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.