资源环境科技发展态势分析平台

The impact of substituting biofuels for fossil fuels on carbon dioxide (CO2) emissions has been debated for many years. A reason for the lack of resolution is that the method widely used to address the question, lifecycle analysis (LCA), is subjective. Its results irreducibly depend on untestable assumptions, notably those pertaining to system boundaries but also those for representing market effects. The best one can do is empirically constrain estimates of net CO2 impact using data that characterize important aspects of the overall system. Our 2016 paper, "Carbon balance effects of U.S. biofuel production and use," took such an approach, using field data to estimate the direct CO2 exchanges for a circumscribed vehicle-fuel system over the 2005-2013 period of expanding US biofuel use. De Kleine and colleagues criticize our work because it does not follow LCA conventions, arguing in particular for the primacy of the assumption that biofuels are inherently carbon neutral. This response refutes their critique; it reminds readers why the lifecycle paradigm fails for a dynamic system involving the terrestrial carbon cycle, stresses the need to bound an analysis of key carbon exchanges, and explains why the circular logic of LCA can be so beguiling.