Popular Science Monthly/Volume 59/July 1901/Climate and Carbonic Acid
|CLIMATE AND CARBONIC ACID.|
By BAILEY WILLIS,
U. S. GEOLOGICAL SURVEY.
THE fact that a very extensive and massive ice sheet covered countries of the northern hemisphere which now enjoy a mild climate is generally known and accepted, although it is little more than fifty years since Agassiz (1840-47) made the then novel suggestion to explain the occurrence of glacial deposits where no glaciers remain. It is not so generally known that the great ice age was characterized by the development of several ice sheets in succession, each of them separated from its forerunner by an interval of mild climate during which the ice retreated far toward its source, and but few realize that these intervals of mildness were longer than the time which has elapsed since the latest glaciers withdrew from New England and the northern Central States.
Since the fact of a glacial period was established, several hypotheses have been framed to account for the phenomena of climatic change. As the sun warms the earth, variations in its condition and distance were postulated. As the poles are now regions of glacial accumulation, it was thought that the earth's axis of rotation might have shifted in such a way as to bring the once glaciated regions into polar relations. Or as heights of land are often mantled in snow and ice under latitudes where lowlands are free, glaciation was connected theoretically with a general elevation of continents and mountains. There are facts to sustain most of the speculations thus suggested. Each contains a possible cause. But no one is free from serious question of its sufficiency, while there is little evidence to show that any was definitely related in time to a glacial epoch, except that one which is based on a general elevation of the land.
Professor Chamberlin, of the University of Chicago, long ago advocated a method of investigation known as the method of multiple hypotheses. It calls upon the student to lay aside a natural preference for the theory which seems plausible and to consider as sincerely that which holds out small promise of development. As an earnest student of the causes of the glacial period, he has thus considered every suggestion that might solve that enigma. The astronomical causes, the shifting of the pole, the variations in altitude of the continent, have all been passed in review,
In considering the climatic conditions which gave to the coast of southern New England the aspect of Greenland at the present time, thought naturally turned to the antithetic phase when Greenland possessed the climate of Florida. And seemingly linked with these were other climatic variations, such as the great humidity of the Coal Measure period and the great aridity of epochs when salt and gypsum deposits accumulated; while the cause of that redness, which in several continents is characteristic of strata of certain geologic ages, might be traced to world-wide atmospheric conditions. The problem was thus greatly broadened in the scope of related phenomena, and the demands to be met by an adequate hypothesis became correspondingly complex.
The investigation upon which the hypothesis under review rests considers the physics and chemistry of the atmosphere in relation to temperature, the physics and chemistry of the ocean, the interaction of the ocean and the air, and those events of geologic history which as cause or effect may be related to the constitution of the atmosphere. It is not here proposed to review critically the several articles in which Professor Chamberlin and his associates have presented the results of profound researches. Suffice it to endeavor clearly to present an outline of their reasoning and conclusions.
The constitution of the atmosphere has long been known, and in a general way is stated for dry air as 21 parts of oxygen and 79 parts of nitrogen by volume. Argon, a newly discovered component, was formerly measured as nitrogen, and frequently there are impurities, though in small amount. There are 3 to 4 parts of carbonic acid in 10,000, and under natural conditions moisture is present in greater or less proportion. It is with these last, the carbonic acid and moisture, that the student of climatic changes has to deal chiefly.
The functions of carbonic acid and moisture in the atmosphere are threefold. They both absorb radiant heat in an unusual degree. By thus raising the temperature of the air, they both increase its capacity for moisture. And they both are chemically active.
Radiant light and heat penetrate the atmosphere to reach the solid earth, and are in part radiated back through the air into space. As the air is transparent toward light, so is it also toward heat, allowing both forms of energy to pass with moderate absorption. A photographer who compares the exposure of his plate at a considerable altitude with that near sea level roughly measures the relative strength of light at the two places and finds it less beneath the greater depth of atmosphere. The direct heat of the sun's rays is correspondingly less by the sea. The energy which the heated earth radiates back toward space is in part also absorbed by the air, which is thus warmed by the passage of rays to and from the earth.
In this absorption the mass of nitrogen and oxygen has but an insignificant part. They are nearly perfectly transparent to heat. Carbonic acid and moisture are the effective constituents, which thicken, as it were, the atmospheric blanket, and being warmed in turn keep warm the earth. If they are decreased the blanket becomes thin and the surface grows cool.
Tyndall first suggested that a lessening of the proportion of carbonic acid might suffice to bring on the cold climate of a glacial epoch. He was followed by several investigators who determined more accurately the parts played by carbonic acid and by moisture, Austrian, German and American scientists competing in the study. In 1896, Dr. Arrhenius, a Swedish physicist, reached definite quantitative estimates of the effects. Employing values for the radiant heat of the full moon at different heights above the horizon, measured by Langley, he computed the heat absorbed by the atmosphere. By elaborate calculations he determined that a decrease of carbonic acid in the atmosphere to an amount ranging from 55 to 67 per cent, of the present content would reduce the average temperature 4 or 5 degrees C, which would bring on a glacial epoch, whereas an increase of carbonic acid to an amount two or three times the present content would elevate the average temperature 8 or 9 degrees C, and bring on a mild climate in high latitudes.
The effects of relatively absorbent or transparent atmospheres are not direct and uniform. They vary with the angle of incidence of the sun's rays and, therefore, with latitude, with seasons, and with day and night. They differ with altitude above the earth's surface; they are unlike on land and sea. But in general result the effect of greater absorptive power is to equalize all differences due to geographical and astronomical relations, whereas that of a relatively transparent condition is to accentuate them.
The physicist having thus indicated a possible solution, the further task of framing a working hypothesis was the geologist's. Chamberlin says: "There are hypotheses and working hypotheses. . . . General suggestions of a possible cause do not reach the dignity of working hypotheses until they are given concrete form, are fitted in detail to the specific phenomena and are made the agents of calling into play effective lines of research." In his attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis, he has nobly met the requirements of his own definition under the difficult conditions imposed by the phenomena of glaciation. However the resulting working hypothesis may hereafter be modified by further research, its presentation must always stand as an example of the highest scientific effort.
Let us briefly review the requirements of the task. The fundamental postulate of the hypothesis is that variations of the atmospheric content of carbonic acid have been the direct cause of variations of climate. It is necessary, therefore, to assign agencies adequate to bring about such alternations of poverty and wealth of carbonic acid. The agencies must operate to produce great cycles of climatic change which are recognized through study of the geologic record. Among others, these comprise an ancient event of extensive glaciation in India, Australia and South Africa, closely following the period of mild climate during which the Coal Measure flora flourished. The agencies must further promote subsidiary action by which minor oscillations of climate may be explained, since within the latest, the Pleistocene, period of glaciation, at least five, and probably more, advances of the ice occurred in alternation with intervals of comparative mildness, during which the ice retreated notably. Depletion and enrichment of the atmosphere must furthermore occur within reasonable limits of geologic periods. And cause must be shown why the atmospheric changes promoted glaciation about peculiarly local centers. In searching the sources of carbonic acid, Chamberlin has been led to reconsider the original constitution of the atmosphere, and thus also theories of the origin of the earth, including the nebular hypothesis. Thither this review may not follow him, but it will be of interest to advert to his views as to the conditions affecting biologic evolution, which are also causally related to variations of the carbonic acid contained in the air.
Carbonic acid, or as it is more accurately called, carbon dioxide, CO2, occurs in many relations and plays many parts in the economy of the world. In some of these activities it enters into permanent combinations and is lost to the atmosphere. In others it passes through cycles of combination and release by which it is temporarily withdrawn from and subsequently returned to the air. If the atmosphere's resources in CO2 be compared with a bank account, we may suppose that the balance follows one or the other of two familiar cases. In the one example there may have been originally a definite though possibly large deposit, which has not since been added to, but upon which many drafts have been and are being drawn. Under this assumption, however rich the atmosphere once was, it is now by comparison poverty stricken. On the other hand, there is reason to believe that the original capital in CO2 was not materially greater than it is now, but that losses have been nearly balanced by gains. The first example represents a view held by geologists who believe that the atmosphere was exceedingly dense, moist and charged with carbon dioxide in early ages of the earth's history; the second illustrates the conceptions based on modern advances of biology and geology, and its acceptance is essential to the hypothesis of glaciation here discussed.
The sources from which fresh contributions may be made to the atmosphere are suggested by the occurrence of carbon dioxide among the gases projected by the sun beyond its gravitative control, in interstellar spaces, in meteorites and in the terrestrial mass. The first three possible sources are too indefinite both in amount and in distribution through the ages to be of any present value to the hypothesis, but the last is important. Crystalline rocks of the superficial crust of the earth are shown by analysis to contain four and one-half times their own volume of gases, of which carbon dioxide, CO2, and monoxide, CO, form a large percentage. During volcanic eruptions gases and vapors are ejected in indefinitely large volumes. What part of these was once of the atmosphere and is returned to it after an underground journey we do not know, but it is believed that a large part may come from the interior. The escape of these internal gases may also occur in some degree continuously by diffusion, and in influential amounts during episodes of mountain growth, when rock masses are strained and riven and upraised. We shall see presently that in the wasting of a mountain range there is serious consumption of carbon dioxide, which in greater or less degree temporarily affects the gain. The Pleistocene glaciation is attributed to a very notable offset of this character, but the exceptional nature of that event as compared with the relatively frequent episodes of mountain growth indicates that the gain of carbon dioxide has commonly equaled or exceeded the resulting temporary loss.
Among the cycles of combination and release, through which carbon dioxide runs, there are two which are both important, though not equally so. The first and less important is the cycle of organic change, involving plant and animal tissues. When grass grows, carbon dioxide is taken from the air. Grass becomes beef, and beef, through, various changes', is resolved into new compounds, yielding back the carbon dioxide to the air. In its brief phases this cycle has no import for the hypothesis, but there are occasions where it is prolonged, as in the accumulations of vegetal substances fossilized as coal. The total amount of carbon dioxide thus abstracted, and now withheld, is very large, and is believed to have been an important factor in promoting at least one instance of glaciation, that which followed closely upon the Coal Measure period.
The more important cycle of combination and release involves the decomposition of rocks by weathering, the solution of certain products and their transportation to the sea, and the reactions through physical, chemical and organic agencies, by which carbon dioxide is either permanently locked up in limestones or is returned to the air.
For the purposes of this statement, the common minerals of rocks may be classified as silicates and carbonates, that is as compounds with silicic acid and compounds with carbonic acid. The former may be typified by the familiar minerals of granite, the latter by limestone. Granite and all similar crystalline aggregates of silicates disintegrate, and the separate minerals are decomposed chemically by the action of carbon dioxide and moisture. Of the various compounds which result, those of carbon dioxide with lime and magnesia are of most direct interest in this connection, and those with lime may be discussed as representative.
The common combinations of lime and carbon dioxide are two: the carbonate of lime, more specifically called the normal or monocarbonate, and the bicarbonate of lime. The carbonate consists of one ion, or chemical unit, of lime, CaO, combined with one ion of carbon dioxide, CO2. The bicarbonate consists of one ion of lime combined with two ions of carbon dioxide. The carbonate is but slightly soluble in water, the bicarbonate is easily dissolved. The carbonate is produced in the decomposition of silicates, and great amounts of it which have been derived from this source in past ages are now contained in limestones and other calcareous sedimentary rocks. Whether it exists for a brief time in the weathering of silicates or is, as limestone, exposed to atmospheric waters, the carbonate very readily combines with carbon dioxide, and the bicarbonate is formed in solution. All surface and underground waters contain bicarbonate of lime in greater or less quantity, and enormous volumes are annually conveyed to the sea. It is estimated roughly that the weight of the carbonate of lime thus dissolved and contributed to the sea annually is 2,700,000,000 tons. This is about one-half of the total saline matter dissolved in surface waters annually, and a portion of the remainder consists of carbonates of magnesia, potash and soda.
It has been computed by Professor Chamberlin and his associates that the present supply of carbon dioxide in the atmosphere would be exhausted by the decomposition of silicates in 5,000 to 18,000 years at the present rate of consumption if there were no source of replenishment. It is evident that the amount of carbon dioxide in the atmosphere at any time is the balance between supply and draft, and that it may be more or less as one or the other preponderates. The next step in forming the hypothesis, therefore, is to consider conditions which may produce fluctuations of consumption and contribution.
The consumption of carbon dioxide in weathering of rocks is an effect of erosion, the familiar process which tends to reduce heights of land to a low slope, declining to sea level. This tendency is opposed by those internal forces of the earth's mass which depress sea bottoms and relatively uplift continents and mountain ranges. The persistent attacks of the sun's energy are directed against.the earthworks raised by terrestrial forces. It is the fabled fight of the powers of light and air against the powers of the dark underworld; and the former never pause, whereas the latter sleep for ages, and, awaking, exert themselves mightily for a brief time only. While they rest, mountains waste to hills and hills to plains, and the sea spreads over the margins of sinking continents. When they put forth their strength, mountains grow and continents rise from the waters. Their intermittent activity exerts a potent influence on the constitution of the atmosphere, and is so important to the hypothesis of glaciation that a more definite account of the evidence of its periodic nature is necessary.
Sediments laid beneath the sea are waste of continents. By their characters and volume they may indicate their derivation from far-stretching lands or from near mountains. They often occur spread across the bases of ranges which have been planed away by air and waves. By evidences such as these the physical history of any province may be made out; by comparison of provinces the major events in the history of a continent are ascertained; and by comparison of continental histories the sequence of world stages is studied. For any province the limits of knowledge depend only on the completeness of the record in the local rock series; for each continent the inferences are qualified by difficulties of correlating successive steps from province to province; and world-wide conclusions must necessarily be restricted to the broadest effects.
In the present condition of geologic investigation we know but incompletely the rhythm of continental and marine oscillation, but certain marked epochs are recognized. Seas were extensive, while lands were low and restricted, during epochs known to geologists as the middle Ordovician, the middle Silurian, the early Carboniferous, the late Jurassic and the upper Cretaceous. At these times the consumption of carbon dioxide by rock weathering was comparatively slight, according to hypothesis. On the other hand, lands were wide and seas confined to their basins during the close of the Silurian and beginning of Devonian time, during the Permian and early Triassic periods, and during the Pliocene and Pleistocene. These were epochs of unusual consumption of carbon dioxide.
The climatic effect of depletion of carbon dioxide depends upon the rate at which it is taken from the atmosphere. If it were abstracted slowly a large loss might be compensated by moderate supply, whereas if it were rapidly removed the effect on the atmospheric content might be decided. In this relation the growth of mountains has an important accelerating influence. Although the rate of weathering is conditioned by many factors, elevation is so important that Chamberlin's estimate is probably near the truth. It is that for continental areas the rate of carbonation varies probably more nearly as the square than as the simple ratio of altitudes.
Modern studies of mountain growth have materially changed the views held within a decade by geologists as to the ages of ranges. Among the rocks of any range there are youngest strata that mark a date earlier than the most remote at which the uplift may have occurred. Impressed with the magnitude and grandeur of mountains, geologists assigned them an antiquity limited only by the age of their component strata, but through the interpretation of landscape forms evidence is now accumulating to show that existing ranges are, as a rule, comparatively young. One interesting conclusion is that we live at a time near the culmination of an epoch of mountain growth, that mountains are now widely distributed and high as compared with those of many preceding periods, and the earth's activity as thus manifested is not materially less now than formerly within known geologic history. The crustal adjustment which produced existing mountain ranges and expanded continents appears to have culminated just before or very early in the Glacial epoch, and the recognition of this fact was the principal basis for the hypothesis that glaciation was related directly to elevation of the land areas. Chamberlin interprets the relation through the influence of rock-weathering upon the carbon dioxide of the air, and attributes the cold period to the resulting thermal transparency of the atmosphere.
The carbon dioxide abstracted from the air by weathering passes into the aqueous circulation of the globe, one-half of it in combination as monocarbonates, the other half superadded to form bicarbonates. A further step in framing the hypothesis is to follow this second part until it shall be returned to the air, which shall thus be reenriched and may promote a period of mild climate.
The ocean is a great reservoir holding carbon dioxide in combination with various bases as bicarbonate. It contains also many other salts. Assuming that all the solids dissolved in sea-water have been derived from the land at a rate of solution equal to that now determined by analyses of river waters, it is possible to make a curious calculation, which shows that the carbonate of lime now in the sea would have accumulated in 60,000 years, whereas the common salt, chloride of sodium, would have required 166,000,000 years. The common salt is not removed from solution, nor is there reason to suppose that there is any special source from which it is concentrated, but which does not supply lime; it may, therefore, be taken as a standard of comparison, which shows that there is much less lime in the sea than we should expect. The deficit is accounted for by the great beds of limestone deposited from the sea at various periods from the long past to the present.
In the ocean, bicarbonate of lime is dissolved in a proportion less than that which the waters can hold in solution, and, according to the principles of the older chemistry, it is under these conditions a fixed combination, which remains dissolved. Should, however, the monocarbonate of lime separate from the water as a solid precipitate, the second part of the carbon dioxide would be free to pass from the water into the atmosphere. We are thus led to consider the agencies which have caused the deposition of lime from dilute solution.
Modern views of chemical combinations regard compounds in solution as going through a constant interchange of reactions, by which ions pass continuously from one association to another, as in the grand chain the dancers weave in and out with touch of hands. The dance of the ions, more technically called dissociation, is most active in dilute solutions, and is promoted by higher, retarded by lower temperatures. It has been shown by experiment that bicarbonate of lime may be dissociated by agitating the solution, and there are occurrences of calcareous formations which indicate that the monocarbonate is deposited as a result of such action. Thus it is probable that, through this process, warm seas surrender to the air a notable amount of carbon dioxide, but that the contribution becomes insignificant or ceases when the waters are chilled.
Under favorable conditions the ocean abounds in organisms which secrete normal carbonate of lime as parts of their structures. They swarm in the warm waters of tropical oceanic currents, they exist in multitudes on the warm shallows where the sea spreads over the margins of continental masses with a depth not exceeding 100 fathoms, but they are rare or are replaced by species without hard parts in cold waters. The physiological reactions by which these organisms obtain the normal carbonate from the water are not definitely known. They may take it from bicarbonate in solution, or by reaction on sulphate of lime setting free sulphuric acid, which attacks the bicarbonate. In any case, the effect is to fix one ion of carbon dioxide in the solid normal carbonate, and to free the second ion, which may pass into the air. The enormous volume of organic calcareous deposits now forming, and the massive limestone strata of past ages, largely or wholly of organic origin, attest the importance of the process. Life may be considered the most important of those agencies which restore carbon dioxide to the atmosphere, but it is narrowly conditioned by limitations of habitat and warmth.
Carbon dioxide absorbed in sea-water is yielded to the atmosphere and returned by it under varying conditions of tension of the gas, of barometric pressure, and of temperature. At moderate temperature the sea gives up the gas freely, and would supply a deficiency gradually brought about in the atmosphere. But colder waters hold it faster, and may even take carbon dioxide from an already depleted atmosphere.
Thus the processes of dissociation by chemical and organic agencies and of absorption depend upon temperature, and through this dependence promote the prevailing tendency of climatic changes. If the change be from warm to cold, cooling waters tighten their grasp on the precious gas that might offset the atmospheric depletion. If they be warmed beneath an air growing rich in carbon dioxide, they become generous of their hoard. The processes are, therefore, auxiliary and intensifying, not initiative.
For the initiative process, which may start the train of effects leading to atmospheric enrichment and a warm epoch, we must refer again to the periodic rest and unrest of the earth's forces.
When, through adjustment of the relations between continental masses and masses beneath the oceans, the internal stresses of the globe have been balanced, the average elevation of lands above sea level is a maximum. The highest rate of consumption of carbon dioxide by weathering may be assumed to follow after a brief but appreciable interval, and from that time forth to diminish. As heights waste and slopes sink low, they become mantled with the residual product of weathering, soil, and efficient contact of carbon dioxide with unaltered rock is limited. When the average height of land is become that of a low plain, carbonation is reduced to a very small part of its maximum activity, and the rate of consumption is slow. At some stage of this change the diminishing rate of depletion may equal and thereafter sink below the rate of supply. Thus the initial condition of a return of milder climate inheres in the transient nature of the cause of a cold epoch. The result might, however, be long delayed, but for the accelerating influence of auxiliary processes, of which, as already stated, life is believed to be the most potent.
It is a recorded fact of geologic history that periods of minimum continental elevation have been periods of extensive marine expansion. These conditions have been associated with remarkable development of marine faunas and with general mildness. Chamberlin was the first to point out a causal relation between these conditions and effects. He entertains the idea that at the climax of an epoch of crustal adjustment, the elevation of continents may be somewhat greater than that required by radial equilibrium. If so, they should in time exhibit a tendency to settle back. In so far as this period of readjustment of balance might suffice for deep general denudation, the subsiding lands would present low plain surfaces to the sea. These conditions would be most favorable for wide migration of the shore from the continental margins far inland, and would result in extensive areas of relatively shallow water. A fauna, which had existed on the narrow slope between the original position of the shore and an oceanic basin, would find its habitat immensely enlarged and favorably conditioned. Responding, it would develop varieties, species and genera until, as sometimes was the case, exuberance ran into unfitness and decadence.
The lowland aspect of continents would be favorable to other ameliorating influences, as well as to life. Before it could have attained its later phases, the acute thermal transparency of the atmosphere must have given place to moderate absorption, and temperate conditions must have succeeded cold. From waters warmed on widening shallows, car- bon dioxide would pass into the air by simple diffusion and by chemical dissociation. But the principal contribution, upon which generally pre- vailing mildness would attend, would be associated with the active development of lime-secreting life, and this relation is firmly estab- lished by observation.
Grand seasons of the eras are thus interpreted by Chamberlin as effects of periodic adjustments of the earth's superficial form to stresses developed within its mass. The causes of these stresses are sought by physicists and geologists in the most profound researches, and for the present, at least, they elude discovery, because the physical and chemical conditions of matter within the earth transcend conditions of observa- tion. But geologic investigation is competent to trace their influence upon aspects of the earth, and not the least valuable result of Cham- berlin's thought is the impulse it imparts to studies into the geography and life of the past.
The general hypothesis being thus promisingly developed, some would have been satisfied there to rest the suggestion, and the general reader may be content with the splendidly panoramic view of effects and causes which it embraces. But its author pursues its analysis and appli- cation with rigorous questioning, limited only by the bounds of existing knowledge, and where knowledge fails he points out the need of research. We shall touch only upon the principal points of his thorough discussion, the competency of the causes, the oscillations of glaciation, the time limits set by the probable duration of glacial and interglacial epochs and the localization of glaciation in Pleistocene and in Car- boniferous times.
As already stated, the Pleistocene glaciation is attributed to deple- tion of the atmospheric carbon dioxide occasioned by the notable expan- sion and elevation of lands late in the Pliocene period. It is estimated that in the preceding warm age the land area was 44,000,000 square miles. That of the succeeding expansion at its maximum is computed at 65,000,000 square miles, and the present extent is taken at 54,000,- 000 square miles. That is to say, the areas are related nearly as 1:11⁄2:11⁄4 - Elevation, which is more important than extent, was at the time of greatest expansion at least two or three times what it shortly before had been when continents were smaller. In the earlier time of mildness the margins of continents were generally submerged, as the eastern portion of North America now is, affording a roomy habitat for lime-secreting marine life. But with the uplift of continents these sea- shelves were reduced to narrow zones along the steeps which descend into oceanic depths. Low, limited lands and wide, warm seas had promoted the flow of carbon dioxide from the waters to the air. Lands elevated and expanded and seas shrunk within their basins reversed the course, and the earth took from the air to give to the waters.
The rate of depletion is capable of reasonable calculation. If the amount of carbon dioxide taken from the atmosphere exceeded by 10 per cent, that supplied to it from all possible sources, 50,000 years would suffice to reduce the content from.18 per cent, to.03 per cent, by weight. And this change would bring on glaciation. There are few students of the earth's history who would be willing to admit that the associated effects of topographic development could have occurred in less, if, indeed, in so short a time, and the causes assigned are thus seen to be fully equal to the task imposed.
The climates of the Glacial period were marked by rhythm recorded in advance and retreat, and re-advance and withdrawal, of the ice front several times repeated. The major changes were as great as that which has intervened between the severest glaciation and the present, and occurred early in the series. The later oscillations declined in both Europe and North America. Such rhythmic rebound from one phase to another and back again is characteristic of phenomena which, though they swing to extremes, themselves set up the action that reverses the movement. The ice sheet itself set the bounds of its possible spread.
Assuming glaciation to be inaugurated and the cold to be intensified by consequent accelerating influences, which need not be detailed here, the depleting process of weathering must be checked by the mantling ice and refrigeration. It is estimated that frost and ice at their maximum effect protected 20 per cent, of the Pleistocene land area. Continued depletion depended on the balance between contribution and abstraction, and it is suggested that 20 per cent, (or whatever may have been the proportion of land area sealed against carbonation) represented the initial preponderance of draft over supply. Whenever the effects of glaciation reduced the consumption of carbon dioxide below the inflow from all sources, the glacial epoch would end and the reaction would begin. Once initiated, it would be accelerated by diffusion and dissociation in the richly stored seas, and by renewed development of life in the warmer waters. The mildness might increase till the great glaciers had vanished, but it could have come to stay only in case the height and area of land had adequately diminished.
Lands remained extensive and elevations great during the Pleistocene period. They were even wider and higher than they are now. As an early ice mantle shrunk it bared rock masses and glacial deposits, which were to a great extent favorably conditioned for chemical attack. The renewed consumption of carbon dioxide in time overbalanced the supply, and glaciation went on again.
What was the period of this climatic pendulum? The answer comes to us vaguely in echoes of Niagara's voices. The cataract began its existence at Lewiston during the retreat of the latest ice sheet. Since that time the gorge has been cut back from Lewiston to the present site of the falls, and it is possible to estimate roughly what time the task has consumed. This episode is one-half or less than one-half of the time elapsed since the beginning of the retreat of the ice from its most advanced position. Thus indefinitely, we may count that something like 40,000 years sped while the climate changed from those Greenland conditions to these which we now enjoy. By similar conservative studies of the effects of deposition and erosion accomplished before the latest glaciation, the duration of the interglacial epoch is found to be several times that of the post-glacial interval; that is, in numbers, 80,000 or 120,000 years or more.
The significance of these figures does not depend upon their precision. They confessedly do but indicate the general magnitude of the times. But they serve to show that those times were more than sufficient for the operation of the causes assigned to produce the observed effects, and thus they sustain the hypothesis. Furthermore, they serve to bring glaciation near to us. In earth history, whose eras are measured by millions of years, events which occurred a hundred thousand years ago are of recent date. We live within the operation of the causes which may hinder or promote glaciation, and, though the present is an age of comparative mildness, we cannot be sure whether this be the spring of a great era or midsummer of an epoch. Are the gnomes of the under-world wearied of mountain building, and have they sunk to rest? Are the shafts of the sun's heat as they traverse the air effectively caught and stored? Does man, consuming fossil carbon in his manifold activities, unconsciously postpone the return of winter?
The cause of a glacial epoch may be found when an adequate cause of cold is linked with the occurrence of glaciation, but the spread of an ice mantle is dependent on snowfall as well as on temperature, and it is through this relation that the peculiar distribution of Pleistocene glaciers may be explained. It will suffice here to state the meteorological conditions which, according to Chamberlin, determined the most striking centers of accumulation, those which were situated in the plains of north-northeastern America.
Studies of polar currents, which free the northern coast of Siberia of ice and crowd it upon the American Arctic Archipelago, combined with the partial data available as to the barometric conditions of the Arctic zone, lead him to the conclusion that in the northern hemisphere the grand movement of the atmosphere from west to east about the globe is oblique to parallels of latitude, and is upon an axis which has its pole at a point located somewhere north of Hudson Bay. One effect of this oblique rotation is to establish in northern latitudes between 50 and GO degrees two areas of low barometer and one of high barometer, a disposition which is in strong contrast to the condition in the southern hemisphere, there being a zone of high pressure along the parallel of 35 degrees, south latitude, with decreasing pressure thence toward the Antarctic. These lows and highs differ from those which are familiar as features of daily weather maps, in that they are nearly stationary. The well-known migrant centers converge toward and run into the great fixed centers. The two permanent lows are situated one across the North Atlantic, from Hudson Bay to Scandinavia, the other in the North Pacific, from Japan to southern Alaska. They are centers of inflowing ascending air currents, and are, therefore, characterized by great precipitation. The region of maximum glaciation at the present time lies between them; one conspicuous development occurring in Greenland in the northwest quarter of the Atlantic low, another lying in Alaska in the northeast quarter of the Pacific low. By an analysis of the winds, it is shown that both Greenland and Alaska lie to leeward of the prevailing currents where they pass ashore. They are not necessarily the provinces of maximum precipitation, rain and snow both considered, but they are areas of copious snowfall, with low annual mean temperatures.
The northern high lies nearly midway between the two lows over Siberia. In contrast to them, it is a center of descending outwardflowing currents, marked by slight precipitation, and it is not now, nor was it in Pleistocene time, a scene of glacial development.
The centers of Pleistocene glaciation were so arranged with reference to the glacial regions of to-day that they would be determined by the oblique circulation and distribution of areas of low pressure, if existing conditions were intensified. An adequate occasion of intensification is found in the thermal transparency of the atmosphere, resulting from depletion of carbon dioxide, and thus the localization of Pleistocene ice sheets is explained in a manner consistent with the major hypothesis of the cause of glaciation.
Chamberlin's hypothesis is framed on an atmospheric basis, but the efficiency of the agencies which it postulates depends upon geographic conditions, upon distribution of land and sea and average heights of continents. The geography of the earth in the closing epochs of the Paleozoic era is known only in its broadest outlines, and they are but vaguely traced. With such imperfect data it is impracticable to explain satisfactorily the extraordinary phenomena of glaciation at that date in intimate association with the development of coal beds and extending within the tropics. Nevertheless, to carry out his purpose of developing a working hypothesis, the author feels obliged to arrange the known facts and to discuss them along the lines so successfully followed in reference to Pleistocene glaciation. An important difference in the argument as applied to the two cases lies in the cause assigned for depletion of the atmospheric carbon dioxide. The postulate of an appropriate time relation between Paleozoic land movements and the epoch of glaciation being conservatively assigned a minor position, the storage of carbon as coal is given major rank in accordance with known relations. This process is not attended by the accelerating and reacting influences, which are due to the equivalent of carbon dioxide contained in bicarbonates, and glaciation would, therefore, result only after depletion had continued longer. In this suggestion is found a possible reason for the wide extension of cool climate and the occurrence of glaciation in remarkably low latitudes.
The broad scope of philosophic thought upon which this working hypothesis rests is indicated by the titles of articles which have flowed from Chamberlin's pen in the last four years. Fortifying his own general researches where needed by those of specialists, he, with reason, challenges fundamental and generally accepted views. He gives the geologist and biologist new clues with which to thread the labyrinth of knowledge, and develops important relations between dynamical geology, stratigraphy, climate and evolution.
- A review of Chamberlin's 'Working Hypothesis of a Cause of Glacial Epochs.'
- A Group of Hypotheses bearing on Climatic Changes, Jour. Geol., Vol. V., No. 7, 1897. The Ulterior Basis of Time Divisions and the Classification of Geologic History, ibid. . Vol. VI., No. 5, 1898. A Systematic Source of Evolution of Provincial Faunas, ibid. . No. 6, 1898. The Influence of Great Epochs of Limestone Formation upon the Constitution of the Atmosphere, ibid. Lord Kelvin's Address on the Age of the Earth as an Abode Fitted for Life, Science, N. S., Vol. IX., No. 235, pp. 889-901, June 30, 1899, and Vol. X., No. 236, pp. 11-18, July 7, 1899.