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,
<
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
graphite.
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.
TABLE 1
THE GEOLOGICAL TIME TABLE[4]
COSMIC TIME
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.
GEOLOGIC TIME
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
age.
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.
PRESENT TIME
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
miles.
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.
FOOTNOTES:
[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.
538-550.]
[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.]
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