## 4 Simulation Models and Sustainability: Characterization
### System Dynamics (SD)  
SD is a top-down modeling framework, originally proposed in 1961, with information feedback aspects. An SD model is used to investigate a dynamic evolution process in various scenarios. Reductionism and holism, as philosophical paradigms, have been cited as the basis for SD modeling. Reductionism, through simplifying ideas, is the act of reducing complicated structures, concepts, or events into their constituent parts. Alternatively, SD is also said to be holistic in its approach to understanding a system's dynamic behavior in comparison to existing issue description and solution methodologies [136]. Causal Loop Diagrams (CLD) maps the feedback loops showing the dynamic interactions of the system variables. The variables are connected using arrows that depict causal relations between them with the indication of positive and negative polarities [137]. The polarities signify the nature of the relationship between the variables as they specify how the dependent variable influences the independent variables. The feedback loops are characterized as balancing and reinforcing. Positive feedback indicates that a change in any of the variables in the causal loop will eventually have a positive effect on itself, whereas negative feedback indicates that a change in any of the variables in the causal loop will have a negative effect on itself. In its methodology, the SD has four major stages, namely, the conceptual stage, causal loop stage, simulation stage, and model evaluation. These stages are depicted in the flowchart in Figure 14.  
Among the three simulation techniques, the SD is considered as being most commensurate with the breadth of analysis that sustainability modeling requires. Appendix A.3 indicates recent representative research wherein SD has been used in modeling sustainability in manufacturing and allied SCs. This has been attributed to two major reasons [138]. Firstly, the idea that sustainable development is conceived as being a \"process\" rather than a \"project\" whereby sustainability is not seen as a blueprint or a pre-conceived end state. Sustainability is a system state ideal and an ongoing process [139]. Secondly, sustainability entails that decisions and measures ought to be focused on the whole system  
Figure 14: The model creation and simulation process is depicted in a flowchart. The system analysis stage, the establishment of the conceptual model stage, the construction of the quantitative model stage, the model verification stage, and the model simulation stage are all covered by the fundamental technique of SD modeling. by sustaining harmony between its sub-systems and perception of cumulative impacts of actions. As sustainability is understood by assessing the state of environmental, social, and economic indicators while adequately addressing the interconnections between them. System dynamics as a paradigm bodes perfectly well to the aforementioned two conditions. As can be discerned from the \"systemic\" predisposition in of SD, its casts a wide net both \"temporally\" as well as \"spatially\". The implication of which is that majority of research done in the SD framework is at the strategic level of operations. One may cite a number of works addressing the strategic aspects of sustainability in SCs. Carrying further the work of [140] on the social responsibility of SCs using SD and incorporating the product lifecycle and capacity planning [141], the SD model is used to model policy effectiveness to achieve optimal sustainability performance across the TBL. Similarly, another study focused on strategic decision-making using SD looks at the modalities of policy-making in case of transition to sustainable manufacturing [142]. SD is used to model the effects of employee training, efficient equipment, and reduce costs and energy consumption as prerequisites to eco-invention. SD has also been used to assess organizational sustainability, [143]. The simulation is carried out at three levels, namely company level, SC level, and social level.