资源环境科技发展态势分析平台

In their recent publication "Carbon balance effects of U.S. biofuel production and use," DeCicco et al. present an empirical assessment of net CO2 emission effects over the period 2005-2013 after the US renewable fuel standard (RFS) came into existence and conclude that biofuels have resulted in a net increase in CO2 emissions over the period. The analysis presented by DeCicco et al. relies on three key assertions. First, that if biofuel carbon combustion emissions are not completely offset by additional net ecosystem production (NEP), then the biofuel should not receive full biogenic carbon credit. Second, that changes in agricultural NEP related to biofuel production can be accurately estimated from national-level agricultural production statistics. Third, that agricultural NEP is a pertinent measure of biofuel global warming impacts. We show that following the conventional definition of NEP the combustion of biofuel by definition leads to an exactly equal increase in NEP; therefore, the first assertion is not meaningful. Regarding the second assertion, we show that estimation of biofuel-related NEP changes from agricultural production statistics is not a robust methodology. Finally, we argue that agricultural NEP is an important parameter for estimating land-use change effects, but in isolation is an irrelevant GHG metric for current biofuels. We find that the conclusions above from DeCicco et al. are unfounded and do not invalidate the application of biogenic carbon offsets in life cycle assessments of biofuels currently used in national and international regulations.