PART III — CLIMATIC CHANGE 



Figure III -4 — VARIATION OF THE MEAN ANNUAL TEMPERATURE 

 OF THE NORTHERN HEMISPHERE 



The graph shows the variation of mean annual temperature near the surface of the 

 northern hemisphere during the past century. Variations in the world mean annual 

 temperature are similar. The curve is based on data from several hundred stations, 

 weighted for the area represented by each station. The data are expressed in 

 terms of ten-year overlapping means of the departures from the 1885 90 mean. 



effect for a given increase in 

 COi concentration is still in dis- 

 pute. (A 1 -centigrade increase 

 might require an increase in 

 concentration of from as little 

 as 25 percent to as much as 80 

 percent; at present rates of COj 

 increase, such a temperature in- 

 crease would take between 50 

 and 300 years.) 



Little is known about the ef- 

 fects of volcanic and man-made 

 dust. The net effect of major 

 eruptions, such as that of Mt. 

 Agung in 1963, on total solar 

 radiation received amounted to 

 a decrease of only 6 percent. 

 Turbidity measurements indi- 

 cate a widespread increase in 

 atmospheric dust content in this 

 century, but the role of man in 

 this, and its meteorological con- 

 sequences, are largely unknown. 



3. Air-sea interaction (meteorol- 

 ogy, oceanography) — Changes 

 of ocean surface temperatures 

 appear to lag two to three years 

 behind long-term atmospheric 



trends, but feedback effects — 

 by which oceanic processes re- 

 inforce an initial atmospheric 

 trend — are of great impor- 

 tance. Short-lived anomalies 

 such as the dry summers in 

 northeastern United States dur- 

 ing the early 1960's can be 

 attributed to persistent sea- 

 temperature anomalies. Little 

 is known about the factors de- 

 termining the duration of a 

 trend or its eventual reversal. 



4. Inherent variability in the at- 

 mospheric circulation (meteor- 

 ology, fluid dynamics) — The 

 year-to-year patterns of devel- 

 opment of the seasonal weather 

 regimes seem to be essentially 

 random. Thus, individual ex- 

 treme seasons may occur in 

 spite of a general trend in the 

 opposite direction. Instances 

 are the severe English winters 

 of 1739-40 and 1878-79, both 

 of which followed a series of 

 mild winter seasons. An exten- 

 sive snow cover or sea-tem- 

 perature anomaly may help to 



trigger circulation changes if 

 other conditions are suitable. 



On the 50- to 200-year time-scale, 

 changes in the disposition of the 

 wind-flow patterns between 10,000 

 and 30,000 feet are important. There 

 is evidence of an approximately 200- 

 year fluctuation in the northern hem- 

 isphere westerlies, with peaks in the 

 early 1300's, 1500's, 1700's, and 

 1900's and with shorter, less regular 

 fluctuations superimposed. Weaken- 

 ing of the westerlies in recent decades 

 has been accompanied by an equator- 

 ward shift of the wind belts. 



Finally, and most fundamentally, 

 the extent to which global climate 

 is precisely determined by the gov- 

 erning physical laws is unknown. 

 Theoretical formulations indicate the 

 possibility that all changes in the 

 atmospheric circulation need not be 

 attributable to specific causes. That 

 is, the atmosphere may not be a 

 completely deterministic system. 



Although much recent attention has 

 been given to changes in atmospheric 

 constituents, it is not yet possible to 

 be at all positive as to which, if any, 

 of these four groups of factors are 

 the major determinants of relatively 

 short-term climatic fluctuations — 

 i.e., those of less than 200 years' dura- 

 tion. Even the secular changes in the 

 strength of the wind belts and their 

 latitudinal location, noted in (4) 

 above, may be determined by forcing 

 of extraterrestrial or terrestrial origin. 



Characteristics of the Fluctuations 



The amplitude of 100- to 200-year 

 fluctuations of temperature is esti- 

 mated to be of the order of 1 J centi- 

 grade; decadal averages of winter 

 temperature have a range of 2 centi- 

 grade. It is generally accepted that 

 the longer the duration of a climatic 

 fluctuation, the larger is the area 

 affected in any given sense and the 

 greater is the response of vegetation, 

 glaciers, and other "indicators." Thus, 

 over the time period from the mid- 



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