In a recent article, Robert Bolman, postulates the following five conditions for sustainability:

  • Renewable resources shall not be used faster than they can regenerate.
  • Pollution and waste shall not be put into the environment faster than the environment can recycle them or render them harmless.
  • Nonrenewable resources shall not be used faster than renewable substitutes (used sustainably) can be developed.
  • The human population and the physical capital plant must be kept at levels low enough to allow conditions 1, 2 and 3 to be met.
  • The previous four conditions must be met through processes that are democratic and equitable enough that people will stand for them.

These conditions can be analyzed using the system dynamics method.


System Dynamics is not the classical scientific method of experimentation under controlled conditions. It is a scientific art intended to analyze and improve the behavior of large and complex systems which usually are not amenable to controlled experimentation. The fundamental premise is that system behavior is driven by its endogenous feddback structure, with information feedback being critical. The objective is then to modify the endogenous feedback structure of the system so as to observe improved behavior in response to exogenous inputs.

This scientific art usually entails some iterative combination of the following steps, not necessarily in this sequence:

Analysis of System Behavior over Time

The goal of the analysis is to identify the undesirable modes of dynamic behavior, and then identify how these behavior modes would have to be modified to improve system behavior. Figure 1 shows patterns of economic growth by region as well as the widening gap between the rich and the poor worldwide. Figure 2 shows the increasing amount of CO2 emissions due to production and consumption activity. The growth pattern is similar for other kinds of pollution.


Figure 1. Global Economic Growth in Rich and Poor Regions
Source: United Nations Development Program (UNDP)


Figure 2. Global CO2 Emissions -- 1950-2000
Source: World Resources Institute (WRI)

Causal-Loop Diagrams

The goal of the causal-loop diagrams is to capture the feedback loop geometry that generates the undesirable dynamic behavior. Simulation models (usually formulated in terms of zero and first order nonlinear difference equations) may be used to augment the causal-loop diagrams. Simulation tests can then be used to determine whether or not the model behaves like the system. Figure 3 is a simple causal loop diagram that shows the feedback loops responsible for the widening gap between the rich and the poor worldwide.


Figure 3. Causal Loop Diagram of Population, Resources, Consumption, and Pollution

Feedback Geometry Redesign, Modeling, and Simulation

This is the most artistic phase of the system dynamics method: how to modify the feedback loop structure so as to suppress the undesirable behavior modes and generate the desirable ones. It is never a matter of adjusting parameter values. The geometry of the loops must be changed. After mathematical formulation and quantification of parameter values, computer simulations are used to assess the modified dynamic behavior of the system. The simulations also provide "visibility" beyond current trends. For instance, the chart below shows that growth may be followed by severe decline in economic activity, population, and other variables.

Simulation Model

Figure 4 is a simulation diagram. It does not include "processes that are democratic and equitable enough that people will stand for them.". Such a political process avoids the extremes of making decisions only for individual gain or only for the common good. A precise formulation of these decision-making practices remains TBD.


Figure 4. Simulation Model Diagram of Population, Resources, Consumption, and Pollution

Simulation Results

Figure 5 is a sample of simulation results. The simulation explores the dynamic interactions between resource depletion, production and consumption oblivious to environmental concerns, and population growth during 500 years, starting with the year 2000. Population grows exponentially, resources and depleted, and the resulting food shortages and environmental contamination induces a catastrophic reversal from positive to negative population growth.


Figure 5. Simulation Results from 2000 to 2050

Note: This is a highly simplified analysis. Donella Meadows et al, in their classical analyses, Limits to Growth, 1972, and Beyond the Limits, 1992, have provided the most detailed analysis of global dynamics available at the moment. Several research teams are currently working on global system dynamics, including Willard Fey and Ann Lam of Ecocosm Dynamics Ltd.

Verification, Validation, and Implementation Plan

Verifications means that the simulation model behaves as intended. Validation means that the model behaves like the current real system. If there is confidence that the re-designed loop geometry will improve system behavior, an implementation plan is prepare to accomplish the transition from the current system to the new system. Then, in most cases, things change that require a new iteration of analysis and redesign. The most important requirement for using the system dynamics method is the availability of updated time histories depicting how the system behaves over time. The most important benefit is the insight gained from the analysis, especially when such insight can be translated into improved system management policies.

|Back to the top|

The following are pointers to some applications of the system dynamics method:

The following are some software tools for system dynamics modeling and simulation:

|Back to the top|

The following are some "ready to run" web simulations:

  • Forio Business Simulations
    • Aftermarket Strategy Simulation
    • B&B Enterprises Management Flight Simulator
    • Brands Management Simulation
    • Leadership in Action Simulation
    • U.S. Oil Import Policy Simulator
    • Supply Chain Management Simulation
    • Price Strategy Simulator: HP/Compaq vs. Dell
  • PriSim Business Simulations
    • Mini-MBA - Strategy & Finance
    • iMPACT - eBusiness
    • Deep Pockets - Mortgage Banking
    • Magnate - Magazine Publishing
    • Cycloan - Market Management
    • DealerSim - Dealership Operations
    • InsurED - Insurance Management
  • Ventana Systems -- Vensim Sample Models
    • Financial Modeling and Risk
    • Maintenance and Service Strategies Explorer
    • Business Process Re-Engineering Explorer
    • Sales Cycle Explorer
    • World3-91 Explorer (Beyond the Limits)
    • Project Model Explorer
    • Urban Game
  • ISEE Systems -- STELLA and iThink Sample Models
    • Ecosystem Dynamics NetSim Demo (ISEE)
    • Growth Dynamics NetSim Demo (ISEE)
    • Other STELLA models (Andrew Ford's Website)
      • The DDT Models
      • The Smolts Migration Model
      • The Tucannon Harvest Manager
      • The Feebate Management Model
      • The Daisyworld Flight Simulator
      • The Idagon Flight Simulators