CHAPTER III
THE CLIMATIC IMPORTANCE OF WATER VAPOUR
When Aristotle, for two thousand years our leading savant in cosmography, about twenty-three centuries ago stated the foundation of his natural science, he laid down as the important principles: moisture and heat and their opposites; because the four elements out of which everything was made, were: earth, characterized by dryness and cold; water, which was moist and cold; air, which combined moisture with heat; and lastly fire, which stood for dryness and heat. Undoubtedly he was considering the requisites of life, which may be designated as humidity and heat. We have seemingly agreed that all life originates in the sea, so that moisture can be considered the first requirement for its appearance on the earth. As to heat, life is destroyed by frost and favoured by increased warmth, at least to a certain point, about 35° to 40° C. (95° to 104° Fahrenheit), which temperature is most propitious to the development of life, while a further increase is detrimental, so that already below the boiling point of water life suffers more harm than at temperatures below freezing. In fact, the geologists have found that the different epochs in earth’s evolution are best characterized by their humidity or dryness. To arrive at a clear conception in these matters we shall briefly survey our present knowledge of the importance for the evolution of life on earth that we should attach to the humid and to the dry periods or localities.
We are all familiar with the heavy, moisture-laden warmth which meets us when we enter a hothouse. It is particularly favourable to the growth of plants and to the prosperity of the lower animals. To the higher animals and to man, the humid heat is not so beneficial. In the open such hothouse air exists only in the tropics. Particularly the Congo region and the parts of Brazil adjoining the Amazon River are remarkable for their humid heat and for their fabulously luxuriant vegetation. From our greatest living climatologist, Julius Hann, I have borrowed the following description of such a clime:
“The changes of temperature between the coldest and the hottest month are very small in the Congo, from .5° to 5° C. (.9° to 9° Fahrenheit), with an average of about 3.5° C. (6.3° F.). The difference between day and night reaches nearly thrice this value, or 9.5° C. (17.1° F.). The dry season becomes shorter the more we approach the equator and in Equatorville and Bangala it shrinks to nothing. During the rainless months, a dense humid fog settles morning and evening over the savannahs. Low hanging clouds of uniform thickness frequently hide the Sun for weeks at a time. It is during the rainy season alone that we see a clear sky between the showers. This season opens and closes with magnificent thunderstorms coming from the east. In Luluaburg, lightning occurs during not less than 106 days in the year. In the dry season the wind carries with it clouds of dust which falls to the ground. The cloudiness is enormous in the Congo basin, so that there are veritably no months with a clear sky in this part of the world. In Vivi, the number of overcast days averages 74 per cent., fluctuating between 63 per cent. in August and 83 per cent. in November. The humidity is very high, varying in Vivi from 70 to 79 per cent., with a mean value of 75 per cent., and in Bolebo the mean itself reaches 79 per cent. During the rainy season, the heat is sometimes unbearably oppressive; suffocating fumes rise from vegetable matter which rapidly decays in the excessive humidity. The annual precipitation does not reach very startling figures; it varies between 120 and 180 cm. (47 to 71 inches). In Gabun, close by, the sky is almost continuously covered with clouds during the dry season.
“Corresponding regions in South America are in parts characterized by an even higher humidity. In Iquitos by the Amazon River, it reaches not less than 83 per cent. of saturation. The annual change of temperature is only about 5° C. (9° F.); in Para (1.08° south latitude on the coast) it shrinks to 1° or 1.5° C. (1.8° to 2.7° F.). In the course of twenty-four hours the variation is considerably larger. The sky is remarkably clear between showers during the rainy season. In the interior of Guiana, the rains continue from the end of April well into July or even into August. Abundant dew is common during the rainless part of the year, thus maintaining the humidity. Sun and Moon are rarely visible, and gigantic lightning storms announce the arrival of the rainy period.”
Similar conditions apparently prevailed during the carboniferous period, which was characterized by a luxuriant vegetation. The mighty tree-trunks of that time fell into the water-covered marshes out of which they had grown and their decay was thereby prevented. Instead they turned into coal like the peat in the mosses of today. This was for some time thought to indicate that the temperature was not particularly high--Frech estimated about 12° C. (53.6° F.) (1910). But since the discovery and subsequent description by Keilhack (1914) of peat-beds on Ceylon, where the average yearly temperature is 26° C. (78.8° F.), a return is to be expected to the older conception, which held that the vegetation during the carboniferous period is evidence of a very warm climate. Judging by the appearance of fossil plants, the temperature should have been nearly the same all over the globe. Carthaus remarks that the air was stirred by only feeble winds because the trees of that age with their great dimensions but frail root-systems could not have withstood a fresh breeze. The sky was hidden behind a continuous thick cover of clouds which only let a faint light sift through to the ground. The motionless air was almost saturated with moisture. The luxuriance of the vegetation, transcending anything existing today, indicates a favourable high percentage of carbonic acid in the air. This combined with the humidity and the dense clouds caused the heat radiation from the Sun to be almost entirely absorbed by the upper strata of the atmosphere in which thereby a strong circulation was maintained. As a result, the heat was nearly equalized between the poles and the equator and under the cloud cover an almost constant temperature reigned day and night, summer and winter. The damp air stood wellnigh still and was filled with dense fog at the smallest changes in temperature. Lack of light prevented the development of flowers, and the thriving plants belonged mainly to the ferns and to the horsetails. Pine and fir trees were yet comparatively few. The conditions in the swampy regions where plant life flourished were nearly identical with those in a hothouse if we were to draw a dense veil in front of the windows in walls and ceiling so that a continuous twilight would prevail.
In this uniform climate, plant life developed enormously faster than animal life. The dense clouds could store considerable quantities of heat in the equatorial belt through evaporation in their upper layers and the violent wind storms above the clouds would carry the aqueous vapours to colder regions where the heat would be liberated through new cloud formations. Currents in the oceans now largely attend to this heat transportation and give for instance to the coast of Norway, and indeed to the whole of Western Europe, its remarkably mild, and to life and civilization, propitious climate, but in the carboniferous age humid air currents fulfilled the same task. They moved considerably faster and more evenly than the ocean currents, were not checked or deflected by coasts or islands, and could therefore produce the extraordinarily uniform temperature and the marine climate all over the globe. Such a heat distribution takes place also in our days at a height of about 10,000 m. (6.2 miles) in the so-called “stratosphere,” but the temperature here is very low, about -60° C. (-76° F.), so that the vapour suspended is hardly worth mentioning, and cannot give rise to cloud formations. The quantities of heat carried in these higher strata of the atmosphere are too insignificant to influence the masses of air below, whose temperature, therefore, is almost entirely governed by that of the sun-heated surface of the earth, except where the ocean currents equalize matters, as for instance in the almost wholly water-covered latitudes south of the 30th parallel on the Southern Hemisphere. Even during the carboniferous period at its height, there existed, of course, a temperature difference between pole and equator, but it was very small, some 10° C. (18° F.) perhaps. Undoubtedly, the formation of coal beds was mainly confined to those regions where the climate was most uniform all the year around.
The opposite extremity, the dry desert climate, is far more pronounced in the present time. This condition is well known in all continents except Europe, where we hardly can claim a desert but instead have steppes, with a vegetation abundant after the spring rains but fast disappearing with the arrival of the burning summer heat. A particular type of plant life has adapted itself to this periodic change from rain to drought, from bitter cold during the winter to parching sun during the summer. Perennial plants, and particularly trees, can rarely endure the rigors of such climatic upheavals. Animal life on the other hand has proved fairly adaptable and displays considerable wealth.
This steppe climate is only an intermediate stage towards the desert climate proper, which is hostile to all life. Its temperature is subject to enormous changes in the course of the day and the year. The annual variation is less pronounced near the equator and the daily variation less on the approach to the poles, on account of the small changes in the sun’s radiation during corresponding periods. The difference between day and night in Sahara is frequently 30° to 40° C. (50° to 70° F.). The lowest temperature observed by Foureau-Lamy, 1898–1899, was -20° C. (-4° F.) or nearly the same as on the Scandinavian coasts. The highest amounted to 48° C. (118.4° F.) or a total variation of nearly 70° C. (126° F.). In Upper Egypt (21.9° N. Lat.) the mean temperature changed from 16.3° C. (61.3° F.) in January to 34.1° C. (93.2° F.) in July, and nearer the equator in Central Africa (8.1° N. Lat., 23.6° E. Long.) the difference amounted to only 6.9° C. (44.6° F.), 22.7° C. (72.5° F.) in December, 29.6° C. (85.1° F.) in April, while in Kiachta (50.4° N. Lat., 106.5° E. Long.) in Siberia, the yearly change reaches 45° C. (81° F.), -26.6° C. (-15.7° F.) in January, 19.1° C. (66.2° F.) in July. The average daily variation at continental stations is about 12° C. (21.6° F.). All this refers to the temperature of the air, while the surface temperature in the course of twenty-four hours may change 50° C. (90° F.) and in the desert even more. Frost occurs in the Sahara as late as May when the maximum temperature may reach 50° C. (122° F.). While in Scandinavia the diurnal difference between highest and lowest temperature averages only 6° to 7° C. (11° to 13° F.), a maximum in July of 10.4° C. (18.7° F.) and a minimum in November of 4° C. (7.2° F.), Hedin on his journey in Tibet, 1899–1902, observed a daily variation of 19° C. (34.2° F.) and no appreciable difference with change in altitude.
The result of such a violent temperature change in the course of a day is a breaking up of the rocks which subsequently and gradually are ground to fine dust by unobstructed winds wherever vegetation does not bind the soil. In this manner the sand deserts are formed. The arid wastes of Asia have lately been vividly described by Sven Hedin. The mountains eroded by the sandstorms resemble dilapidated ruins, standing as monuments of an ancient highland. The sand in East Turkestan is reduced to such a fine powder that it can float in the air for several days after a storm, revealing itself in gorgeous sunsets. Winds sweep the sand into long dunes, which shift in the direction of the blast. It is ferruginous and therefore red or if powderized reddish-yellow. When moistened it assumes a brown to black shade. After rain, the water descends toward the valley, carrying with it the sand in the form of silt. This, through evaporation, is transformed into a plastic black dough, slides like a glacier slowly down the hillsides, and finally comes to rest in some broad hollow which it gradually fills. Such a silt aggregation is called in Persia a “Kevir.” Its surface dries, but the interior remains moist. As evaporation continues it becomes richer in salt so that white crusts of this substance are formed during dry periods. In other districts, as in the basin of the Tarim River, the water occasionally appears in the lowest parts, the so-called “Bayirs” (see Fig. 9), formations similar to the Kevirs, or in salt lakes between the sand dunes. Sand carried by the winds quickly fills these lakes so that they too move in the direction of prevailing winds. They lie with their longest dimensions parallel to each other and at right angle to the course of the Tarim River. The sketch map taken from Hedin’s work shows the Bayirs strung out in line with the lakes somewhat like panels in a tapestry pattern. This reticulation of the landscape is the result of the dune formations. The main dunes with steep western slopes run in the direction N.N.E.-S.S.W. They stand at right angle to the prevailing winds. Nearly perpendicular to their crest lines dunes of smaller height are thrown up by winds in another common direction but less frequent than those which raised the fundamental dunes. This system brings to mind the cloud formations called mackerel sky, clouds rippled in two directions frequently almost at right angle to each other. They owe their peculiarity to two series of wave motions propelled by winds from two different directions in the upper strata of the atmosphere. The cloud patches correspond to the wavecrests on a surging sea. The map of the Bayirs suggests a chessboard with squares somewhat elongated and irregular.
We may now return to a closer study of the largest of these formations, the great Kevir in Persia. This mud-lake, with a dry surface, measures 500 km. (310 miles) in length and 200 km. (124 miles) in width over its largest dimensions. Hedin estimates its area at 55,000 sq. km. (21,142 sq. miles) or the same as that of the great Lake Michigan on the boundary between the United States and Canada. Due to the continuous growth of the salt proportion through the inflow of Kevirs and through superficial evaporation a salt crust of varying thickness is formed near the surface of the lake. Hedin caused a hole to be cut with an iron bar. He first encountered a 10 cm. (3.9 in.) deep covering of clayey paste, and then the salt crust about 7 cm. (2.8 in.) thick resting on a semi-dry layer of clay with a depth of 15 cm. (5.9 in.). Farther below, softer strata of clay followed, becoming more watery the deeper he went. The iron bar carelessly wielded would have disappeared in the mire. Another investigator, Buhse, examined a piece of the crust, which when dry is fairly solid and of a yellowish-grey colour. One half consisted of sand (probably quartz-sand), one sixth of limestone, 6.1 per cent. oxide of iron (causing the yellow colour), 5.3 per cent. common salt, 2.5 per cent. sulphate of sodium, and 2.1 per cent. clay. Rain converts this surface layer into a plastic mass, which persistently sticks to the clothes of the traveller or to the bodies of the camels if they should slip and fall into the mud. Not the slightest trace of vegetation or of any life exists. On the shore of the mud-lake small flat elevations and depressions may be found; otherwise, the surface is as level as that of an ordinary lake.
The Kevir battles with the drifting sand as does the water in East Turkestan. The sand appears to gain in the contest. After storms, vast portions of the Kevir are covered with yellow desert sand. “If the climatic change in Persia continues in the present direction,” says Hedin, who, however, is dealing with large spans of time inasmuch as in his opinion no appreciable alterations have taken place since the invasions of Alexander the Great, “then it may be taken for granted that the slough of the Kevir will lose in moisture and afflux of water and in time will become more solid and that the drift sand with greater ease will gain foothold and territory. The final outcome of the physico-geographical transformation now in progress will no doubt be to convert the entire Kevir into a sand desert of the kind predominating in East Turkestan. And conversely we may infer that East Turkestan, once a part of the central Asiatic Mediterranean sea, in the course of time was filled with the finely divided products of disintegration, such as we now find in the Kevir, and further that its expanse of watery mud and clay finally dried and hardened to such an extent that it could support the load of the encroaching sand. That the extension of the sand formerly was smaller, is also borne out by the archæological discoveries in East Turkestan which several travellers besides myself have made. The hardpan laid bare in the ‘Bayirs’ of the Cherchen desert strongly reminds of the Kevir-ground. In both cases the same dark, fine powder forming a nearly plane surface. In both cases this material when mixed with water is transformed into a slough in which one hopelessly sinks, but in East Turkestan the water has receded to a greater depth and as rains are extremely rare travel all over the smooth ‘Bayir’-ground may be undertaken with impunity.”
These formations are of the greatest interest because they picture the changes taking place on a slowly desiccating planet. In 1858 the Geographical Society in Petrograd despatched an expedition under the command of Khanikoff to visit these regions. From Hedin’s work, _Overland to India_, from which the preceding quotation is taken, we borrow the following vivid description by Khanikoff: “At last, in the morning of April fourth, and during the most oppressive heat, we reached Bala-haus. At this place remnants of a ruined reservoir, long since bereft of its water, could be seen. The desert had here assumed the perfect character of the ‘the accursed land,’ which name it bears among the natives. Not the smallest tuft of grass, not a sign of animal life gladdened the eye, not a sound interrupted the deathlike, awful silence but that of the marching caravan.
“Because of the slow procession of the camels and the delay suffered when we lost our way, we only covered 25 km. (15.5 miles) in the night stage. After four hours’ rest we resumed our march and directed our steps toward the hills, called Kellehper and situated 20 km. (12.5 miles) from Bala-haus; they were plainly visible but positively seemed to take flight on our approach. I was somewhat ahead of the caravan and sat down at the foot of this sand-elevation; and never can I describe the feeling of weariness and of depression that I was unable to ward off as I looked into the ghastly solitude that engulfed me. Scattered clouds intercepted the rays of the sun, but the air was hot and heavy; the diffused light lent a monotonous and disconsolating hue to the greyish, burning surface of the desert; hardly a single variation of colour gave relief to the immense expanses that the vision embraced. The absolute immobility of every point in this mournful landscape, combined with the complete absence of any sound, produced an overwhelming depression of the spirit; one felt as set upon a place that had been struck inanimate for ever, a place to which organic life would never return safe through some terrible catastrophe of nature. One witnessed the beginning, so to speak, of the death-agony of our planet.”
If a drying out has taken place in these regions,--which seems probable from Hedin’s observation that the water in a Tibet lake, Lakker-tso, formerly reached a 133 m. (435 ft.) higher level than at present,--such a process is nevertheless not so obvious here as in the salt inland lakes, for instance the Great Salt Lake in Utah, the Dead Sea, and the Caspian Sea, where the saltness has greatly increased due to evaporation. Concerning the Great Salt Lake we know that even at a comparatively late time it had a much wider extension than now. Its water contains 22 per cent. common salt besides other compounds. The Dead Sea holds 25 per cent. salt. A very variable percentage is to be found in the Caspian Sea. Near the mouth of the Volga it is of course low, only 0.15, and increases southward to 1.32 at the peninsula Apsheron and to 5.63 in the bay of Kaidak. In the gulf of Karaboghaz on the Asiatic side it reaches 28.5 per cent. It has been computed that this gulf receives annually from the inrushing waters of the Caspian Sea 350,000 tons of salt which is partly deposited on its shores and bottom.
This desiccation, however, is a mere trifle when contrasted with the process by which the mighty salt deposits in Germany were formed. It took place, we believe, in a shallow bay extending southward from the Arctic Sea. As the salts, first gypsum, then common salt, and lastly more soluble potassium and magnesium compounds, crystallized by degrees new water masses entered from the sea. At the same time, the bottom of the gulf slowly receded, giving room for fresh evaporating floods. The salt layers deposited in this manner sometimes reach a depth of more than 1000 m. (about one half a mile). We can thus gain a conception of the immense quantities of water evaporated and the enormous time required therefor. The deposits would long ago have been carried away from their original place were it not for the fact that they finally were covered with a layer of slime nearly impervious to water. The most soluble salts, such as chloride of magnesium, have nevertheless been extracted to a great extent.
The extremes of aridity or humidity have of course not occurred during the brief time known to history. Of a special interest is the question in what direction the climate at present is tending. In this connection, Huntington has aroused great attention by propounding the theory that the earth is now in a period of rapid desiccation.
Judging by the testimony of geology, it seems beyond doubt that an age of humidity prevailed simultaneously with the ice-period in northern Europe, over several parts of the globe, in fact everywhere as far as we know except in Australia. This is clearly borne out by the higher levels of the lakes and their consequent greater extension in earlier days. So far as Tibet and Central Asia are concerned we have already mentioned this fact. But in America and Africa the humid period was even more patent. The Great Salt Lake has covered an area many times greater than at present as testify the picturesque terraces in its surroundings (compare Fig. 10). According to the researches by Passage, this period was also strongly pronounced in Africa. A large fresh-water body occupied the Congo basin, the Tsad lake had a far greater expansion than now, and mighty rivers traversed Sahara.
It is often assumed that the climate of Africa has been more humid even in historic times. The geographer Leo Berg in Petrograd, however, is emphatically opposed to this theory. He points out that the writers of antiquity, Diodoros, Polybios, and Pausanias, have given descriptions of the rivers on the North African coast which nearly agree with conditions today. The location of two ancient cities on the shores of Lake Chott-el-Djerid in Tunis (Lacus Tritonis of old), which lake, it is claimed, reached a level very much higher than now 500 years B.C., plainly demonstrates that the shore-line then ran very close to its present position. Students of ancient Egypt are unable to find evidence of any distinct difference in the climate of that country from the earliest times to our days. It is true that marshes in the Nile delta have changed into splendid meadows--but this is the work of man. The humid period must have ended long before history commenced. A few of the old writers, such as Herodotos, Aristophanes, and Philo, assert that rain never falls in Egypt, but this must be classed as an exaggeration when contrasted with references to rain, snow, and hail in this section made by other ancient authors, for instance Plutarch, Pliny, and Ælianus. At any rate it seems as if precipitation was as rare an occurrence in the land of the Pharaohs as in the land of the Nile of today.
Against the statement by Huntington that the climate of Palestine has become very much more torrid during historic times, stands the assertion by Hilderscheid, who has made a thorough study of these questions, that no reason whatever exists for such a conclusion.
Of greatest interest to us in this connection are perhaps Italy and Greece. Huntington believes that the river Alpheios, which flooded Olympia and covered it with a 4 m. to 5 m. (4 or 5 yards) thick sediment, carried a far greater volume of water formerly than today. This flood, however, was caused by an earthquake accompanied by a fall of rocks whereby the river was dammed. There is no necessity for assuming a greater abundance of the waterflow. According to Strabo, the streams Kefissos and Ilissos, between which Athens is situated, dried out during the summer then as they do now. If we are to believe Pausanias, the brooks which traversed the Argive plain behaved similarly, and so they behave today. From all we can judge, the climate of Greece has not changed perceptibly since the days of Homer.
In regard to Sicily, it is stated that several of its rivers were navigable in the Middle Age, while such is not the case now. But this is explained by the devastation of the forests which formerly equalized the flow of these rivers, and perhaps also by the size of the vessels of that time. Cultivation in these regions has sharply declined since antiquity. As a consequence, the loose soil, which formerly was planted, has been washed away and the dams and retaining walls, built to prevent a too rapid drain of the water, have disappeared. Thus the country has grown increasingly arid. Large cities, as Palmyra, existed in desert regions where lack of water now prevents habitation. But water was brought to the metropolitan cities of old through long magnificent aqueducts, the ruins of which partly remain today. We have all reasons to believe that the marked decrease in cultivation and population, laid to changes in conditions of humidity, depended altogether on man’s interference with nature. On certain rocks in Morocco, carvings have been found portraying in a simple way large mammals such as elephants, rhinoceroses, and giraffes, which do not exist in these regions now because of lack of nourishment. But these rough works of art, resembling those of the bushmen of today, date from prehistoric time, the so-called paleolithic era, when the climate admittedly was more humid in these regions than it is now.
Similar conditions obtain, according to Hedin, in Central Asia and in Persia. The climate there has without doubt been more humid but not in historic times. The march of Alexander toward India took place under as adverse conditions as now are found in these regions (Baluchistan). Their cities, now in ruins, received their water supply through conduits from rivers some of which were then adjacent to the cities, although they later have changed their course as pointed out by Leo Berg.
In Western and Central Europe numerous marshes and morasses have indeed been drained and rendered available for cultivation, but this does not prove that the climate is become more dry. On the contrary all observations, for instance those made by Tycho Brahe on the island Hven, indicate that the difference between summer and winter temperature has decreased during historic time; that is the climate is become less continental, or more humid, than formerly. Furthermore, many circumstances, such as the occurrence of hazel and of water chestnut in far more northerly latitudes and the higher altitude of the timber line in earlier time, prove that the summer in prehistoric ages was warmer than now. Simultaneously it was dryer. A study of the lacustrine pile dwellings in Switzerland shows that the levels of the lakes then were not higher than now but very nearly the same, which demonstrates that the precipitation has not changed perceptibly in Switzerland since these buildings were made; the period in question occurred, we believe, about 7000 years ago.
While great climatic changes have taken place since man’s first appearance on earth, presumably before the end of the ice period, historic time is too short to record any distinct modifications. Local ones may be in evidence such as West Europe’s transition to a less continental climate. A variation of this nature has been found not longer back than since thermometrical observations commenced. Thus the winters in Berlin during the period 1746–1847 were colder and the summers warmer than during 1848–1907. The difference for January amounted to -1.5° C. (-2.7° F.) and for May to +0.6° C. (+1.08° F.). The tabulation below, quoted from Ekholm, shows the mean temperature in Stockholm, Lund, London, and Paris, during winter (December-February), spring (March-May), summer (June-August), and autumn (September-November) and for the following periods:
-------+-------------------+------------------ | _Stockholm_ | _Lund_ |-------------------+------------------ | _1799– | _1849– | _1753– | _1799– | 1848_ | 1898_ | 1798_ | 1898_ -------+---------+---------+---------+-------- Winter |+25.5° F.|+26.8° F.|+30.2° F.|30.9° F. | -3.6° C.| -2.9° C.| -1.0° C.|-0.6° C. -------+---------+---------+---------+-------- Spring | 37.9° F.| 37.9° F.| 41.2° F.|41.5° F. | 3.3° C.| 3.3° C.| 5.1° C.| 5.3° C. -------+---------+---------+---------+-------- Summer | 60° F.| 60° F.| 61° F.|60.2° F. | 15.6° C.| 15.6° C.| 16.1° C.|15.7° C. -------+---------+---------+---------+-------- Autumn | 43.9° F.| 43.5° F.| 45.9° F.|45.9° F. | 6.6° C.| 6.4° C.| 7.7° C.| 7.7° C. -------+---------+---------+---------+-------- Year | 41.9° F.| 42.1° F.| 44.6° F.|44.6° F. | 5.5° C.| 5.6° C.| 7.0° C.| 7.0° C. -------+---------+---------+---------+--------
-------+-----------------+----------------- | _London_ | _Paris_ |-----------------+----------------- | _1799– | _1849– | _1806– | _1849- | 1848_ | 1898_ | 1848_ | 1898_ -------+--------+--------+--------+-------- Winter |38.5° F.|39.2° F.|37.9° F.|37.9° F. | 3.6° C.| 4.0° C.| 3.3° C.| 3.3° C. -------+--------+--------+--------+-------- Spring |48.2° F.| 48° F.|50.5° F.|50.3° F. | 9.0° C.| 8.9° C.|10.3° C.|10.2° C. -------+--------+--------+--------+-------- Summer |61.9° F.|62.2° F.|64.6° F.|64.8° F. |16.6° C.|16.8° C.|18.1° C.|18.2° C. -------+--------+--------+--------+-------- Autumn |50.7° F.|50.5° F.|54.2° F.|51.8° F. |10.4° C.|10.3° C.|11.3° C.|11.0° C. -------+--------+--------+--------+-------- Year | 9.8° F.| 50° F.|51.3° F.|51.3° F. | 9.9° C.|10.0° C.|10.7° C.|10.7° C. -------+--------+--------+--------+--------
The difference is not great. For Stockholm the winter has grown warmer, the autumn colder, for London the winter warmer and so slightly also the summer, but spring and autumn a trifle colder, and for Paris the summer a little warmer while the autumn is considerably colder. Lund shows the least variation. The winter has grown 0.4° C. (0.72° F.) warmer and the summer colder by the same amount. The annual mean remains nearly constant, only slightly increased, but the climate is become more marine. (This is hardly apparent from the figures cited as far as Paris is concerned.)
From Tycho Brahe’s observations of the number of days when snow or rain fell in the place where his observatory was situated on the island Hven in Öresund not far from Copenhagen, Ekholm has calculated that the temperature there during the time 1582–1597 was 1.4° C. (2.5° F.) lower in February and 1° C. (1.8° F.) lower in March than in later years (1881–1896). On the other hand the first autumn frost occurred at the same time as now and the same was the case with the last frost in spring, so that the temperatures on these dates in autumn and spring were nearly identical at the end of the sixteenth century and now. Ekholm drew the conclusion that the climate is become more marine.
Hildebrandson makes the objection that Tycho Brahe’s observations were confined to an abnormally cold period judging by the tables Speerschneider has prepared showing the ice formation in Danish navigable waters. Nine out of the sixteen years in which Tycho Brahe gathered his data were notable for abnormally cold winters, while only nineteen out of the hundred years composing the sixteenth century were characterized by equally severe winters.
Thus, the conclusion is not warranted that the winters of the sixteenth century as a whole averaged colder than those of the nineteenth. Later investigations (in 1917) in regard to the dates when the ice would break up in Lake Mälar at Vesterås, in Neva River at Petrograd, and in Dwina River at Riga, have led Ekholm to believe that he has found a periodicity in winter temperatures of not less than 212 years, a conclusion which would agree with Speerschneider’s statistics. If so, we are at present living in a period remarkable for its mild winters while a series of extremely severe winter seasons occurred at the time of Tycho Brahe. This law would also have a bearing on the preceding table of temperatures in Stockholm, Lund, London, and Paris as a succession of severe winters came around in the beginning of the nineteenth century while the reverse is true towards its end. On the whole climatic variations during historic time have been insignificant, if present at all, provided we extend our comparisons over two or more centuries. Such is also the opinion of Hildebrandson.
The idea of a slow deterioration of the climate due to increasing desiccation is of old lineage and is most likely related to the venerable conception of a bygone golden age. Aristotle even at that early date believed that a gradual arefaction of the Earth took place. In recent times this faith has been particularly fostered by Huntington in a number of treatises where he endeavours to prove that Asia, represented for instance by Palestine, Syria, and Persia, and further Africa and North America are subject to a rapid exsiccation clearly traceable through historic time. The contrary, however, is true about Western Europe. It was often said that Southern Russia in recent times suffered a slow arefaction manifesting itself in the formation of steppes. This led to careful investigations showing the assertion to be erroneous and culminating in the work of Leo Berg. Rather, a slight shifting in the opposite direction is detectable as the forest region has expanded at the expense of the steppes in conformity with the development toward the end of prehistoric time. The renowned American astronomer, the late Dr. Lowell, embraced the idea of increasing aridity which he observed himself in Arizona where his observatory is located. The drying out of Arizona undoubtedly took place in long bygone prehistoric time. The disappearance of a high culture in Syria and Mesopotamia was the result of hostile devastation of their waterworks; a compensation is now offered in the reclamation of the deserts along the Nile, in California and Arizona, and in numerous other places.