Whereas Jay Forrester listed his entire model in his book "World Dynamics," Dennis Meadows only talked in "Limits to Growth" about the results obtained using his model. The model itself is not listed.

This was partly done because the **WORLD3** model is considerably more complex than the **WORLD2** model, and consequently, a thorough discussion of all facets of the model would have eaten up much more space in the book, but there is another reason as well.

Stephen Hawking wrote in the introduction to one of his more popular books (A Brief History of Time: From the Big Bang to Black Holes) that he had asked his publisher whether he could include some equations with the book. The publisher answered that he would print anything that Stephen Hawking cared to submit for publication, but that the number of books sold would shrink by a facter of two for every added equation. Hence Hawking decided to refrain from placing *any* equations in his book.

Meadows is an *educator* even more than a *scientist*. He believes strongly in his message, and he wanted to reach as many people with it as he possibly could. Leaving the model equations out was the best way of accomplishing this goal.

Yet, Meadows never tried to hide his model from public scrutiny. In fact, he wrote an internal report of 637 pages that describes all facets of his model. He published that internal report as a separate book in 1974: Dynamics of Growth in a Finite World. The book is still available if anyone cares to buy it. However, whereas Meadows sold millions of copies of his (popular) *Limits to Growth* book, he probably sold no more than a few hundred copies of his (technical) *Dynamics of Growth in a Finite World* report.

Since the **WORLD3** model is fairly complex, it won't fit on a single screen. For this reason, I subdivided the model into 13 different sectors (i.e., sub-models) describing some aspects of:

- arable land dynamics,
- food production,
- Mathis Wackernagel's human ecological footprint,
- human fertility,
- human welfare index,
- industrial investment,
- labor utilization,
- land fertility,
- life expectancy,
- non-recoverable resource utilization,
- pollution dynamics,
- population dynamics, and
- service-sector investment.

The sub-models use green icons. In the main model (yellow icon), I simply dropped one sub-model of each type onto the screen, and connected these models appropriately.

Whereas the **WORLD2** model lumped the entire population into a single state variable, the **WORLD3** model offers a demographic population dynamics model that distinguishes between children and adolescents, young adults of child-bearing age, older adults who are still integrated into the work force, and the retired population.

The capital investment is subdivided into investments in the military/industrial complex, in the service sector, and in agriculture.

Both the natural resources and pollution models have been upgraded by including changes in technology as factors influencing the depletion of resources and the release of pollutants. This is meaningful as improved technology may enable us to use the available resources more efficiently, and may also make it possible to produce goods in a cleaner fashion.

The over-all (compiled) **WORLD3** model contains 41 state variables and 245 algebraic variables.

At this time, all that remains is to thank Prof. Meadows for his support of my efforts. Dennis was kind enough to send me his complete **WORLD3** model (2004 edition, coded in **STELLA**) on a CD. All that I had to do was to translate the model, equation by equation, from **STELLA** to **Modelica**.

Unfortunately, **STELLA** doesn't offer means for documenting models in an explicit way. There was not a single line of comments in the entire model. Meadows compensated for this lack of documentability by choosing very long variable names. These helped tremendously in deciphering the meaning of the model. I hope I didn't goof up too badly in doing so.

In order to make it as easy as possible to compare the **Modelica** version of the **WORLD3** model with the **STELLA**-coded original, I decided to keep the same variable names in my model that Dennis had chosen for his version.

I also wish to acknowledge Prof. J�rgen Randers who provided me with the information necessary to reproduce the 10 scenarios proposed in the book Limits to Growth: The 30-Year Update.

**References:**

- Meadows, D.H., D.L. Meadows, J. Randers, and W.W. Behrens III (1972),
*Limits to Growth: A Report for the Club of Rome's Project on the Predicament of Mankind*, Universe Books, New York, 205p. - Meadows, D.L., W.W. Behrens III, D.H. Meadows, R.F. Naill, J. Randers, and E.K.O. Zahn (1974), Dynamics of Growth in a Finite World, Wright-Allen Press, 637p.
- Meadows, D.H., D.L. Meadows, and J. Randers (1992),
*Beyond the Limits*, Chelsea Green, 300p. - Meadows, D.H., J. Randers, and D.L. Meadows (2004), Limits to Growth: The 30-Year Update, Chelsea Green, 368p.

Name | Description |
---|---|

Arable_Land_Dynamics | Arable land dynamics |

Food_Production | Food production |

Human_Ecological_Footprint | Ecological footprint |

Human_Fertility | Human fertility |

Human_Welfare_Index | Human welfare index |

Industrial_Investment | Investments in the military/industrial sector |

Labor_Utilization | Utilization of the labor force |

Land_Fertility | Land fertility |

Life_Expectancy | Life expectancy |

NR_Resource_Utilization | Utilization of non-recoverable natural resources |

Pollution_Dynamics | Pollution dynamics |

Population_Dynamics | Population dynamics |

Service_Sector_Investment | Investments in the service sector |

Scenario_1 | Original WORLD3 model |

Scenario_2 | More abundant non-recoverable natural resources |

Scenario_3 | More accessible non-recoverable natural resources and pollution control |

Scenario_4 | More accessible non-recoverable natural resources, pollution control, and land yield enhancement |

Scenario_5 | More accessible non-recoverable natural resources, pollution control, land yield enhancement, and erosion protection |

Scenario_6 | More accessible non-recoverable natural resources, pollution control, land yield enhancement, erosion protection, and resource efficiency |

Scenario_7 | More abundant non-recoverable natural resources and birth control |

Scenario_8 | More abundant non-recoverable natural resources, birth control, and stable industrial output |

Scenario_9 | Combining the measures of Scenarios #6 and #8 |

Scenario_10 | Influencing the future 20 years earlier |

Scenario_11 | Influencing the future 10 years later |

Utilities | Utility models of Meadows' WORLD3 model |

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