Because many reasons exist to perform a LCA and different ways exist to define function, as well as choices to include or exclude certain aspects (such as infrastructure), our ability to make straight forward, direct comparisons between LCAs performed by different research groups is compromised. Despite the challenges of comparing different LCAs, a need to make such comparisons frequently exists. Recently, meta-analysis of LCA3,4
has become more common. Meta-analysis is a harmonization process to adjust parameters from different LCAs to ensure methodological consistency to enable comparison. The purpose of the meta-analysis is to provide decisionmakers with a more robust understanding of conflicting studies in the literature, or more simply, to compare results of two studies of similar products with the same function produced with different technologies or from different geographic regions. For example, the National Renewable Energy Laboratory has performed the Lifecycle Assessment Harmonization Project
, which provides additional more detailed guidance on the process undertaken for electricity generation. Additionally, comparison can be strengthened by assessing conclusions and recommendations from different studies.
Based on the preceding description of the stages of a LCA, it is clear what kinds of information are needed, at a minimum, to ensure comparability of two studies: corresponding functional units and system boundaries. In food and agriculture LCAs, numerous functional units have been used. Some common choices include: live or as-harvested weight, at the farm gate for livestock and crops respectively. These may be expressed on a per animal basis or per kg basis. If sufficient information is not provided in the study to allow conversion of the units to correspond, then comparison will not be possible. The guidelines developed by the U.N. Food and Agriculture Organization’s Livestock Environmental Assessment and Performance (LEAP) Partnership provide information on specification of functional units with sufficient detail to enable these types of conversion.5,6 An example of a well characterized functional unit is from the Phase I: More Sustainable Beef Optimization Project.7 In this assessment, the loss in the beef supply chain is described as leading to the chosen functional unit of lean meat consumed (Table 1), enabling other users to compare results they may have for the farm gate production.
For crop production, the moisture content should (but may not) be specified. Other possible functional units for livestock include carcass weight or edible cuts at the packer plant gate. Some studies will report a functional unit of carcass weight at the farm gate – this choice represents two errors which should be corrected. The first is that valuable co-products are produced (non-edible offal, etc.) in processing and an allocation to these co-products may be missing if carcass weight is used at the farm gate; the second error is that energy and other resources expended in the processing stage, and burdens associated with these activities, are excluded at the farm gate. Additional considerations regarding the harmonization of system boundaries include activities which may be excluded in one study or another. In particular, it is common in many studies - but not all - to exclude capital goods (infrastructure).
After harmonization of the functional unit and system boundaries, attention must be given to impact methods used in the studies. Many impact assessment frameworks are available, and each adheres to the ISO standard requirement of a direct causal link between emission and impact. However, various methods can use different estimation techniques even for similar categories. Therefore, it is critically important that the impact methods used in the studies being compared are the same – unless only a qualitative directional comparison is required. Even for evaluation of climate change, which is likely the most commonly reported impact category, care must be taken to ensure that the same global warming potentials (GWP) were used in the studies being compared. The 100-year GWP has changed in the past 20 years; for example, the 100-year GWP for methane was 21 (1996); 25 (2006) and is currently 28 (2013).