The World3 model is a tool used to study how population, industry, food production, and Earth's natural limits interact. It was first created by a group called the Club of Rome, which also wrote the book The Limits to Growth in 1972. Dennis Meadows, the project leader, and 16 other researchers developed the model.

The model was described in the book Dynamics of Growth in a Finite World. It improved upon an earlier model called World2, created by Jay Wright Forrester. Over time, the model was updated to become the World3/91 version, used in the book Beyond the Limits. Later, it was further refined into the World3/2000 model, shared 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 developed worldwide, including the Mesarovic/Pestel Model, Bariloche Model, MOIRA Model, SARU Model, and FUGI Model. It may have inspired the creation of many later models.

Model

The model included several connected parts. Each part focused on a different system within the model. The main systems were:

• the food system, which handled agriculture and food production
• the industrial system
• the population system
• the non-renewable resources system
• the pollution system

A basic way to understand the food system is that land and fertilizer are used for farming. Using more of either can increase food production. However, in the model, land is limited, and industrial production of fertilizer and other farming supplies cannot grow fast enough to meet future needs. This means food production will eventually decrease.

The non-renewable resources system starts with the idea that the total amount of these resources is limited (about 110 times the amount used in the 1990s for the World3/91 model). These resources are taken from the earth and used in other systems. A key assumption is that as more resources are extracted, the remaining resources become harder to reach, which requires more industrial effort for extraction.

The model combines all non-renewable resources into one single variable called "nonrenewable_resources." This includes both energy resources, such as oil and coal, and material resources, such as aluminum and zinc. The model assumes that any non-renewable resource can be replaced by another without cost. It does not consider differences between resources that have been found and those that have not.

The model assumes that as more non-renewable resources are used, more effort is needed to extract them. This effort is represented by a variable called "fraction_of_capital_allocated_to_obtaining_resources" (fcaor). This variable is used in the equation that calculates industrial output. The equation shows that industrial output depends on the amount of industrial capital and other factors, but not directly on the amount of resources used.

The use of non-renewable resources depends on a nonlinear relationship with per capita industrial output. As per capita industrial output increases, so does the use of non-renewable resources.

The book The Dynamics of Growth in a Finite World describes several possible outcomes. The "reference run" represents the most likely scenario if industrialization continues in a way similar to the past and existing technologies and values continue to develop. In this scenario, the world population reaches six billion in 2000 and peaks at seven billion in 2030. After 2030, the population decreases due to higher death rates. In 2015, both per capita industrial output and per capita food production reach their highest levels: $375 per person (1970s dollars, about $2,890 today) and 500 vegetable-equivalent kilograms per person. Pollution levels peak in 2035 at 11 times the levels of the 1970s.

Criticism of the model

The World3 model has faced criticism from various sources, including its creators, economists, and others. In the book Groping in the Dark: The First Decade of Global Modelling, Donella Meadows, one of the authors of The Limits to Growth, provides a detailed critique of the model. A full analysis of the model’s limitations is also found in the book Models of Doom: A Critique of the Limits to Growth.

Vaclav Smil, a Czech-Canadian scientist and policy analyst, disagreed with how the model combines different physical processes into simple equations. However, he also noted that continuous growth in the world’s GDP is a concern.

Other critics, including Henshaw, King, and Zarnikau, wrote in a 2011 paper titled Systems Energy Assessment that while the model’s methods may be valid for studying the world as a whole, they might not help with making decisions. They explained that the data used in the model shows where environmental impacts occur, but it does not track who is responsible for those impacts or who benefits from them. This means:
– The economic reasons behind the impacts, which could also help control them, are not included in the model.
– This might question the usefulness of many economic models for making decisions about sustainability.

The authors of the book Surviving 1,000 Centuries believe some predictions from the model are too negative, but they agree that some of the model’s overall message is correct.

At least one study, published in the journal Global Environmental Change, disagrees with the criticism. The study, led by Turner, states that 30 years of real-world data align well with the "business-as-usual" scenario, called the "standard run," predicted by the World3 model.

Validation

Many scientists have tested the predictions of the World3 model using real-world data, and their findings have been different. A recent study in Yale's Journal of Industrial Ecology shows that current real-world data matches the 1972 predictions. If major changes are not made in how resources are used, economic growth is expected to reach a high point and then quickly decrease by about 2040.

External links and references

• "Basic Literature": A list of books about limits to growth, each with a short summary. Published by the All-Party Parliamentary Group (APPG).
• 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.
• The Limits to Growth (Abstract, 8 pages, by Eduard Pestel). A Report to The Club of Rome (1972), by Donella H. Meadows, Dennis L. Meadows, Jorgen Randers, and William W. Behrens III.
• Limits to Growth, The 30-Year Update, by Dennis Meadows and Eric Tapley. 2004. CD-ROM with the World3-2004 model. ISBN 1-931498-85-7.
• WorldChange Model. This includes a system that helps analyze why sustainability challenges remain unsolved.

• JavaScript version of the World3 simulation.
• Interactive online World3 simulation.
• pyworld3 on GitHub – a Python version of the World3 model.
• MyWorld3 on GitHub – another Python version of the World3 model.
• Macintosh version of the simulation by Kenneth L. Simons.
• Implementation of the World3 model in the simulation language Modelica.
• WorldDynamics.jl on GitHub – a Julia version of the World3 and World2 models.