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PUBLICATIONS

O F THE

Astronomical Society of the Pacific.

VOL. IV.

SAN FRANCISCO, CALIFORNIA, JANUARY 30, 1892.

No. 21.

WHEN SHALL WE HAVE ANOTHER GLACIAL

EPOCH?

BY GARRETT P. SERVISS.

The enunciation by Sir ROBERT BALL of the fact, which seems to have been first clearly brought to light by him, that of the total amount of heat falling from the sun in a year, upon either the Northern or the Southern hemisphere, 63 per cent. is received in summer and 37 per cent. in winter, has awakened renewed interest in the problem of the glacial epochs. Dr. BALL'S theorem not only strengthens the astronomical explanation of the cause of such epochs, but also adds to its clearness. This explanation may be summed up in a few words.

It must be remarked, to begin with, that the summer and winter here spoken of divide the whole year between them, the equinoctial points marking the lines of division, spring and autumn being merged into the greater seasons. Everybody knows that summer in the Northern hemisphere, counting summer as extending in the broader sense just mentioned from the vernal to the autumnal equinox, is about seven days longer than winter. In the Southern hemisphere just the opposite condition prevails. This difference arises from the eccentricity of the earth's orbit. If the orbit were a circle instead of an ellipse, winter and summer would be of equal length.

But the elliptical orbit of the earth is not absolutely fixed, either in form or in its position in space. Owing to the varying attractions of the other planets, and more particularly of Venus and Jupiter, the earth's orbit is alternately rounded up almost into a circle, and then drawn out into a more eccentric ellipse. These changes require vast intervals of time, so that we must go far back into the geological ages in order to obtain evidence of

their effects upon the earth. But it is clear, theoretically, that great seasonal vicissitudes must result from such variations in the form of the earth's orbit. When it approaches nearly to a circle, which is its condition now, the difference in the length of summer and winter is small; when it is in its state of greatest eccentricity, the difference is large, amounting under the most favorable circumstances to as much as thirty-three days, or some authorities say even more, instead of seven days, its present amount.

Now the theory in question asserts that the glacial epochs in the history of the earth have occurred during those periods when, the eccentricity of the orbit being large, the difference in the length of summer and winter was at or near a maximum. Such a maximum will, of course, occur whenever the line of equinoxes is perpendicular to the major axis of the orbit. Under such circumstances one hemisphere would have 199 days of winter and 166 days of snmmer, while the other hemisphere had 166 winter and 199 summer days. But there would be no difference in the distribution of the heat coming from the sun. Just 63 per cent. of it would continue to be received in summer and 37 per cent. in winter in each hemisphere. The hemisphere which had 199 days of winter, would have to spread the 37 per cent. of heat belonging to that season over all those 199 days, while the 63 per cent. of summer heat would be concentrated upon the shorter period of 166 days. Accordingly there would be a very long and cold winter, followed by a short and hot summer. The heat of the latter would not suffice to melt away the snow and ice accumulated during the former, and this accumulation would go on until whole continents were buried under a blanket of ice thousands of feet thick. In the other hemisphere there would be, on the contrary, a short, mild winter and a long summer. So glaciation in one hemisphere would be accompanied by genial climatic conditions in the other.

Such is, in brief, the outline of the astronomical theory of the cause of glacial epochs. I have purposely used the plural in describing such epochs, although we commonly hear only one ice age spoken of, because one of the most interesting points about this theory is that while accounting for that age of ice whose handiwork is so familiar to geologists in the Northern hemisphere, it also demonstrates that there must have been many glacial epochs of varying intensity in the past, and that there will be many more in the future.

This wonderful thing, the burying of half of North America and the greater part of Europe under ice, and the absolute destruction of all their manifold forms of life, can then happen again, nay, must happen again. Naturally, the question arises, when? A precise answer cannot be given, owing to the intricate nature of the causes at work, but it seems possible to give an approximate answer. Dr. JAMES CROLL, in his work on "Climate and Time," has computed the periods of greatest and least eccentricity of the earth's orbit for a million years to come, and for a still greater period of time in the past. About 852,000 years ago the eccentricity attained its greatest possible amount. From 240,000 down to 80,000 years ago the orbit continued to be very eccentric, and Dr. CROLL'S conclusion was that the latest ice age in the Northern hemisphere ended with the close of the period of high eccentricity, 80,000 years ago.

In the future, according to Dr. CROLL'S tables, the earth's orbit will become highly eccentric about 150,000 years from the present epoch. Until that time, it appears, we shall be reasonably secure from any invasion of the ice. But the eccentricity then to be attained will be by no means the greatest possible. It will amount, according to Dr. CROLL, to 0.0353 as against 0.0168 at present, so that even if a glacial epoch then occurs it will probably not be so severe as some of those that have occurred in past time, or some that may be expected in the future. But there are three future periods of very great eccentricity indicated by Dr. CROLL, which will attain their maxima in 800,000, 900,000 and 1,000,000 years respectively from the present time, and when glaciation in its severest form may occur in one hemisphere or the other, or more probably in both alternately. Dr. CROLL'S estimates of the eccentricity of the earth's orbit at the three dates just mentioned are 0.0639, 0.0659 and 0.0528.

It is interesting to note that during the periods of minimum. eccentricity to which Dr. CROLL calls attention as separating these three periods of high eccentricity, the orbit will be even less eccentric than it is at the present. The minima will occur in 850,000 years, with the eccentricity at 0.0144, and in 950,000 years, with the eccentricity at 0.0086. But, both the descent from the high eccentricity of 800,000 years hence to the minimum in 850,000, and the subsequent ascent to the maximum in 900,000, are very sharp and steep, and the same is true of the next following minimum and maximum in 950,000 and 1,000,000. In view of these

facts one is tempted to speculate as to the chances of recovery that the animal and vegetable forms of the regions afflicted by glaciation during these coming periods of high eccentricity would have in the comparatively few thousand years of respite from the ice that would intervene between the maxima.

It will be observed that while ice ages are unquestionably recurrent phenomena, yet they are not separated by anything like regular intervals of time, simply because the conditions favoring their production do not recur at regular intervals, but are the result of exceedingly complex influences. Moreover, ice ages come in pairs or sets, alternating between the Northern and Southern hemispheres. This fact arises from the precession of the equinoxes, by which, once in every 10,500 years, an interchange of condition is effected between the hemispheres. At present, for instance, we in the Northern hemisphere have our winter when the earth is nearest to the sun, and it is seven days shorter than the summer. In the Southern hemisphere, on the other hand, winter occurs when the earth is furthest from the sun and is seven days longer than the summer. If the eccentricity of the earth's orbit were as great now as it will be 150,000 years from now, and more particularly as it will be 800,000 years hence, the Southern hemisphere at present would be suffering from a glacial epoch, while we should enjoy short, mild winters and equable summers, longer than those we have now, but not quite so hot. In about 10,500 years, however, a complete interchange will have taken place, and then our hemisphere will have its winters when the earth is furthest from the sun, and its summers when it is nearest. It will hardly be so comfortable in the United and Europe then as it is in our day, although no glacial invasion is to be expected.

It is because the periods during which the earth's orbit remains greatly eccentric when once drawn out by planetary attraction, are far longer than 10,500 years, that two or more successive ice ages may occur in each hemisphere during the prevalence of a single period of high eccentricity. If the condition of great eccentricity were a phenomena of comparatively brief duration, both hemispheres might escape glaciation during such a period, because it might happen that, while the orbit was drawn out into its extreme state of eccentricity, the equinoxes would nearly coincide with the apsides, and so winter and summer would be of equal duration.

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