In-use Stock of Steel Estimated by Top-down Approach and Bottom-up Approach

Recently, prices of natural resources have rapidly risen, so recovery of materials from the end-of-life products as secondary resources is of great interest. However, it is generally a challenging task to estimate the in-use stock of materials, especially in developing countries, because of lack of data. In this paper, two approaches, the top-down approach and the bottom-up approach, were adopted for estimating the in-use steel stock in end uses. The top-down approach uses time-series data of consumption and trade of materials and product lifetime data, whereas the bottom-up approach uses the numbers of units of a specified product in a designated area and its material intensities. In this paper, steel stock in Japan divided into seven end-uses was estimated by the top-down approach. Steel in-use stock in Japan was estimated as approximately 1000 Tg in 2005. Steel stock in automobiles in 2005 was estimated as 105 Tg by the bottom-up approach and compared with that estimated as 125 Tg by the top-down approach. In addition, applying the bottom-up approach, steel stock used in automobiles in the United States was estimated and compared with that obtained by the previous research using the top-down approach. Steel stock used in automobiles in 2000 in the United States was estimated as 480–870 Tg by the top-down approach and 754–767 Tg by the bottom-up approach. Both approaches have some uncertainties in the parameters used in the estimation. Therefore, complementary use of the two approaches is helpful to estimate in-use stock of materials.