If additional net CO2 uptake occurs as a result of biofuel use, the location of that net uptake is land. Because commodity markets are involved, properly treating land use means that no system boundary short of the entire world will suffice. Once LCA is set aside in favor of direct carbon accounting that examines CO2 sources and sinks separately, better clarity emerges about options for addressing CO2 emissions from liquid fuel use.
CO2 control options
Table 1 lays out a hierarchy of options for mitigating CO2 emitted in the transportation sector. The options are described for motor vehicles but similar logic applies to other sources (such as aircraft or ships) that consume liquid fuels. The first tier in the table gives the standard factorization of emissions as a product of travel demand (vehicle miles of travel, VMT), average vehicle energy intensity and the GHG impact of supplying and using the fuel (DeCicco 2013). The first two factors determine the level of fuel demand. Beyond actions to reduce fuel demand, CO2 reductions must be found through mitigation in the fuel supply system, a task commonly seen as shifting from petroleum fuels to alternatives such as biofuels, electricity or hydrogen.
Table 1 Hierarchy of generalized CO2 mitigation options for transportation
The second tier in the table parses fuel-related mitigation into three categories. The first is to avoid direct release of CO2 from vehicles by using physically carbon-free energy carriers such as electricity or hydrogen, thereby shifting the mitigation task from the transportation sector to energy supply sectors. The second category (capturing CO2 onboard vehicles or utilizing the fuel without CO2 formation) is not practical with any known technology. The third category involves counterbalancing CO2 emissions from the use of carbon-based fuels with CO2 uptake in other locations. It can be further broken down into the three subcategories in the third tier of the table. Two involve the biosphere, either by increasing CO2 uptake in biomass which, as elaborated below, requires raising net ecosystem production (NEP), or by avoiding CO2 releases that otherwise would occur, e.g., by utilizing wastes as a feedstock. The final subcategory involves increasing sequestration in the geosphere.
A first-order model
Biofuels fall under the category of measures that seek to remove CO2 in one location in order to balance the CO2 emitted in other locations where fuels are burned. For analyzing this option it is helpful to define a simple model of globally coupled bio- and fossil-fuel interactions. The analysis examines an increment of biofuel use in the world as it exists today rather than trying to identify an ideal system for the future, and so considers stepwise changes in carbon balance instead of integrating across a future time horizon.
The analytic framework is illustrated in Figure 2, which shows carbon flows to and from the atmosphere with terrestrial and geological sources and sinks. 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. To focus on core terrestrial carbon balance issues, oceans are omitted as are non-CO2 gases and emissions from energy processing (feedstock production, refining and biorefining). This sketch 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.
Consider a change from an initial condition without biofuel use (B0 = 0) to a condition with biofuel use (B1 > 0). Examining here only the last tier of Table 1, liquid fuel consumption is fixed and so emissions from the energy system are unchanged (E1 = Eo). The carbon in extracted fossil fuel (F) is represented separately from the emissions (E) that result from fuel end use. For this analysis neither industrial carbon capture nor industrial direct air capture are considered (C = 0; D = 0). Then, without biofuels, energy-related emissions equal the fossil carbon flow (E0 = F0).
In the initial condition, P0 represents terrestrial net primary production (NPP) on a global basis, which recently has averaged just above 50 PgC/year. NPP was trending upward in earlier decades but evidence suggests a drought-induced decline over 2000–2009 (Zhao and Running 2010). Respiration (R) here refers to generalized heterotrophic respiration, i.e., CO2 from heterotrophs that directly or indirectly consume plants or other carbon-fixing autotrophs. R includes human uses of biomass, other than biofuel, that result in current period CO2 emissions, including food and feed but not conversion to durable products that keep carbon fixed. The net carbon entering the atmosphere is given by:
$$ \mathrm{A}=\mathrm{E}+\mathrm{L}+\mathrm{R}\kern2pt -\kern2pt \mathrm{P}\kern2pt -\kern2pt \mathrm{D} $$
Note that the carbon in produced biomass (B) and fossil (F) feedstocks does not itself directly enter the atmosphere, but is mediated through the energy system to result in emissions (E) from the use of fuel products. Episodic CO2 releases from land-use change (L) are shown in Figure 2; however, assuming that a large CO2 release from the loss of terrestrial carbon stocks is to be avoided, L is treated as zero when deriving conditions for a beneficial increase in biofuel use.
With L = 0, positive global NEP represents the rate of biological carbon fixation, i.e., carbon moving on balance from the atmosphere to the terrestrial pool. Initial NEP is:
$$ {\mathrm{N}}_0={\mathrm{P}}_0-{\mathrm{R}}_0 $$
Net carbon entering the atmosphere (A) is then:
$$ {\mathrm{A}}_0={\mathrm{E}}_0-{\mathrm{N}}_0 $$
that is, the emissions from energy use less NEP. The sign convention here is that P and N are positive as sinks; R and E are positive as sources.
Although B, E and F can be quantified with reasonable certainty by tracking material flows, N, P and R are highly uncertain because their estimation requires complex modeling (e.g., Le Quéré et al 2009). Their global values cannot be measured directly and even site-specific experimental estimates are difficult to obtain. Therefore, determining whether biofuels at real-world, commercial scales have a quantifiable climate benefit involves a severe test.
Counterbalancing CO2 emissions from fuel use
This simplified model enables a systematic look at the third tier of options outlined in Table 1, which involve counterbalancing the direct CO2 emissions from fuel end use.
Increasing CO2 uptake
The first subcategory of options involves increasing CO2 uptake in the biosphere. This case is represented by P1 > P0 while leaving R unchanged. If the gain is devoted to biofuels, then B1 = P1–P0 and F is reduced accordingly (F1 = F0–B1). Thus:
$$ {\mathrm{N}}_1={\mathrm{N}}_0+{\mathrm{B}}_1\kern0.5em \mathrm{and}\kern0.5em {\mathrm{A}}_1={\mathrm{A}}_0-{\mathrm{B}}_1 $$
This effect can be seen as increasing NEP by increasing NPP, resulting in a decrease by B1 of the net flux to the atmosphere. Note that whether or not a biofuel is classified as first or second generationFootnote 3 does not change this basic assessment. In either case, the test is the extent to which NEP is increased during feedstock production. Even if a second-generation processing technology enables more biomass to be converted to fuel than is the case for a sugar, starch or oil crop, it is not the total production (NPP) that factors into improving the carbon balance (as commonly assumed in LCA). Rather, it is only the increase in NEP achieved by whatever management practices might be used to obtain a gain in P (or alternatively, as described next, a decrease in R).
Although this analysis omits process emissions from feedstock production and harvesting through biofuel refining and distribution, those impacts further raise the bar for biofuel’s climate benefit. Consider a process:product carbon ratio of 1:1 (one ton of carbon emitted during processing for every ton of carbon in the finished product), a penalty much lower than that of most biofuel production processes. Each ton of energy-sector carbon displaced by biofuel then requires two tons of increased NPP because both process and end-use emissions must be offset by additional uptake.Footnote 4 Raising NPP (e.g., increasing yields) often involves increasing agricultural inputs and therefore process emissions. In any case, increasing NPP in a manner that increases NEP is one mechanism through which biofuel production can have a carbon cycle benefit.
Avoiding CO2 release
A second way to counterbalance fuel end-use CO2 emissions is by avoiding the emission of CO2 from land where it otherwise would be released. In this case, NPP does not change (P1 = P0); carbon moves from the terrestrial pool to the energy system (B1 > 0) and heterotrophic respiration is reduced accordingly (R1 = R0–B1). Fossil fuel is displaced (F1 = F0–B1) and so:
$$ {\mathrm{N}}_1={\mathrm{P}}_1-{\mathrm{R}}_1={\mathrm{P}}_0-\left({\mathrm{R}}_0-{\mathrm{B}}_1\right)={\mathrm{N}}_0+{\mathrm{B}}_1 $$
Again, NEP is effectively increased by the diversion of biomass to the energy system. The net flux into the atmosphere decreases accordingly:
$$ {\mathrm{A}}_1={\mathrm{E}}_1-{\mathrm{N}}_1={\mathrm{E}}_0-\left({\mathrm{N}}_0+{\mathrm{B}}_1\right)={\mathrm{A}}_0-{\mathrm{B}}_1 $$
This result corresponds to with the common perception of biomass wastes as an environmentally sound feedstock for biofuels. Biomass wastes or residues require second generation processing, which is starting to achieve some pilot-scale production (Wald 2012). The conversion of “marginal” land (not in agricultural production because it is economically marginal regardless of ecological value) to feedstock production can avoid CO2 release through removal of biomass that would otherwise break down. In practice, such land conversion for purpose-grown biofuel feedstocks may involve both an increase in P and a reduction in R, with the carbon cycle benefit still reducing to whatever gain is achieved in NEP.
Benefit of additional fixed carbon
This analysis 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, the effect amounts to increasing NEP as construed here, which means increasing the rate at which carbon is fixed in the biosphere. This reasoning is consistent with known principles for managing the terrestrial carbon stock, which entail either raising the rate of carbon input by increasing the rate of CO2 absorption from the atmosphere or lowering the rate of carbon loss by reducing respiration (Izaurralde et al 2013). When used for bioenergy, the additional biomass is diverted into the energy system and so does not accumulate in the biosphere. Although the gain in NEP does not increase the terrestrial sink, the effect is the same as far as atmospheric benefit is concerned.
Once additional carbon is fixed, sequestering it is just as effective as displacing fossil carbon for reducing CO2 buildup in the atmosphere (Marland and Marland 1992). For example, in the case of avoiding releases that would otherwise occur, R might be decreased through practices that increase carbon accumulation (letting forests regrow, rebuilding soil carbon, etc.). The gain in carbon stock then benefits the atmosphere as much as converting the added biomass into biofuels, and may be more effective if accomplished with lower GHG emissions for processing. In this case R1 < R0, but without biomass diversion into the energy system. Fossil fuel use (F) is unchanged, but N1 > N0 and so the net flux decreases. Similarly, because the atmosphere is indifferent to the location of sources and sinks, biofuel production can be evaluated against Reducing Emissions from Deforestation and Forest Degradation (REDD) and other programs for protecting terrestrial carbon stocks. Guidance can be taken from the longstanding research on how to manage land for carbon storage and directly address the associated land-use challenges (UNFCCC 2012). Mindful of the difficulties of verifying carbon flows, such programs are attentive to baselines, additionality, permanence, leakage and other concerns (Olander 2008), which is not the case for biofuel policies to date.
Whether the additional carbon fixation is achieved by increasing P or decreasing R (or both), determining the best use of the fixed carbon then reduces to a question of economics. For example, making biofuel from wastes is but one way to avoid CO2 releases that would otherwise occur.Footnote 5 Other options include managing the wastes to avoid decay, converting them to long-lived products or utilizing mainly the hydrogen for energy and sequestering the residual biochar. Biofuels do have value in the fuels market; nevertheless, carbon offsets may be more cost effective for mitigation, a likelihood highlighted by the way such offsets are viewed as cost containment measures for climate policy. In all cases, the starting point for comparing options needs to be in situ assessment of the change in NEP, which therefore requires knowledge of baseline NEP including any necessary adjustment for leakage. Options for the use of any additionally fixed carbon can then be evaluated by costs, market value, emissions and other environmental and social considerations.
Geologic sequestration
The third subcategory in Table 1 involves geologic mechanisms for offsetting the CO2 from liquid fuel use. Because CCS is not feasible onboard mobile sources, the “C” option shown in Figure 2 is not available for most forms of liquid fuel use. A “D” pathway of direct air capture (DAC) of CO2 by chemical mechanisms or artificial photosynthesis is also technologically remote (APS 2011). Although DAC is unlikely to make sense until large stationary CO2 sources are nearly eliminated globally, it may be a very-long-term option for offsetting CO2 from dispersed sources such as transportation. For now, the biosphere offers more feasible ways to balance CO2 emissions from liquid fuel use than chemical removal with geologic storage.
That being said, petroleum firms are heavily involved in stationary CCS development, not least because it involves their core competencies in geology, chemical engineering and handling large volumes of liquids and gases. Carbon dioxide has been used for enhanced oil recovery (EOR) for many years. To date most CO2 EOR has not involved sequestration; although efforts to expand this option are underway, the capacity of suitable EOR reservoirs currently appears small compared to the total CO2 emissions from liquid fuel use (NETL 2009). On the other hand, strategies for coordinating carbon sequestration with fossil hydrocarbon production have not seen a level of research on a par with that for traditional oil and gas extraction.
Other geologic options include coal-to-liquids (CTL) and coal + biomass-to-liquids (CBTL) with CCS (NRC 2009). Demonstration is also underway for CCS of CO2 from ethanol biorefineries, exploiting the relatively pure CO2 stream generated by fermentation (ADM 2009). Some studies suggest a long-run potential for negative emissions through bioenergy coupled with carbon capture and sequestration (Rhodes and Keith 2008; Azar et al 2010). In a multisector climate policy context, all such options are relevant to a coordinated program for limiting total GHG emissions from diverse sources including liquid fuels. Nevertheless, for sound implementation, any mechanism must be evaluated by the extent to which the biogenic carbon involved reflects an effective gain in NEP; such evaluation is not seen in the traditional LCA methods that underpin the literature to date (e.g., as reviewed by NRC 2009). A framework is required that prevents double-counting of reductions such as those avoided at stationary CO2 sources and sequestered through CO2 EOR. Explicit, location-specific accounting of net impacts on sources and sinks is crucial for clarity.