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The Variability Of Climate

The variability of the earth's climate is almost as extraordinary as its

uniformity. This variability is made up partly of a long, slow tendency

in one direction and partly of innumerable cycles of every conceivable

duration from days, or even hours, up to millions of years. Perhaps the

easiest way to grasp the full complexity of the matter is to put the

chief types of climatic sequence in the form of a table.

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1. Cosmic uniformity. 7. Brueckner periods.

2. Secular progression. 8. Sunspot cycles.

3. Geologic oscillations. 9. Seasonal alternations.

4. Glacial fluctuations. 10. Pleionian migrations.

5. Orbital precessions. 11. Cyclonic vacillations.

6. Historical pulsations. 12. Daily vibrations.


In assigning names to the various types an attempt has been made to

indicate something of the nature of the sequence so far as duration,

periodicity, and general tendencies are concerned. Not even the rich

English language of the twentieth century, however, furnishes words with

enough shades of meaning to express all that is desired. Moreover,

except in degree, there is no sharp distinction between some of the

related types, such as glacial fluctuations and historic pulsations.

Yet, taken as a whole, the table brings out the great contrast between

two absolutely diverse extremes. At the one end lies well-nigh eternal

uniformity, or an extremely slow progress in one direction throughout

countless ages; at the other, rapid and regular vibrations from day to

day, or else irregular and seemingly unsystematic vacillations due to

cyclonic storms, both of which types are repeated millions of times

during even a single glacial fluctuation.

The meaning of cosmic uniformity has been explained in the preceding

chapter. Its relation to the other types of climatic sequences seems to

be that it sets sharply defined limits beyond which no changes of any

kind have ever gone since life, as we know it, first began. Secular

progression, on the other hand, means that in spite of all manner of

variations, now this way and then the other, the normal climate of the

earth, if there is such a thing, has on the whole probably changed a

little, perhaps becoming more complex. After each period of continental

uplift and glaciation--for such are preeminently the times of

complexity--it is doubtful whether the earth has ever returned to quite

its former degree of monotony. Today the earth has swung away from the

great diversity of the glacial period. Yet we still have contrasts of

what seem to us great magnitude. In low depressions, such as Turfan in

the central deserts of Eurasia, the thermometer sometimes ranges from

0 deg.F. in the morning to 60 deg. in the shade at noon. On a cloudy day in the

Amazon forest close to the seashore, on the contrary, the temperature

for months may rise to 85 deg. by day and sink no lower than 75 deg. at night.

The reasons for the secular progression of the earth's climate appear to

be intimately connected with those which have caused the next, and, in

many respects, more important type of climatic sequence, which consists

of geological oscillations. Both the progression and the oscillations

seem to depend largely on three purely terrestrial factors: first, the

condition of the earth's interior, including both internal heat and

contraction; second, the salinity and movement of the ocean; and third,

the composition and amount of the atmosphere. To begin with the earth's

interior--its loss of heat appears to be an almost negligible factor in

explaining either secular progression or geologic oscillation. According

to both the nebular and the planetesimal hypotheses, the earth's crust

appears to be colder now than it was hundreds or thousands of millions

of years ago. The emission of internal heat, however, had probably

ceased to be of much climatic significance near the beginning of the

geological record, for in southern Canada glaciation occurred very early

in the Proterozoic era. On the other hand, the contraction of the earth

has produced remarkable effects throughout the whole of geological time.

It has lessened the earth's circumference by a thousand miles or more,

as appears from the way in which the rocks have been folded and thrust

bodily over one another. According to the laws of dynamics this must

have increased the speed of the earth's rotation, thus shortening the

day, and also having the more important effect of increasing the bulge

at the equator. On the other hand, recent investigations indicate that

tidal retardation has probably diminished the earth's rate of rotation

more than seemed probable a few years ago, thus lengthening the day and

diminishing the bulge at the equator. Thus two opposing forces have been

at work, one causing acceleration and one retardation. Their combined

effect may have been a factor in causing secular progression of climate.

It almost certainly was of much importance in causing pronounced

oscillations first one way and then the other. This matter, together

with most of those touched in these first chapters, will be expanded in

later parts of the book. On the whole the tendency appears to have been

to create climatic diversity in place of uniformity.

The increasing salinity of the oceans may have been another factor in

producing secular progression, although of slight importance in respect

to oscillations. While the oceans were still growing in volume, it is

generally assumed that they must have been almost fresh for a vast

period, although Chamberlin thinks that the change in salinity has been

much less than is usually supposed. So far as the early oceans were

fresher than those of today, their deep-sea circulation must have been

less hampered than now by the heavy saline water which is produced by

evaporation in warm regions. Although this saline water is warm, its

weight causes it to descend, instead of moving poleward in a surface

current; this descent slows up the rise of the cold water which has

moved along in the depths of the ocean from high latitudes, and thus

checks the general oceanic circulation. If the ancient oceans were

fresher and hence had a freer circulation than now, a more rapid

interchange of polar and equatorial water presumably tended to equalize

the climate of all latitudes.

Again, although the earth's atmosphere has probably changed far less

during geological times than was formerly supposed, its composition has

doubtless varied. The total volume of nitrogen has probably increased,

for that gas is so inert that when it once becomes a part of the air it

is almost sure to stay there. On the other hand, the proportions of

oxygen, carbon dioxide, and water vapor must have fluctuated. Oxygen is

taken out constantly by animals and by all the processes of rock

weathering, but on the other hand the supply is increased when plants

break up new carbon dioxide derived from volcanoes. As for the carbon

dioxide, it appears probable that in spite of the increased supply

furnished by volcanoes the great amounts of carbon which have gradually

been locked up in coal and limestone have appreciably depleted the

atmosphere. Water vapor also may be less abundant now than in the past,

for the presence of carbon dioxide raises the temperature a little and

thereby enables the air to hold more moisture. When the area of the

oceans has diminished, and this has recurred very often, this likewise

would tend to reduce the water vapor. Moreover, even a very slight

diminution in the amount of heat given off by the earth, or a decrease

in evaporation because of higher salinity in the oceans, would tend in

the same direction. Now carbon dioxide and water vapor both have a

strong blanketing effect whereby heat is prevented from leaving the

earth. Therefore, the probable reduction in the carbon dioxide and water

vapor of the earth's atmosphere has apparently tended to reduce the

climatic monotony and create diversity and complexity. Hence, in spite

of many reversals, the general tendency of changes, not only in the

earth's interior and in the oceans, but also in the atmosphere, appears

to be a secular progression from a relatively monotonous climate in

which the evolution of higher organic forms would scarcely be rapid to

an extremely diverse and complex climate highly favorable to progressive

evolution. The importance of these purely terrestrial agencies must not

be lost sight of when we come to discuss other agencies outside the


In Table 2 the next type of climatic sequence is geologic oscillation.

This means slow swings that last millions of years. At one extreme of

such an oscillation the climate all over the world is relatively

monotonous; it returns, as it were, toward the primeval conditions at

the beginning of the secular progression. At such times magnolias,

sequoias, figs, tree ferns, and many other types of subtropical plants

grew far north in places like Greenland, as is well known from their

fossil remains of middle Cenozoic time, for example. At these same

times, and also at many others before such high types of plants had

evolved, reef-making corals throve in great abundance in seas which

covered what is now Wisconsin, Michigan, Ontario, and other equally cool

regions. Today these regions have an average temperature of only about

70 deg.F. in the warmest month, and average well below freezing in winter.

No reef-making corals can now live where the temperature averages below

68 deg.F. The resemblance of the ancient corals to those of today makes it

highly probable that they were equally sensitive to low temperature.

Thus, in the mild portions of a geologic oscillation the climate seems

to have been so equable and uniform that many plants and animals could

live 1500 and at other times even 4000 miles farther from the equator

than now.

At such times the lands in middle and high latitudes were low and small,

and the oceans extended widely over the continental platforms. Thus

unhampered ocean currents had an opportunity to carry the heat of low

latitudes far toward the poles. Under such conditions, especially if the

conception of the great subequatorial continent of Gondwana land is

correct, the trade winds and the westerlies must have been stronger and

steadier than now. This would not only enable the westerlies, which are

really southwesterlies, to carry more heat than now to high latitudes,

but would still further strengthen the ocean currents. At the same time,

the air presumably contained an abundance of water vapor derived from

the broad oceans, and an abundance of atmospheric carbon dioxide

inherited from a preceding time when volcanoes contributed much carbon

dioxide to the air. These two constituents of the atmosphere may have

exercised a pronounced blanketing effect whereby the heat of the earth

with its long wave lengths was kept in, although the energy of the sun

with its shorter wave lengths was not markedly kept out. Thus everything

may have combined to produce mild conditions in high latitudes, and to

diminish the contrast between equator and pole, and between summer and


Such conditions perhaps carry in themselves the seeds of decay. At any

rate while the lands lie quiet during a period of mild climate great

strains must accumulate in the crust because of the earth's contraction

and tidal retardation. At the same time the great abundance of plants

upon the lowlying plains with their mild climates, and the marine

creatures upon the broad continental platforms, deplete the atmospheric

carbon dioxide. Part of this is locked up as coal and part as limestone

derived from marine plants as well as animals. Then something happens so

that the strains and stresses of the crust are released. The sea floors

sink; the continents become relatively high and large; mountain ranges

are formed; and the former plains and emergent portions of the

continental platforms are eroded into hills and valleys. The large size

of the continents tends to create deserts and other types of climatic

diversity; the presence of mountain ranges checks the free flow of winds

and also creates diversity; the ocean currents are likewise checked,

altered, and diverted so that the flow of heat from low to high

latitudes is diminished. At the same time evaporation from the ocean

diminishes so that a decrease in water vapor combines with the previous

depletion of carbon dioxide to reduce the blanketing effect of the

atmosphere. Thus upon periods of mild monotony there supervene periods

of complexity, diversity, and severity. Turn to Table 1 and see how a

glacial climate again and again succeeds a time when relative mildness

prevailed almost everywhere. Or examine Fig. 1 and notice how the lines

representing temperatures go up and down. In the figure Schuchert makes

it clear that when the lands have been large and mountain-making has

been important, as shown by the high parts of the lower shaded area, the

climate has been severe, as shown by the descent of the snow line, the

upper shaded area. In the diagram the climatic oscillations appear

short, but this is merely because they have been crowded together,

especially in the left hand or early part. There an inch in length may

represent a hundred million years. Even at the right-hand end an inch is

equivalent to several million years.

The severe part of a climatic oscillation, as well as the mild part,

will be shown in later chapters to bear in itself certain probable seeds

of decay. While the lands are being uplifted, volcanic activity is

likely to be vigorous and to add carbon dioxide to the air. Later, as

the mountains are worn down by the many agencies of water, wind, ice,

and chemical decay, although much carbon dioxide is locked up by the

carbonation of the rocks, the carbon locked up in the coal is set free

and increases the carbon dioxide of the air. At the same time the

continents settle slowly downward, for the earth's crust though rigid as

steel is nevertheless slightly viscous and will flow if subjected to

sufficiently great and enduring pressure. The area from which

evaporation can take place is thereby increased because of the spread of

the oceans over the continents, and water vapor joins with the carbon

dioxide to blanket the earth and thus tends to keep it uniformly warm.

Moreover, the diminution of the lands frees the ocean currents from

restraint and permits them to flow more freely from low latitudes to

high. Thus in the course of millions of years there is a return toward

monotony. Ultimately, however, new stresses accumulate in the earth's

crust, and the way is prepared for another great oscillation. Perhaps

the setting free of the stresses takes place simply because the strain

at last becomes irresistible. It is also possible, as we shall see, that

an external agency sometimes adds to the strain and thereby determines

the time at which a new oscillation shall begin.

In Table 2 the types of climatic sequences which follow "geologic

oscillations" are "glacial fluctuations," "orbital precessions" and

"historical pulsations." Glacial fluctuations and historical pulsations

appear to be of the same type, except as to severity and duration, and

hence may be considered together. They will be treated briefly here

because the theories as to their causes are outlined in the next two

chapters. Oddly enough, although the historic pulsations lie much closer

to us than do the glacial fluctuations, they were not discovered until

two or three generations later, and are still much less known. The most

important feature of both sequences is the swing from a glacial to an

inter-glacial epoch or from the arsis or accentuated part of an

historical pulsation to the thesis or unaccented part. In a glacial

epoch or in the arsis of an historic pulsation, storms are usually

abundant and severe, the mean temperature is lower than usual, snow

accumulates in high latitudes or upon lofty mountains. For example, in

the last such period during the fourteenth century, great floods and

droughts occurred alternately around the North Sea; it was several times

possible to cross the Baltic Sea from Germany to Sweden on the ice, and

the ice of Greenland advanced so much that shore ice caused the Norsemen

to change their sailing route between Iceland and the Norse colonies in

southern Greenland. At the same time in low latitudes and in parts of

the continental interior there is a tendency toward diminished rainfall

and even toward aridity and the formation of deserts. In Yucatan, for

example, a diminution in tropical rainfall in the fourteenth century

seems to have given the Mayas a last opportunity for a revival of their

decaying civilization.

(After Schuchert, in The Evolution of the Earth and Its Inhabitants,

edited by R. S. Lull.) Diagram showing the times and probable extent of

the more or less marked climate changes in the geologic history of North

America, and of its elevation into chains of mountains.]

Among the climatic sequences, glacial fluctuations are perhaps of the

most vital import from the standpoint of organic evolution; from the

standpoint of human history the same is true of climatic pulsations.

Glacial epochs have repeatedly wiped out thousands upon thousands of

species and played a part in the origin of entirely new types of plants

and animals. This is best seen when the life of the Pennsylvanian is

contrasted with that of the Permian. An historic pulsation may wipe out

an entire civilization and permit a new one to grow up with a radically

different character. Hence it is not strange that the causes of such

climatic phenomena have been discussed with extraordinary vigor. In few

realms of science has there been a more imposing or more interesting

array of theories. In this book we shall consider the more important of

these theories. A new solar or cyclonic hypothesis and the hypothesis of

changes in the form and altitude of the land will receive the most

attention, but the other chief hypotheses are outlined in the next

chapter, and are frequently referred to throughout the volume.

Between glacial fluctuations and historical pulsations in duration, but

probably less severe than either, come orbital precessions. These stand

in a group by themselves and are more akin to seasonal alternations than

to any other type of climatic sequence. They must have occurred with

absolute regularity ever since the earth began to revolve around the sun

in its present elliptical orbit. Since the orbit is elliptical and since

the sun is in one of the two foci of the ellipse, the earth's distance

from the sun varies. At present the earth is nearest the sun in the

northern winter. Hence the rigor of winter in the northern hemisphere is

mitigated, while that of the southern hemisphere is increased. In about

ten thousand years this condition will be reversed, and in another ten

thousand the present conditions will return once more. Such climatic

precessions, as we may here call them, must have occurred unnumbered

times in the past, but they do not appear to have been large enough to

leave in the fossils of the rocks any traces that can be distinguished

from those of other climatic sequences.

We come now to Brueckner periods and sunspot cycles. The Brueckner periods

have a length of about thirty-three years. Their existence was suggested

at least as long ago as the days of Sir Francis Bacon, whose statement

about them is quoted on the flyleaf of this book. They have since been

detected by a careful study of the records of the time of harvest,

vintage, the opening of rivers to navigation, and the rise or fall of

lakes like the Caspian Sea. In his book on Klimaschwankungen seit

1700, Brueckner has collected an uncommonly interesting assortment of

facts as to the climate of Europe for more than two centuries. More

recently, by a study of the rate of growth of trees, Douglass, in his

book on Climatic Cycles and Tree Growth, has carried the subject still

further. In general the nature of the 33-year periods seems to be

identical with that of the 11- or 12- year sunspot cycle, on the one

hand, and of historic pulsations on the other. For a century observers

have noted that the variations in the weather which everyone notices

from year to year seem to have some relation to sunspots. For

generations, however, the relationship was discussed without leading to

any definite conclusion. The trouble was that the same change was

supposed to take place in all parts of the world. Hence, when every sort

of change was found somewhere at any given sunspot stage, it seemed as

though there could not be a relationship. Of late years, however, the

matter has become fairly clear. The chief conclusions are, first, that

when sunspots are numerous the average temperature of the earth's

surface is lower than normal. This does not mean that all parts are

cooler, for while certain large areas grow cool, others of less extent

become warm at times of many sunspots. Second, at times of many sunspots

storms are more abundant than usual, but are also confined somewhat

closely to certain limited tracks so that elsewhere a diminution of

storminess may be noted. This whole question is discussed so fully in

Earth and Sun that it need not detain us further in this preliminary

view of the whole problem of climate. Suffice it to say that a study of

the sunspot cycle leads to the conclusion that it furnishes a clue to

many of the unsolved problems of the climate of the past, as well as a

key to prediction of the future.

Passing by the seasonal alternations which are fully explained as the

result of the revolution of the earth around the sun, we may merely

point out that, like the daily vibrations which bring Table 2 to a

close, they emphasize the outstanding fact that the main control of

terrestrial climate is the amount of energy received from the sun. This

same principle is illustrated by pleionian migrations. The term "pleion"

comes from a Greek word meaning "more." It was taken by Arctowski to

designate areas or periods where there is an excess of some climatic

element, such as atmospheric pressure, rainfall, or temperature. Even if

the effect of the seasons is eliminated, it appears that the course of

these various elements does not run smoothly. As everyone knows, a

period like the autumn of 1920 in the eastern United States may be

unusually warm, while a succeeding period may be unseasonably cool.

These departures from the normal show a certain rough periodicity. For

example, there is evidence of a period of about twenty-seven days,

corresponding to the sun's rotation and formerly supposed to be due to

the moon's revolution which occupies almost the same length of time.

Still other periods appear to have an average duration of about three

months and of between two and three years. Two remarkable discoveries

have recently been made in respect to such pleions. One is that a given

type of change usually occurs simultaneously in a number of well-defined

but widely separated centers, while a change of an opposite character

arises in another equally well-defined, but quite different, set of

centers. In general, areas of high pressure have one type of change and

areas of low pressure the other type. So systematic are these

relationships and so completely do they harmonize in widely separated

parts of the earth, that it seems certain that they must be due to some

outside cause, which in all probability can be only the sun. The second

discovery is that pleions, when once formed, travel irregularly along

the earth's surface. Their paths have not yet been worked out in detail,

but a general migration seems well established. Because of this, it is

probable that if unusually warm weather prevails in one part of a

continent at a given time, the "thermo-pleion," or excess of heat, will

not vanish but will gradually move away in some particular direction. If

we knew the path that it would follow we might predict the general

temperature along its course for some months in advance. The paths are

often irregular, and the pleions frequently show a tendency to break up

or suddenly revive. Probably this tendency is due to variations in the

sun. When the sun is highly variable, the pleions are numerous and

strong, and extremes of weather are frequent. Taken as a whole the

pleions offer one of the most interesting and hopeful fields not only

for the student of the causes of climatic variations, but for the man

who is interested in the practical question of long-range weather

forecasts. Like many other climatic phenomena they seem to represent the

combined effect of conditions in the sun and upon the earth itself.

The last of the climatic sequences which require explanation is the

cyclonic vacillations. These are familiar to everyone, for they are the

changes of weather which occur at intervals of a few days, or a week or

two, at all seasons, in large parts of the United States, Europe, Japan,

and some of the other progressive parts of the earth. They do not,

however, occur with great frequency in equatorial regions, deserts, and

many other regions. Up to the end of the last century, it was generally

supposed that cyclonic storms were purely terrestrial in origin. Without

any adequate investigation it was assumed that all irregularities in the

planetary circulation of the winds arise from an irregular distribution

of heat due to conditions within or upon the earth itself. These

irregularities were supposed to produce cyclonic storms in certain

limited belts, but not in most parts of the world. Today this view is

being rapidly modified. Undoubtedly, the irregularities due to purely

terrestrial conditions are one of the chief contributory causes of

storms, but it begins to appear that solar variations also play a part.

It has been found, for example, that not only the mean temperature of

the earth's surface varies in harmony with the sunspot cycle, but that

the frequency and severity of storms vary in the same way. Moreover, it

has been demonstrated that the sun's radiation is not constant, but is

subject to innumerable variations. This does not mean that the sun's

general temperature varies, but merely that at some times heated gases

are ejected rapidly to high levels so that a sudden wave of energy

strikes the earth. Thus, the present tendency is to believe that the

cyclonic variations, the changes of weather which come and go in such a

haphazard, irresponsible way, are partly due to causes pertaining to the

earth itself and partly to the sun.

From this rapid survey of the types of climatic sequences, it is evident

that they may be divided into four great groups. First comes cosmic

uniformity, one of the most marvelous and incomprehensible of all known

facts. We simply have no explanation which is in any respect adequate.

Next come secular progression and geologic oscillations, two types of

change which seem to be due mainly to purely terrestrial causes, that

is, to changes in the lands, the oceans, and the air. The general

tendency of these changes is toward complexity and diversity, thus

producing progression, but they are subject to frequent reversals which

give rise to oscillations lasting millions of years. The processes by

which the oscillations take place are fully discussed in this book.

Nevertheless, because they are fairly well understood, they are deferred

until after the third group of sequences has been discussed. This group

includes glacial fluctuations, historic pulsations, Brueckner periods,

sunspot cycles, pleionian migrations, and cyclonic vacillations. The

outstanding fact in regard to all of these is that while they are

greatly modified by purely terrestrial conditions, they seem to owe

their origin to variations in the sun. They form the chief subject of

Earth and Sun and in their larger phases are the most important topic

of this book also. The last group of sequences includes orbital

precessions, seasonal alternations, and daily variations. These may be

regarded as purely solar in origin. Yet their influence, like that of

each of the other groups, is much modified by the earth's own

conditions. Our main problem is to separate and explain the two great

elements in climatic changes,--the effects of the sun, on the one hand,

and of the earth on the other.