bined concepts of equilibrium and optimization. This practice has the advantage of producing unique solutions to complex problems but at too high a price: it precludes the consideration of many alternatives that industrial ecologists will want to investigate, alternatives that society may in fact deem more desirable than those that may satisfy the short-term maximization of a single, vaguely defined criterion.

Structural economics is an incomplete theory; this is a difficult status for an economist to defend because practicing economists have come to depend on a complete, although not always operational, conceptual and analytic framework. We choose, however, not to use a high level of abstraction as a mechanism to "complete" the theory. Instead, our models are "open," making use of exogenous information such as technological projections that are provided by engineers and other technical experts rather than being derived through the use of mathematical equations that describe idealized economic mechanisms.

Through our work, my colleagues and I are trying to provide a systematic and operational description of structural economics as an economist's contribution to such new fields as industrial ecology, ecological economics, or the study of global change. This is necessary because I believe that economics needs to undergo, and will undergo, profound transformation not only in theory, but especially in scope and in methodology, as the next generation of economists attempts to understand and resolve environmental problems. The scope of the field is important because it delimits the questions that can be addressed, while the methodology determines the mathematics that will represent the theoretical propositions and the strategy for collecting and using information to implement the mathematical model and help interpret the numerical results.

Industrial ecologists will come from many backgrounds and most cannot afford to be intimately involved in the transformation of other fields such as economics while trying to build their own field. For example, the proliferation of terms used here (input-output analysis, structural economics, neoclassical economics, and ecological economics) is essential for the process of sorting out and building within economics but cannot be allowed to become a jargon that fragments our common efforts. 1 In view of these considerations, I will focus my remaining comments on two related areas of scope and methodology that are of great importance both for structural economics and for industrial ecology. These are (1) the questions that we need to address and (2) the role of case studies.

THE QUESTIONS TO BE ADDRESSED

My colleagues and I have recently completed a quantitative analysis of strategies for sustainable development over the next several decades for the UN Conference on Environment and Development (UNCED) using an input-output model and data base of the world economy (Duchin et al., 1994). We have focused on the use of energy to promote development and on the associated emissions of



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