Musings on IT, data management, whitewater rafting, and backpacking

Monday, May 30, 2011

Adventures with World3 1976-1978

From late 1976 to early 1978, I ran many simulations using the World3 model. World3 was used for the famous book The Limits To Growth.

I was puzzled by my results for many years, until I had an Aha! moment.

In the late 1970s, I had a passion for environmental politics, a full-time data processing job with only half-time of work, and nearly unlimited access to an IBM 360/50 mainframe. The Limits To Growth, based on the World3 model, had caused a major uproar for years after publication.

Our mainframe had the DYNAMO simulation language used by World3, so I keypunched the source code from Dynamics of Growth in a Finite World, and started playing. I had very little background in modeling or computer simulation.

The Limits To Growth showed simple and convincing graphs going out a few decades, which supported the book's main premise that the world was headed toward a dramatic crash in less than 20 years.

One of my first puzzles came from running World3 out a few more decades. Many of the graphs went nuts just after the cutoff dates used in the book. By "nuts", I mean wild fluctuations, sometimes into implausible values, like negative values for quantities that could never be negative. So I restricted my experiments to the same time frames as the book, and started varying other parameters.

I found that very small changes in the starting values could cause dramatic changes in the outputs. I had an sense at the time that this wasn't right, but since I didn't know much about modeling, I thought I was just ignorant.

In early 1978, I changed jobs, and no longer had access to DYNAMO, World3, or free mainframe time. Soon, I had mostly forgotten my experiments.

Many years later, while learning about Chaos theory, I had an Aha! moment.

I wasn't stupid.

World3 was a chaotic system driven by simple non-linear equations.

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