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We live in an era of such transitions, which are under way to varying degrees in specific places and regions around the globe. In the social realm, the transitions that seem most relevant to sustainability include the demographic transitions from high to low birth and death rates; the health transition from early death by infectious diseases to late death by cancer, heart attack, and stroke; the economic transition from state to market control; the civil society transition from single-party, military, or state-run institutions to multiparty politics and a rich mix of nongovernmental institutions. Environmentally, some of the more significant transitions or breaks in trends in specific regions include shifts from the dominance of particular biogeochemical cycles by natural forces to their dominance by human releases, from increasing to decreasing rates of emissions for specific pollutants, and from deforestation to reforestation.

How should societies think of the relationship between major trends or transitions and sustainability? A series of seven interlinked transitions to a more sustainable world has been identified.3 These were later elaborated and amplified4 as demographic, technological, economic, social, institutional, ideological, and informational transitions. For the most part, the researchers present these transitions as requirements for a more sustainable world: if each individual transition is completed successfully the result will constitute a transition to sustainability.

We take a different tack in this study. In Chapter 1 we argued that the path for a transition toward sustainability could not be charted in advance. Instead, we suggested that it would have to be navigated adaptively through trial-and-error experimentation. We remain unconvinced that any specific set of trends or transitions constitutes necessary or sufficient conditions for sustainability. Yet we think that the triad of goals set out in Chapter 1—meeting human needs, preserving life support systems, reducing hunger and poverty—would guide the successful navigation of a transition toward sustainability over the next two generations. Knowledge of trends and emerging transitions may well prove helpful in attaining these goals; societies must first know the directions of present trajectories in the environment and development. Thus, we begin with trends in human development, then turn to the environmental transformations that have been influenced by human actions, emphasizing the interconnectedness of human development and the environment and the needed shifts in trends for attaining a sustainable future. Specialized studies, named in the text that follows, have addressed trends and transitions for particular aspects of environmental change and particular regions of the world. National Research Council studies related to each developmental sector or environmental issue are provided in endnotes keyed to each section. Our purpose is not to duplicate these extensive treatments found through-

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