A stock-and-flow model of biofuels and carbon

Abstract The climate benefit of biofuels depends on how well their production offsets the biogenic carbon emitted by their use. It is commonly assumed that carbon uptake during feedstock growth renders biofuels carbon neutral aside from ancillary fossil carbon and other production-related greenhouse gas (GHG) emissions. This assumption of a feedstock offset is retained in the lifecycle assessment (LCA) models adopted by policymakers for renewable fuels. However, consideration of land-use change (LUC) and other economically induced effects has revealed carbon flow changes more complex than those of a feedstock offset. This paper presents a relatively simple stock-and-flow model that offers a conceptual understanding of the carbon-related dynamics of biofuel use. The model focuses only on key carbon exchanges, omitting the ancillary emissions while capturing the economic impacts of increased crop yields, cropland expansion with LUC, decreased food demand and petroleum market rebound. Applied to a stylized scenario of corn ethanol use, the model transparently depicts effects and trade-offs previously characterized by LCA and other complex methods. It reframes biofuel carbon neutrality in terms of distinct market-mediated mechanisms of net uptake that can offset the biogenic carbon emitted from biofuel use while highlighting the dynamics of the atmospheric carbon response. As a complement to LCA, stock-and-flow analysis can be informative for policymakers and valuable for educational purposes.