Abstract
The use of liquid biofuels has expanded over the past decade in response to policies such as the U.S. Renewable Fuel Standard (RFS) that promote their use for transportation. One rationale is the belief that biofuels are inherently carbon neutral, meaning that only production-related greenhouse gas (GHG) emissions need to be tallied when comparing them to fossil fuels. This assumption is embedded in the lifecycle analysis (LCA) modeling used to justify and administer such policies. LCA studies have often found that crop-based biofuels such as corn ethanol and biodiesel offer at least modest net GHG reductions relative to petroleum fuels. Data over the period of RFS expansion enable empirical assessment of net CO2 emission effects. This analysis evaluates the direct carbon exchanges (both emissions and uptake) between the atmosphere and the U.S. vehicle-fuel system (motor vehicles and the physical supply chain for motor fuels) over 2005–2013. While U.S. biofuel use rose from 0.37 to 1.34 EJ/yr over this period, additional carbon uptake on cropland was enough to offset only 37 % of the biofuel-related biogenic CO2 emissions. This result falsifies the assumption of a full offset made by LCA and other GHG accounting methods that assume biofuel carbon neutrality. Once estimates from the literature for process emissions and displacement effects including land-use change are considered, the conclusion is that U.S. biofuel use to date is associated with a net increase rather than a net decrease in CO2 emissions.
Highlights
Production and consumption of biofuels, meaning biomass-based liquids such as biodiesel and ethanol, has grown steadily in the United States, from 4.2 billion gallons (0.37 EJ/yr) in 2005Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Climatic Change (2016) 138:667–680 to 14.6 billion gallons (1.34 EJ/yr) in 2013 (EIA 2015; higher heating value basis)
Annual Basis Carbon (ABC) accounting reflects the stock-and-flow nature of the carbon cycle, recognizing that changes in the atmospheric stock depend on both inflows and outflows, while lifecycle analysis (LCA) focuses only on inflows (GHGs discharged into the atmosphere)
Such options may enable greater gains in Net ecosystem production (NEP), e.g., by using crop residues that reduce Rh or by using feedstocks that raise net primary production (NPP). Given how different this approach is from the methods commonly used for energy analysis, further work is needed to examine the research and policy implications going forward. This retrospective, national-scale evaluation of substituting biofuels for petroleum fuels applied Annual Basis Carbon accounting to take a circumscribed look at the changes in carbon flows directly associated with a vehicle-fuel system
Summary
Production and consumption of biofuels, meaning biomass-based liquids such as biodiesel and ethanol, has grown steadily in the United States, from 4.2 billion gallons (0.37 EJ/yr) in 2005Electronic supplementary material The online version of this article (doi:10.1007/s10584-016-1764-4) contains supplementary material, which is available to authorized users.Climatic Change (2016) 138:667–680 to 14.6 billion gallons (1.34 EJ/yr) in 2013 (EIA 2015; higher heating value basis). The environmental justification rests on the assumption that, as renewable alternatives to fossil fuels, biofuels are inherently carbon neutral because the CO2 released when they are burned is derived from CO2 uptake during feedstock growth (NRC 2011, 195). That convention is premised on globally complete carbon accounting in which biogenic emissions are not counted in energy sectors when carbon stock changes are counted in land-use sectors. This assumption has been used in cap-and-trade programs and carbon taxes as promulgated to date, which address only fossil-derived CO2 emissions. Errors arise when bioenergy is treated as carbon neutral in national and subnational policies, which do not impose globally coherent accounting that tracks all carbon stock changes (Searchinger et al 2009)
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