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If there actually has been an appreciable decrease in the amount of heat given out by the earth's interior, its effects would agree with the observed conditions of the geological record. It would help to explain the relative mildness of zonal, seasonal, and local contrasts of climate in early geological times, but it would not help to explain the long oscillations from era to era which appear to have been of much greater importance. Those oscillations, so far as we can yet judge, may have been due in part to solar changes, but in large measure they seem to be explained by variations in the extent, distribution, and altitude of the lands. Such variations appear to be the inevitable result of the earth's contraction.

FOOTNOTES:

POST-GLACIAL CRUSTAL MOVEMENTS AND CLIMATIC CHANGES

An interesting practical application of some of the preceding generalizations is found in an attempt by C. E. P. Brooks to interpret post-glacial climatic changes almost entirely in terms of crustal movement. We believe that he carries the matter much too far, but his discussion is worthy of rather full recapitulation, not only for its theoretical value but because it gives a good summary of post-glacial changes. His climatic table for northwest Europe as reprinted from the annual report of the Smithsonian Institution for 1917, p. 366, is as follows:

About 18,000 B. C. the retreat of the ice began in good earnest. Even though no evidence has yet been found, Brooks believes there must have been a change in the distribution of land and sea to account for the diminution of the ice. The ensuing millenniums formed the Magdalenian period in human history, the last stage of the Paleolithic, when man lived in caves and reindeer were abundant in central Europe. At first the ice retreated very slowly and there were periods when for scores of years the ice edge remained stationary or even readvanced. About 10,000 B. C. the edge of the ice lay along the southern coast of Sweden. During the next 2000 years it withdrew more rapidly to about 59?N. Then came the Fennoscandian pause, or Gschnitz stage, when for about 200 years the ice edge remained in one position, forming a great moraine. Brooks suggests that this pause about 8000 B. C. was due to the closing of the connection between the Atlantic Ocean and the Baltic Sea and the synchronous opening of a connection between the Baltic and the White Seas, whereby cold Arctic waters replaced the warmer Atlantic waters. He notes, however, that about 7500 B. C. the obliquity of the ecliptic was probably nearly 1? greater than at present. This he calculates to have caused the climate of Germany and Sweden to be 1?F. colder than at present in winter and 1?F. warmer in summer.

The next climatic stage was marked by a rise of temperature till about 6000 B. C. During this period the ice at first retreated, presumably because the climate was ameliorating, although no cause of such amelioration is assigned. At length the ice lay far enough north to allow a connection between the Baltic and the Atlantic by way of Lakes Wener and Wetter in southern Sweden. This is supposed to have warmed the Baltic Sea and to have caused the climate to become distinctly milder. Next the land rose once more so that the Baltic was separated from the Atlantic and was converted into the Ancylus lake of fresh water. The southwest Baltic region then stood 400 feet higher than now. The result was the Daun stage, about 5000 B. C., when the ice halted or perhaps readvanced a little, its front being then near Ragunda in about latitude 63?. Why such an elevation did not cause renewed glaciation instead of merely the slight Daun pause, Brooks does not explain, although his calculations as to the effect of a slight elevation of the land during the main period of glaciation from 30,000 to 18,000 B. C. would seem to demand a marked readvance.

After 5000 B. C. there ensued a period when the climate, although still distinctly continental, was relatively mild. The winters, to be sure, were still cold but the summers were increasingly warm. In Sweden, for example, the types of vegetation indicate that the summer temperature was 7?F. higher than now. Storms, Brooks assumes, were comparatively rare except on the outer fringe of Great Britain. There they were sufficiently abundant so that in the Northwest they gave rise to the first Peat-Bog period, during which swamps replaced forests of birch and pine. Southern and eastern England, however, probably had a dry continental climate. Even in northwest Norway storms were rare as is indicated by remains of forests on islands now barren because of the strong winds and fierce storms. Farther east most parts of central and northern Europe were relatively dry. This was the early Neolithic period when man advanced from the use of unpolished to polished stone implements.

Not far from 4000 B. C. the period of continental climate was replaced by a comparatively moist maritime climate. Brooks believes that this was because submergence opened the mouth of the Baltic and caused the fresh Ancylus lake to give place to the so-called Litorina sea. The temperature in Sweden averaged about 3?F. higher than at present and in southwestern Norway 2?. More important than this was the small annual range of temperature due to the fact that the summers were cool while the winters were mild. Because of the presence of a large expanse of water in the Baltic region, storms, as our author states, then crossed Great Britain and followed the Baltic depression, carrying the moisture far inland. In spite of the additional moisture thus available the snow line in southern Norway was higher than now.

At this point Brooks turns to other parts of the world. He states that not far from 4000 B. C., a submergence of the lands, rarely amounting to more than twenty-five feet, took place not only in the Baltic region but in Ireland, Iceland, Spitzbergen, and other parts of the Arctic Ocean, as well as in the White Sea, Greenland, and the eastern part of North America. Evidences of a mild climate are found in all those places. Similar evidence of a mild warm climate is found in East Africa, East Australia, Tierra del Fuego, and Antarctica. The dates are not established with certainty but they at least fall in the period immediately preceding the present epoch. In explanation of these conditions Brooks assumes a universal change of sea level. He suggests with some hesitation that this may have been due to one of Pettersson's periods of maximum "tide-generating force." According to Pettersson the varying positions of the moon, earth, and sun cause the tides to vary in cycles of about 9, 90, and 1800 years, though the length of the periods is not constant. When tides are high there is great movement of ocean waters and hence a great mixture of the water at different latitudes. This is supposed to cause an amelioration of climate. The periods of maximum and minimum tide-generating force are as follows:

Brooks thinks that the big trees in California and the Norse sagas and Germanic myths indicate a rough agreement of climatic phenomena with Pettersson's last three dates, while the mild climate of 4000 B. C. may really belong to 3500 B. C. He gives no evidence confirming Pettersson's view at the other three dates.

To return to Brooks' sketch of the relation of climatic pulsations to the altitude of the lands, by 3000 B. C., that is, toward the close of the Neolithic period, further elevation is supposed to have taken place over the central latitudes of western Europe. Southern Britain, which had remained constantly above its present level ever since 30,000 B. C., was perhaps ninety feet higher than now. Ireland was somewhat enlarged by elevation, the Straits of Dover were almost closed, and parts of the present North Sea were land. To these conditions Brooks ascribes the prevalence of a dry continental climate. The storms shifted northward once more, the winds were mild, as seems to be proved by remains of trees in exposed places; and forests replaced fields of peat and heath in Britain and Germany. The summers were perhaps warmer than now but the winters were severe. The relatively dry climate prevailed as far west as Ireland. For example, in Drumkelin Bog in Donegal County a corded oak road and a two-story log cabin appear to belong to this time. Fourteen feet of bog lie below the floor and twenty-six above. This period, perhaps 3000-2000 B. C., was the legendary heroic age of Ireland when "the vigour of the Irish reached a level not since attained." This, as Brooks points out, may have been a result of the relatively dry climate, for today the extreme moisture of Ireland seems to be a distinct handicap. In Scandinavia, civilization, or at least the stage of relative progress, was also high at this time.

It is possible that we have to attribute this damp period in Northwest Europe to some more general cause, for Ellsworth Huntington's curves of tree-growth in California and climate in Western Asia both show moister conditions from about 1000 B. C. to A. D. 200, and the same author believes that the Mediterranean lands had a heavier rainfall about 500 B. C. to A. D. 200. It seems that the phase was marked by a general increase of the storminess of the temperate regions of the northern hemisphere at least, with a maximum between Ireland and North Germany, indicating probably that the Baltic again became the favourite track of depressions from the Atlantic.

Brooks ends his paper with a brief r?sum? of glacial changes in North America, but as the means of dating events are unreliable the degree of synchronism with Europe is not clear. He sums up his conclusions as follows:

On the whole it appears that though there is a general similarity in the climatic history of the two sides of the North Atlantic, the changes are not really contemporaneous, and such relationship as appears is due mainly to the natural similarity in the geographical history of two regions both recovering from an Ice Age, and only very partially to world-wide pulsations of climate. Additional evidence on this head will be available when Baron de Geer publishes the results of his recent investigations of the seasonal glacial clays of North America, especially if, as he hopes, he is able to correlate the banding of these clays with the growth-rings of the big trees.

Brooks' painstaking discussion of post-glacial climatic changes is of great value because of the large body of material which he has so carefully wrought together. His strong belief in the importance of changes in the level of the lands deserves serious consideration. It is difficult, however, to accept his final conclusion that such changes are the main factors in recent climatic changes. It is almost impossible, for example, to believe that movements of the land could produce almost the same series of climatic changes in Europe, Central Asia, the western and eastern parts of North America, and the southern hemisphere. Yet such changes appear to have occurred during and since the glacial period. Again there is no evidence whatever that movements of the land have anything to do with the historic cycles of climate or with the cycles of weather in our own day, which seem to be the same as glacial cycles on a small scale. Also, as Dr. Simpson points out in discussing Brooks' paper, there appears "no solution along these lines of the problem connected with rich vegetation in both polar circles and the ice-age which produced the ice-sheet at sea-level in Northern India." Nevertheless, we may well believe that Brooks is right in holding that changes in the relative level and relative area of land and sea have had important local effects. While they are only one of the factors involved in climatic changes, they are certainly one that must constantly be kept in mind.

FOOTNOTES:

THE CHANGING COMPOSITION OF OCEANS AND ATMOSPHERE

Having discussed the climatic effect of movements of the earth's crust during the course of geological time, we are now ready to consider the corresponding effects due to changes in the movable envelopes--the oceans and the atmosphere. Variations in the composition of sea water and of air and in the amount of air must almost certainly have occurred, and must have produced at least slight climatic consequences. It should be pointed out at once that such variations appear to be far less important climatically than do movements of the earth's crust and changes in the activity of the sun. Moreover, in most cases, they are not reversible as are the crustal and solar phenomena. Hence, while most of them appear to have been unimportant so far as climatic oscillations and fluctuations are concerned, they seemingly have aided in producing the slight secular progression to which we have so often referred.

There is general agreement among geologists that the ocean has become increasingly saline throughout the ages. Indeed, calculations of the rate of accumulation of salt have been a favorite method of arriving at estimates of the age of the ocean, and hence of the earliest marine sediments. So far as known, however, no geologist or climatologist has discussed the probable climatic effects of increased salinity. Yet it seems clear that an increase in salinity must have a slight effect upon climate.

Salinity affects climate in four ways: It appreciably influences the rate of evaporation; it alters the freezing point; it produces certain indirect effects through changes in the absorption of carbon dioxide; and it has an effect on oceanic circulation.

According to the experiments of Mazelle and Okada, as reported by Kr?mmel, evaporation from ordinary sea water is from 9 to 30 per cent less rapid than from fresh water under similar conditions. The variation from 9 to 30 per cent found in the experiments depends, perhaps, upon the wind velocity. When salt water is stagnant, rapid evaporation tends to result in the development of a film of salt on the top of the water, especially where it is sheltered from the wind. Such a film necessarily reduces evaporation. Hence the relatively low salinity of the oceans in the past probably had a tendency to increase the amount of water vapor in the air. Even a little water vapor augments slightly the blanketing effect of the air and to that extent diminishes the diurnal and seasonal range of temperature and the contrast from zone to zone.

Increased salinity means a lower freezing temperature of the oceans and hence would have an effect during cold periods such as the present and the Pleistocene ice age. It would not, however, be of importance during the long warm periods which form most of geologic time. A salinity of about 3.5 per cent at present lowers the freezing point of the ocean roughly 2?C. below that of fresh water. If the ocean were fresh and our winters as cold as now, all the harbors of New England and the Middle Atlantic States would be icebound. The Baltic Sea would also be frozen each winter, and even the eastern harbors of the British Isles would be frequently locked in ice. At high latitudes the area of permanently frozen oceans would be much enlarged. The effect of such a condition upon marine life in high latitudes would be like that of a change to a warmer climate. It would protect the life on the continental shelf from the severe battering of winter storms. It would also lessen the severity of the winter temperature in the water for when water freezes it gives up much latent heat,--eighty calories per cubic centimeter. Part of this raises the temperature of the underlying water.

The expansion of the ice near northern shores would influence the life of the lands quite differently from that of the oceans. It would act like an addition of land to the continents and would, therefore, increase the atmospheric contrasts from zone to zone and from continental interior to ocean. In summer the ice upon the sea would tend to keep the coastal lands cool, very much as happens now near the Arctic Ocean, where the ice floes have a great effect through their reflection of light and their absorption of heat in melting. In winter the virtual enlargement of the continents by the addition of an ice fringe would decrease the snowfall upon the lands. Still more important would be the effect in intensifying the anti-cyclonic conditions which normally prevail in winter not only over continents but over ice-covered oceans. Hence the outblowing cold winds would he strengthened. The net effect of all these conditions would apparently be a diminution of snowfall in high latitudes upon the lands even though the summer snowfall upon the ocean and the coasts may have increased. This condition may have been one reason why widespread glaciation does not appear to have prevailed in high latitudes during the Proterozoic and Permian glaciations, even though it occurred farther south. If the ocean during those early glacial epochs were ice-covered down to middle latitudes, a lack of extensive glaciation in high latitudes would be no more surprising than is the lack of Pleistocene glaciation in the northern parts of Alaska and Asia. Great ice sheets are impossible without a large supply of moisture.

Among the indirect effects of salinity one of the chief appears to be that the low salinity of the water in the past and the greater ease with which it froze presumably allowed the temperature of the entire ocean to be slightly higher than now. This is because ice serves as a blanket and hinders the radiation of heat from the underlying water. The temperature of the ocean has a climatic significance not only directly, but indirectly through its influence on the amount of carbon dioxide held by the oceans. A change of even 1?C. from the present mean temperature of 2?C. would alter the ability of the entire ocean to absorb carbon dioxide by about 4 per cent. This, according to F. W. Clarke, is because the oceans contain from eighteen to twenty-seven times as much carbon dioxide as the air when only the free carbon dioxide is considered, and about seventy times as much according to Johnson and Williamson when the partially combined carbon dioxide is also considered. Moreover, the capacity of water for carbon dioxide varies sharply with the temperature. Hence a rise in temperature of only 1?C. would theoretically cause the oceans to give up from 30 to 280 times as much carbon dioxide as the air now holds. This, however, is on the unfounded assumption that the oceans are completely saturated. The important point is merely that a slight change in ocean temperature would cause a disproportionately large change in the amount of carbon dioxide in the air with all that this implies in respect to blanketing the earth, and thus altering temperature.

Another and perhaps the most important effect of salinity upon climate depends upon the rapidity of the deep-sea circulation. The circulation is induced by differences of temperature, but its speed is affected at least slightly by salinity. The vertical circulation is now dominated by cold water from subpolar latitudes. Except in closed seas like the Mediterranean the lower portions of the ocean are near the freezing point. This is because cold water sinks in high latitudes by reason of its superior density, and then "creeps" to low latitudes. There it finally rises and replaces either the water driven poleward by the winds, or that which has evaporated from the Surface.

During past ages, when the sea water was less salty, the circulation was presumably more rapid than now. This was because, in tropical regions, the rise of cold water is hindered by the sinking of warm surface water which is relatively dense because evaporation has removed part of the water and caused an accumulation of salt. According to Kr?mmel and Mill, the surface salinity of the subtropical belt of the North Atlantic commonly exceeds 3.7 per cent and sometimes reaches 3.77 per cent, whereas the underlying waters have a salinity of less than 3.5 per cent and locally as little as 3.44 per cent. The other oceans are slightly less saline than the North Atlantic at all depths, but the vertical salinity gradients along the tropics are similar. According to the Smithsonian Physical Tables, the difference in salinity between the surface water and that lying below is equivalent to a difference of .003 in density, where the density of fresh water is taken as 1.000. Since the decrease in density produced by warming water from the temperature of its greatest density to the highest temperatures which ever prevail in the ocean is only .004, the more saline surface waters of the dry tropics are at most times almost as dense as the less saline but colder waters beneath the surface, which have come from higher latitudes. During days of especially great evaporation, however, the most saline portions of the surface waters in the dry tropics are denser than the underlying waters and therefore sink, and produce a temporary local stagnation in the general circulation. Such a sinking of the warm surface waters is reported by Kr?mmel, who detected it by means of the rise in temperature which it produces at considerable depths. If such a hindrance to the circulation did not exist, the velocity of the deep-sea movements would be greater.

If in earlier times a more rapid circulation occurred, low latitudes must have been cooled more than now by the rise of cold waters. At the same time higher latitudes were presumably warmed by a greater flow of warm water from tropical regions because less of the surface heat sank in low latitudes. Such conditions would tend to lessen the climatic contrast between the different latitudes. Hence, in so far as the rate of deep-sea circulation depends upon salinity, the slowly increasing amount of salt in the oceans must have tended to increase the contrasts between low and high latitudes. Thus for several reasons, the increase of salinity during geologic history seems to deserve a place among the minor agencies which help to explain the apparent tendency toward a secular progression of climate in the direction of greater contrasts between tropical and subpolar latitudes.

Changes in the composition and amount of the atmosphere have presumably had a climatic importance greater than that of changes in the salinity of the oceans. The atmospheric changes may have been either progressive or cyclic, or both. In early times, according to the nebular hypothesis, the atmosphere was much more dense than now and contained a larger percentage of certain constituents, notably carbon dioxide and water. The planetesimal hypothesis, on the other hand, postulates an increase in the density of the atmosphere, for according to this hypothesis the density of the atmosphere depends upon the power of the earth to hold gases, and this power increases as the earth grows bigger with the infall of material from without.

Whichever hypothesis may be correct, it seems probable that when life first appeared on the land the atmosphere resembled that of today in certain fundamental respects. It contained the elements essential to life, and its blanketing effect was such as to maintain temperatures not greatly different from those of the present. The evidence of this depends largely upon the narrow limits of temperature within which the activities of modern life are possible, and upon the cumulative evidence that ancient life was essentially similar to the types now living. The resemblance between some of the oldest forms and those of today is striking. For example, according to Professor Schuchert: "Many of the living genera of forest trees had their origin in the Cretaceous, and the giant sequoias of California go back to the Triassic, while Ginkgo is known in the Permian. Some of the fresh-water molluscs certainly were living in the early periods of the Mesozoic, and the lung-fish of today is known as far back as the Triassic and is not very unlike other lung-fishes of the Devonian. The higher vertebrates and insects, on the other hand, are very sensitive to their environment, and therefore do not extend back generically beyond the Cenozoic, and only in a few instances even as far as the Oligocene. Of marine invertebrates the story is very different, for it is well known that the horseshoe crab lived in the Upper Jurassic, and Nautilus in the Triassic, with forms in the Devonian not far removed from this genus. Still longer-ranging genera occur among the brachiopods, for living Lingula and Crania have specific representatives as far back as the early Ordovician. Among living foraminifers, Lagena, Globigerina, and Nodosaria are known in the later Cambrian or early Ordovician. In the Middle Cambrian near Field, British Columbia, Walcott has found a most varied array of invertebrates among which are crustaceans not far removed from living forms. Zo?logists who see these wonderful fossils are at once struck with their modernity and the little change that has taken place in certain stocks since that far remote time. Back of the Paleozoic, little can be said of life from the generic standpoint, since so few fossils have been recovered, but what is at hand suggests that the marine environment was similar to that of today."

The testimony of ancient glaciation as to the slight difference in the climate and therefore in the atmosphere of early and late geological times is almost as clear as that of life. Just as life proves that the earth can never have been extremely cold during hundreds of millions of years, so glaciation in moderately low latitudes near the dawn of earth history and at several later times, proves that the earth was not particularly hot even in those early days. The gentle progressive change of climate which is recorded in the rocks appears to have been only in slight measure a change in the mean temperature of the earth as a whole, and almost entirely a change in the distribution of temperature from place to place and season to season. Hence it seems probable that neither the earth's own emission of heat, nor the supply of solar heat, nor the power of the atmosphere to retain heat can have been much greater a few hundred million years ago than now. It is indeed possible that these three factors may have varied in such a way that any variation in one has been offset by variations of the others in the opposite direction. This, however, is so highly improbable that it seems advisable to assume that all three have remained relatively constant. This conclusion together with a realization of the climatic significance of carbon dioxide has forced most of the adherents of the nebular hypothesis to abandon their assumption that carbon dioxide, the heaviest gas in the air, was very abundant until taken out by coal-forming plants or combined with the calcium oxide of igneous rocks to form the limestone secreted by animals. In the same way the presence of sun cracks in sedimentary rocks of all ages suggests that the air cannot have contained vast quantities of water vapor such as have been assumed by Knowlton and others in order to account for the former lack of sharp climatic contrast between the zones. Such a large amount of water vapor would almost certainly be accompanied by well-nigh universal and continual cloudiness so that there would be little chance for the pools on the earth's water-soaked surface to dry up. Furthermore, there is only one way in which such cloudiness could be maintained and that is by keeping the air at an almost constant temperature night and day. This would require that the chief source of warmth be the interior of the earth, a condition which the Proterozoic, Permian, and other widespread glaciations seem to disprove.

As to the absolute amounts of oxygen, Barrell thought that atmospheric oxygen began to be present only after plants had appeared. It will be recalled that plants absorb carbon dioxide and separate the carbon from the oxygen, using the carbon in their tissues and setting free the oxygen. As evidence of a paucity of oxygen in the air in early Proterozoic times, Barrell cites the fact that the sedimentary rocks of that remote time commonly are somewhat greyish or greenish-grey wackes, or other types, indicating incomplete oxidation. He admits, however, that the stupendous thicknesses of red sandstones, quartzite, and hematitic iron ores of the later Proterozoic prove that by that date there was an abundance of atmospheric oxygen. If so, the change from paucity to abundance must have occurred before fossils were numerous enough to give much clue to climate. However, Barrell's evidence as to a former paucity of atmospheric oxygen is not altogether convincing. In the first place, it does not seem justifiable to assume that there could be no oxygen until plants appeared to break down the carbon dioxide, for some oxygen is contributed by volcanoes, and lightning decomposes water into its elements. Part of the hydrogen thus set free escapes into space, for the earth's gravitative force does not appear great enough to hold this lightest of gases, but the oxygen remains. Thus electrolysis of water results in the accumulation of oxygen. In the second place, there is no proof that the ancient greywackes are not deoxidized sediments. Light colored rock formations do not necessarily indicate a paucity of atmospheric oxygen, for such rocks are abundant even in recent times. For example, the Tertiary formations are characteristically light colored, a result, however, of deoxidation. Finally, the fact that sedimentary rocks, irrespective of their age, contain an average of about 1.5 per cent more oxygen than do igneous rocks, suggests that oxygen was present in the air in quantity even when the earliest shales and sandstones were formed, for atmospheric oxygen seems to be the probable source of the extra oxygen they contain. The formation of these particular sedimentary rocks by weathering of igneous rocks involves only a little carbon dioxide and water. Although it seems probable that oxygen was present in the atmosphere even at the beginning of the geological record, it may have been far less abundant then than now. It may have been removed from the atmosphere by animals or by the oxidation of the rocks almost as rapidly as it was added by volcanoes, plants, and other agencies.

After this chapter was in type, St. John announced his interesting discovery that oxygen is apparently lacking in the atmosphere of Venus. He considers that this proves that Venus has no life. Furthermore he concludes that so active an element as oxygen cannot be abundant in the atmosphere of a planet unless plants continually supply large quantities by breaking down carbon dioxide.

But even if the earth has experienced a notable increase in atmospheric oxygen since the appearance of life, this does not necessarily involve important climatic changes except those due to increased atmospheric density. This is because oxygen has very little effect upon the passage of light or heat, being transparent to all but a few wave lengths. Those absorbed are chiefly in the ultra violet.

The distinct possibility that oxygen has increased in amount, makes it the more likely that there has been an increase in the total atmosphere, for the oxygen would supplement the increase in the relatively inert nitrogen and argon, which has presumably taken place. The climatic effects of an increase in the atmosphere include, in the first place, an increased scattering of light as it approaches the earth. Nitrogen, argon, and oxygen all scatter the short waves of light and thus interfere with their reaching the earth. Abbot and Fowle, who have carefully studied the matter, believe that at present the scattering is quantitatively important in lessening insolation. Hence our supposed general increase in the volume of the air during part of geological times would tend to reduce the amount of solar energy reaching the earth's surface. On the other hand, nitrogen and argon do not appear to absorb the long wave lengths known as heat, and oxygen absorbs so little as to be almost a non-absorber. Therefore the reduced penetration of the air by solar radiation due to the scattering of light would apparently not be neutralized by any direct increase in the blanketing effect of the atmosphere, and the temperature near the earth's surface would be slightly lowered by a thicker atmosphere. This would diminish the amount of water vapor which would be held in the air, and thereby lower the temperature a trifle more.

In the second place, the higher atmospheric pressure which would result from the addition of gases to the air would cause a lessening of the rate of evaporation, for that rate declines as pressure increases. Decreased evaporation would presumably still further diminish the vapor content of the atmosphere. This would mean a greater daily and seasonal range of temperature, as is very obvious when we compare clear weather with cloudy. Cloudy nights are relatively warm while clear nights are cool, because water vapor is an almost perfect absorber of radiant heat, and there is enough of it in the air on moist nights to interfere greatly with the escape of the heat accumulated during the day. Therefore, if atmospheric moisture were formerly much more abundant than now, the temperature must have been much more uniform. The tendency toward climatic severity as time went on would be still further increased by the cooling which would result from the increased wind velocity discussed below; for cooling by convection increases with the velocity of the wind, as does cooling by conduction.

Any persistent lowering of the general temperature of the air would affect not only its ability to hold water vapor, but would produce a lessening in the amount of atmospheric carbon dioxide, for the colder the ocean becomes the more carbon dioxide it can hold in solution. When the oceanic temperature falls, part of the atmospheric carbon dioxide is dissolved in the ocean. This minor constituent of the air is important because although it forms only 0.003 per cent of the earth's atmosphere, Abbot and Fowle's calculations indicate that it absorbs over 10 per cent of the heat radiated outward from the earth. Hence variations in the amount of carbon dioxide may have caused an appreciable variation in temperature and thus in other climatic conditions. Humphreys, as we have seen, has calculated that a doubling of the carbon dioxide in the air would directly raise the earth's temperature to the extent of 1.3?C., and a halving would lower it a like amount. The indirect results of such an increase or decrease might be greater than the direct results, for the change in temperature due to variations in carbon dioxide would alter the capacity of the air to hold moisture.

Two conditions would especially help in this respect; first, changes in nocturnal cooling, and second, changes in local convection. The presence of carbon dioxide diminishes nocturnal cooling because it absorbs the heat radiated by the earth, and re-radiates part of it back again. Hence with increased carbon dioxide and with the consequent warmer nights there would be less nocturnal condensation of water vapor to form dew and frost. Local convection is influenced by carbon dioxide because this gas lessens the temperature gradient. In general, the less the gradient, that is, the less the contrast between the temperature at the surface and higher up, the less convection takes place. This is illustrated by the seasonal variation in convection. In summer, when the gradient is steepest, convection reaches its maximum. It will be recalled that when air rises it is cooled by expansion, and if it ascends far the moisture is soon condensed and precipitated. Indeed, local convection is considered by C. P. Day to be the chief agency which keeps the lower air from being continually saturated with moisture. The presence of carbon dioxide lessens convection because it increases the absorption of heat in the zone above the level in which water vapor is abundant, thus warming these higher layers. The lower air may not be warmed correspondingly by an increase in carbon dioxide if Abbot and Fowle are right in stating that near the earth's surface there is enough water vapor to absorb practically all the wave lengths which carbon dioxide is capable of absorbing. Hence carbon dioxide is chiefly effective at heights to which the low temperature prevents water vapor from ascending. Carbon dioxide is also effective in cold winters and in high latitudes when even the lower air is too cold to contain much water vapor. Moreover, carbon dioxide, by altering the amount of atmospheric water vapor, exerts an indirect as well as a direct effect upon temperature.

Other effects of the increase in air pressure which we are here assuming during at least the early part of geological times are corresponding changes in barometric contrasts, in the strength of winds, and in the mass of air carried by the winds along the earth's surface. The increase in the mass of the air would re?nforce the greater velocity of the winds in their action as eroding and transporting agencies. Because of the greater weight of the air, the winds would be capable of picking up more dust and of carrying it farther and higher; while the increased atmospheric friction would keep it aloft a longer time. The significance of dust at high levels and its relation to solar radiation have already been discussed in connection with volcanoes. It will be recalled that on the average it lowers the surface temperature. At lower levels, since dust absorbs heat quickly and gives it out quickly, its presence raises the temperature of the air by day and lowers it by night. Hence an increase in dustiness tends toward greater extremes.

From all these considerations it appears that if the atmosphere has actually evolved according to the supposition which is here tentatively entertained, the general tendency of the resultant climatic changes must have been partly toward long geological oscillations and partly toward a general though very slight increase in climatic severity and in the contrasts between the zones. This seems to agree with the geological record, although the fact that we are living in an age of relative climatic severity may lead us astray.

The significant fact about the whole matter is that the three great types of terrestrial agencies, namely, those of the earth's interior, those of the oceans, and those of the air, all seem to have suffered changes which lead to slow variations of climate. Many reversals have doubtless taken place, and the geologic oscillations thus induced are presumably of much greater importance than the progressive change, yet so far as we can tell the purely terrestrial changes throughout the hundreds of millions of years of geological time have tended toward complexity and toward increased contrasts from continent to ocean, from latitude to latitude, from season to season, and from day to night.

Throughout geological history the slow and almost imperceptible differentiation of the earth's surface has been one of the most noteworthy of all changes. It has been opposed by the extraordinary conservatism of the universe which causes the average temperature today to be so like that of hundreds of millions of years ago that many types of life are almost identical. Nevertheless, the differentiation has gone on. Often, to be sure, it has presumably been completely masked by the disturbances of the solar atmosphere which appear to have been the cause of the sharper, shorter climatic pulsations. But regardless of cosmic conservatism and of solar impulses toward change, the slow differentiation of the earth's surface has apparently given to the world of today much of the geographical complexity which is so stimulating a factor in organic evolution. Such complexity--such diversity from place to place--appears to be largely accounted for by purely terrestrial causes. It may be regarded as the great terrestrial contribution to the climatic environment which guides the development of life.

FOOTNOTES:

THE EFFECT OF OTHER BODIES ON THE SUN

If solar activity is really an important factor in causing climatic changes, it behooves us to subject the sun to the same kind of inquiry to which we have subjected the earth. We have inquired into the nature of the changes through which the earth's crust, the oceans, and the atmosphere have influenced the climate of geological times. It has not been necessary, however, to study the origin of the earth, nor to trace its earlier stages. Our study of the geological record begins only when the earth had attained practically its present mass, essentially its present shape, and a climate so similar to that of today that life as we know it was possible. In other words, the earth had passed the stages of infancy, childhood, youth, and early maturity, and had reached full maturity. As it still seems to be indefinitely far from old age, we infer that during geological times its relative changes have been no greater than those which a man experiences between the ages of perhaps twenty-five and forty.

Similar reasoning applies with equal or greater force to the sun. Because of its vast size it presumably passes through its stages of development much more slowly than the earth. In the first chapter of this book we saw that the earth's relative uniformity of climate for hundreds of millions of years seems to imply a similar uniformity in solar activity. This accords with a recent tendency among astronomers who are more and more recognizing that the stars and the solar system possess an extraordinary degree of conservatism. Changes that once were supposed to take place in thousands of years are now thought to have required millions. Hence in this chapter we shall assume that throughout geological times the condition of the sun has been almost as at present. It may have been somewhat larger, or different in other ways, but it was essentially a hot, gaseous body such as we see today and it gave out essentially the same amount of energy. This assumption will affect the general validity of what follows only if it departs widely from the truth. With this assumption, then, let us inquire into the degree to which the sun's atmosphere has probably been disturbed throughout geological times.

The second bit of evidence is found in recent exhaustive studies of periodicities by Turner and other astronomers. They have sought every possible natural occurrence for which a numerical record is available for a long period. The most valuable records appear to be those of tree growth, Nile floods, Chinese earthquakes, and sunspots. Turner reaches the conclusion that all four types of phenomena show the same periodicity, namely, cycles with an average length of about 260 to 280 years. He suggests that if this is true, the cycles in tree growth and in floods, both of which are climatic, are probably due to a non-terrestrial cause. The fact that the sunspots show similar cycles suggests that the sun's variations are the cause.

These two bits of evidence are far too slight to form the foundation of any theory as to changes in solar activity in the geological past. Nevertheless it may be helpful to set forth certain possibilities as a stimulus to further research. For example, it has been suggested that meteoric bodies may have fallen into the sun and caused it suddenly to flare up, as it were. This is not impossible, although it does not appear to have taken place since men became advanced enough to make careful observations. Moreover, the meteorites which now fall on the earth are extremely small, the average size being computed as no larger than a grain of wheat. The largest ever found on the earth's surface, at Bacubirito in Mexico, weighs only about fifty tons, while within the rocks the evidences of meteorites are extremely scanty and insignificant. If meteorites had fallen into the sun often enough and of sufficient size to cause glacial fluctuations and historic pulsations of climate, it seems highly probable that the earth would show much more evidence of having been similarly disturbed. And even if the sun should be bombarded by large meteors the result would probably not be sudden cold periods, which are the most notable phenomena of the earth's climatic history, but sudden warm periods followed by slow cooling. Nevertheless, the disturbance of the sun by collision with meteoric matter can by no means be excluded as a possible cause of climatic variations.

Allied to the preceding hypothesis is Shapley's nebular hypothesis. At frequent intervals, averaging about once a year during the last thirty years, astronomers have discovered what are known as novae. These are stars which were previously faint or even invisible, but which flash suddenly into brilliancy. Often their light-giving power rises seven or eight magnitudes--a thousand-fold. In addition to the spectacular novae there are numerous irregular variables whose brilliancy changes in every ratio from a few per cent up to several magnitudes. Most of them are located in the vicinity of nebulae, as is also the case with novae. This, as well as other facts, makes it probable that all these stars are "friction variables," as Shapley calls them. Apparently as they pass through the nebulae they come in contact with its highly diffuse matter and thereby become bright much as the earth would become bright if its atmosphere were filled with millions of almost infinitesimally small meteorites. A star may also lose brilliancy if nebulous matter intervenes between it and the observer. If our sun has been subjected to any of these changes some sort of climatic effect must have been produced.

In a personal communication Shapley amplifies the nebular climatic hypothesis as follows:

Within 700 light years of the sun in many directions are great diffuse clouds of nebulosity, some bright, most of them dark. The probability that stars moving in the general region of such clouds will encounter this material is very high, for the clouds fill enormous volumes of space,--e.g., probably more than a hundred thousand cubic light years in the Orion region, and are presumably composed of rarefied gases or of dust particles. Probably throughout all our part of space such nebulosity exists , but only in certain regions is it dense enough to affect conspicuously the stars involved in it. If a star moving at high velocity should collide with a dense part of such a nebulous cloud, we should probably have a typical nova. If the relative velocity of nebulous material and star were low or moderate, or if the material were rare, we should not expect a conspicuous effect on the star's light.

In the nebulous region of Orion, which is probably of unusually high density, there are about 100 known stars, varying between 20% and 80% of their total light--all of them irregularly--some slowly, some suddenly. Apparently they are "friction variables." Some of the variables suddenly lose 40% of their light as if blanketed by nebulous matter. In the Trifid Nebula there are variables like those of Orion, in Messier 8 also, and probably many of the 100 or so around the Rho Ophiuchi region belong to this kind.

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