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The Uniformity Of Climate

The role of climate in the life of today suggests its importance in the

past and in the future. No human being can escape from the fact that his

food, clothing, shelter, recreation, occupation, health, and energy are

all profoundly influenced by his climatic surroundings. A change of

season brings in its train some alteration in practically every phase of

human activity. Animals are influenced by climate even more than man,
r /> for they have not developed artificial means of protecting themselves.

Even so hardy a creature as the dog becomes notably different with a

change of climate. The thick-haired "husky" of the Eskimos has outwardly

little in common with the small and almost hairless canines that grovel

under foot in Mexico. Plants are even more sensitive than animals and

men. Scarcely a single species can flourish permanently in regions which

differ more than 20 deg.C. in average yearly temperature, and for most the

limit of successful growth is 10 deg..[1] So far as we yet know every living

species of plant and animal, including man, thrives best under definite

and limited conditions of temperature, humidity, and sunshine, and of

the composition and movement of the atmosphere or water in which it

lives. Any departure beyond the limits means lessened efficiency, and in

the long run a lower rate of reproduction and a tendency toward changes

in specific characteristics. Any great departure means suffering or

death for the individual and destruction for the species.

Since climate has so profound an influence on life today, it has

presumably been equally potent at other times. Therefore few scientific

questions are more important than how and why the earth's climate has

varied in the past, and what changes it is likely to undergo in the

future. This book sets forth what appear to be the chief reasons for

climatic variations during historic and geologic times. It assumes that

causes which can now be observed in operation, as explained in a

companion volume entitled Earth and Sun, and in such books as

Humphreys' Physics of the Air, should be carefully studied before less

obvious causes are appealed to. It also assumes that these same causes

will continue to operate, and are the basis of all valid predictions as

to the weather or climate of the future.

In our analysis of climatic variations, we may well begin by inquiring

how the earth's climate has varied during geological history. Such an

inquiry discloses three great tendencies, which to the superficial view

seem contradictory. All, however, have a similar effect in providing

conditions under which organic evolution is able to make progress. The

first tendency is toward uniformity, a uniformity so pronounced and of

such vast duration as to stagger the imagination. Superposed upon this

there seems to be a tendency toward complexity. During the greater part

of geological history the earth's climate appears to have been

relatively monotonous, both from place to place and from season to

season; but since the Miocene the rule has been diversity and

complexity, a condition highly favorable to organic evolution. Finally,

the uniformity of the vast eons of the past and the tendency toward

complexity are broken by pulsatory changes, first in one direction and

then in another. To our limited human vision some of the changes, such

as glacial periods, seem to be waves of enormous proportions, but

compared with the possibilities of the universe they are merely as the

ripples made by a summer zephyr.

The uniformity of the earth's climate throughout the vast stretches of

geological time can best be realized by comparing the range of

temperature on the earth during that period with the possible range as

shown in the entire solar system. As may be seen in Table 1, the

geological record opens with the Archeozoic era, or "Age of Unicellular

Life," as it is sometimes called, for the preceding cosmic time has left

no record that can yet be read. Practically no geologists now believe

that the beginning of the Archeozoic was less than one hundred million

years ago; and since the discovery of the peculiar properties of radium

many of the best students do not hesitate to say a billion or a billion

and a half.[2] Even in the Archeozoic the rocks testify to a climate

seemingly not greatly different from that of the average of geologic

time. The earth's surface was then apparently cool enough so that it was

covered with oceans and warm enough so that the water teemed with

microscopic life. The air must have been charged with water vapor and

with carbon dioxide, for otherwise there seems to be no possible way of

explaining the formation of mudstones and sandstones, limestones of vast

thickness, carbonaceous shales, graphites, and iron ores.[3] Although

the Archeozoic has yielded no generally admitted fossils, yet what seem

to be massive algae and sponges have been found in Canada. On the other

hand, abundant life is believed to have been present in the oceans, for

by no other known means would it be possible to take from the air the

vast quantities of carbon that now form carbonaceous shales and


In the next geologic era, the Proterozoic, the researches of Walcott

have shown that besides the marine algae there must have been many other

kinds of life. The Proterozoic fossils thus far discovered include not

only microscopic radiolarians such as still form the red ooze of the

deepest ocean floors, but the much more significant tubes of annelids or

worms. The presence of the annelids, which are relatively high in the

scale of organization, is generally taken to mean that more lowly forms

of animals such as coelenterates and probably even the mollusca and

primitive arthropods must already have been evolved. That there were

many kinds of marine invertebrates living in the later Proterozoic is

indicated by the highly varied life and more especially the trilobites

found in the oldest Cambrian strata of the next succeeding period. In

fact the Cambrian has sponges, primitive corals, a great variety of

brachiopods, the beginnings of gastropods, a wonderful array of

trilobites, and other lowly forms of arthropods. Since, under the

postulate of evolution, the life of that time forms an unbroken sequence

with that of the present, and since many of the early forms differ only

in minor details from those of today, we infer that the climate then was

not very different from that of today. The same line of reasoning leads

to the conclusion that even in the middle of the Proterozoic, when

multicellular marine animals must already have been common, the climate

of the earth had already for an enormous period been such that all the

lower types of oceanic invertebrates had already evolved.




FORMATIVE ERA. Birth and growth of the earth. Beginnings of the

atmosphere, hydrosphere, continental platforms, oceanic basins,

and possibly of life. No known geological record.


ARCHEOZOIC ERA. Origin of simplest life.

PROTEROZOIC ERA. Age of invertebrate origins. An early and a late

ice age, with one or more additional ones indicated.

PALEOZOIC ERA. Age of primitive vertebrate dominance.

Cambrian Period. First abundance of marine animals and dominance

of trilobites.

Ordovician Period. First known fresh-water fishes.

Silurian Period. First known land plants.

Devonian Period. First known amphibians. "Table Mountain" ice


Mississippian Period. Rise of marine fishes (sharks).

Pennsylvanian Period. Rise of insects and first period of marked

coal accumulation.

Permian Period. Rise of reptiles. Another great ice age.

MESOZOIC ERA. Age of reptile dominance.

Triassic Period. Rise of dinosaurs. The period closes with a

cool climate.

Jurassic Period. Rise of birds and flying reptiles.

Comanchean Period. Rise of flowering plants and higher insects.

Cretaceous Period. Rise of archaic or primitive mammalia.

CENOZOIC ERA. Age of mammal dominance.

Early Cenozoic or Eocene and Oligocene time. Rise of higher

mammals. Glaciers in early Eocene of the Laramide Mountains.

Late Cenozoic or Miocene and Pliocene time. Transformation of

ape like animals into man.

Glacial or Pleistocene time. Last great ice age.


PSYCHOZOIC ERA. Age of man or age of reason. Includes the present or

"Recent time," estimated to be probably less than 30,000 years.

Moreover, they could live in most latitudes, for the indirect evidences

of life in the Archeozoic and Proterozoic rocks are widely distributed.

Thus it appears that at an almost incredibly early period, perhaps many

hundred million years ago, the earth's climate differed only a little

from that of the present.

The extreme limits of temperature beyond which the climate of geological

times cannot have departed can be approximately determined. Today the

warmest parts of the ocean have an average temperature of about 30 deg.C. on

the surface. Only a few forms of life live where the average temperature

is much higher than this. In deserts, to be sure, some highly organized

plants and animals can for a short time endure a temperature as high as

75 deg.C. (167 deg.F.). In certain hot springs, some of the lowest unicellular

plant forms exist in water which is only a little below the boiling

point. More complex forms, however, such as sponges, worms, and all the

higher plants and animals, seem to be unable to live either in water or

air where the temperature averages above 45 deg.C. (113 deg.F.) for any great

length of time and it is doubtful whether they can thrive permanently

even at that temperature. The obvious unity of life for hundreds of

millions of years and its presence at all times in middle latitudes so

far as we can tell seem to indicate that since the beginning of marine

life the temperature of the oceans cannot have averaged much above 50 deg.C.

even in the warmest portions. This is putting the limit too high rather

than too low, but even so the warmest parts of the earth can scarcely

have averaged much more than 20 deg. warmer than at present.

Turning to the other extreme, we may inquire how much colder than now

the earth's surface may have been since life first appeared. Proterozoic

fossils have been found in places where the present average temperature

approaches 0 deg.C. If those places should be colder than now by 30 deg.C., or

more, the drop in temperature at the equator would almost certainly be

still greater, and the seas everywhere would be permanently frozen. Thus

life would be impossible. Since the contrasts between summer and winter,

and between the poles and the equator seem generally to have been less

in the past than at present, the range through which the mean

temperature of the earth as a whole could vary without utterly

destroying life was apparently less than would now be the case.

These considerations make it fairly certain that for at least several

hundred million years the average temperature of the earth's surface has

never varied more than perhaps 30 deg.C. above or below the present level.

Even this range of 60 deg.C. (108 deg.F.) may be double or triple the range that

has actually occurred. That the temperature has not passed beyond

certain narrow limits, whatever their exact degree, is clear from the

fact that if it had done so, all the higher forms of life would have

been destroyed. Certain of the lowest unicellular forms might indeed

have persisted, for when dormant they can stand great extremes of dry

heat and of cold for a long time. Even so, evolution would have had to

begin almost anew. The supposition that such a thing has happened is

untenable, for there is no hint of any complete break in the record of

life during geological times,--no sudden disappearance of the higher

organisms followed by a long period with no signs of life other than

indirect evidence such as occurs in the Archeozoic.

A change of 60 deg.C. or even of 20 deg. in the average temperature of the

earth's surface may seem large when viewed from the limited standpoint

of terrestrial experience. Viewed, however, from the standpoint of

cosmic evolution, or even of the solar system, it seems a mere trifle.

Consider the possibilities. The temperature of empty space is the

absolute zero, or -273 deg.C. To this temperature all matter must fall,

provided it exists long enough and is not appreciably heated by

collisions or by radiation. At the other extreme lies the temperature of

the stars. As stars go, our sun is only moderately hot, but the

temperature of its surface is calculated to be nearly 7000 deg.C., while

thousands of miles in the interior it may rise to 20,000 deg. or 100,000 deg. or

some other equally unknowable and incomprehensible figure. Between the

limits of the absolute zero on the one hand, and the interior of a sun

or star on the other, there is almost every conceivable possibility of

temperature. Today the earth's surface averages not far from 14 deg.C., or

287 deg. above the absolute zero. Toward the interior, the temperature in

mines and deep wells rises about 1 deg.C. for every 100 meters. At this rate

it would be over 500 deg.C. at a depth of ten miles, and over 5000 deg. at 100


Let us confine ourselves to surface temperatures, which are all that

concern us in discussing climate. It has been calculated by Poynting[5]

that if a small sphere absorbed and re-radiated all the heat that fell

upon it, its temperature at the distance of Mercury from the sun would

average about 210 deg.C.; at the distance of Venus, 85 deg.; the earth 27 deg.; Mars

-30 deg.; Neptune -219 deg.. A planet much nearer the sun than is Mercury might

be heated to a temperature of a thousand, or even several thousand,

degrees, while one beyond Neptune would remain almost at absolute zero.

It is well within the range of possibility that the temperature of a

planet's surface should be anywhere from near -273 deg.C. up to perhaps

5000 deg.C. or more, although the probability of low temperature is much

greater than of high. Thus throughout the whole vast range of

possibilities extending to perhaps 10,000 deg., the earth claims only 60 deg. at

most, or less than 1 per cent. This may be remarkable, but what is far

more remarkable is that the earth's range of 60 deg. includes what seem to

be the two most critical of all possible temperatures, namely, the

freezing point of water, 0 deg.C., and the temperature where water can

dissolve an amount of carbon dioxide equal to its own volume. The most

remarkable fact of all is that the earth has preserved its temperature

within these narrow limits for a hundred million years, or perchance a

thousand million.

To appreciate the extraordinary significance of this last fact, it is

necessary to realize how extremely critical are the temperatures from

about 0 deg. to 40 deg.C., and how difficult it is to find any good reason for a

relatively uniform temperature through hundreds of millions of years.

Since the dawn of geological time the earth's temperature has apparently

always included the range from about the freezing point of water up to

about the point where protoplasm begins to disintegrate. Henderson, in

The Fitness of the Environment, rightly says that water is "the most

familiar and the most important of all things." In many respects water

and carbon dioxide form the most unique pair of substances in the whole

realm of chemistry. Water has a greater tendency than any other known

substance to remain within certain narrowly defined limits of

temperature. Not only does it have a high specific heat, so that much

heat is needed to raise its temperature, but on freezing it gives up

more heat than any substance except ammonia, while none of the common

liquids approach it in the amount of additional heat required for

conversion into vapor after the temperature of vaporization has been

reached. Again, water substance, as the physicists call all forms of

H{2}O, is unique in that it not only contracts on melting, but

continues to contract until a temperature several degrees above its

melting point is reached. That fact has a vast importance in helping to

keep the earth's surface at a uniform temperature. If water were like

most liquids, the bottoms of all the oceans and even the entire body of

water in most cases would be permanently frozen.

Again, as a solvent there is literally nothing to compare with water. As

Henderson[6] puts it: "Nearly the whole science of chemistry has been

built up around water and aqueous solution." One of the most significant

evidences of this is the variety of elements whose presence can be

detected in sea water. According to Henderson they include hydrogen,

oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur,

phosphorus, which are easily detected; and also arsenic, caesium, gold,

lithium, rubidium, barium, lead, boron, fluorine, iron, iodine, bromine,

potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc,

aluminium, calcium, and strontium. Yet in spite of its marvelous power

of solution, water is chemically rather inert and relatively stable. It

dissolves all these elements and thousands of their compounds, but still

remains water and can easily be separated and purified. Another unique

property of water is its power of ionizing dissolved substances, a

property which makes it possible to produce electric currents in

batteries. This leads to an almost infinite array of electro-chemical

reactions which play an almost dominant role in the processes of life.

Finally, no common liquid except mercury equals water in its power of

capillarity. This fact is of enormous moment in biology, most obviously

in respect to the soil.

Although carbon dioxide is far less familiar than water, it is almost as

important. "These two simple substances," says Henderson, "are the

common source of every one of the complicated substances which are

produced by living beings, and they are the common end products of the

wearing away of all the constituents of protoplasm, and of the

destruction of those materials which yield energy to the body." One of

the remarkable physical properties of carbon dioxide is its degree of

solubility in water. This quality varies enormously in different

substances. For example, at ordinary pressures and temperatures, water

can absorb only about 5 per cent of its own volume of oxygen, while it

can take up about 1300 times its own volume of ammonia. Now for carbon

dioxide, unlike most gases, the volume that can be absorbed by water is

nearly the same as the volume of the water. The volumes vary, however,

according to temperature, being absolutely the same at a temperature of

about 15 deg.C. or 59 deg.F., which is close to the ideal temperature for man's

physical health and practically the same as the mean temperature of the

earth's surface when all seasons are averaged together. "Hence, when

water is in contact with air, and equilibrium has been established, the

amount of free carbonic acid in a given volume of water is almost

exactly equal to the amount in the adjacent air. Unlike oxygen,

hydrogen, and nitrogen, carbonic acid enters water freely; unlike

sulphurous oxide and ammonia, it escapes freely from water. Thus the

waters can never wash carbonic acid completely out of the air, nor can

the air keep it from the waters. It is the one substance which thus, in

considerable quantities relative to its total amount, everywhere

accompanies water. In earth, air, fire, and water alike these two

substances are always associated.

"Accordingly, if water be the first primary constituent of the

environment, carbonic acid is inevitably the second,--because of its

solubility possessing an equal mobility with water, because of the

reservoir of the atmosphere never to be depleted by chemical action in

the oceans, lakes, and streams. In truth, so close is the association

between these two substances that it is scarcely correct logically to

separate them at all; together they make up the real environment and

they never part company."[7]

The complementary qualities of carbon dioxide and water are of supreme

importance because these two are the only known substances which are

able to form a vast series of complex compounds with highly varying

chemical formulae. No other known compounds can give off or take on atoms

without being resolved back into their elements. No others can thus

change their form freely without losing their identity. This power of

change without destruction is the fundamental chemical characteristic of

life, for life demands complexity, change, and growth.

In order that water and carbon dioxide may combine to form the compounds

on which life is based, the water must be in the liquid form, it must be

able to dissolve carbon dioxide freely, and the temperature must not be

high enough to break up the highly complex and delicate compounds as

soon as they are formed. In other words, the temperature must be above

freezing, while it must not rise higher than some rather indefinite

point between 50 deg.C. and the boiling point, where all water finally turns

into vapor. In the whole range of temperature, so far as we know, there

is no other interval where any such complex reactions take place. The

temperature of the earth for hundreds of millions of years has remained

firmly fixed within these limits.

The astonishing quality of the earth's uniformity of temperature becomes

still more apparent when we consider the origin of the sun's heat. What

that origin is still remains a question of dispute. The old ideas of a

burning sun, or of one that is simply losing an original supply of heat

derived from some accident, such as collision with another body, were

long ago abandoned. The impact of a constant supply of meteors affords

an almost equally unsatisfactory explanation. Moulton[8] states that if

the sun were struck by enough meteorites to keep up its heat, the earth

would almost certainly be struck by enough so that it would receive

about half of 1 per cent as much heat from them as from the sun. This is

millions of times more heat than is now received from meteors. If the

sun owes its heat to the impact of larger bodies at longer intervals,

the geological record should show a series of interruptions far more

drastic than is actually the case.

It has also been supposed that the sun owes its heat to contraction. If

a gaseous body contracts it becomes warmer. Finally, however, it must

become so dense that its rate of contraction diminishes and the process

ceases. Under the sun's present condition of size and density a radial

contraction of 120 feet per year would be enough to supply all the

energy now radiated by that body. This seems like a hopeful source of

energy, but Kelvin calculated that twenty million years ago it was

ineffective and ten million years hence it will be equally so. Moreover,

if this is the source of heat, the amount of radiation from the sun

would have to vary enormously. Twenty million years ago the sun would

have extended nearly to the earth's orbit and would have been so tenuous

that it would have emitted no more heat than some of the nebulae in

space. Some millions of years later, when the sun's radius was twice as

great as at present, that body would have emitted only one-fourth as

much heat as now, which would mean that on the earth's surface the

theoretical temperature would have been 200 deg. below the present level.

This is utterly out of accord with the uniformity of climate shown by

the geological record. In the future, if the sun's contraction is the

only source of heat, the sun can supply the present amount for only ten

million years, which would mean a change utterly unlike anything of

which the geological record holds even the faintest hint.[9]

Altogether the problem of how the sun can have remained so uniform and

how the earth's atmosphere and other conditions can also have remained

so uniform throughout hundreds of millions of years is one of the most

puzzling in the whole realm of nature. If appeal is taken to

radioactivity and the breaking up of uranium into radium and helium,

conditions can be postulated which will give the required amount of

energy. Such is also the case if it be supposed that there is some

unknown process which may induce an atomic change like radioactivity in

bodies which are now supposed to be stable elements. In either case,

however, there is as yet no satisfactory explanation of the uniformity

of the earth's climate. A hundred million or a thousand million years

ago the temperature of the earth's surface was very much the same as

now. The earth had then presumably ceased to emit any great amount of

heat, if we may judge from the fact that its surface was cool enough so

that great ice sheets could accumulate on low lands within 40 deg. of the

equator. The atmosphere was apparently almost like that of today, and

was almost certainly not different enough to make up for any great

divergence of the sun from its present condition. We cannot escape the

stupendous fact that in those remote times the sun must have been

essentially the same as now, or else that some utterly unknown factor is

at work.


[Footnote 1: W. A. Setchell: The Temperature Interval in the

Geographical Distribution of Marine Algae; Science, Vol. 52, 1920,

p. 187.]

[Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time;

Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.]

[Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp.


[Footnote 4: From Charles Schuchert in The Evolution of the Earth and

Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with

revisions by Professor Schuchert.]

[Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans.

A, 1903, 202, p. 525.]

[Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.]

[Footnote 7: Henderson: loc. cit., p. 138.]

[Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.]

[Footnote 9: Moulton: loc. cit.]