In his review, DeCicco critically examined four methods that have been used for evaluating the net CO₂ impacts of liquid transportation fuels (but did not compare their results numerically—a useful next step):

• Fuel cycle analysis (FCA) as implemented in GREET and similar models, and which is now used for compliance purposes in policies such as the US RFS, California LCFS and EU RED. FCA is used to evaluate the energy and GHG emission impacts of a wide variety of existing and proposed fuels, including fossil options (coal-to-liquids, gas-to-liquids, unconventional petroleum) as well as electricity, gaseous fuels and biofuels from a range of feedstocks.

  With biofuels, FCA assumes that biogenic CO₂ emissions, both from end-use combustion and during processing, are fully balanced by CO₂ uptake during feedstock growth. This convention that CO₂ emissions from biofuel combustion are not net emissions to the atmosphere is now embedded in public policy.

  The carbon neutrality assumption may be arithmetically correct within a biofuel lifecycle, DeCicco observes, and is also true globally if all biomass used in the world is the subject and terrestrial carbon stock impacts due to land-use change are accounted for separately. However, the assumption is not necessarily true for any particular fuel product system or for biofuel use at a national or other sub-global level.

  Further, because it involves modeling effects that span many years into the future—i.e., even if current data limitations could be resolved, FCA results cannot be empirically verified.

  Despite the “apparent maturity” of FCA methods, there is little consensus on the role of biofuels as a low-carbon replacement for petroleum fuels.

• Terrestrial resource analysis (TRA) methods based on ecology and forest management research. Unlike lifecycle methods, TRA is based on the ecology of the carbon cycle and is designed to handle carbon stocks and flows, counting both sources and sinks while evaluating effects of changing land use and trade-offs between harvesting and letting forests continue to grow.

  TRA starts with natural resources (such as land and fossil fuel reserves) as the basis for the system to be analyzed, including production and use of biofuels, fossil fuels, other products and their associated inputs.

  TRA can model a lifecycle by integrating the combined bio- and fossil-fuel system over time, yielding metrics similar to carbon intensity values from FCA. TRA does not automatically negate biogenic CO₂ emissions from fuel processing and end use.

• “Kyoto accounting,” the method for tallying GHG sources and sinks formalized by the IPCC for reporting of national GHG emissions inventories and as used in the design of cap-and-trade programs and other aspects of international climate policy. This method accounts for all GHG sources and sinks in a region, but generally without without regard to linkages among products produced or consumed in different regions.

  However, DeCicco explains, for biofuels, Kyoto accounting embeds a lifecycle perspective in that biofuel use generates a full CO₂ reduction credit in the transportation sector regardless of the extent to which the assumed carbon neutrality is undone by CO₂ releases in other sectors.

• Integrated assessment modeling (IAM) methods, which examine bioenergy systems in the broader context of combined climatic and economic system modeling at the global level. IAM methods jointly model climatic effects and the global economy, accounting for all GHG sources and sinks, their relationships, and the effects of technology and behavioral changes for mitigating or adapting to climate change.

  IAM thus treats systems at higher levels of aggregation than does FCA, addressing sources and sinks at regional and sectoral levels. It has not been used to specify fuel regulations. While the complete economic and carbon cycle representations make it useful for analyzing policy impacts and informing discussions of mitigation options, IAM also cannot be verified and must be interpreted carefully. But because it is clearly understood to be a scenario tool, IAM is not prone to misapplication as has been seen with FCA, DeCicco says.

Because liquid fuel related carbon stocks and flows must be handled dynamically, FCA’s static product focus invites an ill-posed question. Asking to compute a CI (“carbon footprint”) involves treating an abstract notion—a fuel’s lifecycle—as if it were a well-defined fuel property. It is an example of the logical pitfall that Alfred North Whitehead has termed the fallacy of misplaced concreteness. The fuel comparison question as posed through FCA is not a question that can be unambiguously answered; that is to say, it is scientifically irreducible.

On the other hand, well-grounded methods such as IAM may not seem to offer guidance of the type that policymakers seek (the prescription that all carbon should be priced notwithstanding). A way forward can be found by casting the problem in a different light.

—DeCicco (2014)

Logic tree for options to address CO2 emissions from liquid fuel use. DeCicco (2014). Click to enlarge.

Given the ongoing existence of carbon-based fuels and the policy desire to reduce the carbon dioxide emissions resulting from their combustion, DeCicco notes, there are two options. The first is to to capture the carbon on board the vehicle to avoid releasing CO₂ (mobile capture); the second is to counterbalance resulting vehicle emissions by removing CO₂ from the atmosphere elsewhere.

As the first is not foreseeably plausible, the only option remaining is carbon dioxide removal (CDR) from the atmosphere in locations outside the transportation sector. This is where biofuels come in, as carbon is fixed during feedstock growth. To benefit the climate, however, the amount of CO₂ absorbed from the air must be greater than whatever is already being absorbed through existing activities—i.e., net uptake.

Based on his findings, DeCicco suggests a several types of analysis that would prove useful in assessing the CO₂ impact of liquid fuels:

• Deconstructing FCA models to examine spatially and temporally explicit intermediate results. Such analysis would project CO₂ stocks and flows sector by sector in ways that can be compared to inventory-based estimates.

• Developing independent empirical checks on fuel-related carbon accounting and modeling results.

• Applying TRA methods to actual commercial-scale ethanol and biodiesel production.

• Using IAM with data bases sufficiently disaggregate to calculate the impacts of bioenergy systems as actually deployed at commercial scales and under real-world conditions (versus modeling hypothetical systems with stylized land-use and technology assumptions).

• Examining the applicability of system dynamics methods. Because fuel systems that engage the biosphere involve stock-and-flow interactions, methods designed for dynamic analysis may be able to represent carbon cycle and economic effects effectively.

• Systematically evaluating current and proposed GHG accounting methods according to intended application (e.g., compiling emissions inventories, analyzing policy options, guiding R&D, specifying regulations or developing voluntary programs).

He also proposes a different policy agenda, with a greater focus on CDR mechanisms than on biofuel synthesis—i.e., the first task is securing the additional fixed carbon on which the climate benefit of any biofuel depends. This entails developing ways of removing CO₂ from the atmosphere at faster rates and larger scales, and not only raising productivity (higher yields) but also managing land with carbon in mind.

… In treating biofuels as tautologically carbon neutral within their lifecycle, FCA obscures the concrete reality that end-use CO₂ emissions from biofuels differ but little from those of the fossil fuels they replace. Although the carbon in a biofuel was recently removed from the atmosphere, that does not guarantee that it represents a net removal, the extent of which can only be ascertained by quantifying the relevant CO₂ sources and sinks. A similar mistake occurs in the Kyoto convention of treating biomass as carbon neutral. The use of this assumption in two accounting methods that have greatly shaped public policy creates a serious cognitive challenge. Like any fallacy worthy of the label, the grip on reasoning can be quite firm, and so it may take some time before the error is taken to heart by the community of researchers, environmentalists, businesses and policymakers who are wedded to the lifecycle concept of carbon intensity and the methods that propagate it.

Setting the lifecycle paradigm aside clarifies the CO₂ mitigation task for transportation fuels. The liquid carbon challenge is, in fact, a CO₂ removal problem. It requires increasing net CO₂ uptake, in the biosphere or elsewhere, in ways that counterbalance the end-use CO₂ emissions from fuel consumption. An implication is that research should be ramped up on options for increasing the rate at which CO₂ is removed from the atmosphere and on programs to manage and utilize carbon fixed in the biosphere, which offers the best CO₂ removal mechanism now at hand. Such strategies can complement measures that control the demand for liquid fuels by reducing travel activity, improving vehicle efficiency and shifting to non-carbon fuels. The need for new paradigms to address CO₂ emissions from liquid fuels is an issue that clearly warrants further analysis and discussion. Nevertheless, methods well grounded in the realities of the carbon cycle may identify mitigation options that are more effective, more accessible and less costly than those rationalized by FCA and pursued for many years with little meaningful success.

—DeCicco (2014)

Resources

• DeCicco, J.M. (2014) “The liquid carbon challenge: evolving views on transportation fuels and climate,” Wiley Interdisciplinary Reviews: Energy and Environment(WIREs E&E) doi: 10.1002/wene.133