Chain of custody modelling in carbon footprinting and life cycle assessment

Abstract Purpose Carbon footprinting and life cycle assessment (LCA) are moving from average secondary data towards supply chain specific primary data, which can be modelled via different chain of custody (CoC) models. Contrary to common LCA practice, there is a push towards CoC models that enable a separation of physical and accounted products: the mass balance–credit method and the book and claim model. This article deepens the understanding of CoC modelling options in LCA and identifies corresponding challenges. Methods A three-step research procedure is defined. First, the five different CoC models, as described in ISO 22095, are illustrated with the example of a hypothetical organization that uses both inputs with specified characteristics, such as biogenic materials, and conventional inputs, such as fossil materials. Second, the fundamental LCA standards—ISO 14040/44—are analyzed for specifications that are relevant for CoC modelling. The five CoC models are then analyzed for their conformity with these specifications. Furthermore, standards, such as ISO 14067 and the GHG Protocol, as well as the carbon footprint guidelines from Catena-X, Pathfinder, and Together for Sustainability (TfS), are analyzed for relevant specifications. Finally, challenges and potential requirements for the use of CoC models in LCA are identified and elaborated. Results and discussion The mass balance–credit method and the book and claim model can be interpreted as not adhering to the LCA approach of describing physical systems, as specified in ISO 14040. Further, the mass balance–credit method leads to co-products, while not applying the allocation hierarchy of ISO 14044. However, the market-based method for electricity accounting, which resembles the book and claim model, is allowed and even required by some standards, such as the GHG Protocol and the ISO 14067. This may set a precedent for a more general separation of physical and accounted products. Main challenges for CoC modelling in LCA are the avoidance of double counting, the definition of consistent allocation rules, and the definition and enforcement of adequate requirements. The decision on which CoC models are to be allowed in LCA and the corresponding requirements depend on value choices. Thus, stakeholder processes are required for consensus finding. Conclusion While CoC modelling may gain relevance in LCA, it is important for the LCA community to be aware of potential challenges and work towards adequate solutions and requirements. Further research and specification are necessary to harmonize the standards for CoC modelling and LCA.