Popular Science Monthly/Volume 8/April 1876/The Polar Glaciers I
|THE POLAR GLACIERS.|
THE centre of gravity of the earth is the centre of the sphere formed by the surface of the oceans; or rather, owing to the flattening of the earth at the poles, it is a point equally distant, in opposite directions, from the level of the sea. The waters, being free to move, must of necessity conform themselves to this equidistance from the gravitating centre of the whole mass. Inasmuch, then, as any plane which cuts the earth into two parts through its centre of gravity must equally divide the weight of the whole earth, it follows also that the same plane would exactly bisect the great spheroid of the oceans. In each hemisphere the sea-level in all corresponding parts would be at the same distance from this centre; and whatever land and mountains there might be above the ocean in one half would have to be counter-balanced by land, or an excess of weight of some sort, in the other half. And this counterpoising weight must itself rise above the level of the sea, unless we say that one side of the world is composed of heavier materials than the other, of which there is not the least evidence or probability.
If the plane thus dividing the earth be that of its equator, there will be found in the northern hemisphere about 44,000,000 square miles of land, and in the southern, so far as is known, about 16,000,000 square miles. Now, the great problem in physical geography is. What is there in the southern hemisphere to counterbalance this great excess of land in the northern?
Humboldt has estimated that, if the mountains and highlands of Asia were leveled down and made to fill up evenly the low places, the whole continent would have a uniform height of 1,150 feet above the sea. In like manner, South America would have a height of 1,130 feet; North America of 750 feet; and Europe of 670 feet. The average of the whole he estimates at 920 feet. Of the mainlands not included in the above—namely, Africa, Australia, the polar lands, and islands—about as much is north as south of the equator. So that we may safely estimate that there is in the northern hemisphere an excess of 28,000,000 square miles of land, of the average height above-mentioned, to be counterpoised by something yet to be found in the southern hemisphere.
If there is an excess in the quantity or bulk of water south of the equator over that north of it, then the difference of weight between this excess and so much land, which is about in the proportion of one to two and a half, must be added to the unknown quantity which we are soon to look for above the southern seas. As there is, of course, the same excess of water-surface south of the equator that there is of land-surface north of it, and as we may very safely assume that the oceans have a mean depth of at least 3,220 feet (31 X 920) and that the southern waters average as deep as the northern, it follows that our unknown quantity is at the very least doubled by the above considerations. We have, therefore, to seek in the southern hemisphere what will balance 28,000,000 square miles of land at least 1,840 feet high.
We look over the map of the world, and down near the bottom we find some uncertain landmarks with many breaks, but on the whole tracing out very nearly the antarctic circle, and indicating that there is, covering nearly all that zone, an unexplored and scarcely discovered country. This impenetrable region is estimated to be as large as the continent of North America, about 8,000,000 square miles. A very little arithmetic will now prove the bold claim which I here make, that, even supposing the whole of this region to be land of the average continental height, there is still required over it all an average thickness of two and a half miles of solid ice to make the southern hemisphere equal the northern in weight.
This result of calculation is well confirmed by the information which all southern navigators have brought back from those most desolate and ice-bound regions. The zone of the antarctic has been encroached upon only in a small space south of the Pacific. On every other side, so far as has been discovered, mountains of ice block the way on and near the polar circle, which seems to be the great ice-barrier of the south pole. Discoverers suppose what they have looked upon to be land, but rarely have they ever seen any thing but rolling ranges of ice and snow rising higher and higher as far as the eye could reach. In the most open of the south-polar seas, Sir James Ross, in 1841, sailed 450 miles along an unbroken cliff of ice from 150 to 250 feet high, and of unknown depth beneath the water. It was one of the vast antarctic glaciers pushing down into the sea, from which some of those southern icebergs were broken off, that navigators have frequently laid down for islands, while the next sailor that voyaged that way found open water where they were charted.
Not a sign of vegetation, not an indication of thawing, has ever been discovered within or near the antarctic circle, whereas there are aboriginal races and numerous settlements of civilized communities on every side within the arctic circle. The whaleboat or the dog-sledge has traversed the arctics and found the sea-level in almost every degree of high latitude. In the south no adventurer has yet penetrated within probably 1,500 miles of the centre of greatest cold. Whence comes this great difference in the climate and ice accumulations of the two poles of the earth? It is the object of this article to inquire if in the astronomical relations of our planet there are found any sufficient causes for such differences.
The path of the earth about the sun once every year is an ellipse, with the sun in one of the foci or centres. An ellipse is a circular figure having two centres instead of one; that is, the circumference is everywhere equally distant from the two centres taken together—the sum of the two distances is always the same. Therefore, the sun being in one of these centres, the earth is nearer to it in one half of the year than in the other. At the present time the nearest approach, or the perigee, occurs about the 1st day of January; and the earth is at that time 3,200,000 miles nearer to the sun than it is on the 1st day of July.
It is a peculiar property of bodies revolving in elliptical orbits, that they travel faster when near the centre of attraction than when farther away. It follows, from the second of the three great laws of planetary motion discovered by Kepler, that the line connecting the two bodies must pass over equal areas in equal times. The earth passes through our winter portion of its orbit, that is, from autumnal to vernal equinox, in eight days less time than through the summer part of it. In the southern hemisphere, of course, the condition of things is reversed, and the winter there is eight days longer than the summer. Moreover, the sun is at its greatest distance from the earth during the long southern winter, and at its least in the short northern winter.
Of the two causes, I regard the first as of main importance. Distance from the sun, whatever theory may be, does not seem to have much effect upon climate. The southern summers, when the sun is over 3,000,000 miles nearer the earth, are said to be even some degrees cooler than the same seasons in corresponding localities of the northern hemisphere. And to take an extreme example, Mars, which is 50,000,000 miles farther from the sun than the earth is, has snow-lines about its poles which reach no nearer the equator than on our planet in corresponding seasons. But the excess or diminution of eight days, in the winters of climates which even in their warmest seasons barely balance on the thawing point of ice, is a true cause in polar conditions and differences. Considering that these days affect chiefly the period of briefest sunshine, it amounts to quite one-twentieth of the whole power of the sun on a hemisphere. This difference would not be apparent in the warm regions of the globe, where there is always an excess of heat which is carried off by evaporation and ocean-currents; but it would exert nearly its full force in polar regions which are unaffected by those influences.
It cannot be denied that it is the sun's heat which prevents the temperature of the earth from sinking to, or very near to, the absolute zero of cold, wherever in the thermometrical scale that may be. Chemists have produced a cold estimated at 257° below zero, of Fahr. It is not by any means probable that this reaches the entire absence of heat. But, on the supposition that it is so, and that polar regions are unaffected by the air or water currents of the tropics, then an excess of eight winter days would lessen a polar temperature 15°, and unquestionably amount to the difference of an accumulation of ice and snow year after year, instead of the annual thawing during each summer, of the winter's increase.
This is precisely what is, or has been, taking place at the respective poles of the earth. Year after year, probably for a long period, there has been a steady accumulation of ice-material about the south pole, adding weight to that hemisphere. Then, in proportion to this increase, the centre of gravity of the earth has moved a little toward the south; and the waters, always obedient to this controlling point, have gradually gathered into the southern seas, covering the lowlands and plains of islands and continents. At the same time the waters were drawn away from the north-polar regions, uncovering lands, and leaving bays and sounds and inlets innumerable. The geography of the countries fully corresponds to these inferences. The seas of the arctics are comparatively shallow and deeply cut up, and the lands are low-lying. In the antarctics the oceans are deep and bayless, and all the mainlands and islands are precipitous and craggy, as if they were the peaks and table-lands of mountain-ranges.
It is now the question whether this state of things is a permanent arrangement—whether we of the north side are always to have the advantage of extent of territory, of fertile lands and healthful homes in middle latitudes, in short, of all that makes the rivalry of nations, and civilization a necessity. To answer this question it will be necessary to turn again to astronomy, and to study for a few moments some of its more abstruse problems.
In addition to the rotation of the earth on its axis once every day, and its revolution about the sun once in a year, there is also a slow, rolling motion of the equator, caused by the attraction of the sun on the excess of matter in equatorial diameters over the polar. It is precisely as when one touches the rim of a top in rapid motion: there is set up at once a slow, gyrating or tilting roll, and the upper end of the stem describes a small circle. Just so the sun lays hold of the protuberant rim of the great terrestrial top, and immediately it begins to oscillate in the long secular period of 25,868 years; while the polar axis, extended to the heavens, describes in the same length of time a small circle of 231° radius among the northern or southern stars. This is the motion which occasions what is called the precession of the equinoxes. The plane of the earth's equator crosses the plane of its orbit; and, when the earth is at the points of junction, the days and nights are equal the world over. These two points, therefore, are the equinoxes; and the earth passes through them about the 21st days of March and September. Owing to the rolling motion of the equator, above described, these points, always in the line of intersection of the two planes, pass successively through the twelve signs or constellations, making slowly the entire circuit of the heavens. The vernal equinox, which now points to, or is on a line between, the sun and the constellation of the Fish, after about 26,000 years will have traveled the great circle of the heavens and come back again to point to the same cluster of stars which is now overhead at midnight on the 21st of March.
But the time of this revolution, so far as it affects the climate of the earth, is modified by the following circumstance: The ellipse or oblong circle in which the earth revolves about the sun is itself all the time slowly revolving. The long diameter of it—the major axis—makes a complete revolution in the heavens once in 110,000 years. Now, as this revolution is forward, or in the same direction among the constellations that the sun appears to move, while that of the equinoxes is retrograde, it follows that the extremities of the major axis, which are the perigee and the apogee, advance to meet the equinoctial points; so that the revolutions, or rather the conjunctions, of the equinoxes, which have to do with terrestrial climate, are accomplished in the shorter period of 21,000 years.
Now, all this astronomy amounts simply to this: that in the year of our Lord 1248 the earth was at its nearest approach to the sun on the 21st day of December, our winter solstice; and that in 10,500 years from that time the same thing will happen on the 21st day of July, our summer solstice. In the period comprising the first case, our winters are short and mild, and our summers long and sunny. During the cycle which shall comprise the latter case, our winters will be rigorous and our summers short. The northern hemisphere is now having its great summer. In about 10,000 years it will be in the midst of its great winter; and whatever differences there may be between the two hemispheres, owing to astronomical causes, will then be in full force against the northern.
A distinguished Scotch mathematician, Mr. James Croll, has estimated that the melting of a mile in thickness of the present antarctic ice would raise the sea-level at the north pole 300 feet, and at Glasgow 280 feet. We have calculated, from data which were intended to be under-estimates in every case, that there were at least two and a half miles of average thickness in what geographers call the great ice-cupola of the south pole. If, therefore, not only this were removed, but an equal quantity of ice were deposited at the north pole, there would be a deepening of the sea at the arctic circle of 1,500 feet.
Thus it is seen that, as certainly as terrestrial revolutions continue, in the course of 10,000 years there must come an entire reversal of polar conditions. The southern waters must be drained off to make the oceans of an opposite hemisphere. New lands, enriched with the sediment of a hundred centuries, will rise up to extend the borders of the old south continents, and islands joining together, will expand into mainlands. At the same time the northern continents must be in great part submerged, and their summits and ranges become the bleak islands and the bold headlands of a tempestuous ocean. Central Asia, with its broad table-lands, may still retain the name of a continent; but, beyond a few outlying islands, there will be no Europe, and but little of North America left. The Atlantic waters will stand five hundred feet over Lake Superior, and will wash the base of the Rocky Mountains in all their length. A new Gulf Stream may again, as it must often have done before, flow up the valley of the Mississippi, returning the deltas to the prairies, and remaking the beds of the garden of the world. These are no idle or impossible fancies. Not only are they the results of rigorous calculation, but they accord perfectly with the unmistakable evidences which the ocean has left, all over our land, of its recent work and presence.
The time-honored geologist, Sir Charles Lyell, lays great stress on the quantity of land and the configurations of continents, as chiefly efficacious in the great climatic changes. But it may be pertinently asked, What becomes of his continents and configurations when the seas of one pole advance to the other, as they unquestionably do, as they cannot but do, every 10,500 years, obedient to the transfer of vast ice weights from one end of the world to the other? On all the mountains of New England there are sea-lines at elevations of 2,000 and 3,000 feet, and Lyell himself has recorded the facts. When the ocean was that deep over Boston, there were no continents in the northern hemisphere. Undoubtedly the height and direction of mountain-ranges, the trending of sea-shores, and the course of ocean currents, have much to do with local climates. But, instead of the relative quantity or location of land and sea having any agency in producing the glacial periods, it is these periods which produce the land and the sea.
So much for the causes and conditions which pertain to the geography of the present and the future. When, now, we turn back a few of the leaves which tell of the past condition of our planet, we immediately see that the same causes have been at work in recent geological times on a much more extensive scale—in fact, that they have been the chief agents in composing and modifying the present surface of the earth outside of the tropics. Over all the northern portions of Europe, Asia, and North America, are found the unmistakable evidences of extensive and recent ice-work. Bowlders of every size, some worn and some angular, are scattered in immense quantities over all the country, on the hills, on the plains, in places where the only possible explanation is that they were lifted up, carried, and dropped, just where they are found; and the great iceberg was the carrier. The face of the rock-beds, wherever brought to view, in the valleys or on the mountains, is almost always found to be ground or polished, and, over that, grooved and furrowed with nearly parallel scratches. The Alpine glaciers are doing exactly the same work today. Erratic blocks of foreign origin, and sometimes of enormous dimensions, are frequently found perched on the very tops of hills, or stranded high up the mountain-sides; and the quarries from which they came are invariably found to the northward, sometimes fifty or even a hundred miles. It is argued that nothing but polar glaciers could thus have moved them in uniformly meridional lines. The scrapings of grounding ice-floes, the marks of ancient sea-shores, and marine relics and shells, are found at elevations of several thousand feet above the present ocean-level. There is no escaping the conclusion that the northern continents have been, in not remote ages, deeply submerged beneath an ice-laden sea; and that the entire polar and north temperate regions, extending in some places south of the fortieth parallel of latitude, have been capped with one massive covering of ice of great thickness. Precisely the same evidences are found in South America, and, according to Agassiz, even much nearer the equator than in North America. We have again to search our astronomy for causes many times more powerful than any thing we have yet found, for differences of polar temperatures.
The earth is made to revolve in an orbit drawn out of the circular form by the combined attractions of the other planets, Jupiter carrying the controlling influence. When the average of all these forces for long periods is more in one direction than in another, our planet is drawn away from the sun on that side. Now, it must occasionally happen, with the various periods of revolution of the planets, that they unite at times to produce extreme irregularities. The present difference between the nearest and farthest distance of the sun from us is 3,200,000 miles. It is found, by calculating back the planetary orbits and conjunctions, that this focal distance has been as much as 14,000,000 miles. There was, then, an excess of thirty-nine winter days during each year of the great secular winter of either pole. This exceptionally high eccentricity occurred, according to the calculations of Mr. James Croll, about 850,000 years ago. But it is now generally thought that we have no need to go back as far as that for the period of the last glacial epoch: 200,000 years ago the focal distance was 10,500,000 miles, and the winter excess twenty-eight days. This, on the supposition heretofore made of the absolute zero of cold being at least 257° below the freezing-point, would lower the mean temperature in polar regions 50° Fahr., and would unquestionably extend the permanent ice-limits far into the temperate zone. From that time, down to 70,000 years ago, the eccentricity was continually from two to four times greater than now. Since about 70,000 years ago, it has been nearly all the time less than at present. Thus it may fairly be concluded that the great glacial period of the Post-tertiary era came to an end with the fourth secular winter in the past, or b. c. 67,000.
This is a very interesting date to us of the genus homo; for it must have been about this time, according to all accounts, that our forefathers made their appearance on the earth. Man, with the longhaired mammoth, the woolly rhinoceros, the huge cave-bear, the great horned reindeer, and numerous other species now extinct, followed close upon the retreating ice-fields of the bowlder period. Our primeval ancestors were a race of hunters, and they subsisted on the most abundant and magnificent game that the world has ever seen. They lived in caves or under projecting ledges, and with only flint-headed weapons contested their lives and homes with savage beasts. They cracked the bones of animals for their marrow, or crushed them in stone mortars for the fats and the juices which they contained. It was the lingering carnivorous instinct to gnaw the bones of their prey. They had fires at their funeral feasts, but there is little evidence of their indulging often in the luxury of cooked meats. It was a rude life, and a hard struggle they must have had for it; but their history is read in the drift-beds and cave-deposits of Europe, as plainly as if there had been an Herodotus to write it.
The effect and bearing of the great ice periods on geological work and time will be further considered in a second article in continuation of this.
- The temperature of stellar space is estimated by Sir John Herschel and others at -239° Fahr.
- This article was written before the publication of Mr. Croll's recent work on "Climate and Time." The reference here is to an article published some years since in the Philosophical Magazine.