Review of weighting methods for life cycle impact assessment under GLAM

Purpose
Weighting is the process of assigning relative importance to life cycle inventory results or indicator results across impact categories, using weighting factors based on value choices. It is an optional step within Life Cycle Assessment (LCA) but plays an important role in interpreting and communicating the relative importance of different environmental impacts. As part of the Global LCIA Guidance (GLAM) project under the UN Life Cycle Initiative, a comprehensive review of weighting methods was conducted to better understand which approaches are most appropriate for different applications in LCA.

Methods
Members of the GLAM weighting subtask identified and reviewed twenty-seven weighting methods. These methods were grouped into four categories: Multiple Criteria Decision Analysis (MCDA), monetary, data-driven and distance-to-target methods. Classifiers based on inherent features of the weighting methods were applied to support their inclusion or exclusion from further considerations. Each method then was assessed against a set of evaluation criteria defined by the subtask members. A color-code system (green, yellow or red) was applied to indicate the degree to which each method met each criterion to facilitate comparison and communication.
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Results and discussion
Each method was briefly described with appropriate references, including examples of usage in LCA studies where available. The review results are summarized in a table that highlights the performance of each method against the evaluation criteria. All monetary methods are classified as trade-off rates, whereas there are MCDA methods and data-driven methods that can be either trade-off rates or importance coefficients. All distance-to-target methods are classified as importance coefficients. The ability of each method to incorporate temporal discounting or cultural differentiation varies, depending on the data availability and study design. None of the methods reviewed fully met all evaluation criteria, especially within the scope of the GLAM project. Some criteria (like Scientific validity) are sufficiently met by almost all of these methods.

Conclusions
Existing weighting methods based on different approaches have both advantages and limitations. No single method is universally sufficient, and their validity depends on context. This comprehensive overview of available weighting methods provides a valuable starting point for practitioners seeking to identify suitable weighting method for specific LCA applications. To facilitate easy use, a software was also developed based on this review to support the selection of the most appropriate weighting method for LCA studies.