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6 HISTORICAL PERSPECTIVES: CLIMATIC CHANGES THROUGHOUT THE MILLENNIA John E. Kutzbach We are here to consider the prospects of global change and our common future. Our aim is to look forward. I want to share with you some per- spectives about global change that we can gain from first looking back- ward to our common past. Why is it important to look backward first? The first reason is that the global changes that may occur in the next century may be larger than any changes that have occurred in recent centuries. We need to look to more distant times for examples of large global change. We will see that large global climatic changes have had large effects on plants, animals, and humans. The second reason for looking to the past is that the past is a laboratory in which we can study these global processes and develop our predictive capabilities. If we can identify the factors that have caused global change in the past, and if we can successfully estimate past climates and climatic changes using our computer models, then we will gain confidence in our ability to estimate and anticipate future changes. I will describe five examples of large global change from the past: one from about a billion years ago, another from several hundred million years ago, and others from one million years, ten thousand years, and a few centuries ago. Some common themes in all of these examples are that climate and life have been intertwined since the dawn of earth history, that relatively small causes have had large and often unexpected con- sequences, and that the magnitude of some of the possible global changes of the next century rival the magnitude of some of the biggest changes from our past. Of course, the global changes of the past were not caused by humans. Nor could the animals of the earth, or the humans, do anything about these changes. Animals, plants, and humans moved, adapted, or died. In contrast, the global changes of the present and near future may be caused by humans, and perhaps we can do something about them. The first example of global change is taken from more than a billion years ago. Early, innovative forms of life, blue-green algae, used the energy from the sun to split molecules of water and carbon dioxide and then recombined them differently to form organic compounds and oxygen--a process we call photosynthesis. Fossilized deposits from this period are called stromatolites; present-day structures that resemble stromatolites grow today in warm, shallow seas. With the burial of the organic matter produced by photosynthesis, and with other geochemical changes, the 50
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51 FIGURE 6.1 Locations of continents for various times during the past several hundred million years. (Adapted from N. Calder. 1983. Time- scale: An Atlas of the Fourth Dimension. Viking Press, New York.) amount of oxygen in our ancient atmosphere began to increase and the amount of carbon dioxide began to decrease. High in the atmosphere oxygen was split apart by ultraviolet radiation from the sun and then recombined to form ozone. This marked the birth of our ozone shield. That shield is now about a billion years old. It has protected the earth's surface from harmful ultraviolet radiation and has permitted life to flourish on the continents ever since. It has been an old friend, and of course we need to know why it is becoming thinner now. A second example of global change is taken from several hundred million years ago. The drifting and rifting of our restless continents created a grand sequence of global changes over hundreds of millions of years. Figure 6.1 shows the continents' locations from 600 million to 100 million years ago. During a particularly fascinating period between 300 million and 200 million years ago, the continents came together to form one supercontinent, called Pangaea, and then parted again. We are using climate models, the same sort of models used to study possible future climates, to calculate the climate of an idealized Pangaean world (Figure 6.2~. This one-continent world existed at the start of the age of the dinosaur. North America, Greenland, and parts of Eurasia were in the northern hemisphere. South America, Africa, India, Australia, and Antarctica were in the southern hemisphere. There was one ocean and one large sea, the Tethys Sea, on the eastern tropical shores. The huge land mass, according to the climate model, experienced huge seasonal swings of temperature. The continental interior was hot and dry, especially in summer, with temperatures exceeding 35°C, or 100°F.
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90 60 30 of -30 -60 -an 90 So -30 -en 52 l l l l l I ~ r I I ~ l . j , ~ l. v~ A _ LAURASIA ,''_' PANTHAtASSIC ',' i___ _'' " TETHYS SEA GONDWANALAND ~ OCEA N - 9 0 -180 -120 -60 10 m/s 5 mls ) ~ o 60 120 180 FIGURE 6.2 (Top) The idealized Pangaean continent with Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Panthalassa, the world ocean, and the Tethys Sea are indicated. Fine dashed lines indicate very approximately the outlines of modern land masses, but these outlines are only schematic. (Bottom) Surface winds (arrows) and features of surface temperature (warmer than 30°C, stipple; colder than 0°C, hatch) for June-July-August based on a climate model simulation. Polar regions were cold in winter, with temperatures below freezing. It was humid along the coasts of the Tethys Sea ? where monsoon winds were strong. Geologic evidence of the location of plant and animal fossils, ancient sand dunes, and mineral deposits supports much of this computer- estimated climate scenario. This agreement of the simulated climate with geologic data is important. It shows that we are beginning to under- stand, and model, the processes of early global change. Although that world was vastly different from ours in many respects, the climate model calculates that the average temperature of the Pangaean world was only about 5°C, or 9°F, higher than earth's present temperature. That is, the Pangaean world was only marginally warmer than some projections of global temperature for the next century. Throughout this period of drifting continents there was a general rise in the diversity and abundance of life on our planet. Figure 6.3
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53 800 _ ? In _ ~ ~ _ ~ ~ _ ~ 5 1°' ~ ~ ~/ 34 o 500 Myr O FIGURE 6.3 Over the past 500 million years, the general rise in the number of families of marine life has been interrupted at least five times by major extinctions. (Adapted from T. H. van Andel. 1985. New Views on an Old Planet: Continental Drift and the History of Earth, p. 282. Cambridge University Press, New York.) illustrates the increasing number of families of marine life over the period from 500 million years ago to the present. However, the general rise has been interrupted at least five times by major extinctions of life. Around the time of Pangaea, the greatest extinction of all time occurred. By some estimates, about one-half of all families and three-fourths of all species became extinct. This great dying may have been related to the drastic changes in climate that accompanied the formation of Pangaea. Perhaps the Pangaean world had too few unique habitats, perhaps the heat and aridity were too extreme? perhaps ocean currents were different, or perhaps the amounts of oxygen and carbon dioxide were different. We do not know what caused the great extinction, but we know that it happened. Can we learn, from such unexplained catastrophic extinctions, any lessons about the delicate balances that govern life on our planet? I hope so. Because some ecologists estimate that the destruction of tropical rain forests, along with other habitats elsewhere, may produce extinction rates over the next century that will rival the five great extinctions of the past. Another great dying, the most recent, occurred around 65 million years ago. The dinosaurs died. Perhaps changes internal to our globe caused this event, too; or perhaps, as some have argued, an asteroid hit the earth. According to one theory, a giant dust cloud may have been thrown into the atmosphere by the impact. This cloud could have screened out the sunlight and caused a brief but deadly cold? dark winter over the face of the earth. For whatever reason, and there are many theories, the
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54 10xCO~ CO-\ T' 100 Myr I2XCO2 ? O Myr FIGURE 6.4 Over the past 100 million years, earth's climate has experienced a long cooling trend, as indicated by the schematic arrow labeled T for temperature. Over the same period, the atmospheric concentration of carbon dioxide (C02) may also have declined from levels 10 times the present. For comparison, the estimated abrupt doubling of CO2 concentration is shown with a dashed line. dinosaurs are gone! If this great dying had not occurred, dinosaurs might still be the dominant species. But it did occur, and soon thereafter, mammals became the dominant species. A third example of global change encompasses a long slide from a warm to a cool climate (Figure 6.4~. It began around 100 million years ago, when the climate was still much warmer than it is at present. Why was it warmer? One strong possibility is that the carbon dioxide content of the atmosphere may have been about 10 times the present level. This high level could have been caused by great volcanic eruptions associated with rapid spreading of the seafloor and rifting of the continents at that time. However, as the continents moved toward their modern locations, the rate of seafloor spreading slowed, and volcanic activity and outgas sing of carbon dioxide decreased. This, according to one theory, caused the atmospheric concentration of carbon dioxide to fall. And as the greenhouse effect diminished, the earth cooled. In other words, this ancient global change may also have linked changes of carbon dioxide and climate, but with falling levels of greenhouse gases and temperature. If the amount of carbon dioxide doubles in the next century, it will possibly mark a return to the higher carbon dioxide levels, and higher temperature levels, of several million years ago. This is a graphic illustration of the potential magnitude of the experiment that we embarked on with the rapid burning of fossil fuels--fossil fuels that were formed, in many cases, about 100 million years ago. A fourth example of global change comes from the glacial cycles of the past million years (Figure 6.5~. The diminished greenhouse effect mentioned above, perhaps aided by mountain uplift of the Rockies and in Tibet, seemed to help set the stage for the growth of glaciers and huge ice sheets. Geologic data for the most recent glacial age, which occurred about 18,000 years ago, indicate that glacial ice covered much
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5s DATA , . . . . . . . . .. . . . . . . . . ,3 ~ ,N,~ SURFACE PRESENT TYPE PRECIPITATION O ICE SHEET ~ DATA ANNUAL f~ YEAR ROUND ~ I`IOOEL ANNUAL ~ SEA ICE _ < 1000 mm pit WINTER ONLY Ed S" 1" O LAND O OCEAN The Earths Orbit June 2: POLLEN OAK SPRUCE FORAMS ATLANTIC I `: r``har ? 20% N. AMER. EUROPE 20Yo > 10% > 20°h > 5% INDIAN OCEAN > 20q~o G bu/Io~des — > 30% : G pachyderms > 75Yo March 21 -.'.' · - ,I~ ~ \ it_ 2 3 1/2. I_'_ ~ December 21 ~J 1'~: ) September 23 '/ FIGURE 6.5 (Left) Changes in the earth's climate and vegetation that accompanied the transition from glacial conditions (18 ka, around 18,000 years ago) to interglacial conditions (present), as illustrated by geologic and paleoecologic evidence. (Right) Changes in the earth's orbit, shown here for modern conditions, are thought to pace the timing of glacials and interglacials. The minimum earth-sun distance occurs now in northern winter but cycles through the calendar year with a period of about 20,000 years. The axial tilt, now 23~°, varies between about 22° and 25° with a period of about 40,000 years. (Left: Reprinted, by permission, from CORMAP Members, 1988. Copyright (c) 1988 by the AAAS. Right: Reprinted, by permission, from N. G. Pisias and J. Imbrie. 1986/1987. Orbital geometry, CO2, and Pleistocene climate, Oceanus 29~4~:43. Copyright (c) 1986 by Woods Hole Oceanographic Institution.) of North America and Europe and reached south to the locations of modern-day cities such as Chicago, New York, London, Berlin, and Moscow. For comparison, current maps show glacial ice only on Greenland. In the past decade, great progress has been made in understanding the cause of these huge swings from glacial to nonglacial climates. Almost certainly, the swings are triggered, or paced, by relatively small changes in the earth's orbit, small changes that alter the amount of sunlight reaching the earth. These small changes in amount of sunlight equal only a few percent, but the consequences are large. What are these
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56 GLOBAL TEMPERATURE CHANGE 5 'C 18 ka 9 ka 0 ka GLACIAL AGE PRESENT FIGURE 6.6 The simulated gradual increase in surface temperature, averaged for the globe, from the glacial age (18,000 years ago) to present is about 5°C (9°F). For comparison, the estimated abrupt increase in temperature of about 3°C over the next century due to greenhouse warming is shown with a dashed line. Orbital changes? The earth's axis wobbles, like the axis of a spinning top, completing one wobble cycle in about 20,000 years. Also, the tilt of the earth's axis changes slightly, taking about 4O, 000 years for one cycle. And the orbit itself alternates between being almost circular and slightly egg-shaped, with a cycle time of about 100,000 years. When the earth's orbit favors less sunlight in summer, the climate cools, mile-high mountains of ice rise, and the sea level falls by several hundred feet. When the orbit favors more sunlight in summer, the climate warms, the ice melts, and the sea level rises. What lessons can we learn from our glacial past? One lesson is that small causes (such as small changes in the earth's orbit) may have large consequences. There are also potential amplifiers in the global system. In the case of glacial cycles, we now know that the amount of carbon dioxide in the earth's atmosphere drops during the glacial swings, making them even colder, and climbs as the ice melts. Thus changes in greenhouse gases may amplify the consequences of a relatively small initial change in the amount of sunlight. Our roots, and our common past, have links to these glacial ages. Some of our ancestors painted pictures of elk on the walls of the caves of Southern France, sheltered from the cold northerly winds blowing across the tundra of Ice Age Europe. Others took advantage of the lowered sea level to cross the Bering Straits from the Old World to the New World. Then, we humans could only react to nature's moves. Now we are making the moves; we are causing global changes. What happened when the recent glacial age ended? Starting about 18,000 years ago there was a gradual global warming trend of about 5°C, or about 9°F (Figure 6.6~. (Of course, much greater warming occurred in polar regions.) The warming trend extended over more than 10,000 years--until roughly 6,000 years ago. That gradual warming trend is contrasted with a projected, abrupt, 3°C warming trend in the next
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57 DATA - FIGURE 6.7 Changes in the earth's climate and vegetation that accompanied the transition from glacial conditions (18 ka, around 18,000 years ago) to rapid deglaciation (9 ka, around 9,000 years ago) to the present, as illustrated by geologic and paleoecologic evidence (see Figure 6.5 for key). (Reprinted, by permission, from COHMAP Members, 1988. Copyright (c) 1988 by the AAAS.) century. This is a rather startling comparison. The temperature increase during the next century could be similar, in magnitude, to the entire temperature increase that has occurred since the last glacial age. How have natural communities responded to these climatic changes of the past? One example is that whole forest communities have marched across the land. About 18,000 years ago, spruce forests were located in the central United States south of the ice sheet (Figure 6.7~. As the ice melted and the climate warmed, the spruce forest moved to the Great Lakes area 9,000 years ago and is currently located in southern and northwestern Canada.
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58 Some forests moved more than 500 miles to the north in the 10,000-year period of gradual warming, or about 5 miles per century. For comparison, the projected midlatitude warming trend in the next century might force our forests to try to march 250 miles per century--in other words about 50 times faster than the most recent natural rate. The inability of plant and animal communities to move together at such rates might literally tear communities apart. Even at the more sedate pace of the warming at the close of the last glacial age, there were extinctions--of the mammoth, for example, perhaps due to its failure to adjust to the changing environment, or to human impacts (such as hunting pressure), or to both; we do not know. Climate in the tropics undergoes changes, too. Tombstone-like towers that are eroding fragments of lake sediments and that contain the skeletons of fish and crocodile bones are standing today in North Africa in the heart of the Sahara Desert. Around 5,000 to 10,000 years ago a vast lake covered the region, and a whole network of lakes and Neolithic fishermen occupied the Sahara. We have assembled geologic data to describe the climate of that time. Some 9,000 years ago, the area of wetter climate included much of North Africa, the Middle East, and parts of southern and eastern Asia (Figure 6.8~. Experiments with climate models have shown that these huge changes in precipitation were also caused by small changes in the earth's orbit. The important point is that, with the help of climate models, we now know how these changes came to be. About 9,000 years ago, the earth's orbit favored more summer sunlight. The land became hotter and the monsoon winds blew more strongly from sea to land. Rainfall increased and pushed farther inland. The increased rain created lakes in shallow depressions of the desert floor and, in parts of North Africa and the Middle East, created conditions more favorable for the agricultural revolution then under way. The fair agreement between model results and the geologic record of this global change gives us confidence that we are beginning to have a crude predictive capability for understanding what happened and why. A fifth and last set of examples of global change comes from the present millennium. The golden age of the Mesa Verde Indians of Colorado may have been cut short by problems of overpopulation and overuse of the land. These problems were perhaps accentuated by persistent drought that began abruptly in the late thirteenth and early fourteenth centuries. The droughts in Mesa Verde began about the time that temperatures in Europe fell (Figure 6.9~. An important point is that these relatively recent climatic changes can be dated very accurately. And from this we have learned that the climate can change abruptly. Starting in the fourteenth century and continuing through the nineteenth century, Europe was colder than it is now (Figure 6.9~. This period, called the Little Ice Age, has been illustrated by a nineteenth- century artist's sketch of the advance of a mountain glacier in Switzerland. In the nineteenth century, the glacier had descended to a valley floor, threatening a village. In the twentieth century, as a recent photograph shows, the glacier has retreated many miles up to the head of the valley. The climate has warmed from the nineteenth to the
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59 DATA 9 ka i ~Idr88 ~ea~onalit,) | PAST EFFECTIVE MOISTURE O THAN PRESENT Fj /A LESS THAN LO PRESENT F I SAME AS 1~ PRESENT O INDETERMINATE FIGURE 6.8 (Left) Features of the earth's climate around 9,000 years ago (9 ka) based on geologic and paleoecologic evidence (top panel) and climate model simulations of enhanced monsoonal circulations (bottom panel). (Right) Changes in the earth's orbit from the present configuration, where perihelion (minimum earth-sun distance) is in northern winter, to the configuration for 9,000 years ago, where perihelion was in northern summer and the axial tilt was 24°, account in climate model simulations for the enhanced monsoons. (Left: Reprinted, by permission, from COHMAP Members, 1988. Copyright (c) 1988 by the AAAS. Right: Reprinted, by permission, from N. G. Pisias and J. Imbri~ 1986/1987. Orbital geometry, CO2, and Pleistocene climate, Oceanus 29~4~:43. Copyright (c) 1986 by Woods Hole Oceanographic Institution.) F O R A M S oCEAAN > 20% G bu/loldes > 30% ,,,,, ~ G. pachyderms The Earths Orbit March 21 June 2~ cember hi September 23 twentieth century, and present-day temperatures are already warmer than they were at any time in the last millennium. We do not know, for certain, the causes of the climatic changes of recent centuries and decades. Perhaps there have been small changes in the amount of sunlight, or small changes in the frequency of volcanic eruptions, or subtle internal oscillations of atmosphere and ocean. These recent climatic changes are smaller in magnitude than the changes suggested by our models for the twenty-first century. Nevertheless, even these small changes are obviously large enough to have major regional
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60 I i I I I I I 1 r 1.5 °C LITTLE ICE AGE r i / ~ I l 1 1 1 1 1 1 900 1 100 1300 1500 1700 1900 YEAR A.D. L ess glaci o 1 More I glacial FIGURE 6.9 Estimates of the changes in temperature in Europe over the past 1,000 years. (Reprinted, by permission, from J. Imbrie and K. P. Imbrie. 1986. Ice Ages: Solving the Mystery, p. 181. Harvard University Press, Cambridge, Mass. Copyright (c) 1988 by John Imbrie and Katherine Palmer Imbrie.) impacts, as indicated by the advance of glaciers in Switzerland of the thirteenth century or by the Dust Bowl years in America of the twentieth century. This backdrop of ongoing natural climate variability has another very serious consequence. It complicates our task of recognizing the initial phases of human-caused climate change--witness the discussions of the hot, dry summer of 1988. To sum up, I have highlighted several examples of global change from the past: the dawn of life on our planet, the restless rifting of the continents, the waxing and waning of ice ages and monsoons, and the droughts and cold spells of recent centuries. There are a number of lessons to be learned, I think, from this brief historical perspective on global change throughout the millennia. 1. Climate and life have been intertwined since the dawn of earth's history. 2. Relatively small causes (such as orbital changes) have had large consequences. 3. Global change has sometimes been accompanied by the growth or catastrophic decline of species; only five great dyings in the past may compare in magnitude to some estimates of near-future extinctions from the diverse global changes now under way. 4. The potential magnitude of climatic change in the next century, caused by human activities, is comparable to that of some of the large natural climatic changes of the past, but human-caused changes may occur at much faster rates. 5. The global system we need to understand is complicated, but we are making progress in understanding how it works and in constructing predictive models.
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61 Perhaps the most important lesson is that even though there is still much that we do not know about global change, past or future, we already know enough to begin to act now. REFERENCES FOR ADDITIONAL READING COHMAP Members. 1988. Climatic changes of the last 18,000 years: Observations and model simulations. Science 241:1043-1052. Crowley, T.J. 1983. The geological record of climatic change. Geophys. Space Phys. 21:828-877. Kutzbach, J.E., and R.G. Gallimore. 1989. Pangaean climates: Megamonsoons of the megacontinent. J. Geophys. Res. 94:3341-3358 Schneider, S., and R. Lander. 1984. The Coevolution of Climate and Life. Sierra Club Books, San Francisco, 317 pp. Rev.
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