Reply: Downstream cut-offs in integrated hybrid life-cycle assessment

In the comment by Peters and Hertwich (Peters, G.P., Hertwich, E., 2006-this issue), the authors provide two interpretations on one of the block matrices that is a part of the integrated hybrid Life Cycle Assessment (LCA) framework presented in Suh (2004a) (Suh, S., 2004a. Functions, commodities and environmental impacts in an ecological economic model. Ecological Economics 48 (4), 451-467). The authors argue that the contribution of the downstream cut-off matrix to the overall results might be small, or the amount of resources needed to compile the matrix will be too high, depending on the type of the LCA databases that the authors distinguish. In this response, I confirm that the contribution of the downstream cut-off matrix is generally minor, which can be easily recognized from the structure of the system. However, one should realize that there are the cases when it is not, leaving little reason to automatically preset the downstream cut-off matrix as a zero matrix. A set of conditions under which a downstream cut-off matrix becomes an important part is discussed, and a practical method for checking the potential importance of a downstream cut-off matrix before actually compiling data is presented. The method is tested using the numerical example shown in Suh (2004a). The distinction of LCA databases based on their capabilities of handling arbitrary demands, which is proposed by Peters and Hertwich, is discussed from an LCA practitioner's point of view, and it is argued that the actual process of data collection for the downstream cut-off matrix is far from what the authors suggest. Finally, a step-wise strategy for an efficient data collection under the budget and time constraints is discussed.