CHAPTER V

THE CLIMATE OF HISTORY[16]

We are now prepared to consider the climate of the past. The first period to claim attention is the few thousand years covered by written history. Strangely enough, the conditions during this time are known with less accuracy than are those of geological periods hundreds of times more remote. Yet if pronounced changes have occurred since the days of the ancient Babylonians and since the last of the post-glacial stages, they are of great importance not only because of their possible historic effects, but because they bridge the gap between the little variations of climate which are observable during a single lifetime and the great changes known as glacial epochs. Only by bridging the gap can we determine whether there is any genetic relation between the great changes and the small. A full discussion of the climate of historic times is not here advisable, for it has been considered in detail in numerous other publications.[17] Our most profitable course would seem to be to consider first the general trend of opinion and then to take up the chief objections to each of the main hypotheses.

In the hot debate over this problem during recent decades the ideas of geographers seem to have gone through much the same metamorphosis as have those of geologists in regard to the climate of far earlier times.

As every geologist well knows, at the dawn of geology people believed in climatic uniformity--that is, it was supposed that since the completion of an original creative act there had been no important changes. This view quickly disappeared and was superseded by the hypothesis of progressive cooling and drying, an hypothesis which had much to do with the development of the nebular hypothesis, and which has in turn been greatly strengthened by that hypothesis. The discovery of evidence of widespread continental glaciation, however, necessitated a modification of this view, and succeeding years have brought to light a constantly increasing number of glacial, or at least cool, periods distributed throughout almost the whole of geological time. Moreover, each year, almost, brings new evidence of the great complexity of glacial periods, epochs, and stages. Thus, for many decades, geologists have more and more been led to believe that in spite of surprising uniformity, when viewed in comparison with the cosmic possibilities, the climate of the past has been highly unstable from the viewpoint of organic evolution, and its changes have been of all degrees of intensity.

Geographers have lately been debating the reality of historic changes of climate in the same way in which geologists debated the reality of glacial epochs and stages. Several hypotheses present themselves but these may all be grouped under three headings; namely, the hypotheses of (1) progressive desiccation, (2) climatic uniformity, and (3) pulsations. The hypothesis of progressive desiccation has been widely advocated. In many of the drier portions of the world, especially between 30° and 40° from the equator, and preëminently in western and central Asia and in the southwestern United States, almost innumerable facts seem to indicate that two or three thousand years ago the climate was distinctly moister than at present. The evidence includes old lake strands, the traces of desiccated springs, roads in places now too dry for caravans, other roads which make detours around dry lake beds where no lakes now exist, and fragments of dead forests extending over hundreds of square miles where trees cannot now grow for lack of water. Still stronger evidence is furnished by ancient ruins, hundreds of which are located in places which are now so dry that only the merest fraction of the former inhabitants could find water. The ruins of Palmyra, in the Syrian Desert, show that it must once have been a city like modern Damascus, with one or two hundred thousand inhabitants, but its water supply now suffices for only one or two thousand. All attempts to increase the water supply have had only a slight effect and the water is notoriously sulphurous, whereas in the former days, when it was abundant, it was renowned for its excellence. Hundreds of pages might be devoted to describing similar ruins. Some of them are even more remarkable for their dryness than is Niya, a site in the Tarim Desert of Chinese Turkestan. Yet there the evidence of desiccation within 2000 years is so strong that even so careful and conservative a man as Hann,[18] pronounces it "überzeugend."

A single quotation from scores that might be used will illustrate the conclusions of some of the most careful archæologists.[19]

Among the regions which were once populous and highly civilized, but which are now desert and deserted, there are few which were more closely connected with the beginnings of our own civilization than the desert parts of Syria and northern Arabia. It is only of recent years that the vast extent and great importance of this lost civilization has been fully recognized and that attempts have been made to reduce the extent of the unexplored area and to discover how much of the territory which has long been known as desert was formerly habitable and inhabited. The results of the explorations of the last twenty years have been most astonishing in this regard. It has been found that practically all of the wide area lying between the coast range of the eastern Mediterranean and the Euphrates, appearing upon the maps as the Syrian Desert, an area embracing somewhat more than 20,000 square miles, was more thickly populated than any area of similar dimensions in England or in the United States is today if one excludes the immediate vicinity of the large modern cities. It has also been discovered that an enormous desert tract lying to the east of Palestine, stretching eastward and southward into the country which we know as Arabia, was also a densely populated country. How far these settled regions extended in antiquity is still unknown, but the most distant explorations in these directions have failed to reach the end of ruins and other signs of former occupation.

The traveler who has crossed the settled, and more or less populous, coast range of northern Syria and descended into the narrow fertile valley of the Orontes, encounters in any farther journey toward the east an irregular range of limestone hills lying north and south and stretching to the northeast almost halfway to the Euphrates. These hills are about 2,500 feet high, rising in occasional peaks from 3,000 to 3,500 feet above sea level. They are gray and unrelieved by any visible vegetation. On ascending into the hills the traveler is astonished to find at every turn remnants of the work of men's hands, paved roads, walls which divided fields, terrace walls of massive structure. Presently he comes upon a small deserted and partly ruined town composed of buildings large and small constructed of beautifully wrought blocks of limestone, all rising out of the barren rock which forms the ribs of the hills. If he mounts an eminence in the vicinity, he will be still further astonished to behold similar ruins lying in all directions. He may count ten or fifteen or twenty, according to the commanding position of his lookout. From a distance it is often difficult to believe that these are not inhabited places; but closer inspection reveals that the gentle hand of time or the rude touch of earthquake has been laid upon every building. Some of the towns are better preserved than others; some buildings are quite perfect but for their wooden roofs which time has removed, others stand in picturesque ruins, while others still are level with the ground. On a far-off hilltop stands the ruin of a pagan temple, and crowning some lofty ridge lie the ruins of a great Christian monastery. Mile after mile of this barren gray country may be traversed without encountering a single human being. Day after day may be spent in traveling from one ruined town to another without seeing any green thing save a terebinth tree or two standing among the ruins, which have sent their roots down into earth still preserved in the foundations of some ancient building. No soil is visible anywhere except in a few pockets in the rock from which it could not be washed by the torrential rains of the wet season; yet every ruin is surrounded with the remains of presses for the making of oil and wine. Only one oasis has been discovered in these high plateaus.

Passing eastward from this range of hills, one descends into a gently rolling country that stretches miles away toward the Euphrates. At the eastern foot of the hills one finds oneself in a totally different country, at first quite fertile and dotted with frequent villages of flat-roofed houses. Here practically all the remains of ancient times have been destroyed through ages of building and rebuilding. Beyond this narrow fertile strip the soil grows drier and more barren, until presently another kind of desert is reached, an undulating waste of dead soil. Few walls or towers or arches rise to break the monotony of the unbroken landscape; but the careful explorer will find on closer examination that this region was more thickly populated in antiquity even than the hill country to the west. Every unevenness of the surface marks the site of a town, some of them cities of considerable extent.

We may draw certain very definite conclusions as to the former conditions of the country itself. There was soil upon the northern hills where none now exists, for the buildings now show unfinished foundation courses which were not intended to be seen; the soil in depressions without outlets is deeper than it formerly was; there are hundreds of olive and wine presses in localities where no tree or vine could now find footing; and there are hillsides with ruined terrace walls rising one above the other with no sign of earth near them. There was also a large natural water supply. In the north as well as in the south we find the dry beds of rivers, streams, and brooks with sand and pebbles and well-worn rocks but no water in them from one year's end to the other. We find bridges over these dry streams and crudely made washing boards along their banks directly below deserted towns. Many of the bridges span the beds of streams that seldom or never have water in them and give clear evidence of the great climatic changes that have taken place. There are well heads and well houses, and inscriptions referring to springs; but neither wells nor springs exist today except in the rarest instances. Many of the houses had their rock-hewn cisterns, never large enough to have supplied water for more than a brief period, and corresponding to the cisterns which most of our recent forefathers had which were for convenience rather than for dependence. Some of the towns in southern Syria were provided with large public reservoirs, but these are not large enough to have supplied water to their original populations. The high plateaus were of course without irrigation; but there are no signs, even in the lower flatter country, that irrigation was ever practiced; and canals for this purpose could not have completely disappeared. There were forests in the immediate vicinity, forests producing timbers of great length and thickness; for in the north and northeast practically all the buildings had wooden roofs, wooden intermediate floors, and other features of wood. Costly buildings, such as temples and churches, employed large wooden beams; but wood was used in much larger quantities in private dwellings, shops, stables, and barns. If wood had not been plentiful and cheap--which means grown near by--the builders would have adopted the building methods of their neighbors in the south, who used very little wood and developed the most perfect type of lithic architecture the world has ever seen. And here there exists a strange anomaly: Northern Syria, where so much wood was employed in antiquity, is absolutely treeless now; while in the mountains of southern Syria, where wood must have been scarce in antiquity to have forced upon the inhabitants an almost exclusive use of stone, there are still groves of scrub oak and pine, and travelers of half a century ago reported large forests of chestnut trees.[20] It is perfectly apparent that large parts of Syria once had soil and forests and springs and rivers, while it has none of these now, and that it had a much larger and better distributed rainfall in ancient times than it has now.

Professor Butler's careful work is especially interesting because of its contrast to the loose statements of those who believe in climatic uniformity. So far as I am aware, no opponent of the hypothesis of climatic changes has ever even attempted to show by careful statistical analysis that the ancient water supply of such ruins was no greater than that of the present. The most that has been done is to suggest that there may have been sources of water which are now unknown. Of course, this might be true in a single instance, but it could scarcely be the case in many hundreds or thousands of ruins.

Although the arguments in favor of a change of climate during the last two thousand years seem too strong to be ignored, their very strength seems to have been a source of error. A large number of people have jumped to the conclusion that the change which appears to have occurred in certain regions occurred everywhere, and that it consisted of a gradual desiccation.

Many observers, quite as careful as those who believe in progressive desiccation, point to evidences of aridity in past times in the very regions where the others find proof of moisture. Lakes such as the Caspian Sea fell to such a low level that parts of their present floors were exposed and were used as sites for buildings whose ruins are still extant. Elsewhere, for instance in the Tian-Shan Mountains, irrigation ditches are found in places where irrigation never seems to be necessary at present. In Syria and North Africa during the early centuries of the Christian era the Romans showed unparalleled activity in building great aqueducts and in watering land which then apparently needed water almost as much as it does today. Evidence of this sort is abundant and is as convincing as is the evidence of moister conditions in the past. It is admirably set forth, for example, in the comprehensive and ably written monograph of Leiter on the climate of North Africa.[21] The evidence cited there and elsewhere has led many authors strongly to advocate the hypothesis of climatic uniformity. They have done exactly as have the advocates of progressive change, and have extended their conclusions over the whole world and over the whole of historic times.

The hypotheses of climatic uniformity and of progressive change both seem to be based on reliable evidence. They may seem to be diametrically opposed to one another, but this is only when there is a failure to group the various lines of evidence according to their dates, and according to the types of climate in which they happen to be located. When the facts are properly grouped in both time and space, it appears that evidence of moist conditions in the historic Mediterranean lands is found during certain periods; for instance, four or five hundred years before Christ, at the time of Christ, and 1000 A. D. The other kind of evidence, on the contrary, culminates at other epochs, such as about 1200 B. C. and in the seventh and thirteenth centuries after Christ. It is also found during the interval from the culmination of a moist epoch to the culmination of a dry one, for at such times the climate was growing drier and the people were under stress. This was seemingly the case during the period from the second to the fourth centuries of our era. North Africa and Syria must then have been distinctly better watered than at present, as appears from Butler's vivid description; but they were gradually becoming drier, and the natural effect on a vigorous, competent people like the Romans was to cause them to construct numerous engineering works to provide the necessary water.

The considerations which have just been set forth have led to a third hypothesis, that of pulsatory climatic changes. According to this, the earth's climate is not stable, nor does it change uniformly in one direction. It appears to fluctuate back and forth not only in the little waves which we see from year to year or decade to decade, but in much larger waves, which take hundreds of years or even a thousand. These in turn seem to merge into and be imposed on the greater waves which form glacial stages, glacial epochs, and glacial periods. At the present time there seems to be no way of determining whether the general tendency is toward aridity or toward glaciation. The seventh century of our era was apparently the driest time during the historic period--distinctly drier than the present--but the thirteenth century was almost equally dry, and the twelfth or thirteenth before Christ may have been very dry.

The best test of an hypothesis is actual measurements. In the case of the pulsatory hypothesis we are fortunately able to apply this test by means of trees. The growth of vegetation depends on many factors--soil, exposure, wind, sun, temperature, rain, and so forth. In a dry region the most critical factor in determining how a tree's growth shall vary from year to year is the supply of moisture during the few months of most rapid growth.[22] The work of Douglass[23] and others has shown that in Arizona and California the thickness of the annual rings affords a reliable indication of the amount of moisture available during the period of growth. This is especially true when the growth of several years is taken as the unit and is compared with the growth of a similar number of years before or after. Where a long series of years is used, it is necessary to make corrections to eliminate the effects of age, but this can be done by mathematical methods of considerable accuracy. It is difficult to determine whether the climate at the beginning and end of a tree's life was the same, but it is easily possible to determine whether there have been pulsations while the tree was making its growth. If a large number of trees from various parts of a given district all formed thick rings at a certain period and then formed thin ones for a hundred years, after which the rings again become thick, we seem to be safe in concluding that the trees have lived through a long, dry period. The full reasons for this belief and details as to the methods of estimating climate from tree growth are given in _The Climatic Factor_.

The results set forth in that volume may be summarized as follows: During the years 1911 and 1912, under the auspices of the Carnegie Institution of Washington, measurements were made of the thickness of the rings of growth on the stumps of about 450 sequoia trees in California. These trees varied in age from 250 to nearly 3250 years. The great majority were over 1000 years of age, seventy-nine were over 2000 years, and three over 3000. Even where only a few trees are available the record is surprisingly reliable, except where occasional accidents occur. Where the number approximates 100, accidental variations are largely eliminated and we may accept the record with considerable confidence. Accordingly, we may say that in California we have a fairly accurate record of the climate for 2000 years and an approximate record for 1000 years more. The final results of the measurements of the California trees are shown in Fig. 4, where the climatic variations for 3000 years in California are indicated by the solid line. The high parts of the line indicate rainy conditions, the low parts, dry. An examination of this curve shows that during 3000 years there have apparently been climatic variations more important than any which have taken place during the past century. In order to bring out the details more clearly, the more reliable part of the California curve, from 100 B. C. to the present time, has been reproduced in Fig. 5. This is identical with the corresponding part of Fig. 4, except that the vertical scale is three times as great.

The curve of tree growth in California seems to be a true representation of the general features of climatic pulsations in the Mediterranean region. This conclusion was originally based on the resemblance between the solid line of Fig. 4, representing tree growth, and the dotted line representing changes of climate in the eastern Mediterranean region as inferred from the study of ruins and of history before any work on this subject had been done in America.[24] The dotted line is here reproduced for its historical significance as a stage in the study of climatic changes. If it were to be redrawn today on the basis of the knowledge acquired in the last twelve years, it would be much more like the tree curve. For example, the period of aridity suggested by the dip of the dotted line about 300 A. D. was based largely on Professor Butler's data as to the paucity of inscriptions and ruins dating from that period in Syria. In the recent article, from which a long quotation has been given, he shows that later work proves that there is no such paucity. On the other hand, it has accentuated the marked and sudden decay in civilization and population which occurred shortly after 600 A. D. He reached the same conclusion to which the present authors had come on wholly different grounds, namely, that the dip in the dotted line about 300 A. D. is not warranted, whereas the dip about 630 A. D. is extremely important. In similar fashion the work of Stein[25] in central Asia makes it clear that the contrast between the water supply about 200 B.C. and in the preceding and following centuries was greater than was supposed on the basis of the scanty evidence available when the dotted line of Fig. 4 was drawn in 1910.

Since the curve of the California trees is the only continuous and detailed record yet available for the climate of the last three thousand years, it deserves most careful study. It is especially necessary to determine the degree of accuracy with which the growth of the trees represents (1) the local rainfall and (2) the rainfall of remote regions such as Palestine. Perhaps the best way to determine these matters is the standard mathematical method of correlation coefficients. If two phenomena vary in perfect unison, as in the case of the turning of the wheels and the progress of an automobile when the brakes are not applied, the correlation coefficient is 1.00, being positive when the automobile goes forward and negative when it goes backward. If there is no relation between two phenomena, as in the case of the number of miles run by a given automobile each year and the number of chickens hatched in the same period, the coefficient is zero. A partial relationship where other factors enter into the matter is represented by a coefficient between zero and one, as in the case of the movement of the automobile and the consumption of gasoline. In this case the relation is very obvious, but is modified by other factors, including the roughness and grade of the road, the amount of traffic, the number of stops, the skill of the driver, the condition and load of the automobile, and the state of the weather. Such partial relationships are the kind for which correlation coefficients are most useful, for the size of the coefficients shows the relative importance of the various factors. A correlation coefficient four times the probable error, which can always be determined by a formula well known to mathematicians, is generally considered to afford evidence of some kind of relation between two phenomena. When the ratio between coefficient and error rises to six, the relationship is regarded as strong.

Few people would question that there is a connection between tree growth and rainfall, especially in a climate with a long summer dry season like that of California. But the growth of the trees also depends on their position, the amount of shading, the temperature, insect pests, blights, the wind with its tendency to break the branches, and a number of other factors. Moreover, while rain commonly favors growth, great extremes are relatively less helpful than more moderate amounts. Again, the roots of a tree may tap such deep sources of water that neither drought nor excessive rain produces much effect for several years. Hence in comparing the growth of the huge sequoias with the rainfall we should expect a correlation coefficient high enough to be convincing, but decidedly below 1.00. Unfortunately there is no record of the rainfall where the sequoias grow, the nearest long record being that of Sacramento, nearly 200 miles to the northwest and close to sea level instead of at an altitude of about 6000 feet.

Applying the method of correlation coefficients to the annual rainfall of Sacramento and the growth of the sequoias from 1863 to 1910, we obtain the results shown in Table 3. The trees of Section A of the table grew in moderately dry locations although the soil was fairly deep, a condition which seems to be essential to sequoias. In this case, as in all the others, the rainfall is reckoned from July to June, which practically means from October to May, since there is almost no summer rain. Thus the tree growth in 1861 is compared with the rainfall of the preceding rainy season, 1860-1861, or of several preceding rainy seasons as the table indicates.

+-------------------------------------------------------------------+ | TABLE 3 | | | | CORRELATION COEFFICIENTS BETWEEN RAINFALL AND | | GROWTH OF SEQUOIAS IN CALIFORNIA[26] | | | | (_r_) = _Correlation coefficient_ | | (_e_) = _Probable error_ | | (_r_/_e_) = _Ratio of coefficient to probable error_ | | | | A. SACRAMENTO RAINFALL AND GROWTH OF 18 SEQUOIAS IN DRY | | LOCATIONS, 1861-1910 | | | | (_r_) (_e_) (_r_/_e_) | | ------ ------ ----- | | 1 year of rainfall -0.059 ±0.096 0.6 | | 2 years of rainfall +0.288 ±0.090 3.2 | | 3 years of rainfall +0.570 ±0.066 8.7 | | 4 years of rainfall +0.470 ±0.076 6.2 | | | | B. SACRAMENTO RAINFALL AND GROWTH OF 112 SEQUOIAS MOSTLY IN | | MOIST LOCATIONS, 1861-1910 | | | | 3 years of rainfall +0.340 ±0.087 3.9 | | 4 years of rainfall +0.371 ±0.084 4.5 | | 5 years of rainfall +0.398 ±0.082 4.9 | | 6 years of rainfall +0.418 ±0.079 5.3 | | 7 years of rainfall +0.471 ±0.076 6.2 | | 8 years of rainfall (+0.520) ±0.071 7.3 | | 9 years of rainfall +0.575 ±0.065 8.8 | | 10 years of rainfall +0.577 ±0.065 8.8 | | | | C. SACRAMENTO RAINFALL AND GROWTH OF 80 SEQUOIAS IN MOIST | | LOCATIONS, 1861-1910 | | | | 10 years of rainfall +0.605 ±0.062 9.8 | | | | D. ANNUAL SEQUOIA GROWTH AND RAINFALL OF PRECEDING 5 YEARS | | AT STATIONS ON SOUTHERN PACIFIC RAILROAD | | | | 1 = _Years_ | | 2 = _Altitude_ (_feet_) | | 3 = _Rainfall_ (_inches_) | | 4 = _Approximate distance from sequoias_ (_miles_) | | | | 1 2 3 4 (_r_) (_e_) (_r_/_e_) | | --------- ---- ----- --- ------ ------ --------- | | Sacramento, 1861-1910 70 19.40 200 +0.398 ±0.081 4.9 | | Colfax, 1871-1909 2400 48.94 200 +0.122 ±0.113 1.1 | | Summit, 1871-1909 7000 48.07 200 +0.148 ±0.113 1.3 | | Truckee, 1871-1909 5800 27.12 200 +0.300 ±0.105 2.9 | | Boca, 1871-1909 5500 20.34 200 +0.604 ±0.076 8.0 | | Winnemucca, 1871-1909 4300 8.65 300 +0.492 ±0.089 5.5 | | | +-------------------------------------------------------------------+

In the first line of Section A a correlation coefficient of only -0.056, which is scarcely six-tenths of the probable error, means that there is no appreciable relation between the rainfall of a given season and the growth during the following spring and summer. The roots of the sequoias probably penetrate so deeply that the rain and melted snow of the spring months do not sink down rapidly enough to influence the trees before the growing season comes to an end. The precipitation of two preceding seasons, however, has some effect on the trees, as appears in the second line of Section A, where the correlation coefficient is +0.288, or 3.2 times the probable error. When the rainfall of three seasons is taken into account the coefficient rises to +0.570, or 8.7 times the probable error, while with four years of rainfall the coefficient begins to fall off. Thus the growth of these eighteen sequoias on relatively dry slopes appears to have depended chiefly on the rainfall of the second and third preceding rainy seasons. The growth in 1900, for example, depended largely on the rainfall in the rainy seasons of 1897-1898 and 1898-1899.

Section B of the table shows that with 112 trees, growing chiefly in moist depressions where the water supply is at a maximum, the correlation between growth and rainfall, +0.577 for ten years' rainfall, is even higher than with the dry trees. The seepage of the underground water is so slow that not until four years' rainfall is taken into account is the correlation coefficient more than four times the probable error. When only the trees growing in moist locations are employed, the coefficient between tree growth and the rainfall for ten years rises to the high figure of +0.605, or 9.8 times the probable error, as appears in Section C. These figures, as well as many others not here published, make it clear that the curve of sequoia growth from 1861 to 1910 affords a fairly close indication of the rainfall at Sacramento, provided allowance be made for a delay of three to ten years due to the fact that the moisture in the soil gradually seeps down the mountain-sides and only reaches the sequoias after a considerable interval.

If a rainfall record were available for the place where the trees actually grow, the relationship would probably be still closer.

The record at Fresno, for example, bears out this conclusion so far as it goes. But as Fresno lies at a low altitude and its rainfall is of essentially the Sacramento type, its short record is of less value than that of Sacramento. The only rainfall records among the Sierras at high levels, where the rainfall and temperature are approximately like those of the sequoia region, are found along the main line of the Southern Pacific railroad. This runs from Oakland northeastward seventy miles across the open plain to Sacramento, then another seventy miles, as the crow flies, through Colfax and over a high pass in the Sierras at Summit, next twenty miles or so down through Truckee to Boca, on the edge of the inland basin of Nevada, and on northeastward another 160 miles to Winnemucca, where it turns east toward Ogden and Salt Lake City. Section D of Table 3 shows the correlation coefficients between the rainfall along the railroad and the growth of the sequoias. At Sacramento, which lies fairly open to winds from the Pacific and thus represents the general climate of central California, the coefficient is nearly five times the probable error, thus indicating a real relation to sequoia growth. Then among the foothills of the Sierras at Colfax, the coefficient drops till it is scarcely larger than the probable error. It rises rapidly, however, as one advances among the mountains, until at Boca it attains the high figure of +0.604 or eight times the probable error, and continues high in the dry area farther east. In other words the growth of the sequoias is a good indication of the rainfall where the trees grow and in the dry region farther east.

In order to determine the degree to which the sequoia record represents the rainfall of other regions, let us select Jerusalem for comparison. The reasons for this selection are that Jerusalem furnishes the only available record that satisfies the following necessary conditions: (1) its record is long enough to be important; (2) it is located fairly near the latitude of the sequoias, 32°N versus 37°N; (3) it is located in a similar type of climate with winter rains and a long dry summer; (4) it lies well above sea level (2500 feet) and somewhat back from the seacoast, thus approximating although by no means duplicating the condition of the sequoias; and (5) it lies in a region where the evidence of climatic changes during historic times is strongest. The ideal place for comparison would be the valley in which grow the cedars of Lebanon. Those trees resemble the sequoias to an extraordinary degree, not only in their location, but in their great age. Some day it will be most interesting to compare the growth of these two famous groups of old trees.

+-------------------------------------------------------------------+ | TABLE 4 | | | | CORRELATION COEFFICIENTS BETWEEN | | RAINFALL RECORDS IN CALIFORNIA | | AND JERUSALEM | | | | (_r_) = _Correlation coefficient_ | | (_e_) = _Probable error_ | | (_r_/_e_) = _Ratio of coefficient to probable error_ | | | | A. JERUSALEM RAINFALL FOR 3 YEARS AND VARIOUS GROUPS OF | | SEQUOIAS[27] | | | | (_r_) (_e_) (_r_/_e_)| | ------ ----- ---------| | 11 trees measured by Douglass +0.453 ±0.078 5.8 | | 80 trees, moist locations, Groups IA, | | IIA, IIIA, VA +0.500 ±0.073 6.8 | | 101 trees, 69 in moist locations, 32 in | | dry, I, II, III +0.616 ±0.061 10.1 | | 112 trees, 80 in moist locations, 32 in | | dry, I, II, III, V +0.675 ±0.053 12.7 | | | | B. RAINFALL AT JERUSALEM AND AT STATIONS IN CALIFORNIA AND NEVADA | | | | 1 = _Altitude_ (_feet_) | | 2 = _Years_ | | | | -- 3 years -- -- 5 years -- | | 1 2 (_r_) (_r_/_e_) (_r_) (_r_/_e_) | | ---- --------- ------ ------- ------ ------- | | Sacramento, 70 1861-1910 +0.386 4.7 +0.352 4.2 | | Colfax, 2400 1871-1909 +0.311 3.1 +0.308 3.0 | | Summit, 7000 1871-1909 +0.099 0.9 +0.248 2.3 | | Truckee, 5800 1871-1909 +0.229 2.2 +0.337 3.3 | |[A]Boca, 5500 1871-1909 +0.482 6.4 +0.617 8.6 | | Winnemucca, 4300 1871-1909 +0.235 2.2 +0.260 2.4 | | San Bernardino, 1050 1871-1909 +0.275 2.7 +0.177 1.8 | | | | C. RAINFALL FOR 3 YEARS AT CALIFORNIA AND NEVADA STATIONS, | | 1871-1909 | | | | (_r_) (_r_/_e_) | | ------ ------- | | Sacramento and San Bernardino +0.663 10.7 | | San Bernardino and Winnemucca +0.291 2.8 | | | +-------------------------------------------------------------------+

The correlation coefficients for the sequoia growth and the rainfall at Jerusalem are given in Section A, Table 4. They are so high and so consistent that they scarcely leave room for doubt that where a hundred or more sequoias are employed, as in Fig. 5, their curve of growth affords a good indication of the fluctuations of climate in western Asia. The high coefficient for the eleven trees measured by Douglass suggests that where the number of trees falls as low as ten, as in the part of Fig. 4 from 710 to 840 B. C., the relation between tree growth and rainfall is still close even when only one year's growth is considered. Where the unit is ten years of growth, as in Figs. 4 and 5, the accuracy of the tree curve as a measure of rainfall is much greater than when a single year is used as in Table 4. When the unit is raised to thirty years, as in the smoothed part of Fig. 4 previous to 240 B. C., even four trees, as from 960 to 1070, probably give a fair approximation to the general changes in rainfall, while a single tree prior to 1110 B. C. gives a rough indication.

Table 4 shows a peculiar feature in the fact that the correlations of Section A between tree growth and the rainfall of Jerusalem are decidedly higher than those between the rainfall in the two regions. Only at Sacramento and Boca are the rainfall coefficients high enough to be conclusive. This, however, is not surprising, for even between Sacramento and San Bernardino, only 400 miles apart, the correlation coefficient for the rainfall by three-year periods is only 10.7 times the probable error, as appears in Section C of Table 4, while between San Bernardino and Winnemucca 500 miles away, the corresponding figure drops to 2.8. It must be remembered that in some respects the growth of the sequoias is a much better record of rainfall than are the records kept by man. The human record is based on the amount of water caught by a little gauge a few inches in diameter. Every gust of wind detracts from the accuracy of the record; a mile away the rainfall may be double what it is at the gauge. Each sequoia, on the other hand, draws its moisture from an area thousands of times as large as a rain gauge. Moreover, the trees on which Figs. 4 and 5 are based were scattered over an area fifty miles long and several hundred square miles in extent. Hence they represent the summation of the rainfall over an area millions of times as large as that of a rain gauge. This fact and the large correlation coefficients between sequoia growth and Jerusalem rainfall should be considered in connection with the fact that all the coefficients between the rainfall of California and Nevada and that of Jerusalem are positive. If full records of the complete rainfall of California and Nevada on the one hand and of the eastern Mediterranean region on the other were available for a long period, they would probably agree closely.

Just how widely the sequoias can be used as a measure of the climate of the past is not yet certain. In some regions, as will shortly be explained, the climatic changes seem to have been of an opposite character from those of California. In others the Californian or eastern Mediterranean type of change seems sometimes to prevail but is not always evident. For example, at Malta the rainfall today shows a distinct relation to that of Jerusalem and to the growth of the sequoias. But the correlation coefficient between the rainfall of eight-year periods at Naples, a little farther north, and the growth of the sequoias at the end of the periods is -0.132, or only 1.4 times the probable error and much too small to be significant. This is in harmony with the fact that although Naples has summer droughts, they are not so pronounced as in California and Palestine, and the prevalence of storms is much greater. Jerusalem receives only 8 per cent of its rain in the seven months from April to October, and Sacramento 13, while Malta receives 31 per cent and Naples 43. Nevertheless, there is some evidence that in the past the climatic fluctuations of southern Italy followed nearly the same course as those of California and Palestine. This apparent discrepancy seems to be explained by our previous conclusion that changes of climate are due largely to a shifting of storm tracks. When sunspots are numerous the storms which now prevail in northern Italy seem to be shifted southward and traverse the Mediterranean to Palestine just as similar storms are shifted southward in the United States. This perhaps accounts for the agreement between the sequoia curve and the agricultural and social history of Rome from about 400 B. C. to 100 A. D., as explained in _World Power and Evolution_. For our present purposes, however, the main point is that since rainfall records have been kept the fluctuations of climate indicated by the growth of the sequoias have agreed closely with fluctuations in the rainfall of the eastern Mediterranean region. Presumably the same was true in the past. In that case, the sequoia curve not only is a good indication of climatic changes or pulsations in regions of similar climate, but may serve as a guide to coincident but different changes in regions of other types.

An enormous body of other evidence points to the same conclusion. It indicates that while the average climate of the present is drier than that of the past in regions having the Mediterranean type of winter rains and summer droughts, there have been pronounced pulsations during historic times so that at certain times there has actually been greater aridity than at present. This conclusion is so important that it seems advisable to examine the only important arguments that have been raised against it, especially against the idea that the general rainfall of the eastern Mediterranean was greater in the historic past than at present. The first objection is the unquestionable fact that droughts and famines have occurred at periods which seem on other evidence to have been moister than the present. This argument has been much used, but it seems to have little force. If the rainfall of a given region averages thirty inches and varies from fifteen to forty-five, a famine will ensue if the rainfall drops for a few years to the lower limit and does not rise much above twenty for a few years. If the climate of the place changes during the course of centuries, so that the rainfall averages only twenty inches, and ranges from seven to thirty-five, famine will again ensue if the rainfall remains near ten inches for a few years. The ravages of the first famine might be as bad as those of the second. They might even be worse, because when the rainfall is larger the population is likely to be greater and the distress due to scarcity of food would affect a larger number of people. Hence historic records of famines and droughts do not indicate that the climate was either drier or moister than at present. They merely show that at the time in question the climate was drier than the normal for that particular period.

The second objection is that deserts existed in the past much as at present. This is not a real objection, however, for, as we shall see more fully, some parts of the world suffer one kind of change and others quite the opposite. Moreover, deserts have always existed, and when we talk of a change in their climate we merely mean that their boundaries have shifted. A concrete example of the mistaken use of ancient dryness as proof of climatic uniformity is illustrated by the march of Alexander from India to Mesopotamia. Hedin gives an excellent presentation of the case in the second volume of his _Overland to India_. He shows conclusively that Alexander's army suffered terribly from lack of water and provisions. This certainly proves that the climate was dry, but it by no means indicates that there has been no change from the past to the present. We do not know whether Alexander's march took place during an especially dry or an especially wet year. In a desert region like Makran, in southern Persia and Beluchistan, where the chief difficulties occurred, the rainfall varies greatly from year to year. We have no records from Makran, but the conditions there are closely similar to those of southern Arizona and New Mexico. In 1885 and 1905 the rainfall for five stations in that region was as follows:

+------------------------------------------------------------+ | | | _Mean rainfall | | during period | | _1885_ _1905_ since | | observations | | began_ | | Yuma, Arizona, 2.72 11.41 3.13 | | Phoenix, Arizona, 3.77 19.73 7.27 | | Tucson, Arizona, 5.26 24.17 11.66 | | Lordsburg, New Mexico, 3.99 19.50 8.62 | | El Paso, Texas (on New | | Mexico border), 7.31 17.80 9.06 | | ---- ----- ----- | | Average, 4.61 18.52 7.95 | | | +------------------------------------------------------------+

These stations are distributed over an area nearly 500 miles east and west. Manifestly a traveler who spent the year 1885 in that region would have had much more difficulty in finding water and forage than one who traveled in the same places in 1905. During 1885 the rainfall was 42 per cent less than the average, and during 1905 it was 134 per cent more than the average. Let us suppose, for the sake of argument, that the average rainfall of southeastern Persia is six inches today and was ten inches in the days of Alexander. If the rainfall from year to year varied as much in the past in Persia as it does now in New Mexico and Arizona, the rainfall during an ancient dry year, corresponding in character to 1885, would have been about 5.75 inches. On the other hand, if we suppose that the rainfall then averaged less than at present,--let us say four inches,--a wet year corresponding to 1905 in the American deserts might have had a rainfall of about ten inches. This being the case, it is clear that our estimate of what Alexander's march shows as to climate must depend largely on whether 325 B. C. was a wet year or a dry year. Inasmuch as we know nothing about this, we must fall back on the fact that a large army accomplished a journey in a place where today even a small caravan usually finds great difficulty in procuring forage and water. Moreover, elephants were taken 180 miles across what is now an almost waterless desert, and yet the old historians make no comment on such a feat which today would be practically impossible. These things seem more in harmony with a change of climate than with uniformity. Nevertheless, it is not safe to place much reliance on them except when they are taken in conjunction with other evidence, such as the numerous ruins, which show that Makran was once far more densely populated than now seems possible. Taken by itself, such incidents as Alexander's march cannot safely be used either as an argument for or against changes of climate.

The third and strongest objection to any hypothesis of climatic changes during historic times is based on vegetation. The whole question is admirably set forth by J. W. Gregory,[28] who gives not only his own results, but those of the ablest scholars who have preceded him. His conclusions are important because they represent one of the few cases where a definite statistical attempt has been made to prove the exact condition of the climate of the past. After stating various less important reasons for believing that the climate of Palestine has not changed, he discusses vegetation. The following quotation indicates his line of thought. A sentence near the beginning is italicized in order to call attention to the importance which Gregory and others lay on this particular kind of evidence:

Some more certain test is necessary than the general conclusions which can be based upon the historical and geographical evidence of the Bible. In the absence of rain gauge and thermometric records, _the most precise test of climate is given by the vegetation; and fortunately the palm affords a very delicate test of the past climate of Palestine and the eastern Mediterranean_.... The date palm has three limits of growth which are determined by temperature; thus it does not reach full maturity or produce ripe fruit of good quality below the mean annual temperature of 69°F. The isothermal of 69° crosses southern Algeria near Biskra; it touches the northern coasts of Cyrenaica near Derna and passes Egypt near the mouth of the Nile, and then bends northward along the coast lands of Palestine.

To the north of this line the date palm grows and produces fruit, which only ripens occasionally, and its quality deteriorates as the temperature falls below 69°. Between the isotherms of 68° and 64°, limits which include northern Algeria, most of Sicily, Malta, the southern parts of Greece and northern Syria, the dates produced are so unripe that they are not edible. In the next cooler zone, north of the isotherm of 62°, which enters Europe in southwestern Portugal, passes through Sardinia, enters Italy near Naples, crosses northern Greece and Asia Minor to the east of Smyrna, the date palm is grown only for its foliage, since it does not fruit.

Hence at Benghazi, on the north African coast, the date palm is fertile, but produces fruit of poor quality. In Sicily and at Algiers the fruit ripens occasionally and at Rome and Nice the palm is grown only as an ornamental tree.

The date palm therefore affords a test of variations in mean annual temperature of three grades between 62° and 69°.

This test shows that the mean annual temperature of Palestine has not altered since Old Testament times. The palm tree now grows dates on the coast of Palestine and in the deep depression around the Dead Sea, but it does not produce fruit on the highlands of Judea. Its distribution in ancient times, as far as we can judge from the Bible, was exactly the same. It grew at "Jericho, the city of palm trees" (Deut. xxxiv: 3 and 2 Chron. xxviii: 15), and at Engedi, on the western shore of the Dead Sea (2 Chron. xx: 2; Sirach xxiv: 14); and though the palm does not still live at Jericho--the last apparently died in 1838--its disappearance must be due to neglect, for the only climatic change that would explain it would be an increase in cold or moisture. In olden times the date palm certainly grew on the highlands of Palestine; but apparently it never produced fruit there, for the Bible references to the palm are to its beauty and erect growth: "The righteous shall flourish like the palm" (Ps. xcii: 12); "They are upright as the palm tree" (Jer. x: 5); "Thy stature is like to a palm tree" (Cant. vii: 7). It is used as a symbol of victory (Rev. vii: 9), but never praised as a source of food.

Dates are not once referred to in the text of the Bible, but according to the marginal notes the word translated "honey" in 2 Chron. xxxi: 5 may mean dates....

It appears, therefore, that the date palm had essentially the same distribution in Palestine in Old Testament times as it has now; and hence we may infer that the mean temperature was then the same as now. If the climate had been moister and cooler, the date could not have flourished at Jericho. If it had been warmer, the palms would have grown freely at higher levels and Jericho would not have held its distinction as _the_ city of palm trees.[29]

In the main Gregory's conclusions seem to be well grounded, although even according to his data a change of 2° or 3° in mean temperature would be perfectly feasible. It will be noticed, however, that they apply to temperature and not to rainfall. They merely prove that two thousand years ago the mean temperature of Palestine and the neighboring regions was not appreciably different from what it is today. This, however, is in no sense out of harmony with the hypothesis of climatic pulsations. Students of glaciation believe that during the last glacial epoch the mean temperature of the earth as a whole was only 5° or 6°C. lower than at present. If the difference between the climate of today and of the time of Christ is a tenth as great as the difference between the climate of today and that which prevailed at the culmination of the last glacial epoch, the change in two thousand years has been of large dimensions. Yet this would require a rise of only half a degree Centigrade in the mean temperature of Palestine. Manifestly, so slight a change would scarcely be detectable in the vegetation.

The slightness of changes in mean temperature as compared with changes in rainfall may be judged from a comparison of wet and dry years in various regions. For example, at Berlin between 1866 and 1905 the ten most rainy years had an average precipitation of 670 mm. and a mean temperature of 9.15°C. On the other hand, the ten years of least rainfall had an average of 483 mm. and a mean temperature of 9.35°. In other words, a difference of 137 mm., or 39 per cent, in rainfall was accompanied by a difference of only 0.2°C. in temperature. Such contrasts between the variability of mean rainfall and mean temperature are observable not only when individual years are selected, but when much longer periods are taken. For instance, in the western Gulf region of the United States the two inland stations of Vicksburg, Mississippi, and Shreveport, Louisiana, and the two maritime stations of New Orleans, Louisiana, and Galveston, Texas, lie at the margins of an area about 400 miles long. During the ten years from 1875 to 1884 their rainfall averaged 59.4 inches,[30] while during the ten years from 1890 to 1899 it averaged only 42.4 inches. Even in a region so well watered as the Gulf States, such a change--40 per cent more in the first decade than in the second--is important, and in drier regions it would have a great effect on habitability. Yet in spite of the magnitude of the change the mean temperature was not appreciably different, the average for the four stations being 67.36°F. during the more rainy decade and 66.94°F. during the less rainy decade--a difference of only 0.42°F. It is worth noticing that in this case the wetter period was also the warmer, whereas in Berlin it was the cooler. This is probably because a large part of the moisture of the Gulf States is brought by winds having a southerly component. Similar relationships are apparent in other places. We select Jerusalem because we have been discussing Palestine. At the time of writing, the data available in the _Quarterly Journal of the Palestine Exploration Fund_ cover the years from 1882-1899 and 1903-1909. Among these twenty-five years the thirteen which had most rain had an average of 34.1 inches and a temperature of 62.04°F. The twelve with least rain had 24.4 inches and a temperature of 62.44°. A difference of 40 per cent in rainfall was accompanied by a difference of only 0.4°F. in temperature.

The facts set forth in the preceding paragraphs seem to show that extensive changes in precipitation and storminess can take place without appreciable changes of mean temperature. If such changed conditions can persist for ten years, as in one of our examples, there is no logical reason why they cannot persist for a hundred or a thousand. The evidence of changes in climate during the historic period seems to suggest changes in precipitation much more than in temperature. Hence the strongest of all the arguments against historic changes of climate seems to be of relatively little weight, and the pulsatory hypothesis seems to be in accord with all the known facts.

Before the true nature of climatic changes, whether historic or geologic, can be rightly understood, another point needs emphasis. When the pulsatory hypothesis was first framed, it fell into the same error as the hypotheses of uniformity and of progressive change--that is, the assumption was made that the whole world is either growing drier or moister with each pulsation. A study of the ruins of Yucatan, in 1912, and of Guatemala, in 1913, as is explained in _The Climatic Factor_, has led to the conclusion that the climate of those regions has changed in the opposite way from the changes which appear to have taken place in the desert regions farther south. These Maya ruins in Central America are in many cases located in regions of such heavy rainfall, such dense forests, and such malignant fevers that habitation is now practically impossible. The land cannot be cultivated except in especially favorable places. The people are terribly weakened by disease and are among the lowest in Central America. Only a hundred miles from the unhealthful forests we find healthful areas, such as the coasts of Yucatan and the plateau of Guatemala. Here the vast majority of the population is gathered, the large towns are located, and the only progressive people are found. Nevertheless, in the past the region of the forests was the home of by far the most progressive people who are ever known to have lived in America previous to the days of Columbus. They alone brought to high perfection the art of sculpture; they were the only American people who invented the art of writing. It seems scarcely credible that such a people would have lived in the worst possible habitat when far more favored regions were close at hand. Therefore it seems as if the climate of eastern Guatemala and Yucatan must have been relatively dry at some past time. The Maya chronology and traditions indicate that this was probably at the same time when moister conditions apparently prevailed in the subarid or desert portions of the United States and Asia. Fig. 3 shows that today at times of many sunspots there is a similar opposition between a tendency toward storminess and rain in subtropical regions and toward aridity in low latitudes near the heat equator.

Thus our final conclusion is that during historic times there have been pulsatory changes of climate. These changes have been of the same type in regions having similar kinds of climate, but of different and sometimes opposite types in places having diverse climates. As to the cause of the pulsations, they cannot have been due to the precession of the equinoxes nor apparently to any allied astronomical cause, for the time intervals are too short and too irregular. They cannot have been due to changes in the percentage of carbon dioxide in the atmosphere, for not even the strongest believers in the climatic efficacy of that gas hold that its amount could fluctuate in any such violent way as would be necessary to explain the pulsations shown in the California curve of tree growth. Volcanic activity seems more probable as at least a partial cause, and it would be worth while to investigate the matter more fully. Nevertheless, it can apparently be only a minor cause. In the first place, the main effect of a cloud of dust is to alter the temperature, but Gregory's summary of the palm and the vine shows that variations in temperature are apparently of very slight importance during historic times. Again, ruins on the bottoms of enclosed salt lakes, old beaches now under the water, and signs of irrigation ditches where none are now needed indicate a climate drier than the present. Volcanic dust, however, cannot account for such a condition, for at present the air seems to be practically free from such dust for long periods. Thus we now experience the greatest extreme which the volcanic hypothesis permits in one direction, but there have been greater extremes in the same direction. The thermal solar hypothesis is likewise unable to explain the observed phenomena, for neither it nor the volcanic hypothesis offers any explanation of why the climate varies in one way in Mediterranean climates and in an opposite way in regions near the heat equator.

This leaves the cyclonic hypothesis. It seems to fit the facts, for variations in cyclonic storms cause some regions to be moister and others drier than usual. At the same time the variations in temperature are slight, and are apparently different in different regions, some places growing warm when others grow cool. In the next chapter we shall study this matter more fully, for it can best be appreciated by examining the course of events in a specific century.

FOOTNOTES:

[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]

[Footnote 17: Ellsworth Huntington: Explorations in Turkestan, 1905; The Pulse of Asia, 1907; Palestine and Its Transformation, 1911; The Climatic Factor, 1915; World Power and Evolution, 1919.]

[Footnote 18: J. Hann: Klimatologie, Vol. 1, 1908, p. 352.]

[Footnote 19: H. C. Butler: Desert Syria, the Land of a Lost Civilization; Geographical Review, Feb., 1920, pp. 77-108.]

[Footnote 20: This is due to the fact that where these forests occur, in Gilead for example, the mountains to the west break down, so that the west winds with water from the Mediterranean are able to reach the inner range without having lost all their water. It is one of the misfortunes of Syria that its mountains generally rise so close to the sea that they shut off rainfall from the interior and cause the rain to fall on slopes too steep for easy cultivation.]

[Footnote 21: H. Leiter: Die Frage der Klimaanderung waherend geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen Gesellschaft, Wien, 1909, p. 143.]

[Footnote 22: A most careful and convincing study of this problem is embodied in an article by J. W. Smith: The Effects of Weather upon the Yield of Corn; Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the basis of the yield of corn in Ohio for 60 years and in other states for shorter periods, he shows that the rainfall of July has almost as much influence on the crop as has the rainfall of all other months combined. See his Agricultural Meteorology, New York, 1920.]

[Footnote 23: See chapter by A. E. Douglass in The Climatic Factor; and his book on Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also article by M. N. Stewart: The Relation of Precipitation to Tree Growth, in the Monthly Weather Review, Vol. 41, 1913.]

[Footnote 24: The dotted line is taken from Palestine and Its Transformation, pp. 327 and 403.]

[Footnote 25: M. A. Stein: Ruins of Desert Cathay, London, 1912.]

[Footnote 26: In the preparation and interpretation of this table the help of Mr. G. B. Cressey is gratefully acknowledged.]

[Footnote 27: For the tree data used in these comparisons, see The Climatic Factor P. 328, and A. E. Douglass: Climatic Cycles and Tree Growth, p. 123.]

[Footnote A: One year interpolated.]

[Footnote 28: J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol. 43, 1914, pp. 148-172 and 293-318.]

[Footnote 29: Geog. Jour., Vol. 43, pp. 159-161.]

[Footnote 30: See A. J. Henry: Secular Variation of Precipitation in the United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.]