World 3 was the first comprehensive integrated global simulation model. It became known to a wide audience through the publication of Limits to Growth (Meadows, Meadows et al., 1972) and in its revised form through the publication of Beyond the Limits (Meadows, Meadows et al., 1991).
World 3 is a system dynamics model that covers a wide range of population, food, energy, environmental and economic issues. System dynamics models are based on sets of difference equations. These equations are used to express levels of stock variables and rates as a measure of change in the stock variables. Auxiliary variables, mostly parameters, quantitatively describe the relationships between the different stocks and flows. This modelling technique enables modellers to model complex interactions without having to tackle massive sets of time-series data. Scenarios can be forecasted and backcasted from a base year, which is the only year for which data is needed for a scenario run (for validation and calibration other data sets might be needed). In the case of the revised World 3 model, the base year is 1990. The forms of the equations, the interactions and the values for the auxiliaries and the constants are based on statistical analysis and scientific judgement without participative input.
Scenario simulations run from 1900 until 2100. The so-called backcasting is used in order to validate and calibrate the model to actual history. A comprehensive technical description of the original World 3 model can be found in Dynamics of Growth in a Finite World (Meadows, Meadows et al., 1972). World 3 is the first comprehensive example of horizontal integration. There is a plethora of feedbacks between the different domains of the models. However, the socio-cultural domain and the institutional domain are not represented in the model at all. The level vertical integration is limited. It is not possible to trace clear cause-effect chains as the descriptions of the different processes are too crude to allow for a detailed analysis. A major drawback of the model is the fact that it can only provide scenarios on a global level without a regional disaggregation. The scenarios generated are of limited use for policy makers as they allow only very general conclusions about the possible effects of different policy strategies. As each new scenario run requires adjustments in the code of the program, the ease of use for people other than the developers is very limited.
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