|Cropland adjoining patches of forest. All arable land
removes carbon from the atmosphere at varying rates.
This quarrel reflects a new stage in the long-running debate because it does not involve disputes about net energy use or even the food-versus-fuel and land-use change issues raised over the past decade. It is instead a disagreement about the core assumptions to use when examining the question, particularly whether or not biofuels should be treated as inherently carbon neutral. That's the assumption that the CO2 emitted when biofuels are burned does not count because it is biogenic, i.e., newly removed from the atmosphere when feedstocks are grown. My work challenges this assumption, showing that it only holds under certain conditions. De Kleine and colleagues defend the assumption, arguing that it is true unconditionally.
The disagreement is not merely academic. Because new oil production technologies have expanded the supply of economically attractive fossil-based liquid fuels, the business case for biofuels rests increasingly on their value for mitigating CO2 emissions. The stakes are high for both the biofuels industry and for policies to address global warming.
To dig into the details, start with our paper from last year, "Carbon balance effects of U.S. biofuel production and use." Using field data, we found that the rate at which CO2 was removed from the air was enough to offset only 37% of the CO2 emitted from biofuel use, rather than 100% as commonly assumed. Once production-related emissions are considered, we concluded that biofuels (corn ethanol and soy biodiesel) worsened CO2 emissions rather than reduced them over the 2005-2013 period examined.
The commentary by De Kleine and colleagues walks readers through reasoning that defends the proposition that biofuels are, by definition, fully carbon neutral based on how carbon cycles through the global ecosystem. They say that our carbon balance analysis has inconsistencies and fails to consider economic effects. They also state that our method is not robust because it uses a term that is not pertinent for measuring the direct CO2 impact of biofuels and depends on the time period chosen for analysis. De Kleine and colleagues conclude that our results are unfounded and that their examination of our data and assumptions falsifies the approach we used.
In response, I explain the mistakes in the logic used by De Kleine and colleagues, as well as by the majority of biofuel researchers and government officials who apply lifecycle analysis for policy. They assume that biogenic carbon flows are always in balance over a biomass product's lifecycle; in other words, that flows of CO2 between the atmosphere and biosphere (which includes crop and forest lands as well as all other ecosystems) always even out. They therefore believe that lifecycle analysis, which treats carbon flows as being steady (not varying over time) is suitable for evaluating biofuels.
In reality, however, carbon flows are dynamic (vary over time). The atmosphere and biosphere are not in balance because mankind is consuming ever more biogenic carbon for food and other economic uses. It is therefore necessary to examine the dynamics of biofuel use and determine the extent to which biogenic CO2 emissions are balanced by CO2 uptake on a case-by-case basis.
My response invites readers to visualize how the rate at which CO2 is removed from the atmosphere by farmland changed as U.S. biofuel production ramped up over the years. One can compare that change in biomass carbon uptake to the fact that biofuel use does not appreciably change the rate at which CO2 is emitted from motor vehicles. This thought experiment underscores the need to look at how carbon flows change through time, i.e., to analyze the dynamics of the situation, as we did in our 2016 paper.
To put the debate in perspective, I explain why it is so appealing to idealize biofuels as part of a circular economy in which inputs always balance outputs. But the world is far from that ideal state and so cannot be analyzed correctly using methods which assume that carbon flows are already in balance. Moreover, transitioning from today's unsteady (and unsustainable) state to one where CO2 levels are no longer rising is itself a dynamic process. Lifecycle logic breaks down in the real world and so does the assumption that biofuels are inherently carbon neutral.
Readers are invited to judge for themselves which view they find to be more convincing.
DeCicco, J.M., D.Y. Liu, J. Heo, R. Krishnan, A. Kurthen and L. Wang. 2016. Carbon balance effects of U.S. biofuel production and use. Climatic Change 138(3): 667-80.
De Kleine R., T.J. Wallington, J.E. Anderson and H.C. Kim. 2017. Commentary on "Carbon balance effects of U.S. biofuel production and use" by DeCicco et al. (2016). Climatic Change, published online 16 Aug 2017. http://doi.org/10.1007/s10584-017-2032-y
DeCicco, J.M. 2017. Author's response to commentary on "Carbon balance effects of U.S. biofuel production and use." Climatic Change, published online 16 Aug 2017. http://dx.doi.org/10.1007/s10584-017-2026-9