Methodological issues regarding biofuels and carbon uptake
Questions regarding the net effect of biofuels on carbon dioxide (CO2) emissions have been difficult to resolve because of methodological uncertainties. One method of choice is lifecycle assessment (LCA), which takes a fuel product system as its object of analysis. LCA uses a static system model, with carbon flows averaged over a defined "lifecycle." Although it may evaluate some carbon stock changes, the LCA convention of treating biogenic CO2 emissions as fully offset by the carbon embodied in a biofuel's feedstock renders its results independent of the dominant portion of carbon uptake on the land from which the feedstock is sourced. An application of material flow analysis termed annual basis carbon (ABC) accounting captures system dynamics and is fully sensitive to changes in carbon uptake. This paper compares the LCA and ABC methods and contrasts their respective results for a case study of real-world biofuel production. It highlights the large impact of baseline carbon uptake, which can affect the sign of the results from either a likely decrease or a likely increase in net CO2 emissions even before considering economically induced effects. Implications include the need for further methodological work, new program-scale model development, empirical re-analysis of biofuel systems, and reconsideration of existing public policies and research priorities.