Metal stocks in conventional mineral deposits form the basis for sustainability studies. The quantification of these stocks is geologically challenging. Kesler (this volume) regards the reserve base estimates of the U.S. Geological Survey as the most reliable.
The accounting of resource flows is based on the mass balance principle. The origin of recent material flow studies is provided by Kneese et al. (1970). Systematic applications to major industrial sectors (Ayres and Ayres 1998) and to major industrial substance (Ayres and Ayres 1999) have been compiled, and there have been many case studies of specific material fl ows over the last decade.
Increasing attention is being paid to human-made stocks of valuable material resources, including in-use stocks in durable products, infrastructures, and waste deposits. These stocks are often called "urban mines," in contrast to natural reserves.
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