NATURE 



I4X 



THURSDAY, JUNE 24, 1875 



CROLLS ''CLIMATE AND TIME"* 

 Climate and Time in their Geological Relations j a theory 

 of Secular Chatiges of the Earth's Climate. By James 

 Croll, of H.M. Geological Survey of Scotland. (London : 

 Daldy, Isbister, and Co., 1875.) 

 II. 



MR. CROLL'S own theory about the distribution of 

 heat by means of ocean currents is in intimate 

 connection with his ideas as to the variation of climate in 

 past time. His theory may be summarised as follows : — 

 The Gulf Stream and 'other warm or cold currents are 

 due entirely to the prevailing system of winds, which 

 force the water along the surface, or even make it take a 

 lower course ; the return of the colder water from the 

 Arctic regions being assisted by the difference of level 

 caused by driving up the waters into a narrow basin, such 

 as he supposes those regions to be. The result of this 

 theory is, that if one hemisphere is colder than the~ other, 

 the trades on that hemisphere will be strongest, and the 

 resulting warm current will flow into the warmer hemi- 

 sphere ; any difference, therefore, in the mean temperature 

 of one hemisphere from that of the other is augmented 

 according to this theory by ocean circulation, whereas on 

 Dr. Carpenter's theory the latter would have a counter- 

 acting influence. When, however, we take both theories 

 into account, and also the effect of the distribution of land 

 and sea, which is remarkably manifested by the two facts 

 of the South Atlantic being coldest and the North Pacific 

 also coldest, we see that we are not in a position to 

 estimate the effect, if any of much consequence, of the 

 different forms of ocean circulation on the occurrence of 

 a glacial epoch, but must look for the causes of the latter 

 independently. 



Now what are the known facts to be explained ? They 

 are well described in various parts of this book, and the 

 proofs of the less known are carefully given. We have first 

 the widespread indications of a sheet of land ice in the 

 northern hemisphere, reaching in some parts far down 

 into temperate, if not into tropical regions ; secondly, 

 similar indications in the southern hemisphere during the 

 same geological period, but without any proof of their 

 being contemporaneous even in centuries with those in 

 the northern ; thirdly, a much milder climate than at 

 present prevailing in very high latitudes in comparatively 

 modern geological periods, though anterior to the glacial 

 epoch ; fourthly, that these changes from more intense cold 

 to more intense heat have been going on throughout the 

 whole of geological time ; and lastly, that in the midst 

 even of the glacial epoch, warm interglacial periods oc- 

 curred. No satisfactory theory of the cause of the glacial 

 epoch can leave any of these facts unaccounted for, still 

 less contradicted. Sir Charles Lyell's theory, referring it 

 to an alteration of the distribution of land and sea, does 

 not well adapt itself to the magnitude of the phenomena 

 indicated above in the first and second facts, and requires 

 very sudden and violent changes to account for the fifth ; 

 and, moreover, it is shown by Mr. Croll that the distribu- 

 tion he indicates would have the very opposite effect to 



* Continued from p. 123. 



Vol. XII.— No. 295 



that supposed ; geologists are therefore driven, however 

 reluctantly, to consider the action of cosmical causes. 

 Four theories founded on such causes have been pro- 

 posed. 



The first, that the solar system was passing through a 

 cold region of space, may be dismissed at once ; the 

 second is that the sun is a variable star, and therefore 

 the amount of heat received from him is variable ; the 

 third is, that the glacial epoch was due to a great obliquity 

 of the ecliptic ; and the fourth, Mr. CroU's, is that it 

 depended on an increased eccentricity of the orbit com- 

 bined with aphelion winters. We will discuss the last 

 theory first, and examine Mr. Croll's proofs of it. In order 

 to show how the eccentricity has varied in past time, and 

 to find the periods at which it was a maximum or minimum, 

 Mr. Croll has calculated by means of Leverrier's formula 

 what its amount has been or will be, from 3,000,000 years 

 past time to 1,000,000 years in the future, for intervals of 

 50,000 years, and has given a diagram and tables to 

 illustrate the result. This must have been a most labo- 

 rious task, but we are sorry to say that the results re- 

 quire confirmation. We have repeated the calculations 

 for two of the most remarkable dates, near which the 

 change is represented by Mr. Croll as very rapid from a 

 maximum to a minimum, viz., 850,000 and 900,000 years 

 ago respectively, and find that at the former date the 

 eccentricity was "0697 instead of '0747, and at the latter 

 date was '0278 instead of "0102 as expressed in the table. 

 To satisfy ourselves that the mistakes are Mr. Croll's and 

 not ours, we have recalculated also one of Mr. Stone's 

 and one of M. Leverrier's results which have been used 

 by Mr. Croll for the completion of his table, and in both 

 instances have exactly verified them. The fact that the 

 eccentricity was large when he represents it so, and small 

 when he makes it small, seems to indicate that some 

 approximating progress has been followed, and that pos- 

 sibly his diagram may give a rough idea of the changes 

 of eccentricity for past time, provided of course that we 

 agree to Leverrier's formula being used for such remote 

 periods. 



Assuming, however, that at some past date the eccen- 

 tricity of the earth's orbit approached its maximum value, 

 and that at the same time the winter of one hemisphere 

 occurred in aphelion, what would be the result ? In the 

 first place the total annual heat received from the sun, 

 which varies inversely as the minor axis of the earth's 

 orbit, would be 'slightly increased, but not sufficiently to 

 have much, effect upon chmate. The more important 

 result would be that the hemisphere whose winter was in 

 aphelion would have it very rigorous, and its summers 

 would be very hot, while the other hemisphere would be 

 enjoying a perpetual summer. It is on this that Mr. Croll 

 relies for producing a glacial epoch, and we see that it 

 involves the statement that the two hemispheres were 7iot 

 glaciated at the same time, while the other theories 

 assume that they were. 



Our question therefore is : Will an extreme difference 

 between the winter and summer temperature produce a 

 glacial epoch ? The actual amount of heat received by 

 either hemisphere may easily be shown to be the same, 

 whether there are great or little differences between 

 summer and winter, whether as to their length or their 

 intensity, so that a glacial epoch could not be the direct 



