トップダウン手法とボトムアップ手法による用途別鋼材蓄積量の推計(社会・環境)  [in Japanese] In-use Stock of Steel Estimated by Top-down Approach and Bottom-up Approach(Social and Environmental Engineering)  [in Japanese]

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, a top-down approach and a bottom-up approach, were adopted for estimating the in-use steel stock in end uses. A top-down approach uses time-series data of consumption and trade of materials and product lifetime data, whereas a bot-tom-up approach uses the numbers of units of a specified product in a designated area and its material intensities. In this paper, the steel stock in Japan divided into six end uses was estimated by the top-down approach. The steel in-use stock in Japan was estimated as approximately 1,000Tg in 2005. Steel stock in automobiles in 2005 was estimated as 105Tg by the bottom-up approach and compared with that estimated as 125Tg by the top-down approach. In addition, applying the bottom-up approach, steel stock used in automobiles in U.S. was estimated and compared with that obtained by the previous research using a top-down approach. Steel stock used in automobiles in 2000 in U.S. was estimated as 480-870Tg by the top-down approach and 754-767Tg 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.