CYCLICAL BEHAV! 



nineteenth century to the present, the 

 amplitude and incidence of tem- 

 perature fluctuations in Europe and 

 eastern North America are broadly 

 similar. But certain of the minor 

 fluctuations lasting perhaps 25 to 30 

 years are missing, or of opposite 

 direction, in some localities in these 

 areas. Minor fluctuations within 

 North America itself also show spatial 

 and temporal irregularities. 



There is no clear evidence that any 

 of the fluctuations are strictly peri- 

 odic — i.e., that they are rhythms. 

 Around the North Atlantic, the ampli- 

 tude of the fluctuations over the past 

 300 years or so appears to have in- 

 creased while their duration has de- 

 creased by comparison to the previous 

 centuries. Periods of about 23, 45-60, 

 100, and 170 years have been sug- 

 gested, but not statistically estab- 

 lished, from observational data and 

 indirect historical records in Europe. 

 Recent analyses of cores from the 

 Greenland ice-cap also indicate fluctu- 

 ations in ls O (oxygen-isotope) con- 

 tents with a period of about 120 years. 



The spatial pattern of climatic 

 change during this century is com- 

 plex. Annual and winter tempera- 

 tures increased over much of the 

 globe from approximately 1890 to 

 1940, especially in high latitudes of 

 the northern hemisphere; net cooling 

 has occurred over about 80 percent 

 of the globe since the latter date. 

 There is virtually no correlation be- 

 tween the over-all global changes and 

 trends in particular areas, which 

 strongly suggests that the controls 

 of regional fluctuations are distinct 

 from those for global changes. 



In low latitudes, the most impor- 

 tant fluctuations involve precipitation, 

 with a decrease of the order of 30 

 percent in many parts of the tropics 

 around 1900. During most of the first 

 half of this century, the equatorial 

 rain belt tended to be narrower, and 

 the tropical arid zone wider, than 

 either during the preceding half- 

 century or since the 1940/50's. This 



change did not affect monsoon Asia, 

 but the same pattern of change oc- 

 curred on the east coasts of Australia 

 and North America up to about 40 

 latitude. The other major area affected 

 by precipitation change is central 

 Asia, where the 1950/60's have been 

 much drier. 



Interactions with Society 



Fluctuations in climate — either nat- 

 ural or man-induced — can have im- 

 portant economic and social implica- 

 tions for man. For example, studies 

 in England and the eastern United 

 States since about 1940 indicate a 

 return to conditions of the early nine- 

 teenth century. What would be the 

 implications of such a sustained de- 

 terioration of climate in the middle 

 and high latitudes of the northern 

 hemisphere? What would be the 

 effects of man's intentional or acci- 

 dental modification of large-scale and 

 local climate? 



Effects of Continuing Deterioration 

 — Even beyond such frost-suscepti- 

 ble, high-value crops as the Florida 

 citrus fruits, farming activities can be 

 markedly affected by changes in tem- 

 perature and moisture balance, espe- 

 cially those occurring in spring and 

 fall. The growing season in England, 

 for example, has shortened by an 

 average of two weeks since 1950, as 

 compared with the years 1920 to 

 1940. Cereal cultivation was revived 

 in Iceland only in the 1920's, after 

 a gap of four centuries or more. Many 

 more summers like that of 1969 — 

 when sea-ice persisted along the 

 northern coast for most of the sum- 

 mer and grain harvests were ruined — 

 could seriously threaten that coun- 

 try's marginal economy. The effect 

 on the fishing industry of Iceland 

 (and other European countries) could 

 also be serious. 



The increased frequency of severe 

 winters in northwest Europe since 

 1939-40, as compared with the pre- 

 ceding twenty years, has been re- 



flected in greater disruption ol 

 port and increased requirements ior 

 domestic heating, winter fodder for 

 cattle, and the like. Through a series 

 of chain reactions, for example, the 

 winter of 1^69-70 had a serious effect 

 on the whole East German economy. 



The relationships between climatic 

 changes and farming are generally 

 nonlinear. In view of the second- 

 er third-order interactions among 

 weather, pests and diseases, soil, and 

 crops, the implications of recent 

 changes may be more significant agri- 

 culturally than the basic climatic fluc- 

 tuations might suggest. This is as true 

 in temperate middle latitudes as it is 

 in semi-arid or other marginal cli- 

 mates. For example, lower air and sea 

 temperatures in spring in northeastern 

 North America are believed to have 

 affected fish (especially salmon) by 

 accentuating the sublethal effects of 

 DDT. 



Effect of Man's Activities — Non- 

 meteorologists tend to base estimates 

 on assumptions of a constant mean 

 and variance of the climatic elements. 

 But there is a serious need to scruti- 

 nize long-term weather/climate modi- 

 fication schemes with respect to their 

 possible interaction with climatic 

 trends. Cloud-seeding programs de- 

 signed to augment snowfall in moun- 

 tain areas, for example, may increase 

 avalanche hazard and spring-summer 

 runoff. If the planned increase coin- 

 cides with an unrecognized trend to 

 greater precipitation (or falling tem- 

 peratures), the effects may exceed 

 expectations. 



Unintentional effects of man, 

 through increased atmospheric pollu- 

 tion (dust, carbon dioxide and other 

 gases, supersonic aircraft trails in the 

 stratosphere), are of international 

 concern, particularly with respect to 

 their health implications. The possi- 

 ble broader effects on global climate 

 and, directly or indirectly, on man's 

 economic activities may be even more 

 critical. Although the role of CO2 is 

 reasonably well understood, the effect 



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