World3


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. 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 W. 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 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 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. 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 nonrenewable resources are extracted, the remaining resources are increasingly difficult to extract, thus diverting more and more industrial output to resource extraction.

Criticism of the model

There has been quite a bit of criticism of the World3 model. Some has come from the model creators themselves, some has come from economists and some has come from other places. At least one study, however, claims that "30 years of historical data compare favorably with key features of a business-as-usual scenario called the 'standard run' scenario" produced by the World3 model.
In the book Groping in the Dark: The First Decade of Global Modelling, Donella Meadows states:
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
  • the residual nature of the investment function
  • the generally ineffective labour contribution to output
A detailed criticism of the model is in the book Models of Doom: A Critique of the Limits to Growth.
Vaclav Smil disagreed with the combination of physically different processes into simplified equations:
He does however consider continuous growth in world GDP a problem:
Others have put forth criticisms, such as Henshaw, King, and Zarnikau who in a 2011 paper, Systems Energy Assessment point out that the methodology of such models may be valid empirically as a world model, but might not then also be useful for decision making. The impact data being used is generally collected according to where the impacts are recorded as occurring, following standard I/O material processes accounting methods. It is not reorganized according to who pays for or profits from the impacts, so who is actually responsible for economic impacts is never determined.
The authors of the book Surviving 1,000 Centuries consider some of the predictions too pessimistic, but some of the overall message correct.
... come to the well-known study, Limits to Growth, published under the sponsorship of the 'Club de Rome' - an influential body of private individuals. A first attempt was made to make a complete systems analysis of the rapidly growing human-biological-resource-pollution system. In this analysis the manifold interactions between the different parts were explicitly taken into account. The conclusion was that disaster was waiting around the corner in a few decades because of resource exhaustion, pollution and other factors. Now, 35 years later, our world still exists,... So the 'growth lobby' has laughed and proclaimed that Limits to Growth and, by extension, the environmental movements may be forgotten. This entirely misses the point. Certainly the timescale of the problems was underestimated in Limits to Growth, giving us a little more time than we thought. Moreover, during the last three decades a variety of national or collaborative international measures have been taken that have forced reductions in pollution, as we shall discuss. A shining example of this is the Montreal Protocol that limited the industrial production of fluorocarbons that damage the ozone layer and generated the 'ozone hole' over Antarctica. The publication of Limits to Growth has greatly contributed towards creating the general willingness of governments to consider such issues. Technological developments have also lead to improvements in the efficiency of the use of energy and other resources, but, most importantly, the warnings from Malthus onward have finally had their effect as may be seen from the population-limiting policies followed by China and, more hesitantly, by India. Without such policies all other efforts would be in vain. However, the basic message of Limits to Growth, that exponential growth of our world civilization cannot continue very long and that a very careful management of the planet is needed, remain as valid as ever.