Many policymakers view biofuels as necessary for addressing the large portion of transportation demand likely to require liquid energy carriers for the foreseeable future, particularly as automobile, truck and aircraft use rise in developing economies. Nevertheless, biofuels are controversial because of the uncertainties that surround their net climatic benefits; the conditions that need to be met for beneficial biofuel systems; and differences in methods, data and assumptions used for evaluation.

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. LCA methods specifically designed for analyzing fuels compute a carbon intensity metric (lifecycle GHG emissions per unit of energy) for “fuels” as defined by feedstock and fuel product pathways involving particular production technology, land-use and spatio-temporal boundary assumptions.

...The other approach is integrated assessment modeling (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) results in a very divergent literature. Ideally, 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.

To bridge the gap between biofuel-related policies rationalized by incomplete interpretations of LCA versus the guidance implied by IAM and more sophisticated LCA studies, it would be useful to have a conceptual framework that is transparent and well grounded in science but also accessible for policymakers. Such an approach can indicate policy directions consistent with fostering the conditions that comprehensive analyses imply are necessary for climate protection. To develop such a framework, this paper (a) takes a global view, to avoid the system boundary problems inherent to LCA; (b) anchors accounting in the “here and now,” focusing on stepwise changes in CO₂ sources and sinks while avoiding assumptions about the future (such as payback of carbon debt); and (c) avoids invoking the closed-loop model of biofuel-related carbon flows that results in a difficult verification problem regarding net CO₂ uptake.

—DeCicco 2013

The end-use CO₂ emissions from an engine burning liquid fuel—whether a biofuel or a petroleum fuel—vary little, DeCicco notes, falling within a range of 73 (±2) gCO₂/MJ for common liquid fuels. Therefore, he notes, fuel end-use is not where CO₂ reductions can be found, aside from more fundamental shifts such as lower travel demand, higher vehicle efficiency and shifts to electricity or hydrogen.

Furthermore, all current and near-commercial processes for synthesizing biofuels release more CO₂ than petroleum refining—regardless of whether fossil fuels or biomass wastes are used for process energy. LCA analysis treats biogenic process CO₂ as being fully offset by the CO₂ taken up during feedstock growth. However, DeCicco observes, these emissions enter the atmosphere just as do those from other industrial sources. “In short, fuel processing is not a location of additional CO₂ uptake.” On top of that, indirect land use change (ILUC) emissions appear to be as much as an order-of-magnitude larger than end-use CO₂ emissions from fuel use.

Setting aside LCA in favor of direct carbon accounting that examines CO₂ sources and sinks separately offers better clarity emerges about options for addressing CO₂ emissions from liquid fuel use, DeCicco asserts.

DeCicco presents an analytic framework using a model with carbon flows to and from the atmosphere with terrestrial and geological sources and sinks. [Diagram below.] The energy system engages all three pools and represents the technologies through which energy services (such as mobility) are delivered using carbon-based liquid fuels. This broad model enables a transparent analysis that replaces the LCA framework with a carbon cycle framework that emphasizes the separate locations of sources and sinks while examining current period changes in carbon flows, DeCicco says.

	B C D E F L P R	Biofuel carbon Carbon captured from energy system Direct carbon capture from the atmosphere Carbon emissions from energy use Fossil fuel carbon Land-use change Net primary production Heterotrophic respiration
Simplified carbon balance diagram for the globally coupled fossil and biofuel system associated with the use of carbon-based liquid fuels. DeCicco 2013. Click to enlarge.

An analysis based on this model shows that biofuels have a current-period climate benefit only if their feedstocks are derived from a higher rate of primary production (P) or a lower rate of heterotrophic respiration (R) than would otherwise occur. Either way, DeCicco finds, the result amounts to increasing net ecosystem production (NEP)—i.e., increasing the rate at which carbon is fixed in the biosphere.

The approach given here works forward incrementally from present conditions instead of modeling hypothetical future systems. By construction, it precludes an accumulation of carbon debt because it identifies conditions under which an increase in biofuel use yields an immediate atmospheric benefit. The analysis contrasts with LCA methods that justify a large near-term release of carbon stocks due to land-use change by invoking assumptions about future system dynamics under which the resulting carbon debt gets repaid. Some may raise a concern that the near-term focus of this analysis creates a distorted (overly pessimistic) view of the mitigation potential of biofuels relative to continued fossil fuel use.

In fact, this analytic approach does allow for an increasing and potentially extensive use of biofuels for long-term climate mitigation. However, it constrains the rate of biofuel expansion to one that benefits the atmosphere a year at a time as determined by the availability of biomass that demonstrably reflects an increase in NEP. Thus, this framework can be viewed as offering policy guidance for achieving land use and feedstock sourcing conditions in line with mitigation scenarios involving biofuels (e.g., as reviewed in IPCC 2011).

...Broadly speaking, removing CO₂ from the air through biological or other means requires carbon management. That in turn requires a “here and now” (in situ) attention to land use (terrestrial carbon management), geologic mechanisms or other options while measuring the quantity of carbon fixed and tracking it (including any released during processing) through its ultimate disposition. That could be as a feedstock for biofuels or other synthetic fuels, but also could be any forms of disposition including sequestration or long-lived products that keep carbon from re- entering the air. Thus, the carbon mitigation challenge for liquid fuels has been incorrectly seen as a fuel synthesis and substitution problem. In reality, it is a net carbon uptake problem.

—DeCicco 2013

Resources

• John M. DeCicco (2013) Biofuel’s carbon balance: doubts, certainties and implications. Climactic Change doi: 10.1007/s10584-013-0927-9