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Svante Arrhenius
"On the Influence of Carbonic Acid in the Air upon the Temperature
of the Ground"
Philosophical Magazine 41, 237 (1896)[1]
I. Introduction: Observations of Langley on Atmospherical Absorption
A great deal has been written on the influence of the absorption of the atmosphere upon
the climate. Tyndall[2] in particular has pointed out the enormous
importance of this question. To him it was chiefly the diurnal and annual variation of the
temperature that were lessened by this circumstance. Another side of the question, that
has long attracted the attention of physicists, is this: Is the mean temperature of the
ground in any way influenced by the presence of heat-absorbing gases in the atmosphere?
Fourier[3] maintained that the atmosphere acts like the glass of a
hot-house, because it lets through the light rays of the sun but retains the dark rays
from the ground. This idea was elaborated by Pouillet[4]; and Langley
was by some of his researches led to the view, that "the temperature of the earth
under direct sunshine, even though our atmosphere were present as now, would probably fall
to -200°C., if that atmosphere did not possess the quality of selective absorption"[5]. This view, which was founded on too wide a use of Newton's law of
cooling, must be abandoned, as Langley himself in a later memoir showed that the full
moon, which certainly does not posses any sensible heat-absorbing atmosphere, has a
"mean effective temperature" of about 45°C.[6]
The air retains heat (light or dark) in two different ways. On the one hand, the heat
suffers a selective diffusion on its passage through the air; on the other hand, some of
the atmospheric gases absorb considerable quantities of heat. These two actions are very
different. ...
The selective absorption of the atmosphere is, according to the researches of Tyndall,
Lecher and Pernter, Röntgen, Heine, Langley, Ångstöm, Paschen, and others[7], of a wholly different kind. It is not exerted by the chief mass of
the air, but in a high degree by aqueous vapour and carbonic acid, which are present in
the air in small quantities. Further, this absorption is not continuous over the whole
spectrum, but nearly insensible in the light part of it, and chiefly limited to the
long-waved part, where it manifests itself in very well-defined absorption-bands, which
fall off rapidly on both sides[8]. The influence of this absorption
is comparatively small on the heat from the sun, but must be of great importance in the
transmission of rays from the earth. Tyndall held the opinion that the water-vapour has
the greatest influence, whilst other authors, for instance Lecher and Pernter, are
inclined to think that the carbonic acid plays the more important part. The researches of
Paschen show that these gases are both very effective, so that probably sometimes the one,
sometimes the other, may have the greater effect according to the circumstances.
...
II. The Total Absorption by Atmospheres of Varying Composition
As we have no determined, in the manner described, the values of the
absorption-coefficients for all kinds of rays, it will with the help of Langley's figures[9] be possible to calculate the fraction of the heat from a body at
15°C. (the earth) which is absorbed by an atmosphere that contains specified quantities
of carbonic acid and water-vapour. ...
III. Thermal Equilibrium on the Surface and in the Atmosphere of the Earth
As we now have a sufficient knowledge of the absorption of heat by the atmosphere, it
remains to examine how the temperature of the ground depends on the absorptive power of
the air. Such an investigation has been already performed by Pouillet[10],
but it must be made anew, for Pouillet used hypotheses that are not in agreement with our
present knowledge.
In our deductions we will assume that the heat that is conducted from the interior of
the earth to its surface may be wholly neglected. If a change occurs in the temperature of
the earth's surface, the upper layers of the earth's crust will evidently also change
their temperature; but this later process will pass away in a very short time in
comparison with the time that is necessary for the alteration of the surface temperature,
so that at any time the heat that is transported from the interior to the surface
(positive in the winter, negative in the summer) must remain independent of the small
secular variations of the surface temperature, and in the course of a year be very nearly
equal to zero.
Likewise we will suppose that the heat that is conducted to a given place on the
earth's surface or in the atmosphere in consequence of atmospheric or oceanic currents,
horizontal or vertical, remains the same in the course of the time considered, and we will
also suppose that the clouded part of the sky remains unchanged. It is only the variation
of the temperature with the transparency of the air that we shall examine.
All authors agree in the view that there prevails an equilibrium in the temperature of
the earth and of its atmosphere. The atmosphere must, therefore, radiate as much heat to
space as it gains partly through the absorption of the sun's rays, partly through the
radiation from the hotter surface of the earth and by means of ascending currents of air
heated by contact with the ground. On the other hand, the earth loses just as much heat by
radiation o space and to the atmosphere as it gains by absorption of the sun's rays. If we
consider a given place in the atmosphere or on the ground, we must also take into
consideration the quantities of heat that are carried to this place by means of oceanic or
atmospheric currents. For this radiation we will suppose that Stefan's law of radiation,
which is now generally accepted, holds good, or in other words that the quantity of heat
(W) that radiates from a body of the albedo (1-n) and
temperature T (absolute) to another body of the absorption-coefficient b
and absolute temperature q is
W = nbg(T4-q4),
where g is the so-called radiation constant (1.21x10-12
per sec. and cm.2) Empty space may be regarded as having the absolute
temperature 0[11].
...
IV. Calculation of the Variation of Temperature that would ensue in consequence of a
given Variation of the Carbonic Acid in the Air.
...
By means of these values, I have calculated the mean alteration of temperature that
would follow if the quantity of carbonic acid varied from its present mean value (K=1) to
another, viz. to K=0.67, 1.5, 2, 2.5, and 3 respectively. This calculation is made for
every tenth parallel, and separately for the four seasons of the year. The variation is
given in Table VII.
Table VII.--Variation of Temperature caused by a given Variation of Carbonic Acid.
[Editor's note: Arrhenius' Table VII actually consists of five tables side by side,
each representing a different concentration of 'carbonic acid'. For ease of viewing, I
have decoupled these tables. I have also written explicitly the latitude ranges, whereas
the author placed the data between listed latitudes. CJG]
Carbonic Acid=0.67.
Latitude. |
Dec.-Feb. |
March-May. |
June-Aug. |
Sept.-Nov. |
Mean of the year. |
60 to 70 |
-2.9 |
-3.0 |
-3.4 |
-3.1 |
-3.1 |
50 to 60 |
-3.0 |
-3.2 |
-3.4 |
-3.3 |
-3.22 |
40 to 50 |
-3.2 |
-3.3 |
-3.3 |
-3.4 |
-3.3 |
30 to 40 |
-3.4 |
-3.4 |
-3.2 |
-3.3 |
-3.32 |
20 to 30 |
-3.3 |
-3.2 |
-3.1 |
-3.1 |
-3.17 |
10 to 20 |
-3.1 |
-3.1 |
-3.0 |
-3.1 |
-3.07 |
0 to 10 |
-3.1 |
-3.0 |
-3.0 |
-3.0 |
-3.02 |
-10 to 0 |
-3.0 |
-3.0 |
-3.1 |
-3.0 |
-3.02 |
-10 to -20 |
-3.1 |
-3.1 |
-3.2 |
-3.1 |
-3.12 |
-20 to -30 |
-3.1 |
-3.2 |
-3.3 |
-3.2 |
-3.2 |
-30 to -40 |
-3.3 |
-3.3 |
-3.4 |
-3.4 |
-3.35 |
-40 to -50 |
-3.4 |
-3.4 |
-3.3 |
-3.4 |
-3.37 |
-50 to -60 |
-3.2 |
-3.3 |
-- |
-- |
-- |
Carbonic Acid=1.5.
Latitude. |
Dec.-Feb. |
March-May. |
June-Aug. |
Sept.-Nov. |
Mean of the year. |
60 to 70 |
3.3 |
3.4 |
3.8 |
3.6 |
3.52 |
50 to 60 |
3.4 |
3.7 |
3.6 |
3.8 |
3.62 |
40 to 50 |
3.7 |
3.8 |
3.4 |
3.7 |
3.65 |
30 to 40 |
3.7 |
3.6 |
3.3 |
3.5 |
3.52 |
20 to 30 |
3.5 |
3.3 |
3.2 |
3.5 |
3.47 |
10 to 20 |
3.5 |
3.2 |
3.1 |
3.2 |
3.25 |
0 to 10 |
3.2 |
3.2 |
3.1 |
3.1 |
3.15 |
-10 to 0 |
3.1 |
3.1 |
3.2 |
3.2 |
3.15 |
-10 to -20 |
3.2 |
3.2 |
3.2 |
3.2 |
3.2 |
-20 to -30 |
3.2 |
3.2 |
3.4 |
3.3 |
3.27 |
-30 to -40 |
3.4 |
3.5 |
3.7 |
3.5 |
3.52 |
-40 to -50 |
3.6 |
3.7 |
3.8 |
3.7 |
3.7 |
-50 to -60 |
3.8 |
3.7 |
-- |
-- |
-- |
Carbonic Acid=2.0.
Latitude. |
Dec.-Feb. |
March-May. |
June-Aug. |
Sept.-Nov. |
Mean of the year. |
60 to 70 |
6.0 |
6.1 |
6.0 |
6.1 |
6.05 |
50 to 60 |
6.1 |
6.1 |
5.8 |
6.1 |
6.02 |
40 to 50 |
6.1 |
6.1 |
5.5 |
6.0 |
5.92 |
30 to 40 |
6.0 |
5.8 |
5.4 |
5.6 |
5.7 |
20 to 30 |
5.6 |
5.4 |
5.0 |
5.2 |
5.3 |
10 to 20 |
5.2 |
5.0 |
4.9 |
5.0 |
5.02 |
0 to 10 |
5.0 |
5.0 |
4.9 |
4.9 |
4.95 |
-10 to 0 |
4.9 |
4.0 |
5.0 |
5.0 |
4.97 |
-10 to -20 |
5.0 |
5.0 |
5.2 |
5.1 |
5.07 |
-20 to -30 |
5.2 |
5.3 |
5.5 |
5.4 |
5.35 |
-30 to -40 |
5.5 |
5.6 |
5.8 |
5.6 |
5.62 |
-40 to -50 |
5.8 |
6.0 |
6.0 |
6.0 |
5.95 |
-50 to -60 |
6.0 |
6.1 |
-- |
-- |
-- |
Carbonic Acid=2.5.
Latitude. |
Dec.-Feb. |
March-May. |
June-Aug. |
Sept.-Nov. |
Mean of the year. |
60 to 70 |
7.9 |
8.0 |
7.9 |
8.0 |
7.95 |
50 to 60 |
8.0 |
8.0 |
7.6 |
7.9 |
7.87 |
40 to 50 |
8.0 |
7.9 |
7.0 |
7.9 |
7.7 |
30 to 40 |
7.9 |
7.6 |
6.9 |
7.3 |
7.42 |
20 to 30 |
7.2 |
7.0 |
6.6 |
6.7 |
6.87 |
10 to 20 |
6.7 |
6.6 |
6.3 |
6.6 |
6.52 |
0 to 10 |
6.6 |
6.4 |
6.3 |
6.4 |
6.42 |
-10 to 0 |
6.4 |
6.4 |
6.6 |
6.6 |
6.5 |
-10 to -20 |
6.6 |
6.6 |
6.7 |
6.7 |
6.65 |
-20 to -30 |
6.7 |
6.8 |
7.0 |
7.0 |
6.87 |
-30 to -40 |
7.0 |
7.2 |
7.4 |
7.4 |
7.32 |
-40 to -50 |
7.7 |
7.9 |
7.9 |
7.9 |
7.85 |
-50 to -60 |
7.9 |
8.0 |
-- |
-- |
-- |
Carbonic Acid=3.0.
Latitude. |
Dec.-Feb. |
March-May. |
June-Aug. |
Sept.-Nov. |
Mean of the year. |
60 to 70 |
9.1 |
9.3 |
9.4 |
9.4 |
9.3 |
50 to 60 |
9.3 |
9.5 |
8.9 |
9.5 |
9.3 |
40 to 50 |
9.5 |
9.4 |
8.6 |
9.2 |
9.17 |
30 to 40 |
9.3 |
9.0 |
8.2 |
8.8 |
8.82 |
20 to 30 |
8.7 |
8.3 |
7.5 |
7.9 |
8.1 |
10 to 20 |
7.9 |
7.5 |
7.2 |
7.5 |
7.52 |
0 to 10 |
7.4 |
7. |
7.2 |
7.3 |
7.3 |
-10 to 0 |
7.3 |
7.3 |
7.4 |
7.4 |
7.35 |
-10 to -20 |
7.4 |
7.5 |
8.0 |
7.6 |
7.62 |
-20 to -30 |
7.9 |
8.1 |
8.6 |
8.3 |
8.22 |
-30 to -40 |
8.6 |
8.7 |
9.1 |
8.8 |
8.8 |
-40 to -50 |
9.1 |
9.2 |
9.4 |
9.3 |
9.25 |
-50 to -60 |
9.4 |
9.5 |
-- |
-- |
-- |
A glance at this Table shows that the influence is nearly the same over the whole
earth. The influence has a minimum near the equator, and increases from this to a flat
maximum that lies the further from the equator the higher the quantity of carbonic acid in
the air. For K=0.67 the maximum effect lies about the 40th parallel, for K=1.5 on the
50th, for K=2 on the 60th, and for higher K-values above the 70th parallel. The influence
is in general greater in the winter than in the summer, except in the case of the parts
that lie between the maximum and the pole. The influence will also be greater the higher
the value of n, that is in general somewhat greater for land
than for ocean. On account of the nebulosity of the Southern hemisphere, the effect will
be less there than in the Northern hemisphere. An increase in the quantity of carbonic
acid will of course diminish the difference in temperature between day and night. A very
important secondary elevation of the effect will be produced in those places that alter
their albedo by the extension or regression of the snow-covering (see p. 257 [omitted from
this excerpt--CJG]), and this secondary effect will probably remove the maximum effect
from lower parallels to the neighbourhood of the poles[12].
...
We may now inquire how great must the variation of the carbonic acid in the atmosphere
be to cause a given change of the temperature. The answer may be found by interpolation in
Table VII. To facilitate such an inquiry, we may make a simple observation. If the
quantity of carbonic acid decreases from 1 to 0.67, the fall of temperature is nearly the
same as the increase of temperature if this quantity augments to 1.5. And to get a new
increase of this order of magnitude (3°.4), it will be necessary to alter the quantity of
carbonic acid till it reaches a value nearly midway between 2 and 2.5. Thus if the
quantity of carbonic acid increases in geometric progression, the augmentation of the
temperature will increase nearly in arithmetic progression. This rule--which naturally
holds good only in the part investigated--will be useful for the following summary
estimations.
5. Geological Consequences.
I should certainly not have undertaken these tedious calculations if an extraordinary
interest had not been connected with them. In the Physical Society of Stockholm there have
been occasionally very lively discussions on the probable causes of the Ice Age; and these
discussions have, in my opinion, led to the conclusion that there exists as yet no
satisfactory hypothesis that could explain how the climatic conditions for an ice age
could be realized in so short a time as that which has elapsed from the days of the
glacial epoch. The common view hitherto has been that the earth has cooled in the lapse of
time; and if one did not know that the reverse has been the case, one would certainly
assert that this cooling must go on continuously. Conversations with my friend and
colleague Professor Högbom, together with the discussions above referred to, led me to
make a preliminary estimate of the probable effect of a variation of the atmospheric
carbonic acid on the belief that one might in this way probably find an explanation for
temperature variations of 5°-10°C., I worked out the calculation more in detail, and lay
it now before the public and the critics.
From geological researches the fact is well established that in Tertiary times there
existed a vegetation and an animal life in the temperate and arctic zones that must have
been conditioned by a much higher temperature than the present in the same regions.[13] The temperature in the arctic zones appears to have exceeded the
present temperature by about 8 or 9 degrees. To this genial time the ice age succeeded,
and this was one or more times interrupted by interglacial periods with a climate of about
the same character as the present, sometimes even milder. When the ice age had its
greatest extent, the countries that now enjoy the highest civilization were covered with
ice. This was the case with Ireland, Britain (except a small part in the south), Holland,
Denmark, Sweden and Norway, Russia (to Kiev, Orel, and Nijni-Novgorod), Germany and
Austria (to the Harz, Erz-Gebirge, Dresden, and Cracow). At the same time an ice-cap from
the Alps covered Switzerland, parts of France, Bavaria south of the Danube, the Tyrol,
Styria, and other Austrian countries, and descended into the northern part of Italy.
Simultaneously, too, North America was covered with ice on the west coast to the 47th
parallel, on the east coast to the 40th, and in the central part to the 37th (confluence
of the Mississippi and Ohio rivers). In the most different parts of the world, too, we
have found traces of a great ice age, as in the Caucasus, Asia Minor, Syria, the
Himalayas, India, Thian Shan, Altai, Atlas, on Mount Kenia and Kilimandjaro (both very
near to the equator), in South Africa, Australia, New Zealand, Kerguelen, Falkland
Islands, Patagonia and other parts of South America. The geologists in general are
inclined to think that these glaciations were simultaneous on the whole earth[14]; and this most natural view would probably have been generally
accepted, if the theory of Croll, which demands a genial age on the Southern Hemisphere at
the same time as an ice age on the Northern and vice versa, had not influenced
opinion. By measurements of the displacement of the snow-line we arrive at the
result,--and this is very concordant for different places--that the temperature at that
time must have been 4°-5°C. lower than at present. The last glaciation must have taken
place in rather recent times, geologically speaking, so that the human race certainly had
appeared at that period. Certain American geologists hold the opinion that since the close
of the ice age only some 7000 to 10,000 years have elapsed, but this most probably is
greatly underestimated.
One may now ask, How much must the carbonic acid vary according to our figures, in
order that the temperature should attain the same values as in the Tertiary and Ice ages
respectively? A simple calculation shows that the temperature in the arctic regions would
rise about 8° to 9°C., if the carbonic acid increased to 2.5 or 3 times its present
value. In order to get the temperature of the ice age between the 40th and 50th parallels,
the carbonic acid in the air should sink to 0.62--0.55 of its present value (lowering of
temperature 4°-5°C.). The demands of the geologists, that at the genial epochs the
climate should be more uniform than now, accords very well with our theory. The
geographical annual and diurnal ranges of temperature would be partly smoothed away, if
the quantity of carbonic acid was augmented. The reverse would be the case (at least to a
latitude of 50 from the equator), if the carbonic acid diminished in amount. But in both
these cases I incline to think that the secondary action (see p. 257 [omitted from this
excerpt--CJG]) due to the regress or the progress of the snow-covering would play the most
important role. The theory demands also that, roughly speaking, the whole earth should
have undergone about the same variations of temperature, so that according to it genial or
glacial epochs must have occurred simultaneously on the whole earth. Because of the
greater nebulosity of the Southern hemisphere, the variations must there have been a
little less (about 15 per cent.) than in the Northern hemisphere. The ocean currents, too,
must there, as in the present time, have effaced the differences in temperature at
different latitudes to a greater extent than in the Northern hemisphere. This effect also
results from the greater nebulosity in the arctic zones than in the neighbourhood of the
equator.
There is now an important question which should be answered, namely:--Is it probable
that such great variations in the quantity of carbonic acid as our theory requires have
occurred in relatively short geological times? The answer to this question is given by
Prof. Högbom. As his memoir on this question may not be accessible to most readers of
these pages, I have summed up an translated his utterances which are of most importance to
our subject[15]:--
"Although it is not possible to obtain exact quantitative expressions for the
reactions in nature by which carbonic acid is developed or consumed, nevertheless there
are some factors, of which one may get an approximately true estimate, and from which
certain conclusions that throw light on the question may be drawn. In the first place, it
seems to be of importance to compare the quantity of carbonic acid now present in the air
with the quantities that are being transformed. If the former is insignificant with the
latter, then the probability for variations is wholly other than in the opposite case.
"On the supposition that the mean quantity of carbonic acid in the air reaches
0.03 vol. per cent., this number represents 0.045 per cent. by weight, or 0.342 millim.
partial pressure, or 0.466 gramme of carbonic acid for every cm.2 of the
earth's surface. The quantity of carbon that is fixed in the living organic world can
certainly not be estimated with the same degree of exactness; but it is evident that the
numbers that might express this quantity ought to be of the same order of magnitude, so
that the carbon in the air can neither be conceived of as very great nor as very little,
in comparison with the quantity of carbon occurring in organisms. With regard to the great
rapidity with which the transformation in organic nature proceeds, the disposable quantity
of carbonic acid is not so excessive that changes caused by climatological or other
reasons in the velocity and value of that transformation might be not able to cause
displacements of the equilibrium.
"The following calculation is also very instructive for the appreciation of the
relation between the quantity of carbonic acid in the air and the quantities that are
transformed. The world's present production of coal reaches in round numbers 500 millions
of tons per annum, or 1 ton per km.2 of the earth's surface. Transformed into
carbonic acid, this quantity would correspond to about a thousandth part of the carbonic
acid in the atmosphere. It represents a layer of limestone of 0.003 millim. thickness over
the whole globe, or 1.5 km.3 in cubic measure. This quantity of carbonic acid,
which is supplied to the atmosphere chiefly by modern industry, may be regarded as
completely compensating the quantity of carbonic acid that is consumed in the formation of
limestone (or other mineral carbonates) by the weathering or decomposition of silicates.
From the determination of the amounts of dissolved substances, especially carbonates, in a
number of rivers in different countries and climates, and of the quantity of water flowing
in these rivers and of their drainage-surface compared with the land-surface of the globe,
it is estimated that the quantities of dissolved carbonates that are supplied to the ocean
in the course of a year reach at most the bulk of 3 km.3 As it is also proved
that the rivers the drainage regions of which consist of silicates convey very unimportant
quantities of carbonates compared with those that flow through limestone regions, it is
permissible to draw the conclusion, which is also strengthened by other reasons, that only
an insignificant part of these 3 km.3 of carbonates is formed directly by
decomposition of silicates. In other words, only an unimportant part of this quantity of
carbonate of lime can be derived from the process of weathering in a year. Even though the
number given were on account of inexact or uncertain assumptions erroneous to the extent
of 50 per cent. or more, the comparison instituted is of very great interest, as it proves
that the most important of all the processes by means of which carbonic acid has been
removed from the atmosphere in all times, namely the chemical weathering of siliceous
minerals, is of the same order of magnitude as a process of contrary effect, which is
caused by the industrial development of our time, and which must be conceived of as being
of a temporary nature.
"In comparison with the quantity of carbonic acid which is fixed in limestone (and
other carbonates), the carbonic acid of the air vanishes. With regard to the thickness of
sedimentary formations and the great part of them that is formed by limestone and other
carbonates, it seems not improbable that the total quantity of carbonates would cover the
whole earth's surface to a height of hundreds of metres. If we assume 100 metres,- a
number that may be inexact in a high degree, but probably is underestimated,--we find that
about 25,000 times as much carbonic acid is fixed to lime in the sedimentary formations as
exists free in the air. Every molecule of carbonic acid in this mass of limestone has,
however, existed in and passed through the atmosphere in the course of time. Although we
neglect all other factors which may have influenced the quantity of carbonic acid in the
air, this number lends but very slight probability to the hypothesis, that this quantity
should in former geological epochs have changed within limits which do not differ much
from the present amount. As the process of weathering has consumed quantities of carbonic
acid many thousand times greater than the amount now disposable in the air, and as this
process from different geographical, climatological and other causes has in all likelihood
proceeded with very different intensity at different epochs, the probability of important
variations in the quantity of carbonic acid seems to be very great, even if we take into
account the compensating processes which, as we shall see in what follows, are called
forth as soon as, for one reason or another, the production or consumption of carbonic
acid tends to displace the equilibrium to any considerable degree. One often hears the
opinion expressed, that the quantity of carbonic acid in the air ought to have been very
much greater formerly than now, and that the diminution should arise from the circumstance
that carbonic acid has been taken from the air and stored in the earth's crust in the form
of coal and carbonates. In many cases this hypothetical diminution is ascribed only to the
formation of coal, whilst the much more important formation of carbonates is wholly
overlooked. This whole method of reasoning on a continuous diminution of the carbonic acid
in the air loses all foundation in fact, notwithstanding that enormous quantities of
carbonic acid in the course of time have been fixed in carbonates, if we consider more
closely the processes by means of which carbonic acid has in all times been supplied to
the atmosphere. From these we may well conclude that enormous variations have occurred,
but not that the variation has always proceeded in the same direction.
"Carbonic acid is supplied to the atmosphere by the following processes:--(1)
volcanic exhalations and geological phenomena connected therewith; (2) combustion of
carbonaceous meteorites in the higher regions of the atmosphere; (3) combustion and decay
of organic bodies; (4) decomposition of carbonates; (5) liberation of carbonic acid
mechanically inclosed in minerals on their fracture or decomposition. The carbonic acid of
the air is consumed chiefly by the following processes:--(6) formation of carbonates from
silicates on weathering; and (7) the consumption of carbonic acid by vegetative processes.
The ocean, too, plays an important role as a regulator of the quantity of carbonic acid in
the air by means of the absorptive power of its water, which gives off carbonic acid as
its temperature rises and absorbs it as it cools. The processes named under (4) and (5)
are of little significance, so that they may be omitted. So too the processes (3) and (7),
for the circulation of matter in the organic world goes on so rapidly that their
variations cannot have any sensible influence. From this we must except periods in which
great quantities of organisms were stored up in sedimentary formations and thus subtracted
from the circulation, or in which such stored-up products were, as now, introduced anew
into the circulation. The source of carbonic acid named in (2) is wholly incalculable.
"Thus the processes (1), (2), and (6) chiefly remain as balancing each other. As
the enormous quantities of carbonic acid (representing a pressure of many atmospheres)
that are now fixed in the limestone of the earth's crust cannot be conceived to have
existed in the air but as an insignificant fraction of the whole at any one time since
organic life appeared on the globe, and since therefore the consumption through weathering
and formation of carbonates must have been compensated by means of continuous supply, we
must regard volcanic exhalations as the chief source of carbonic acid for the atmosphere.
"But this source has not flowed regularly and uniformly. Just as single volcanoes
have their periods of variation with alternating relative rest and intense activity, in
the same manner the globe as a whole seems in certain geological epochs to have exhibited
a more violent and general volcanic activity, whilst other epochs have been marked by a
comparative quiescence of the volcanic forces. It seems therefore probable that the
quantity of carbonic acid in the air has undergone nearly simultaneous variations, or at
least that this factor has had an important influence.
"If we pass the above-mentioned processes for consuming and producing carbonic
acid under review, we find that they evidently do not stand in such a relation to or
dependence on one another that any probability exists for the permanence of an equilibrium
of the carbonic acid in the atmosphere. An increase or decrease of the supply continued
during geological periods must, although it may not be important, conduce to remarkable
alterations of the quantity of carbonic acid in the air, and there is no conceivable
hindrance to imagining that this might in a certain geological period have been several
times greater, or on the other hand considerably less, than now."
As the question of the probability of quantitative variation of the carbonic acid in
the atmosphere is in the most decided manner answered by Prof. Högbom, there remains only
one other point to which I wish to draw attention in a few words, namely: Has no one
hitherto proposed any acceptable explanation for the occurrence of genial and glacial
periods? Fortunately, during the progress of the foregoing calculations, a memoir was
published by the distinguished Italian meteorologist L. De Marchi which relieves me from
answering the last question[16]. He examined in detail the different
theories hitherto proposed- -astronomical, physical, or geographical, and of these I here
give a short résumé. These theories assert that the occurrence of genial or
glacial epochs should depend on one or other change in the following circumstances:--
- The temperature of the earth's place in space.
- The sun's radiation to the earth (solar constant).
- The obliquity of the earth's axis to the ecliptic.
- The position of the poles on the earth's surface.
- The form of the earth's orbit, especially its eccentricity (Croll).
- The shape and extension of continents and oceans.
- The covering of the earth's surface (vegetation).
- The direction of the oceanic and aerial currents.
- The position of the equinoxes.
De Marchi arrives at the conclusion that all these hypotheses must be rejected (p.
207). On the other hand, he is of the opinion that a change in the transparency of the
atmosphere would possibly give the desired effect. According to his calculations, "a
lowering of this transparency would effect a lowering of the temperature on the whole
earth, slight in the equatorial regions, and increasing with the latitude into the 70th
parallel, nearer the poles again a little less. Further, this lowering would, in
non-tropical regions, be less on the continents than on the ocean and would diminish the
annual variations of the temperature. This diminution of the air's transparency ought
chiefly to be attributed to a greater quantity of aqueous vapour in the air, which would
cause not only a direct cooling but also copious precipitation of water and snow on the
continents. The origin of this greater quantity of water- vapour is not easy to
explain." De Marchi has arrived at wholly other results than myself, because
selective absorption which is possessed by aqueous vapour. And, further, he has forgotten
that if aqueous vapour is supplied to the atmosphere, it will be condensed till the former
condition is reached, if no other change has taken place. As we have seen, the mean
relative humidity between the 40th and 60th parallels on the northern hemisphere is 76 per
cent. If, then, the mean temperature sank from its actual value +5.3 by 4°-5°C., i.
e. to +1.3 or +0.3, and the aqueous vapour remained in the air, the relative humidity
would increase to 101 or 105 per cent. This is of course impossible, for the relative
humidity cannot exceed 100 per cent. in the free air. A fortiori it is impossible
to assume that the absolute humidity could have been greater than now in the glacial
epoch.
As the hypothesis of Croll still seems to enjoy a certain favour with English
geologists, it may not be without interest to cite the utterance of De Marchi on this
theory, which he, in accordance with its importance, has examined more in detail than the
others. He says, and I entirely agree with him on this point:--"Now I think I may
conclude that from the point of view of climatology or meteorology, in the present state
of these sciences, the hypothesis of Croll seems to be wholly untenable as well in its
principles as in its consequences"[17].
It seems that the great advantage which Croll's hypothesis promised to geologists, viz.
of giving them a natural chronology, predisposed them in favour of its acceptance. But
this circumstance, which at first appeared advantageous, seems with the advance of
investigation rather to militate against the theory, because it becomes more and more
impossible to reconcile the chronology demanded by Croll's hypothesis with the facts of
observation.
I trust that after the foregoing pages will prove useful in explaining some points in
geological climatology which have hitherto proved most difficult to interpret.
[Notes--CJG]
[1] Extract from a paper presented to the Royal Swedish Academy of
Sciences, 11th December, 1895. Communicated by the Author.
[2] 'Heat a Mode of Motion', 2nd ed. p. 405 (Lond., 1865).
[3] Mém. de l'Ac. R. d. Sci. de l'Inst. de France, t.
vii. 1827.
[4] Comptes rendus, t. vii. p. 41 (1838).
[5] Langley, 'Professional Papers of the Signal Service,' No. 15.
"Researches on Solar Heat," p. 123 (Washington, 1884).
[6] Langley, "The Temperature of the Moon," Mem. of the
National Academy of Sciences, vol. iv, 9th mem. p. 193 (1890).
[7] Vide Winkelmann, Handbuch der Physik.
[8] Cf., e.g., Trabert, Meteorologische Zeitschrift,
Bd. ii. p. 238 (1894).
[9] 'Temperature of the Moon,' plate 5.
[10] Pouillet, Comptes rendus, t. vii. p. 41 (1838).
[11] Langley, 'Prof. Papers,' No. 15, p. 122. "The
Temperature of the Moon," p. 206.
[12] See Addendum, p. 275. [Omitted from this excerpt.--CJG]
[13] For details cf. Neumayr, Erdgeschichte,
Bd. 2, Leipzig, 1887; and Geikie, "The Great Ice-Age,' 3rd ed. London, 1894;
Nathorst, Jirdens historia, p. 989, Stockholm, 1894.
[14] Neumayr, Erdgeschichte, p. 648; Nathorst, l.
c. p. 992.
[15] Högbom, Svensk kemisk Tidskrift, Bd. vi. p. 169
(1894).
[16] Luigi De Marchi: Le cause dell'era glaciale,
premiato dal R. Istituto Lombardo, Pavia, 1895.
[17] De Marchi, l. c. p. 166.
Carmen Giunta's classical chemistry page.
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