Global Systems Simulator

We chose as the objective for the design of the GSS model the representation of the minimum structure needed to explore the concepts of 'sustainability' and 'carrying capacity' at the global level. The World3 model we recognized as Newtonian insofar as it represents a closed system governed by time-invariant parameters that capture the laws of motion of the system in such a way that the trajectory of the system, past and future, is uniquely determined. It makes the user of the model an observer of the system rather than an integral part of it. The GSS belongs to the evolutionary paradigm in which the laws governing the system identify the ensemble of possibilities within which the evolutionary processes unfold. The evolutionary approach was deemed more appropriate as the Earth system is open: both to energy from the sun and to human learning and adaptation.  The GSS is positioned as a tool that can be used to explore alternative scenarios. Exploration involving interaction between the model and its user community is a learning process and the source of novelty for adaptation.

The GSS represents the physical substrate of the Earth system. It consists of a number of processes including:


  • population growth

  • food consumption

  • artifact use

  • food production

  • durable goods production

  • materials recycling 

  • pollution treatment

  • energy production from renewable and non-renewable sources

  • natural resource production from forestry, agriculture and mining

  • a research process that adds to the stock of knowledge.

Energy, raw materials, finished goods, waste materials, pollutants, human effort, and technology embodied in artifacts including production capacity, are the flows that link the various processes.

The Earth system has an endowment of natural resources, a human population, and stocks of artifacts, production capacity and knowledge that serve as the starting point for scenarios. The processes are subject to user control resulting in a system that is over-determined in terms of its control variables.

The GSS is designed in such a way that the system of feedbacks among the processes is incomplete: population driven requirements for raw materials, energy, crops and wood from the natural resource base are not made coherent with their availability. Availability may be limited by original endowments,  regeneration rates, or insufficient investment in exploration or in production capacity; nor are requirements for labour  made coherent with the availability of labour from the population. Differences between requirements and availability are tensions that must be resolved by the user, a surrogate for the society of which he is a member, who provides alternative settings for the control variables.  It is this idea of tension arising from disequilibrium that makes the user of the GSS an integral part of the system as a source of novelty. The interactions between the GSS and the user are illustrated in figure 1 below.

Figure 1

The structure of the GSS is depicted in the figure 2 below. The yellow boxes are sub-models, each of which contains the representation of one or more processes. The arrows indicate the sequence of calculations. Each sub-model is calculated over the full time horizon of the simulation before proceeding to subsequent sub-models. This sequence of calculations is unlike most models that complete the calculations for all sub-models in the first time period before proceeding to calculations for the second and subsequent time periods. It follows that the user sees results for all time periods and all sub-models before proceeding to adjust the settings of the control variables for all time periods. This proves to be advantageous as the user has information about future time periods when setting the control variables for earlier time periods. Furthermore, experiments conducted using an 'automated user' that follows prescribed rules for completing the feedback structure, indicate that the block-recursive strategy for performing the calculations may be as much as three orders of magnitude more computationally efficient than the usual time-recursive methods.

Figure 2

In order to meet the objective of representing the minimum structure needed to explore the concept of sustainability, the GSS was designed to incorporate and illustrate the following principles:

  • The problem being addressed is the sustainability of the population of human beings living on the planet Earth.

  • Earth receives a flow energy from the sun.

  • Earth has finite endowments of natural resources. In the GSS, these endowments consist of land, coal, oil, gas, hydro-electric potential, and a mineral.

  • The population requires a flow nutrition and a stock of artifacts for its survival and well-being.

  • Land supports the growth of trees or crops; crops are the source of nutrition; trees are a source of both wood from which artifacts can be made and energy.

  • Coal, oil and gas are non-renewable endowment-limited sources of energy; hydro-electric potential, trees and solar radiation are renewable and rate-limited sources of energy.

  • The mineral resource is a non-renewable endowment-limited source of material, called metal, from which artifacts can be made.

  • Two kinds of processes must be represented: those that are naturally occurring and those that are purposeful in the sense that they have been put in place and are operated to serve human ends (in the GSS, tree growth and forest regeneration are naturally occurring; most other processes such as mining, ore concentration, exploration, recycling, pollution treatment, planting and harvesting, materials transformations, and artifact production and use are purposeful.

  • Purposeful processes require effort to put them in place and to operate them.

  • There are two sources of effort: human labour and energy. The availability of human labour is limited by the size of the source population.

  • In some processes, energy used in combination with an appropriate artifact, such as an engine, can substitute for human labour as a source of effort.

  • Improvements in process efficiency are not free; rather they require an investment of effort in the form of human labour devoted to increasing the stock of knowledge.

  • The efficiency of processes is embedded in the stocks of productive capacity and artifacts and these stocks reflect the knowledge available at the time they were put in place.

  • Production from endowment-limited resources are subject to diminishing returns to effort; this is represented in different ways in different process. For non-renewable energy production, the amount of effort required to find resources increases per unit producible energy as the limit of the resource is approached. For mineral production the amount of effort required for ore concentration increases as a function of accumulated production. In agriculture three kinds of land are distinguished according to yield potential and the highest yield land is produced first. Sites for hydro-electric energy production are ordered by quality and highest quality sites are used first (by quality is meant the investment per unit of generating capacity).

  • The concept of 'externality' is represented in the GSS in its treatment of pollutants. Some processes generate a waste by-product or pollutant that, if released untreated into the environment, may accumulate and adversely impact other processes: in agriculture, yields are a function of pollutants released; in forests tree mortality is increased as a function of accumulated pollutants.

  • It is also necessary to represent the concepts of reuse and recycling in a world in which there is a finite source of materials In the GSS, all artifacts are made from a variable combination of metal and wood (metal is non-renewable and endowment-limited; wood is renewable but rate-limited and subject to the competing use of wood as a source of energy). As artifacts reach the end of their lives, the metal component of the discarded artifacts may be recovered with the expenditure of effort - the amount of which is a function of the recovery ratio. The GSS is parameterized in such a way that the cross-over between the amount of effort per unit metal from the mine and from recycling occurs in the future.

The GSS has proved to be of interest well beyond its original intended role as a demonstration model.

The GSS has been made available to the Canadian Association for the Club of Rome. A group of CACOR members saw the GSS as a significant advancement over the World3 model upon which the Limits to Growth was based and saw the potential of the GSS approach to engage a wide range of interest groups in furthering understanding the interrelatedness of global issues and fostering the common understanding required for effective action. A not-for-profit entity called Global Systems Centre was proposed  to realize this potential and funding was sought to launch the project. The project foresaw the development of the GSS both as an educational tool for use in high schools and universities that could be used to enhance learning in the domain of global issues and sustainable development and as an analytic tool that could be used in the development of strategies and policies to address the issues of the global problematique.

The concepts and methods embodied in the GSS to explore the concept of sustainability have been the source of inspiration for the design of a number of custom models including the Australian Stocks and Flows Framework.


There has been significant experience in the use of the GSS as an educational tool  at both the high school and university levels and many lessons have been learned. Currently, whatIf? Technologies is supporting the use of the GSS in a distance learning course on systems methods that is offered by Royal Roads University. Web-based instructional material has been developed in cooperation with Pille Bunnel, the course instructor, long time friend of the company and colleague. The course material is available here.


Forrester, Jay. World Dynamics.  Wright-Allen Press, 1971. 

Hoffman, Robert. “Concepts for a New Generation of Global Modelling Tools: Expanding our Capacity for Perception”. Report of the CACOR Global Modelling Project Team published in CACOR Proceedings, Series 1, No. 7, September 1993.

Hoffman, Robert and Bert McInnis, An Overview of the Global Systems Simulator,  January, 1995.

Hoffman, Robert. “Interacting with the Global Systems Simulator”.  CACOR Proceedings, Series 3, No. 2, March, 2001.

Meadows, Donella H., Dennis L. Meadows, J. Randers, and W. W. Behrens.  The Limits to Growth.  Universe Books, New York, 1972.

Meadows, Dennis L. Dynamics of Growth in a Finite World. Wright-Allen Press, 1974.

Meadows, Donella and J. M. Robinson.  The Electronic Oracle: Computer Models and Social Decisions.  John Wiley and Sons, 1985.

Meadows, Donella, Gerhart Bruckmann and John. Robinson. Groping in the Dark:  The First Decade of Global Modelling.  John Wiley and Sons, 1982.

Meadows, Donella, Dennis L. Meadows, Jorgen Randers. Beyond the Limits. McClelland and Stewart, 1992.

Meadows, Donella, Dennis L. Meadows, Jorgen Randers. Limits to Growth: The 30 Year Update. Chelsea Green, 2004.