E^ 



/veryone is aware that our climate is 

 changing. Most people are aware that 

 such temperate areas as Illinois are 

 growing notably warmer. We see car- 

 dinals and mockingbirds in the Chicago 

 area, whereas forty years ago they never 

 came north of southern Illinois. Opos- 

 sums have invaded as far north as cen- 

 tral New York State. 



From the more serious economic view- 

 point, it has become clear that the ma- 

 ple-sugar industry of northern Ohio is 

 dying out because the sugar-maple trees 

 cannot endure the warmer climate. 

 New Englanders who previously fished 

 on the Grand Banks must now ply the 

 less favorable waters off Greenland for 

 their codfish. Siberia and Canada be- 

 come more prosperous every decade, as 



only a few degrees cooler than the 

 equator. 



Looking more closely at the last 100 

 million years, as shown in Chart 2, we 

 see that there has been a cool time about 

 every 20 million years, with warmer peri- 

 ods between. (Each chart is a blow-up 

 of the right end of the preceding one, 

 giving the added detail we are able to 

 determine as we approach recent time.) 

 A more important trend shows up on 

 this second graph, however. About 35 

 million years ago, a cooling began that 

 continued far longer than before. The 

 warm time of 20 million years ago served 

 merely to interrupt this general cooling 

 trend. Progressively colder temperatures 

 led finally to the Pleistocene Ice Age. 



Even during the Ice Age, as shown in 



CLIMATES OF THE PAST 

 and i—i i-ri inr 



FUTURE 



JOHN CLARK 



ASSOCIATE CURATOR 

 SEDIMENTARY PETROLOGY 



the warming climate permits barley and 

 wheat to grow farther and farther north. 



How much warmer will our hemi- 

 sphere become, and what will be the 

 consequences? Even more important, 

 how rapidly will these changes take 

 place? 



A glance at the history of climates may 

 help us to predict the future. 



The first of the four charts on page 7 

 shows that we are living in a very abnor- 

 mal time, geologically speaking. We are 

 either just emerging from a period of 

 glaciation, or are in an interglacial phase 

 of that period. Glaciations have oc- 

 curred not more than four or five times 

 in the last billion years (the earliest one 

 is problematical both as to time and as 

 to its actual existence), and have been 

 spaced about 200 to 250 million years 

 apart. Most of earth history has seen 

 warm, equable climates, with the poles 



Page 6 MARCH 



Chart 3, the climate did not remain con- 

 tinuously cold. Continental glaciers 

 formed four times, with longer intergla- 

 cial episodes of much warmer weather 

 than we now enjoy. 



The fourth graph shows climate dur- 

 ing the last 10,000 years, since the wan- 

 ing of the last ice sheet. Warmer and 

 cooler periods of a few hundred years' 

 duration have alternated through this 

 brief span of human history and prehis- 

 tory. For almost a thousand years, from 

 4000 to 3000 B.C., the climate was much 

 warmer than it is now. The present 

 warming period began about 1850, fol- 

 lowing 300 years of cold. 



These charted records tell us that cli- 

 mates have fluctuated on rhythms of a 

 few hundred years, several thousand 

 years, 20 million years, and 200-250 

 million years. The briefer rhythms may 

 have occurred throughout time, but our 



imperfect understanding of the geologic 

 record prevents our recognizing them. 

 Continental ice sheets were produced, 

 apparently, only when cold episodes of 

 the three larger rhythms coincided. 

 Since we are now in a warming period, 

 and ice sheets have all but disappeared, 

 the question is, which one or more of the 

 rhythms have passed into a warming 

 phase? Unfortunately, we have no way 

 of knowing. 



The next question is of more immedi- 

 ate interest. How long will this warm- 

 ing continue, and how fast will it pro- 

 gress? Also, what will be its conse- 

 quences? We do have some of the an- 

 swers to these questions. 



We know that essentially our atmos- 

 phere is a thin, fluid film surrounding 

 the earth. The equator receives more 

 heat than do the poles; this sets up a con- 

 vection system, somewhat like what hap- 

 pens when a broad, shallow pan of water 

 is set partly over a gas burner. This con- 

 vection system of warm and cool winds, 

 modified by local factors, determines 

 rainfall, temperature, storms, in fact all 

 aspects of weather and climate. 



At present, the equatorial "burner" is 

 much warmer than the polar cold spots, 

 so we have an actively moving system 

 like water boiling in a pan. If the poles 

 were much warmer than at present, this 

 "boiling" would partly break down to a 

 series of weaker, local movements like 

 water simmering in a pot. Some places 

 now arid would receive more rain from 

 local, wet winds, while other regions at 

 present well-watered might become arid. 



As shown in Chart 4, our climate is 

 warming. At the latitude of Chicago, 

 the increase is about 1 ° Fahrenheit every 

 35 years, but farther north it is almost 

 twice as fast. At this rate, how long will 

 it be until Chicago's climate becomes 

 not only warmer, but actually different, 

 due to the setting up of a new convec- 

 tion system? Will this changed climate 

 be like the warm, wet, jungle-producing 

 climate of Florida, or more like the hot, 

 dry summers and dry, cool winters of 

 west-central Texas? Will the change be 

 permanent, or will we revert to our pres- 

 ent climate? 



In the Museum section on sedimen- 

 tary petrology we are seeking the answers 

 to some of these questions by looking at 

 the past. We are trying to determine 



