Temporal issues in life cycle assessment-a systematic review

Purpose Case studies on life cycle assessments frequently admit that the precision of their outcome could be undermined due to temporal issues, though they usually refrain from offering much more detail. In addition, available overview papers and reviews on problems and challenges in LCA do not address the whole range of temporal issues. As those are major sources of inaccuracies and influence each other, it is important to get a clear picture of them, to close gaps in definitions, to systemize temporal issues, and to show their interdependencies and proposed solutions. In order to identify the state of science on those questions, we conducted a systematic literature review. Methods We first systematized temporal issues based on ISO 14040ff and divided them into six types: time horizon, discounting, temporal resolution of the inventory, time-dependent characterization, dynamic weighting, and time-dependent normalization. Building on that, we identified suitable search terms and developed an analysis grid for the content analysis. We included only methodological papers and case studies with original findings on solutions for temporal issues. Bibliographic data, impact types, industrial fields, and methodological contributions were analyzed. Results and discussion Literature differentiates between different types of time horizons. There is one for the whole assessment, defined in goal and scope, one for the life cycle inventory, and one for the impact characterization. Setting a time horizon for the assessment is regarded as equivalent to the application of discounting. Both very long and very short time horizons of the assessment are not practical depending on the topic assessed in the LCA. Very short ones would offend the principle of intergenerational equality, while very long ones would marginalize short-term actions and thereby reduce incentives to act. There is consensus in the literature that temporally differentiated life cycle inventories and time-dependent, or at least time horizon dependent, characterization improve the accuracy of LCA. Generally, dynamic life cycle assessments are attractive for companies because the calculation results are not only more accurate but are often also lower than in static life cycle assessments. Conclusion The main questions where we did not find consensus are the issue of the length of the time horizon of the assessment and the issue of discounting. Those are regarded as subjective and are encountered with sensitivity or scenario analysis. Further investigations should be taken for a better understanding of this issue and for concrete solutions because their influence on the results of life cycle assessments is often fundamental.