Hypotheses Of Climatic Change

The next step in our study of climate is to review the main hypotheses

as to the causes of glaciation. These hypotheses apply also to other

types of climatic changes. We shall concentrate on glacial periods,

however, not only because they are the most dramatic and well-known

types of change, but because they have been more discussed than any

other and have also had great influence on evolution. Moreover, they

stand nea the middle of the types of climatic sequences, and an

understanding of them does much to explain the others. In reviewing the

various theories we shall not attempt to cover all the ground, but shall

merely state the main ideas of the few theories which have had an

important influence upon scientific thought.



The conditions which any satisfactory climatic hypothesis must satisfy

are briefly as follows:



(1) Due weight must be given to the fact that changes of climate are

almost certainly due to the combined effect of a variety of causes, both

terrestrial and solar or cosmic.



(2) Attention must also be paid to both sides in the long controversy as

to whether glaciation is due primarily to a diminution in the earth's

supply of heat or to a redistribution of the heat through changes in

atmospheric and oceanic circulation. At present the great majority of

authorities are on the side of a diminution of heat, but the other view

also deserves study.



(3) A satisfactory hypothesis must explain the frequent synchronism

between two great types of phenomena; first, movements of the earth's

crust whereby continents are uplifted and mountains upheaved; and,

second, great changes of climate which are usually marked by relatively

rapid oscillations from one extreme to another.



(4) No hypothesis can find acceptance unless it satisfies the somewhat

exacting requirements of the geological record, with its frequent but

irregular repetition of long, mild periods, relatively cool or

intermediate periods like the present, and glacial periods of more or

less severity and perhaps accompanying the more or less widespread

uplifting of continents. At least during the later glacial periods the

hypothesis must explain numerous climatic epochs and stages superposed

upon a single general period of continental upheaval. Moreover, although

historical geology demands cycles of varied duration and magnitude, it

does not furnish evidence of any rigid periodicity causing the cycles to

be uniform in length or intensity.



(5) Most important of all, a satisfactory explanation of climatic

changes and crustal deformation must take account of all the agencies

which are now causing similar phenomena. Whether any other agencies

should be considered is open to question, although the relative

importance of existing agencies may have varied.



I. Croll's Eccentricity Theory. One of the most ingenious and most

carefully elaborated scientific hypotheses is Croll's[10] precessional

hypothesis as to the effect of the earth's own motions. So well was this

worked out that it was widely accepted for a time and still finds a

place in popular but unscientific books, such as Wells' Outline of

History, and even in scientific works like Wright's Quaternary Ice

Age. The gist of the hypothesis has already been given in connection

with the type of climatic sequence known as orbital precessions. The

earth is 93 million miles away from the sun in January and 97 million in

July. The earth's axis "precesses," however, just as does that of a

spinning top. Hence arises what is known as the precession of the

equinoxes, that is, a steady change in the season at which the earth is

in perihelion, or nearest to the sun. In the course of 21,000 years the

time of perihelion varies from early in January through the entire

twelve months and back to January. Moreover, the earth's orbit is

slightly more elliptical at certain periods than at others, for the

planets sometimes become bunched so that they all pull the earth in one

direction. Hence, once in about one hundred thousand years the effect of

the elliptical shape of the earth's orbit is at a maximum.



Croll argued that these astronomical changes must alter the earth's

climate, especially by their effect on winds and ocean currents. His

elaborate argument contains a vast amount of valuable material. Later

investigation, however, seems to have proven the inadequacy of his

hypothesis. In the first place, the supposed cause does not seem nearly

sufficient to produce the observed results. Second, Croll's hypothesis

demands that glaciation in the northern and southern hemisphere take

place alternately. A constantly growing collection of facts, however,

indicates that glaciation does not occur in the two hemispheres

alternately, but at the same time. Third, the hypothesis calls for the

constant and frequent repetition of glaciation at absolutely regular

intervals. The geological record shows no such regularity, for sometimes

several glacial epochs follow in relatively close succession at

irregular intervals of perhaps fifty to two hundred thousand years, and

thus form a glacial period; and then for millions of years there are

none. Fourth, the eccentricity hypothesis provides no adequate

explanation for the glacial stages or subepochs, the historic

pulsations, and the other smaller climatic variations which are

superposed upon glacial epochs and upon one another in bewildering

confusion. In spite of these objections, there can be little question

that the eccentricity of the earth's orbit and the precession of the

equinoxes with the resulting change in the season of perihelion must

have some climatic effect. Hence Croll's theory deserves a permanent

though minor place in any full discussion of the causes of climatic

changes.



II. The Carbon Dioxide Theory. At about the time that the eccentricity

theory was being relegated to a minor niche, a new theory was being

developed which soon exerted a profound influence upon geological

thought. Chamberlin,[11] adopting an idea suggested by Tyndall, fired

the imagination of geologists by his skillful exposition of the part

played by carbon dioxide in causing climatic changes. Today this theory

is probably more widely accepted than any other. We have already seen

that the amount of carbon dioxide gas in the atmosphere has a decided

climatic importance. Moreover, there can be little doubt that the amount

of that gas in the atmosphere varies from age to age in response to the

extent to which it is set free by volcanoes, consumed by plants,

combined with rocks in the process of weathering, dissolved in the ocean

or locked up in the form of coal and limestone. The main question is

whether such variations can produce changes so rapid as glacial epochs

and historical pulsations.



Abundant evidence seems to show that the degree to which the air can be

warmed by carbon dioxide is sharply limited. Humphreys, in his excellent

book on the Physics of the Air, calculates that a layer of carbon

dioxide forty centimeters thick has practically as much blanketing

effect as a layer indefinitely thicker. In other words, forty

centimeters of carbon dioxide, while having no appreciable effect on

sunlight coming toward the earth, would filter out and thus retain in

the atmosphere all the outgoing terrestrial heat that carbon dioxide is

capable of absorbing. Adding more would be like adding another filter

when the one in operation has already done all that that particular kind

of filter is capable of doing. According to Humphreys' calculations, a

doubling of the carbon dioxide in the air would in itself raise the

average temperature about 1.3 deg.C. and further carbon dioxide would have

practically no effect. Reducing the present supply by half would reduce

the temperature by essentially the same amount.



The effect must be greater, however, than would appear from the figures

given above, for any change in temperature has an effect on the amount

of water vapor, which in turn causes further changes of temperature.

Moreover, as Chamberlin points out, it is not clear whether Humphreys

allows for the fact that when the 40 centimeters of CO{2} nearest the

earth has been heated by terrestrial radiation, it in turn radiates half

its heat outward and half inward. The outward half is all absorbed in

the next layer of carbon dioxide, and so on. The process is much more

complex than this, but the end result is that even the last increment of

CO{2}, that is, the outermost portions in the upper atmosphere, must

apparently absorb an infinitesimally small amount of heat. This fact,

plus the effect of water vapor, would seem to indicate that a doubling

or halving of the amount of CO{2}, would have an effect of more than

1.3 deg.C. A change of even 2 deg.C. above or below the present level of the

earth's mean temperature would be of very appreciable climatic

significance, for it is commonly believed that during the height of the

glacial period the mean temperature was only 5 deg. to 8 deg.C. lower than now.



Nevertheless, variations in atmospheric carbon dioxide do not

necessarily seem competent to produce the relatively rapid climatic

fluctuations of glacial epochs and historic pulsations as distinguished

from the longer swings of glacial periods and geological eras. In

Chamberlin's view, as in ours, the elevation of the land, the

modification of the currents of the air and of the ocean, and all that

goes with elevation as a topographic agency constitute a primary cause

of climatic changes. A special effect of this is the removal of carbon

dioxide from the air by the enhanced processes of weathering. This, as

he carefully states, is a very slow process, and cannot of itself lead

to anything so sudden as the oncoming of glaciation. But here comes

Chamberlin's most distinctive contribution to the subject, namely, the

hypothesis that changes in atmospheric temperature arising from

variations in atmospheric carbon dioxide are able to cause a reversal of

the deep-sea oceanic circulation.



According to Chamberlin's view, the ordinary oceanic circulation of the

greater part of geological time was the reverse of the present

circulation. Warm water descended to the ocean depths in low latitudes,

kept its heat while creeping slowly poleward, and rose in high latitudes

producing the warm climate which enabled corals, for example, to grow in

high latitudes. Chamberlin holds this opinion largely because there

seems to him to be no other reasonable way to account for the enormously

long warm periods when heat-loving forms of life lived in what are now

polar regions of ice and snow. He explains this reversed circulation by

supposing that an abundance of atmospheric carbon dioxide, together with

a broad distribution of the oceans, made the atmosphere so warm that the

evaporation in low latitudes was far more rapid than now. Hence the

surface water of the ocean became a relatively concentrated brine. Such

a brine is heavy and tends to sink, thereby setting up an oceanic

circulation the reverse of that which now prevails. At present the polar

waters sink because they are cold and hence contract. Moreover, when

they freeze a certain amount of salt leaves the ice and thereby

increases the salinity of the surrounding water. Thus the polar water

sinks to the depths of the ocean, its place is taken by warmer and

lighter water from low latitudes which moves poleward along the surface,

and at the same time the cold water of the ocean depths is forced

equatorward below the surface. But if the equatorial waters were so

concentrated that a steady supply of highly saline water kept descending

to low levels, the direction of the circulation would have to be

reversed. The time when this would occur would depend upon the delicate

balance between the downward tendencies of the cold polar water and of

the warm saline equatorial water.



Suppose that while such a reversed circulation prevailed, the

atmospheric CO{2} should be depleted, and the air cooled so much that

the concentration of the equatorial waters by evaporation was no longer

sufficient to cause them to sink. A reversal would take place, the

present type of circulation would be inaugurated, and the whole earth

would suffer a chill because the surface of the ocean would become cool.

The cool surface-water would absorb carbon dioxide faster than the

previous warm water had done, for heat drives off gases from water. This

would hasten the cooling of the atmosphere still more, not only directly

but by diminishing the supply of atmospheric moisture. The result would

be glaciation. But ultimately the cold waters of the higher latitudes

would absorb all the carbon dioxide they could hold, the slow

equatorward creep would at length permit the cold water to rise to the

surface in low latitudes. There the warmth of the equatorial sun and the

depleted supply of carbon dioxide in the air would combine to cause the

water to give up its carbon dioxide once more. If the atmosphere had

been sufficiently depleted by that time, the rising waters in low

latitudes might give up more carbon dioxide than the cold polar waters

absorbed. Thus the atmospheric supply would increase, the air would

again grow warm, and a tendency toward deglaciation, or toward an

inter-glacial condition would arise. At such times the oceanic

circulation is not supposed to have been reversed, but merely to have

been checked and made slower by the increasing warmth. Thus

inter-glacial conditions like those of today, or even considerably

warmer, are supposed to have been produced with the present type of

circulation.



The emission of carbon dioxide in low latitudes could not permanently

exceed the absorption in high latitudes. After the present type of

circulation was finally established, which might take tens of thousands

of years, the two would gradually become equal. Then the conditions

which originally caused the oceanic circulation to be reversed would

again destroy the balance; the atmospheric carbon dioxide would be

depleted; the air would grow cooler; and the cycle of glaciation would

be repeated. Each cycle would be shorter than the last, for not only

would the swings diminish like those of a pendulum, but the agencies

that were causing the main depletion of the atmospheric carbon dioxide

would diminish in intensity. Finally as the lands became lower through

erosion and submergence, and as the processes of weathering became

correspondingly slow, the air would gradually be able to accumulate

carbon dioxide; the temperature would increase; and at length the

oceanic circulation would be reversed again. When the warm saline waters

of low latitudes finally began to sink and to set up a flow of warm

water poleward in the depths of the ocean, a glacial period would

definitely come to an end.



This hypothesis has been so skillfully elaborated, and contains so many

important elements that one can scarcely study it without profound

admiration. We believe that it is of the utmost value as a step toward

the truth, and especially because it emphasizes the great function of

oceanic circulation. Nevertheless, we are unable to accept it in full

for several reasons, which may here be stated very briefly. Most of them

will be discussed fully in later pages.



(1) While a reversal of the deep-sea circulation would undoubtedly be of

great climatic importance and would produce a warm climate in high

latitudes, we see no direct evidence of such a reversal. It is equally

true that there is no conclusive evidence against it, and the

possibility of a reversal must not be overlooked. There seem, however,

to be other modifications of atmospheric and oceanic circulation which

are able to produce the observed results.



(2) There is much, and we believe conclusive, evidence that a mere

lowering of temperature would not produce glaciation. What seems to be

needed is changes in atmospheric circulation and in precipitation. The

carbon dioxide hypothesis has not been nearly so fully developed on the

meteorological side as in other respects.



(3) The carbon dioxide hypothesis seems to demand that the oceans should

have been almost as saline as now in the Proterozoic era at the time of

the first known glaciation. Chamberlin holds that such was the case, but

the constant supply of saline material brought to the ocean by rivers

and the relatively small deposition of such material on the sea floor

seem to indicate that the early oceans must have been much fresher than

those of today.



(4) The carbon dioxide hypothesis does not attempt to explain minor

climatic fluctuations such as post-glacial stages and historic

pulsations, but these appear to be of the same nature as glacial epochs,

differing only in degree.



(5) Another reason for hesitation in accepting the carbon dioxide

hypothesis as a full explanation of glacial fluctuations is the highly

complex and non-observational character of the explanation of the

alternation of glacial and inter-glacial epochs and of their constantly

decreasing length.



(6) Most important of all, a study of the variations of weather and of

climate as they are disclosed by present records and by the historic

past suggests that there are now in action certain other causes which

are competent to explain glaciation without recourse to a process whose

action is beyond the realm of observation.



These considerations lead to the conclusion that the carbon dioxide

hypothesis and the reversal of the oceanic circulation should be

regarded as a tentative rather than a final explanation of glaciation.

Nevertheless, the action of carbon dioxide seems to be an important

factor in producing the longer oscillations of climate from one

geological era to another. It probably plays a considerable part in

preparing the way for glacial periods and in making it possible for

other factors to produce the more rapid changes which have so deeply

influenced organic evolution.



III. The Form of the Land. Another great cause of climatic change

consists of a group of connected phenomena dependent upon movements of

the earth's crust. As to the climatic potency of changes in the lands

there is practical agreement among students of climatology and

glaciation. That the height and extent of the continents, the location,

size, and orientation of mountain ranges, and the opening and closing of

oceanic gateways at places like Panama, and the consequent diversion of

oceanic currents, exert a profound effect upon climate can scarcely be

questioned. Such changes may be introduced rapidly, but their

disappearance is usually slow compared with the rapid pulsations to

which climate has been subject during historic times and during stages

of glacial retreat and advance, or even in comparison with the epochs

into which the Pleistocene, Permian, and perhaps earlier glacial periods

have been divided. Hence, while crustal movements appear to be more

important than the eccentricity of the earth's orbit or the amount of

carbon dioxide in the air, they do not satisfactorily explain glacial

fluctuations, historic pulsations, and especially the present little

cycles of climatic change. All these changes involve a relatively rapid

swing from one extreme to another, while an upheaval of a continent,

which is at best a slow geologic process, apparently cannot be undone

for a long, long time. Hence such an upheaval, if acting alone, would

lead to a relatively long-lived climate of a somewhat extreme type. It

would help to explain the long swings, or geologic oscillations between

a mild and uniform climate at one extreme, and a complex and varied

climate at the other, but it would not explain the rapid climatic

pulsations which are closely associated with great movements of the

earth's crust. It might prepare the way for them, but could not cause

them. That this conclusion is true is borne out by the fact that vast

mountain ranges, like those at the close of the Jurassic and Cretaceous,

are upheaved without bringing on glacial climates. Moreover, the marked

Permian ice age follows long after the birth of the Hercynian Mountains

and before the rise of others of later Permian origin.



IV. The Volcanic Hypothesis. In the search for some cause of climatic

change which is highly efficient and yet able to vary rapidly and

independently, Abbot, Fowle, Humphreys, and others,[12] have concluded

that volcanic eruptions are the missing agency. In Physics of the Air,

Humphreys gives a careful study of the effect of volcanic dust upon

terrestrial temperature. He begins with a mathematical investigation of

the size of dust particles, and their quantity after certain eruptions.

He demonstrates that the power of such particles to deflect light of

short wave-lengths coming from the sun is perhaps thirty times more than

their power to retain the heat radiated in long waves from the earth.

Hence it is estimated that if a Krakatoa were to belch forth dust every

year or two, the dust veil might cause a reduction of about 6 deg.C. in the

earth's surface temperature. As in every such complicated problem, some

of the author's assumptions are open to question, but this touches their

quantitative and not their qualitative value. It seems certain that if

volcanic explosions were frequent enough and violent enough, the

temperature of the earth's surface would be considerably lowered.



Actual observation supports this theoretical conclusion. Humphreys

gathers together and amplifies all that he and Abbot and Fowle have

previously said as to observations of the sun's thermal radiation by

means of the pyrheliometer. This summing up of the relations between the

heat received from the sun, and the occurrence of explosive volcanic

eruptions leaves little room for doubt that at frequent intervals during

the last century and a half a slight lowering of terrestrial temperature

has actually occurred after great eruptions. Nevertheless, it does not

justify Humphreys' final conclusion that "phenomena within the earth

itself suffice to modify its own climate, ... that these and these alone

have actually caused great changes time and again in the geologic past."

Humphreys sees so clearly the importance of the purely terrestrial point

of view that he unconsciously slights the cosmic standpoint and ignores

the important solar facts which he himself adduces elsewhere at

considerable length.



In addition to this the degree to which the temperature of the earth

as a whole is influenced by volcanic eruptions is by no means so clear

as is the fact that there is some influence. Arctowski,[13] for example,

has prepared numerous curves showing the march of temperature month

after month for many years. During the period from 1909 to 1913, which

includes the great eruption of Katmai in Alaska, low temperature is

found to have prevailed at the time of the eruption, but, as Arctowski

puts it, on the basis of the curves for 150 stations in all parts of the

world: "The supposition that these abnormally low temperatures were due

to the veil of volcanic dust produced by the Katmai eruption of June 6,

1912, is completely out of the question. If that had been the case,

temperature would have decreased from that date on, whereas it was

decreasing for more than a year before that date."



Koeppen,[14] in his comprehensive study of temperature for a hundred

years, also presents a strong argument against the idea that volcanic

eruptions have an important place in determining the present temperature

of the earth. A volcanic eruption is a sudden occurrence. Whatever

effect is produced by dust thrown into the air must occur within a few

months, or as soon as the dust has had an opportunity to be wafted to

the region in question. When the dust arrives, there will be a rapid

drop through the few degrees of temperature which the dust is supposed

to be able to account for, and thereafter a slow rise of temperature. If

volcanic eruptions actually caused a frequent lowering of terrestrial

temperature in the hundred years studied by Koeppen, there should be more

cases where the annual temperature is decidedly below the normal than

where it shows a large departure in the opposite direction. The contrary

is actually the case.



A still more important argument is the fact that the earth is now in an

intermediate condition of climate. Throughout most of geologic time, as

we shall see again and again, the climate of the earth has been milder

than now. Regions like Greenland have not been the seat of glaciers, but

have been the home of types of plants which now thrive in relatively low

latitudes. In other words, the earth is today only part way from a

glacial epoch to what may be called the normal, mild climate of the

earth--a climate in which the contrast from zone to zone was much less

than now, and the lower air averaged warmer. Hence it seems impossible

to avoid the conclusion that the cause of glaciation is still operating

with considerable although diminished efficiency. But volcanic dust is

obviously not operating to any appreciable extent at present, for the

upper air is almost free from dust a large part of the time.



Again, as Chamberlin suggests, let it be supposed that a Krakatoan

eruption every two years would produce a glacial period. Unless the most

experienced field workers on the glacial formations are quite in error,

the various glacial epochs of the Pleistocene glacial period had a joint

duration of at least 150,000 years and perhaps twice as much. That would

require 75,000 Krakatoan eruptions. But where are the pits and cones of

such eruptions? There has not been time to erode them away since the

Pleistocene glaciation. Their beds of volcanic ash would presumably be

as voluminous as the glacial beds, but there do not seem to be

accumulations of any such size. Even though the same volcano suffered

repeated explosions, it seems impossible to find sufficient fresh

volcanic debris. Moreover, the volcanic hypothesis has not yet offered

any mechanism for systematic glacial variations. Hence, while the

hypothesis is important, we must search further for the full explanation

of glacial fluctuations, historic pulsations, and the earth's present

quasi-glacial climate.



V. The Hypothesis of Polar Wandering. Another hypothesis, which has

some adherents, especially among geologists, holds that the position of

the earth's axis has shifted repeatedly during geological times, thus

causing glaciation in regions which are not now polar. Astrophysicists,

however, are quite sure that no agency could radically change the

relation between the earth and its axis without likewise altering the

orbits of the planets to a degree that would be easily recognized.

Moreover, the distribution of the centers of glaciation both in the

Permian and Pleistocene periods does not seem to conform to this

hypothesis.



VI. The Thermal Solar Hypothesis. The only other explanations of the

climatic changes of glacial and historic times which now seem to have

much standing are two distinct and almost antagonistic solar hypotheses.

One is the idea that changes in the earth's climate are due to

variations in the heat emitted by the sun and hence in the temperature

of the earth. The other is the entirely different idea that climatic

changes arise from solar conditions which cause a redistribution of the

earth's atmospheric pressure and hence produce changes in winds, ocean

currents, and especially storms. This second, or "cyclonic," hypothesis

is the subject of a book entitled Earth and Sun, which is to be

published as a companion to the present volume. It will be outlined in

the next chapter. The other, or thermal, hypothesis may be dismissed

briefly. Unquestionably a permanent change in the amount of heat emitted

by the sun would permanently alter the earth's climate. There is

absolutely no evidence, however, of any such change during geologic

time. The evidence as to the earth's cosmic uniformity and as to secular

progression is all against it. Suppose that for thirty or forty thousand

years the sun cooled off enough so that the earth was as cool as during

a glacial epoch. As glaciation is soon succeeded by a mild climate, some

agency would then be needed to raise the sun's temperature. The impact

of a shower of meteorites might accomplish this, but that would mean a

very sudden heating, such as there is no evidence of in geological

history. In fact, there is far more evidence of sudden cooling than of

sudden heating. Moreover, it is far beyond the bounds of probability

that such an impact should be repeated again and again with just such

force as to bring the climate back almost to where it started and yet to

allow for the slight changes which cause secular progression. Another

and equally cogent objection to the thermal form of solar hypothesis is

stated by Humphreys as follows: "A change of the solar constant

obviously alters all surface temperatures by a roughly constant

percentage. Hence a decrease of the heat from the sun would in general

cause a decrease of the interzonal temperature gradients; and this in

turn a less vigorous atmospheric circulation, and a less copious rain or

snowfall--exactly the reverse of the condition, namely, abundant

precipitation, most favorable to extensive glaciation."



This brings us to the end of the main hypotheses as to climatic changes,

aside from the solar cyclonic hypothesis which will be discussed in the

next chapter. It appears that variations in the position of the earth at

perihelion have a real though slight influence in causing cycles with a

length of about 21,000 years. Changes in the carbon dioxide of the air

probably have a more important but extremely slow influence upon

geologic oscillations. Variations in the size, shape, and height of the

continents are constantly causing all manner of climatic complications,

but do not cause rapid fluctuations and pulsations. The eruption of

volcanic dust appears occasionally to lower the temperature, but its

potency to explain the complex climatic changes recorded in the rocks

has probably been exaggerated. Finally, although minor changes in the

amount of heat given out by the sun occur constantly and have been

demonstrated to have a climatic effect, there is no evidence that such

changes are the main cause of the climatic phenomena which we are trying

to explain. Nevertheless, in connection with other solar changes they

may be of high importance.



FOOTNOTES:



[Footnote 10: James Croll: Climate and Time, 1876.]



[Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis

of the cause of glacial periods on an atmospheric basis; Jour. Geol.,

Vol. VII, 1899, pp. 545-584, 667-685, 757-787.



T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp.

93-106, 655-677, and Vol. III, pp. 432-446.



S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some

investigations on carbon dioxide which have had a pronounced effect on

later conclusions.



F. Frech adopted Arrhenius' idea and developed it in a paper entitled

Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte

Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.



The exact origin of the carbon dioxide theory has been stated so

variously that it seems worth while to give the exact facts. Prompted by

the suggestion, of Tyndall that glaciation might be due to depletion of

atmospheric carbon dioxide, Chamberlin worked up the essentials of his

early views before he saw any publication from Arrhenius, to whom the

idea has often been attributed. In 1895 or earlier Chamberlin began to

give the carbon dioxide hypothesis to his students and to discuss it

before local scientific bodies. In 1897 he prepared a paper on "A Group

of Hypotheses Bearing on Climatic Changes," Jour. Geol., Vol. V (1897),

to be read at the meeting of the British Association at Toronto, basing

his conclusions on Tyndall's determination of the competency of carbon

dioxide as an absorber of heat radiated from the earth. He had

essentially completed this when a paper by Arrhenius, "On the influence

of carbonic acid in the air upon the temperature of the ground," Phil.

Mag., 1896, pp. 237-276, first came to his attention. Chamberlin then

changed his conservative, tentative statement of the functions of carbon

dioxide to a more sweeping one based on Arrhenius' very definite

quantitative deductions from Langley's experiments. Both Langley and

Arrhenius were then in the ascendancy of their reputations and seemingly

higher authorities could scarcely have been chosen, nor a finer

combination than experiment and physico-mathematical development.

Arrhenius' deductions were later proved to have been overstrained, while

Langley's interpretation and even his observations were challenged.

Chamberlin's latest views are more like his earlier and more

conservative statement.]



[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate;

Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.



W. J. Humphreys: Volcanic dust and other factors in the production of

climatic and their possible relation to ice ages; Bull. Mount Weather

Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air,

1920.]



[Footnote 13: H. Arctowski: The Pleonian Cycle of Climatic Fluctuations;

Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New

York Academy of Sciences, Vol. 24, 1914.]



[Footnote 14: W. Koeppen: Ueber mehrjaehrige Perioden der Witterung

ins besondere uezer die II-jaehrige Periode der Temperatur. Also,

Lufttemperaturen Sonnenflecke und Vulcanausbrueche; Meteorologische

Zeitschrift, Vol. 7, 1914, pp. 305-328.]