsummer would probably melt the snow at heights where the present temperature rises but little above the melting-point. Hence the conditions in the interior and eastern part of a continent like Asia would be less favorable than they are now to marked glaciation. The western parts of continents and islands would be more fully under the influence of the sea; and as there is no reason to suppose that its surface temperature would be lower than now, it follows that there would not, all other things being equal, be more snow than now in countries where rain is the rule, even in winter. The effect of the combination would be in any case but slight, and not by far, in M. Woeikoff's opinion, to be compared to that of such geographical conditions as the distribution of land and sea, and of mountains and lowlands.
The idea that glaciation was dependent on extreme cold has been rejected by other students. J. de Charpentier recognized the conditions as inconsistent. Lecoq, of Clermont, "affirmed a correlation between a great solar heat, provoking a powerful evaporation, and the formation of glaciers." Tyndall has shown that the ice of the Alps "derives its origin from the heat of the sun," and that if that were diminished their source of supply would be cut off. The thoughts of some other writers, as Le Blanc, Forbes, and Charles Martins, have been turned to showing that the depression of temperature, if there was any, need not have been great.
Another group of writers, whose views have been summarized by M. Millot, of Nancy, hold that warmer climates than now prevail were more favorable to glaciation, and gave character to the Glacial period; and that the present conditions of limited glaciation are the result of the sun's cooling, whereby the supply of evaporated moisture has fallen off. They claim that their theory furnishes the simplest explanation of the presence of warmth-loving plants and animals along with evidences of ice-action. The hot and the glaciated region were so close to one another that the mixture easily took place.
Prof. G. F. Becker, of the United States Geological Survey, has also expressed the opinion (Popular Science Monthly, February, 1884) that the Glacial period was one of higher mean temperature at the sea-level than the present; that while the formation of glaciers may have been affected by all contemporaneous changes, including secular revolutions, it is not necessary to have recourse to such causes; the question is chiefly one of differences between the temperatures at the sea-level and those at the level where the glacier was formed.
M. Blytt, studying the distribution of the Scandinavian fauna, has found it subject to considerable local variations at short distance, which have relation to differences in conditions of exposure and the character of the soil. He concludes that no great changes, but only small variations in the extremes of temperature and rainfall, are required to explain these departures. Such variations may be produced, for his country, by fluctuations in the direction, force, and temperature of ocean currents and winds that need in no case be great; but he believes that these variations are coincident with periodical changes of climate corresponding with secular incidents.
The considerable effects of exposure on local climates are exemplified at the winter seaside resorts on the south coast of England, where certain spots enjoying conditions of shelter from cold winds, combined with exposures favoring the concentration of the sun's rays and the warm winds upon them, enjoy a springlike mildness through much of the winter. Prof. W. Mattieu Williams speaks of Torbay, Torquay, Broadstairs, and Hastings as possessing these characteristics. A considerable difference has been noticed in the winter temperatures of places east and west of a certain point on the coast, though all are nearly in the same latitude.
Dr. D. Hart Merriam has described a succession of temperature zones in descending from the plateau level to the bottom of the Colorado Cañon equivalent to those stretching from the coniferous forests of northern Canada to the cactus plains of Mexico, with marked variations of climatic conditions under apparently very slight diversities of exposure.
A variation of only 5·3° Fahr. in the mean annual temperature at Uskfield, England, is shown by Mr. C. Leeson Prince to be sufficient to exert an enormous influence on the general character of the seasons, the produce of the soil, and the health of the population.
The fact of changes in climate being admitted, discussion turns upon their extent, and the laws by which they are governed. In many cases they are brought about by changes in local conditions, of which the removal or replacement of forests, or the relations of land and water, are among the most important. In other cases a periodical law is supposed. The attempt has been made by some meteorologists and astronomers to show that there is a connection between such changes and an eleven-year period of abundance and scarcity of sun-spots. It was believed by an observer in Ceylon in 1872 that that island was on the eve of an important change of climate depending upon a cycle of thirty or thirty-five years. The previous thirty years, he asserted, had shown a complete contrast to the thirty years preceding them, with manifestly different effects on animal and vegetable life. It had been a period of relatively lighter rainfall, and the next cycle of thirty years was expected to be, above the average, wet. This theory of changes by thirty or thirty-six years is often met in following the discussions on this subject. A paper published recently in the Archives des Sciences Physiques et Naturelles deduced from a total of twenty thousand years of observations, at about five hundred stations, that the climates of all the continents, excepting only a few maritime coast regions, were subject to simultaneous variations, which became more and more pronounced toward the interior regions. The years 1815, 1850, and 1881 came about the middle of relatively wet periods, and 1830 and 1860 of dry periods. The mean period of the oscillations was deduced from records of vintages, going back to the year 1400, to be thirty-six years. The changes appeared to be dependent on certain relations of atmospheric pressure, the wet periods being characterized by lesser differences, and the dry periods by increased differences, in that factor. The theory of a period of thirty-five or thirty-six years is fully elaborated by Prof. E. Brückner, of the University of Basle, in his book Klimaschwankungen (Vienna and Olmutz, 1890). Approaching the question from nearly every conceivable point of view—of temperature, precipitation, atmospheric pressure, the rise and fall and freezing and thawing of rivers, vintages and harvests—he is led to the same conclusion in every case. The period is nearly equivalent to three of the supposed eleven year sun-spot periods. Herr G. Hellman has counted thirty-four seasons since 1755 when December and January in Berlin were warmer than the average; but the warm seasons came at irregular intervals, and did not suggest any law.
Dr. W. Koppen, of Hamburg, records, as the outcome of an investigation which he made of the periodicity of weather changes, "that for certain intervals strongly marked periodical influences make their appearance and then vanish entirely, at times being replaced by others of a totally different character. No law has, as yet, been discovered for these changes."
The presence of forests has not been shown to contribute directly to the increase of rainfall, nor their removal to diminish it. Yet their influence on climate must be considerable. This is confessed when the farmer on the prairies plants belts of trees between his fields and the quarters from which cold winds and destructive storms are expected. They stand like a wall to protect the localities they overhang against sudden extremes of temperature and other accidents of violent weather. Although they may not increase the amount of precipitation to a perceptible extent, they, by means of their matted roots and the undergrowth which they promote, and by their beneficent shade, convert the ground on which they stand into a kind of reservoir, and husband the moisture which, without them, would run off or dry up at once. Thus they contribute to prevent sudden floods in the wet season, and, permitting a slow exudation of moisture into the streams, to keep them lively and the rivers to which they are tributary full during dry seasons. Many persons believe, too, that they diffuse a coolness and vaporous moisture in the atmosphere, the presence and influence of which, although they are not manifested in rain, are nevertheless real. Whether they may not exert an influence on the distribution of rain through the seasons, as they certainly do on that of ground moisture, does not appear to have been yet adequately investigated.
In connection with the influence upon climate of the relations of land and water, the speculations respecting the probable effect upon the climate of Europe of flooding the Desert of Sahara deserve to be noticed. It has usually been taken for granted that a cooler condition would follow. But Prof. Hennessey argued several years ago that, as vapor, rather than dry air, is the chief vehicle of wind-borne heat, the result would be the opposite of this. While the midday heats of the desert are intense, the nights are cold. Hence a uniformly warm breeze can not come from there. The warm southwest winds of central and southern Europe have been found to be connected with the currents of the Atlantic, and not to come from the desert. The substitution of water for barren sands and rocks would be followed by the storing up of the heat of the sun which is now partly dissipated by radiation at night, and would furnish a source of constant warmth.