1. CYCLICAL BEHAVIOR OF CLIMATE 



Long-Term Temperature Cycles and Their Significance 



The earth's climate results from 

 three fundamental factors: 



1. The earth's mass, which pro- 

 vides a gravitational field of 

 sufficient strength to hold all 

 gases released from the interior 

 except hydrogen and helium; 



2. The amount of energy emitted 

 by the sun, the distance of the 

 earth from the sun, and the 

 earth's reflectivity, which com- 

 bine to provide surface temper- 

 atures on earth suitable for the 

 existence of a substantial hydro- 

 sphere, including oceans, rivers, 

 lakes, and, at certain times, con- 

 spicuous ice masses; 



3. The astronomical motions of 

 the earth which, together with 

 the inclination of the earth's 

 axis on the plane of the ecliptic, 

 provide diurnal and seasonal 

 cycles. 



If these three fundamental factors 

 (and their components) were to re- 

 main constant through time, the 

 earth's climate would not change 

 except for short-range phenomena re- 

 lated to the hydro-atmosphere. Geo- 

 logical history and direct human ob- 

 servation show, however, that climate 

 has changed and is changing conspic- 

 uously, with variations ranging from 

 a few to many millions of years. The 

 causes for these changes are numer- 

 ous and varied, and often multiple. 



Affecting mankind most, either 

 favorably or unfavorably, are the 

 changes that occur across time inter- 

 vals ranging from tens of years to 

 50,000 years. The former may en- 

 courage men to undertake great agri- 

 cultural and industrial activity in 



regions affected by climatic ameliora- 

 tion, only to have their efforts de- 

 stroyed when climate deteriorates; the 

 latter have brought about the great 

 glacial/interglacial cycles of the past 

 million years, which strongly affected 

 the entire biosphere and directed the 

 course of human evolution. 



Short-Range Climatic Change 



Short-range climatic variations 

 (years to centuries) have been moni- 

 tored by direct observation since the 

 dawn of recorded history, but accurate 

 climatic measurements date only from 

 the middle of the seventeenth century 

 when the Accademia del Cimento of 

 Florence and the Royal Society of 

 London began their works. For more 

 than a hundred years these observa- 

 tions were restricted to Europe. 



Global climatic cycles for which an 

 explanation is immediately clear are 

 the diurnal cycle, due to the rotation 



of the earth, and the yearly cycle, due 

 to the revolution of the earth around 

 the sun. A 2.2-year cycle due to alter- 

 nating easterlies and westerlies in the 

 equatorial stratosphere also appears 

 rather well established. If the effect 

 of the daily, seasonal, and yearly 

 cycles is eliminated, climatic records 

 — including temperatures, pressure, 

 precipitation, wind strength, and 

 storm occurrences — may also exhibit 

 apparent periodicities. Thus, in 1964 

 Schove listed a dozen possible cycles, 

 which ranged in wavelength from 2 

 to 200 years. An apparent 20-year 

 periodicity, for instance, is shown by 

 the 10-year moving average tempera- 

 ture record for July in Lancashire, 

 England. A similar periodicity is not 

 visible, however, in the temperature 

 record for January. The problem is 

 that an infinite record would be neces- 

 sary in order to prove that a cyclical 

 phenomenon is really stationary — 

 i.e., that conditions at the end of a 

 cycle are identical to those at the 

 beginning. 



Figure 111-1 — AVERAGE WATER LEVEL IN LAKE VICTORIA 



This graph indicates that the rise and fall of water level in Lake Victoria from 1900 

 to the middle of the 1920's was correlated with the 11-year sunspot cycle. After 

 that period, however, the correlation broke down. 



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