site logo

The Changing Composition Of Oceans And Atmosphere

Having discussed the climatic effect of movements of the earth's crust

during the course of geological time, we are now ready to consider the

corresponding effects due to changes in the movable envelopes--the

oceans and the atmosphere. Variations in the composition of sea water

and of air and in the amount of air must almost certainly have occurred,

and must have produced at least slight climatic consequences. It should

be pointed out at once that such variations appear to be far less

important climatically than do movements of the earth's crust and

changes in the activity of the sun. Moreover, in most cases, they are

not reversible as are the crustal and solar phenomena. Hence, while most

of them appear to have been unimportant so far as climatic oscillations

and fluctuations are concerned, they seemingly have aided in producing

the slight secular progression to which we have so often referred.

There is general agreement among geologists that the ocean has become

increasingly saline throughout the ages. Indeed, calculations of the

rate of accumulation of salt have been a favorite method of arriving at

estimates of the age of the ocean, and hence of the earliest marine

sediments. So far as known, however, no geologist or climatologist has

discussed the probable climatic effects of increased salinity. Yet it

seems clear that an increase in salinity must have a slight effect upon


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

Kruemmel,[97] 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


(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 deg.C. below that of fresh water. If the ocean were

fresh and our winters as cold as now, all the harbors of New England and

the Middle Atlantic States would be icebound. The Baltic Sea would also

be frozen each winter, and even the eastern harbors of the British Isles

would be frequently locked in ice. At high latitudes the area of

permanently frozen oceans would be much enlarged. The effect of such a

condition upon marine life in high latitudes would be like that of a

change to a warmer climate. It would protect the life on the continental

shelf from the severe battering of winter storms. It would also lessen

the severity of the winter temperature in the water for when water

freezes it gives up much latent heat,--eighty calories per cubic

centimeter. Part of this raises the temperature of the underlying water.

The expansion of the ice near northern shores would influence the life

of the lands quite differently from that of the oceans. It would act

like an addition of land to the continents and would, therefore,

increase the atmospheric contrasts from zone to zone and from

continental interior to ocean. In summer the ice upon the sea would tend

to keep the coastal lands cool, very much as happens now near the Arctic

Ocean, where the ice floes have a great effect through their reflection

of light and their absorption of heat in melting. In winter the virtual

enlargement of the continents by the addition of an ice fringe would

decrease the snowfall upon the lands. Still more important would be the

effect in intensifying the anti-cyclonic conditions which normally

prevail in winter not only over continents but over ice-covered oceans.

Hence the outblowing cold winds would he strengthened.[98] The net

effect of all these conditions would apparently be a diminution of

snowfall in high latitudes upon the lands even though the summer

snowfall upon the ocean and the coasts may have increased. This

condition may have been one reason why widespread glaciation does not

appear to have prevailed in high latitudes during the Proterozoic and

Permian glaciations, even though it occurred farther south. If the ocean

during those early glacial epochs were ice-covered down to middle

latitudes, a lack of extensive glaciation in high latitudes would be no

more surprising than is the lack of Pleistocene glaciation in the

northern parts of Alaska and Asia. Great ice sheets are impossible

without a large supply of moisture.

(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 deg.C. from the present mean

temperature of 2 deg.C. would alter the ability of the entire ocean to

absorb carbon dioxide by about 4 per cent. This, according to F. W.

Clarke,[99] 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[100] when the partially combined carbon dioxide

is also considered. Moreover, the capacity of water for carbon dioxide

varies sharply with the temperature.[101] Hence a rise in temperature of

only 1 deg.C. would theoretically cause the oceans to give up from 30 to 280

times as much carbon dioxide as the air now holds. This, however, is on

the unfounded assumption that the oceans are completely saturated. The

important point is merely that a slight change in ocean temperature

would cause a disproportionately large change in the amount of carbon

dioxide in the air with all that this implies in respect to blanketing

the earth, and thus altering temperature.

(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.[102]

During past ages, when the sea water was less salty, the circulation was

presumably more rapid than now. This was because, in tropical regions,

the rise of cold water is hindered by the sinking of warm surface water

which is relatively dense because evaporation has removed part of the

water and caused an accumulation of salt. According to Kruemmel and

Mill,[103] 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 deg.C.) to the highest temperatures which ever

prevail in the ocean (30 deg.C. or 86 deg.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 Kruemmel, who detected

it by means of the rise in temperature which it produces at considerable

depths. If such a hindrance to the circulation did not exist, the

velocity of the deep-sea movements would be greater.

If in earlier times a more rapid circulation occurred, low latitudes

must have been cooled more than now by the rise of cold waters. At the

same time higher latitudes were presumably warmed by a greater flow of

warm water from tropical regions because less of the surface heat sank

in low latitudes. Such conditions would tend to lessen the climatic

contrast between the different latitudes. Hence, in so far as the rate

of deep-sea circulation depends upon salinity, the slowly increasing

amount of salt in the oceans must have tended to increase the contrasts

between low and high latitudes. Thus for several reasons, the increase

of salinity during geologic history seems to deserve a place among the

minor agencies which help to explain the apparent tendency toward a

secular progression of climate in the direction of greater contrasts

between tropical and subpolar latitudes.

Changes in the composition and amount of the atmosphere have presumably

had a climatic importance greater than that of changes in the salinity

of the oceans. The atmospheric changes may have been either progressive

or cyclic, or both. In early times, according to the nebular hypothesis,

the atmosphere was much more dense than now and contained a larger

percentage of certain constituents, notably carbon dioxide and water.

The planetesimal hypothesis, on the other hand, postulates an increase

in the density of the atmosphere, for according to this hypothesis the

density of the atmosphere depends upon the power of the earth to hold

gases, and this power increases as the earth grows bigger with the

infall of material from without.[104]

Whichever hypothesis may be correct, it seems probable that when life

first appeared on the land the atmosphere resembled that of today in

certain fundamental respects. It contained the elements essential to

life, and its blanketing effect was such as to maintain temperatures not

greatly different from those of the present. The evidence of this

depends largely upon the narrow limits of temperature within which the

activities of modern life are possible, and upon the cumulative evidence

that ancient life was essentially similar to the types now living. The

resemblance between some of the oldest forms and those of today is

striking. For example, according to Professor Schuchert:[105] "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. Zooelogists who see these

wonderful fossils are at once struck with their modernity and the little

change that has taken place in certain stocks since that far remote

time. Back of the Paleozoic, little can be said of life from the generic

standpoint, since so few fossils have been recovered, but what is at

hand suggests that the marine environment was similar to that of today."

At present, as we have repeatedly seen, little growth takes place either

among animals or plants at temperatures below 0 deg.C. or above 40 deg.C., and

for most species the limiting temperatures are about 10 deg. and 30 deg.. 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 deg.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 near the dawn of earth

history and at several later times, proves that the earth was not

particularly hot even in those early days. The gentle progressive change

of climate which is recorded in the rocks appears to have been only in

slight measure a change in the mean temperature of the earth as a whole,

and almost entirely a change in the distribution of temperature from

place to place and season to season. Hence it seems probable that

neither the earth's own emission of heat, nor the supply of solar heat,

nor the power of the atmosphere to retain heat can have been much

greater a few hundred million years ago than now. It is indeed possible

that these three factors may have varied in such a way that any

variation in one has been offset by variations of the others in the

opposite direction. This, however, is so highly improbable that it seems

advisable to assume that all three have remained relatively constant.

This conclusion together with a realization of the climatic significance

of carbon dioxide has forced most of the adherents of the nebular

hypothesis to abandon their assumption that carbon dioxide, the heaviest

gas in the air, was very abundant until taken out by coal-forming plants

or combined with the calcium oxide of igneous rocks to form the

limestone secreted by animals. In the same way the presence of sun

cracks in sedimentary rocks of all ages suggests that the air cannot

have contained vast quantities of water vapor such as have been assumed

by Knowlton and others in order to account for the former lack of sharp

climatic contrast between the zones. Such a large amount of water vapor

would almost certainly be accompanied by well-nigh universal and

continual cloudiness so that there would be little chance for the pools

on the earth's water-soaked surface to dry up. Furthermore, there is

only one way in which such cloudiness could be maintained and that is by

keeping the air at an almost constant temperature night and day. This

would require that the chief source of warmth be the interior of the

earth, a condition which the Proterozoic, Permian, and other widespread

glaciations seem to disprove.

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;[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 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 their

action. 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, 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 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[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

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,[108] 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,[109] suggests that oxygen was present in the air in quantity even

when the earliest shales and sandstones were formed, for atmospheric

oxygen seems to be the probable source of the extra oxygen they contain.

The formation of these particular sedimentary rocks by weathering of

igneous rocks involves only a little carbon dioxide and water. Although

it seems probable that oxygen was present in the atmosphere even at the

beginning of the geological record, it may have been far less abundant

then than now. It may have been removed from the atmosphere by animals

or by the oxidation of the rocks almost as rapidly as it was added by

volcanoes, plants, and other agencies.

After this chapter was in type, St. John[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 quantities

by breaking down carbon dioxide.

But even if the earth has experienced a notable increase in atmospheric

oxygen since the appearance of life, this does not necessarily involve

important climatic changes except those due to increased atmospheric

density. This is because oxygen has very little effect upon the passage

of light or heat, being transparent to all but a few wave lengths. Those

absorbed are chiefly in the ultra violet.

The distinct possibility that oxygen has increased in amount, makes it

the more likely that there has been an increase in the total atmosphere,

for the oxygen would supplement the increase in the relatively inert

nitrogen and argon, which has presumably taken place. The climatic

effects of an increase in the atmosphere include, in the first place, an

increased scattering of light as it approaches the earth. Nitrogen,

argon, and oxygen all scatter the short waves of light and thus

interfere with their reaching the earth. Abbot and Fowle,[110] who have

carefully studied the matter, believe that at present the scattering is

quantitatively important in lessening insolation. Hence our supposed

general increase in the volume of the air during part of geological

times would tend to reduce the amount of solar energy reaching the

earth's surface. On the other hand, nitrogen and argon do not appear to

absorb the long wave lengths known as heat, and oxygen absorbs so little

as to be almost a non-absorber. Therefore the reduced penetration of the

air by solar radiation due to the scattering of light would apparently

not be neutralized by any direct increase in the blanketing effect of

the atmosphere, and the temperature near the earth's surface would be

slightly lowered by a thicker atmosphere. This would diminish the amount

of water vapor which would be held in the air, and thereby lower the

temperature a trifle more.

In the second place, the higher atmospheric pressure which would result

from the addition of gases to the air would cause a lessening of the

rate of evaporation, for that rate declines as pressure increases.

Decreased evaporation would presumably still further diminish the vapor

content of the atmosphere. This would mean a greater daily and seasonal

range of temperature, as is very obvious when we compare clear weather

with cloudy. Cloudy nights are relatively warm while clear nights are

cool, because water vapor is an almost perfect absorber of radiant heat,

and there is enough of it in the air on moist nights to interfere

greatly with the escape of the heat accumulated during the day.

Therefore, if atmospheric moisture were formerly much more abundant than

now, the temperature must have been much more uniform. The tendency

toward climatic severity as time went on would be still further

increased by the cooling which would result from the increased wind

velocity discussed below; for cooling by convection increases with the

velocity of the wind, as does cooling by conduction.

Any persistent lowering of the general temperature of the air would

affect not only its ability to hold water vapor, but would produce a

lessening in the amount of atmospheric carbon dioxide, for the colder

the ocean becomes the more carbon dioxide it can hold in solution. When

the oceanic temperature falls, part of the atmospheric carbon dioxide is

dissolved in the ocean. This minor constituent of the air is important

because although it forms only 0.003 per cent of the earth's atmosphere,

Abbot and Fowle's[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 1.3 deg.C.,

and a halving would lower it a like amount. The indirect results of such

an increase or decrease might be greater than the direct results, for

the change in temperature due to variations in carbon dioxide would

alter the capacity of the air to hold moisture.

Two conditions would especially help in this respect; first, changes in

nocturnal cooling, and second, changes in local convection. The presence

of carbon dioxide diminishes nocturnal cooling because it absorbs the

heat radiated by the earth, and re-radiates part of it back again. Hence

with increased carbon dioxide and with the consequent warmer nights

there would be less nocturnal condensation of water vapor to form dew

and frost. Local convection is influenced by carbon dioxide because this

gas lessens the temperature gradient. In general, the less the gradient,

that is, the less the contrast between the temperature at the surface

and higher up, the less convection takes place. This is illustrated by

the seasonal variation in convection. In summer, when the gradient is

steepest, convection reaches its maximum. It will be recalled that when

air rises it is cooled by expansion, and if it ascends far the moisture

is soon condensed and precipitated. Indeed, local convection is

considered by C. P. Day to be the chief agency which keeps the lower air

from being continually saturated with moisture. The presence of carbon

dioxide lessens convection because it increases the absorption of heat

in the zone above the level in which water vapor is abundant, thus

warming these higher layers. The lower air may not be warmed

correspondingly by an increase in carbon dioxide if Abbot and Fowle are

right in stating that near the earth's surface there is enough water

vapor to absorb practically all the wave lengths which carbon dioxide is

capable of absorbing. Hence carbon dioxide is chiefly effective at

heights to which the low temperature prevents water vapor from

ascending. Carbon dioxide is also effective in cold winters and in high

latitudes when even the lower air is too cold to contain much water

vapor. Moreover, carbon dioxide, by altering the amount of atmospheric

water vapor, exerts an indirect as well as a direct effect upon


Other effects of the increase in air pressure which we are here assuming

during at least the early part of geological times are corresponding

changes in barometric contrasts, in the strength of winds, and in the

mass of air carried by the winds along the earth's surface. The increase

in the mass of the air would 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

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.


[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.


[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 shores 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


[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.]