CHANGES OF CLIMATE 465 



It is apparent, on examining the evidence thus far at hand, that 

 the fact of permanent, progressive changes in climate during his- 

 torical times has not yet been definitely established. 



Periodic Oscillations of Climate: Sunspot Period. — The discovery 

 of a distinct eleven-year periodicity in the magnetic phenomena of the 

 earth, naturally led to investigations of similar periods in meteorology. 

 Numerous and varied studies along this line, extending back even into 

 the seventeenth century, but beginning actively about 1870, have been 

 and are still being prosecuted by a considerable number of persons, and 

 the literature on the subject has assumed large proportions. The 

 results, however, have not been satisfactory. The problem is difficult 

 and obscure. It is natural to expect a relation of this sort, and some 

 relation certainly exists. But the results have not come up to expecta- 

 tions. Fluctuations in temperature and rainfall, occurring in an 

 eleven-year period, have been made out for certain stations, but the 

 variations are slight, and it is not yet clear that they are sufficiently 

 marked, uniform and persistent over large areas to make practical 

 application of the periodicity in forecasting possible. In some cases, 

 the relation to sunspot periodicity is open to debate; in others, the 

 results are contradictory. 



Koppen has brought forward evidence of a sunspot period in the 

 mean annual temperature, especially in the tropics, the maximum 

 temperatures coming in the years of sunspot minima. The whole 

 amplitude of the variation in the mean annual temperatures, from 

 sunspot minimum to sunspot maximum, is, however, only 1.3° in the 

 tropics and a little less than 1° in the extra-tropics. There are, how- 

 ever, long periods during which there appears to be no influence, or at 

 least, an obscure one, and the relation before 1816 seems to have been 

 opposite to that since then. More recently Nordmann (for the years 

 1870-1900) has continued Koppems investigation, using the mean 

 annual temperatures of certain tropical stations, and finds that the 

 mean temperatures run parallel with the sunspot curve, but that the 

 minimum temperatures occur with the sunspot maxima (amplitude 

 0.7°). This seems to contradict Koppen's conclusion, and also the 

 fact that the sun is hotter at a time of maximum sunspots. The latter 

 difficulty has been explained on the ground that the rainfall and cloudi- 

 ness, both of which are at a maximum with the sunspot curve, lower 

 the temperature, especially in the tropics. It is obvious that the situa- 

 tion in this matter is rather confusing just at the present time, and 

 that the relation of sunspots and terrestrial temperatures is not wholly 

 clear. The sunspots themselves are probably not the immediate or 

 sole control. " There seems little doubt," says Sir Norman Lockyer, 

 ' that we must look to the study of the solar prominences not only as 

 the primary factors in the magnetic and atmospheric changes in our 



vol,, lxix. — 30. 



