There is only one region in central Asia where the population appears to have been less in classical times than now, and that is the high basin of Kashmir. Huntington points out that this basin is at present near the upward limit of agriculture, and any fall of temperature and increase of snowfall would drive out the inhabitants. But local legends point to such a period in the remote past, corresponding to the period of increased habitability of the central Asian deserts; at its close there were extensive migrations from Turkestan into Kashmir.
Passing to America, we come to interesting evidence of a very different class—I refer to the “big trees” (Sequoia) of California. Since these trees live in a semi-arid climate, the amount of rainfall is the chief factor in their growth, which finds an expression in the breadth of the annual rings measured on the stump of the tree when it is cut down. The method of utilizing the data was due to A. E. Douglass. A careful comparison was first made between the measurements of rings and the rainfall measured at neighbouring stations, and a formula was developed by which the rainfall of each year could be reconstructed from the tree-growth with a high degree of accuracy. In extrapolating to find the rainfall for earlier years before rainfall measurements began, various corrections had to be applied, for instance trees grow more rapidly when young than when they are old, while trees which are likely to live to a great age grow more slowly at first than trees which die younger. These methods were applied to nearly two thousand “big trees,” some of which were found to be four thousand years old, but it is pointed out that the corrections eliminate any progressive variation of climate which may have occurred, so that the results show only “cycles” of greater or lesser length. Summing up, Huntington says: “Judging from what we have seen of the rainfall of to-day and its relation to the growth of the Sequoias, high portions of their curve (of growth) seem to indicate periods when the winters were longer than now, when storms began earlier in the fall and lasted later into the spring, and when mid-winter was characterized by the great development of a cold continental high-pressure area, which pushed the storms of the prevailing zone of westerly winds far down into sub-tropical regions and thus caused sub-tropical conditions to invade what is now the zone of equatorial rains.” Neglecting later favourable periods, which are relatively short and unimportant, it is found that these conditions prevailed very markedly between 1200 B.C. and A.D. 200, with maxima about 1150 B.C., 700 B.C., and from 450 B.C. to 250 B.C.
Thus over the greater part of the temperate regions of the northern hemisphere we have evidence of an important rainy period between the extreme limits of 1800 B.C. and A.D. 400 or 500. This period was best developed from 1200 B.C. to A.D. 200, and reached its maximum about 400 B.C. It constitutes a remarkable wave of climatic variation, which is hitherto without adequate explanation. A somewhat similar, though less intense, wave which occurred about A.D. 1200-1300, and which is described in the following chapter, was associated by Wolf to a great outburst of sunspots which took place about A.D. 1200. It is well known that sunspots are an index of solar activity, the sun’s radiation being greater at times of spot maximum than at times of spot minimum. Greater solar radiation increases the evaporation over the oceans, so that the air becomes more humid. This moist air is carried by atmospheric currents over the land, where the moisture is condensed into clouds and greatly increases the rainfall. At the same time the cloud canopy shuts off some of the direct heat of the sun, and we have the curious paradox that at times of sunspot maximum, or greatest solar radiation, the temperature of the earth’s surface is lowest.
The connexion outlined above is, however, extremely problematical for temperate regions. Since the absolute sunspot maximum at A.D. 1200 is also very doubtful, it will be realized that the evidence for the sunspot hypothesis of the mediæval rainfall maximum is extremely slender. Furthermore, since we know nothing whatever about the solar activity during the classical rainfall maximum, we are still less in a position to extend the sunspot hypothesis to that period also.
The interesting theory recently put forward by O. Pettersson, already alluded to, provides a plausible alternative explanation of the severe stormy climate of the Peat-bog period, which reached a maximum near 400 B.C. Without going into details this theory is that the strength of the tides depends on the relative positions of the sun and moon, and the tides are greatest when these act in conjunction, and also when they are nearest to the earth. This fluctuation of strength passes through various cyclic variations with periods of nine years, about ninety years and about 1800 years, though the lengths of the periods are not constant. The latter cycle is most important to our purposes; according to Pettersson’s calculations the fluctuations of the “tide-generating force” were as follow:
| Maxima | 3500 b.c. | 2100 b.c. | 350 b.c. | a.d. 1434 |
| Minima | 2800 b.c. | 1200 b.c. | a.d. 530. |
Increased range of the tides means increased circulation in the waters of the oceans, especially an increased interchange between the warm North Atlantic and the cold Arctic waters. It also means than an unusual amount of ice is brought down from high into low latitudes. Wide local variations of temperature of the surface waters of the oceans cause increased cyclonic activity, and hence we may expect a generally increased storminess at times of maximum “tide-generating force,” and the reverse at times of minimum.
For the last maximum (A.D. 1434) Pettersson is able to adduce a good deal of historical evidence of increased storminess in north-west Europe and bad ice-conditions near Iceland and Greenland, while Huntington has found an increase of rainfall shown by the big trees of California. The next preceding maximum, that of 360 B.C., marks the culminating point of the Peat-bog phase. The Norse sagas and the Germanic myths point to a severe climate about 650 B.C., which destroyed an early civilization. This was the “Twilight of the Gods,” when frost and snow ruled the world for generations. The period was the Early Iron Age, when civilization deteriorated greatly in north-west Europe.
Of the maximum of 2100 B.C. there is no trace. It is possible that the great Atlantic submergence of the Maritime phase is connected with the tidal maximum of 3500 B.C., but the phenomena were on a scale so much greater than those of the more recent maxima that this can hardly have been the sole cause.
The minima should have been characterized by periods of relatively quiet stable climate with little ice near Iceland and Greenland. That the last minimum, in A.D. 530, was such a period there is considerable evidence in the high level reached by civilization at that period in Scandinavia and by the revival in Ireland. Again, about 1200 B.C., in the early part of the Peat-bog phase, there is evidence of considerable traffic by sea between Scandinavia and Ireland. The Irish Museum has lately discovered a hoard of gold objects dated about 1000 B.C., in which the designs show a Scandinavian origin. The minimum of 2800 B.C., which occurred in the Forest phase, may have contributed to the dry climate of that period, but otherwise has left no trace.