Popular Science Monthly/Volume 29/July 1886/Geological Climate in High Latitudes

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By C. B. WARRING, Ph. D.

THE peculiar climate of geological times has hitherto been treated as if it were a question of temperature only. Scientists have sought the cause of the remarkable warmth in arctic regions, but have left untouched other questions of equal and perhaps greater importance.

One can hardly contemplate the climatic conditions of that remote period without inquiring how there could be other than a great difference of temperature between the summers and winters of lands less than 8° from the pole; and how could circumstances—environments—so unlike as the four or five months of day of those regions, and the twelve-hour day of the tropics, fail to induce great specific differences in their fauna and flora. The questions spontaneously arise: Is it possible that the days and nights in high latitudes were then as they are now? Must not the climate have been warm in January as well as in July? Must not the influences of the solar rays—the actinic force—have been distributed through the year with at least approximate uniformity in high as well as low latitudes? It is these questions, as well as those of temperature, that I shall consider in this paper. I propose to study the record left by the plants and animals which lived in those remote days. Some of their more obvious teachings are startling enough. Regions where now vegetation is of the scantiest character, where no trees exist save a few dwarf willows, where the winters are cold almost beyond endurance, were, as late as the Miocene, covered with magnificent forests of magnolias, oaks, cypresses, and a hundred other species. In more remote periods they abounded in plants and animals, whose fellows of identical species lived at the same time,[2] or at least in the same geological period, near the equator.

These statements are so extraordinary that they need to be established by unanswerable evidence. Of this there is a great abundance.

In latitude 81° 40' Captain Nares found the remains of corals in vast quantities. These creatures require not merely a warm but an equable temperature. Those of to-day can not live where the temperature falls below 66° Fahr. Sir Charles Lyell says: "The same genera, and to some extent the same species, of Ammonites are found in those high latitudes and in India. Remains of a large ichthyosaurus were brought by Sir Edward Belcher from latitude 77° 16'. Others were found by the Swedish expedition in Spitzbergen, latitude 78° 10'." In Dana's "Manual of Geology," under the heading "Climate," in all the early periods, abundant illustration is given of the uniformity of climate in high and in low latitudes. On page 181 he sums up in these words: "No marked difference between the life of the primordial period in warm and cold latitudes has been observed"; and again, on page 253, "The living species, from 30° to 80°, were in part the same, or closely allied."

It is unnecessary to multiply proofs. All geologists agree that, all over the world, the plants and animals of any particular horizon were exceedingly alike, and very often identical. The living species to which they are most nearly allied are peculiarly sensible to changes of temperature.

So far, therefore, as it is possible to judge the past by the present, the fossils indicate a warm and uniform temperature almost to the poles, such as is now found in regions inhabited by similar species. Geologists are forced to this conclusion. In that wonderful work, Professor Dana's "Manual of Geology," it crops out everywhere: Page 266, "There is no sufficient evidence of cold arctic seas"; page 289, "There was little difference of temperature between temperate and arctic seas." (See pages 352, 452, 480, 488, 514, 521, 526, etc.) All tell the same story. "No zones of climate." Warm arctic seas all the year round.

It may, however, be thought that no very certain conclusions can be drawn from these facts, because the identical species which flourished in those remote times are no longer extant, and perhaps they were not so sensitive to cold as are their closely allied successors. There is some force in this, but we must not give it too much weight, for all progress in knowledge of the world's history is based upon the belief that, in general, corals in Palæozoic times indicate such conditions as exist where we now find corals; saurians, where we now find saurians; tree-ferns where we now find tree-ferns; and so of other organisms. As soon as we leave this principle, we are at sea without a compass, and almost without a star to guide us. There is direct evidence of the warmth of climate in the Tertiary, and, if this be established, there will hardly be dispute as to the climate of the earlier periods. Plants of living species which require not only a mild temperature, but one of great evenness, have been found in very high latitudes. In Spitzbergen, latitude 78° 56', there have been found the remains of a Miocene flora remarkable for its variety and luxuriance. One species, Libocedrus decurrens (Heer), now lives with the redwoods of California; another now occurs in the Andes of Chili; while a third, according to Dr. Gray, is the common Taxodium, or cypress of the Southern States. In Greenland, latitude 70°, were found magnolias and zamias.[3] All these require not merely a warmth but an evenness of temperature that in such high latitudes is extraordinary; extraordinary and incomprehensible, if then, as now, the solar heat was wholly shut out for more than four months. It will help to realize the difficulty of a uniform climate in regions 75° to 85° from the equator, if we consider what now would be the effect of a four months night covering the torrid zone, and remember that the cold of arctic countries is not due to their position, but to the absence of the sun's rays caused thereby. The accumulated heat of summer, great as it is at the equator, would soon be radiated into space, and, when the sun returned, not a living plant or animal would remain to greet it.

The effect would be no less fatal if the long nights occurred in a zone extending, say, 20° north or south of the Gulf States. Consider the effect produced now by a slight lengthening of the night, and then say how complete would be the destruction if the night's duration was increased from a few hours to four months!

In geological times, if the axis of the earth had its present obliquity, the midwinter nights in Spitzbergen, where the plants I have mentioned were found, were four months long. The resulting changes of temperature must have been very great. At the present time they are enormous. Captain Nares[4] says that the thermometer at his winter quarters fell in March to—73.7°. For thirteen consecutive days it showed—59°; for over five days—66½°. The variation between that and summer must be something enormous, for Mr. Meech has shown in his paper on "Solar Heat," published in the "Smithsonian Contributions to Knowledge," that the amount of heat from the sun received in latitude regions, during the three middle months of the arctic day, is greater than is received in the same time at the equator.

In Dr. Kane's "Arctic Exploration," we read that the difference between maximum and minimum (summer and winter) temperature, in latitude 78° 37', was 120° Fahr. At St. Michael's, latitude 631/2°, the thermometer ranged from +76° to-55°, a variation of 131°. It would, I think, be a moderate estimate, should we attribute at Spitzbergen a variation of 100° to the changes in the sun's declination, or, in other words, to the obliquity of the earth's axis.

The cold undoubtedly was greatly modified by the latent heat of the surrounding ocean, and by the inflow of ocean-currents. But the same capacity for giving off heat exists now, and the same currents continue to flow; yet in Spitzbergen—a not large island, surrounded by a broad expanse of water—the cold is very intense. The specific heat of water has undergone no change; so far as that is concerned, the surrounding ocean does as much now, as then, to make the Spitzbergen winters mild.

Did the Gulf Stream, or the Japan Current, in those remote times, have a greater flow than now? Their effective cause is the difference between polar and tropical temperature. If this was nothing, the flow would be nothing. In geological times the difference of the temperatures must have been small, since the same species of plants and animals extended from the tropics to as near the poles as has been explored. Hence the flow of these streams, to say the least, could not, in those times, have been greater than it is at present.

Whether 100° is an overestimate of the difference between the summer and winter temperature at Spitzbergen, due to the long days and nights, it is certain that the sun produces a great effect upon the temperature in high latitudes. Whatever other thermal influences may have existed in the Miocene, or in other and earlier periods of geology, their effect was no greater in winter than in summer. Admitting it to have been the same—a matter of great doubt the temperature, as the nights grew longer, must have fallen until it reached a point at which the loss of heat, by radiation into space, was just equal to that brought in by the ocean-streams, and by such aerial currents as might blow from warmer regions.[5] In summer there were the same sources of heat plus a sun shining not twelve nor fifteen hours, but for months. Calling the winter heat A, and the increment from the sun B, the heat during summer equaled A plus B. The difference at the present day between the temperature of arctic seasons is enormous. It is difficult to see how it could have been so reduced as to render life possible for plants whose fellows of the same species were, at that very time, growing in regions thousands of miles nearer the equator.

The fact of their continued existence, not dwarfed and scanty, but with greatness of size and luxuriance of growth, seems to indicate that there could not have been a total cessation of solar heat for months in winter, and an uninterrupted influx for months in summer. In other words, the evidence of plants and animals points to the absence of present long days and nights.

However it may be, as late as the Tertiary, geologists are agreed that at least to the end of the Palæozoic there is a lack of any indications of zones of climate,[6] or, to put it in another form, that there is evidence only of evenness of climate.

I next inquire, What is indicated as to the length of the arctic day by the effects of light upon plants?

In all discussions of these curious facts—at least so far as I have seen—no attention has been paid to the effect upon vegetable and animal life, of the great difference between the length of the days and nights in high and low latitudes, even though the temperature were kept up. In Spitzbergen, for example, the sun shines uninterruptedly for four months, and for an equal time its rays are cut off, while in tropical regions a day of twelve hours is followed by a night of the same length. In the temperate zone the day is at most but a few hours longer. If the earth's axis in preglacial times was inclined 231/2°, the same inequality prevailed then. Light is as necessary to plant-life as heat, and, in respect to the character of the polar day, its evidence is more important, since light is affected only by the inclination of the earth's axis. The flow of the Gulf Stream, the lay of the land, or the relative amount and arrangement of the land and water—matters of great moment when considering questions of temperature—have no effect whatever upon the length of the day, or, in other words, upon the mode of light distribution.

Mr. Darwin and his followers have called attention to the influence of environments in destroying old species and in the production of new. In view of all that they have established, it seems incredible that species identically the same could have lived and shown luxuriant growth, say in Spitzbergen and Florida, through thousands and millions of years, unaffected by such difference as now exists in the length of the days and nights. The arguments against the reality of such a difference become stronger when we reflect that in both high and low latitudes there were from period to period enormous changes in species, old ones passing away and new ones appearing, not once, nor twice, but a great many times, and yet at each epoch the new species, from the shores of the Arctic Ocean to the coasts of Bolivia, were everywhere largely the same; always enough of identical species to show that arctic and tropical environments were essentially alike. It seems, if possible, still more incredible that in later times—say, in the Miocene—species which originated in Spitzbergen and upper Greenland could migrate to low latitudes, and still show no change in specific characters.

It certainly was to be expected that conditions so unlike—I refer, now, only to the long days and nights—should have been attended by widely diverse plant-life.

The belief that such would have been the case is strengthened by the fact noticed by Mr. H. C. Watson, and quoted approvingly by Mr. Darwin in his "Origin of Species," that, "in receding from polar toward equatorial latitudes, the Alpine or mountain floras really become less and less arctic."

But, were they truly arctic, and identical with those now in Spitzbergen, such floras, accustomed to a hibernation of nine months, might well be indifferent as to where that time was spent, whether in the cold and continuous darkness of an arctic night, or in the cold of a winter on a low-latitude mountain-top. On the other hand, the plants, e. g., of the Carboniferous, were not arctic plants, but were warm-temperate, if not tropical, and there was no arctic cold, but "a warm, moist, equable atmosphere,"[7] in which they "flourished luxuriantly."[8] Another corroborative fact is found in the peculiar structure of certain post-glacial arctic trees. A conifer, found standing in latitude 721/2°, and of post-glacial origin, was brought to England by Sir E. Belcher, where Sir William Hooker made a microscopical examination of its structure. He found that it differed remarkably from any other conifer with which he was acquainted. Each annual ring consisted of two zones of tissue: the inner zone was narrow, of a dark color, formed of more slender, woody fibers, with few or no disks upon them; the outer zone was broader, of a pale color, and consisted of ordinary tubes of fiber of wood marked with disks such as are common to all coniferæ. These characters he found in all parts of the wood. They suggest, as he says, the annual recurrence of some special cause that modified the first and last formed fibers of each year's deposit, and this cause, he thinks, is found in the peculiar conditions of an arctic climate, where the days were at first very short, a few hours only of sunshine. Then the first and imperfectly developed fibers were formed. As the days grew longer and longer, and the solar rays at last became continuous, the woody fibers became more perfect, and were studded with disks of a more highly organized structure than are usual in the natural order to which this tree belongs.[9]

Since Spitzbergen is nearly 5° farther north, such or similar effects ought to show themselves there in greater intensity in the conifers of early geological times, if the same causes then existed. Whether they have been specially searched for, I do not know; but their absence, if established, would strengthen the conviction that the conditions of arctic climate which produced such a peculiar mode of growth did not exist in the time many thousand years earlier when libocedrus, magnolia, and zamia were denizens of high latitudes.

There are other facts whose tendency is in the same direction.

It is admitted by all that the climate in the earlier geological times and down to the end of the Miocene was warm through the whole year. If, therefore, the earth's axis was then inclined 231/2° as now, the plants of Spitzbergen and other high latitudes must have spent during every year of their existence more than four consecutive months without a ray of sunshine, and surrounded by an atmosphere moist and warm.

Their condition resembled that of plants in a warm, dark, and moist cellar. Modern vegetation so placed soon bleaches and dies. Undoubtedly it was possible for a specially adapted flora to exist under such circumstances. And a special flora is what we should expect. But the flora of Spitzbergen was not special; it was cosmopolitan in all the earlier periods, and in the Miocene some of the identical species flourished there with "amazing luxuriance," whose descendants, with specific character unchanged, are now found in the Southern States of our own country. It seems to me that this is presumptive evidence, if not demonstration, that as late as the Miocene the long arctic nights were unknown.

Moreover, this very luxuriance of foliage, which so surprised Lyell and other geologists, tends to the same conclusion. It is a matter of common observation that plants exposed to the full force of the sun's rays have smaller leaves than others of the same species which are somewhat protected. It would seem as though Nature compensated for the inferior intensity of the solar action by giving more surface to be acted upon. Now, since the intensity of the sun's rays varies as the cosine of the latitude, it is evident, in case the sun underwent no change in declination, that, while the length of the day in Spitzbergen and Florida would be the same, the intensity of the light in the latter would be almost double. Hence, if the earth's axis really was nearly or quite perpendicular, with the same conditions as to moisture and warmth, we ought to look for greater breadth and length of leaves in Spitzbergen than in regions much farther south, and we find them.

I know of but one fact in the geological record which seems to point to the existence of changing seasons. Fossil exogenous trees of very early times have been found with well-developed growth-rings, and, as these are usually attributed to seasonal changes, it has been said that they prove the existence of seasons; and, as these are due to the obliquity of the earth's axis, any inference to the contrary from other facts must be wrong. But growth-rings do not of necessity indicate summer and winter. They may occur several times in a summer, or not at all, or once in several years, or where there is absolutely no change of seasons. I have seen a hard and woody stem of Chenopodium album, not more than four months from the seed, in which were eight well-formed rings. Dr. Gray says there is "a woody Phytolacca which makes at least twice as many layers as it is years old," and that cycads require several years for one layer. The orange and lemon, in green-houses, where seasons can hardly be said to be known, form ring's as well defined as those of our forest-trees. On the Amazon, as may be seen in a collection of woods now in Vassar College, the rings are very apparent in some species, while in others equally exogenous none can be seen. The mangrove, which grows in the tropics on the sea-shore between high and low water mark, where by no possibility can there be any annual change either in temperature, or from wet to dry, has the rings well developed.[10] These facts suffice to prove that the existence of growth-rings is independent of the existence of seasons.

I think it must be admitted that the teachings of geology are in harmony with what would have been the climatic conditions in high latitudes if the axis of the earth was then perpendicular (or nearly so) to the ecliptic, provided that in some way the temperature could have been kept up sufficiently. And, if there be anything in the influence of environments, the lack of results corresponding to days and nights so different as those, e. g., of Spitzbergen and Florida, is evidence that the days and nights in those countries did not differ then as they now do. If there was no such difference, the earth's axis then did not incline as it does at the present day.

The tilting of the earth, or, in other words, changing the direction of its axis—if gradual—would occasion no perceptible disturbance. Hence no conclusion is to be drawn from the absence of traces of such a cataclysm as would have attended a change of the geographical position of the poles. The latter, however slowly brought about, would have necessitated a change in the position of the equatorial protuberance, or, if the crust was too rigid for that, a change in the ocean sufficient to overwhelm the land. The only possible effect of an increase in the obliquity of the axis would be an increase in the length of the days and nights in high latitudes followed by corresponding climatic changes. These would have registered themselves in the plants and animals of high latitudes, while near the equator the effect would be scarcely perceptible. Days and nights in low latitudes would be only slightly affected, consequently animal and vegetable life would continue as before. It is corroborative of such a tilting, that the plants and animals in high latitudes, which, till near the end of the Tertiary, had been the same as in low latitudes, became wholly changed to new species after the Tertiary, while those in low latitudes remain as they were in general character, and, in some cases, the identical species yet survive.

In our Southern States, for example, the flora is closely allied to, and as to some species identical with, those of the Miocene of arctic regions. From the Miocene back, the geological record tells of life conditions—environments—the same all over the world.

So far, therefore, as geology is concerned, the evidence appears to be all one way, and I think I am justified in saying that the conclusion to which it points would be readily adopted, were it not for reasons derived from another science.

Astronomers say that a permanent change in the inclination of the earth's axis by means of any force known to science is impossible. We know, however, very little of the means by which our system was brought into its present state.

The only theory that attempts to explain, on purely mechanical principles, the existence of the solar system is the nebular hypothesis in some of its forms, although even that requires a self-existent entity back of it. According to this hypothesis, the earth and moon were once one body, which revolved, of course, on one axis. At some remote time a separation occurred. But no force of avulsion, whether the moon was merely left behind as the mass contracted, or whether, as Mr. Darwin thinks, it was thrown off after the earth had become solid, and pushed back to its present distance, could affect the plane of rotation, or the direction of the axes. On mechanical principles, the moon when it left the earth must have moved in the plane of the earth's equator, and the three axes—that of the earth, that of the lunar orbit, and that of the moon itself—must have been parallel to each other. But such is not the case now. The axis of the moon is inclined about 1° 30', that of its orbit 5° 9', and that of the earth 231/2°. It is evident that at some time the axes, or some of them, have undergone a change of direction. On purely mechanical principles, the change did not occur before the earth and moon separated, nor at the moment of separation, nor, in fact, at any time.

Astronomy, therefore, proves too much! It proves that the present condition is not eternal; that the earth was not created with its present oblique axes—in fact, that normally it was perpendicular to the ecliptic; and that, once in any position, it was impossible for it to become more oblique "by any force known to science." To all of which those of us who are not astronomers can only answer: "What you say may all be true, but, nevertheless, the earth's axis is inclined, and, if we can not show the cause—an inability which extends to a great many other things—our business is to discover, if we can, the time when it became inclined. It is not a question of possibilities, but of chronology."

The fact that the earth's axis has a different obliquity from that of the moon proves that a change occurred in the one or the other after their separation; and, since the moon remains so nearly in the normal position, it must have been the earth that was changed. The uniformity of biological conditions in all latitudes indicates that the present obliquity had not been attained in Archæan time, nor in Palæozoic, nor in Mesozoic, nor in the Eocene, nor in the Miocene, nor in the earlier Pliocene; then comes a blank during which the Glacial epoch came and went, and, when again the record begins to be legible, there are, for the first time in the world's history, indications of alternating seasons.[11]

In view of all these facts, it seems most probable that, in that blank interval, the Glacial epoch, or, more largely, between the end of the Miocene and the beginning of the Champlain, that movement occurred which gave the earth seasons, unequal days and nights, and greatly enlarged its limits of inhabitability.

It requires no argument to show that an axis nearly perpendicular would account for the otherwise inexplicable evenness of geological climate. Although the Gulf Stream, or other currents, might bend the isotherms, the temperature at any point would, with such an axis, have remained constant. The conditions as to light and actinic force would have been the same everywhere, save the variation due to greater or less latitude. All this, however, is compatible with great cold; hence it remains to inquire why the polar climate was so warm. Many theories have been advanced to solve this problem. I have neither space nor time to discuss them now, and will only say that six or seven of the earlier ones are ably treated by Searles V. Wood, Jr., in the "Geological Magazine" for September and October, 1876; also by Dr. Croll, in his "Climate and Time." Dr. Croll's own theory I have discussed at large in "The Three Climates of Geology" ("Penn Monthly," June, July, and August, 1880), and have there pointed out what seem to me insuperable objections to it.[12]

Professor Whitney has lately put forth another theory, attributing the early warmth to the sun itself being hotter in geological times than it now is. All conclusions, however, in regard to the sun's former temperature must be hypothetical; but, if it be a gaseous body, as suggested by Professor Young, it has been growing hotter all the time it has been giving out heat.[13] To this, as to all other theories heretofore advanced, there lies the serious objection that they ignore the world-wide uniformity of light and actinic force, and no theory that fails here can be satisfactory.

A perpendicular axis alone does not account for the warmth in polar regions. On the contrary, with such an axis, they would receive during the year less heat than they now do, and hence Dr. Croll infers that a perpendicular axis would make the polar climate less genial. This is true, if temperature depends solely on the amount of heat received. But, as every one knows, it depends far more upon the amount retained. Green-houses and drying-houses are often uncomfortably warm when the mercury without indicates a temperature near freezing. The solar rays readily enter through the glass, and are absorbed by the floors, walls, etc., while the heat radiated back is unable to escape. # Many substances possess this property, and Professor Tyndall has shown that among them are carbonic acid and aqueous vapor.

In the present state of our knowledge, it is impossible to form even an approximate estimate as to the actual amount of carbon stored in the earth's crust as graphite, coal, lignite, bitumen, petroleum, etc., but it must be many times—probably many hundred times—greater than that now remaining in the atmosphere. All these forms of carbon are directly or indirectly of vegetable origin, and hence it once existed in the form of carbonic acid. It has been said, however, that so much CO2 was not found in the air at any one time, but that it was given out by volcanoes just about fast enough to take the place of that which was stored in the earth's crust. But, as Professor Dana remarks, volcanoes do not originate this gas; they give it out only as their fires come in contact with limestone, and this occurs but rarely now, and was still more uncommon in Palæozoic times.[14]

It appears, therefore, that at some remote period all the carbon which has since been a portion of animal or vegetable forms, existed as free carbonic acid, and formed a part of the then atmosphere. With the beginning of plant-life a process of elimination commenced. It continued till about the close of the Tertiary, when the amount taken out by living forms and that restored to the atmosphere by decomposition became equal—a condition which still exists.

The CO2 is now diffused with great uniformity over the earth, and, for lack of reason to the contrary, we must believe that it was equally uniform in geological times. It acted as glass does in a green-house; it retained the heat radiated from the earth's surface, and consequently caused a rise in temperature. This increased in a higher ratio the capacity of the air for water, and that in its turn aided still further in retaining the heat, and of course made the climate warmer. In this, I think, lies the secret of the warm climate in high latitudes in those early times, the otherwise cold polar regions being protected by this double "blanket." The effect of such a covering is well set forth by Professor Tyndall in "Heat considered as a Mode of Motion," pp. 405, 406. I quote only one sentence: "The removal, for a single summer night, of the aqueous vapor which covers England would be attended by the destruction of every plant which a freezing temperature could kill."

In contrast with this, I add one illustration of the temperature possible were the earth covered with a "warm blanket" equal in heat-retaining power to glass. I quote from Professor Langley's summary of work on Mount Whitney to ascertain the amount of heat the sun sends to the earth: "On the summit of Mount Whitney the temperature in a blackened copper vessel, covered by two sheets of common window-glass, rose above the boiling point. With such a vessel water could be boiled in the snow-fields of Mount Whitney by the direct solar rays."

Besides carbonic acid and water, there probably were in the early atmosphere other gases and vapors. Ammonia would produce thirteen times the effect of CO2 at the same density, and marsh-gas four and one half times, and so of others ("Heat as a Mode of Motion," p. 362). Whatever there was of these, their influence tended to increase the "warm blanket." The amazingly slow change of temperature in the early periods finds a reasonable explanation in the effect of those gases and vapors in the atmosphere.

Professor Tyndall has shown that, commencing with a vacuum, and adding a small number of very small increments, the absorption is sensibly proportional to the increments, but, as the quantity increases, the deviation from proportionality augments (idem, p. 356); at length a condition is reached in which further increments produce very little effect. The converse must also be true. Commencing with a great amount of the gas, or vapor, a very great number of decrements will be needed to produce any sensible effect; then a smaller number, and so on, until toward the end, and then the decrement needed will be very small, and the effect comparatively large.

The following diagram, made from a table on page 35 of the same work, shows this more clearly. The curved line indicates temperature for equal increments of the gas.

The diagram is for sulphuric ether and olefiant gas. All other (compound) gases and vapors observe the same law, but differ in the rapidity of descent of the curve. The abscissas correspond to the equal decrements in equal times; the ordinates, to the fall in heat-retaining power. Up to the heavy line, counting from the right, the ordinates are proportional to the numbers in Professor Tyndall's table (page 354), which were obtained by measurements. The others are estimated by continuing the curve to the left. If we suppose the first abscissa on the right represents the Tertiary, the next three will represent the Mesozoic; the next twelve the Palæozoic, and an unknown number beyond will represent the Archæan.

The Upper Curve is for the Ether, and the Lower One for Olefiant Gas.

All that is known about the changes of temperature in the geological climate is derived from observing the changes in the plants and animals. These changes were world-wide, and hence were due to a world-wide cause. None sufficiently broad has been imagined other than a change of temperature, or a change in the purity of the atmosphere. The peculiar life of the Palæozoic lasted several times as long as that of the Mezosoic; and that, in turn, lasted several times as long as that of the Tertiary. Or, calling the last 1, the ratio was 12:3:1. The millions of years of the Palæozoic brought a certain increase of purity and decrease of temperature, and were followed by an almost complete extermination of species, owing to their non-adaptation to the new conditions. The Mesozoic, although only one fourth as long, brought probably quite as great a change, and was followed by "one of the most complete exterminations on record." Through the Eocene and Miocene, although not more than one fifteenth as long as the preceding, the fall of temperature was actually greater than in them all combined, and this was far exceeded in the rest of the Tertiary, for, in the Miocene, cypresses and magnolias grew within 101/2° of the pole, and, at the end of the Pliocene, that dread winter began to set in which is known as the Glacial epoch. There was a correspondingly rapid change in the plants and animals. Whether it is a matter of accident, or whether they stand in relation of cause and effect, the record of life is such as it would be if the warmth were due to a blanket of carbonic acid and water-vapor, and the temperature fell in accordance with Tyndall's law.

It is also very suggestive that, while, in the earlier periods, the changes in plants and animals were world-wide, the Tertiary changes were more and more confined to high latitudes, as if the cold were setting down from the poles toward the equator. Such was the effect to be expected if the early warmth was due to the warm blanket of CO2 and aqueous vapor. If that was growing thinner, it would be long before any sensible effect would be produced; but, when it did appear, it would first manifest itself near the poles, where less solar heat was received, and where so much depended upon the heat being retained, and from the polar regions it would spread toward the equator. With these facts in view, there is no difficulty in seeing why the flora of temperate or even warmer regions should have had their origin in very high latitudes, since it was there that a temperature first appeared which was adapted to their needs.

I have purposely avoided speaking of how much CO2 the air can contain and support life. I doubt very much the possibility of saying what the limits were in those remote times when not merely every species, but every genus, was exceedingly unlike any now living. They may have been adapted to conditions fatal to any creature known to us. It is certain that as the air grew purer the early animals died, and were replaced by others more like those now living. Present animals, or even human beings, according to Professor Remsen, of Johns Hopkins University, can breathe an atmosphere containing five per cent of carbonic acid "without experiencing serious or even disagreeable effects." That is, the present amount of CO2 could be increased one hundred and fifty times, and more, without "even disagreeable effects." If this be true, the fact that the animals of those early times flourished, is no reason why we may not believe that the atmosphere contained many hundred times as much carbonic acid as it does now.

Accounting for the uniformity of biological conditions, including in that term heat, light, and actinic forces, solves only a part of the climatic problem. The cold which followed must also be accounted for, as well as the return of a mild climate to regions so long covered with ice. The former was a corollary of the causes already discussed. It was due to the combined effect of a perpendicular axis and a purer atmosphere, aided by those high latitude uplifts which occurred at or soon after the close of the Pliocene. The warm blanket being removed, the natural effects of an upright axis began to show themselves. It was the same as if the sun got no farther north than it does now on the 21st of March. Since the cold of the vernal equinox is in part the residuum of winter, it will be near the truth to say that, with the axis perpendicular, the temperature would be the same as now in April. The present flora would die out, and it would not be necessary to go far to the north to meet perpetual snow. For snow once fallen, or ice once formed, would never melt, but, accumulating through the ages, would force its way by its own weight equatorward until it reached a region where the heat of the sun was sufficient to melt it away. The uplifts in high latitudes intensified the results. Vice versa, when the axis became oblique, more solar heat fell within the polar circle, those regions became warmer, and the Glacial epoch departed. If these conditions—a perpendicular axis and high uplifts—could be to-day restored, the atmosphere remaining as it is, the Glacial epoch would return.

The removal of the ice was hastened by the depression of high latitude lands. This depression was very extensive. Vast tracts were submerged. In this is found, I think, the cause of the mild climate of the Champlain, while the cold (the minor Glacial epoch) that followed was due to another upward movement, or movements, comparatively limited in extent—in fact, confined mostly to Europe and Asia.

Subsequent changes left us the climate of to-day.

In the briefest possible space, I sum up as follows:

The uniformity of plant-life, regardless of latitude, to near the close of the Tertiary, indicates uniformity of biological conditions i. e., of light, or actinic force, and heat.

The former indicates that the earth's axis was approximately perpendicular—a conclusion in harmony with the belief that the moon and earth were once one body, and consequently that their axes were originally parallel.

The uniformity of temperature in high latitudes through the year is also accounted for by a perpendicular axis, and, so far as I can see, only by that.

Astronomy tells only of present conditions. As to how or when the axis took its present obliquity, it is unable to say.

Geology fixes the date by the record which solar influences have left on organic forms, and places it near or just after the close of the Tertiary.

The warmth of arctic regions—for there might have been uniformity without warmth—was due to the "double blanket" of carbonic acid and aqueous vapor.

The cold of the Glacial epoch was due to the loss of CO2 and aqueous vapor, aided by high-latitude uplifts.

The disappearance of the ice and cold was due to the earth's axis being made to incline as now, and to the reduction of the uplifts.

The warmth of the Champlain was due to depression of the land.

The second or minor ice period was due to another uplift confined to (or at least much greater in) Europe and Asia.

The depression of the land to somewhat near the present level, with present amount of CO2 and H2O in the atmosphere, resulted in present climatic conditions.

The influence of gases and vapors upon climate was to some extent considered by its eminent discoverer, Professor Tyndall, and I presume by every one that has read his account of his experiments on the passage of heat through these bodies. Theorizers on climate have been fond of changing the earth's axis, confining themselves, however, for the most part to altering the geographical position of the poles—i. e., increasing one set of latitudes and decreasing another, to suit their needs; and a few have invoked an increased or a decreased obliquity.

The present explanation differs widely from all that have preceded it, and in its entirety has the merit of novelty, whatever that may be.

  1. Read before the New York Academy of Science.
  2. It is of small importance, in reference to these questions, whether Huxley's "Homotaxy" is true or not. If true, then these same species lived first in high latitudes, and afterward in low, or vice versa. The important point for my present inquiry is, that the same species lived and flourished in places where life-conditions now are so extremely unlike.
  3. Dana, "Manual of Geology," revised edition, p. 526.
  4. Captain Nares's report in "Nature," vol. xv, p. 35.
  5. Stellar heat need not be considered, as it does not vary, and, besides, it is very small.
  6. Dana's "Manual of Geology," third edition, page 352, says: "If we draw any conclusions from the facts, it must be that the temperature of the arctic zone differed little from that of Europe and America. Through the whole hemisphere—and we may say world—there was a genial atmosphere" (and corresponding conditions as to actinic influence) "for one uniform type of vegetables, and there were genial waters for corals and brachiopods."
  7. Dana.
  8. Lyell.
  9. See this account in Croll's "Climate and Time," pp. 264, 265.
  10. See "American Journal of Science," 1878, Article XLV, "Is the Existence of Growth-Rings in the Exogenous Plant Proof of Alternating Seasons?" by the present writer.
  11. There probably were zones of climate in the latter half of the Tertiary, or at least in the Pliocene, but these are quite compatible with the absence of seasons, since, with a perpendicular axis, temperature, however it might differ on different parallels, would be constant in each.
  12. Since writing the above I have read Professor Woeikof's article in the "American Journal of Science" for March, 1886, entitled "An Examination of Dr. Croll's Hypotheses of Geological Climates." It is a careful testing of Dr. Croll's theories by applying them to present facts, as to summer and winter variations of climate. He shows that no such differences exist as Dr. Croll's theories demand. He sums up his conclusions (page 178) as follows: "The main points on which rests, so to say, the whole fabric in its explanation of glaciation and geological climates generally—the influence of winter in aphelion and in perihelion—during high eccentricity, and the calculation of temperatures in proportion to the sun-heat received, are unfortunately fallacious." The article will well repay the student of geological climate for its careful study.
  13. For proof of this curious paradox, discovered by Mr. J. H. Lane, see "American Journal of Science," July, 1880; also, Newcomb's "Astronomy," p. 508.
  14. "Manual of Geology," p. 363. Some think that there has been an accretion of CO2 from interstellar space. This appears to be too hypothetical to seriously affect the argument.