Abstract
FOR some time past it has been generally believed that the climate of Central Asia was once less arid than at present, but we now know, as Dr. Sven Hedin explained to the Royal Geographical Society on December 8 (p. 134), that important changes have taken place since the Christian era began. He found in the Lob Nor region forests with the trees long dead, traces of a road, ruined villages, coins, manuscripts and other relics which proved the northern shore of the old salt lake (now dry) to have been cultivated and occupied, down to about sixteen centuries ago, by a fairly civilised people. This, I think, implies a rainfall, less inappreciable than the present one, during the earlier centuries of that era, and the change, as he found dead forests, cannot be attributed (as in parts of southern Europe and Syria) to reckless destruction by the hand of man. But, besides this, Sir Norman and Dr. W. J. S. Lockyer have recently proved (in a communication to the Royal Society) a very remarkable correspondence to exist between the distribution of the periodic rains in India, Mauritius and elsewhere and the amount of solar activity, and they have, within the last few days, drawn the attention of the same Society to the fact that zones of abnormally high and low mean barometric pressure exist on opposite sides of the earth and oscillate from the one position to the other in accordance with the periodic small variations of solar activity, Dr. Sven Hedin's discovery apparently indicates a change secular rather than periodic, but may not both operate independently, as in the case of changes due to variations of eccentricity in the earth's orbit and to precession of the axis of rotation? The authors of those papers admit the existence of disturbing causes, some of which may be local, but not necessarily all. Is it, then, possible that these discoveries may afford a clue to the solution of two great geological puzzles—the abnormal temperatures of the Pleistocene and of early Tertiary times? In regard to the former, many now believe that the climate of North Central Europe when the loess was deposited more nearly resembled that of the Caspian steppes, and all maintain that in the Glacial epoch the mean temperature of the whole continent was much below what it is now. How much this, was, at the time of greatest cold, is not easily estimated, but a few years ago I attempted a rough approximation. This will be found in a volume of the Contemporary Science Series called “Ice Work” (part iii. chap, i.), and the results (for Europe) are as follows:—Supposing the British Isles to be at their present level (in order to avoid the controversy as to the origin of Boulder-clays and Glacial gravels), the mean temperature of these islands at the present Ordnance Datum would have to be lowered by about 20° F. The same would probably hold good of Scandinavia—at any rate, that would suffice to make either country much more closely resemble a corresponding part of Greenland. In the more central parts of Europe, the problem is rather easier, for here we are undoubtedly dealing with “land-ice.” A fall of 18° in the mean temperature would suffice for the Alps; perhaps rather less, 15° or 16°, for the Pyrenees, the Sierras Guadarrama and Nevada, possibly also for the breccia-producing age on the Rock of Gibraltar. A reduction of 18° at most, and more probably about 16° or 15°, would bring back small glaciers to Auvergne, the Schwarzwald, Vosges, Apennines, Corsican mountains, the Caucasus and even the Atlas. I may add that a reduction of 15° appears sufficient to form a great ice-sheet in North America, and that in the southern hemisphere and at Mount Kenya in Africa distinctly smaller change suffices. All these estimates assume the present levels maintained; they may be corrected at the rate of 1° for each 300 feet of elevation or depression. But geologists too often forget that the anomaly of early Tertiary heat is not less difficult to explain than that of Pleistocene cold, for in later Eocene ages the mean temperature of southern England can hardly have been less than 20° above that which it now enjoys. The explanations which have been offered for the Glacial epoch—a different arrangement of sea and land, variations in eccentricity, precessional movements (none of which, in my opinion, are more than partially successful)—cannot be applied to the latter case, so that we seem compelled to seek for some other cause. Variations in solar heat have been already suggested, but hitherto this hypothesis has seemed too much a Deus ex machinâ. But as Dr. Sven Hedin's discoveries show that important alterations in climate have been in progress during the last fifteen or sixteen centuries, and Sir Norman Lockyer's researches indicate that comparatively small changes in solar activity produce rather important meteorological effects upon the earth, geologists qualified for the investigation may find it not unprofitable to follow up the clue.
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BONNEY, T. Secular Changes of Climate . Nature 67, 150 (1902). https://doi.org/10.1038/067150a0
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DOI: https://doi.org/10.1038/067150a0
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