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. 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. 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. 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.

中文翻译：

---

生命周期评估中的时间问题——系统评价

生命周期评估的案例研究经常承认，由于时间问题，其结果的准确性可能会受到损害，尽管他们通常不会提供更多细节。此外，现有的关于 LCA 中问题和挑战的概述论文和评论并没有解决所有时间问题。由于这些是不准确的主要来源并相互影响，因此重要的是要清楚地了解它们，缩小定义中的差距，将时间问题系统化，并显示它们的相互依赖性和建议的解决方案。为了确定这些问题的科学状况，我们进行了系统的文献综述。我们首先根据 ISO 14040ff 将时间问题系统化，并将它们分为六种类型：时间范围、折扣、库存的时间分辨率、时间相关表征、动态加权和时间相关归一化。在此基础上，我们确定了合适的搜索词并开发了一个用于内容分析的分析网格。我们只收录了方法论论文和案例研究，其中包含有关时间问题解决方案的原始发现。分析了书目数据、影响类型、工业领域和方法论贡献。文学区分不同类型的时间范围。一项针对整体评估，在目标和范围中进行定义，一项针对生命周期清单，一项针对影响特征。为评估设定时间范围被视为等同于应用贴现。根据 LCA 中评估的主题，评估的非常长和非常短的时间范围都不切实际。很短的会违反代际平等的原则，而很长的会边缘化短期行动，从而减少行动的动力。文献中有一个共识，即时间差异化的生命周期清单和时间相关的，或者至少是时间范围相关的，特征描述提高了 LCA 的准确性。一般来说，动态生命周期评估对公司很有吸引力，因为计算结果不仅更准确，而且往往低于静态生命周期评估。我们未能达成共识的主要问题是评估时间范围的长度问题和折扣问题。这些被认为是主观的，会遇到敏感性或情景分析。