Fig. 9. Cretaceous Paleogeography.(After Schuchert.)Although there is much evidence of periodic increaseof the sea in equatorial latitudes and of land in high latitudes, it has remained for the zoölogist Metcalf to present a very pretty bit of evidence that at certain times submergence along the equator coincided with emergence in high latitudes, and vice versa. Certain fresh water frogs which carry the same internal parasite are confined to two widely separated areas in tropical and south temperate America and in Australia. The extreme improbability that both the frogs and the parasites could have originated independently in two unconnected areas and could have developed by convergent evolution so that they are almost identical in the two continents makes it almost certain that there must have been a land connection between South America and Australia, presumably by way of Antarctica. The facts as to the parasites seem also to prove that while the land connection existed there was a sea across South America in equatorial latitudes. The parasite infests not only the frogs but the American toads known as Bufo. Now Bufo originated north of the equator in America and differs from the frogs which originated in southern South America in not being found in Australia. This raises the question of how the frogs could go to Australia via Antarctica carrying the parasite with them, while the toads could not go. Metcalf's answer is that the toads were cut off from the southern part of South America by an equatorial sea until after the Antarctic connection between the Old World and the New was severed.As Patagonia let go of Antarctica by subsidence of the intervening land area, there was a probable concomitant rise of land through what is now middle South America and the northern and southern portions of this continent came together.[89]These various changes in the earth's crust have given rise to certain specific types of distribution of the lands, which will now be considered. We shall inquire what climatic conditions would arise from changes in (a) the continuity of the lands from north to south, (b) the amount of land in tropical latitudes, and (c) the amount of land in middle and high latitudes.(a) At present the westward drift of warm waters, set in motion by the trade winds, is interrupted by land masses and turned poleward, producing the important Gulf Stream Drift and Japan Current in the northern hemisphere, and corresponding, though less important, currents in the southern hemisphere. During the past, quite different sets of ocean currents doubtless have existed in response to a different distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and several other periods, the present American barrier to the westward moving tropical current was broken in Central America. Even if the supposed continent of "Gondwana Land" extended from Africa to South America in equatorial latitudes, strong currents must still have flowed westward along its northern shore under the impulse of the peculiarly strong trade winds which the equatorial land would create. Nevertheless at such times relatively little warm tropical water presumably entered the North Atlantic, for it escaped into the Pacific. At several other times, such as the late Ordovician and mid-Devonian, when the isthmian barrier existed, it probably turned an important current northward into what is now the Mississippi Basin instead of into the Atlantic. There it traversed an epeiric, or mid-continental sea open to both north and south. Hence its effectiveness in warming Arctic regions must have been quite different from that of the present Gulf Stream.(b) We will next consider the influences of changes in the amount of equatorial and tropical land. As such lands are much hotter than the corresponding seas, the intensity and width of the equatorial belt of low pressure must be great when they are extensive. Hence the trade winds must have been stronger than now whenever tropical lands were more extensive than at present. This is because the trades are produced by the convection due to excessive heat along the heat equator. There the air expands upward and flows poleward at high altitudes. The trade wind consists of air moving toward the heat equator to take the place of the air which there rises. When the lands in low latitudes were wide the trade winds must also have dominated a wide belt. The greater width of the trade-wind belt today over Africa than over the Atlantic illustrates the matter. The belt must have been still wider when Gondwana Land was large, as it is believed to have been during the Paleozoic era and the early Mesozoic.An increase in the width of the equatorial belt of low pressure under the influence of broad tropical lands would be accompanied not only by stronger and more widespread trade winds, but by a corresponding strengthening of the subtropical belts of high pressure. The chief reason would be the greater expansion of the air in the equatorial low pressure belt and the consequent more abundant outflow of air at high altitudes in the form of anti-trades or winds returning poleward above the trades. Such winds would pile up the air in the region of the high-pressure belt. Moreover, since the meridians converge as one proceeds away from the equator, the air of the poleward-moving anti-trades tends to be crowded as it reaches higher latitudes, thus increasing the pressure. Unless there were a corresponding increase in tropicalcyclones, one of the most prominent results of the strengthened trades and the intensified subtropical high-pressure belt at times of broad lands in low latitudes would be great deserts. It will be recalled that the trade-wind lowlands and the extra-tropical belt of highs are the great desert belts at present. The trade-wind lowlands are desert because air moving into warmer latitudes takes up water except where it is cooled by rising on mountain-sides. The belt of highs is arid because there, too, air is being warmed, but in this case by descending from aloft.Again, if the atmospheric pressure in the subtropical belt should be intensified, the winds flowing poleward from this belt would necessarily become stronger. These would begin as southwesterlies in the northern hemisphere and northwesterlies in the southern. In the preceding chapter we have seen that such winds, especially when cyclonic storms are few and mild, are a powerful agent in transferring subtropical heat poleward. If the strength of the westerlies were increased because of broad lands in low latitudes, their efficacy in transferring heat would be correspondingly augmented. It is thus evident that any change in the extent of tropical lands during the geologic past must have had important climatic consequences in changing the velocity of the atmospheric circulation and in altering the transfer of heat from low latitudes to high. When the equatorial and tropical lands were broad the winds and currents must have been strong, much heat must have been carried away from low latitudes, and the contrast between low and high latitudes must have been relatively slight. As we have already remarked, leading paleogeographers believe that changes in the extent of the lands have been especially marked in low latitudes, and that on the averagethere has been a decrease in the extent of land within the tropics. Gondwana Land is the greatest illustration of this. In the same way, on the numerous paleogeographic maps of North America, most paleogeographers have shown fairly extensive lands south of the latitude of the United States during most of the geologic epochs.[90](c) There is evidence that during geologic history the area of the lands in middle and high latitudes, as well as in low latitudes, has changed radically. An increase in such lands would cause the winters to grow colder. This would be partly because of the loss of heat by radiation into the cold dry air over the continents in winter, and partly because of increased reflection from snow and frost, which gather much more widely upon the land than upon the ocean. Furthermore, in winter when the continents are relatively cold, there is a strong tendency for winds to blow out from the continent toward the ocean. The larger the land the stronger this tendency. In Asia it gives rise to strong winter monsoons. The effect of such winds is illustrated by the way in which the westerlies prevent the Gulf Stream from warming the eastern United States in winter. The Gulf Stream warms northwestern Europe much more than the United States because, in Europe, the prevailing winds are onshore.Another effect of an increase in the area of the lands in middle and high latitudes would be to interpose barriers to oceanic circulation and thus lower the temperature of polar regions. This would not mean glaciation in high latitudes, however, even when the lands were widespread as in the Mesozoic and early Tertiary. Students of glaciology are more and more thoroughly convincedthat glaciation depends on the availability of moisture even more than upon low temperature.In conclusion it may be noted that each of the several climatic influences of increased land area in the high latitudes would tend to increase the contrasts between land and sea, between winter and summer, and between low latitudes and high. In other words, so far as the effect upon high latitudes themselves is concerned, an expansion of the lands there would tend in the same direction as a diminution in low latitudes. In so far as the general trend of geological evolution has been toward more land in high latitudes and less in low, it would help to produce a progressive increase in climatic diversity such as is faintly indicated in the rock strata. On the other hand, the oscillations in the distribution of the lands, of which geology affords so much evidence, must certainly have played an important part in producing the periodic changes of climate which the earth has undergone.III. Throughout geological history there is abundant evidence that the process of contraction has led to marked differences not only in the distribution and area of the lands, but in their height. On the whole the lands have presumably increased in height since the Proterozoic, somewhat in proportion to the increased differentiation of continents and oceans.[91]If there has been such an increase, the contrast between the climate of ocean and land must have been accentuated, for highlands have a greater diurnal and seasonal range of temperature than do lowlands. The ocean has very little range of either sort. The large range at high altitudes is due chiefly to the small quantity of water vapor, for this declinessteadily with increased altitude. A diminution in the density of the other constituents of the air also decreases the blanketing effect of the atmosphere. In conformity with the great seasonal range in temperature at times when the lands stand high, the direction of the wind would be altered. When the lands are notably warmer than the oceans, the winds commonly flow from land to sea, and when the continents are much colder than the oceans, the direction is reversed. The monsoons of Asia are examples. Strong seasonal winds disturb the normal planetary circulation of the trade winds in low latitudes and of the westerlies in middle latitudes. They also interfere with the ocean currents set in motion by the planetary winds. The net result is to hinder the transfer of heat from low latitudes to high, and thus to increase the contrasts between the zones. Local as well as zonal contrasts are also intensified. The higher the land, the greater, relatively speaking, are the cloudiness and precipitation on seaward slopes, and the drier the interior. Indeed, most highlands are arid. Henry's[92]recent study of the vertical distribution of rainfall on mountain-sides indicates that a decrease sets in at about 3500 feet in the tropics and only a little higher in mid-latitudes.In addition to the main effects upon atmospheric circulation and precipitation, each of the many upheavals of the lands must have been accompanied by many minor conditions which tended toward diversity. For example, the streams were rejuvenated, and instead of meandering perhaps over vast flood plains they intrenched their channels and in many cases dug deep gorges. The water table was lowered, soil was removed from considerable areas, the bare rock was exposed, and the type of dominantvegetation altered in many places. An almost barren ridge may represent all that remains of what was once a vast forested flood plain. Thus, increased elevation of the land produces contrasted conditions of slope, vegetation, availability of ground water, exposure to wind and so forth, and these unite in diversifying climate. Where mountains are formed, strong contrasts are sure to occur. The windward slopes may be very rainy, while neighboring leeward slopes are parched by a dry foehn wind. At the same time the tops may be snow-covered. Increased local contrasts in climatic conditions are known to influence the intensity of cyclonic storms,[93]and these affect the climatic conditions of all middle and high latitudes, if not of the entire earth. The paths followed by cyclonic storms are also altered by increased contrast between land and water. When the continents are notably colder than the neighboring oceans, high atmospheric pressure develops on the lands and interferes with the passage of lows, which are therefore either deflected around the continent or forced to move slowly.The distribution of lofty mountains has an even more striking climatic effect than the general uplift of a region. In Proterozoic times there was a great range in the Lake Superior region; in the late Devonian the Acadian mountains of New England and the Maritime Provinces of Canada possibly attained a height equal to the present Rockies. Subsequently, in the late Paleozoic a significant range stood where the Ouachitas now are. Accompanying the uplift of each of these ranges, and all others, the climate of the surrounding area, especially to leeward, must have been altered greatly. Many extensive salt depositsfound now in fairly humid regions, for example, the Pennsylvanian and Permian deposits of Kansas and Oklahoma, were probably laid down in times of local aridity due to the cutting off of moisture-bearing winds by the mountains of Llanoria in Louisiana and Texas. Hence such deposits do not necessarily indicate periods of widespread and profound aridity.When the causes of ancient glaciation were first considered by geologists, about the middle of the nineteenth century, it was usually assumed that the glaciated areas had been elevated to great heights, and thus rendered cold enough to permit the accumulation of glaciers. The many glaciers occurring in the Alps of central Europe where glaciology arose doubtless suggested this explanation. However, it is now known that most of the ancient glaciation was not of the alpine type, and there is adequate proof that the glacial periods cannot be explained as due directly and solely to uplift. Nevertheless, upheavals of the lands are among the most important factors in controlling climate, and variations in the height of the lands have doubtless assisted in producing climate oscillations, especially those of long duration. Moreover, the progressive increase in the height of the lands has presumably played a part in fostering local and zonal diversity in contrast with the relative uniformity of earlier geological times.IV. The contraction of the earth has been accompanied by volcanic activity as well as by changes in the extent, distribution, and altitude of the lands. The probable part played by volcanic dust as a contributory factor in producing short sudden climatic variations has already been discussed. There is, however, another though probably less important respect in which volcanic activity may have had at least a slight climatic significance. The oldestknown rocks, those of the Archean era, contain so much igneous matter that many students have assumed that they show that the entire earth was once liquid. It is now considered that they merely indicate igneous activity of great magnitude. In the later part of Proterozoic time, during the second quarter of the earth's history according to Schuchert's estimate, there were again vast outflowings of lava. In the Lake Superior district, for example, a thickness of more than a mile accumulated over a large area, and lavas are common in many areas where rocks of this age are known. The next quarter of the earth's history elapsed without any correspondingly great outflows so far as is known, though several lesser ones occurred. Toward the end of the last quarter, and hence quite recently from the geological standpoint, another period of outflows, perhaps as noteworthy as that of the Proterozoic, occurred in the Cretaceous and Tertiary.The climatic effects of such extensive lava flows would be essentially as follows: In the first place so long as the lavas were hot they would set up a local system of convection with inflowing winds. This would interfere at least a little with the general winds of the area. Again, where the lava flowed out into water, or where rain fell upon hot lava, there would be rapid evaporation which would increase the rainfall. Then after the lava had cooled, it would still influence climate a trifle in so far as its color was notably darker or lighter than that of the average surface. Dark surfaces absorb solar heat and become relatively warm when the sun shines upon them. Dark objects likewise radiate heat more rapidly than light-colored objects. Hence they cool more rapidly at night, and in the winter. As most lavas are relatively dark they increase the average diurnal range of temperature.Hence even after they are cool they increase the climatic diversity of the land.The amount of heat given to the atmosphere by an extensive lava flow, though large according to human standards, is small compared with the amount received from the sun by a like area, except during the first few weeks or months before the lava has formed a thick crust. Furthermore, probably only a small fraction of any large series of flows occurred in a given century or millennium. Moreover, even the largest lava flows covered an area of only a few hundredths of one per cent of the earth's surface. Nevertheless, the conditions which modify climate are so complicated that it would be rash to state that this amount of additional heat has been of no climatic significance. Like the proverbial "straw that broke the camel's back," the changes it would surely produce in local convection, atmospheric pressure, and the direction of the wind may have helped to shift the paths of storms and to produce other complications which were of appreciable climatic significance.V. The last point which we shall consider in connection with the effect of the earth's interior upon climate is internal heat. The heat given off by lavas is merely a small part of that which is emitted by the earth as a whole. In the earliest part of geological history enough heat may have escaped from the interior of the earth to exert a profound influence on the climate. Knowlton,[94]as we have seen, has recently built up an elaborate theory on this assumption. At present, however, accurate measurements show that the escape of heat is so slight that it has no appreciable influence except in a few volcanicareas. It is estimated to raise the average temperature of the earth's surface less than 0.1°C.[95]In order to contribute enough heat to raise the surface temperature 1°C., the temperature gradient from the interior of the earth to the surface would need to be ten times as great as now, for the rate of conduction varies directly with the gradient. If the gradient were ten times as great as now, the rocks at a depth of two and one-half miles would be so hot as to be almost liquid according to Barrell's[96]estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate that such high temperatures have not prevailed at such slight depths since the Proterozoic. Furthermore, the fact that the climate was cold enough to permit glaciation early in the Proterozoic era and at from one to three other times before the opening of the Paleozoic suggests that the rate of escape of heat was not rapid even in the first half of the earth's recorded history. Yet even if the general escape of heat has never been large since the beginning of the better-known part of geological history, it was presumably greater in early times than at present.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 beexplained by variations in the extent, distribution, and altitude of the lands. Such variations appear to be the inevitable result of the earth's contraction.CHAPTER XIIPOST-GLACIAL CRUSTAL MOVEMENTS AND CLIMATIC CHANGESAn interesting practical application of some of the preceding generalizations is found in an attempt by C. E. P. Brooks[97]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:PhaseClimateDate1. The Last Great Glaciation.Arctic climate.30,000-18,000 B. C.2. The Retreat of the Glaciers.Severe continental climate.18,000-6000 B. C.3. The Continental Phase.Continental climate.6000-4000 B. C.4. The Maritime Phase.Warm and moist.4000-3000 B. C.5. The Later Forest Phase.Warm and dry.3000-1800 B. C.6. The Peat-Bog Phase.Cooler and moister.1800 B. C.-300 A. D.7. The Recent Phase.Becoming drier.300 A. D.-Brooks bases his chronology largely on De Geer's measurements of the annual layers of clay in lakebottoms but makes much use of other evidence. According to Brooks the last glacial epoch lasted roughly from 30,000 to 18,000 B. C., but this includes a slight amelioration of climate followed by a readvance of the ice, known as the Buhl stage. During the time of maximum glaciation the British Isles stood twenty or thirty feet higher than now and Scandinavia was "considerably" more elevated. The author believes that this caused a fall of 1°C. in the temperature of the British Isles and of 2°C. in Scandinavia. By an ingenious though not wholly convincing method of calculation he concludes that this lowering of temperature, aided by an increase in the area of the lands, sufficed to start an ice sheet in Scandinavia. The relatively small area of ice cooled the air and gave rise to an area of high barometric pressure. This in turn is supposed to have caused further expansion of the ice and to have led to full-fledged glaciation.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.[98]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 about200 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:Maxima 3500 B. C. ———— 2100 B. C. ———— 350 B. C. ———— A. D. 1434Minima ————— 2800 B. C. ———— 1200 B. C. ———— A. D. 530 ————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., thatis, 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.By 1600 B. C. the land had assumed nearly its present level in the British Isles and the southern Baltic region, while northern Scandinavia still stood lower than now. The climate of Britain and Germany was so humid that there was an extensive formation of peat even on high ground not before covered. This moist stage seems to have lasted almost to the time of Christ, and may have been the reason why the Romans described Britain aspeculiarly wet and damp. At this point Brooks again departs from northwest Europe to a wider field: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 résumé 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.When we turn to the northwest of North America, this is brought out very markedly. For in Yukon and Alaska the Ice Age was a very mild affair compared with its severity in eastern America and Scandinavia. As the land had not a heavy ice-load to recover from, there were no complicated geographicalchanges. Also, there were no fluctuations of climate, but simply a gradual passage to present conditions. The latter circumstance especially seems to show that the emphasis laid on geographical rather than astronomical factors ofgreatclimatic changes is not misplaced.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.CHAPTER XIIITHE CHANGING COMPOSITION OF OCEANS AND ATMOSPHEREHaving 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 effectsof increased salinity. Yet it seems clear that an increase in salinity must have a slight effect upon climate.Salinity affects climate in four ways: (1) It appreciably influences the rate of evaporation; (2) it alters the freezing point; (3) it produces certain indirect effects through changes in the absorption of carbon dioxide; and (4) it has an effect on oceanic circulation.(1) According to the experiments of Mazelle and Okada, as reported by Krümmel,[99]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.(2) 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. TheBaltic 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.[100]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 thecoasts 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.(3) 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,[101]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[102]when the partially combined carbon dioxide is also considered. Moreover, the capacity of water for carbon dioxide varies sharply with the temperature.[103]Hence a rise in temperature of only 1°C. would theoretically cause the oceans to give up from 30to 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.(4) 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.[104]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 coldwater 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 Krümmel and Mill,[105]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 (4°C.) to the highest temperatures which ever prevail in the ocean (30°C. or 86°F.) 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 Krümmel, 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 bythe 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.[106]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, andits 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:[107]"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 (Ceratodus) 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 (Limulus) 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. Zoölogists who see these wonderful fossils are at oncestruck 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."At present, as we have repeatedly seen, little growth takes place either among animals or plants at temperatures below 0°C. or above 40°C., and for most species the limiting temperatures are about 10° and 30°. The maintenance of so narrow a scale of temperature is a function of the atmosphere, as well as of the sun. Without an atmosphere, the temperature by day would mount fatally wherever the sun rides high in the sky. By night it would fall everywhere to a temperature approaching absolute zero, that is -273°C. Some such temperature prevails a few miles above the earth's surface, beyond the effective atmosphere. Indeed, even if the atmosphere were almost as it is now, but only lacked one of the minor constituents, a constituent which is often actually ignored in statements of the composition of the air, life would be impossible. Tyndall concludes that if water vapor were entirely removed from the atmosphere for a single day and night, all life—except that which is dormant in the form of seeds, eggs, or spores—would be exterminated. Part would be killed by the high temperature developed by day when the sun was high, and part, by the cold night.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 nearthe 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 andthat 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.Thus there appears to be strong evidence against the radical changes in the atmosphere which are sometimes postulated. Yet some changes must have taken place, and even minor changes would be accompanied by some sort of climatic effect. The changes would take the form of either an increase or a decrease in the atmosphere as a whole, or in its constituent elements. The chief means by which the atmosphere has increased appear to be as follows: (a) By contributions from the interior of the earth via volcanoes and springs and by the weathering of igneous rocks with the consequent release of their enclosed gases;[108](b) by the escape of some of the abundant gases which the ocean holds in solution; (c) by the arrival on the earth of gases from space, either enclosed in meteors or as free-flying molecules; (d) by the release of gases from organic compounds by oxidation, or by exhalation from animals and plants. On the other hand, one or another of the constituents of the atmosphere has presumably decreased (a) by being locked up in newly formed rocks or organic compounds; (b) by being dissolved in the ocean; (c) by the escape of molecules into space; and (d) by the condensation of water vapor.The combined effect of the various means of increase and decrease depends partly on the amount of each constituent received from the earth's interior or from space, and partly on the fact that the agencies which tend to deplete the atmosphere are highly selective in theiraction. Our knowledge of how large a quantity of new gases the air has received is very scanty, but judging by present conditions the general tendency is toward a slow increase chiefly because of meteorites, volcanic action, and the work of deep-seated springs. As to decrease, the case is clearer. This is because the chemically active gases, oxygen, CO2, and water vapor, tend to be locked up in the rocks, while the chemically inert gases, nitrogen and argon, show almost no such tendency. Though oxygen is by far the most abundant element in the earth's crust, making up more than 50 per cent of the total, it forms only about one-fifth of the air. Nitrogen, on the other hand, is very rare in the rocks, but makes up nearly four-fifths of the air. It would, therefore, seem probable that throughout the earth's history, there has been a progressive increase in the amount of atmospheric nitrogen, and presumably a somewhat corresponding increase in the mass of the air. On the other hand, it is not clear what changes have occurred in the amount of atmospheric oxygen. It may have increased somewhat or perhaps even notably. Nevertheless, because of the greater increase in nitrogen, it may form no greater percentage of the air now than in the distant past.As to the absolute amounts of oxygen, Barrell[109]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 remotetime 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,[110]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,[111]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 theycontain. 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[112]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 allscatter the short waves of light and thus interfere with their reaching the earth. Abbot and Fowle,[113]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 atmosphericmoisture 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[114]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 radiatedby 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 geologicaltimes 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 reënforce 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 thanthe 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.
Fig. 9. Cretaceous Paleogeography.(After Schuchert.)
Although there is much evidence of periodic increaseof the sea in equatorial latitudes and of land in high latitudes, it has remained for the zoölogist Metcalf to present a very pretty bit of evidence that at certain times submergence along the equator coincided with emergence in high latitudes, and vice versa. Certain fresh water frogs which carry the same internal parasite are confined to two widely separated areas in tropical and south temperate America and in Australia. The extreme improbability that both the frogs and the parasites could have originated independently in two unconnected areas and could have developed by convergent evolution so that they are almost identical in the two continents makes it almost certain that there must have been a land connection between South America and Australia, presumably by way of Antarctica. The facts as to the parasites seem also to prove that while the land connection existed there was a sea across South America in equatorial latitudes. The parasite infests not only the frogs but the American toads known as Bufo. Now Bufo originated north of the equator in America and differs from the frogs which originated in southern South America in not being found in Australia. This raises the question of how the frogs could go to Australia via Antarctica carrying the parasite with them, while the toads could not go. Metcalf's answer is that the toads were cut off from the southern part of South America by an equatorial sea until after the Antarctic connection between the Old World and the New was severed.
As Patagonia let go of Antarctica by subsidence of the intervening land area, there was a probable concomitant rise of land through what is now middle South America and the northern and southern portions of this continent came together.[89]
As Patagonia let go of Antarctica by subsidence of the intervening land area, there was a probable concomitant rise of land through what is now middle South America and the northern and southern portions of this continent came together.[89]
These various changes in the earth's crust have given rise to certain specific types of distribution of the lands, which will now be considered. We shall inquire what climatic conditions would arise from changes in (a) the continuity of the lands from north to south, (b) the amount of land in tropical latitudes, and (c) the amount of land in middle and high latitudes.
(a) At present the westward drift of warm waters, set in motion by the trade winds, is interrupted by land masses and turned poleward, producing the important Gulf Stream Drift and Japan Current in the northern hemisphere, and corresponding, though less important, currents in the southern hemisphere. During the past, quite different sets of ocean currents doubtless have existed in response to a different distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and several other periods, the present American barrier to the westward moving tropical current was broken in Central America. Even if the supposed continent of "Gondwana Land" extended from Africa to South America in equatorial latitudes, strong currents must still have flowed westward along its northern shore under the impulse of the peculiarly strong trade winds which the equatorial land would create. Nevertheless at such times relatively little warm tropical water presumably entered the North Atlantic, for it escaped into the Pacific. At several other times, such as the late Ordovician and mid-Devonian, when the isthmian barrier existed, it probably turned an important current northward into what is now the Mississippi Basin instead of into the Atlantic. There it traversed an epeiric, or mid-continental sea open to both north and south. Hence its effectiveness in warming Arctic regions must have been quite different from that of the present Gulf Stream.
(b) We will next consider the influences of changes in the amount of equatorial and tropical land. As such lands are much hotter than the corresponding seas, the intensity and width of the equatorial belt of low pressure must be great when they are extensive. Hence the trade winds must have been stronger than now whenever tropical lands were more extensive than at present. This is because the trades are produced by the convection due to excessive heat along the heat equator. There the air expands upward and flows poleward at high altitudes. The trade wind consists of air moving toward the heat equator to take the place of the air which there rises. When the lands in low latitudes were wide the trade winds must also have dominated a wide belt. The greater width of the trade-wind belt today over Africa than over the Atlantic illustrates the matter. The belt must have been still wider when Gondwana Land was large, as it is believed to have been during the Paleozoic era and the early Mesozoic.
An increase in the width of the equatorial belt of low pressure under the influence of broad tropical lands would be accompanied not only by stronger and more widespread trade winds, but by a corresponding strengthening of the subtropical belts of high pressure. The chief reason would be the greater expansion of the air in the equatorial low pressure belt and the consequent more abundant outflow of air at high altitudes in the form of anti-trades or winds returning poleward above the trades. Such winds would pile up the air in the region of the high-pressure belt. Moreover, since the meridians converge as one proceeds away from the equator, the air of the poleward-moving anti-trades tends to be crowded as it reaches higher latitudes, thus increasing the pressure. Unless there were a corresponding increase in tropicalcyclones, one of the most prominent results of the strengthened trades and the intensified subtropical high-pressure belt at times of broad lands in low latitudes would be great deserts. It will be recalled that the trade-wind lowlands and the extra-tropical belt of highs are the great desert belts at present. The trade-wind lowlands are desert because air moving into warmer latitudes takes up water except where it is cooled by rising on mountain-sides. The belt of highs is arid because there, too, air is being warmed, but in this case by descending from aloft.
Again, if the atmospheric pressure in the subtropical belt should be intensified, the winds flowing poleward from this belt would necessarily become stronger. These would begin as southwesterlies in the northern hemisphere and northwesterlies in the southern. In the preceding chapter we have seen that such winds, especially when cyclonic storms are few and mild, are a powerful agent in transferring subtropical heat poleward. If the strength of the westerlies were increased because of broad lands in low latitudes, their efficacy in transferring heat would be correspondingly augmented. It is thus evident that any change in the extent of tropical lands during the geologic past must have had important climatic consequences in changing the velocity of the atmospheric circulation and in altering the transfer of heat from low latitudes to high. When the equatorial and tropical lands were broad the winds and currents must have been strong, much heat must have been carried away from low latitudes, and the contrast between low and high latitudes must have been relatively slight. As we have already remarked, leading paleogeographers believe that changes in the extent of the lands have been especially marked in low latitudes, and that on the averagethere has been a decrease in the extent of land within the tropics. Gondwana Land is the greatest illustration of this. In the same way, on the numerous paleogeographic maps of North America, most paleogeographers have shown fairly extensive lands south of the latitude of the United States during most of the geologic epochs.[90]
(c) There is evidence that during geologic history the area of the lands in middle and high latitudes, as well as in low latitudes, has changed radically. An increase in such lands would cause the winters to grow colder. This would be partly because of the loss of heat by radiation into the cold dry air over the continents in winter, and partly because of increased reflection from snow and frost, which gather much more widely upon the land than upon the ocean. Furthermore, in winter when the continents are relatively cold, there is a strong tendency for winds to blow out from the continent toward the ocean. The larger the land the stronger this tendency. In Asia it gives rise to strong winter monsoons. The effect of such winds is illustrated by the way in which the westerlies prevent the Gulf Stream from warming the eastern United States in winter. The Gulf Stream warms northwestern Europe much more than the United States because, in Europe, the prevailing winds are onshore.
Another effect of an increase in the area of the lands in middle and high latitudes would be to interpose barriers to oceanic circulation and thus lower the temperature of polar regions. This would not mean glaciation in high latitudes, however, even when the lands were widespread as in the Mesozoic and early Tertiary. Students of glaciology are more and more thoroughly convincedthat glaciation depends on the availability of moisture even more than upon low temperature.
In conclusion it may be noted that each of the several climatic influences of increased land area in the high latitudes would tend to increase the contrasts between land and sea, between winter and summer, and between low latitudes and high. In other words, so far as the effect upon high latitudes themselves is concerned, an expansion of the lands there would tend in the same direction as a diminution in low latitudes. In so far as the general trend of geological evolution has been toward more land in high latitudes and less in low, it would help to produce a progressive increase in climatic diversity such as is faintly indicated in the rock strata. On the other hand, the oscillations in the distribution of the lands, of which geology affords so much evidence, must certainly have played an important part in producing the periodic changes of climate which the earth has undergone.
III. Throughout geological history there is abundant evidence that the process of contraction has led to marked differences not only in the distribution and area of the lands, but in their height. On the whole the lands have presumably increased in height since the Proterozoic, somewhat in proportion to the increased differentiation of continents and oceans.[91]If there has been such an increase, the contrast between the climate of ocean and land must have been accentuated, for highlands have a greater diurnal and seasonal range of temperature than do lowlands. The ocean has very little range of either sort. The large range at high altitudes is due chiefly to the small quantity of water vapor, for this declinessteadily with increased altitude. A diminution in the density of the other constituents of the air also decreases the blanketing effect of the atmosphere. In conformity with the great seasonal range in temperature at times when the lands stand high, the direction of the wind would be altered. When the lands are notably warmer than the oceans, the winds commonly flow from land to sea, and when the continents are much colder than the oceans, the direction is reversed. The monsoons of Asia are examples. Strong seasonal winds disturb the normal planetary circulation of the trade winds in low latitudes and of the westerlies in middle latitudes. They also interfere with the ocean currents set in motion by the planetary winds. The net result is to hinder the transfer of heat from low latitudes to high, and thus to increase the contrasts between the zones. Local as well as zonal contrasts are also intensified. The higher the land, the greater, relatively speaking, are the cloudiness and precipitation on seaward slopes, and the drier the interior. Indeed, most highlands are arid. Henry's[92]recent study of the vertical distribution of rainfall on mountain-sides indicates that a decrease sets in at about 3500 feet in the tropics and only a little higher in mid-latitudes.
In addition to the main effects upon atmospheric circulation and precipitation, each of the many upheavals of the lands must have been accompanied by many minor conditions which tended toward diversity. For example, the streams were rejuvenated, and instead of meandering perhaps over vast flood plains they intrenched their channels and in many cases dug deep gorges. The water table was lowered, soil was removed from considerable areas, the bare rock was exposed, and the type of dominantvegetation altered in many places. An almost barren ridge may represent all that remains of what was once a vast forested flood plain. Thus, increased elevation of the land produces contrasted conditions of slope, vegetation, availability of ground water, exposure to wind and so forth, and these unite in diversifying climate. Where mountains are formed, strong contrasts are sure to occur. The windward slopes may be very rainy, while neighboring leeward slopes are parched by a dry foehn wind. At the same time the tops may be snow-covered. Increased local contrasts in climatic conditions are known to influence the intensity of cyclonic storms,[93]and these affect the climatic conditions of all middle and high latitudes, if not of the entire earth. The paths followed by cyclonic storms are also altered by increased contrast between land and water. When the continents are notably colder than the neighboring oceans, high atmospheric pressure develops on the lands and interferes with the passage of lows, which are therefore either deflected around the continent or forced to move slowly.
The distribution of lofty mountains has an even more striking climatic effect than the general uplift of a region. In Proterozoic times there was a great range in the Lake Superior region; in the late Devonian the Acadian mountains of New England and the Maritime Provinces of Canada possibly attained a height equal to the present Rockies. Subsequently, in the late Paleozoic a significant range stood where the Ouachitas now are. Accompanying the uplift of each of these ranges, and all others, the climate of the surrounding area, especially to leeward, must have been altered greatly. Many extensive salt depositsfound now in fairly humid regions, for example, the Pennsylvanian and Permian deposits of Kansas and Oklahoma, were probably laid down in times of local aridity due to the cutting off of moisture-bearing winds by the mountains of Llanoria in Louisiana and Texas. Hence such deposits do not necessarily indicate periods of widespread and profound aridity.
When the causes of ancient glaciation were first considered by geologists, about the middle of the nineteenth century, it was usually assumed that the glaciated areas had been elevated to great heights, and thus rendered cold enough to permit the accumulation of glaciers. The many glaciers occurring in the Alps of central Europe where glaciology arose doubtless suggested this explanation. However, it is now known that most of the ancient glaciation was not of the alpine type, and there is adequate proof that the glacial periods cannot be explained as due directly and solely to uplift. Nevertheless, upheavals of the lands are among the most important factors in controlling climate, and variations in the height of the lands have doubtless assisted in producing climate oscillations, especially those of long duration. Moreover, the progressive increase in the height of the lands has presumably played a part in fostering local and zonal diversity in contrast with the relative uniformity of earlier geological times.
IV. The contraction of the earth has been accompanied by volcanic activity as well as by changes in the extent, distribution, and altitude of the lands. The probable part played by volcanic dust as a contributory factor in producing short sudden climatic variations has already been discussed. There is, however, another though probably less important respect in which volcanic activity may have had at least a slight climatic significance. The oldestknown rocks, those of the Archean era, contain so much igneous matter that many students have assumed that they show that the entire earth was once liquid. It is now considered that they merely indicate igneous activity of great magnitude. In the later part of Proterozoic time, during the second quarter of the earth's history according to Schuchert's estimate, there were again vast outflowings of lava. In the Lake Superior district, for example, a thickness of more than a mile accumulated over a large area, and lavas are common in many areas where rocks of this age are known. The next quarter of the earth's history elapsed without any correspondingly great outflows so far as is known, though several lesser ones occurred. Toward the end of the last quarter, and hence quite recently from the geological standpoint, another period of outflows, perhaps as noteworthy as that of the Proterozoic, occurred in the Cretaceous and Tertiary.
The climatic effects of such extensive lava flows would be essentially as follows: In the first place so long as the lavas were hot they would set up a local system of convection with inflowing winds. This would interfere at least a little with the general winds of the area. Again, where the lava flowed out into water, or where rain fell upon hot lava, there would be rapid evaporation which would increase the rainfall. Then after the lava had cooled, it would still influence climate a trifle in so far as its color was notably darker or lighter than that of the average surface. Dark surfaces absorb solar heat and become relatively warm when the sun shines upon them. Dark objects likewise radiate heat more rapidly than light-colored objects. Hence they cool more rapidly at night, and in the winter. As most lavas are relatively dark they increase the average diurnal range of temperature.Hence even after they are cool they increase the climatic diversity of the land.
The amount of heat given to the atmosphere by an extensive lava flow, though large according to human standards, is small compared with the amount received from the sun by a like area, except during the first few weeks or months before the lava has formed a thick crust. Furthermore, probably only a small fraction of any large series of flows occurred in a given century or millennium. Moreover, even the largest lava flows covered an area of only a few hundredths of one per cent of the earth's surface. Nevertheless, the conditions which modify climate are so complicated that it would be rash to state that this amount of additional heat has been of no climatic significance. Like the proverbial "straw that broke the camel's back," the changes it would surely produce in local convection, atmospheric pressure, and the direction of the wind may have helped to shift the paths of storms and to produce other complications which were of appreciable climatic significance.
V. The last point which we shall consider in connection with the effect of the earth's interior upon climate is internal heat. The heat given off by lavas is merely a small part of that which is emitted by the earth as a whole. In the earliest part of geological history enough heat may have escaped from the interior of the earth to exert a profound influence on the climate. Knowlton,[94]as we have seen, has recently built up an elaborate theory on this assumption. At present, however, accurate measurements show that the escape of heat is so slight that it has no appreciable influence except in a few volcanicareas. It is estimated to raise the average temperature of the earth's surface less than 0.1°C.[95]
In order to contribute enough heat to raise the surface temperature 1°C., the temperature gradient from the interior of the earth to the surface would need to be ten times as great as now, for the rate of conduction varies directly with the gradient. If the gradient were ten times as great as now, the rocks at a depth of two and one-half miles would be so hot as to be almost liquid according to Barrell's[96]estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate that such high temperatures have not prevailed at such slight depths since the Proterozoic. Furthermore, the fact that the climate was cold enough to permit glaciation early in the Proterozoic era and at from one to three other times before the opening of the Paleozoic suggests that the rate of escape of heat was not rapid even in the first half of the earth's recorded history. Yet even if the general escape of heat has never been large since the beginning of the better-known part of geological history, it was presumably greater in early times than at present.
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 beexplained by variations in the extent, distribution, and altitude of the lands. Such variations appear to be the inevitable result of the earth's contraction.
An interesting practical application of some of the preceding generalizations is found in an attempt by C. E. P. Brooks[97]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:
Brooks bases his chronology largely on De Geer's measurements of the annual layers of clay in lakebottoms but makes much use of other evidence. According to Brooks the last glacial epoch lasted roughly from 30,000 to 18,000 B. C., but this includes a slight amelioration of climate followed by a readvance of the ice, known as the Buhl stage. During the time of maximum glaciation the British Isles stood twenty or thirty feet higher than now and Scandinavia was "considerably" more elevated. The author believes that this caused a fall of 1°C. in the temperature of the British Isles and of 2°C. in Scandinavia. By an ingenious though not wholly convincing method of calculation he concludes that this lowering of temperature, aided by an increase in the area of the lands, sufficed to start an ice sheet in Scandinavia. The relatively small area of ice cooled the air and gave rise to an area of high barometric pressure. This in turn is supposed to have caused further expansion of the ice and to have led to full-fledged glaciation.
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.[98]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 about200 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:
Maxima 3500 B. C. ———— 2100 B. C. ———— 350 B. C. ———— A. D. 1434Minima ————— 2800 B. C. ———— 1200 B. C. ———— A. D. 530 ————
Maxima 3500 B. C. ———— 2100 B. C. ———— 350 B. C. ———— A. D. 1434Minima ————— 2800 B. C. ———— 1200 B. C. ———— A. D. 530 ————
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., thatis, 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.
By 1600 B. C. the land had assumed nearly its present level in the British Isles and the southern Baltic region, while northern Scandinavia still stood lower than now. The climate of Britain and Germany was so humid that there was an extensive formation of peat even on high ground not before covered. This moist stage seems to have lasted almost to the time of Christ, and may have been the reason why the Romans described Britain aspeculiarly wet and damp. At this point Brooks again departs from northwest Europe to a wider field:
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.
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 résumé 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.When we turn to the northwest of North America, this is brought out very markedly. For in Yukon and Alaska the Ice Age was a very mild affair compared with its severity in eastern America and Scandinavia. As the land had not a heavy ice-load to recover from, there were no complicated geographicalchanges. Also, there were no fluctuations of climate, but simply a gradual passage to present conditions. The latter circumstance especially seems to show that the emphasis laid on geographical rather than astronomical factors ofgreatclimatic changes is not misplaced.
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.
When we turn to the northwest of North America, this is brought out very markedly. For in Yukon and Alaska the Ice Age was a very mild affair compared with its severity in eastern America and Scandinavia. As the land had not a heavy ice-load to recover from, there were no complicated geographicalchanges. Also, there were no fluctuations of climate, but simply a gradual passage to present conditions. The latter circumstance especially seems to show that the emphasis laid on geographical rather than astronomical factors ofgreatclimatic changes is not misplaced.
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.
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 effectsof increased salinity. Yet it seems clear that an increase in salinity must have a slight effect upon climate.
Salinity affects climate in four ways: (1) It appreciably influences the rate of evaporation; (2) it alters the freezing point; (3) it produces certain indirect effects through changes in the absorption of carbon dioxide; and (4) it has an effect on oceanic circulation.
(1) According to the experiments of Mazelle and Okada, as reported by Krümmel,[99]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.
(2) 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. TheBaltic 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.[100]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 thecoasts 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.
(3) 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,[101]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[102]when the partially combined carbon dioxide is also considered. Moreover, the capacity of water for carbon dioxide varies sharply with the temperature.[103]Hence a rise in temperature of only 1°C. would theoretically cause the oceans to give up from 30to 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.
(4) 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.[104]
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 coldwater 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 Krümmel and Mill,[105]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 (4°C.) to the highest temperatures which ever prevail in the ocean (30°C. or 86°F.) 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 Krümmel, 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 bythe 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.[106]
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, andits 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:[107]"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 (Ceratodus) 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 (Limulus) 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. Zoölogists who see these wonderful fossils are at oncestruck 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."
At present, as we have repeatedly seen, little growth takes place either among animals or plants at temperatures below 0°C. or above 40°C., and for most species the limiting temperatures are about 10° and 30°. The maintenance of so narrow a scale of temperature is a function of the atmosphere, as well as of the sun. Without an atmosphere, the temperature by day would mount fatally wherever the sun rides high in the sky. By night it would fall everywhere to a temperature approaching absolute zero, that is -273°C. Some such temperature prevails a few miles above the earth's surface, beyond the effective atmosphere. Indeed, even if the atmosphere were almost as it is now, but only lacked one of the minor constituents, a constituent which is often actually ignored in statements of the composition of the air, life would be impossible. Tyndall concludes that if water vapor were entirely removed from the atmosphere for a single day and night, all life—except that which is dormant in the form of seeds, eggs, or spores—would be exterminated. Part would be killed by the high temperature developed by day when the sun was high, and part, by the cold night.
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 nearthe 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 andthat 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.
Thus there appears to be strong evidence against the radical changes in the atmosphere which are sometimes postulated. Yet some changes must have taken place, and even minor changes would be accompanied by some sort of climatic effect. The changes would take the form of either an increase or a decrease in the atmosphere as a whole, or in its constituent elements. The chief means by which the atmosphere has increased appear to be as follows: (a) By contributions from the interior of the earth via volcanoes and springs and by the weathering of igneous rocks with the consequent release of their enclosed gases;[108](b) by the escape of some of the abundant gases which the ocean holds in solution; (c) by the arrival on the earth of gases from space, either enclosed in meteors or as free-flying molecules; (d) by the release of gases from organic compounds by oxidation, or by exhalation from animals and plants. On the other hand, one or another of the constituents of the atmosphere has presumably decreased (a) by being locked up in newly formed rocks or organic compounds; (b) by being dissolved in the ocean; (c) by the escape of molecules into space; and (d) by the condensation of water vapor.
The combined effect of the various means of increase and decrease depends partly on the amount of each constituent received from the earth's interior or from space, and partly on the fact that the agencies which tend to deplete the atmosphere are highly selective in theiraction. Our knowledge of how large a quantity of new gases the air has received is very scanty, but judging by present conditions the general tendency is toward a slow increase chiefly because of meteorites, volcanic action, and the work of deep-seated springs. As to decrease, the case is clearer. This is because the chemically active gases, oxygen, CO2, and water vapor, tend to be locked up in the rocks, while the chemically inert gases, nitrogen and argon, show almost no such tendency. Though oxygen is by far the most abundant element in the earth's crust, making up more than 50 per cent of the total, it forms only about one-fifth of the air. Nitrogen, on the other hand, is very rare in the rocks, but makes up nearly four-fifths of the air. It would, therefore, seem probable that throughout the earth's history, there has been a progressive increase in the amount of atmospheric nitrogen, and presumably a somewhat corresponding increase in the mass of the air. On the other hand, it is not clear what changes have occurred in the amount of atmospheric oxygen. It may have increased somewhat or perhaps even notably. Nevertheless, because of the greater increase in nitrogen, it may form no greater percentage of the air now than in the distant past.
As to the absolute amounts of oxygen, Barrell[109]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 remotetime 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,[110]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,[111]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 theycontain. 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[112]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 allscatter the short waves of light and thus interfere with their reaching the earth. Abbot and Fowle,[113]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 atmosphericmoisture 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[114]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 radiatedby 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 geologicaltimes 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 reënforce 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 thanthe 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.