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Causes Of Mild Geological Climates

Hypotheses Of Climatic Change

The Climate Of History

Some Problems Of Glacial Periods

The Variability Of Climate

The Climatic Stress Of The Fourteenth Century

The Uniformity Of Climate

The Solar Cyclonic Hypothesis

Glaciation According To The Solar-cyclonic Hypothesis[38]

Least Viewed

The Changing Composition Of Oceans And Atmosphere

The Sun's Journey Through Space

Terrestrial Causes Of Climatic Changes

Post-glacial Crustal Movements And Climatic Changes

The Earth's Crust And The Sun

The Effect Of Other Bodies On The Sun

The Origin Of Loess

The Solar Cyclonic Hypothesis

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Uniformity Of Climate

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
stages, 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view quickly disappeared and was superseded by the hypothesis of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.

Next: The Climatic Stress Of The Fourteenth Century

Previous: The Solar Cyclonic Hypothesis

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