Do Carbon Footprint Estimates Depend on the LCA Modelling Approach Adopted? A Case Study of Bread Wheat Grown in a Crop-Rotation System

This study aims to assess the impact of global warming on winter wheat cultivation under different rotation systems with potato, maize or oilseed rape over a six-year period in the region of Galicia, Spain, to identify the rotation system most favorable from a climate change perspective. An attributional life cycle assessment (ALCA) with economic allocation (retrospective assessment of impacts) and a consequential life cycle assessment (CLCA) with system expansion (impacts of a change) were performed to identify discrepancies and differences in the results in this impact category and thus in the decision supported by the farmers, whose main goal is to produce wheat grain for bread purposes with the lowest carbon footprint. The global warming results modelled with ALCA and CLCA can be contradictory. In general, the climate change impact was considerably higher when modelled with CLCA than with ALCA. Farming activities were consistently identified as hotspots when using both CLCA and ALCA, but other hotspots differed in terms of their contributions. Concerning the ranking of cropping systems that produce grain with the lowest greenhouse gases emission levels, contradictory results were identified in some cases between the LCA modelling approaches. Nevertheless, the cultivation of native winter wheat under ecological management is always the preferred choice, regardless of the approach. However, wheat rotation with potato is preferrable in the ALCA, and with maize in the CLCA. The assumptions required to perform a CLCA have a large impact on results. The allocation of burdens between the co-products in the ALCA involves a level of uncertainty since discrepancies arise with the selection of the allocation procedure. Thus, the assumptions made affect the results considerably and have a direct effect on the final conclusions.