US study challenges 'first principles' of biofuels carbon accounting
Wed 25 September 2013
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A new study from the Energy Institute at the University of Michigan challenges the conventional analysis that “biofuels recycle carbon”. Published in the journal Climactic Change the author concludes that strategies should move away from a downstream focus on replacing fuel products to an upstream focus on achieving additional CO2 uptake through the most cost-effective and least damaging means possible.
In its report on the paper, Green Car Congress suggests that the analysis could have a significant impact on climate policies for transport fuels.
Dr. John M. DeCicco of the Energy Institute at the University of Michigan says in his paper that many policymakers view biofuels as necessary for addressing the large portion of transport demand likely to require liquid energy carriers for the foreseeable future, particularly as automobile, truck and aircraft use rises in developing economies.
Broadly speaking, two approaches have been used to examine the greenhouse gas (GHG) impacts of biofuels. As commonly used for energy policy, lifecycle analysis (LCA) calculates trajectories of GHG fluxes for specified bioenergy product systems in comparison to reference fossil-based systems.
The other approach is integrated assessment modelling (IAM), which offers comprehensive guidance regarding the climate impacts of biofuels but does so at a highly aggregate level. Such models examine bioenergy within a broader climate mitigation context that incorporates globally coupled climatic, biogeochemical and economic systems.
Although the most thorough LCA results are broadly consistent with many IAM findings, the dependence of LCA on system boundary assumptions (among other sources of uncertainty) gives very divergent results. Ideally, the author says that carefully qualified results from IAM would be used to guide public policy. What has happened instead is that policymakers have embraced certain LCA results, relying on simplistic, or at best inadequately qualified, interpretations of the fact that biofuels “recycle” carbon, i.e., that end-use CO2 emissions from combustion are fully balanced by CO2 uptake in feedstock growth.
This closed-loop model of carbon flows is easy to understand and its intuitive appeal fosters a widespread popular belief that biofuels are inherently carbon neutral. Thus, policies have been designed under the assumption that carbon accounting need only address production-related, fossil-derived CO2 and other GHGs while excluding biogenic CO2 emissions throughout the fuel cycle.
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