(2) How Salt Lakes Show Changes of Rainfall.—Salt lakes are a particularly sensitive index of changes of climate since, having no outlets, they rise and fall in response to increased or decreased rainfall. For example, Owens Lake at the eastern base of the Sierras in Cali fornia must formerly have been fresh, for an old outlet channel, rem nants of a great beach, and cliffs cut by the waves prove that it once stood 180 feet higher than now and sent out an abundant overflow. When the Los Angeles aqueduct was being built the amount of salt now in the lake was found to be only as much as would be brought in by the Owens and other tributaries in not much more than 2000 years. Therefore geologists conclude that at the time of Christ or a few centuries earlier the lake must have been a body of fresh water with an outlet. If that is so, the climatic cycle which enabled Palmyra to prosper so greatly must have had a similar effect in south ern California. Other lakes in our own country and especially in Asia show similar indications of changes of climate so that their evi dence is as widespread as that of ruins.
How the Caspian Sea Shows Alternate Wet and Dry Epochs.— The changes of climate during historic times do not seem to belong to one cycle but to several. Since records of the level of the Caspian Sea are available for 2000 years that salt lake furnishes an uncom monly good measure of the climatic cycles of the Christian Era. These records relate to three main kinds of facts: (1) the distance from the lake shore to known land-marks; (2) old walls like the Great Chinese Wall and old buildings which were built on dry land but are now submerged beneath the lake; (3) old buildings which the lake once reached, but which are now above its level. All these kinds of facts indicate that at the time of Christ or earlier the Caspian Sea stood at a level 75 or 100 feet above the level of to-day. Six or seven hundred years later the climate was so dry that the lake stood even lower than now, as is indicated by the ends of two great walls constructed to keep out barbarians. A few hundred years later, how ever, in the tenth century, the wet part of a cycle was reached, for a Persian geographer tells us that the Caspian Sea had then risen some 40 or 50 feet to the level of a certain tower, and a little later the water probably rose still higher. It is interesting to know that at this same time Palmyra partially recovered.
This does not end the fluctuations of the Caspian Sea, however, for in the twelfth or thirteenth century a second dry period again lowered the lake level. A century later the old records tell us that the Caspian again rose to a height of nearly 40 feet above the present level. Thus it appears that the Caspian Sea stood at a surprisingly high level at the time of Christ, at a low level six or seven hundred years later, high again about 1000 A.D., low in the thirteenth cen
tury, high in the fourteenth, and now low once more. Hence it is clear that there have been two complete main cycles of rainfall since the time of Christ, and we are now in a third.
(3) How Trees Show Climatic Cycles.—Within 50 miles of Owens Lake, but on the other side of the Sierras, the famous Big Trees of California furnish a still fuller record of these same climatic cycles and of many smaller ones during a period of over 3000 years. The rings of trees vary in thickness for several causes, but in a dry climate like that of southern California the chief cause is the amount of rain and the season at which it falls. As hundreds of the Big Trees have been cut down for fence posts and matchwood, it is possible to measure the thickness of the rings as they appear on the stumps. By count ing in from the edge it is easy to find a ring that was formed in 1776, for instance, in 1492, or at the birth of Christ.
The rings dating from the time of Christ are thick and indicate that at that time, when Palmyra had an abundant supply of water, when Owens Lake overflowed and there was high water in the Caspian Sea, the Big Trees also had plenty of water and grew rapidly. Six or seven hundred years later when Palmyra was abandoned and when the Caspian Sea stood 15 or more feet lower than at present, the trees formed only narrow rings, because the climate was dry. The way in which the growth of the trees has varied is shown in Fig. 116. The high parts of the curve indicate abundant rainfall. The black shading at the bottom indicates periods of comparative aridity in the subtropical belt.
Variations in Climatic Cycles According to Latitude.—Climatic cycles, whether large or small, produce uniform effects in similiar climatic regions, but quite different effects in different zones. The wonderful Maya ruins of Guatemala, for example, appear to indicate that when California had abundant rainfall, the northern parts of Central America suffered a decrease. Thus at the time of Christ and for a few centuries thereafter, and again about A.D. 1000 Guatemala and Yucatan appear to have been drier than now. Many of the most remarkable ruins are located in places where the forest is of the equatorial type and so dense and malarial that agriculture and civili zation are now apparently impossible. In the days of the Mayas, however, the climate was apparently dry enough so that conditions were like those of the more favorable jungle regions instead of like those of the dense rain forests. All these changes, both in the sub tropical and equatorial regions, were apparently due to a shifting of the location of the climatic belts.