Author’s response to commentary on “Carbon balance effects of U.S. biofuel production and use”

Abstract

The impact of substituting biofuels for fossil fuels on carbon dioxide (CO 2 ) 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 CO 2 impact using data that characterize important aspects of the overall system. Our 2016 paper, BCarbon balance effects of U.S. biofuel production and use,^took such an approach, using field data to estimate the direct CO 2 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.Our recent paper, DeCicco et al. (2016), evaluated carbon exchanges between the atmosphere and the US energy and agricultural sectors over 2005-2013, a period of large growth in biofuel production and use. The method involved a straightforward estimation of annual CO 2 flows directly to and from a deliberately circumscribed vehicle-fuel system. The analysis emphasized material carbon flows, referring to the carbon chemically bound in the fuels and feedstocks, both fossil (crude oil) and biomass (corn, soybeans). This focus distinguishes carbon exchanges that can be estimated using available data from those that can only be projected through economic modeling. The results constrain (place a lower bound on) the net emissions of CO 2 and other greenhouse gas (GHG) emissions associated with US biofuel use over the period examined.