1896 Arrhenius (1119300), страница 9
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Oneoften hears the opinion expressed, that the quantity ofcarbonic acid in the air ought to have been very muchgreater fbrmerly than now, and that the diminution shouldarise from the circumstance that carbonic acid has been takenfrom the air and stored in the earth's crust in the form ofcoal and carbonates. In many cases this hypothetical diminution'is ascribed only to the ibrmation of coal, whilst themuch more important formation of carbonates is wholly overlooked. This whole method of reasoning on a continuousdiminution of the carbonic acid in the air loses all foundationin fact~ notwithstanding that enormous quantities of carbonicU227"2 P:of. S.
Arrhenius on the Influence of Carbonic Acidacid in the course of time have been fixed in carbonates, ifwe consider more closely the processes by means of whichcarbonic acid has in all times been supplied to the atmosphere.From these we may well conclude that enormous variationshave occurred, but not that the variation has always proceededin the same direction.~cCarbonic acid is supplied to the atmosphere by the following processes :--(1) volcanic exhalations and geological phenomena connected therewith; (2) combustion of carbonaceousmeteorites in the higher regions of the atmosphere ; (3) combustion and decay of organic bodies; (4) decomposition ofcarbonates; (5) liberation oi' carbonic acid mechanicallyinclosed in minerals on their fracture or decomposition.The carbonic acid of the air is consumed chiefly by thefollowing processes :--(6) formation of carbonates fromsilicates on weathering ; and (7) the consumption of carbonicacid by vegetative processes.
The ocean, too, plays animportant r61e as a regulator of the quantity of carbonic acidin the air by means of the absorptive power of its water,which gives off carbonic acid as its temperature rises andabsorbs 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 matterin the organic world goes on so rapidly that their variationscannot have any sensible influence.
From this we mustexcept periods in which great quantities of organisms wereskored up in sedimentary formations and thus subtractedfrom the circulation, or in which such stored-up productswere, as now, introduced anew into the circulation.
Thesource of carbonic acid named in (2) is wholly incalculable."Thus the processes (1), (2), and (6) chiefly remain asbalancing 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 crustcannot be conceived to have existed in the air but as an insignificant fraction of the whole at any one time since organiclife appeared on the globe, and since therefore the consumption through weathering and formation of carbonates musthave been compensated by means of continuous supply, wemust 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 withalternating relative rest and intense activity, in the samemanner the globe as a whole seems in certain geologicalepochs to have exhibited a more violent and general volcanicin the A i r upon the Temperature o.f the Ground.273activity, 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 hasundergone nearly simultaneous variaiions, or at least thatthis factor has had an important influence." If we pass the above-mentioned processes for consumingand producing carbonic acid under review, we find that theyevidently do not stand in such a relation to or dependence onone another that any probability exists for the permanenceof an equilibrium of the carbonic acid in the atmosphere.An increase or decrease of the supply continued duringgeolo~cal periods must, although it may not be important,conduce to remarkable a]terations of the quantity of carbonicacid in the air, and there is no conceivable hindrance toimagining that this might in a certain geological period havebeen several times greater~ or on the other hand considerablyless, than now."As the question of the probability of quantitative variationof the carbonic acid in the atmosphere is in the most decidedmanner answered by Prof.
HSgbom, there remains only oneother 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 Italianmeteorologist L. De Marchi which relieves me from answering the last question*.
He examined in detail the differenttheories hitherto proposed--astronomical, physical, or geegraphical, and of these I here give a short rdsumd. Thesetheories assert that the occurrence of genial or glacial epochsshould depend on one or other change in the following circumstances : - (1) The temperature of the earth's place in space.(2) The sun s radiation to the earth (solar constant).(3) The obliquity of the earth's axis to the ecliptic.(4) The position of the poles on the earth's surface.(5) The form of the earth's orbit, especially its eccentricity(Croll).(6) The shape and extension of continents and oceans.(7) The covering of the earth's surface (vegetation).(8) The direction of the oceanic and a~rial currents.(9) The position of the equinoxes.De Marchi arrives at the conclusion that all these hypothesesmust be rejected (p.
207). On the other hand~ he is of the, Luigi De Marchi : Ze cause dell' era glaclale, premiatodal R. IstitutoLombardo, Pavia, 1895.274Prof. 8. Arrhenius on the I~fluence of Carbonic Acidopinion that a change in the transparency of the atmospherewould possibly give the desired effect. According to hisequatorial regions, and increasing with the latitude into the70th parallel, nearer the poles again a little less.
Further,this lowering would, in non-tropical regions, be less on thecontinents than on the ocean and would diminish the annualvariations of the temperature. This diminution of the air'stransparency ought chiefly to be attributed to a greaterquantity of aqueous vapour in the air, which would causenot only a direct cooling but also copious precipitati,n ofwater and snow on the continents.The origin of thisgreater quantity ofwater-vapour is not easy to explain." DeMarehi has arrived at wholly other results than myself, becausehe has not sufficiently considered the important quality ofselective 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 formercondition is reached, if no other change has taken place.
Aswe have seen, the mean relative humidity between the 40thand 60th parallels on the northern hemisphere is 76 per cent.If, then, the mean temperature sank from its actual value + 5"3by 4°--5° C., i. e. to + 1"3 or +0"3, and the aqueous vapourremained in the air, the relative humidity would increase to101 or 105 per cent. This is of course impossible, for therelative humidity cannot exceed 100 per cent. in the free air.A fortiori it is impossible to assume that the absolutehmnidity could have been greater than now in the glacialepoch.As the hypothesis of Croll still seems to enjoy a certainfavour with English geologists, it may not be without interestto cite the utterance of De Marchi on this theory, whichhe, in accordance with its importance, has examined more indetail than the others.
He says, and I entirely agree withhim on this point : - - " :Now I think I may conclude that fromthe point of view of climatology or meteorology, in thepresent state of these sciences, the hypothesis of Croll seemsto be wholly,untenable as well in its principles as in itsconsequencesIt seems that the great advantage which Croll's hypothesispromised to geologists, viz. of giving them a natm'al 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* D e ~archi, l.
v. p. 166.in the Ai~" upon the Temperature of the Grou~d.275against the theory, because it becomes more and more inapossible to reconcile the chronology demanded by (',roll'shypothesis with the facts of observation.1 trust that after what has been said the theory proposed in the foregoing pages will prove useful in explainingsome points in geological climatology which have hithertoproved most difficult to interpret.ADDENDUM*.As the nebulosity is very different in different latitudes,and also different over the sea and over the continents, it isevident that the influence of a variation in the carbonic acidof the air will be somewhat different from that calculatedabove, where it is assumed that the nebulosity is the sameover the whole globe.
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