Climatic Changes Part 13

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 const.i.tuent 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;[106]

(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 s.p.a.ce, 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 const.i.tuents 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 s.p.a.ce; 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 const.i.tuent received from the earth's interior or from s.p.a.ce, and partly on the fact that the agencies which tend to deplete the atmosphere are highly selective in their action. Our knowledge of how large a quant.i.ty 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, CO_{2}, 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 ma.s.s 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[107] thought that atmospheric oxygen began to be present only after plants had appeared.

It will be recalled that plants absorb carbon dioxide and separate the carbon from the oxygen, using the carbon in their tissues and setting free the oxygen. As evidence of a paucity of oxygen in the air in early Proterozoic times, Barrell cites the fact that the sedimentary rocks of that remote time commonly are somewhat greyish or greenish-grey wackes, or other types, indicating incomplete oxidation. He admits, however, that the stupendous thicknesses of red sandstones, quartzite, and hemat.i.tic 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 a.s.sume that there could be no oxygen until plants appeared to break down the carbon dioxide, for some oxygen is contributed by volcanoes,[108] and lightning decomposes water into its elements. Part of the hydrogen thus set free escapes into s.p.a.ce, 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 acc.u.mulation 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,[109] suggests that oxygen was present in the air in quant.i.ty even when the earliest shales and sandstones were formed, for atmospheric oxygen seems to be the probable source of the extra oxygen they contain.

The formation of these particular sedimentary rocks by weathering of igneous rocks involves only a little carbon dioxide and water. Although it seems probable that oxygen was present in the atmosphere even at the beginning of the geological record, it may have been far less abundant then than now. It may have been removed from the atmosphere by animals or by the oxidation of the rocks almost as rapidly as it was added by volcanoes, plants, and other agencies.

After this chapter was in type, St. John[C] 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 quant.i.ties 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 pa.s.sage of light or heat, being transparent to all but a few wave lengths. Those absorbed are chiefly in the ultra violet.

The distinct possibility that oxygen has increased in amount, makes it the more likely that there has been an increase in the total atmosphere, for the oxygen would supplement the increase in the relatively inert nitrogen and argon, which has presumably taken place. The climatic effects of an increase in the atmosphere include, in the first place, an increased scattering of light as it approaches the earth. Nitrogen, argon, and oxygen all scatter the short waves of light and thus interfere with their reaching the earth. Abbot and Fowle,[110] who have carefully studied the matter, believe that at present the scattering is quant.i.tatively 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 acc.u.mulated during the day.

Therefore, if atmospheric moisture were formerly much more abundant than now, the temperature must have been much more uniform. The tendency toward climatic severity as time went on would be still further increased by the cooling which would result from the increased wind velocity discussed below; for cooling by convection increases with the velocity of the wind, as does cooling by conduction.

Any persistent lowering of the general temperature of the air would affect not only its ability to hold water vapor, but would produce a lessening in the amount of atmospheric carbon dioxide, for the colder the ocean becomes the more carbon dioxide it can hold in solution. When the oceanic temperature falls, part of the atmospheric carbon dioxide is dissolved in the ocean. This minor const.i.tuent of the air is important because although it forms only 0.003 per cent of the earth's atmosphere, Abbot and Fowle's[111] 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 1.3C., and a halving would lower it a like amount. The indirect results of such an increase or decrease might be greater than the direct results, for the change in temperature due to variations in carbon dioxide would alter the capacity of the air to hold moisture.

Two conditions would especially help in this respect; first, changes in nocturnal cooling, and second, changes in local convection. The presence of carbon dioxide diminishes nocturnal cooling because it absorbs the heat radiated by the earth, and re-radiates part of it back again. Hence with increased carbon dioxide and with the consequent warmer nights there would be less nocturnal condensation of water vapor to form dew and frost. Local convection is influenced by carbon dioxide because this gas lessens the temperature gradient. In general, the less the gradient, that is, the less the contrast between the temperature at the surface and higher up, the less convection takes place. This is ill.u.s.trated 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 lat.i.tudes 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 a.s.suming during at least the early part of geological times are corresponding changes in barometric contrasts, in the strength of winds, and in the ma.s.s of air carried by the winds along the earth's surface. The increase in the ma.s.s of the air would reenforce the greater velocity of the winds in their action as eroding and transporting agencies. Because of the greater weight of the air, the winds would be capable of picking up more dust and of carrying it farther and higher; while the increased atmospheric friction would keep it aloft a longer time. The significance of dust at high levels and its relation to solar radiation have already been discussed in connection with volcanoes. It will be recalled that on the average it lowers the surface temperature. At lower levels, since dust absorbs heat quickly and gives it out quickly, its presence raises the temperature of the air by day and lowers it by night. Hence an increase in dustiness tends toward greater extremes.

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

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

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

FOOTNOTES:

[Footnote 97: Encyclopaedia Britannica, 11th edition: article "Ocean."]

[Footnote 98: C. E. P. Brooks: The Meteorological Conditions of an Ice sheet and Their Bearing on the Desiccation of the Globe; Quart. Jour.

Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70.]

[Footnote 99: Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U.

S. Geol. Survey.]

[Footnote 100: Quoted by Schuchert in The Evolution of the Earth.]

[Footnote 101: Smithsonian Physical Tables, Sixth Revision, 1914, p.

142.]

[Footnote 102: Chamberlin, in a very suggestive article "On a possible reversal of oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373, 1906), discusses the probable climatic consequences of a reversal in the direction of deep-sea circulation. It is not wholly beyond the bounds of possibility that, in the course of ages the increasing drainage of salt from the lands not only by nature but by man's activities in agriculture and drainage, may ultimately cause such a reversal by increasing the ocean's salinity until the more saline tropical portion is heavier than the cooler but fresher subpolar waters. If that should happen, Greenland, Antarctica, and the northern sh.o.r.es of America and Asia would be warmed by the tropical heat which had been transferred poleward beneath the surface of the ocean, without loss _en route_. Subpolar regions, under such a condition of reversed deep-sea circulation, might have a mild climate. Indeed, they might be among the world's most favorable regions climatically.]

[Footnote 103: Encyclopaedia Britannica: article "Ocean."]

[Footnote 104: Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, 1906; and T. C. Chamberlin: The Origin of the Earth, 1916.]

[Footnote 105: Personal communication.]

[Footnote 106: R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of Wash., No. 106, 1908.]

[Footnote 107: J. Barrell: The Origin of the Earth, in Evolution of the Earth and Its Inhabitants, 1918, p. 44, and more fully in an unpublished ma.n.u.script.]

[Footnote 108: F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920, Bull. No. 695, U. S. Geol. Survey, p. 256.]

[Footnote 109: F. W. Clarke: _loc. cit._, pp. 27-34 et al.]

[Footnote C: Chas. E. St. John: Science Service Press Reports from the Mt. Wilson Observatory, May, 1922.]

[Footnote 110: Abbot and Fowle: Annals Astrophysical Observatory; Smiths. Inst., Vol. II, 1908, p. 163.

F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst., Vol. 69, 1918.]

[Footnote 111: Abbot and Fowle: _loc. cit._, p. 172.]

CHAPTER XIV

THE EFFECT OF OTHER BODIES ON THE SUN

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

Similar reasoning applies with equal or greater force to the sun.

Because of its vast size it presumably pa.s.ses through its stages of development much more slowly than the earth. In the first chapter of this book we saw that the earth's relative uniformity of climate for hundreds of millions of years seems to imply a similar uniformity in solar activity. This accords with a recent tendency among astronomers who are more and more recognizing that the stars and the solar system possess an extraordinary degree of conservatism. Changes that once were supposed to take place in thousands of years are now thought to have required millions. Hence in this chapter we shall a.s.sume that throughout geological times the condition of the sun has been almost as at present.

It may have been somewhat larger, or different in other ways, but it was essentially a hot, gaseous body such as we see today and it gave out essentially the same amount of energy. This a.s.sumption will affect the general validity of what follows only if it departs widely from the truth. With this a.s.sumption, then, let us inquire into the degree to which the sun's atmosphere has probably been disturbed throughout geological times.

In _Earth and Sun_, as already explained, a detailed study has led to the conclusion that cyclonic storms are influenced by the electrical action of the sun. Such action appears to be most intense in sunspots, but apparently pertains also to other disturbed areas in the sun's atmosphere. A study of sunspots suggests that their true periodicity is almost if not exactly identical with that of the orbital revolution of Jupiter, 11.8 years. Other investigations show numerous remarkable coincidences between sunspots and the orbital revolution of the other planets, including especially Saturn and Mercury. This seems to indicate that there is some truth in the hypothesis that sunspots and other related disturbances of the solar atmosphere owe their periodicity to the varying effects of the planets as they approach and recede from the sun in their eccentric orbits and as they combine or oppose their effects according to their relative positions. This does not mean that the energy of the solar disturbances is supposed to come from the planets, but merely that their variations act like the turning of a switch to determine when and how violently the internal forces of the sun shall throw the solar atmosphere into commotion. This hypothesis is by no means new, for in one form or another it has been advocated by Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.

The agency through which the planets influence the solar atmosphere is not yet clear. The suggested agencies are the direct pull of gravitation, the tidal effect of the planets, and an electro-magnetic effect. In _Earth and Sun_ the conclusion is reached that the first two are out of the question, a conclusion in which E. W. Brown acquiesces.

Unless some unknown cause is appealed to, this leaves an electro-magnetic hypothesis as the only one which has a reasonable foundation. Schuster inclines to this view. The conclusions set forth in _Earth and Sun_ as to the electrical nature of the sun's influence on the earth point somewhat in the same direction. Hence in this chapter we shall inquire what would happen to the sun, and hence to the earth, on their journey through s.p.a.ce, if the solar atmosphere is actually subject to disturbance by the electrical or other effects of other heavenly bodies. It need hardly be pointed out that we are here venturing into highly speculative ground, and that the verity or falsity of the conclusions reached in this chapter has nothing to do with the validity of the reasoning in previous chapters. Those chapters are based on the a.s.sumption that terrestrial causes of climatic changes are supplemented by solar disturbances which produce their effect partly through variations in temperature but also through variations in the intensity and paths of cyclonic storms. The present chapter seeks to shed some light on the possible causes and sequence of solar disturbances.

Let us begin by scanning the available evidence as to solar disturbances previous to the time when accurate sunspot records are available. Two rather slender bits of evidence point to cycles of solar activity lasting hundreds of years. One of these has already been discussed in Chapter VI, where the climatic stress of the fourteenth century was described. At that time sunspots are known to have been unusually numerous, and there were great climatic extremes. Lakes overflowed in Central Asia; storms, droughts, floods, and cold winters were unusually severe in Europe; the Caspian Sea rose with great rapidity; the trees of California grew with a vigor unknown for centuries; the most terrible of recorded famines occurred in England and India; the Eskimos were probably driven south by increasing snowiness in Greenland; and the Mayas of Yucatan appear to have made their last weak attempt at a revival of civilization under the stimulus of greater storminess and less constant rainfall.

The second bit of evidence is found in recent exhaustive studies of periodicities by Turner[112] and other astronomers. They have sought every possible natural occurrence for which a numerical record is available for a long period. The most valuable records appear to be those of tree growth, Nile floods, Chinese earthquakes, and sunspots.

Turner reaches the conclusion that all four types of phenomena show the same periodicity, namely, cycles with an average length of about 260 to 280 years. He suggests that if this is true, the cycles in tree growth and in floods, both of which are climatic, are probably due to a non-terrestrial cause. The fact that the sunspots show similar cycles suggests that the sun's variations are the cause.

These two bits of evidence are far too slight to form the foundation of any theory as to changes in solar activity in the geological past.

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

Nevertheless, the disturbance of the sun by collision with meteoric matter can by no means be excluded as a possible cause of climatic variations.