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.50These 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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