BIBLIOGRAPHY
Reid, C. “Submerged forests.” Cambridge University Press, 1913.
Lewis, F. J. “The history of the Scottish peat-mosses and their relation to the Glacial period.” Edinburgh, Scot. Geogr. Mag., 22, 1906, p. 241.
CHAPTER XVI
THE “CLASSICAL” RAINFALL MAXIMUM, 1800 B.C. TO A.D. 500
About 1800 B.C., or the beginning of the Bronze Age in Britain, the subsiding land finally attained approximately its present level. At the same time the climate of western Europe deteriorated, becoming much more humid and rainy, and there set in a period of intense peat-formation in Ireland, Scotland and northern England, Scandinavia and North Germany, known as the Peat-Bog Period or Upper Turbarian. The peat-beds choked and killed the forests which had developed on the older peat-bogs, and grew up above the stools and fallen trunks, so that we have two layers of peat separated by an old forest. The forest level contains neolithic articles, the peat contains gold collars, bronze swords and pins, and other objects of the Bronze Age. This growth also went on even over high ground, which had not previously been covered by peat, for Professor Henry informs us that on Copped Mountain, near Enniskillen, and at other places in Ireland, Bronze Age cairns and tumuli are found resting on rock and covered by several feet of bog. Peat-beds on the Frisian dunes between two layers of blown sand are dated about 100 B.C., and some bogs in northern France were formed during the Roman period. There is also some much-disputed contemporary Latin evidence that at the time of the Roman occupation the climate of Britain was damp and boggy, while Gibbon (“Decline and Fall of the Roman Empire”), referring to the climate of central Europe at the beginning of the Christian era, points to some evidence that the climate was colder. This is, that the Rhine and the Danube were frequently frozen over, so that the natives crossed them with cavalry and wagons without difficulty, although at the present time this never happens. It is possible that this severe climate is referred to in the Germanic legend of the “Twilight of the Gods,” when frost and snow ruled the world for generations. The Norse sagas point to a similar cold period in Scandinavia. This lapse of climate occurred in the Early Iron Age, about 650 to 400 B.C., when there was a rapid deterioration from the high Scandinavian civilization of the Bronze Age. This deterioration of culture was probably the direct result of the increased severity of the climate.
This Pluvial period has been made the subject of special studies by Ellsworth Huntington in several important books and papers; he finds evidence of a distinctly Pluvial period in three regions—the Mediterranean, central and south-western Asia, and an area including the southern United States and northern Mexico. In the first of these, the Mediterranean, Huntington considers that the Græco-Roman civilizations grew up in a period of increased rainfall which lasted from about 500 B.C. to A.D. 200. These states were able to develop in comparative peace because during this time there were no great invasions of nomadic peoples from eastern Europe or central Asia, a fact which points to good rainfall in these comparatively dry regions, so that their inhabitants had no need to emigrate in quest of a living. In the Mediterranean itself the heavier rainfall allowed a solid agricultural basis which produced a sturdy race of peasants who made good soldiers. Owing to the greater cyclonic control of climate and consequent changeable weather, these inhabitants were more vigorous in mind and body, for Huntington’s researches have demonstrated that long spells of monotonous weather, either fine or rainy, are unfavourable for human energy. Finally the heavier rainfall maintained a perennial flow in the rivers, giving plentiful supplies of good drinking water. These conditions broke down earlier in Greece than in Italy, as the latter naturally has a heavier rainfall. Huntington considers that the decline of Greece was largely due to malarial poisoning, the decreasing rainfall causing the river-flow to break down in summer, leaving isolated pools forming a breeding ground for mosquitoes.
After A.D. 200 the climate of Italy also deteriorated. The decrease of rainfall, combined with gradual exhaustion of the soil, made wheat-growing more and more difficult for the small agriculturalist, and the farms came into the hands of large landowners, who worked them by slave labour, and in place of wheat either grew vines or olives or raised flocks and herds. The agricultural population gravitated to Rome and a few other large cities, and had to be fed by imported wheat. The decline was probably aided by the introduction of malaria, as in Greece.
In north Africa and Palestine the question is more debatable. C. Negro, who has investigated the supposed desiccation of Cyrenaica, concludes that there has been no change of climate since Roman times, but a careful study of his evidence suggests that his conclusions are open to criticism. All that he has proved is that there has been no marked progressive decrease of rainfall since about A.D. 200; he has ignored the possibility of great fluctuations before and since that date. In north Africa it seems difficult to believe that the great cities of antiquity could have existed under present climatic conditions, but when we turn to Palmyra in the Syrian desert we have practically incontrovertible proof in the great aqueducts, built to carry from the hill-springs to the city large volumes of water which these springs no longer deliver, so that even where they are intact the aqueducts now carry only the merest trickle.
In Persia we find numerous ruins, which point to a much greater population two thousand or more years ago. This population lived by agriculture, and the remains of their irrigation works are now found in regions where running water never comes. Even the scanty population of to-day can hardly live on the present rainfall of the country, and it is unbelievable that the much greater population indicated by these ruined cities could have existed without a very much greater supply of water. The same condition is indicated by the ruined cities of the great deserts of central Asia. These cities were inhabited by agriculturalists, and the remains of tilled fields, terraces and irrigation works abound in places where the supply of brackish water would now be barely sufficient for drinking purposes for such a large population. Huntington has also made a careful study of the water-level of the Caspian Sea in classical times, and finds that there was a great period of high water extending from unknown antiquity to about A.D. 400.
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.
Although at first sight the effect which Pettersson sets out to explain seems out of all proportion to the smallness of his cause, the coincidences after 2000 B.C. are extremely interesting, and suggest that after the land and sea distribution reached its present form the astronomical cause adduced by Pettersson was possibly effective, but before that date the astronomical cause, if it existed, was masked by the much greater climatic variations due to changes in the land and sea distribution.
The opinion has frequently been expressed that the “Classical” and “Mediæval” rainfall maxima were phenomena similar to the Glacial period, but less intensive. This view is often carried to its logical conclusion, that the thirty-five-year cycle, the eleven-year, and still smaller cycles of climate, are also part of the same series, and that the Glacial period and, let us say, the three-year periodicity of rainfall are therefore due to variations of the same agent, in this case the sun. This logical extension of the theory is, however, completely untenable. The eleven-year periodicity is admittedly connected with variations in the solar activity, but there are other cycles which are completely independent of such variations, such as, for instance, the annual variation undergone by all meteorological elements, which depends entirely on the inclination of the earth’s axis. There is a well-marked 4.8 year period in the amount of ice off Iceland, the half-cycle of which is exactly equal to the distance travelled by the water taking part in the North Atlantic circulation, divided by the velocity with which it travels. There is, therefore, no a priori reason for assuming that the cause of the Glacial period was identical with the cause of the Classical and Mediæval rainfall maxima. Further, in the latter case, the chief phenomenon was the increase of rainfall; the decrease of temperature was merely incidental, but in the Glacial period the outstanding feature was a great lowering of temperature in the polar and temperate regions, and in this case it was the increase of rainfall which was incidental.