World3
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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 creaters 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.
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Model
The model consisted of several interacting parts. Each of these dealt with a different system of the model. The main systems where the food system, dealing with agriculture and food production, the industrial system, the population system, the non-renewable resources system and the pollution system.
Agricultural system
The simplest useful view of this system is that there is 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 will be a food collapse at some point in the future.
Nonrenewable resources system
Main Article: World3 nonrenewable resource sector
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 later resources are increasingly difficult to extract.
Criticism of the model
There have been quite a bit of criticism of the world3 model. Some has come from the model creators themseleves, 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 enought 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 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 subsitutes, 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).
The criticisms that have been made of the model do need to be considered, however, even if the finite natural resource assumption is removed from the model, the model still show a collapse due to pollution. The model cannot be discounted because of the criticism of it made so far.
As a rebuttal, critics have argued that Julian Simon, an economist, and Bjørn Lomborg, a political scientist, lack the required scientific credentials. Both authors produced books that have been heavily criticised by environmental scientists.
As to the Limits to Growth, fact remains that this Earth is finite and resources and space are running out.
External links
- The Institute for Policy and Social Science Research (http://www.unh.edu/ipssr/)
- Macintosh version of the Simulation by Kenneth L. Simons (http://www.rpi.edu/~simonk/ESP/BTL4MacDownload.html)