losses included $2 billion to $5 billion by international airline and hotel chains from lost tourism.

Extreme weather events compounding local vulnerabilities (multiple stresses) can disrupt predator/prey relationships (functional biological diversity) and can generate biological surprises, such as population explosions of pests and pathogens that can affect human, plant, and animal health. The impacts of extreme events and epidemics can ripple through economies, affecting agriculture, productivity, trade, and tourism, in addition to their direct effects on regional human health and well-being.

There is, of course, much uncertainty about the role of climatic change in causing ecological changes that have costly effects on humans. A major recent example that highlights the difficulty in assigning causation is the collapse of the commercially important northern cod populations off the coast of eastern Newfoundland and Labrador, Canada, in the late 1980s and early 1990s. This collapse led to a costly program to compensate the over 30,000 people who could no longer work as fishers or fish processors. Debates continue about the roles of the North Atlantic Oscillation and other, more specific, climatic and oceanographic changes relative to the role of overfishing.18 There is also uncertainty about the links from ecological consequences to human consequences because of gaps in knowledge about the ability of human communities to respond effectively to anticipated ecological changes.

In those regions where climatic variability is associated with El Niño-Southern Oscillation (ENSO) events, there is hope that improved understanding of sea surface temperatures and associated changes in atmospheric circulation will result in advance warnings of droughts, floods, and epidemics and reduced losses.19 This type of human dimensions research highlights the importance of improved understanding of climate change and variability, the need to consider social vulnerability and adaptive capacity when forecasting the consequences of global change, the potential benefits of predicting climatic extremes, and the need to evaluate carefully options for reducing greenhouse gas emissions.

Key Scientific Questions

Key scientific questions for research on the human dimensions of global change can be grouped into four broad interrelated interdisciplinary categories:

  • What are the major human causes of changes in the global environment and how do they vary over time, across space, and between economic sectors and social groups?
  • What are the human consequences of global environmental change for key life support systems, such as water, health, and agriculture, and for economies and political systems?
  • What are the potential human responses to global change? How effective


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losses included $2 billion to $5 billion by international airline and hotel chains from lost tourism. Extreme weather events compounding local vulnerabilities (multiple stresses) can disrupt predator/prey relationships (functional biological diversity) and can generate biological surprises, such as population explosions of pests and pathogens that can affect human, plant, and animal health. The impacts of extreme events and epidemics can ripple through economies, affecting agriculture, productivity, trade, and tourism, in addition to their direct effects on regional human health and well-being. There is, of course, much uncertainty about the role of climatic change in causing ecological changes that have costly effects on humans. A major recent example that highlights the difficulty in assigning causation is the collapse of the commercially important northern cod populations off the coast of eastern Newfoundland and Labrador, Canada, in the late 1980s and early 1990s. This collapse led to a costly program to compensate the over 30,000 people who could no longer work as fishers or fish processors. Debates continue about the roles of the North Atlantic Oscillation and other, more specific, climatic and oceanographic changes relative to the role of overfishing.18 There is also uncertainty about the links from ecological consequences to human consequences because of gaps in knowledge about the ability of human communities to respond effectively to anticipated ecological changes. In those regions where climatic variability is associated with El Niño-Southern Oscillation (ENSO) events, there is hope that improved understanding of sea surface temperatures and associated changes in atmospheric circulation will result in advance warnings of droughts, floods, and epidemics and reduced losses.19 This type of human dimensions research highlights the importance of improved understanding of climate change and variability, the need to consider social vulnerability and adaptive capacity when forecasting the consequences of global change, the potential benefits of predicting climatic extremes, and the need to evaluate carefully options for reducing greenhouse gas emissions. Key Scientific Questions Key scientific questions for research on the human dimensions of global change can be grouped into four broad interrelated interdisciplinary categories: What are the major human causes of changes in the global environment and how do they vary over time, across space, and between economic sectors and social groups? What are the human consequences of global environmental change for key life support systems, such as water, health, and agriculture, and for economies and political systems? What are the potential human responses to global change? How effective

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are they and at what cost? How do we value and decide among the range of options? What are the underlying social processes or driving forces behind the human relationship to the global environment, such as human attitudes and behavior, population dynamics, and economic transformation? How do they function to alter the global environment? Research on the human dimensions of global change has value both as basic science and for informing environmental decisions. It increases basic understanding of how past human activities have created present environmental conditions, how past environmental changes and variations have affected human well-being, and how people have responded to these variations and changes. By developing understanding of human-environment dynamics, human dimensions research improves the knowledge base for anticipating future environmental changes and for informing policies aimed at reshaping the environmental future. Studies of the human consequences of and responses to global change help inform judgments about what kinds of responses would be most desirable (e.g., mitigation, adaptation options) and about how to organize those responses to achieve the desired effects. Below we describe the major science issues, review progress that has been made in understanding them, and identify some lessons that have been learned from previous research. What Are the Major Causes of Changes in the Global Environment What has been learned in recent years about human causes of global environmental change? One major focus of research has been the explanation of changes in the composition of the Earth's atmosphere. Looking at the atmosphere through human history, one finds that the concentrations of several gases (carbon dioxide, methane, nitrous oxide) changed only a little for more than a thousand years and then started to increase rapidly around 1800. The obvious hypothesis to explain these data is that prior to industrialization in the nineteenth century the related basic cycles of the Earth's environment were in approximate equilibrium and aggregate human activity was too small to be detectable in globally averaged data; then, increasingly since the Industrial Revolution, aggregate human activity has changed the composition of the atmosphere, in particular adding measurably to the concentrations of certain gases. Similarly, looking at the history of land use and land cover, one finds significant changes occurring, although over longer time periods. The obvious hypothesis to explain these observations again is that human beings altered the land and used resources to meet the needs of a rapidly growing population and an expanding industrial economy. Research into the direct human causes of global change has thus focused on changes in land and

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energy use. But there is also a growing body of work on the fundamental social processes that drive human use of the environment. Human Activity and Land Use Change Interest in the causes of local and regional land use changes is long standing in the social sciences.20 Significant steps have been made in documenting the long history of human transformation of land cover and in explaining the major forces that drive land use. These studies are of interest to a wide range of social and environmental scientists because land is a key factor in social relationships and resource use. But these studies also provide specific contributions to scientific understanding of biogeochemical cycles (especially the carbon cycle), regional climate modification, and alterations in natural ecosystems and are a critical basis for policies to mitigate and adapt to climate change, conserve biodiversity, and reduce land degradation.21 Land use studies provide a powerful rationale for maintaining land and marine remote sensing satellite systems and suggest ways in which these technologies can be made more germane to decision making. The global change research community has made considerable progress in recent years on several important questions, such as the social causes of deforestation in regions like the Amazon River basin and Southeast Asia; the role of social, political, and economic institutions in land use decisions; and the relationships between population and land use (and land cover) change.22 There have also been tremendous improvements in the ability to combine social, physical, and remote sensing data within geographic information systems, often with the explicit purpose of understanding how processes at local scales are nested in regional, national, and global scales.23 Additionally, human dimensions research has highlighted the important distinction between land use and land cover. Whereas land cover refers to the land's physical attributes (e.g., forest, grassland), land use expresses the way such attributes have been transformed by human action (e.g., ranching, crop production, logging); that is, land use measures provide a socioeconomic portrait of a landscape.24 Land cover is directly represented in global climate models. Land use links land cover to the human activities that transform the land. The emerging field of environmental history has provided important data on the trajectories and causes of land use changes in the past. For example, historical studies of the U.S. Great Plains have shown how changes in the use and management of grazing and croplands relate to government policy and economics and in turn influence the cycling of carbon and nutrients.25 Historians and geographers have also reconstructed the history of human use of such regions as the Mediterranean, Caribbean, and Latin America.26 Historical studies of land use have altered scientific thinking on the past and the present in a variety of ways. For instance, many observers have presumed that much of the humid tropical forests is pristine or that human impacts on the

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global environment mainly occurred in recent decades. However, research has shown that many forests were cleared in the distant past or have been managed for centuries and that their current rich biodiversity may be a product of past human manipulation, resulting in higher frequencies of species with economic, medicinal, and other human uses than might be expected to result from natural processes of secondary succession.27 Although human population growth is commonly seen as the major cause of land cover change and destruction of habitats for biota, particularly because of land clearing to grow food, the role of population is in fact far more complex. Numerous cases do suggest that population growth and/or migration are correlated with increasing rates of tropical deforestation, but just as many suggest that population growth need not lead to increasing deforestation—when alternative employment, settlement concentration, and other processes are available as alternatives to land clearing, to provide a population with an acceptable standard of living.28 In fact, there is considerable evidence that only at higher population densities does one find more intensive and efficient use of land.29 Research on land management practices has demonstrated that overexploitation of common-pool natural resources—the so-called tragedy of the commons30—is not an inevitable consequence of human nature and the spatial distribution of resources but is contingent on the structure of human communities and the condition of social institutions that effectively govern access to a resource, monitor its condition, and establish sanctions for overexploitation.31 Both cultural traditions and contemporary legal rules, such as land tenure rules, are critical in influencing how land can be used and by whom. The emergence of integrated and interdisciplinary approaches to understanding land use and environmental issues has resulted in a series of studies that show how political and economic structures constrain individual choices about management of land and resources.32 For example, colonial legacies of unequal land tenure and export-oriented production, combined with current unfavorable terms of trade and debt, have driven many peasants to overuse their land, adopt polluting technologies, or cut their forests.33 Social scientists have begun to make greater use of orbital Earth-observing satellites in recent years. The interest in understanding the social dimensions of land use change has challenged some of the inferences about land use drawn by natural scientists by showing, for example, the importance of secondary growth and the likely miscalculations of biomass and carbon pools resulting from overly aggregated analyses that fail to quantify the differences between mature and 10-year-old regrowth vegetation.34 Social scientists have explained the processes underlying various patterns of forest change seen on satellite images in terms of the development of transportation networks, land tenure, and export agriculture. Social scientists have also made important contributions to explaining satellite observations of vegetation dynamics in Africa and to understanding land use change in areas undergoing urbanization.35

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Field studies of land use have provided information of great relevance to global and regional atmosphere-biosphere modeling. For example, coarse-resolution satellite data tend to represent the predominant soil type or vegetation in each grid cell, even if a minor soil or vegetation type is of major economic or ecological significance. Such a representation of the data can seriously misrepresent land use and productivity potential as well as biogeochemical cycles. Another important development is the focus on explaining trends and patterns in land use intensification, in which crop yields are increased through the use of agricultural chemicals and irrigation, resulting in alterations in regional and global biogeochemical cycles and ecosystems. Progress in the past decade is evident in the rise of an International Human Dimensions Programme/International Geosphere-Biosphere Programme (IHDP/IGBP) core project on land use/land cover change, with a coordinated, comparative, multilevel strategy for understanding, monitoring, and modeling land use.36 In developing frameworks, case studies, and models of how social forces drive changes in land use and land cover, this type of comparative research program has the potential to explain and predict land use change but also to assist in identifying strategies for managing land use and protecting ecosystems. Recent important U.S. initiatives include the expansion of the population program at the National Institute of Child Health and Human Development (NICHD) into population and environmental research in 1995, the creation in 1996 of an NSF-funded research center that works on land use—the Center for the Study of Institutions, Population and Environmental Change at Indiana University—and the National Aeronautics and Space Administration's (NASA) Land Use Cover Change request for proposals. In summary, there has been considerable progress in understanding the human causes of land use change, including the following insights: Humans have been altering land cover and use for centuries. Some regions that now appear pristine have been subject to human management since prehistoric times. There is no simple relationship between population and deforestation or between common property rights and resource degradation. The analysis of institutions—in their broadest sense, including political, legal, economic, and traditional institutions—and their interactions with individual decision making is critical in explaining land use. Satellite images can provide important insights for social science, and social science can help guide satellite programs to useful applications. The age and gender structure of landholding households affects how much forest is cut for farming. Tax incentives affect Amazonian deforestation.37 Secure land tenure is important to long-term resource conservation.38

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Road construction in forests leads to increased deforestation not only by farmers claiming land but also by logging companies. However, there is still inadequate knowledge on such key issues as these: How to develop land management institutions that both respond to local needs and mitigate global environmental change. How to aggregate in-depth studies of land cover and land use to provide global projections of use in large-scale modeling and international management of global change. The role of population mobility in land use change. How to best use the expanding range of satellite data in land use/land cover change research. Human Impacts on Coastal and Marine Ecosystems Global change research encompasses the study of changes in coastal and marine ecosystems insofar as they are affected by physical and socioeconomic processes that are global in scale and effect. Social and applied scientists have investigated the importance of coastal and marine ecosystems for many communities, regions, and nations. They have also addressed the ways in which resource use and pollution have altered the condition and biodiversity of coastal ecosystems in many regions of the world, including the destruction of protective and productive mangrove ecosystems, the degradation of coastal lagoons and estuaries and species that live or reproduce in them, and the minor contamination of even the deep and remote oceans. Steady increases in demand, technological capacity, and effort have led to a long-term trend of increasing fish catches, which is believed to have leveled off during the 1990s, indicating limits to sustainable harvests.39 Heavy fish mortality means that environmental fluctuations as well as other human impacts, such as pollution and degradation of habitat, make fisheries even more vulnerable.40 Social scientists and others have documented the roles of technological change, population growth, institutional structures, and social attitudes in driving demand for fish and other marine resources, as well as in shaping the nature and effectiveness of fisheries management, and they have sought ways to use these resources more sustainably.41 They have also contributed to understanding the ecological and social concerns associated with mariculture, which is increasing throughout the world as a way to compensate for declining natural resources.42 This research also contributes to several related themes identified in this chapter, including the links between economic globalization (e.g., of industrial shrimp farming), conflicts over common property resources and loss of forest lands (mangroves); the emergence of new social institutions (social movements in resistance to industrial aquaculture); and the use of new information technologies (communications and

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spatial) in resource management.43 Research on common property management, discussed in a later section of this chapter, has drawn many important examples from marine ecosystem use.44 Some important insights of this research include the following: People have responded to problems in coastal marine systems primarily by intensifying, diversifying, and expanding the areal extent of their uses of those systems, tending to extend such problems to the global level. Globalized systems of production and marketing, combined with increases in population and consumer demand and patterns of subsidization, increase competition between countries and communities for scarce marine resources. Rules of free and open access, combined with the weaknesses of international management regimes, make it difficult to control harvesting in deep ocean and other multinational fisheries. Restricting access is a necessary but not sufficient approach to reducing incentives to overharvest and pollute marine ecosystems. The technical and institutional tools of marine resource management have not adequately incorporated the effects of coastal development, wetlands drainage, dams, and pollution of rivers and oceans in diminishing breeding habitat and degrading marine resources. The success of fisheries and coastal management depends on functional interdependence between local institutions and regional, national, and international institutions. Current knowledge is not adequate to achieve several essential goals: Provide complete geographic coverage of the status of human use of marine and coastal resources. Analyze and model changes in the abundance of fish and marine mammal populations as a function of multiple social and environmental stresses, including interannual, decadal, and longer-term climatic change. Evaluate the full range of institutions, including traditional systems, to understand how they increase or reduce human impacts on coasts and oceans. Changes in Energy and Materials Use Fossil fuel use is the most prominent human activity that alters the composition of the global atmosphere. Since the 1970s, a burst of human dimensions research seeking to understand the consumption of fossil fuels has been proceeding simultaneously at several levels. The methods developed for studying energy use have more recently been applied to human transformations of the global

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nitrogen cycle and to human consumption of other environmentally significant materials. The results are useful as inputs to climate models, for anticipating future rates of environmental change, and for identifying effective ways to mobilize social and economic forces to alter trajectories of environmental change. First, fossil fuel use has been disaggregated by fuel type, geographic region, mediating technology, and social purpose (lighting, water heating, transportation, steel making, etc.). It has been shown that patterns and environmental impacts vary greatly by country and that national-level consumption varies with technology, population, and other factors, such as industrialization and degree of central planning of economies.45 Second, progress has been made in understanding patterns and changes in energy and materials use across countries and over time.46 Energy use and its environmental impacts, for example, generally increase as a function of economic prosperity, but there are exceptions. Countries beyond a certain level of affluence experience declines in per capita environmental impact, although considerable dispute remains about where the turning point lies.47 Also, the energy-affluence link breaks down in certain periods, including those characterized by rapidly increasing relative energy prices and significant policy interventions.48 Thus, changes in prices and policies allowed economic growth to continue in the United States without increases in energy use or carbon emissions between the mid-1970s and the mid-1980s, but energy use has been increasing since then, driven by increasing travel demand, shifts in the vehicle fleet, and other factors, and similar trends have been occurring in other developed countries.49 Long-term trends show decreasing carbon emissions per unit of energy use due to fuel switching and electrification, decreasing materials use per unit of economic output, and replacement of dense materials such as steel with lighter-weight materials such as plastics.50 These rates of change have an autonomous dynamic and respond to the prices of inputs, but little is known about how public policy might alter the trends to enhance environmental quality. Third, patterns of energy and materials use have been studied in relation to particular variables that may account for changes and variations in use, and some of these variables can be affected by public policy. At the household level, for example, energy use is affected by income and fuel prices, household structure and social group membership, and by individual knowledge, beliefs, and habits, as well as by the energy-using technologies that households possess.51 Research on the determinants of consumer decisions to take advantage of technical and economic possibilities to improve energy efficiency indicates that more is required than favorable attitudes and accurate information. There is significant potential to improve residential energy efficiency with appropriately designed interventions. The research strongly suggests that the most effective interventions are specific to consumers' situations and that they use combinations of information, incentives, and social influence. Participation of affected consumers in program design can greatly increase effectiveness.52

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Research has focused on identifying how the energy consumption patterns of firms and individuals change as a function of changes in information, incentives, technology, and social organization, thus illuminating the potential for reducing society's reliance on fossil fuels by promoting the adoption of new technologies or changing behaviors and preferences. Specific areas of extensive research include technology-focused research on energy consumption, energy efficiency, renewable energy, and nuclear power; research on price elasticities and response to incentives; and research on behavioral and informational factors affecting change in consumer choice.53 Consumer energy choices are also shaped by political and economic structures that influence regulations and incentives for different types of energy, transportation, and housing policy, as well as the reach of advertising to different regions and social groups. Research on energy conservation has blended behavioral and technological analyses to compare the technical potential for reducing the energy use required to provide an energy service, such as indoor climate control, with actual reductions in energy consumption. It has examined ways to achieve more of this potential reduction by identifying and removing barriers to energy conservation, such as subsidies and other market distortions, principal-agent problems, incomplete consumer knowledge and misinformation, and problems related to the early stages of the introduction of new technology. This research provides a basis for selecting promising policy options to achieve national commitments to stabilize greenhouse gas emissions. Materials balance analysis provides the basis of an accounting system that tracks the stocks and flows of certain materials, particularly the chemical elements, through the human economy. Analysis of material flows in this fashion has been called industrial metabolism and industrial ecology.54 As in energy research, the analysis begins with descriptive work that clarifies the principal human activities dominating each materials flow; proceeds to explorations of behavioral, economic, technological, and policy-related determinants of these activities; and expands to include prospects for changes in consumption patterns over time, including changes related to economic development.55 One element that has been productively studied is nitrogen, which in the form of nitrous oxide acts as a greenhouse gas and affects the chemistry of stratospheric ozone and in several chemical forms plays a role in nitrogen fertilization of the biosphere. The predominant role of fertilizer production in human-induced changes to the global nitrogen cycle was only recently recognized.56 Our understanding of energy use is far more sophisticated than it was two decades ago. It has led to the broader concept of environmentally significant consumption and to the idea of applying analyses like those used for energy to various nonelemental materials of environmental importance, such as wood, steel, cement, glass, and plastics.57 An important part of research on energy use is scenario making, which seeks to extrapolate current energy use patterns into the future.58 Time horizons of

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prominent scenarios range from a decade to a century. Two important goals of scenario making are transparency (explicit reviewable assumptions) and self-consistency. For global change studies, most scenarios are built on the basis of models of the evolution of national economies, often assuming a similar evolution for large groups of countries at a similar stage of economic development and structure. Typically, population growth is exogenous to the models, and per capita energy consumption is the subject of investigation. The most significant uncertainties relate to the determinants of the rate of introduction of new technologies.59 Rarely addressed to date but presenting major sources of uncertainty are changes in preferences over time, such as those that might accompany new environmental information.60 Scenario making is a conservative activity in that it assumes only slow changes from established trends; it is not well suited to exploring the significance of surprises and catastrophes. Nonetheless, over the past two decades, scenario making to elucidate energy consumption has become a highly developed art, featuring dialogues among modelers to ensure quality control and intercomparisons and to highlight debatable assumptions. Scenario building has been an essential basis for IPCC assessment models of future climate and analyses of mitigation options, the latter employing models used for scenario building in policy analysis of greenhouse gas emission control strategies.61 Important insights of such activities include the following: Estimates of future emissions of greenhouse gases are highly sensitive to assumptions about future economic, technological, and social changes, particularly about the autonomous rates of decarbonization and improvement in the energy efficiency of technology, about the likelihood of further large-scale economic transformations, and about the stability of preferences. Energy and materials uses are determined by multiple factors: they are not simple functions of population or economic activity but depend on complex interactions of these factors and others. Future emissions of greenhouse gases will be driven by pressures from increasing affluence and population, with countervailing trends that reduce the amount of energy and materials used per unit of economic activity and the rate of emissions per unit of energy and materials used. Current knowledge is inadequate to accomplish some tasks critical to understanding consumption trends, their potential environmental consequences, and the possibilities for altering them. These tasks include: Clarifying the determinants of ''autonomous'' change in energy and materials efficiency and thus improving the accuracy of projections of change in greenhouse gas emissions and in the pressure on depletable resources.

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Specifying the ways in which population, technology, affluence, preferences, policies, and other forces interact to change the rates of environmentally significant consumption in high-consuming developed economies and particularly in developing economies, where large increases in consumption are anticipated. Identifying and quantifying important sources of variation in the adoption of environmentally beneficial technology among firms within industries. What Are the Human Consequences of Global Environmental Change? Human dimensions research has made important progress in understanding the consequences of global change for people and ecosystems. Drawing on earlier research in applied climatology and natural hazards, the past 10 years have seen a major effort to understand the potential impacts of climate change on human activity, as well as studies of the impacts of past and present climate variability, the impacts of ozone depletion on human health, and the effects of land degradation and biodiversity loss on society. Credible climate impact assessments are a basis for developing policy responses to global climate change and for successful application of information on current climate variability to resource management. Consequences of Future Climate Changes The first studies of potential global warming impacts analyzed how crop yields and water resources would change in developed countries in response to climate scenarios of monthly changes in temperature and precipitation, based on coarse and uncertain output from climate models simulating the equilibrium response to a doubling of carbon dioxide levels in the atmosphere.62 Later crop modeling efforts have incorporated the direct physiological effects of higher carbon dioxide levels, employed transient climate scenarios and daily data, covered developing countries, and replaced the concept of the unresponsive farmer with that of people capable of flexible adaptation to climate change.63 It appears that many U.S. farmers will be able to adapt to the climate changes expected from a doubling of atmospheric carbon dioxide levels by shifts in technology and crop mix but that others, especially in developing countries, will experience lower yields because they cannot afford technology and may be farming more biophysically vulnerable land.64 Some studies of economy-wide impacts arrive at similar conclusions;65 however, these conclusions may be sensitive to some of the assumptions underlying the analyses, as discussed in more detail in the section below on integrated assessment. A major conceptual advance occurred in moving from impact assessments based on climate model scenarios to analyses based on an understanding of vulnerability.66 The lack of consensus about how climate may change at the

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satisfactory practical value, analysis must develop to the point at which it can give confidence in the validity of particular analytical techniques for estimating nonmarket values and provide a satisfactory way of analyzing equity issues, particularly those relating to intergenerational equity. Understanding Risk, Uncertainty, and Complex Choices Responding to the prospect of global change requires interventions in complex systems that are not fully understood. Decision makers can benefit from recent advances in understanding human judgment and decision processes regarding complex environmental choices. Over the past decade this research has increasingly clarified why scientific efforts to analyze and assess global environmental threats do not easily lead to social consensus on policy responses to those threats. This research shows that, while analyses normally focus on a few critical outcomes, such as sea level rise or species extinction, nonexperts commonly consider multiple dimensions of environmental conditions and decisions, such as risks to human health, economic costs and benefits, the extent of scientific uncertainty and ignorance, catastrophic potential, and threats to aspects of the environment with intrinsic value. Moreover, even single dimensions such as human health are multifaceted; people assess risks partly depending on whether they believe they can control personal exposure and their own emotional responses to the specific hazard.108 The factors that matter most to people can vary with the situation and with their social position, and these differences then influence their policy preferences.109 These findings have implications for scientific choices and the policy process. Chief among them is the idea that the information that results from science-driven research agendas is not necessarily considered useful or relevant by those whose decisions the scientific analysis is intended to inform. For scientists to know what information will be considered useful and relevant, they must have input from those who participate in environmental decisions. Each technique used to assess environmental risks inevitably makes judgments about what the problem is that needs scientific input, which dimensions of the problem should be investigated, and their relative importance.110 Consequently, decisions that rely on any particular analytical technique are often rejected by people who do not accept its underlying judgments. Moreover, decisions made without the participation of some of the interested and affected parties tend to be rejected by those parties. Consequently, decision processes that are too narrowly based, either in terms of analysis or participation, often fail to meet decision makers' needs for information and involvement.111 The ability to reach an implementable decision depends on the process that combines analysis and deliberation to frame scientific questions, gather and interpret information, and present it to participants in the decision in ways that address their needs for information and understanding. The critical elements of this

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decision process and some important related research questions have been identified.112 The decision process is also a major concern of the new National Center for Environmental Decision Making Research, established by NSF. Additionally, research has begun to illuminate how scientific and technical information is incorporated into environmental decision making at local, national, and international levels.113 A number of important findings have been made in this research area: Whereas many risk assessments consider only a few dimensions of risk (e.g., mortality risk, economic loss), nonspecialists' judgments about risk typically consider multiple dimensions. Objections to the results of risk assessments often arise from disagreement about judgments underlying the assessments or from restricted participation in making those judgments. The adequacy and acceptability of a judgment about risk depend on both the underlying analysis and the deliberative process that judged which analysis to do, how to collect information, and how to interpret it. Knowledge is not yet adequate in this field to accomplish several essential tasks, such as: Adequately characterizing uncertainties and scientific disagreements about the nature and extent of risks. Designing processes that combine analysis and deliberation to ensure that information is developed and organized to meet the needs of the range of decision participants. Structuring procedures that inform scientists' work and decision makers' understanding with a combination of formal analysis and the information, perspectives, and judgments of others involved in risk decision making. Integrated Assessment One approach used to understand the implications of policy responses to global change is known as "integrated assessment." In integrated assessment, methods or processes are applied to combine knowledge from multiple domains, such as socioeconomic and biophysical fields, within a consistent framework to inform policy and decision making. Integrated assessments of environmental issues have been conducted since the 1970s,114 but the past 10 years have seen a flood of interest and activity, particularly to address global climate change. Since 1990 the number of integrated assessment projects relating to climate change has grown from only a few to more than 40.115 Although the concept of integration has been very broadly applied with

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regard to what is integrated and how, recent practice in the area of climate change has been rather narrow. Integration can mean "end-to-end" connection of a causal chain from fossil fuel emissions and land use to their impacts, with weighing of climate change impacts against measures to reduce them or to adapt. (Some amount of such "vertical" integration is often taken as a requirement for integrated assessment.) Integration can also denote expanding each link of this chain to consider more diverse source activities and emissions, more atmospheric and biotic processes, more forms and sectors of impacts, or more spatial detail or heterogeneity of agents. Integrated assessments may also examine social and biophysical linkages between climate change and other issues (e.g., ozone depletion, tropospheric air pollution) or include linkages to other social or policy issues such as public health or economic development. In addition to formal modeling, methods for integration can also include structured cross-disciplinary discourse; judgmental integration of data, theory, and formal models from separate domains; and structured heuristic processes such as simulations, scenario exercises, and policy exercises. A major purpose of integrated assessment is to provide a consistent framework for the representation, propagation, analysis, and communication of uncertainties. A striking result of the few attempts to integrate uncertainty quantitatively across biophysical and socioeconomic domains has been that, among the various kinds of uncertainties, socioeconomic uncertainties appear to predominate in assessing aggregate impacts and net benefits of policies and decisions. Key socioeconomic uncertainties include future population growth and migration, social and political determinants of environmentally relevant consumption, rate and character of technological change, adaptation-mediated regional impacts of climate and environmental change, effects of policies, and variation in preferences. For example, in an early assessment that integrated energy-economic and carbon cycle models, it was found that the largest contribution to uncertainty in atmospheric CO2 concentrations at the end of the next century came from estimates of the ease of substitution of fossil and nonfossil energy inputs in the economy and general productivity growth;116 uncertainty in the airborne CO2 fraction and in total fossil fuel resources ranked near the bottom of all contributions to uncertainty. In 1993 and 1996, studies using a stochastic integrated-assessment model found that differences in preferences dominated climate uncertainty in determining preferred policy choice.117 Recent integrated climate assessment, however, has stressed vertical, or end-to-end, integration, primarily by building single integrating computer models. These models typically combine and modify preexisting models of energy and the economy, atmospheric chemistry and dynamics, oceans, the terrestrial biosphere, and/or agriculture and other forms of land use. In each project some domains are represented richly, others very schematically. Most integrated as-

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sessment projects have a national to global scale, rather coarse spatial and sectoral resolution, and weak representation of policies and political processes. Early work on integrated assessment of climate change combined energy-economic models with either accounting or input-output systems to develop comprehensive emissions scenarios or with simple highly parameterized atmospheric models to project the effect of specific economic and control scenarios on atmospheric trace gas concentrations.118 More recent projects have added climate and impacts modules. Projects differ in their conceptual emphasis and the potential insights they can offer. Some concentrate on the dynamics of emissions, atmospheric accumulation, impacts, and responses. These projects postulate a single global optimizing producer-consumer and require a common metric for abatement costs and climate damages, so they normally represent regional or global climate damages by simple aggregate functions of temperature change. These models allow the investigation of dynamically optimal abatement strategies that balance, over time, the costs of emissions abatement and damages from climate change or that meet a specified environmental target at minimum cost. They also permit study of how preferred policies depend on alternative specifications of damage functions, discount rates, the dynamics of impacts and technological change, or the structure of world regions and of bargaining.119 Other integrated assessment projects concentrate on the specification and propagation of uncertainty, allowing identification and ranking of key policy-relevant uncertainties or the elaboration of adaptive and learning strategies for responding to progressively resolved uncertainty over time.120 Still other projects concentrate on the elaboration of spatial and sectoral detail for climate impacts, human adaptation and responses, and human-mediated feedbacks through land use change to the climate system.121 Integrated assessment practitioners have claimed insights such as the following: that a large near-term abatement effort for climate change is not justified; that the market impacts of climate change in high-income countries (but not low-income ones) will be small; that optimal abatement paths would reduce gross domestic product by only a few percent, compared with unconstrained paths, and can be accomplished with carbon taxes of a few dollars per ton; and that delays of a few decades in controlling emissions are preferable to immediate action, even if stringent reductions are subsequently determined to be needed.122 These conclusions, however, depend on several particular characteristics of most assessment models: they offer very limited representation of the possibility of extreme events; they only reference doubled CO2 scenarios and so fail to include the concentrations likely by the end of the next century under aggressive fossil fuel growth, which drives atmospheric, ecosystem, and impacts models all far out of their validated ranges; they include weak or no representation of multiple interacting environmental stresses; and they assume limited learning in technological change or policy.

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Important advances in knowledge from integrated assessment modeling include the following: The finding that socioeconomic uncertainties dominate biophysical uncertainties in contributing to aggregate uncertainty about future climate impacts and preferable response strategies.123 Initial quantitative estimates of the benefits available from various levels of international cooperation to manage climate change. An evaluation of the implications of sulfate aerosols in climate change for alternative abatement strategies. A preliminary characterization of the effects of linked demographic, economic, and climatic pressures on land cover and atmosphere.124 Knowledge is not yet adequate in this field to achieve the following: Reduce major socioeconomic uncertainties in integrated assessment models. Estimate impacts and preferable policies from models that relax some of the most important restrictive assumptions of existing models (e.g., doubling of CO2 concentrations). Provide acceptable techniques for choosing among model simplifications, so that outputs best meet users' needs. What Are the Underlying Social Processes, or Driving Forces, Behind the Human Relationship to the Global Environment? Human dimensions research has also examined fundamental questions about the broader political, social, technological, and economic forces that shape the human activities that cause environmental change and influence its consequences. The number of such forces that may directly or indirectly alter the global environment has no limit. This section focuses on several driving forces about which important scientific progress has been made and which are often mentioned as arenas for policies to mitigate environmental change. There are many other important social forces and phenomena whose direct or indirect environmental effects may also be large and that may also have policy significance. These include national taxation policies, economic inequality within and between countries, war and the international arms trade, and societies' treatment of women. Important scientific progress has been made in understanding how humans perceive global change; the ways that individuals and institutions cope with environmental changes; and the dynamics of human populations, technological change, and economic transformations.

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Public Attitudes and Values Public support is necessary for any collective response to global environmental threats, whether through policy decisions or the aggregated actions of large numbers of individuals and organizations. A series of studies shows strong and persistent concern and support for environmental quality and protection in a variety of countries, rich.and poor;125 in the United States and other countries where relevant data are available, this support cuts across socioeconomic lines. In some developed countries, concern is strongly correlated with education; in some it is strongest in younger age cohorts. Concern about global environmental problems relative to local and national ones is strongest in developed countries, whereas in countries with highly visible pollution problems, environmental issues closer to home are seen as relatively more serious.126 Environmental concern is strongest in countries with serious objective pollution problems and in countries with strong environmentalist values.127 Research on the factors underlying environmental concern finds that it is partly rooted in basic psychological values, particularly concerns with the welfare of others and of future generations and a widespread belief in the sacredness of nature.128 This work draws on extensive basic research that has developed a comprehensive typology of human values.129 Additionally, environmental concern reflects beliefs about how environmental conditions may affect those things that an individual values, suggesting that public response to newly identified environmental conditions may depend on the kinds of consequences projected for those conditions.130 Despite some widely held misconceptions about the causes of climate change,131 such variation from accepted scientific accounts does not seem to diminish levels of public concern with the environmental problems that also concern scientists. The other side of the coin of environmental concern is an apparent unconcern by individuals about the environment, as reflected in increasing levels of materials and energy consumption associated with increased income. Critics of "consumer society" point to advertising and the mass media as drivers of materialist attitudes and desires and argue that these forces and others are driving the emerging middle classes in many developing countries to emulate North American styles of consumption. These plausible arguments have not yet been supported by careful quantitative studies of the relevant social forces, attitudes, and behaviors.132 Important advances in knowledge in this area are documentation of widespread support for environmental protection across countries and socioeconomic groups and initial identification of the ways that values, beliefs, and attitudes affect political support for environmental policy. Knowledge is not yet adequate to relate the development of public attitudes to mass media coverage of environmental issues and the roles of elites, interest groups, advertising, and social movement organizations and to model the development of public support for

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action on emerging global environmental issues as a function of new scientific knowledge. Individual and Household Behavior Household consumption of energy and certain materials is important both in causing and in responding to global change. Consumer behavior is determined partly by values, attitudes, and beliefs but is strongly mediated by nonattitudinal factors, including the cost and inconvenience of making environmentally significant behavioral changes, the availability of relevant technologies, institutional barriers, knowledge about which behaviors are effective, and the presence or absence of supporting public policies and social pressures. Consequently, the determinants of consumption are highly situation specific, and efforts to change the environmentally relevant consumption of households require multifaceted approaches that identify and address the barriers to change that are most important for the specific behavioral change and target actor.133 Considerable progress has been made in understanding certain key classes of consumption, such as residential energy use in some high-income countries. A major research challenge, only now beginning to be addressed, is to understand how the factors that drive such consumption vary with national and cultural context. Political behavior is also important to responses to global change. As in the case of consumption, the connections between individual concerns and political influence are complex and imperfect. Political action reflects opportunities for effective political participation individually and through environmental organizations, changing value priorities, the framing of issues in the mass media and by interested parties, and the actions of scientific experts individually and through epistemic communities.134 Research linking environmental attitudes to political participation and influence is helping build understanding of the political feasibility of policies to meet international commitments. Important advances in knowledge of individual and household behavior include the following: Improved understanding of the many factors affecting specific types of environmentally significant consumption at the household level (especially energy use) in high-income countries and recognition of the situation specificity of these effects. Recognition of the various factors affecting individuals' political behavior on environmental issues. Appreciation of the need for multifaceted approaches in policies aimed at altering consumption patterns. Knowledge is not yet adequate to achieve several ends:

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Project environmentally significant consumption in developing countries as a function of economic, demographic, and other changes. Model the causes and trajectories of environmentally significant household consumption other than energy. Develop more realistic assessments of likely environmental policy outcomes that take behavioral responses into account. Economic Transformations Various large-scale economic transformations around the world may have major implications for the generation of environmental change and for human vulnerability to it. These transformations include the dependence of an increasing proportion of the world's population on global markets for necessities such as food and fuel that were previously produced locally, much of them outside the money economy; increasing liberalization of international trade;135 the emergence of service economies in place of manufacturing-based ones in most high-income countries; and the transformation of formerly socialist economies from a central command model to a more decentralized market-based one. One of the most important themes in the past 10 years of social science research has been the ''globalization'' of economies and cultures.136 The increasing mobility of capital and labor has facilitated the expansion of transnational corporations and massively restructured the geography of industry, agriculture, human settlements, and all of their associated environmental impacts.137 The environmental effects of trade liberalization are more complex than sometimes realized. Despite claims that trade liberalization has predictably negative environmental impacts, the limited existing evidence suggests that environmental impacts are sometimes positive (e.g., better allocation of soil and water resources in agriculture) and sometimes negative (e.g., foreign investment in countries with lax environmental regulations). Analyses of overall impacts must consider the effects on resource allocation, the scale of overall economic activity, the composition of output (e.g., manufacturing vs. services), effects on developing "green" technologies, and the interactions of trade with policy.138 The North American Free Trade Agreement (NAFTA) stimulated some important work on the environmental implications of changing trading regimes. Although some scholars claimed that NAFTA would result in improved environmental conditions, especially in Mexico, others suggested that free trade would result in environmental degradation as communities relaxed regulations to attract industry or as polluting industries moved to Mexico to take advantage of lower wage rates.139 NAFTA was also predicted to alter agricultural production patterns in ways that would increase Mexico's vulnerability to U.S. droughts.140 Perhaps the most important and dramatic change in the global political economy in the past 10 years is the collapse of the Soviet bloc and the transfor-

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mation of Eastern European economies. These economies had previously been among the most energy and pollution intensive in the world. Studies showed that in the immediate aftermath of political changes in such countries as Russia and Poland, greenhouse gas emissions decreased as industrial production and consumption fell in the ensuing economic crisis.141 Now, as foreign investment and privatization transform these economies, the implications for the global environment in terms of emissions, land use, and resource management are unclear. Important advances have been made in understanding the effects of economic transformations: Most of the world's food is now produced within a global system, in which most of the basic grain on the world market is produced in very few countries. The fact that many countries depend on food imports greatly enhances the regional and global impacts of climatic change and variation in those grain-producing regions on which much of the world depends. Industrial production is shifting from core industrial countries to the developing world. The service sector has grown dramatically, especially in urban areas, contributing to increased vulnerability of human settlements, as poor people move into cities for work and must often live in hazard-prone environments. Knowledge is still inadequate for several needs: Establishing the theoretical and empirical links among economic globalization, global environmental change, and the consequences of global change. Estimating the net overall and regional environmental effects of trade liberalization. Estimating the likely long-term environmental effects of ongoing economic transformation in the former socialist bloc. Human Population Dynamics The past decade has seen substantial progress in understanding fundamental population processes: fertility, mortality, and migration as well as the relationships among them that determine population growth, age structure, and geographic distribution. This research is important to global change because population dynamics are some of the most important driving forces behind land use change and energy use and a factor in increasing demands for food, water, and living space that increase vulnerability. Efforts to reduce fertility (i.e., the num-

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ber of births per woman) have received the most research and policy attention, with Asia, the Middle East, Africa, and Latin America being the areas of interest. Based on evidence from censuses, the World Fertility Survey, the Demographic and Health Surveys, and other surveys, it is now conclusive that fertility rates have dropped in a sufficient number of formerly high-fertility countries to produce substantial reductions in the world's fertility. The world's total fertility rate has dropped to approximately 3.0 today—thus having achieved most of the reductions needed to reach replacement-level fertility. The Middle East and sub-Saharan Africa are still the regions with the highest levels of fertility, but even there evidence is emerging that fertility reductions have begun. Considerable research has examined the causes of this fertility decline. Almost all countries that have achieved substantial fertility declines in the past 25 years have had concerted family planning programs. The effectiveness of these programs in reducing fertility levels, as opposed to other factors, such as rising education levels, has been rigorously debated.142 Most agree that family planning programs have been one of many factors leading to fertility decline; the disagreement revolves around the size of the family planning program's effect. With respect to mortality, most reductions in the past were attributable to declines in infant and child mortality. That trend is now shifting, and attention has been turning to questions of how long people might live. The debate on the limits has not been resolved,143 but the research fueling the debate has helped to increase our focus on morbidity associated with increasing longevity and the need to have global change research include the effects of increased longevity. Human migration is an issue of emerging importance for global change because of the possible environmental impacts of concentrated populations and the vulnerability of these populations to extreme events, especially when people are concentrated in coastal zones or floodplains.144 Research progress in understanding migration has been hampered because accurate data are hard to acquire, and when they can be collected, they tend to be aggregated. Finally, household size has been declining in a number of countries as affluence increases. For example, in the United States the proportion of all households with just one or two members increased from 46 percent in 1970 to 57 percent in 1995. Since households have effects on the environment from production and consumption that are somewhat independent of the number of household members, models should consider both population growth and growth in the number of households. Important findings in human population dynamics include the following two fundamental ones: Total fertility rates are declining worldwide, particularly in countries that have had concerted family planning programs. Human migration, particularly urbanization and movement to vulnerable environments, has been identified as a major potential influence on future environmental change.

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Knowledge is not yet adequate to estimate the environmental effects of particular types of migration or to model environmental impacts as a function of household size and composition as distinct from population effects. Technological Change A major source of uncertainty in projecting future human contributions to global change and analyzing response costs is the rate at which improved technology will lead to the substitution of abundant natural resources for scarce ones and of reproducible capital for depletable resources. Economists and technologists have typically viewed technical change as widening the possibility of substitution among resources. This has frequently led to a bias in favor of assuming adaptation strategies for response rather than mitigation strategies. Ecologists and other biologists have typically regarded substitutability as being narrowly restricted. The argument about biodiversity is, in part, a reflection of these alternative views. The problem has not yet been modeled satisfactorily, nor has sufficient empirical research been conducted to test the alternative perspectives. However, dialogue between the two theoretical camps is increasing and signs of a conceptual synthesis are beginning to appear, in which the questions are formulated in terms of the relationship between rates of substitution and rates of resource consumption.145 Past research has documented some regularities in the time path of change in environmentally significant technologies, including rates of technology diffusion and secular trends toward so-called dematerialization and decarbonization; it has also documented variations around general time trends.146 There has been a lively empirically based debate about the extent to which scarcity may induce innovations that reduce costs and find substitutes, a debate that may be heading toward synthesis.147 Extensive studies have also been conducted of the conditions favoring adoption of technological innovations. This research is starting to make it possible for modelers of global change effects and builders of integrated assessment models to replace ad hoc coefficients of technological change with numbers based on empirical analysis and sound theory. Important advances in this field include the following: identification of secular trends toward dematerialization and decarbonization of economies, along with variations around these trends; identification of factors influencing the rates of adoption of technological innovations; and identification of the substitution rate of inexhaustible resources for depletable ones as a key parameter for studies of sustainability. Knowledge is not yet adequate to model the factors influencing variations in average rates of decline in national energy intensity and related indicators and variations around the average among industries and firms or to model the effects of environmental policies on rates of innovation in environmentally benign technologies.