In-use product stocks link manufactured capital to natural capital

Significance The determination of long-term in-use stocks of manufactured products can complement existing monetary approaches to measuring manufactured capital and helps to explore the linkage between manufactured capital and natural capital in terms of materials transfer. The development of new products, substitution among products, and the historical evolution of in-use product stocks in the United States reveal the increase in variety, improvement in quality, and growth in quantity of US manufactured capital. Because products are produced from materials, and products developed more recently tend to use a greater diversity of materials, this study also reveals that US modern manufactured capital relies on the use of more diverse materials and on the increasing use of materials that originate from natural capital. In-use stock of a product is the amount of the product in active use. In-use product stocks provide various functions or services on which we rely in our daily work and lives, and the concept of in-use product stock for industrial ecologists is similar to the concept of net manufactured capital stock for economists. This study estimates historical physical in-use stocks of 91 products and 9 product groups and uses monetary data on net capital stocks of 56 products to either approximate or compare with in-use stocks of the corresponding products in the United States. Findings include the following: (i) The development of new products and the buildup of their in-use stocks result in the increase in variety of in-use product stocks and of manufactured capital; (ii) substitution among products providing similar or identical functions reflects the improvement in quality of in-use product stocks and of manufactured capital; and (iii) the historical evolution of stocks of the 156 products or product groups in absolute, per capita, or per-household terms shows that stocks of most products have reached or are approaching an upper limit. Because the buildup, renewal, renovation, maintenance, and operation of in-use product stocks drive the anthropogenic cycles of materials that are used to produce products and that originate from natural capital, the determination of in-use product stocks together with modeling of anthropogenic material cycles provides an analytic perspective on the material linkage between manufactured capital and natural capital.