Popular Science Monthly/Volume 9/July 1876/The Causes of the Cold of the Ice Period
|THE CAUSES OF THE COLD OF THE ICE PERIOD.|
A FEW years ago the scientific world was startled by the assertion—made by Charpentier and Agassiz, who had been studying the glacial phenomena of Switzerland—that at no very remote period, geologically speaking, the climate of the northern hemisphere had been very much colder than at present; and that the arctic conditions which now prevail in Greenland—with perpetual snow-sheets, and glaciers reaching the sea—extended as far south as the middle of the present temperate zone.
At first, seriously questioned by most, strenuously denied by some, this theory was found to be sustained by such abundant and indisputable evidence—the inscriptions left by the glaciers themselves—that it was not long before it had secured a general acceptance from geologists. Since then there has been a vast amount of. theorizing and investigation, to determine if possible the causes of these remarkable changes of climate.
Up to the present time, however, no theory has been proposed which has been sustained by really satisfying evidence, and there is still much difference of opinion on the question among those who know most about it.
As the subject is one of peculiar geological significance, and great dramatic interest, I venture to bring forward some notes upon it, taken from the geologist's standpoint, hoping that they may contribute in some slight degree to the solution of the problem.
The theories which have been proposed to account for the cold of the Ice period divide themselves into two groups, viz., the cosmical and terrestrial; or those which invoke extraneous or astronomical influences, and those which look to changes in the earth itself, or on its surface, for a sufficient cause or causes.
In the first category may be enumerated the theory of Prof. Croll, that variations in the eccentricity of the earth's orbit have induced great alternations of climate on portions of the earth's surface; that of Belt and Drayson, which supposes the known variability of the angle of the pole with the ecliptic to have been at times sufficiently great to have brought arctic conditions locally down into the temperate zone; also, the speculations that the heat evolved from the sun has been variable in quantity, that the earth has at various times passed through cold spaces in the universe, etc.
In the second category are the views first put forth by Lyell, according to which all the variations of climate recorded in geological history have been induced by changes in the earth itself or on its surface.
In this paper I shall consider only the latter theory, leaving the discussion of the astronomical aspects of the subject to astronomers, mathematicians, and physicists, who alone are competent to thoroughly investigate them.
The explanation given by Lyell of the cold of the Ice period is in conformity with his characteristic conservatism. It is well known that the climatic conditions of all parts of the earth's surface are profoundly affected by their topographical features. This may be seen at a glance by reference to any map on which the isothermal lines are delineated. Continental surfaces are known to be productive of extremes of temperature, while the climate of sea areas is comparatively equable; and the general character of the climate of land and water surfaces is further and locally affected by the configuration and altitude of the land, by the breadth and depth of the oceanic basins, and especially by the ocean-currents. The sea forms the great evaporating surface, and the source from which is derived the enormous quantity of water transported by the system of atmospheric circulation. The local climate of continents is also largely influenced by the winds which blow over them; for these determine, to a considerable degree, the temperature and the annual rainfall; hence the volume and excavating power of rivers, etc. The higher portions of continents, as mountain-chains and plateaux, are colder than the lowlands, and hence become condensers of moisture—places where snow accumulates and glaciers are formed.
A striking illustration of the influence of topography on climate is shown by the high mountains of the tropics, where perpetual snow and glaciers are coexistent with extreme tropical conditions, not only on the same parallel, but within a narrow area. It is evident, then, that topographical changes such as could be easily conceived would readily and perfectly accomplish all the alternations of climate of which we have any evidence in geological history. Recognizing the potency of topographical causes, Lyell sought for, and thought he had found, a sufficient explanation of the contrast between the climates of the Ice period and the present, in changes in the physical geography of the northern hemisphere; assuming and believing that the Glacial period was marked and caused by great elevation and breadth of land-surface about the pole, and, as a corollary and consequence of this proposition, a depression of land and a broadening of oceanic surfaces in the temperate and tropical zones.
This theory affords so simple an explanation of the problem of the Ice period, that it at first strongly commends itself to those who are most cautious and logical in their modes of thought and investigation. Modern science is eminently conservative, and one of the first lessons learned by the investigator of this age is, to exhaust all known causes of phenomena before appealing to the unknown. Still, however plausible this view may be, it must be sustained by solid and substantial proof before it deserves to be regarded as anything but a theory, and before it can be accepted as a rule of faith and practice among geologists. Unfortunately, such proof is not only yet wanting, but there are many facts which, in the light of our present knowledge, seem to indicate that it will never be obtained. The theory of Lyell has, however, been adopted by Prof. Dana, in the last edition of his "Manual," where he says (p. 541), "The occurrence of an Ice period was probably dependent mainly, as suggested by Lyell, on the extension and elevation of the land over the higher latitudes." Prof. Dana has further elaborated and applied the Lyellian hypothesis by suggesting that in the Glacial period barriers of land connected the continents of the two hemispheres, and excluded the tropical currents from the polar seas, in this way cutting off the most powerful equalizing influences, and inducing an exaggeration of the heat of the tropics and the cold of the polar regions. He also claims that high and broad land-surfaces in the circumpolar areas formed great condensers and refrigerators, upon which the moisture, freely and rapidly evaporated from the seething caldron of the circumscribed tropical seas, was precipitated to form almost universal snow-fields and glaciers; certainly very favorable conditions for the production of many of the phenomena which characterized the Glacial period. It must be remembered, however, that this theory presupposes barriers established not only across the North Atlantic and Pacific Oceans, but in the southern hemisphere as well—for this also had its Ice period—barriers connecting the widely-separated promontories of Cape Horn, Cape of Good Hope, and the islands of the East Indian Archipelago; also that, simultaneously with the existence of such barriers, the tropical lands were depressed, and the sea spread its sediments over much of what is in the present age terra firma.
In reviewing the theory proposed by Lyell and Dana, I have been impressed with the conviction that if the physical geography of the northern and southern hemispheres had been either alternately or simultaneously such as this theory requires, we should find some evidence of it, apart from the inscriptions made by glaciers nearer the equator than any now exist. In the search for such evidence, however, I have not only failed to find it, but have, as it seems to me, found other things which go far to disprove the theory.
In order to fully state the case, it will be necessary to review several chapters in geological history, and compare the preceding and also the succeeding age with that in which the climate of Greenland came as far south as New York.
The results of such comparisons may be given as follows:
I. It is known to most students of geology that, during the Tertiary age, the climate of all the arctic regions was warm-temperate. A luxuriant forest then covered Greenland, and all the northern portion of this continent; such a forest as could only flourish in a climate as mild as that of our Middle and Southern States.
According to the Lyellian hypothesis this should have been a period of great depression of arctic, and elevation of tropical lands; but we have proof that such was not the case. On the contrary, the land area at the north was broader then than now, while in the tropics it was narrower.
It can be shown, too, that land-connection then existed in northern latitudes between Europe and America, and also between America and Asia. The Atlantic bridge stretched from Greenland to Iceland, thence to the Hebrides and Scotland, which was then part of the European Continent. The Pacific bridge was where Behring's Straits now are.
These conclusions are deducible from the following facts:
1. Our American flora, which began in the Cretaceous, spread in the Tertiary age to Europe on the one hand, and to China and Japan on the other; and this could only have taken place when the continents were connected. The characteristic plants of this flora have been found fossilized on the Upper Missouri, on Mackenzie's River, Disco Island, Greenland, Iceland, the island of Mull, and on the continent of Europe as far south as Italy. No collection has been made of Tertiary plants in Japan and China, but the living flora of these countries contains a large number of species identical with those found, either living or fossil, in North America. The remarkable similarity between the flora of Northeastern Asia and that of America, so clearly shown by Prof. Gray, is such as to demonstrate a community of origin, and that its place of origin was America may be fairly inferred from the character of the present American flora and from the facts that a large part of the most characteristic genera are found here in the Cretaceous rocks, and many of the living species in our fresh-water Tertiaries.
2. Marine Tertiary deposits are almost completely absent from the arctic lands, while they now skirt or cover most tropical continents and islands.
Pocks containing marine Tertiary fossils are conclusive evidence of the submergence in Tertiary times of the land in the localities where they occur; and they would not fail to exist over great areas in the arctic, had the land there been more depressed in the Tertiary age than now; since most of the country which borders the Arctic Sea, both in America and Asia, lies but little above the sea-level.
The Tertiary strata, that have yielded more than three hundred species of land-plants at the far north are generally fresh-water and marsh deposits, containing fresh-water shells and beds of lignite similar to those of the central portions of our own continent. In contrast to the state of things thus indicated, the marine Tertiaries, which form the margins of our South Atlantic and Gulf States, the West Indies, the Isthmus, and the northern part of South America, are automatic records of high sea or low land level, in the tropical regions during Tertiary times.
These facts seem to prove that in the period when a warm-temperate climate prevailed over all the arctic regions, the land was broader and higher than now at the north, lower and narrower at the south; and that barriers did then exist which excluded the tropical ocean-currents from the arctic sea.
II. Just what the topography of the arctic regions was during the Glacial period, we have as yet no very full and accurate information. It has been generally supposed that at least certain areas in the north were then high, but this cannot he said to he proved. That the arctic lands have been at some time raised higher than now, is shown by the fiords of the northern coasts, which, as first pointed out by Dana, must have been excavated by subaërial erosion; but a large part of that erosion may have been effected in the Tertiary age, and perhaps it was chiefly accomplished then.
When a dense forest clothed the arctic lands, and spread over continuous land-surfaces to Europe and Asia, these now half-submerged fiords were valleys traversed by flowing streams; for the abundant Tertiary vegetation of the far north proves the country to have been well watered. That these fiords were filled with glaciers during the Ice period is certain, as the bottoms and sides of many of them are glaciated, but this would happen again with a depression of temperature, and without a depression of sea-level. The fact that the glaciated surface of the bottoms of fiords in Sweden and America passes under the sea, and reaches as far as observation can be carried, is not the proof of elevation it has been claimed to be, for the glaciers that now reach the sea must score their beds to the depth of several hundred feet, before their extremities are lifted up by the one-tenth greater gravity of water, and are floated off as icebergs.
Prof. J. W. Dawson holds the view that the Glacial period was one of depression at the north, as he finds marine shells in the bowlder clay of the St. Lawrence Valley; and he attributes much of the glaciation of Eastern North America to icebergs dragged over the submerged land.
Croll says ("Climate and Time," p. 391):
According to the investigations of Bohtlingk and Kjerulf, Scandinavia was 600 feet lower during the Glacial period than now. Erdmann, on the contrary, supposes that Sweden was higher during the Glacial epoch than at the present day, from the fact that polished rock-surfaces extend beneath the sea; but this, as we have seen, proves no such thing.
Dana bases his statement that the northern portion of our continent was highest in the Ice period on the system of deep, now-buried channels, by which its surface was once furrowed, and upon the fiords which fringe the northern coast; but, as elsewhere stated, we have no proof that all, or nearly all, this erosion was not effected previous to the Glacial epoch. Reviewing all the facts that have been cited, we can at least say that the indications of elevation are not nearly so well marked in the Quaternary as in the Tertiary; and the evidence of such elevation as would shut out the tropical currents from the Arctic Sea in the Quaternary age is wholly wanting.
In the Champlain epoch the northern land was greatly depressed, as we learn from the fact that the clays containing marine shells are found on the present land at a constantly-increasing elevation as we go toward the north. About New York the Champlain clays reach from 50 to 100 feet above the ocean-level; on Lake Champlain they are 400 feet, at Montreal nearly 500 feet, at Labrador 800, in Barrow's Straits 1,000, and at the extreme point reached by the Polaris Expedition, on the coast of Greenland, 1,800 feet above the sea (Bessel).
On the European coast of the Atlantic we have proof of an elevation of the land during the Tertiary, and a subsidence in the Quaternary, similar to those described above. Hence we may infer that in the Champlain epoch the topography of the arctic regions was just that which would be favorable for the transfer by ocean-currents of the heat of the tropics to the arctic, and a prevalence over the arctic regions of a warm climate. But it must be said that all the shells found in the Champlain clays, from Lake Champlain to Greenland, are of a decided boreal character, which indicates that during the entire deposition of that formation a climate scarcely warmer than that of Greenland prevailed from New England northward.
If it is true that the Glacial epoch was one of elevation at the north—an elevation of the land much greater than the present—the change to the depressed condition of the Champlain epoch, when the sea stood from 1,500 to 1,800 feet higher on the coast of Greenland than it now does, must have been comparatively sudden; and if, as has been asserted, the depression of the Champlain epoch was common to the whole northern hemisphere, it could have been effected only by a great change in the figure of the earth, or by a flow of the ocean-waters into the polar regions, such as has been suggested by Adhemar and Croll. These writers hold the view that the effect of the extreme cold of the Glacial period was to form an ice-cap some miles in thickness over the arctic regions, and that this ice-cap moved the centre of gravity of the earth toward the pole, so that the oceanic waters flowed into this hemisphere and thus elevated the sea-level.
One result of the formation of an ice-cap over the polar regions alternately in one and the other hemisphere might very well he, as claimed by Croll and admitted by Sir William Thomson, such great ebbs and flows of the ocean-waters as we find recorded in the Champlain clays, and the present depressed sea-level; hut some more conclusive evidence of an ice-cap will be asked by cautious reasoners than these alternations of level: such evidence, for example, as universal glaciation over all of North America of 40° north latitude. No such evidence has as yet been adduced; but, on the contrary, observers report an absence of ice-marks in the interior of the continent northwest of the Great Lakes. This we might take to be proof that the glaciers of the Ice period were limited to the highlands comparatively near the ocean, the source of evaporation, and that the interior was so dry then and now that no glaciers could be formed there. This is, however, a subject which requires further investigation. Whatever be its cause, the uniformity and magnitude of the change of sea-level from the Tertiary emergence to the Champlain submergence, and then to the present, render it one of the most remarkable phenomena recorded in geological history, and one that with careful study will probably throw much light upon the great dynamical influences that have produced changes on the earth's surface.
III. Either simultaneously or alternately with the extremes of warmth and cold, which we find recorded in the northern, warm and cold periods prevailed in the southern hemisphere. The evidences of a Glacial period in South America are as conclusive as on our own continent; but it is difficult to conceive how barriers could, at that time, have been thrown across the great open oceans—the South Atlantic and South Pacific—in such a way as to confine the tropical currents to the central portions of these oceans.
We are, perhaps, not justified in saying that such barriers never did exist, but it will be conceded that the difficulties which oppose their erection there are much greater than in the northern hemisphere; and the hypothesis which supposes their existence in the Glacial period of the southern hemisphere is so entirely unsupported by facts, that we are compelled to regard it as mere conjecture.
In any discussion of the phenomena and causes of the Ice period we are, up to the present time, somewhat limited and embarrassed for want of a wider range of observation. The facts are not yet all in. Nearly all the detailed and careful observations made on the glacial phenomena of the northern hemisphere have been limited to the eastern half of North America and the western part of the European Continent. Here the traces left by the glaciers are really stupendous in their magnitude and extent; and we have demonstrative evidence that, during the Ice period, the glaciers and snow-fields of Greenland stretched continuously clown along the Atlantic coast of North America to and below New York, and that the highlands of New England and Eastern Canada were completely covered, and probably deeply buried, in sheets of ice and snow. In the British Islands and Norway the inscriptions made by ancient glaciers are scarcely less broad and profound, and it is even conjectured that the bed of the shallow North Sea is itself glaciated throughout. These evidences of vast accumulations of ice and snow on the borders of the Atlantic have led some theorists to suppose that the Ice period was attended, if not in part caused, by a far more abundant evaporation from the surface of the Atlantic than takes place at present; and it has even been conjectured that submarine volcanoes in the tropics might have loaded the atmosphere with an unusual amount of moisture. This speculation seems to me, however, both improbable and superfluous; improbable, because no traces of any such cataclysm have been discovered, and it is more than doubtful whether the generation of steam in the tropics, however large the quantity, would produce glaciation of the polar regions. The ascent of steam and heated air loaded with vapor to the altitude of refrigeration, would, as it seems to me, result in the rapid radiation of the heat into space, and the local precipitation of unusual quantities of rain; and the effect of such a catastrophe would be slowly propagated and feebly felt in the arctic and antarctic regions. The hypothesis is superfluous, because all we want, to restore the conditions recorded in the glaciated area, is simply a depression of temperature; by this the climate of Greenland, with all the attending phenomena, would be brought down on both sides of the Atlantic to the lowest point where the average annual temperature of Greenland prevailed.
This is, I think, proved by the condition of Greenland itself; remote as it is from evaporating surfaces of warm water, the precipitation of moisture upon that continent is, however, sufficient to cover it deeply under sheets of snow and ice; the whole interior being occupied by a continental glacier; and it is easy to see that, with a depression of the average annual temperature 10°, the highlands of Labrador would be brought into the same condition. With a still further depression the elevated portions of New England, the Adirondacks, and the highlands north of the lakes, would be completely encased in snow and ice. If the flow of the St. Lawrence were arrested, and the annual precipitation of the region drained by it were congealed, and retained from year to year, glaciers would soon form, and creep down from the highlands into the valleys, until the basins of the great lakes and the troughs of the Hudson and St. Lawrence would be completely filled with ice. On the eastern side of the Atlantic this state of things would be still more rapidly reached, inasmuch as, from the effect of the Gulf Stream, the coast climate is considerably more moist.
So far, then, as the region bordering the North Atlantic is concerned, a simple depression of temperature from any cause whatever, terrestrial or cosmical, would produce all the phenomena of the Ice period.
Before we can certainly determine, however, what the nature of the cause producing the cold of the Ice period was, we must know more accurately where and how the cause operated. To accomplish this, observations must be made over all those portions of the northern and southern hemispheres where the traces of former glaciers are visible.
In a general way we know that there was a cold period throughout the northern hemisphere, as glacial phenomena are reported from Siberia and Northwestern America, somewhat similar to those which we find on the Atlantic coast. In regard to Siberia very much remains to be learned. Nearly the whole of the northern portion of this great area is flat, and is deeply covered with Quaternary deposits; and it has been conjectured that in the Ice period the shallow sea off the Siberian coast was solidly frozen throughout a great portion of its breadth, and thus formed an ice-dam, behind which the drainage of the northern slope accumulated alternately as sheets of ice and bodies of fresh water.
The northern portion of the interior of our own continent is said to be without 'distinct marks of glacial action. Should this statement be confirmed by further observation, it would not, however, be a formidable argument against a general Glacial period; for intense cold would leave no permanent record there, unless there was sufficient precipitation of moisture to form glaciers. As this region is now very dry and sterile, it was perhaps so through the Ice period, and snow at no time fell there in sufficient quantity to form glaciers. On the mountains of British America and Alaska, of Oregon and California, there are abundant evidences of glaciers far more numerous and extensive than any now existing; and these furnish demonstrative evidence that this region shared in the effects of a distinct Ice period. The slopes of the Cascade Mountains in Oregon are everywhere glaciated, and perhaps no more impressive record of the Ice period exists than that formed by the planed and furrowed surfaces, the roches moutonnées, etc., by which all the higher portions of the belt, twenty to thirty miles in width, are marked. No ice-sheet moved in that region from the north, as there was no district of northern highlands where continental glaciers could be generated; but the glaciers radiated east and west from various centres along the crests of the chain, and descended at least 2,500 feet below the present snow line. This I determined by actual barometric observation in many places, and I nowhere found the lower limit of glacial action, as the planed and furrowed surfaces passed beneath the alluvium of the lower valleys.
Whether there was a depression of the Western coast during the Champlain epoch, corresponding to that recorded along the shores of the Atlantic, we are as yet unable to say, as careful observations on this interesting subject are wanting; and these are not easily made on this iron-hound and earthquake-shaken coast, where there has been so little low and level land upon which Champlain clays could be deposited.
That this portion of the continent—like the Eastern side—has been higher than now, we learn from the deeply-excavated channels of the Golden Gate, the straits of Carquines, the mouth of the Columbia, the Canal De Haro, etc. But this erosion was produced in part if not altogether in Tertiary times. At Shoalwater Bay and about Steilacoom, there are raised beaches, apparently of ancient date, but farther south the changes of level have been so frequent and local that nothing like system has been educed from a comparison of the old shore-lines.
Taken as a whole, the glacial inscriptions of the West coast, as studied by King and Le Conte in California, and myself in Oregon, prove an Ice period as distinctly as do the glacial marks of the Atlantic coast and the Mississippi Valley; but the peculiar topography of the Western country has made the record a somewhat different one.
From the foregoing facts it seems to me that we are justified in concluding:
1. That however simple and plausible the Lyellian hypothesis may be, or however ingenious the extension or application of it suggested by Dana, it is not sustained by any proof, and the testimony of the rocks seems to be decidedly against it.
2. Though much may yet be learned from a more extended and careful study of the glacial phenomena of all parts of both hemispheres, the facts already gathered seem to be incompatible with any theory yet advanced which makes the Ice period simply a series of telluric phenomena, and so far strengthens the arguments of those who look to extraneous and cosmical causes for the origin of these phenomena.
- It has been suggested that the warmth of the Tertiary climate was simply the effect of the residual heat of a globe cooling from incandescence, but many facts disprove this. For example, the fossil plants found in our Lower Cretaceous rocks in Central North America indicate a temperate climate in latitude 35° to 40° in the Cretaceous age. The coal-flora, too, and the beds of coal, indicate a moist, equable, and warm but not hot climate in the Carboniferous age, millions of years before the Tertiary, and 3,000 miles farther south than localities where magnolias, tulip-trees, and deciduous cypresses, grew in the latter age. Some learned and cautious geologists even assert that there have been several Ice periods, one as far back as the Devonian.
- Some of the huge tabular icebergs, which have been observed off the Antarctic Continent, projected more than 500 feet above the surface of the ocean; and as for every foot above water there must have been 8.7 feet submerged, the whole thickness of the ice-sheet, from which these bergs were detached, must have been over 5,000 feet, and such a glacier must grind the sea-bottom to a depth of over 4,000 feet. (See Croll, "Climate and Time," p. 385.)