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Thematic Guide to Integrated Assessment Modeling



World Systems Models

The World Systems Modeling effort (Forrester, 1971) captured scientific and popular imagination and led to acrimonious debate in the early 1970's. The studies initiated by Jay Forrester and his co-workers attempted to quantify over long time scales the impact of increasing consumption and population on future global resources and human well being. In a short period of time, Forrester and others moved from treating the results of their modeling efforts as tentative to a more aggressive stance of policy advocacy. Not surprisingly, this generated intense discussion and debate and resulted in the funding of several computer modeling efforts. In retrospect, however, there is a clear consensus that the efforts fell short of their original expectations (Morgan and Henrion, 1990).

World Systems Models contained three implicit underlying assumptions:

  1. It is possible to adequately model all relevant phenomena.
  2. All the modeling can be done in a particular manner (in this case, using Systems Dynamics).
  3. The results of the models are relevant for policy decisions.

Each of these assumptions is problematic. Though modelers don't always make the first claim explicitly, it is often implicit in their interpretation and use of models. It is not possible to adequately model all phenomena, because as Ayres (1984) and Land and Schneider (1987) point out, indeterminacy at the individual and institutional level will lead to macro-indeterminacy. From this it follows that models may only be used only for describing outcomes for a given set of input assumptions. Model results may eventually inform the policy process through the diffusion of ideas. It is also quite clear, that systems dynamics approaches are not the only way to model phenomena, the scientific literature is replete with alternative approaches.

Integrated Assessments share two (1 and 3) of these three characteristics with World Systems Models. IA models attempt to represent the relevant causal factors and aspire to provide policy relevant information, as is evident from definitions of IA in the literature (Rotmans et al., 1995). Despite the overlap with World Systems Models, there is little debate about the implications of that effort for global change integrated assessment modeling. One reason for this may be the fact that world systems models were so heavily discredited. An acknowledgment of the similarities between world systems models and IA models might be seen by IA modelers as unhealthy; after all few researchers would be willing to draw parallels between our own work and work in a field with an already tarnished reputation[FN]. Nonetheless, we feel that there are several important lessons that we in the IA community can learn from a retrospective examination of the reasons for failure. Otherwise we run the risk of repeating the mistakes of our predecessors.

Morgan and Henrion (1990) list five important shortcomings of the world systems modeling efforts. They write:

for the more policy focused efforts we believe that five important shortcomings figured importantly in contributing to the limited policy utility of these methods: (1) inadequate and incomplete understanding of the systems being modeled and a concomitant lack of attention to model verification; (2) failure to be sufficiently specific about the objectives of the modeling projects; (3) failure to carefully examine the implications of uncertainty in input variables and model time constants; (4) inability to deal with the stochastic elements in the systems being modeled; (5) difficulties arising from the ideological perspectives of the analyst.

As we shall see in the next section, many of these shortcomings are also applicable to IA endeavours.





Parson, E.A. and K. Fisher-Vanden, Searching for Integrated Assessment: A Preliminary Investigation of Methods, Models, and Projects in the Integrated Assessment of Global Climatic Change. Consortium for International Earth Science Information Network (CIESIN). University Center, Mich. 1995.


Suggested Citation

Center for International Earth Science Information Network (CIESIN). 1995. Thematic Guide to Integrated Assessment Modeling of Climate Change [online]. Palisades, NY: CIESIN. Available at [accessed DATE].



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