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COMPUTING SCIENCE

Computation and the Human Predicament

The Limits to Growth and the limits to computer modeling

Brian Hayes

Limits of Limits

After three immersions in The Limits to Growth, at intervals of 20 years, I feel entitled to state some opinions.

First, the book’s message is worth listening to. There are limits, and exponential growth is unsustainable. A society that measures well-being by the first derivative of GDP is asking for trouble. But I am more optimistic than the Limits authors are about our ability to deal with these issues before the world turns into the set of a Mad Max movie.

As for the mathematical model behind the book, I believe it is more a polemical tool than a scientific instrument. Forrester and the Limits group have frequently said that the graphs drawn by their computer programs should not be taken as predictions of the future, but only as indicating “dynamic tendencies” or “behavior modes.” But despite these disclaimers, Limits is full of blunt statements about the future: “If the present growth trends continue unchanged,... the limits to growth on this planet will be reached sometime in the next one hundred years.” And whether the models are supposed to be predictive or not, they are offered as an explicit guide to public policy. For example, in testimony before a congressional committee in the 1970s Forrester recommended curtailing investment in industrialization and food production as a way of slowing population growth.

It’s possible that Forrester was offering wise advice, and someday we’ll regret not taking it. But when a mathematical or scientific argument is brought forward to justify taking such a painful and troubling action, standards of rigor will surely be set very high.

In an unpublished paper on the testing of system dynamics models, Forrester and a student wrote: “The ultimate objective of validation is transferred confidence in a model’s usefulness as a basis for policy change.” That has yet to happen for World3.

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