World3

The World3 model was a 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 "Limits to Growth". The principal creators of the model were Donella Meadows, Dennis Meadows, and Jørgen Randers.

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" and later was tweaked to get the World3/2000 model distributed by the Institute for Policy and Social Science Research.

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 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.

One of the major criticisms of the model is that it simply has not reflected the reality of the world since the 1970s when the model was first published. This criticism is in general false, since most of the predictions of doom or collapse do not begin to occur until around 2015 in the reference run. The model predicted that humanity would run up against the fundamental limits to economic growth about a century after the publication of the book: i.e. 2072, with extremely serious ecological problems only beginning to become obvious in the 2030s and 2040s. Moreover, some of the other runs in the model had even later dates for the beginning of the collapse. The 1992 book, "Beyond the Limits", describes several values of the model that were revised to fit what had happened. None of those changes were sufficient to change the general qualitative conclusions reached by the Meadows group.

Since the modelers that created the model are most familiar with it, their criticism is most relevant. In the book "Groping in the Dark: The First Decade of Global Modelling" (Page 129), 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 (which assumes no diminishing returns to capital),
*the residual nature of the investment function,
*the generally ineffective labour contribution to output.

Of course, since Donella Meadows was very closely associated with the model, she would be unlikely to see problem in the basic qualitative assumptions, so outside criticism needs to be looked at.

The most detailed criticism of the model is in the book "Models of Doom: A Critique of the Limits to Growth".

Both Julian Lincoln Simon and Bjørn Lomborg have discussed the assumptions that the model makes. The first assumption that they criticize is the assumption of finite natural resources. Since the model has a hard limit and no method of switching to substitutes, this is probably a valid complaint about the model. They also state that the limits on agriculture are invalid since they are based on the limit of the amount of land. However, this is incorrect, since the model does allow more food to be grown with the same amount land but with increases of other agricultural inputs (such as fertilizer).

Vaclav Smil disagreed with the combination of physically different processes into simplified equations: :But those of us who knew the DYNAMO language in which the simulation was written and those who took the model apart line-by-line quickly realized that we had to deal with an exercise in misinformation and obfustication rather than with a model delivering valuable insights. I was particularly astonished by the variables labelled "Nonrenewable Resources" and "Pollution". Lumping together (to cite just a few scores of possible examples) highly substitutable but relatively limited resources of liquid oil with unsubstitutable but immense deposits of sedimentary phosphate rocks, or short-lived atmospheric gases with long-lived radioactive wastes, struck me as extraordinarily meaningless. "Energy at the Crossroads" ISBN 0-262-69324-0 pg 168

He does however consider continuous growth in world GDP a problem:

:Only the widespread scientific illiteracy and innumeracy -- all you need to know in this case is how to execute the equation $y\; =\; x\; *\; e^\{rt\}$-- prevents most of the people from dismissing the idea of sustainable growth at healthy rates as an oxymoronic stupidity whose pursuit is, unfortunately, infinitely more tragic than comic. After all, even cancerous cells stop growing once they have destroyed the invaded tissues. "Energy at the Crossroads" ISBN 0-262-69324-0 pg 338-339

External links and References

* [http://www.unh.edu/ipssr/ The Institute for Policy and Social Science Research]
* [http://www.rpi.edu/~simonk/ESP/BTL4MacDownload.html Macintosh version of the Simulation by Kenneth L. Simons]
* [http://www.modelica.org/libraries/SystemDynamics/releases/2.0 Implementation of the World3 model] in the simulation language Modelica
* Models of Doom, A Critique of the Limits to Growth, edited by H.S.D. Cole, Christoper Freeman, Marie Jahoda, and K.L.R. Pavitt. 1973 ISBN 0-87663-905-8
* Dynamics of Growth in a Finite World, by Dennis L. Meadows, William W. Behrens III, Donella H. Meadows, Roger F. Naill, Jorgen Randers, and Erich K.O. Zahn. 1974 ISBN 0-9600294-4-3
* World Dynamics, by Jay Wright Forrester. 1973 ISBN 0-262-56018-6
*Limits to Growth
* [http://www.chelseagreen.com/2005/items/ltgcd Limits to Growth, The 30-Year Update] , by Dennis Meadows and Eric Tapley. 2004 CDRom with World3-2004 model. ISBN 1-931498-85-7