The World3 model is a system dynamics model for computer simulation of interactions between population, industrial growth, food production and limits in the ecosystems of the earth. It was originally produced and used by a Club of Rome study that produced the model and the book The Limits to Growth (1972). The creators of the model were Dennis Meadows, project manager, and a team of 16 researchers.
The model was documented in the book Dynamics of Growth in a Finite World. It added new features to Jay Wright Forrester's World2 model. Since World3 was originally created, it has had minor tweaks to get to the World3/91 model used in the book Beyond the Limits, later improved to get the World3/2000 model distributed by the Institute for Policy and Social Science Research and finally the World3/2004 model used in the book Limits to Growth: the 30 year update.
World3 is one of several global models that have been generated throughout the world (Mesarovic/Pestel Model, Bariloche Model, MOIRA Model, SARU Model, FUGI Model) and is probably
the model that generated the spark for all later models .
Model
The model consisted of several interacting parts. Each of these dealt with a different system of the model. The main systems were
- the food system, dealing with agriculture and food production
- the industrial system
- the population system
- the non-renewable resources system
- the pollution system
Agricultural system
The simplest useful view of this system is that land and fertilizer are used for farming, and more of either will produce more food. In the context of the model, since land is finite, and industrial output required to produce fertilizer and other agricultural inputs can not keep up with demand, there necessarily will be a food collapse at some point in the future.
Nonrenewable resources system
The nonrenewable resource system starts with the assumption that the total amount of resources available is finite (about 110 times the consumption at 1990s rates for the World3/91 model). These resources can be extracted and then used for various purposes in other systems in the model. An important assumption that was made is that as the non-renewable resources are extracted, the remaining resources are increasingly difficult to extract, thus diverting more and more industrial output to resource extraction.
The model combines all possible nonrenewable resources into one aggregate algorithmic variable, . This combines both energy resources and non-energy resources. Examples of nonrenewable energy resources would include oil and coal. Examples of material nonrenewable resources would include aluminum and zinc. This assumption allows costless substitution between any nonrenewable resource. The model ignores differences between discovered resources and undiscovered resources. In this scenario, in 2000, the world population reaches six billion, and then goes on to peak at seven billion in 2030. After that population declines because of an increased death rate. In 2015, both industrial output per capita and food per capita peak at US$375 per person (1970s dollars, about $ today) and 500 vegetable-equivalent kilograms/person. Persistent pollution peaks in the year 2035 at 11 times 1970s levels. Donella Meadows (a Limits author) writes: <blockquote>We have great confidence in the basic qualitative assumptions and conclusions about the instability of the current global socioeconomic system and the general kinds of changes that will and will not lead to stability. We have relatively great confidence in the feedback-loop structure of the model, with some exceptions which I list below. We have a mixed degree of confidence in the numerical parameters of the model; some are well-known physical or biological constants that are unlikely to change, some are statistically derived social indices quite likely to change, and some are pure guesses that are perhaps only of the right order of magnitude. The structural assumptions in World3 that I consider most dubious and also sensitive enough to be of concern are:
- the constant capital-output ratio (which assumes no diminishing returns to capital)
- the residual nature of the investment function
- the generally ineffective labour contribution to output</blockquote>
A detailed criticism of the model is in the book Models of Doom: A Critique of the Limits to Growth.
Czech-Canadian scientist and policy analyst Vaclav Smil disagreed with the combination of physically different processes into simplified equations:
He does however consider continuous growth in world GDP a problem: