PART III — CLIMATIC CHANGE 



such periodicity. These terrestrial 

 phenomena last either much shorter 

 times, like volcanic eruptions or dust 

 storms, or much longer times, like 

 changes in the relative position or 

 extent of continents and oceans. 

 There are, however, certain astronom- 

 ical motions of the earth that occur in 

 cycles of tens of thousands of years. 

 Because of the attraction of the moon, 

 the sun, and the planets on the bulge 

 of the earth, the earth's axis precesses 

 with a periodicity of 26,000 years; the 

 obliquity of the ecliptic with respect 

 to the terrestrial equatorial plane 

 changes with a periodicity of 40,000 

 years; and the eccentricity of the 

 earth's orbit changes with a periodic- 

 ity of 92,000 years. The result of 

 these motions is that, in the high 

 latitudes, periods of warm summers 

 and cold winters alternate every 

 40,000 years with periods during 

 which summers are colder and winters 

 warmer. 



Long before research on deep-sea 

 sediments indicated the probable oc- 

 currence of climatic cycles 40,000 

 years long, the Serbian physicist Mi- 

 lankovitch and the German meteor- 

 ologist Kcippen had suggested that 

 long periods of cool summers could 

 trigger a glaciation even if accom- 

 panied by warmer winters. They 

 reasoned that winter is cold enough 

 anyway at high latitudes for snow to 

 accumulate on the ground, while cool 

 summers would allow permanent 

 snow to expand year after year. The 

 earth's reflectivity would thus in- 

 crease, temperature would decrease, 

 more snow would accumulate, and a 

 major glaciation would rapidly de- 

 velop. 



This theory was enlarged by Geiss 

 and Emiliani to include plastic ice- 

 flow, heat absorption by ice-melting, 

 and downbuckling of the earth's 

 crust under the weight of the ice- 

 sheets in order to explain the disap- 

 pearance of the major ice-sheets 

 (Greenland and Antarctica excluded) 

 at the end of each glaciation. As it 

 now stands, the theory seems to ac- 



count for glacial and interglacial 

 events and their time-scale during the 

 recent past. It also accounts for the 

 timing of high interglacial sea levels 

 related to ice melting. That is, the 

 times when summers were warmest, 

 as calculated from astronomical con- 

 stants, were also the times when sea 

 level stood high as determined by 

 radioactive dating of fossil shells and 

 corals. 



The generalized temperature curve 

 of Figure 1II-2 shows, superimposed 

 on the major oscillations, a number 

 of smaller oscillations. Mathematical 

 analysis of the original isotopic curves 

 of the deep-sea cores has shown 

 that these smaller oscillations are re- 

 lated to the precession of the equi- 

 noxes. Precession of the equinoxes is 

 apparently also responsible for the 

 occurrence of more than one high sea 

 level during interglacial intervals, oc- 

 curring whenever northern summers 

 coincide with perihelion and resulting 

 from partial or even total melting of 

 Greenland ice. 



Figure III-3 shows the original oxy- 

 gen isotopic curves for two deep-sea 

 cores from the Caribbean. The hori- 

 zontal scale shows the depth below 

 the top of the core, the tops of the 

 cores being on the left side (0 cm). 



The tops represent modern sediments, 

 and the time-scale for the various 

 cores can be evaluated by comparing 

 each curve with the generalized tem- 

 perature curve of Figure III-2. The 

 vertical axis represents the 18 0/ u '0 

 concentrations, which are inversely 

 proportional to temperature. The 

 more negative values, therefore, rep- 

 resent higher temperatures. 



As shown in Figure III-3, isotopic 

 values as negative as the ones occur- 

 ring at the tops of the cores, repre- 

 senting the present interglacial con- 

 ditions, occur only occasionally below. 

 Temperature was considerably lower 

 than today during much of the previ- 

 ous interglacial intervals, with periods 

 of temperatures as high as today oc- 

 curring only for a short time (a few 

 thousand years at most) at the peaks 

 of the previous interglacial ages. The 

 present period of high temperature 

 began about 8,000 years ago and 

 reached a peak 2,000 years later. It 

 was followed by a 2°-centigrade tem- 

 perature decrease about 4,000 years 

 ago, in turn followed by a l°-centi- 

 grade increase. 



The Present Situation 



All these changes, short-term as 

 well as long-term, regional as well as 



Figure 111-3 — TEMPERATURE CURVES DERIVED FROM OXYGEN 

 ISOTOPE RATIOS OF DEEP-SEA CORES 



400 



500 600 700 800 900 

 DEPTH BELOW TOP (cm) 



1000 1100 1200 1300 1400 



Sediments formed from shells of microscopic protozoans are known to have a high 

 concentration of '~0 when formed during cold periods. Therefore, high values of 

 the ratio ls 0/"O indicate cool temperatures; low values indicate warm temperatures. 

 The point at which the core is sampled can be dated by other means. Several 

 centimeters of core represent a thousand years. 



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