A semantic study of the Emergy Sustainability Index in the hybrid lifecycle-emergy framework

• Different versions of the Emergy Sustainability Index (ESI) are compared.
• A specific algorithm is developed to compute the ESI's lifecycle-oriented version.
• Although operational, the definition of foreground inputs needs further discussion.
• Sensitivity analysis can identify specific potential actions to increase the ESI.
• ESI can be efficiently increased by influencing suppliers’ environmental policy.

Emergy-based indicators are claimed to be useful outcomes of the emergy evaluation framework, which aims at guiding decision-makers toward environmental sustainability. The calculation of the Emergy Sustainability Index (ESI), in particular, seems widely consensual among emergy scholars, but several variants actually exist in the scientific literature, which may lead to different interpretations or misunderstanding of the ESI result. This paper proposes a semantic study of two variants in both components of the ESI (the Emergy Yield Ratio and the Environmental Loading Ratio, respectively EYR and ELR), to enhance standardization and reproducibility in the calculation of emergy indicators. It is shown that ESI can be consistently defined at the level of the production site as well as from a lifecycle perspective, although several case studies in the literature use an intermediary version with inconsistent system boundaries. A recent definition of lifecycle-oriented EYR is made operational by the development of an algorithm to be implemented in the emergy accounting software SCALE. However, the classification of foreground inputs needs further precision. ESI is also decomposed using partial derivatives, in order to analyze the influence of each input category and retrieve generic recommendations. These multiple outcomes demonstrate the added value of hybrid lifecycle-emergy evaluation to identify specific potential actions toward enhancing ESI of human activities.