A Dynamic Model for Determining the Temporal Distribution of Environmental Burden

In an article on the role of temporal information in life‐cycle assessment in this journal, Field and colleagues argued that frequently it is not the single product but the “fleet” (or cohort) of products that “is the appropriate unit of analysis,” and that in focusing on the fleet one “explicitly introduces the notion of time as a critical element of comparative life‐cycle assessments. …” Major transitions, such as replacement of one fleet of products by an alternative fleet, correspond to a system in a transient rather than steady state, and explicit consideration of time is central to transient analysis. One tool increasingly used as part of life‐cycle assessment, economic input‐output (EIO) analysis, at best deals with time in an implicit fashion. This article illustrates how the sequential interindustry model (SIM), a formulation of the EIOmodel that explicitly represents time, might be utilized in life‐cycle assessment. SIM introduces this temporal component by explicitly accounting for the time required by production activities and the resulting sequencing of the inputs. This can be thought of as engineering rather than accounting information. The data demands of such a model are not likely to be met at present or at any time in the near future. Even so, simulation methods and the use of so‐called synthetic data have a history of productive use in a number of fields, including the social sciences. SIM also utilizes the contribution of Joshi on the application of the EIO model to environmental impact and the inclusion of the use as well as the production phases of a product in EIO analysis. The possibility of accounting for discounting of future events, with its impact on decision making, is also briefly discussed.