value of a Pennsylvania corn and soybean farm as $75 per acre per year, but that the value drops to only $2 per acre per year when calculated by natural resource accounting methods. Finally, Repetto et al. demonstrate that estimates of industrial productivity growth can be spuriously low if they do not account for emissions reductions. Using natural resource accounting methods, they estimate productivity growth in the electric power industry to be two to three times as high as conventional estimates of productivity growth during the same period.

National Materials Use and Waste Production

Ayres and Ayres (this volume) and Wernick and Ausubel (this volume) focus on aggregate waste generation by the U.S. economy. Ayres and Ayres make detailed calculations to estimate this aggregate waste production. They use a mass balance approach that combines information on the import and extraction of materials in the United States with information on production of goods. They calculate waste generation by subtracting the quantity of materials in goods produced from the materials imported and extracted domestically. This is similar to 3M's system of calculating the mass of products, by-products, and wastes produced in its plants (Zosel), but it is applied to the nation as a whole. Such information can be instructive for a variety of reasons. First, measurements of this type provide a comprehensive baseline by which to measure future progress in waste reduction. Second, they tally the various types of waste production in the various industries. Third, these measurements may help to identify previously overlooked opportunities to reduce waste production because although data are regularly collected on materials extraction and production of products, rates of waste production often become apparent only through deliberate, detailed calculation.

Wernick and Ausubel suggest a variety of metrics that can be used to track changes in national environmental performance. Their metrics measure a variety of features of national materials use and waste production, such as aggregate consumption of materials per capita, ratios of uses of various fuels, consumption of materials per unit of economic production, growth-versus-harvesting ratios for natural resources, and inputs of agricultural chemicals per unit of agricultural production. Cox and Offutt (this volume) describe recent efforts to establish similar metrics for farming and ranching. Such measures should prove useful for tracking national and sectoral progress in waste reduction and identifying opportunities for improvements within various sectors of the economy.

Global Estimates of Environmental Performance: Appropriation of Primary Production

An interesting aggregate measure of global human impacts is Vitousek et al.'s (1986) estimate of the proportion of net primary production1 "appropriated"



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