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Lessons from the Geologic Past Conspicuous though we now are, Homo sapiens have emerged only within the last few hundreds of thousands of years, a flash of an instant in the planet's 5-billion-year history. The results of our actions are merely superimposed on natural variations in the earth's network of oceanic, atmospheric, and biological systems. As scientists try to predict the future global environment, they constantly confront an enormous obstacle: incomplete un- derstanding of how physical, chemical, and biological processes affect each other and shape the planet today. What is known about the past, however, demonstrates that climate and the fortunes of earth's inhabitants have been intertwined since life began on earth, and that relatively small changes can have large and unexpected consequences. Michael McElroy, chairman of the Department of Earth and Planetary Sciences at Harvard University, notes that if earth scientists could work in the laboratory, "We would have hy- potheses, we would do experiments, we would manipulate the experiment and we would learn about processes by the conven- tional iteration of theory and experiment. But as geophysicists 20
LESSONS FROM THE GEOLOGIC PAST 21 or geochemists, we do not have that luxury. We cannot manip- ulate the earth. Our laboratory is the past. If we imaginatively attempt to understand the changes that took place, in the long term we will have our best chance to predict and to guide our future." In particular, several phases in earth's history shed light on what may be in store with future changes in climate. FORMATION OF THE OZONE SHIELD Our lessons from the past begin with a development that occurred more than ~ billion years ago. Early aquatic organ- isms, blue-green algae, began to use energy from the sun to split molecules of water and carbon dioxide and then recom- b~ne them into organic compounds and oxygen the process known as photosynthesis. Oxygen was used up as organic car- bon was converted to carbon dioxide, but not all of it. In a fateful development, oxygen-poisonous to organisms then- began to accumulate in the atmosphere, touching off a massive ecological disaster for primordial organisms. As oxygen built up, the carbon dioxide content began to cirop. High in the at- mosphere, some molecules of oxygen (02) were split as they absorbed energy from ultraviolet rays and formed single atoms of oxygen. When these recombined with oxygen, ozone (03) molecules formed, which are very effective absorbers of ultra- violet rays emitted by the sun. The ozone formed a protective shield around the earth, eliminating the threat of irradiation by ultraviolet light. With this development, the land was fit for more complex life. The organisms that first began to cast off the oxygen did not survive the switch to an oxygenated atmo- sphere, but some others did, and made the pivotal transition from water to land. PANGAEA A paroxysmal change that occurred about 300 million years ago can help us understand more about our climate today. At that time, when the age of dinosaurs was just beginning, the movement of the earth's crustal plates caused the two major
22 THE EARTH AS A SYSTEM PANGAEA 1 Do Myr Locations of continents for various times during me past several hundred million years. (Adapted from N. Calder. 1983. Timescale: An Atlas of the Fourth Dimension. Viking Press, New York.) continents at the time Laurasia in the north and Gondwana- land in the south to mass together for one unique, relatively brief period of less than 100 million years into a megacontinent called Pangaea, or "all lands." What was it like on Pangaea? The earth in the age of Pan- gaea, of course, was very different from the earth today, and from the earth in the preceding and following geologic times, when the continents were not so closely joined. Nevertheless, scientists use this ancient natural experiment to help understand how the distribution of lands and oceans affects climate. John E. Kutzbach and Robert G. Gallunore, both of the Center for Climatic Research at the University of Wisconsin at Madison, are using a general circulation climate mocle! to cal- culate the climate of Pangaea. Results from their model, along
LESSONS FROM THE GEOLOGIC PAST 23 with evidence from the fossil and geologic record, support ear- lier speculations that the megacontinent was beset by large-scale monsoon conditions in both summer and winter and that these seasons were typified, respectively, by extreme heat and cold. The continental interior was hot and dry, with monthly average temperatures in summer of 35°C, or well over 100°F. (Since these were average temperatures, many days were probably as warm as almost 50°C, or 120°F.) it was relatively humid in the polar regions and along the coasts of the vast, continental-scale em- bayment called the Tethys Ocean, where monsoon winds were strong; the tropics, except along the coasts, were dry. These computer simulations of the climate are supported by geologic evidence of the location of plant and animal fossils and mineral deposits; the degree of agreement between model and geologic data is an indication of scientists' growing understanding of how the climate system works. Although the Pangaean world bore little semblance to ours, the global average temperature was only about 5°C (or 9°F) higher than at present. Before Pangaea formed, the diversity and abundance of life on our planet rose, but as the continents converged into a sin- gle mass, the greatest extinction of all time occurred. By some estimates, more than half of all families and three quarters of all species became extinct. Scientists believe that perhaps this great dying between the Permian and Triassic periods was somehow related to the marked changes in climate that accompanied the development of Pangaea. One possibility is that most organisms could not adapt to the extreme fluctuations in temperature and moisture between summer and winter. Today, only Siberia and northern Canada experience as wide a range of seasonal vari- ation. Other possible explanations abound: Did the Pangaean world have too few unique habitats? Did some sort of catastro- phe occur? Did changes in the ocean currents, temperature, or salinity disrupt the global climate or the chemical and biological balances? Or did the atmospheric concentrations of oxygen and carbon dioxide perhaps create conditions that the plants and organisms living then simply could not tolerate?
24 THE EARTH AS A SYSTEM THE ICE AGES Another significant and telling event in the earth's history was the long slide of climate from warm to cold, beginning about 100 million years ago, when the climate was still much warmer than present and the level of carbon dioxide may have been 10 times greater than it is today. What natural processes could have produced such a high level of carbon dioxide in the earth's atmosphere? It is possible that widespread volcanism had infused the atmosphere with carbon dioxide. The continents dispersed as Pangaea broke apart, and volcanism may have been much more active than today along the mid-ocean ridges where the seafloor was forming and on the continental margins where the seafloor was being subducted. As continents drifted toward their current locations, the seafloor spread apart at a slower rate. Many researchers believe that, consequently, there was less volcanic activity and related carbon dioxide emissions, which led to a cooler climate. This cooling trend that began almost 100 million years ago, coupled with continued movements of continental plates, led to the growth of huge ice sheets on Antarctica and Greenland. Co- incidentally, the North American plate buckled and the Rocky Mountains began to rise. Halfway around the world, the Indian continent collided with the Eurasian continent, giving rise to the Tibetan Plateau, a process still under way. This phase of moun- tain building may have contributed to conditions conducive to further glaciation of North America and Eurasia in the past million to 2 million years. To earth scientists, ice ages are in many ways the flip side of the warming that may be in store for us now. In the past decade, great progress has been made toward understanding the cause of the glacial cycles during the most recent ~ million to 2 million years of earth history. While researchers have attempted to explain the causes of ice ages for more than a century, recently developed scientific tools are yielding major new findings. Some of the most important new information comes from cores carefully extracted by drilling deep into the ice caps of
LESSONS FROM THE GEOLOGIC PAST 25 Greenland and Antarctica. By analyzing the gas in bubbles trapped in the ice, scientists have learned that the atmosphere during the ice ages was quite different from that of today and that the concentrations of greenhouse gases and dust in the atmosphere have undergone wide fluctuations. While several ice cores have been drilled in ice caps around the world, one more than 2 kilometers (~.2 molest long is particularly valuable because it covers more than an entire glacial cycle. This core, recovered from a drillhole at Vostok in Antarctica, contains the record of 160,000 years of climate history, from the present warm period, or "interglacial," through the most recent 100,000-year- long ice age, through a previous warm period and back into an . . even ear 1er ice age. The Vostok ice core is being analyzed by Claude Lorius and colleagues at the Laboratory of Glaciology and Geophysics of the Environment in St. Martin d'Heres and at the Laboratory of Isotopic Geochemistry in Gif sur Yvette, both in France. While researchers in recent years had already learned that carbon diox- ide levels during the most recent ice age were lower than they are during today's interglacial, the French group reports an even stronger relationship between this greenhouse gas and tempera- ture: Atmospheric greenhouse gases and climate generally shift in lockstep throughout the glacial cycle. As the earth moves into an interglacial period, for instance, temperatures rise, and so do concentrations of carbon dioxide. During the deepest part of the ice age, temperatures plummet, and so does carbon dioxide, to perhaps 60 percent of that during the interglacial periods. But researchers do not yet know which is cause and which is effect. Ice cores are far from the only tool providing insights into the earth's climatic history. Researchers glean clues from other sources such as fossilized pollen grains, annual growth rings of trees, records of changing sea level in coral reefs, and even fossilized middens, the junk piles left by packets. Cores of sediment extracted from the floor of the deep sea are especially useful because their chemical composition and the warm- or cold-water fossils they contain reflect changes in ocean temper- ature and the volume of the polar ice caps. 1, ,
26 THE EARTH AS A SYSTEM Information from these diverse sources adds up to a picture of growing complexity but increasing clarity. Scientists have established that ice ages are almost certainly triggered by rel- atively small changes in the amount of sunlight reaching the earth at different latitudes and seasons. These small changes in sunlight, only a few percent, are caused by three orbital effects: slight changes in the earth's elliptical orbit around the sun from nearly circular to more elliptical, over a cycle of about 100,000 years; shifts in the degree at which the earth's axis is tilted, over a cycle of about 40,000 years; and wobbling of the axis itself, over a cycle of about 20,000 years. When the orbital conditions result in less sunlight in summer, the climate cools, ice may gradually accumulate into mountains of ice over 2 miles tall, and, because water is transferred to the ice caps, sea level may drop by several hundred feet. When the orbital conditions yield more sunlight in summer, the climate warms, the ice melts, and sea level rises. While orbital changes may trigger the glacial cycles, the shifts in sunlight are not great enough in themselves to force cli- mate change of ice age magnitude. Oceans, with their vast capacity for storing heat and carbon, also may play a crit- ical role in causing climate change. Lorius and colleagues suggest that two kinds of ocean fluctuations-"deep changes possibly driven by sea level and surface changes driven by atmospheric circulation"~rive the carbon dioxide variations between glacial and nonglacial tunes. Not only physics but also chemistry and biology may be catalysts in the climate cycle. Plankton and other photosyn- thetic microorganisms living In the ocean may help regulate the world's climate. These microbes absorb carbon dioxide in the process of photosynthesis. According to one scenario, the plankton may flourish, or "bloom," as polar ice caps grow and nutrients in the water become more concentrated or as ocean currents change, bringing nutrient-rich bottom waters to the surface. The explosion in plankton concentrations would mean more photosynthesis. Carbon dioxide levels in the ocean would
LESSONS FROM THE GEOLOGIC PAST 27 drop, and consequently more wouIcl be pulled from the atmo- sphere, cooling the earth. Plankton may also be involved in another mechanism, along with the clouds that form over the ocean and at any given time cover 30 percent of the world. Researchers studying both arctic and antarctic ice say that the concentration of sulfate particles varies with temperature as temperatures drop, sulfate concen- trations rise. One possible reason is that some kinds of plankton excrete a sulfur compound called dimethyIsulfide (DMS). When dimethylsulfide diffuses from the ocean to the atmosphere, it oxidizes into sulfate particles, which act as condensation nuclei for water droplets that form marine stratus clouds over the open ocean. As the plankton bloom, the number of nuclei increases. With more nuclei, more incoming solar radiation is reflected back to space by the clouds, lowering the water's surface tem- perature and cooling the earth. THE CURRENT WARM PHASE The geologic record shows that the latest act of the glacial drama opened as the most recent glaciation began to wind down about 1S,000 years ago. As has been the pattern, the cold period lasted about 100,000 years; the present balmy climate is a brief warm spell in a typically icy cycle. This most recent switch from a glacial to a warm phase is of special interest to scientists grappling to make sense of the complexities of modern climate because the amount of carbon dioxide that has accumulated in the atmosphere from when the melting began to the present is roughly equal to the amount of greenhouse gases projected to build up from the present to about the middle of the next century. To determine what happened when the most recent glacia- tion ended and why, a number of academic institutions have pooled their efforts through the Cooperative Holocene Map- ping Project (COHMAP). As Kutzbach explains, they use both geologic data and general circulation climate models to identify
28 THE EARTH AS A SYSTEM 1.5 °C I I I L e s s LITTLE ICE A G E ,'- r ~l ~J\: I 1 1 1 1 1 1 900 1 100 1300 1500 1700 1900 Y E A R 9 1 a ci a I ore glacial Estimates of the changes in temperature in Europe over the past 1000 years. (Reprinted, by permission, from J. Imbrie and K. P. Imbrie. 1986. Ice Ages: Solving the Mystery. Harvard University Press, Cambridge, Mass. Copyright (A 1986 by John Imbrie and Katherine Palmer Imbrie.) and evaluate the causes and mechanisms of this most recent change from glacial to the current interglacial. Although the ex- act sequence of events is unclear, COHMAP results suggest that the orbitally caused increase of summer sunlight, the rise in car- bon dioxide, and the melting of glacial ice all began about 1S,000 years ago. The global warming trend over the past 13,000 years has been about 5°C (or 9°F). In high latitudes, such as Canada, the warming since IS,000 years ago has been much greater- 10°, 20°, even 30°C. Arctic permafrost and sea ice receded and sea level rose. Spruce forests migrated from the central United States to southern Canada, and lake beds clried up in California and Nevada. These and other environmental changes influ- encecl human communities throughout the world. This most recent global warming of about 5°C occurred over a period of many millennia. In contrast, the projected future warming from human-producec! greenhouse gases may occur within a century or so, in other words, perhaps 10 to 50 times faster. Results from COHMAP also show that climate changes in
LESSONS FROM THE GEOLOGIC PAST 29 the mid-latitudes and the tropics likewise seem to be linked to small changes in the earth's orbit. As radiation in summer increased, the land became hotter, creating greater contrast be- tween ocean and land temperatures. This produced strong sum- mer monsoons from 12,000 to 6,000 years ago in the Northern Hemisphere tropics and subtropics, and drought in the interi- ors of North America and central Asia. Thus, at a time when the American desert basins were dry, lakes flooded much of North Africa that is today covered by the shifting sands of the Sahara Desert. These lakes formed in shallow depressions of the desert floor. In parts of North Africa and the Middle East, climatic conditions became more favorable for the agricultural revolution then under way. Agreement between mode! results and the geologic record of this relatively recent global change reassures scientists that they are now beginning to understand what happens in the climate system. The climate has also varied in recent centuries and decades. Although these changes were not as dramatic as those of earlier times, nor as large as those expected in the next century, their beginnings and endings are accurately known. From this, scien- tists know that climate can change abruptly and that the changes can be large enough to have regional impacts. The golden age of the Anasazi Indians on Mesa Verde in the southwestern United States, for instance, may have been cut short by overpopulation and overuse of land, coupled with the persistent drought that began suddenly in the late thirteenth century. This drought be- gan about the time that Europe was gripped by a cold snap, known as the Little Ice Age, that persisted until the nineteenth century. In the 1930s dry conditions led to the North American Dust Bowl. Whether these variations in climate were caused by changes in the amount of sunlight or in the frequency of vol- canic eruptions or by subtle internal oscillations of atmosphere and ocean is not known. As evident from these examples of natural climate variabil- ity, it is difficult to recognize the initial phases of human-caused
30 THE EARTH AS A SYSTEM climate change. Nonetheless, it is clear that throughout the his- tory of life on earth, the fortunes of earth's inhabitants have been inextricably tied to variations in climate. To be sure, photosyn- thesizing algae helped create and maintain the conditions that allowed life to persist without pause for more than 3.5 billion years. We, on the other hand, have produced conditions that could push the earth to the brink of climate change at a rate unprecedented In the planet's history. It is highly probable that life will survive. After all, life has survived all the past changes of climate. But it may not be life as we know it now. Will plant and animal communities respond quickly enough to the projected environmental change, or will the uneven pace of adjustment literally tear communities apart? Will humans be able to adapt as planetary conditions change? The answer may lie in the planet's past, and in understanding the complex, interdependent components that make up the earth system.
