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The Climate Of History

We are now prepared to consider the climate of the past. The first

period to claim attention is the few thousand years covered by written

history. Strangely enough, the conditions during this time are known

with less accuracy than are those of geological periods hundreds of

times more remote. Yet if pronounced changes have occurred since the

days of the ancient Babylonians and since the last of the post-glacial

they are of great importance not only because of their possible

historic effects, but because they bridge the gap between the little

variations of climate which are observable during a single lifetime and

the great changes known as glacial epochs. Only by bridging the gap can

we determine whether there is any genetic relation between the great

changes and the small. A full discussion of the climate of historic

times is not here advisable, for it has been considered in detail in

numerous other publications.[17] Our most profitable course would seem

to be to consider first the general trend of opinion and then to take up

the chief objections to each of the main hypotheses.

In the hot debate over this problem during recent decades the ideas of

geographers seem to have gone through much the same metamorphosis as

have those of geologists in regard to the climate of far earlier times.

As every geologist well knows, at the dawn of geology people believed in

climatic uniformity--that is, it was supposed that since the completion

of an original creative act there had been no important changes. This

view quickly disappeared and was superseded by the hypothesis of

progressive cooling and drying, an hypothesis which had much to do with

the development of the nebular hypothesis, and which has in turn been

greatly strengthened by that hypothesis. The discovery of evidence of

widespread continental glaciation, however, necessitated a modification

of this view, and succeeding years have brought to light a constantly

increasing number of glacial, or at least cool, periods distributed

throughout almost the whole of geological time. Moreover, each year,

almost, brings new evidence of the great complexity of glacial periods,

epochs, and stages. Thus, for many decades, geologists have more and

more been led to believe that in spite of surprising uniformity, when

viewed in comparison with the cosmic possibilities, the climate of the

past has been highly unstable from the viewpoint of organic evolution,

and its changes have been of all degrees of intensity.

Geographers have lately been debating the reality of historic changes of

climate in the same way in which geologists debated the reality of

glacial epochs and stages. Several hypotheses present themselves but

these may all be grouped under three headings; namely, the hypotheses of

(1) progressive desiccation, (2) climatic uniformity, and (3)

pulsations. The hypothesis of progressive desiccation has been widely

advocated. In many of the drier portions of the world, especially

between 30 deg. and 40 deg. from the equator, and preeminently in western and

central Asia and in the southwestern United States, almost innumerable

facts seem to indicate that two or three thousand years ago the climate

was distinctly moister than at present. The evidence includes old lake

strands, the traces of desiccated springs, roads in places now too dry

for caravans, other roads which make detours around dry lake beds where

no lakes now exist, and fragments of dead forests extending over

hundreds of square miles where trees cannot now grow for lack of water.

Still stronger evidence is furnished by ancient ruins, hundreds of which

are located in places which are now so dry that only the merest fraction

of the former inhabitants could find water. The ruins of Palmyra, in the

Syrian Desert, show that it must once have been a city like modern

Damascus, with one or two hundred thousand inhabitants, but its water

supply now suffices for only one or two thousand. All attempts to

increase the water supply have had only a slight effect and the water is

notoriously sulphurous, whereas in the former days, when it was

abundant, it was renowned for its excellence. Hundreds of pages might be

devoted to describing similar ruins. Some of them are even more

remarkable for their dryness than is Niya, a site in the Tarim Desert of

Chinese Turkestan. Yet there the evidence of desiccation within 2000

years is so strong that even so careful and conservative a man as

Hann,[18] pronounces it "ueberzeugend."

A single quotation from scores that might be used will illustrate the

conclusions of some of the most careful archaeologists.[19]

Among the regions which were once populous and highly civilized,

but which are now desert and deserted, there are few which were

more closely connected with the beginnings of our own civilization

than the desert parts of Syria and northern Arabia. It is only of

recent years that the vast extent and great importance of this lost

civilization has been fully recognized and that attempts have been

made to reduce the extent of the unexplored area and to discover

how much of the territory which has long been known as desert was

formerly habitable and inhabited. The results of the explorations

of the last twenty years have been most astonishing in this regard.

It has been found that practically all of the wide area lying

between the coast range of the eastern Mediterranean and the

Euphrates, appearing upon the maps as the Syrian Desert, an area

embracing somewhat more than 20,000 square miles, was more thickly

populated than any area of similar dimensions in England or in the

United States is today if one excludes the immediate vicinity of

the large modern cities. It has also been discovered that an

enormous desert tract lying to the east of Palestine, stretching

eastward and southward into the country which we know as Arabia,

was also a densely populated country. How far these settled regions

extended in antiquity is still unknown, but the most distant

explorations in these directions have failed to reach the end of

ruins and other signs of former occupation.

The traveler who has crossed the settled, and more or less

populous, coast range of northern Syria and descended into the

narrow fertile valley of the Orontes, encounters in any farther

journey toward the east an irregular range of limestone hills lying

north and south and stretching to the northeast almost halfway to

the Euphrates. These hills are about 2,500 feet high, rising in

occasional peaks from 3,000 to 3,500 feet above sea level. They are

gray and unrelieved by any visible vegetation. On ascending into

the hills the traveler is astonished to find at every turn remnants

of the work of men's hands, paved roads, walls which divided

fields, terrace walls of massive structure. Presently he comes upon

a small deserted and partly ruined town composed of buildings large

and small constructed of beautifully wrought blocks of limestone,

all rising out of the barren rock which forms the ribs of the

hills. If he mounts an eminence in the vicinity, he will be still

further astonished to behold similar ruins lying in all directions.

He may count ten or fifteen or twenty, according to the commanding

position of his lookout. From a distance it is often difficult to

believe that these are not inhabited places; but closer inspection

reveals that the gentle hand of time or the rude touch of

earthquake has been laid upon every building. Some of the towns are

better preserved than others; some buildings are quite perfect but

for their wooden roofs which time has removed, others stand in

picturesque ruins, while others still are level with the ground. On

a far-off hilltop stands the ruin of a pagan temple, and crowning

some lofty ridge lie the ruins of a great Christian monastery. Mile

after mile of this barren gray country may be traversed without

encountering a single human being. Day after day may be spent in

traveling from one ruined town to another without seeing any green

thing save a terebinth tree or two standing among the ruins, which

have sent their roots down into earth still preserved in the

foundations of some ancient building. No soil is visible anywhere

except in a few pockets in the rock from which it could not be

washed by the torrential rains of the wet season; yet every ruin is

surrounded with the remains of presses for the making of oil and

wine. Only one oasis has been discovered in these high plateaus.

Passing eastward from this range of hills, one descends into a

gently rolling country that stretches miles away toward the

Euphrates. At the eastern foot of the hills one finds oneself in a

totally different country, at first quite fertile and dotted with

frequent villages of flat-roofed houses. Here practically all the

remains of ancient times have been destroyed through ages of

building and rebuilding. Beyond this narrow fertile strip the soil

grows drier and more barren, until presently another kind of desert

is reached, an undulating waste of dead soil. Few walls or towers

or arches rise to break the monotony of the unbroken landscape; but

the careful explorer will find on closer examination that this

region was more thickly populated in antiquity even than the hill

country to the west. Every unevenness of the surface marks the site

of a town, some of them cities of considerable extent.

We may draw certain very definite conclusions as to the former

conditions of the country itself. There was soil upon the northern

hills where none now exists, for the buildings now show unfinished

foundation courses which were not intended to be seen; the soil in

depressions without outlets is deeper than it formerly was; there

are hundreds of olive and wine presses in localities where no tree

or vine could now find footing; and there are hillsides with ruined

terrace walls rising one above the other with no sign of earth near

them. There was also a large natural water supply. In the north as

well as in the south we find the dry beds of rivers, streams, and

brooks with sand and pebbles and well-worn rocks but no water in

them from one year's end to the other. We find bridges over these

dry streams and crudely made washing boards along their banks

directly below deserted towns. Many of the bridges span the beds of

streams that seldom or never have water in them and give clear

evidence of the great climatic changes that have taken place. There

are well heads and well houses, and inscriptions referring to

springs; but neither wells nor springs exist today except in the

rarest instances. Many of the houses had their rock-hewn cisterns,

never large enough to have supplied water for more than a brief

period, and corresponding to the cisterns which most of our recent

forefathers had which were for convenience rather than for

dependence. Some of the towns in southern Syria were provided with

large public reservoirs, but these are not large enough to have

supplied water to their original populations. The high plateaus

were of course without irrigation; but there are no signs, even in

the lower flatter country, that irrigation was ever practiced; and

canals for this purpose could not have completely disappeared.

There were forests in the immediate vicinity, forests producing

timbers of great length and thickness; for in the north and

northeast practically all the buildings had wooden roofs, wooden

intermediate floors, and other features of wood. Costly buildings,

such as temples and churches, employed large wooden beams; but wood

was used in much larger quantities in private dwellings, shops,

stables, and barns. If wood had not been plentiful and cheap--which

means grown near by--the builders would have adopted the building

methods of their neighbors in the south, who used very little wood

and developed the most perfect type of lithic architecture the

world has ever seen. And here there exists a strange anomaly:

Northern Syria, where so much wood was employed in antiquity, is

absolutely treeless now; while in the mountains of southern Syria,

where wood must have been scarce in antiquity to have forced upon

the inhabitants an almost exclusive use of stone, there are still

groves of scrub oak and pine, and travelers of half a century ago

reported large forests of chestnut trees.[20] It is perfectly

apparent that large parts of Syria once had soil and forests and

springs and rivers, while it has none of these now, and that it had

a much larger and better distributed rainfall in ancient times than

it has now.

Professor Butler's careful work is especially interesting because of its

contrast to the loose statements of those who believe in climatic

uniformity. So far as I am aware, no opponent of the hypothesis of

climatic changes has ever even attempted to show by careful statistical

analysis that the ancient water supply of such ruins was no greater than

that of the present. The most that has been done is to suggest that

there may have been sources of water which are now unknown. Of course,

this might be true in a single instance, but it could scarcely be the

case in many hundreds or thousands of ruins.

Although the arguments in favor of a change of climate during the last

two thousand years seem too strong to be ignored, their very strength

seems to have been a source of error. A large number of people have

jumped to the conclusion that the change which appears to have occurred

in certain regions occurred everywhere, and that it consisted of a

gradual desiccation.

Many observers, quite as careful as those who believe in progressive

desiccation, point to evidences of aridity in past times in the very

regions where the others find proof of moisture. Lakes such as the

Caspian Sea fell to such a low level that parts of their present floors

were exposed and were used as sites for buildings whose ruins are still

extant. Elsewhere, for instance in the Tian-Shan Mountains, irrigation

ditches are found in places where irrigation never seems to be necessary

at present. In Syria and North Africa during the early centuries of the

Christian era the Romans showed unparalleled activity in building great

aqueducts and in watering land which then apparently needed water almost

as much as it does today. Evidence of this sort is abundant and is as

convincing as is the evidence of moister conditions in the past. It is

admirably set forth, for example, in the comprehensive and ably written

monograph of Leiter on the climate of North Africa.[21] The evidence

cited there and elsewhere has led many authors strongly to advocate the

hypothesis of climatic uniformity. They have done exactly as have the

advocates of progressive change, and have extended their conclusions

over the whole world and over the whole of historic times.

The hypotheses of climatic uniformity and of progressive change both

seem to be based on reliable evidence. They may seem to be diametrically

opposed to one another, but this is only when there is a failure to

group the various lines of evidence according to their dates, and

according to the types of climate in which they happen to be located.

When the facts are properly grouped in both time and space, it appears

that evidence of moist conditions in the historic Mediterranean lands is

found during certain periods; for instance, four or five hundred years

before Christ, at the time of Christ, and 1000 A. D. The other kind of

evidence, on the contrary, culminates at other epochs, such as about

1200 B. C. and in the seventh and thirteenth centuries after Christ. It

is also found during the interval from the culmination of a moist epoch

to the culmination of a dry one, for at such times the climate was

growing drier and the people were under stress. This was seemingly the

case during the period from the second to the fourth centuries of our

era. North Africa and Syria must then have been distinctly better

watered than at present, as appears from Butler's vivid description; but

they were gradually becoming drier, and the natural effect on a

vigorous, competent people like the Romans was to cause them to

construct numerous engineering works to provide the necessary water.

The considerations which have just been set forth have led to a third

hypothesis, that of pulsatory climatic changes. According to this, the

earth's climate is not stable, nor does it change uniformly in one

direction. It appears to fluctuate back and forth not only in the little

waves which we see from year to year or decade to decade, but in much

larger waves, which take hundreds of years or even a thousand. These in

turn seem to merge into and be imposed on the greater waves which form

glacial stages, glacial epochs, and glacial periods. At the present time

there seems to be no way of determining whether the general tendency is

toward aridity or toward glaciation. The seventh century of our era was

apparently the driest time during the historic period--distinctly drier

than the present--but the thirteenth century was almost equally dry, and

the twelfth or thirteenth before Christ may have been very dry.

The best test of an hypothesis is actual measurements. In the case of

the pulsatory hypothesis we are fortunately able to apply this test by

means of trees. The growth of vegetation depends on many factors--soil,

exposure, wind, sun, temperature, rain, and so forth. In a dry region

the most critical factor in determining how a tree's growth shall vary

from year to year is the supply of moisture during the few months of

most rapid growth.[22] The work of Douglass[23] and others has shown

that in Arizona and California the thickness of the annual rings affords

a reliable indication of the amount of moisture available during the

period of growth. This is especially true when the growth of several

years is taken as the unit and is compared with the growth of a similar

number of years before or after. Where a long series of years is used,

it is necessary to make corrections to eliminate the effects of age, but

this can be done by mathematical methods of considerable accuracy. It is

difficult to determine whether the climate at the beginning and end of a

tree's life was the same, but it is easily possible to determine whether

there have been pulsations while the tree was making its growth. If a

large number of trees from various parts of a given district all formed

thick rings at a certain period and then formed thin ones for a hundred

years, after which the rings again become thick, we seem to be safe in

concluding that the trees have lived through a long, dry period. The

full reasons for this belief and details as to the methods of estimating

climate from tree growth are given in The Climatic Factor.

The results set forth in that volume may be summarized as follows:

During the years 1911 and 1912, under the auspices of the Carnegie

Institution of Washington, measurements were made of the thickness of

the rings of growth on the stumps of about 450 sequoia trees in

California. These trees varied in age from 250 to nearly 3250 years. The

great majority were over 1000 years of age, seventy-nine were over 2000

years, and three over 3000. Even where only a few trees are available

the record is surprisingly reliable, except where occasional accidents

occur. Where the number approximates 100, accidental variations are

largely eliminated and we may accept the record with considerable

confidence. Accordingly, we may say that in California we have a fairly

accurate record of the climate for 2000 years and an approximate record

for 1000 years more. The final results of the measurements of the

California trees are shown in Fig. 4, where the climatic variations for

3000 years in California are indicated by the solid line. The high parts

of the line indicate rainy conditions, the low parts, dry. An

examination of this curve shows that during 3000 years there have

apparently been climatic variations more important than any which have

taken place during the past century. In order to bring out the details

more clearly, the more reliable part of the California curve, from 100

B. C. to the present time, has been reproduced in Fig. 5. This is

identical with the corresponding part of Fig. 4, except that the

vertical scale is three times as great.

and in western and central Asia (dotted line).

Note. The curves of Figs. 4 and 5 are reproduced as published in The

Solar Hypothesis in 1914. Later work, however, has indicated that in

the Asiatic curve the dash lines, which were tentatively inserted in

1914, are probably more nearly correct than the dotted lines. Still

further evidence indicates that the Asiatic curve is nearly like that of

California in its main features.]

The curve of tree growth in California seems to be a true representation

of the general features of climatic pulsations in the Mediterranean

region. This conclusion was originally based on the resemblance between

the solid line of Fig. 4, representing tree growth, and the dotted line

representing changes of climate in the eastern Mediterranean region as

inferred from the study of ruins and of history before any work on this

subject had been done in America.[24] The dotted line is here reproduced

for its historical significance as a stage in the study of climatic

changes. If it were to be redrawn today on the basis of the knowledge

acquired in the last twelve years, it would be much more like the tree

curve. For example, the period of aridity suggested by the dip of the

dotted line about 300 A. D. was based largely on Professor Butler's data

as to the paucity of inscriptions and ruins dating from that period in

Syria. In the recent article, from which a long quotation has been

given, he shows that later work proves that there is no such paucity. On

the other hand, it has accentuated the marked and sudden decay in

civilization and population which occurred shortly after 600 A. D. He

reached the same conclusion to which the present authors had come on

wholly different grounds, namely, that the dip in the dotted line about

300 A. D. is not warranted, whereas the dip about 630 A. D. is extremely

important. In similar fashion the work of Stein[25] in central Asia

makes it clear that the contrast between the water supply about 200 B.C.

and in the preceding and following centuries was greater than was

supposed on the basis of the scanty evidence available when the dotted

line of Fig. 4 was drawn in 1910.

as measured by growth of Sequoia trees.

Fig. 5 is the same as the later portion of Fig. 4, except that the

vertical scale has been magnified threefold. It seems probable that the

dotted line at the right is more nearly correct than the solid line.

During the thirty years since the end of the curve the general tendency

appears in general to have been somewhat upward.]

Since the curve of the California trees is the only continuous and

detailed record yet available for the climate of the last three thousand

years, it deserves most careful study. It is especially necessary to

determine the degree of accuracy with which the growth of the trees

represents (1) the local rainfall and (2) the rainfall of remote regions

such as Palestine. Perhaps the best way to determine these matters is

the standard mathematical method of correlation coefficients. If two

phenomena vary in perfect unison, as in the case of the turning of the

wheels and the progress of an automobile when the brakes are not

applied, the correlation coefficient is 1.00, being positive when the

automobile goes forward and negative when it goes backward. If there is

no relation between two phenomena, as in the case of the number of miles

run by a given automobile each year and the number of chickens hatched

in the same period, the coefficient is zero. A partial relationship

where other factors enter into the matter is represented by a

coefficient between zero and one, as in the case of the movement of the

automobile and the consumption of gasoline. In this case the relation is

very obvious, but is modified by other factors, including the roughness

and grade of the road, the amount of traffic, the number of stops, the

skill of the driver, the condition and load of the automobile, and the

state of the weather. Such partial relationships are the kind for which

correlation coefficients are most useful, for the size of the

coefficients shows the relative importance of the various factors. A

correlation coefficient four times the probable error, which can always

be determined by a formula well known to mathematicians, is generally

considered to afford evidence of some kind of relation between two

phenomena. When the ratio between coefficient and error rises to six,

the relationship is regarded as strong.

Few people would question that there is a connection between tree growth

and rainfall, especially in a climate with a long summer dry season like

that of California. But the growth of the trees also depends on their

position, the amount of shading, the temperature, insect pests, blights,

the wind with its tendency to break the branches, and a number of other

factors. Moreover, while rain commonly favors growth, great extremes are

relatively less helpful than more moderate amounts. Again, the roots of

a tree may tap such deep sources of water that neither drought nor

excessive rain produces much effect for several years. Hence in

comparing the growth of the huge sequoias with the rainfall we should

expect a correlation coefficient high enough to be convincing, but

decidedly below 1.00. Unfortunately there is no record of the rainfall

where the sequoias grow, the nearest long record being that of

Sacramento, nearly 200 miles to the northwest and close to sea level

instead of at an altitude of about 6000 feet.

Applying the method of correlation coefficients to the annual rainfall

of Sacramento and the growth of the sequoias from 1863 to 1910, we

obtain the results shown in Table 3. The trees of Section A of the table

grew in moderately dry locations although the soil was fairly deep, a

condition which seems to be essential to sequoias. In this case, as in

all the others, the rainfall is reckoned from July to June, which

practically means from October to May, since there is almost no summer

rain. Thus the tree growth in 1861 is compared with the rainfall of the

preceding rainy season, 1860-1861, or of several preceding rainy seasons

as the table indicates.





(r) = Correlation coefficient

(e) = Probable error

(r/e) = Ratio of coefficient to probable error


LOCATIONS, 1861-1910

(r) (e) (r/e)

------ ------ -----

1 year of rainfall -0.059 +-0.096 0.6

2 years of rainfall +0.288 +-0.090 3.2

3 years of rainfall +0.570 +-0.066 8.7

4 years of rainfall +0.470 +-0.076 6.2



3 years of rainfall +0.340 +-0.087 3.9

4 years of rainfall +0.371 +-0.084 4.5

5 years of rainfall +0.398 +-0.082 4.9

6 years of rainfall +0.418 +-0.079 5.3

7 years of rainfall +0.471 +-0.076 6.2

8 years of rainfall (+0.520) +-0.071 7.3

9 years of rainfall +0.575 +-0.065 8.8

10 years of rainfall +0.577 +-0.065 8.8


LOCATIONS, 1861-1910

10 years of rainfall +0.605 +-0.062 9.8



1 = Years

2 = Altitude (feet)

3 = Rainfall (inches)

4 = Approximate distance from sequoias (miles)

1 2 3 4 (r) (e) (r/e)

--------- ---- ----- --- ------ ------ ---------

Sacramento, 1861-1910 70 19.40 200 +0.398 +-0.081 4.9

Colfax, 1871-1909 2400 48.94 200 +0.122 +-0.113 1.1

Summit, 1871-1909 7000 48.07 200 +0.148 +-0.113 1.3

Truckee, 1871-1909 5800 27.12 200 +0.300 +-0.105 2.9

Boca, 1871-1909 5500 20.34 200 +0.604 +-0.076 8.0

Winnemucca, 1871-1909 4300 8.65 300 +0.492 +-0.089 5.5


In the first line of Section A a correlation coefficient of only -0.056,

which is scarcely six-tenths of the probable error, means that there is

no appreciable relation between the rainfall of a given season and the

growth during the following spring and summer. The roots of the sequoias

probably penetrate so deeply that the rain and melted snow of the spring

months do not sink down rapidly enough to influence the trees before the

growing season comes to an end. The precipitation of two preceding

seasons, however, has some effect on the trees, as appears in the second

line of Section A, where the correlation coefficient is +0.288, or 3.2

times the probable error. When the rainfall of three seasons is taken

into account the coefficient rises to +0.570, or 8.7 times the probable

error, while with four years of rainfall the coefficient begins to fall

off. Thus the growth of these eighteen sequoias on relatively dry slopes

appears to have depended chiefly on the rainfall of the second and third

preceding rainy seasons. The growth in 1900, for example, depended

largely on the rainfall in the rainy seasons of 1897-1898 and 1898-1899.

Section B of the table shows that with 112 trees, growing chiefly in

moist depressions where the water supply is at a maximum, the

correlation between growth and rainfall, +0.577 for ten years' rainfall,

is even higher than with the dry trees. The seepage of the underground

water is so slow that not until four years' rainfall is taken into

account is the correlation coefficient more than four times the probable

error. When only the trees growing in moist locations are employed, the

coefficient between tree growth and the rainfall for ten years rises to

the high figure of +0.605, or 9.8 times the probable error, as appears

in Section C. These figures, as well as many others not here published,

make it clear that the curve of sequoia growth from 1861 to 1910 affords

a fairly close indication of the rainfall at Sacramento, provided

allowance be made for a delay of three to ten years due to the fact that

the moisture in the soil gradually seeps down the mountain-sides and

only reaches the sequoias after a considerable interval.

If a rainfall record were available for the place where the trees

actually grow, the relationship would probably be still closer.

The record at Fresno, for example, bears out this conclusion so far as

it goes. But as Fresno lies at a low altitude and its rainfall is of

essentially the Sacramento type, its short record is of less value than

that of Sacramento. The only rainfall records among the Sierras at high

levels, where the rainfall and temperature are approximately like those

of the sequoia region, are found along the main line of the Southern

Pacific railroad. This runs from Oakland northeastward seventy miles

across the open plain to Sacramento, then another seventy miles, as the

crow flies, through Colfax and over a high pass in the Sierras at

Summit, next twenty miles or so down through Truckee to Boca, on the

edge of the inland basin of Nevada, and on northeastward another 160

miles to Winnemucca, where it turns east toward Ogden and Salt Lake

City. Section D of Table 3 shows the correlation coefficients between

the rainfall along the railroad and the growth of the sequoias. At

Sacramento, which lies fairly open to winds from the Pacific and thus

represents the general climate of central California, the coefficient is

nearly five times the probable error, thus indicating a real relation to

sequoia growth. Then among the foothills of the Sierras at Colfax, the

coefficient drops till it is scarcely larger than the probable error. It

rises rapidly, however, as one advances among the mountains, until at

Boca it attains the high figure of +0.604 or eight times the probable

error, and continues high in the dry area farther east. In other words

the growth of the sequoias is a good indication of the rainfall where

the trees grow and in the dry region farther east.

In order to determine the degree to which the sequoia record represents

the rainfall of other regions, let us select Jerusalem for comparison.

The reasons for this selection are that Jerusalem furnishes the only

available record that satisfies the following necessary conditions: (1)

its record is long enough to be important; (2) it is located fairly near

the latitude of the sequoias, 32 deg.N versus 37 deg.N; (3) it is located in a

similar type of climate with winter rains and a long dry summer; (4) it

lies well above sea level (2500 feet) and somewhat back from the

seacoast, thus approximating although by no means duplicating the

condition of the sequoias; and (5) it lies in a region where the

evidence of climatic changes during historic times is strongest. The

ideal place for comparison would be the valley in which grow the cedars

of Lebanon. Those trees resemble the sequoias to an extraordinary

degree, not only in their location, but in their great age. Some day it

will be most interesting to compare the growth of these two famous

groups of old trees.






(r) = Correlation coefficient

(e) = Probable error

(r/e) = Ratio of coefficient to probable error



(r) (e) (r/e)

------ ----- ---------

11 trees measured by Douglass +0.453 +-0.078 5.8

80 trees, moist locations, Groups IA,

IIA, IIIA, VA +0.500 +-0.073 6.8

101 trees, 69 in moist locations, 32 in

dry, I, II, III +0.616 +-0.061 10.1

112 trees, 80 in moist locations, 32 in

dry, I, II, III, V +0.675 +-0.053 12.7


1 = Altitude (feet)

2 = Years

-- 3 years -- -- 5 years --

1 2 (r) (r/e) (r) (r/e)

---- --------- ------ ------- ------ -------

Sacramento, 70 1861-1910 +0.386 4.7 +0.352 4.2

Colfax, 2400 1871-1909 +0.311 3.1 +0.308 3.0

Summit, 7000 1871-1909 +0.099 0.9 +0.248 2.3

Truckee, 5800 1871-1909 +0.229 2.2 +0.337 3.3

[A]Boca, 5500 1871-1909 +0.482 6.4 +0.617 8.6

Winnemucca, 4300 1871-1909 +0.235 2.2 +0.260 2.4

San Bernardino, 1050 1871-1909 +0.275 2.7 +0.177 1.8



(r) (r/e)

------ -------

Sacramento and San Bernardino +0.663 10.7

San Bernardino and Winnemucca +0.291 2.8


The correlation coefficients for the sequoia growth and the rainfall at

Jerusalem are given in Section A, Table 4. They are so high and so

consistent that they scarcely leave room for doubt that where a hundred

or more sequoias are employed, as in Fig. 5, their curve of growth

affords a good indication of the fluctuations of climate in western

Asia. The high coefficient for the eleven trees measured by Douglass

suggests that where the number of trees falls as low as ten, as in the

part of Fig. 4 from 710 to 840 B. C., the relation between tree growth

and rainfall is still close even when only one year's growth is

considered. Where the unit is ten years of growth, as in Figs. 4 and 5,

the accuracy of the tree curve as a measure of rainfall is much greater

than when a single year is used as in Table 4. When the unit is raised

to thirty years, as in the smoothed part of Fig. 4 previous to

240 B. C., even four trees, as from 960 to 1070, probably give a fair

approximation to the general changes in rainfall, while a single tree

prior to 1110 B. C. gives a rough indication.

Table 4 shows a peculiar feature in the fact that the correlations of

Section A between tree growth and the rainfall of Jerusalem are

decidedly higher than those between the rainfall in the two regions.

Only at Sacramento and Boca are the rainfall coefficients high enough to

be conclusive. This, however, is not surprising, for even between

Sacramento and San Bernardino, only 400 miles apart, the correlation

coefficient for the rainfall by three-year periods is only 10.7 times

the probable error, as appears in Section C of Table 4, while between

San Bernardino and Winnemucca 500 miles away, the corresponding figure

drops to 2.8. It must be remembered that in some respects the growth of

the sequoias is a much better record of rainfall than are the records

kept by man. The human record is based on the amount of water caught by

a little gauge a few inches in diameter. Every gust of wind detracts

from the accuracy of the record; a mile away the rainfall may be double

what it is at the gauge. Each sequoia, on the other hand, draws its

moisture from an area thousands of times as large as a rain gauge.

Moreover, the trees on which Figs. 4 and 5 are based were scattered over

an area fifty miles long and several hundred square miles in extent.

Hence they represent the summation of the rainfall over an area millions

of times as large as that of a rain gauge. This fact and the large

correlation coefficients between sequoia growth and Jerusalem rainfall

should be considered in connection with the fact that all the

coefficients between the rainfall of California and Nevada and that of

Jerusalem are positive. If full records of the complete rainfall of

California and Nevada on the one hand and of the eastern Mediterranean

region on the other were available for a long period, they would

probably agree closely.

Just how widely the sequoias can be used as a measure of the climate of

the past is not yet certain. In some regions, as will shortly be

explained, the climatic changes seem to have been of an opposite

character from those of California. In others the Californian or eastern

Mediterranean type of change seems sometimes to prevail but is not

always evident. For example, at Malta the rainfall today shows a

distinct relation to that of Jerusalem and to the growth of the

sequoias. But the correlation coefficient between the rainfall of

eight-year periods at Naples, a little farther north, and the growth of

the sequoias at the end of the periods is -0.132, or only 1.4 times the

probable error and much too small to be significant. This is in harmony

with the fact that although Naples has summer droughts, they are not so

pronounced as in California and Palestine, and the prevalence of storms

is much greater. Jerusalem receives only 8 per cent of its rain in the

seven months from April to October, and Sacramento 13, while Malta

receives 31 per cent and Naples 43. Nevertheless, there is some evidence

that in the past the climatic fluctuations of southern Italy followed

nearly the same course as those of California and Palestine. This

apparent discrepancy seems to be explained by our previous conclusion

that changes of climate are due largely to a shifting of storm tracks.

When sunspots are numerous the storms which now prevail in northern

Italy seem to be shifted southward and traverse the Mediterranean to

Palestine just as similar storms are shifted southward in the United

States. This perhaps accounts for the agreement between the sequoia

curve and the agricultural and social history of Rome from about

400 B. C. to 100 A. D., as explained in World Power and Evolution. For

our present purposes, however, the main point is that since rainfall

records have been kept the fluctuations of climate indicated by the

growth of the sequoias have agreed closely with fluctuations in the

rainfall of the eastern Mediterranean region. Presumably the same was

true in the past. In that case, the sequoia curve not only is a good

indication of climatic changes or pulsations in regions of similar

climate, but may serve as a guide to coincident but different changes in

regions of other types.

An enormous body of other evidence points to the same conclusion. It

indicates that while the average climate of the present is drier than

that of the past in regions having the Mediterranean type of winter

rains and summer droughts, there have been pronounced pulsations during

historic times so that at certain times there has actually been greater

aridity than at present. This conclusion is so important that it seems

advisable to examine the only important arguments that have been raised

against it, especially against the idea that the general rainfall of the

eastern Mediterranean was greater in the historic past than at present.

The first objection is the unquestionable fact that droughts and famines

have occurred at periods which seem on other evidence to have been

moister than the present. This argument has been much used, but it seems

to have little force. If the rainfall of a given region averages thirty

inches and varies from fifteen to forty-five, a famine will ensue if the

rainfall drops for a few years to the lower limit and does not rise much

above twenty for a few years. If the climate of the place changes during

the course of centuries, so that the rainfall averages only twenty

inches, and ranges from seven to thirty-five, famine will again ensue if

the rainfall remains near ten inches for a few years. The ravages of the

first famine might be as bad as those of the second. They might even be

worse, because when the rainfall is larger the population is likely to

be greater and the distress due to scarcity of food would affect a

larger number of people. Hence historic records of famines and droughts

do not indicate that the climate was either drier or moister than at

present. They merely show that at the time in question the climate was

drier than the normal for that particular period.

The second objection is that deserts existed in the past much as at

present. This is not a real objection, however, for, as we shall see

more fully, some parts of the world suffer one kind of change and others

quite the opposite. Moreover, deserts have always existed, and when we

talk of a change in their climate we merely mean that their boundaries

have shifted. A concrete example of the mistaken use of ancient dryness

as proof of climatic uniformity is illustrated by the march of Alexander

from India to Mesopotamia. Hedin gives an excellent presentation of the

case in the second volume of his Overland to India. He shows

conclusively that Alexander's army suffered terribly from lack of water

and provisions. This certainly proves that the climate was dry, but it

by no means indicates that there has been no change from the past to the

present. We do not know whether Alexander's march took place during an

especially dry or an especially wet year. In a desert region like

Makran, in southern Persia and Beluchistan, where the chief difficulties

occurred, the rainfall varies greatly from year to year. We have no

records from Makran, but the conditions there are closely similar to

those of southern Arizona and New Mexico. In 1885 and 1905 the rainfall

for five stations in that region was as follows:


Mean rainfall

during period

1885 1905 since



Yuma, Arizona, 2.72 11.41 3.13

Phoenix, Arizona, 3.77 19.73 7.27

Tucson, Arizona, 5.26 24.17 11.66

Lordsburg, New Mexico, 3.99 19.50 8.62

El Paso, Texas (on New

Mexico border), 7.31 17.80 9.06

---- ----- -----

Average, 4.61 18.52 7.95


These stations are distributed over an area nearly 500 miles east and

west. Manifestly a traveler who spent the year 1885 in that region would

have had much more difficulty in finding water and forage than one who

traveled in the same places in 1905. During 1885 the rainfall was 42 per

cent less than the average, and during 1905 it was 134 per cent more

than the average. Let us suppose, for the sake of argument, that the

average rainfall of southeastern Persia is six inches today and was ten

inches in the days of Alexander. If the rainfall from year to year

varied as much in the past in Persia as it does now in New Mexico and

Arizona, the rainfall during an ancient dry year, corresponding in

character to 1885, would have been about 5.75 inches. On the other hand,

if we suppose that the rainfall then averaged less than at present,--let

us say four inches,--a wet year corresponding to 1905 in the American

deserts might have had a rainfall of about ten inches. This being the

case, it is clear that our estimate of what Alexander's march shows as

to climate must depend largely on whether 325 B. C. was a wet year or a

dry year. Inasmuch as we know nothing about this, we must fall back on

the fact that a large army accomplished a journey in a place where today

even a small caravan usually finds great difficulty in procuring forage

and water. Moreover, elephants were taken 180 miles across what is now

an almost waterless desert, and yet the old historians make no comment

on such a feat which today would be practically impossible. These things

seem more in harmony with a change of climate than with uniformity.

Nevertheless, it is not safe to place much reliance on them except when

they are taken in conjunction with other evidence, such as the numerous

ruins, which show that Makran was once far more densely populated than

now seems possible. Taken by itself, such incidents as Alexander's march

cannot safely be used either as an argument for or against changes of


The third and strongest objection to any hypothesis of climatic changes

during historic times is based on vegetation. The whole question is

admirably set forth by J. W. Gregory,[28] who gives not only his own

results, but those of the ablest scholars who have preceded him. His

conclusions are important because they represent one of the few cases

where a definite statistical attempt has been made to prove the exact

condition of the climate of the past. After stating various less

important reasons for believing that the climate of Palestine has not

changed, he discusses vegetation. The following quotation indicates his

line of thought. A sentence near the beginning is italicized in order to

call attention to the importance which Gregory and others lay on this

particular kind of evidence:

Some more certain test is necessary than the general conclusions

which can be based upon the historical and geographical evidence of

the Bible. In the absence of rain gauge and thermometric records,

the most precise test of climate is given by the vegetation; and

fortunately the palm affords a very delicate test of the past

climate of Palestine and the eastern Mediterranean.... The date

palm has three limits of growth which are determined by temperature;

thus it does not reach full maturity or produce ripe fruit of good

quality below the mean annual temperature of 69 deg.F. The isothermal of

69 deg. crosses southern Algeria near Biskra; it touches the northern

coasts of Cyrenaica near Derna and passes Egypt near the mouth of

the Nile, and then bends northward along the coast lands of


To the north of this line the date palm grows and produces fruit,

which only ripens occasionally, and its quality deteriorates as the

temperature falls below 69 deg.. Between the isotherms of 68 deg. and 64 deg.,

limits which include northern Algeria, most of Sicily, Malta, the

southern parts of Greece and northern Syria, the dates produced are

so unripe that they are not edible. In the next cooler zone, north

of the isotherm of 62 deg., which enters Europe in southwestern

Portugal, passes through Sardinia, enters Italy near Naples, crosses

northern Greece and Asia Minor to the east of Smyrna, the date palm

is grown only for its foliage, since it does not fruit.

Hence at Benghazi, on the north African coast, the date palm is

fertile, but produces fruit of poor quality. In Sicily and at

Algiers the fruit ripens occasionally and at Rome and Nice the palm

is grown only as an ornamental tree.

The date palm therefore affords a test of variations in mean annual

temperature of three grades between 62 deg. and 69 deg..

This test shows that the mean annual temperature of Palestine has

not altered since Old Testament times. The palm tree now grows dates

on the coast of Palestine and in the deep depression around the Dead

Sea, but it does not produce fruit on the highlands of Judea. Its

distribution in ancient times, as far as we can judge from the

Bible, was exactly the same. It grew at "Jericho, the city of palm

trees" (Deut. xxxiv: 3 and 2 Chron. xxviii: 15), and at Engedi, on

the western shore of the Dead Sea (2 Chron. xx: 2; Sirach xxiv: 14);

and though the palm does not still live at Jericho--the last

apparently died in 1838--its disappearance must be due to neglect,

for the only climatic change that would explain it would be an

increase in cold or moisture. In olden times the date palm certainly

grew on the highlands of Palestine; but apparently it never produced

fruit there, for the Bible references to the palm are to its beauty

and erect growth: "The righteous shall flourish like the palm" (Ps.

xcii: 12); "They are upright as the palm tree" (Jer. x: 5); "Thy

stature is like to a palm tree" (Cant. vii: 7). It is used as a

symbol of victory (Rev. vii: 9), but never praised as a source of


Dates are not once referred to in the text of the Bible, but

according to the marginal notes the word translated "honey" in

2 Chron. xxxi: 5 may mean dates....

It appears, therefore, that the date palm had essentially the same

distribution in Palestine in Old Testament times as it has now; and

hence we may infer that the mean temperature was then the same as

now. If the climate had been moister and cooler, the date could not

have flourished at Jericho. If it had been warmer, the palms would

have grown freely at higher levels and Jericho would not have held

its distinction as the city of palm trees.[29]

In the main Gregory's conclusions seem to be well grounded, although

even according to his data a change of 2 deg. or 3 deg. in mean temperature

would be perfectly feasible. It will be noticed, however, that they

apply to temperature and not to rainfall. They merely prove that two

thousand years ago the mean temperature of Palestine and the neighboring

regions was not appreciably different from what it is today. This,

however, is in no sense out of harmony with the hypothesis of climatic

pulsations. Students of glaciation believe that during the last glacial

epoch the mean temperature of the earth as a whole was only 5 deg. or 6 deg.C.

lower than at present. If the difference between the climate of today

and of the time of Christ is a tenth as great as the difference between

the climate of today and that which prevailed at the culmination of the

last glacial epoch, the change in two thousand years has been of large

dimensions. Yet this would require a rise of only half a degree

Centigrade in the mean temperature of Palestine. Manifestly, so slight a

change would scarcely be detectable in the vegetation.

The slightness of changes in mean temperature as compared with changes

in rainfall may be judged from a comparison of wet and dry years in

various regions. For example, at Berlin between 1866 and 1905 the ten

most rainy years had an average precipitation of 670 mm. and a mean

temperature of 9.15 deg.C. On the other hand, the ten years of least

rainfall had an average of 483 mm. and a mean temperature of 9.35 deg.. In

other words, a difference of 137 mm., or 39 per cent, in rainfall was

accompanied by a difference of only 0.2 deg.C. in temperature. Such

contrasts between the variability of mean rainfall and mean temperature

are observable not only when individual years are selected, but when

much longer periods are taken. For instance, in the western Gulf region

of the United States the two inland stations of Vicksburg, Mississippi,

and Shreveport, Louisiana, and the two maritime stations of New Orleans,

Louisiana, and Galveston, Texas, lie at the margins of an area about 400

miles long. During the ten years from 1875 to 1884 their rainfall

averaged 59.4 inches,[30] while during the ten years from 1890 to 1899

it averaged only 42.4 inches. Even in a region so well watered as the

Gulf States, such a change--40 per cent more in the first decade than in

the second--is important, and in drier regions it would have a great

effect on habitability. Yet in spite of the magnitude of the change the

mean temperature was not appreciably different, the average for the four

stations being 67.36 deg.F. during the more rainy decade and 66.94 deg.F. during

the less rainy decade--a difference of only 0.42 deg.F. It is worth noticing

that in this case the wetter period was also the warmer, whereas in

Berlin it was the cooler. This is probably because a large part of the

moisture of the Gulf States is brought by winds having a southerly

component. Similar relationships are apparent in other places. We select

Jerusalem because we have been discussing Palestine. At the time of

writing, the data available in the Quarterly Journal of the Palestine

Exploration Fund cover the years from 1882-1899 and 1903-1909. Among

these twenty-five years the thirteen which had most rain had an average

of 34.1 inches and a temperature of 62.04 deg.F. The twelve with least rain

had 24.4 inches and a temperature of 62.44 deg.. A difference of 40 per cent

in rainfall was accompanied by a difference of only 0.4 deg.F. in


The facts set forth in the preceding paragraphs seem to show that

extensive changes in precipitation and storminess can take place without

appreciable changes of mean temperature. If such changed conditions can

persist for ten years, as in one of our examples, there is no logical

reason why they cannot persist for a hundred or a thousand. The evidence

of changes in climate during the historic period seems to suggest

changes in precipitation much more than in temperature. Hence the

strongest of all the arguments against historic changes of climate seems

to be of relatively little weight, and the pulsatory hypothesis seems to

be in accord with all the known facts.

Before the true nature of climatic changes, whether historic or

geologic, can be rightly understood, another point needs emphasis. When

the pulsatory hypothesis was first framed, it fell into the same error

as the hypotheses of uniformity and of progressive change--that is, the

assumption was made that the whole world is either growing drier or

moister with each pulsation. A study of the ruins of Yucatan, in 1912,

and of Guatemala, in 1913, as is explained in The Climatic Factor, has

led to the conclusion that the climate of those regions has changed in

the opposite way from the changes which appear to have taken place in

the desert regions farther south. These Maya ruins in Central America

are in many cases located in regions of such heavy rainfall, such dense

forests, and such malignant fevers that habitation is now practically

impossible. The land cannot be cultivated except in especially favorable

places. The people are terribly weakened by disease and are among the

lowest in Central America. Only a hundred miles from the unhealthful

forests we find healthful areas, such as the coasts of Yucatan and the

plateau of Guatemala. Here the vast majority of the population is

gathered, the large towns are located, and the only progressive people

are found. Nevertheless, in the past the region of the forests was the

home of by far the most progressive people who are ever known to have

lived in America previous to the days of Columbus. They alone brought to

high perfection the art of sculpture; they were the only American people

who invented the art of writing. It seems scarcely credible that such a

people would have lived in the worst possible habitat when far more

favored regions were close at hand. Therefore it seems as if the climate

of eastern Guatemala and Yucatan must have been relatively dry at some

past time. The Maya chronology and traditions indicate that this was

probably at the same time when moister conditions apparently prevailed

in the subarid or desert portions of the United States and Asia. Fig. 3

shows that today at times of many sunspots there is a similar opposition

between a tendency toward storminess and rain in subtropical regions and

toward aridity in low latitudes near the heat equator.

Thus our final conclusion is that during historic times there have been

pulsatory changes of climate. These changes have been of the same type

in regions having similar kinds of climate, but of different and

sometimes opposite types in places having diverse climates. As to the

cause of the pulsations, they cannot have been due to the precession of

the equinoxes nor apparently to any allied astronomical cause, for the

time intervals are too short and too irregular. They cannot have been

due to changes in the percentage of carbon dioxide in the atmosphere,

for not even the strongest believers in the climatic efficacy of that

gas hold that its amount could fluctuate in any such violent way as

would be necessary to explain the pulsations shown in the California

curve of tree growth. Volcanic activity seems more probable as at least

a partial cause, and it would be worth while to investigate the matter

more fully. Nevertheless, it can apparently be only a minor cause. In

the first place, the main effect of a cloud of dust is to alter the

temperature, but Gregory's summary of the palm and the vine shows that

variations in temperature are apparently of very slight importance

during historic times. Again, ruins on the bottoms of enclosed salt

lakes, old beaches now under the water, and signs of irrigation ditches

where none are now needed indicate a climate drier than the present.

Volcanic dust, however, cannot account for such a condition, for at

present the air seems to be practically free from such dust for long

periods. Thus we now experience the greatest extreme which the volcanic

hypothesis permits in one direction, but there have been greater

extremes in the same direction. The thermal solar hypothesis is likewise

unable to explain the observed phenomena, for neither it nor the

volcanic hypothesis offers any explanation of why the climate varies in

one way in Mediterranean climates and in an opposite way in regions near

the heat equator.

This leaves the cyclonic hypothesis. It seems to fit the facts, for

variations in cyclonic storms cause some regions to be moister and

others drier than usual. At the same time the variations in temperature

are slight, and are apparently different in different regions, some

places growing warm when others grow cool. In the next chapter we shall

study this matter more fully, for it can best be appreciated by

examining the course of events in a specific century.


[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of

Climatic Changes; Bull. Geol. Soc.