PART III — CLIMATIC CHANGE 



The Role of Solar Activity — An 

 example of the erroneous conclusions 

 to which inadequate analysis of cycli- 

 cal phenomena may lead is shown in 

 Figure III-l. A relationship between 

 sunspots and water level in Lake Vic- 

 toria may be inferred from the record 

 between 1900 and 1925, but this rela- 

 tionship breaks down completely after 

 1925. As a matter of fact, the search 

 for causal relationship between cli- 

 mate and the solar sunspot cycle, 

 which averages 11.2 years but ranges 

 from 8 to 18 years, has proved rather 

 unsuccessful. A climatic effect prob- 

 ably does exist, but it is small and 

 masked by other phenomena. 



Changes in solar activity may in- 

 duce changes up to a factor of a 

 thousand in the short wavelength re- 

 gion of the solar spectrum, but this 

 region represents only a hundred- 

 thousandth of the total energy emit- 

 ted by the sun. Thus, the change in 

 solar energy output produced by vari- 

 ations in solar activity is at most one 

 percent. Work by the Smithsonian 

 Institution has shown, however, that 

 the amount of solar energy received 

 at the outer boundary of the earth's 

 atmosphere at the mean distance from 

 the sun (the so-called "solar con- 

 stant," equal to 1.3 million ergs per 

 square centimeter per second) has re- 

 mained constant within the limits of 

 error of the observations during the 

 past 50 years. 



Examples From the Past — Secular 

 climatic changes are often impressive. 

 For example, Lake Constance froze 

 completely in the winter of 1962-63 

 for the first time since 1829-30; Lake 

 Chad poured water into the Sahara in 

 1959 for the first time in 80 years; 

 precipitation in northeast Brazil has 

 decreased 50 percent during the past 

 50 years; arctic temperatures rose 

 some 2° centigrade between 1885 and 

 1940; and the average temperature of 

 the atmosphere and ocean surfaces 

 increased 0.7° centigrade during the 

 same time. Some of these changes are 

 regional (i.e., temperature rises in one 

 region while decreasing in an adja- 



cent one), but others, including the 

 latter one just mentioned, are not. 



Global climatic changes ranging 

 across time intervals of decades to 

 many centuries are known from his- 

 torical records and geological or pale- 

 ontological observations. Exception- 

 ally good weather prevailed in Europe 

 between a.d. 800 and 1200, when 

 glacier boundaries were about 200 

 meters higher, when the Vikings sailed 

 across the northern seas, and when 

 Greenland received its name. A few 

 centuries of colder climate followed: 

 the Baltic froze solid in the winter of 

 1322-23, an event that has not been 

 repeated since; Iceland was blocked 

 by ice for six months of the year dur- 

 ing the first half of the seventeenth 

 century (compared to 1-3 weeks 

 today); and all Alpine glaciers read- 

 vanced substantially in the same 

 period. Since the beginning of the 

 nineteenth century, climate has im- 

 proved again. Whether these climatic 

 changes are cyclical or not is not 

 known, although "cycles" of 80 and 

 200 years, presumably induced by 

 solar changes, have been mentioned 

 in the literature. 



Long-Range Climatic Change 



Climatic changes across longer time 

 intervals (a few thousand years to 

 millions of years) can only be inferred 

 from the geological and paleontologi- 

 cal records. The occurrence of mod- 

 ern-looking blue-green algae in chert 

 deposits dating from two billion years 

 ago indicates that the radiation bal- 

 ance of the earth has not changed 

 much over this extremely long time 

 interval. However, three times since 

 the beginning of the Cambrian era, 

 about 600 million years ago, the radia- 

 tion balance of the earth has been 

 sufficiently disturbed to produce con- 

 spicuous glaciations. This happened 

 during the Early Paleozoic (about 450 

 million years ago) , Late Paleozoic 

 (about 250 million years ago), and 

 Late Cenozoic (the past few million 

 years). At these times, ice-sheets 

 some 2 kilometers thick repeatedly 



covered as much as 30 percent of the 

 continental surface. 



Why Glaciation Occurs — For these 

 major glaciations to develop, the radi- 

 ation balance of the earth must have 

 become negative with respect to its 

 normal state during nonglacial times. 

 That is, the amount of solar radiation 

 reflected back into outer space must 

 have become greater. Cooling of the 

 earth by a decrease of incoming solar 

 radiation does not seem likely be- 

 cause, according to the 1953 calcu- 

 lations of Opik, formation of the ice- 

 sheets to the extent known would 

 have entailed cooling the equatorial 

 belt down to 8 centigrade, whereas 

 the paleontological record indicates 

 that warm-water faunas have existed 

 ever since the beginning of the Cam- 

 brian. Therefore, the radiative bal- 

 ance of the earth must have become 

 negative through the effect of terres- 

 trial phenomena alone. 



Many such phenomena could have 

 done the trick. For instance, an in- 

 crease in continentality would have 

 increased the earth's reflectivity and 

 produced cooling, since land absorbs 

 less solar energy than the sea. Dis- 

 placement of continental masses to- 

 ward high latitudes should favor 

 glaciation. Finally, an increase in 

 atmospheric haze produced by vol- 

 canic activity and dust storms could 

 have reduced the amount of solar 

 energy reaching the earth's surface 

 and, at the same time, reflected into 

 space a portion of the incoming solar 

 radiation. Once the earth's surface 

 temperature is reduced below a certain 

 critical value by one or another or a 

 combination of these factors, ice may 

 begin to develop. Ice is highly reflec- 

 tive, of course, so that more ice means 

 more solar energy reflected, lower 

 temperatures, and even more ice. In- 

 deed, ice appears to be self-expanding 

 and to come to a stop only when the 

 ocean has cooled so much as to pro- 

 vide insufficient evaporation for feed- 

 ing the ice-sheets. 



The pattern of glaciation is best 

 known for the past few hundred thou- 



52 



