Boundary matters: the potential of system dynamics to support sustainability?

System Dynamics can be a powerful tool for understanding dynamics, especially feedback behaviours, in many social-ecological systems where sustainable pathways are sought. Along with quantitative simulations and optimization packages, System Dynamics offers qualitative tools (e.g. system archetypes, Causal Loop Diagrams, and Stock and Flow Diagrams) for further enhancing understanding. Like all methods, System Dynamics can be applied effectively and successfully for specific objectives in specific contexts by some analysts and modellers, but can also be used poorly and potentially unethically by others. This paper investigates the potential of System Dynamics to support Sustainability and the role played by modellers, including their practices and assumptions, in developing this potential. This is achieved by reviewing the application of System Dynamics within the context of the sustainability literature and how there are multiple levels of boundary judgements to be made by System Dynamics modellers, their commissioners and stakeholders: specifically around the sustainability principles that provide the criteria by which policy options from the modelling are evaluated; definitions of problem scope; and around the extent of dynamic processes explicitly considered in both the qualitative and quantitative System Dynamics thinking and modelling practice. To illustrate how boundary judgements matter for sustainability modelling, we draw on three examples, and discuss the potential risks and challenges these raise. The paper also discusses how and why System Dynamics needs to be used by analysts in a more complete manner (i.e. engaging qualitative, quantitative, and participatory approaches) to ensure that in any given applied case, the models, and the judgements underpinning their development, are effectively embedded in societal and political contexts that allow their use for supporting sustainability.