Part 8 (1/2)

V. The occurrence of widespread glaciation near the tropics during the Permian, as shown in Fig. 7, has given rise to much discussion. The recent discovery of glaciation in lat.i.tudes as low as 30 in the Proterozoic is correspondingly significant. In all cases the occurrence of glaciation in low and middle lat.i.tudes is probably due to the same general causes. Doubtless the position and alt.i.tude of the mountains had something to do with the matter. Yet taken by itself this seems insufficient. Today the loftiest range in the world, the Himalayas, is almost unglaciated, although its southern slope may seem at first thought to be almost ideally located in this respect. Some parts rise over 20,000 feet and certain lower slopes receive 400 inches of rain per year. The small size of the Himalayan glaciers in spite of these favorable conditions is apparently due largely to the seasonal character of the monsoon winds. The strong outblowing monsoons of winter cause about half the year to be very dry with clear skies and dry winds from the interior of Asia. In all low lat.i.tudes the sun rides high in the heavens at midday, even in winter, and thus melts snow fairly effectively in clear weather. This is highly unfavorable to glaciation.

The inblowing southern monsoons bring all their moisture in midsummer at just the time when it is least effective in producing snow. Conditions similar to those now prevailing in the Himalayas must accompany any great uplift of the lands which produces high mountains and large continents in subtropical and middle lat.i.tudes. Hence, uplift alone cannot account for extensive glaciation in subtropical lat.i.tudes during the Permian and Proterozoic.

[Ill.u.s.tration: _Fig. 7. Permian geography and glaciation._ (_After Schuchert._)]

The a.s.sumption of a great general lowering of temperature is also not adequate to explain glaciation in subtropical lat.i.tudes. In the first place this would require a lowering of many degrees,--far more than in the Pleistocene glacial period. The marine fossils of the Permian, however, do not indicate any such condition. In the second place, if the lands were widespread as they appear to have been in the Permian, a general lowering of temperature would diminish rather than increase the present slight efficiency of the monsoons in producing glaciation.

Monsoons depend upon the difference between the temperatures of land and water. If the general temperature were lowered, the reduction would be much less p.r.o.nounced on the oceans than on the lands, for water tends to preserve a uniform temperature, not only because of its mobility, but because of the large amount of heat given out when freezing takes place, or consumed in evaporation. Hence the general lowering of temperature would make the contrast between continents and oceans less than at present in summer, for the land temperature would be brought toward that of the ocean. This would diminish the strength of the inblowing summer monsoons and thus cut off part of the supply of moisture. Evidence that this actually happened in the cold fourteenth century has already been given in Chapter VI. On the other hand, in winter the lands would be much colder than now and the oceans only a little colder, so that the dry outblowing monsoons of the cold season would increase in strength and would also last longer than at present. In addition to all this, the mere fact of low temperature would mean a general reduction in the amount of water vapor in the air. Thus, from almost every point of view a mere lowering of temperature seems to be ruled out as a cause of Permian glaciation. Moreover, if the Permian or Proterozoic glacial periods were so cold that the lands above lat.i.tude 30 were snow-covered most of the time, the normal surface winds in subtropical lat.i.tudes would be largely equatorward, just as the winter monsoons now are. Hence little or no moisture would be available to feed the snowfields which give rise to the glaciers.

It has been a.s.sumed by Marsden Manson and others that increased general cloudiness would account for the subtropical glaciation of the Permian and Proterozoic. Granting for the moment that there could be universal persistent cloudiness, this would not prevent or counteract the outblowing anti-cyclonic winds so characteristic of great snowfields.

Therefore, under the hypothesis of general cloudiness there would be no supply of moisture to cause glaciation in low lat.i.tudes. Indeed, persistent cloudiness in all higher lat.i.tudes would apparently deprive the Himalayas of most of their present moisture, for the interior of Asia would not become hot in summer and no inblowing monsoons would develop. In fact, winds of all kinds would seemingly be scarce, for they arise almost wholly from contrasts of temperature and hence of atmospheric pressure. The only way to get winds and hence precipitation would be to invoke some other agency, such as cyclonic storms, but that would be a departure from the supposition that glaciation arose from cloudiness.

Let us now inquire how the cyclonic hypothesis accounts for glaciation in low lat.i.tudes. We will first consider the terrestrial conditions in the early Permian, the last period of glaciation in such lat.i.tudes.

Geologists are almost universally agreed that the lands were exceptionally extensive and also high, especially in low lat.i.tudes. One evidence of this is the presence of abundant conglomerates composed of great boulders. It is also probable that the carbon dioxide in the air during the early Permian had been reduced to a minimum by the extraordinary amount of coal formed during the preceding period. This would tend to produce low temperature and thus make the conditions favorable for glaciation as soon as an accentuation of solar activity caused unusual storminess. If the storminess became extreme when terrestrial conditions were thus universally favorable to glaciation, it would presumably produce glaciation in low lat.i.tudes. Numerous and intense tropical cyclones would carry a vast amount of moisture out of the tropics, just as now happens when the sun is active, but on a far larger scale. The moisture would be precipitated on the equatorward slopes of the subtropical mountain ranges. At high elevations this precipitation would be in the form of snow even in summer. Tropical cyclones, however, as is shown in _Earth and Sun_, occur in the autumn and winter as well as in summer. For example, in the Bay of Bengal the number recorded in October is fifty, the largest for any month; while in November it is thirty-four, and December fourteen as compared with an average of forty-two for the months of July to September. From January to March, when sunspot numbers averaged more than forty, the number of tropical hurricanes was 143 per cent greater than when the sunspot numbers averaged below forty. During the months from April to June, which also would be times of considerable snowy precipitation, tropical hurricanes averaged 58 per cent more numerous with sunspot numbers above forty than with numbers below forty, while from July to September the difference amounted to 23 per cent. Even at this season some snow falls on the higher slopes, while the increased cloudiness due to numerous storms also tends to preserve the snow. Thus a great increase in the frequency of sunspots is accompanied by increased intensity of tropical hurricanes, especially in the cooler autumn and spring months, and results not only in a greater acc.u.mulation of snow but in a decrease in the melting of the snow because of more abundant clouds. At such times as the Permian, the general low temperature due to rapid convection and to the scarcity of carbon dioxide presumably joined with the extension of the lands in producing great high-pressure areas over the lands in middle lat.i.tudes during the winters, and thus caused the more northern, or mid-lat.i.tude type of cyclonic storms to be s.h.i.+fted to the equatorward side of the continents at that season. This would cause an increase of precipitation in winter as well as during the months when tropical hurricanes abound. Many other circ.u.mstances would cooperate to produce a similar result. For example, the general low temperature would cause the sea to be covered with ice in lower lat.i.tudes than now, and would help to create high-pressure areas in middle lat.i.tudes, thus driving the storms far south. If the sea water were fresher than now, as it probably was to a notable extent in the Proterozoic and perhaps to some slight extent in the Permian, the higher freezing point would also further the extension of the ice and help to keep the storms away from high lat.i.tudes. If to this there is added a distribution of land and sea such that the volume of the warm ocean currents flowing from low to high lat.i.tudes was diminished, as appears to have been the case, there seems to be no difficulty in explaining the subtropical location of the main glaciation in both the Permian and the Proterozoic. An increase of storminess seems to be the key to the whole situation.

One other possibility may be mentioned, although little stress should be laid on it. In _Earth and Sun_ it has been shown that the main storm track in both the northern and southern hemispheres is not concentric with the geographical poles. Both tracks are roughly concentric with the corresponding magnetic poles, a fact which may be important in connection with the hypothesis of an electrical effect of the sun upon terrestrial storminess. The magnetic poles are known to wander considerably. Such wandering gives rise to variations in the direction of the magnetic needle from year to year. In 1815 the compa.s.s in England pointed 24-1/2 W. of N. and in 1906 17 45' W. Such a variation seems to mean a change of many miles in the location of the north magnetic pole. Certain changes in the daily march of electromagnetic phenomena over the oceans have led Bauer and his a.s.sociates to suggest that the magnetic poles may even be subject to a slight daily movement in response to the changes in the relative positions of the earth and sun.

Thus there seems to be a possibility that a p.r.o.nounced change in the location of the magnetic pole in Permian times, for example, may have had some connection with a s.h.i.+fting in the location of the belt of storms. It must be clearly understood that there is as yet no evidence of any such change, and the matter is introduced merely to call attention to a possible line of investigation.

Any hypothesis of Permian and Proterozoic glaciation must explain not only the glaciation of low lat.i.tudes but the lack of glaciation and the acc.u.mulation of red desert beds in high lat.i.tudes. The facts already presented seem to explain this. Glaciation could not occur extensively in high lat.i.tudes partly because during most of the year the air was too cold to hold much moisture, but still more because the winds for the most part must have blown outward from the cold northern areas and the cyclonic storm belt was pushed out of high lat.i.tudes. Because of these conditions precipitation was apparently limited to a relatively small number of storms during the summer. Hence great desert areas must have prevailed at high lat.i.tudes. Great aridity now prevails north of the Himalayas and related ranges, and red beds are acc.u.mulating in the centers of the great deserts, such as those of the Tarim Basin and the Transcaspian. The redness is not due to the original character of the rock, but to intense oxidation, as appears from the fact that along the edges of the desert and wherever occasional floods carry sediment far out into the midst of the sand, the material has the ordinary brownish shades. As soon as one goes out into the places where the sand has been exposed to the air for a long time, however, it becomes pink, and then red. Such conditions may have given rise to the high degree of oxidation in the famous Permian red beds. If the air of the early Permian contained an unusual percentage of oxygen because of the release of that gas by the great plant beds which formed coal in the preceding era, as Chamberlin has thought probable, the tendency to produce red beds would be still further increased.

It must not be supposed, however, that these conditions would absolutely limit glaciation to subtropical lat.i.tudes. The presence of early Permian glaciation in North America at Boston and in Alaska and in the Falkland Islands of the South Atlantic Ocean proves that at least locally there was sufficient moisture to form glaciers near the coast in relatively high lat.i.tudes. The possibility of this would depend entirely upon the form of the lands and the consequent course of ocean currents. Even in those high lat.i.tudes cyclonic storms would occur unless they were kept out by conditions of pressure such as have been described above.

The marine faunas of Permian age in high lat.i.tudes have been interpreted as indicating mild oceanic temperatures. This is a point which requires further investigation. Warm oceans during times of slight solar activity are a necessary consequence of the cyclonic hypothesis, as will appear later. The present cold oceans seem to be the expectable result of the Pleistocene glaciation and of the present relatively disturbed condition of the sun. If a sudden disturbance threw the solar atmosphere into violent commotion within a few thousand years during Permian times, glaciation might occur as described above, while the oceans were still warm. In fact their warmth would increase evaporation while the violent cyclonic storms and high winds would cause heavy rain and keep the air cool by constantly raising it to high levels where it would rapidly radiate its heat into s.p.a.ce.

Nevertheless it is not yet possible to determine how warm the oceans were at the actual time of the Permian glaciation. Some faunas formerly reported as Permian are now known to be considerably older. Moreover, others of undoubted Permian age are probably not strictly contemporaneous with the glaciation. So far back in the geological record it is very doubtful whether we can date fossils within the limits of say 100,000 years. Yet a difference of 100,000 years would be more than enough to allow the fossils to have lived either before or after the glaciation, or in an inter-glacial epoch. One such epoch is known to have occurred and nine others are suggested by the inter-stratification of glacial till and marine sediments in eastern Australia. The warm currents which would flow poleward in inter-glacial epochs must have favored a prompt reintroduction of marine faunas driven out during times of glaciation. Taken all and all, the Permian glaciation seems to be accounted for by the cyclonic hypothesis quite as well as does the Pleistocene. In both these cases, as well as in the various pulsations of historic times, it seems to be necessary merely to magnify what is happening today in order to reproduce the conditions which prevailed in the past. If the conditions which now prevail at times of sunspot minima were magnified, they would give the mild conditions of inter-glacial epochs and similar periods. If the conditions which now prevail at times of sunspot maxima are magnified a little they seem to produce periods of climatic stress such as those of the fourteenth century. If they are magnified still more the result is apparently glacial epochs like those of the Pleistocene, and if they are magnified to a still greater extent, the result is Permian or Proterozoic glaciation. Other factors must indeed be favorable, for climatic changes are highly complex and are unquestionably due to a combination of circ.u.mstances. The point which is chiefly emphasized in this book is that among those several circ.u.mstances, changes in cyclonic storms due apparently to activity of the sun's atmosphere must always be reckoned.

FOOTNOTES:

[Footnote 46: W. H. Hobbs: Characteristics of Existing Glaciers, 1911.

The Role of the Glacial Anticyclones in the Air Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.]

[Footnote 47: R. D. Salisbury: Physiography, 1919.]

[Footnote 48: Griffith Taylor: Australian Meteorology, 1920, p. 283.]

[Footnote 49: J. D. Whitney: Climatic Changes of the Later Geological Times, 1882.]

[Footnote 50: E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914.

Mr. Free has prepared a summary of this Bulletin which appears in The Solar Hypothesis, Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.]

[Footnote 51: G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol.

Surv.]

[Footnote 52: C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914, pp. 63-66.]

[Footnote 53: H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.

Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc., Vol. 42, Sept., 1910, pp. 641-661.]

[Footnote 54: S. S. Visher: The Bajada of the Tucson Bolson of Southern Arizona; Science, N. S., Mar. 23, 1913.