BIBLIOGRAPHY

Lewis, F. J. “The history of the Scottish peat-mosses and their relation to the Glacial period.” Edinburgh, Scot. Geogr. Mag., 22, 1916, p. 241.

Brooks, C. E. P. “The correlation of the Quaternary deposits of Great Britain with those of the Continent of Europe.” Ann. Rep. Smithsonian Inst., 1917, p. 277.

Huntington, Ellsworth. “The pulse of Asia.” Boston and New York, 1907.

——. “The climatic factor as illustrated in arid America.” Washington, Carnegie Institution, 1914.

——. “World power and evolution.” New Haven, 1919, pp. 186-207.

Pettersson, O. “Climatic variations in historic and prehistoric time.” Svenska Hydrogr.-Biol. Komm. Skrifter, Heft 5.

CHAPTER XVII
THE CLIMATIC FLUCTUATIONS SINCE A.D. 500

The question of climatic changes during the historic period has been the subject of much discussion, and several great meteorologists and geographers have endeavoured to prove that at least since about 500 B.C. there has been no appreciable variation. It is admitted that there have been shiftings of the centres of population and civilization, first from Egypt and Mesopotamia to the Mediterranean regions, and later to northern and western Europe, but these have been attributed chiefly to political causes, and especially to the rise of Islam and the rule of the “accursed Turk.” Recently, however, there has arisen a class of evidence which cannot be explained away on political grounds, and which appears to have decided the battle in favour of the supporters of change; I refer to the evidence of the trees, explained in the preceding chapter. The conclusions derived from the big trees of California have fallen admirably into line with archæological work in central America, in central Asia and other regions, and have shown that the larger variations even of comparatively recent times have been very extensive, if not world-wide, in their development.

Let us consider first the evidence of the trees. These indicate that after the moist period ending about A.D. 400, described in the preceding chapter, the rainfall was generally light until about A.D. 1000, when it showed a sharp rise, probably to the level attained in A.D. 1. (The correction for age renders an exact comparison between periods a thousand years apart difficult.) This period of abundant rainfall lasted some fifty years, followed by a gradual decline to a brief minimum, shortly before A.D. 1200. About 1300 occurred another rapid rise, reaching a maximum before 1350; the period of heavy rain continued a short while after 1400, when a decline set in, reaching a minimum at 1500, after which the rainfall recovered somewhat, and subsequently maintained approximately its present level, with a slight maximum about 1600 to 1645.

In the desert of Arizona, in regions at present too dry for agriculture, there are abundant ruins, which are attributed by Huntington to three periods:

(a) Pueblo ruins, dating back to just before the coming of the Spaniards (i.e. about A.D. 1600), and indicating merely an increase of population at the present centres.

(b) Ruins of an older civilization, termed by Huntington the Pajaritan, during which numerous inhabitants lived in places where at present no crops can be raised. “These people, as appears from their pottery, their skulls and their methods of agriculture, belong to a different civilization from that of the modern Pueblos who inhabited Gran Quivera at the time of the coming of the Spaniards. They had evidently disappeared long before that date, as is evident from the present ruins of their villages, and from the absence of any hint of their existence in the early annals of the country” (Geogr. Journal, 40, 1912, p. 396).

The largest ruins of this type invariably lie near the main lines of drainage. They consist of villages with houses of several storeys. But digging down beneath these ruins we find (c) traces of an older occupation, and ruins of a primitive type are also found on the plateaus remote from any except small valleys. “They are usually small, and are greatly ruined, and seem to belong to a time long anterior to the main large ruins.” Huntington terms this type the Hohokam; unfortunately this and the Pajaritan occupations cannot be accurately dated, but it is reasonable to connect them with the rainfall maxima shown by the trees, about the time of Christ, and in A.D. 1000 to A.D. 1300.

A similar succession has been found in the neighbourhood of Mexico City. The earliest trace of occupation is a crude “mountain pottery,” in ordinary river sand and gravel. These deposits are succeeded by finer sand with better pottery known as the “San Juan” type, above which comes a culture layer with the remains of houses. This is covered by a bed of “tepetate,” a white calcareous deposit frequently found in dry regions where much water evaporates. The gravels suggest the occasional heavy rains of arid countries. The San Juan pottery extends throughout the “tepetate,” which probably corresponds to the dry period of A.D. 400-1000 in California.

Historical records in Mexico date back to the coming of the Aztecs in A.D. 1325. They show that in 1325 and again in 1446 the level of the lake of Mexico was high, but towards the end of the fifteenth century the water was much lower. In 1520 it was high again; in 1600 it was low, but high from 1629 to 1634. From 1675 to 1755 was a long dry period. On the whole the climate from 300 to 600 years ago seems to have been moister than that of to-day.

Still further south in the Peninsula of Yucatan recent explorations have yielded results of extreme interest. Yucatan lies within the tropical rain-belt, and is covered by almost impenetrable forests. The climate is enervating and unhealthy, and the present inhabitants are greatly lacking in vigour. In the forest, however, have been found the ruins of ninety-two towns, some of them of great size, and all remarkable for the beauty as well as the solidity of their architecture.

These ruins belong to the great Mayan civilization. Mayan history has been briefly summarized by Huntington as follows: “First we have a long period of active development, during which the calendar was evolved and the arts of architecture and sculpture were gradually developed.... This time of marked growth must have preceded the Christian era. Then comes ... the building of the great cities of Copan, Quirigua, Tikal and others. These first great cities were in the southern part of the Maya area, on the borders of Honduras or in eastern Guatemala. They lasted perhaps three or four centuries; then quickly declined. So far as we have any evidence, civilization never revived in this southern area, for the structures of the great period have not been rebuilt by later inhabitants. Towards the end of the period of greatness the centre of Mayan culture moved northward.... The great period, according to Bowditch, lasted from 100 B.C. to A.D. 350 ... then came a time of very low civilization, lasting for centuries.... A revival ensued about A.D. 900 or A.D. 1000, and architecture once more reached a high pitch, but ... only in northern Yucatan; all the rest of the country seems to have remained in darkness. Moreover, this mediæval revival was relatively shortlived. Since that time the condition of the Mayas has fluctuated more or less, but on the whole there has been a decline.”

Now at the present day the densest and most progressive population in Yucatan is found in the driest part of the country, where the forest gives place to jungle. If the line of separation between jungle and forest were moved southward 300 miles, the former would include all the districts where ruins are now found. We see from the above summary that the prosperous periods of Mayan history were just those periods which in California were moist; in Yucatan they must have been dry. Huntington’s explanation is the theory of the “shifting of climatic belts”; during the rainy period in California the temperate storm-tracks were shifted further southward. At the same time the sub-tropical high-pressure belt, which at present lies over the West Indies, was also shifted southwards, and this brought a dry cool winter to Yucatan, with an increased contrast of seasons, and consequently a more invigorating climate.

In Asia, Huntington and other explorers have found similar traces of past variations of climate, a fascinating account of which is given in “The Pulse of Asia.” Space will not permit of a summary in detail, but the following general conclusions may be quoted:[6]

“If we omit the Volga and the European portions of the Caspian drainage area, the limits (of the six basins considered) lie over sixteen hundred miles apart from north to south and over three thousand from east to west. All this great area seems to have been subject to the same great waves of climatic change.

“In the ancient days when the Oxus River entered the Scythian Gulf of the expanded Caspian Sea, and Lake Gyoljuk discharged permanently to the Tigris, the lake of Seyistan had not been converted into dry land by the giants. Kashmir was so cold and snowy that agriculture was impossible.... In the Lop basin the rivers were full of water; Lop-Nor was the “Great Salt Lake”; the desert was comparatively small and the zone of vegetation extensive; and on all sides there was a density of population and a degree of prosperity far beyond those of to-day. And in the Turfan basin the same was probably true.

“A great change took place throughout the six basins during the early centuries of the Christian era. The lakes of Gyoljuk, Seyistan, the Caspian, Lop-Nor and presumably Turfan were greatly reduced in size. In the case of the first three, parts of the old lake-beds were used as sites for villages. Except in Kashmir, the change of climate appears to have brought disaster....

“Again there came a change (about A.D. 700). The process of desiccation gave place to a slight but important tendency toward increased rainfall and lower temperature. Kashmir became colder and more snowy, and hence more isolated; the rivers of Lop and Turfan gained greater volume; and the lakes of Lop, the Caspian and Turfan expanded once more. The habitability of the arid regions began to increase; migrations came to an end; and central Asia was prosperous for a time. Finally (about 1350) a latest and slightest change took place in the other direction, and we seem to-day to be in the midst of an epoch of comparative equilibrium, with no marked tendency towards climatic change in either direction.” There was, however, a period of comparatively high water in the Caspian in the early part of the seventeenth century.

In Europe the evidence for climatic changes during historical times is more difficult to follow, since variations of rainfall leave fewer traces in a moist than in an arid or semi-arid country. A certain amount of material is given by Brückner in his “Klimaschwankungen.” He finds that there was a great advance of the Alpine glaciers from 1595 to 1610, while two Italian lakes without outlet, the Lago di Fucina and Lake Trasimeno, attained a high level about the same time. Other evidence for western Europe is derived from the date of the wine-harvest and from the records of severe winters. Like the growth-curves of the big trees, they need a secular correction to alter the general slope of the curve, especially in the case of severe winters, but the larger irregularities probably correspond to real variations of climate. I have added in column 4 the numbers of winters with sea-ice on some part of the Danish coast, as tabulated from the records compiled by Captain C. I. H. Speerschneider. The results are in general agreement with column 3, particularly as showing that the period 1401-50 was relatively mild; but the first half of the seventeenth century is less instead of greater than its two neighbours in this column.

The figures for the wine-harvest refer to the average for the period 1816-80; - indicates that the harvest was so many days earlier than normal, corresponding to a high summer temperature (May to August). The table shows that cold winters were especially numerous in the first half of the twelfth century and again in the thirteenth. The end of the fifteenth century was marked by hot summers and mild winters, or a warmer climate; the beginning of the seventeenth century by cold (presumably snowy) winters and cool summers. Thus the periods of increased rainfall in the arid regions of Asia and America were marked by a colder climate in the rainy regions of western Europe.

1.2.3.4.5.
Period, 50 years about.Date of Wine Harvest.No. of Severe Winters.Winters with Ice on Danish coast.Remarks.
825 4
875 7
925 5
975 6
1025 6
107510
112515Cold winters
117510
122513 }
127513 }Cold winters
132513 7 }
1375114
1425+5 137
1475+1 72 Warm
1525+2.9105
1575+2.21414 }
1625+4.11711 }Cold
1675+2.71514 }
1725+0.1105
1775- 0.2—22
1825- 0.9—21

The date of the break up of the River Dwina at Mitau was recorded intermittently from 1530 to 1709, and regularly since that date, and the figures have been discussed by Rykatchef. Recasting them in our unit of fifty years we find the mean dates to be:

1551-16001601-501651-17001701-501751-18001801-50
March 29March 30March 5March 26March 26March 28

This again points to a cold period about the beginning of the seventeenth century.

The climate of Iceland and Greenland in the Middle Ages has been the subject of much controversy, the view that there were extensive changes during that period being warmly upheld by one party and as warmly combated by the other party. The case for climatic change has been well set out by O. Pettersson[7]. The Roman authors (Pliny, Solinus, etc.) wrote that there was a frozen sea about Thule (Iceland), but a party of monks who visited the island about A.D. 795 during the months of February to August, in which the ice is normally most abundant in Icelandic waters, found the coast free, though they met with a frozen sea a day’s journey to the northward. In the ninth century the Norsemen visited Iceland regularly, and at times sailed round it, apparently without interference from ice. The early settlers practised agriculture with some success. In the thirteenth century, however, the reports of ice off Iceland became frequent—apparently the conditions were worse than those of the present day, and much more so than in the eighth and ninth centuries. According to Rabot, it appears from ancient records that considerable areas cultivated in the tenth century are now covered with ice. The first spread of the glaciers took place in the first half of the fourteenth century. In the fifteenth and sixteenth centuries the climate of Iceland ameliorated somewhat, but in the seventeenth there was a readvance, which destroyed several farms about 1640 or 1650. Since then there has been a slight retreat.

The ice-conditions of Greenland are closely related to those of Iceland, and the records of the Norse colonization of Greenland bear out the conclusions drawn from the latter island. Up to the close of the twelfth century ice is hardly ever mentioned in the accounts of voyages, though it is now a great hindrance. Eric, the pioneer explorer of West Greenland, spent three successive winters on the islands in Juliaanehaab Bay (latitude 60° 45′ N.), and explored the country during the summer; “this cannot be explained otherwise than by assuming that the Polar ice did not reach Cape Farewell and the west coast of Greenland in those days.” In the thirteenth century ice is first specifically mentioned as a danger to navigation, and at the end of the fourteenth century the old Norse sailing route was on account of ice definitely abandoned in favour of one further south. Shortly afterwards the Norse colonies were wiped out by a southward migration of the Eskimos. Even in Norway itself the fourteenth century was a time of dearth, short harvests and political troubles, when corn had to be imported from Germany instead of being exported to Iceland as in former years.

It should be noted that Pettersson’s conclusions are considered invalid by H. H. Hildebrandsson[8] on the ground of the incompleteness of the records.

For the southern hemisphere our records are naturally much rarer and of less antiquity than for the northern hemisphere, and until the tree-rings are investigated we cannot carry our study back beyond the sixteenth century. From some researches into the municipal archives of Santiago de Chile, latitude 33½° S., published by B. V. Mackenna in 1877, we can infer, however, that the general course of variation since 1520 was similar to that of corresponding regions in North America. Santiago lies in a semi-arid region where a temporary shortage of water is severely felt, the average annual rainfall being only 364 mm. (14.3 inches). The early travellers, however, make no specific mention of drought, and in 1540 Pedro de Valdivia crossed the desert of Atacama with a column of troops and cattle without inconvenience—a feat which would be difficult nowadays. In 1544 there were heavy rains and great floods in June. The next record is for the year 1609, recording another heavy flood on the Mapocho, which was repeated nine years later in 1618. The first recorded drought occurred in the years 1637 to 1640; there was another flood in 1647, after which came a series of severe droughts interrupted by occasional floods, which lasted until the close of the eighteenth century. The first half of the nineteenth century was again comparatively rainy. The records thus indicate a wet period centred about 1600, followed by a dry period during the eighteenth century, exactly parallel to the records from the United States and Europe.