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4 System Interactions: Atmosphere, Oceans, Land, and Humans Over the past several decades, scientists' understancting of the complexities of the earth system has evolved to the point where they now recognize that the components of the system- the atmosphere, oceans, land, and associated living beings in- cluding humans are inextricably intertwined. A change in one part of the earth system has repercussions for other parts often In ways that are neither obvious nor immediately apparent. It is beyond the human ken, however, to study the whole, multicli- mensional system at once. As the following sections attest, the effort to understand the dynamics driving change in the global environment is clesigned along the academic lines that essen- tially define classical disciplines. In fact, though, researchers are ever-aware that the various sciences of the atmosphere, ocean, land, and water are connected in countless ways. The intrica- cies of the earth system range from the obvious links between currents in the ocean and atmosphere, to the all-encompassing global cycles of carbon and water, to the subtle, distant effect of clearing a tract of tropical forest on the amount of carbon in the atmosphere. While each major component of the earth system holds its 31

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32 THE EARTH AS A SYSTEM mysteries, the effect of human activity on the system can be the greatest wildcard of all. For the first time, the social sciences are assuming substantial weight in the study of the earth sys- tem as researchers and policymakers struggle to discern how humankind, this relatively recent, terribly powerful feature of the earth, affects the age-old forces that also dictate our planet's future. ATMOSPHERE Many of the earth's inhabitants live far from the oceans; concerns about tropical forests may seem remote to farmers on the American Plains, or to women gathering firewood in the Himalayas. But the atmosphere touches each of us. The atmosphere, a gaseous envelope that surrounds the earth, is the engine of the physical climate system. When ra- diation from the sun enters the atmosphere, some is reflected back upward by clouds and dust, and some continues on to the land surface. Of that radiation that strikes the surface, some is absorbed by the earth, but some is reflected back to space by ice, snow, water, and other reflective surfaces. In addition, infrared radiation is emitted by the earth. A portion of this en- ergy gets trapped by certain atmospheric gases whose particular chemistries do not allow the outgoing, longer-wave infrared ra- diation to escape. Insteacl, this bounces back to the earth, raising the surface temperature. This phenomenon, which has operated throughout earth's history, is well known as the greenhouse ef- fect. Without the atmosphere and the greenhouse effect, the earth's surface would be frozen, and life would not be possible. At the other end of the spectrum, the atmosphere on Venus is so dense with carbon dioxide and the greenhouse effect is so intense that the planet's surface is everywhere as hot as cooling lava on earth. The composition of the atmosphere determines the earth's ability to maintain a balance between the energy coming in and the energy released. The main gases in dry air are nitrogen (79

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SYSTEM INTERACTIONS 33 (79 percent), oxygen (20 percent), and argon (1 percent). Water vapor, present in variable concentrations up to a few percent, is the major gas responsible for the greenhouse effect on earth. Other greenhouse gases are present in trace amounts, usually measured in parts per billion (ppb). The trace gas that has recently received the most attention is carbon dioxide, which currently constitutes 0.034 percent, or 344 parts per million, of the atmosphere. In addition to carbon dioxide, other trace gases-two chIo- rofluorocarbons (CFC-~1 and CFC-12, which also destroy the protective ozone layer that shields us from harmful ultraviolet radiation), methane, nitrous oxide, and tropospheric ozone are efficient at absorbing infrared radiation emitted by the earth. They are of special interest now because their concentrations in the atmosphere are rising. As they do, less radiation escapes from the surface into space, and the earth's temperature rises. The future of the earth's climate and, perhaps, its inhabi- tants, depends on how much concentrations of carbon dioxide and other trace gases are likely to rise. Carbon dioxide poses the single greatest threat because it is the most abundant of these gases. It occurs naturally in the atmosphere and is cy- cled through nearly all living organisms. Animals, including humans, exhale it as a waste product, whereas plants "breathe" it, using the carbon to make the carbohydrates they require in the processes known as photosynthesis. Analysis of air bubbles trapped in glacial ice and contem- porary measurements reveal that carbon dioxide concentrations have increased by nearly 25 percent since the eighteenth century, when industrialization began. The main cause is the combustion of fossil fuels, which produces compounds that also contribute to problems such as local air pollution and acid deposition. During combustion, carbon is oxidized to carbon dioxide and released to the atmosphere. The destruction of forests for settIe- ments or cultivation contributes to this rise also. When land is cleared, the trees either decompose or are burned, and the car- bon stored in the plant material is released to the atmosphere. We have accurate records of modem carbon dioxide levels

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SYSTEM INTEMCTIONS 35 since 195S, when Charles D. Keeling, of Scripps Institution of Oceanography in La lolIa, California, began measuring concen- trations of atmospheric gases from a research station high on Mauna Loa in Hawaii. His records cover a relatively brief in- terval, but are treasured by scientists: They clearly show that carbon dioxide is increasing in the global atmosphere, anct they also show a striking sawtooth pattern that reflects the entire biosphere of the Northern Hemisphere "breathing in" as plants grow in the warm months and "exhaling" when they are dor- mant. From studies of glacial ice samples, scientists know that the level of carbon dioxide during ice ages was about 200 parts per million. in between glacial periods, when the earth was warm, it was about 280 parts per million. Today we are at 350 parts per million and climbing. The other trace greenhouse gases methane, nitrous ox- ide, chIorofluorocarbons, and ozone-absorb infrared radiation much more effectively than carbon dioxide cloes, but they are present in much smaller quantities. Their combined effect may well cause half of the global warming projected for the next century. The atmosphere's methane content is particularly worri- some because it is rising at a much faster rate than even carbon dioxide. Systematic measurements of methane concentrations did not begin until the late 1960s. During the 1980s, levels of this gas rose sharply, at a rate of about l.1 percent per year. Stud- ies of ice cores show that the methane increase over the centuries parallels the swelling of human population, a logical connection because methane is produced through the rumination of increas- ing numbers of cattle and through rice paddy cultivation, which is also increasing. Like carbon dioxicle, methane concentrations in the atmosphere vary with the glacial cycle. During the ice ages, methane was present in the atmosphere at roughly 300 parts per billion. During interglacial periods, the atmospheric levels doubled to perhaps 600 parts per billion. Now we are at 1800 parts per billion and climbing. The sources for this rise include melting of tundra permafrost, biomass burning, leaks

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36 355 350 - Q o z C: z o on 320 345 340 335 330 325 315 310 THE EARTH AS A SYSTEM If__ ~ 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 YEAR Concentration of atmospheric carbon dioxide in parts per million of dry air (ppm) versus time for the years 1958 to 1989 at Mauna Loa Observatory, Hawaii. The dots indicate monthly average concentration. (Reprinted, by permission, from C. D. Keeling et al. 1989. "A Three Dimensional Model of Atmospheric CO2 Transport Based on Observed Winds: Observational Data and Preliminary Analysis," Appendix A, in Aspects of Climate Variability in the Pacific and the Western Americas, Geophysical Monograph, vol. 55, Nov. Copyright (3 1989 by me American Geophysical Union.) in natural gas pipelines, and emissions from rice paddies and cattle, but they are far from quantified. Atmospheric scientists try to decipher the workings of the physical climate system by constructing what are known as general circulation models. These computer models use math- ematical equations to express the basic physical principles that govern the global atmosphere and then use actual data to test whether the models adequately simulate reality. The general circulation models, as they now exist, simu- late the physical climate and geographical features on a very coarse scale. A country the size of Japan, for example, does not appear on the computer-generated maps. Vast numbers of calculations and large amounts of computer time and money would be required to refine the scale.

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SYSTEM INTERACTIONS 37 It is not only the coarse scale of the general circulation mod- els that is proving problematic in using the models to answer questions about climate change. The current models clo not in- corporate other components of the earth system that are known to exert strong influences on the physical climate. Scientists are attempting to incorporate the dynamics of the ocean, and its enormous abilities to absorb heat and carbon, into the climate models. Cloud cover, too, has a strong moderating influence on the greenhouse effect, but it is difficult to characterize and incorporate into coarse-scale models. Even more clifficult to model, and perhaps more important, are the living parts of the world the forests, which store carbon and moisture, and the marine biota, which sequester carbon. Scientists look longingly to the day when enough is understood about these processes to include them in the models. Perhaps such a grand mode! can never be constructed, but the conceptual approach embedded in the attempt lays the cornerstone for earth system science. OCEANS The worId's oceans are the atmosphere's partner in the phys- ical climate system. Just as atmospheric chemistry fluctuates, so does ocean chemistry, though not in the same ways. While much is known about ocean circulation and its coupling to at- mospheric currents and pressure, less is certain about its ability to store additional carbon or about how much heat it will store in response to rising surface temperatures. The ocean is an immense reservoir of heat, holding the heat it absorbs from solar radiation longer than the land does. As the ocean water moves through its grand circulation scheme, heat is transferred vertically from the surface waters to the deep ocean and back, and horizontally from high latitude to Tow latitude and from longitude to longitude. As heat is released by the ocean in a region remote from where it was absorbed, it interacts with the overlying atmo- sphere, moderating the daily and seasonal cycles and tempera- ture on the earth's surface areas. Thus the ocean helps to shape

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38 THE EARTH AS A SYSTEM the regional features of weather and climate. The episodic cli- mate phenomenon known as the E} Nino/Southern Oscillation, a change in atmospheric circulation that occurs irregularly every 2 to 7 years above the tropical Pacific Ocean, is the most notable example that a local disturbance in the balance between ocean and atmosphere can interact to cause an abrupt and dramatic change in the circulation of the tropical oceans and the global atmosphere. Kevin Trenberth, of the National Center for At- mospheric Research in Boulder, Colorado, and colleagues have shown that the hot and dry conditions in central North America in the summer of 1988 could have been triggered by unusual distributions in sea surface temperatures that occurred in the aftermath of the 1987 E! Nino. A critical unanswered question is, what is the ocean's role in storing the carbon dioxide added to the atmosphere by hu- man activity? Preliminary calculations suggest that about half of the carbon dioxide added to the atmosphere by fossil fuel combustion and deforestation remains there. At least part of the carbon dioxide has been absorbed by the ocean, which holds 60 times as much carbon as there is In all of the atmospheric carbon dioxide. The ocean's carbon largely resides at the bottom of the sea and has accumulated over billions of years. Photosynthetic plankton in the ocean's surface waters are consumed by other organisms; some of that carbon is returned to the atmosphere through respiration, and part goes into storage in the deep-sea sediment as detritus and shells or skeletons of marine organ- isms. The free-fall of organisms from the surface to the ocean floor and the subsequent release of carbon as deep ocean waters are slowly recycled up to the surface waters have a profound effect on the way carbon is apportioned throughout the earth system. The movement of carbon through the earth system would be quite different if noting lived in the ocean. If one could con- sider the influence of physics and chemistry alone, carbon diox- ide in the surface waters would be evenly distributed. In fact, however, there is a distinct physical and chemical difference be- tween the capacities of waters at different latitudes to sequester

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SYSTEM INTERACTIONS 39 and release carbon dioxide. The food webs of organisms deter- mine to what degree the carbon that is fixed photosynthetically will go back into the water and to what degree it will go into the creep ocean. in other words, the physics of the system that provides nutrients such as carbon, nitrogen, and other elements from the deep ocean to the surface and that moves surface wa- ters from one location to another also influences the nature of the food web. In turn, the nature of the food web influences the partitioning of carbon dioxide. lames McCarthy, a biological oceanographer at Harvard University, believes that assumptions about the ocean's capacity for storing added carbon must be looked at carefully. What is it, he asks, that determines the capacity of the ocean today to absorb carbon? Why is it not half that amount, or twice that amount? How might the capacity of the ocean to absorb the car- bon dioxide that is being released from fossil fuel combustion change in the future? What are the implications of this for the ocean carbon cycle? What would happen if the surface ocean conditions were to change? Scientists have not yet answered these questions, but the record of the past provides some valuable clues in addressing them. The distinct correlation between the concentration of car- bon dioxide and the surface temperature of the planet during the glacial cycles over the last 160,000 years must have involvecl the ocean. Researchers believe the carbon cannot move through any other reservoirs in the earth system efficiently enough over those time periods to account for these changes in carbon diox- ide concentrations. While there are many questions in urgent need of answers, in the last decade the ocean science community has developed new and powerful techniques for addressing them. Scientists have increased their understancting of the coupled nature of the atmosphere-ocean system, and of ocean physics and bio- geochemistry. increased computing and modeling capability improves researchers' ability to handle large data sets and to be able to put those data into forms that can be subjected to critical analysis. The developments in remote sensing in the last decade have

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40 THE EARTH AS A SYSTEM been extraordinary. Until fairly recently, oceanographers based their studies of ocean processes on samples of ocean water gath- ered while aboard ships an extremely slow, labor-intensive process. Ships move at roughly 10 knots, but weather patterns can move across the surface of the earth much faster. Indeed, much of the data collected from the ocean surface is biased be- cause of problems of space and time scale. Now, satellites have made it possible to measure not only the ocean surface tem- perature but also how the surface currents are moving. Surface winds can be tracked with instruments aboard satellites, anct the height of the ocean surface can be precisely gauged. These measurements reveal valuable information about ocean circu- lation. And, finally, the color of the ocean can be assessed to approximate the concentration of plankton pigment, and thus biological activity, at the ocean's surface. LAND Nothing seems more solid than a tract of land, and yet the plants and animals, the soil, and the life-supporting nutrients provided by that land make up a single interdependent unit an ecosystem that is dynamic on time scales ranging from days to seasons to years to millennia. Over days and seasons, the earth's plant communities absorb and release carbon in a breath-like rhythm. Over years and decades, ecosystems respond to the natural patterns of plant succession and occasional events such as E1 Nino or drought. At the far extreme, ecosystems on land change on time scales of tens to thousands of years according to the earth's glacial cycles. Ecosystems function metabolically, producing and consum- ing many of the gases that drive the earth system. Plants capture energy from the sun and carbon dioxide from the atmosphere in their growth process. Terrestrial plants take up more than 100 billion metric tons of carbon each year and return approx- imately as much to the atmosphere as plants die and decay. This cyclical exchange involves 20 times the amount of carbon released through combustion of fossil fuels. Microorganisms in

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42 THE EARTH AS A SYSTEM the soil release carbon dioxide and methane as end products and nitrogen-containing trace gases as by-products. As is the case with carbon, the amount of nitrous oxide cycled through terrestrial ecosystems is much greater than the amount released through combustion of fossil fuels. Of the three main components of the earth system-atmo- sphere, oceans, and land the land is the most heterogeneous. The earth's surface is a mosaic of different types of ecosys- tems ranging from arid desert to tropical forests to tundra to the more familiar temperate forests. Each harbors distinct plant and animal communities, and each uniquely contributes to the func- tioning of the earth system. Tropical rain forests, for example, with abundant moisture and high temperatures that facilitate ex- ceedingly rapid plant growth and decomposition of dead plant material, cover about 7 percent of the earth's land area but con- tribute a much larger share of the worId's annual turnover of biomass. At the other end of the spectrum, the cold tempera- tures In the tundra inhibit decomposition of plant material, and so the carbon in the biomass is stored there for long periods. Though change is a quality intrinsic to all ecosystems, changes to the plant cover from agriculture, clearing of forests, and other human activities are not just another sort of change imposed on the background of natural variation. Rather, they profoundly alter the amount of light reflected back to the atmo- sphere from the land, the roughness of the land surface, which influences wind patterns, and the cycling of materials through the earth system. For studies of the short-term dynamics of terrestrial ecosys- tems, biologists, like oceanographers and climatologists, have benefited from advances in satellite technology. One of the most important short-term dynamic effects is the seasonal variation in vegetation, which can be seen from space and recorded in snapshots. Some of these images show where plants are active at any given time and are extremely useful because the informa- tion can be accumulated daily, summed annually, and compared with measurements of the atmosphere. Peter Vitousek, a biolo- gist at Stanford University, explains that results are particularly

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SYSTEM INTERACTIONS 43 striking when the seasonal variation in the amount of light ab- sorbed globally by vegetation is compared to the relative carbon dioxide concentration over the same seasons. "If you do this," Vitousek says, "not only do you see the biosphere inhale and exhale seasonally, but you actually see the distribution of the or- ganisms over the surface of the earth engaged in that process." In trying to assess what is in store for terrestrial ecosystems, researchers are drawn to the most recent instance when global climate changed on a massive scale: an ice age. The ice age is of particular interest in light of projections for the planet in the next century. Even during glaciation and the retreat of glaciers, which occurred much more slowly than the rate of warming projected for the planet in the next 100 years, the rate of change was so fast that only some species were able to adapt to the changes. Associations between species were severed. Eventually those species that survived recolonized into new communities, often in unfamiliar areas and in different combinations of members. As a result, many ecosystems were composed of wholly different combinations of species than are found anywhere today. During the ice age, the major vegetation zones shifted thou- sands of kilometers from their current positions, and so the frac- tion of the earth's surface covered by specific types of vegetation also was altered substantially. What is in store for ecosystems in the future, and how these changes will feed back to other parts of the earth system, are open questions. THE WATER CYCLE it is easy to take water for granted. Rain, a lake, dew, waves crashing along a shoreline, snow, fog, a freshwater spring sur- rounded by desert palms-water in these and many other fa- miliar forms means that life can be sustained. Nowhere else in the solar system does water currently exist in its liquid state; nowhere else has life taken root and flourished. Here water connects the various components of the biosphere, driving pro- cesses on land, sea, and air. Like the other components of the earth system, water is

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44 THE EARTH AS A SYSTEM mobile on time scales ranging from the gradual advance of glaciers to the pelting of raindrops. It resides temporarily in oceans, groundwater, lakes, ice, and clouds and flows between them through rainfall and snow, evaporation from surfaces and through plants, and runoff across the earth's surface. Nearly ev- ery process in the earth system requires it. It sculpts the earth's topography, pushing vast amounts of debris ahead of advanc- ing glaciers, compressing the land beneath mountains of ice. Soil particles caught up in river flows traverse great distances to the oceans and lakes, where they settle to the bottom and eventually harden into sedimentary rock. Water also destroys rocks, acting as a solvent in the weathering process or splitting them mechanically, pushing into crevasses where it freezes and expands. Most aspects of the water cycle are poorly understood: There is simply too much of it in too many places for the many reservoirs, flows, and fluxes to be measured accurately. We do know that oceans hold the lion's share, over 97 percent, of the earth's water, followed by glaciers and ice caps. Lakes, rivers, and other surface water hold a mere one or two ten-thousandths of the global water stock. People have affected the water cycle by constructing dams and reservoirs, which alter river flow and evaporation. Cities are built and paved, creating new patterns of runoff and pre- venting rainwater from entering the ground. Forests are cleared, reducing the ability of the soil and plants to retain water. Peo- ple also consume water for drinking, cooking, and bathing and use it to irrigate their fields and to cool industrial plants. Such human actions raise the possibility that availability of water for future human use will be altered. in light of the massive transformation under way in the global environment, water is of special interest because it exerts a strong moderating influence on the global climate system. Oceans, ice and snow, and clouds determine the earth's ability to reflect incoming radiation back to space, thereby helping to regulate temperature. in the form of water vapor a greenhouse gas water joins the other trace gases to absorb radiation leaving

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SYSTEM INTERACTIONS 45 the earth's surface. Scientists are fairly certain that the water cycle, which transports and distributes most of the solar energy reaching the planet, will change in response to a warmer climate. Changes in factors such as the area of the earth covered by reflective polar ice or the abundance of clouds over the oceans, for example, would have a further effect on global temperature. As temperature and hence evaporation from oceans and land increase, global precipitation is expected to increase by 5 to 10 percent. The timing and quantity of runoff may change, as will the amount of moisture stored in soil, with implications for world agriculture. Changes in vegetation in response to a warmer climate may profoundly affect patterns of evaporation and also whether precipitation seeps into the soil anct ground- water for future use or runs off directly once it hits the ground. With current understanding, scientists cannot say how large the shifts in precipitation will be or where they will occur. HUMAN INTERACTIONS The recent furor over the changes humanity has wrought in the global environment since industrialization began invites the assumption that human alteration of the earth's landscape is a fairly recent phenomenon. In fact, many of our effects on the environment did not reach their global scale until the latter half of the twentieth century. But studies of many parts of the world suggest that as we extended our natural abilities with tools and later learned to cultivate plants, we became an effective agent of environmental change. Ecologists Robert Peters, of the World Wildlife Fund, and Thomas Lovejoy, of the Smithsonian Institution, traced the rec- ord of human activity and its effect on terrestrial plant and animal life in several regions of the world. One of the areas they studiecl, the Mecliterranean, provides a telling example. Destruction of natural habitats around the Mediterranean began at least 7000 years B.C. Excavations show that by 6000 B.C., the bones of wild animals in kitchen refuse heaps were replaced by the bones of domestic sheep. In the fifth and fourth

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46 THE EARTH AS A SYSTEM centuries B.C., forests began to dwindle as wood was harvested for fuel and construction. Around the Mediterranean, the re- searchers explain, humans have disrupted natural communities for so long that "it is difficult to determine which plants are natural or introduced, or what the original vegetation was like." Throughout the region, degradation of forested areas is so ex- treme that even if an area is protected, original vegetation often will not regenerate. Over the centuries, forests were converted to pasture, and grazing pasture was then replaced by thorny plants over enormous areas. Animal communities, displaced as their habitat disappeared, shrank in size and diversity. Scientists have found patterns of human-induced change in other regions. Aborigines are thought to have walked into Australia from Indonesia about 40,000 years ago, when sea level was lowered during a glacial episode. Almost immediately, Australian vegetation became dominated by the fire-resistant eucalyptus tree. In Britain, habitat destruction over the last 3,000 to 4,000 years has caused 90 percent of its forest and most of its wilderness to vanish. In North America, as in Europe, marshes were clrained, rivers dammed, and prairies plowed. And in Brazil's Atlantic forest, clearing began in earnest in the seventeenth century and continues today. -From an original one million square kilometers, the Atlantic forest has been cleared until now, only fragments remain less than 7 percent in any condition and less than ~ percent undisturbed. The message in these examples is clear: With longer human occupation and greater population density, the influence of hu- mans on other parts of the earth system grows. Now we know that human activities have become so pervasive that the effects are no longer local but are regional and even global in scale. Forest clearing is eliminating habitats where millions of species reside, acid rain is affecting lakes and streams in North Amer- ica and Europe, and pollutants are changing the makeup of the atmosphere in ways that can affect climate and the protective ozone layer. This awareness that humanity is an intrinsic part of the

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SYSTEM INTERACTIONS 47 earth system is causing a fundamental shift In the way science is pursued. No longer is it sufficient to explore only the physical dynamics of the earth system. This effort, ciaunting in itself, may be dwarfed by the effort to decipher the confounding be- havior of Homo sapiens, the planet's most powerful inhabitant. Thus, as physical scientists join together to study, model, and predict changes on the earth's surface and in its atmosphere, their traditional focus on the physical and biological aspects of change is shifting to include the social sciences. For the first time, scientists from disciplines ranging from geochemistry to ecology are realizing that human action is the critical element in their studies. So potent is the human impact on the earth system that knowledge of physical processes ruling terrestrial or atmospheric change will be incomplete until scientists better understand the human dimensions of that change. While studies in fielcls including economics, psychology, and communication provide an invaluable research foundation, they have, for the most part, focused on what determines and controls individual behavior. Roberta Balstad Miller, director of the Division of Social and Economic Science of the National Science Foundation, stresses that the study of human aspects of global change must consider not only individual behavior but also entire institutions national laws and regulations, profit margins, transportation patterns, agricultural markets, and tax structures that are significant for the environment. The re- search must also address the history of environmental change, dealing with human and institutional activities over long pe- riods of time. "Research on the human dimensions of global change that ignores these factors would be nearly as inadequate as research that ignores the human dimension altogether," Miller said. "Will a social science research effort on global change be expensive? No question. But we must never forget that the costs of cloing nothing are even greater." The effort to discern the human causes of global change is complicated because the target changes over time: humans both act on and react to their environment. Using their unique

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48 THE EARTH AS A SYSTEM capability to choose, people can perceive and assess possible future changes that they hope to encourage or avoid. Accord- ing to Harvard University's William C. Clark, "Ultimately, it is certain patterns of human behavior that lead to environmental degradation, and other patterns that result in sustainable devel- opment. We need to establish how relevant human behaviors are shaped, and how they can be altered as part of efforts to manage the long-term, large-scale interactions between people and their environments."