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Causes Of Mild Geological Climates

Hypotheses Of Climatic Change

Some Problems Of Glacial Periods

The Climate Of History

The Variability Of Climate

The Climatic Stress Of The Fourteenth Century

The Uniformity Of Climate

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Origin Of Loess

Least Viewed

The Sun's Journey Through Space

The Changing Composition Of Oceans And Atmosphere

Terrestrial Causes Of Climatic Changes

Post-glacial Crustal Movements And Climatic Changes

The Earth's Crust And The Sun

The Effect Of Other Bodies On The Sun

The Solar Cyclonic Hypothesis

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Origin Of Loess

The Uniformity Of Climate

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.]

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