This section has three main parts. The first evaluates the material carbon balance for the vehicle-fuel system to estimate the biogenic carbon offset, comprising the paper’s main result. The second and third parts address GHG emissions related to fuel processing and displacement effects, respectively.

Material carbon balance

We calculate vehicle end-use CO 2 emissions using fuel carbon content data from EPA (2010b) and fuel consumption data from EIA (2015). Figure 2 shows the resulting estimates in TgC/yr (carbon rather than CO 2 mass basis; 1 Tg = 1012 g). The rate at which motor fuel carbon flows into the air declined by 23 TgC/yr, or 5 %, from 455 to 432 TgC/yr over 2005–2013, due to the 2008 recession and vehicle efficiency gains [A1(e)]. However, the biofuel portion of tailpipe CO 2 emissions rose from 6.5 TgC/yr in 2005 to 24.1 TgC/yr in 2013. In 2013, biofuels accounted for 5.8 % of motor fuel energy end-use and 5.6 % of tailpipe CO 2 emissions, up from a 1.4 % share in 2005.

Fig. 2 Direct carbon emissions from U.S. motor fuel use, 2000–2014. Source: derived from EIA (2015) Full size image

The other component of biogenic emissions occurs during ethanol fermentation, which yields one mole of CO 2 per mole of C 2 H 5 OH produced. This release reached 10.2 TgC/yr in 2013. Combined with biofuel end-use CO 2 emissions, the overall increase in motor fuel-related biogenic emissions was 25 TgC/yr. In policy-oriented carbon accounting to date, these biogenic emissions are treated as carbon neutral. In ABC accounting, how much they are actually “neutralized” (offset) by gains in carbon uptake is a question to be addressed.

Carbon uptake on cropland

To estimate CO 2 uptake on cropland we used Annual Crop Production (ACP) data from the National Agricultural Statistical Service (NASS; USDA 2015), including planted area, harvested area, average yield and production by crop. For tractability, the analysis was limited to crops that covered at least 95 % of U.S. cropland according to the USDA Cropland Data Layer in 2013. We did not attempt to estimate overseas carbon uptake for the small portion of biofuel that was imported, which averaged 5 % of U.S. biofuel consumption over 2005–13 [A1(f)]. Uptake was calculated by multiplying crop production by the fraction of carbon in each crop from composition data adjusted for moisture content [A2]. As shown in Fig. 3, net CO 2 uptake rose from 195 to 215 TgC/yr over 2005–2013. These estimates of NEP reflect the downward flow of carbon from the atmosphere into the part of the biosphere occupied by U.S. cropland.

Fig. 3 Carbon uptake on U.S. cropland, 2005–2013. Source: derived from USDA (2015) Full size image

Carbon uptake is dominated by corn, which has the largest planted area and a higher yield than other crops. The carbon harvest from corn alone rose by 25 TgC/yr over the analysis period due to a 17 % increase in planted area and a 7 % increase in yield. The corn-soy rotation is the most extensive U.S. farming practice and soybeans are second to corn basis in planted area. However, soybean yields average less than one-third those of corn by volume and only about 25 % those of corn on a carbon basis. With increases of 6 % in planted area and 2 % in yield, soybeans saw a carbon harvest gain of 2 TgC/yr. Nearly all other U.S. field crops saw their planted areas decline over the period. Sorghum was an exception; however, its yield fell and so its harvest did not change significantly. Among other crops, only wheat had a measurable gain in carbon harvest, but by only 0.3 TgC/yr. Collectively, harvests fell for all other major crops, mainly because of smaller planted area, netting out to an aggregate carbon harvest increase of 20 TgC/yr (about 10 %) over 2005–2013.

The biogenic carbon offset

The observed increases in carbon harvest provide estimates of the increases in NEP over the analysis period. Being smaller than the 25 TgC/yr increase in biogenic CO 2 emissions associated with biofuel use, it is not enough to fully offset those emissions. Because cropland NEP varies annually with economically-driven crop planting decisions and weather-dependent harvest outcomes, the overall offset is estimated by comparing cumulative gains in NEP to cumulative biogenic emissions. These calculations are given in Table 1.

Table 1 Biogenic carbon emissions compared to net gains in carbon uptake Full size table

The first section of the table shows year-by-year ∆NEP (first differences of the annual carbon harvest values shown in Fig. 3) and the gains in NEP and biogenic emissions relative to 2005. Because NEP is a flow (TgC∙yr−1), ∆NEP is the derivative of a flow and has TgC∙yr−2 as its unit. Being based on harvest data, the annual ∆NEP can be positive (a gain in uptake) or negative (e.g., due to a poor growing season). As shown in the table, the aggregate harvest as measured on a carbon basis fell in 2006, meaning that the flow rate of CO 2 from the atmosphere to cropland declined, giving a negative value for ∆NEP that year. It jumped in 2007 due to a better growing season but also because notably more corn was planted that year. The annual variability of NEP is reflected in the changing sign of ∆NEP throughout the period.

Integrating ∆NEP gives the net change in the rate of carbon uptake since the base year (2005), as shown by the “Net NEP gain” row in Table 1. By 2013, the net gain in NEP was nearly 20 TgC/yr, as can be seen in Fig. 3. To determine cumulative additional CO 2 removal from the atmosphere, we integrate again by taking the running sum of the annual gain in NEP. As the integral of a mass flow rate, the resulting values have units of mass (TgC, i.e., millions of metric tons). These results for additional CO 2 removal are shown as “Additional C uptake” in the cumulative effects section of the table and plotted as the green line in Fig. 4.

Fig. 4 Cumulative carbon emitted by U.S. biofuel use compared to cumulative additional carbon uptake on cropland Full size image

Similar calculations are performed for the biogenic CO 2 emissions. As shown in Table 1, biogenic emissions increase annually because biofuel production rose steadily over the 2005–2013 period. The cumulative amount of biogenic CO 2 that entered the atmosphere is obtained by integrating this flow, yielding the values plotted in black in Fig. 4. By the end of the period, cumulative biogenic emissions reach 132 TgC. Cumulative net uptake, which reflects the additional amount of carbon removed from the atmosphere by the cropland beyond what was removed in the base year, sums to 49 TgC. The difference between the biogenic carbon emitted and the additional carbon uptake is shown in Fig. 4 as the carbon neutrality “gap,” which reaches 83 TgC by 2013. This value reflects the extent to which biogenic emissions exceeded additional carbon uptake over the analysis period.

The last line of Table 1 compares cumulative carbon uptake and biogenic emissions in percentage terms, indicating that the additional uptake was enough to offset only 37 % of the increase in biogenic emissions from 2005 to 2013. This result shows that full carbon neutrality (a 100 % offset) fails for renewable fuel use in the United States over this period. It also shows how the extent of offset depends on the growing season. Because harvests fell in 2006 compared to 2005 (when the RFS was passed), the percent offset is very negative in 2006 and does not become positive until 2008. The cumulative offset reaches a high of 66 % of cumulative biogenic emissions in 2009 before falling again. Although subsequent years of data are needed to make a longer-term estimate, even if biofuel production levels off it seems unlikely that the cumulative offset would reach 100 % anytime soon.

Vehicle-fuel system carbon balance

Estimates of the material carbon flows defined in Fig. 1 can be used to construct a carbon mass balance for the vehicle-fuel system, showing inputs by source and outputs according to their disposition [A1(b)]. These balances, which exclude non-material-carbon process emissions, are depicted in Fig. 5.

Fig. 5 Material carbon flows through the U.S. vehicle-fuel system (TgC/yr) Full size image

The carbon harvest is either output from the system as food and feed or refined into biofuel. Some carbon is emitted as CO 2 during biorefining and petroleum refining. In biorefining, the primary coproduct is distiller’s grain, which is supplied for use as animal feed [A1(c)]. For petroleum refining, the process CO 2 emissions estimate assumes a well-to-tank energy efficiency of 81.7 % [A1(d)].

In Fig. 5, the sum of input carbon flows matches the sum of the output flows each year. The total rate of carbon flow through the system fell from 745 TgC/yr in 2005 to 715 TgC/yr in 2013, largely due to lower motor fuel demand. Although all of the biogenic carbon emitted comes from NEP (the gross carbon harvest), the gain in NEP over 2005–2013 does not produce enough additional carbon to cover the sum of that which substitutes for fossil carbon in motor fuel plus what gets released during processing. Because the increase in carbon harvest is less than the decrease in fossil carbon input, fuel demand is met at the expense of carbon supplied to the food and feed system. Thus, Fig. 5 reflects how ABC accounting respects conservation of mass (carbon), in contrast to LCA, which does not ensure conservation of mass because it fails to properly assess carbon uptake.

Process GHG emissions

In addition to fuel-related material carbon emissions, GHGs are emitted from feedstock and fuel processing operations. These emissions are the traditional focus of LCA and there is no need to revisit their estimation here. For comparison purposes, we use process emission factors from EPA (2010b).

Adding process emissions to material CO 2 emissions yields total net GHG emissions from the vehicle-fuel system, which dropped by 38 TgC/yr from 2005 to 2013, i.e., by about 10 % of base year emissions (calculations given in appendix Table A2). This drop is explained by a combination of greater carbon uptake (tallied as negative emissions), lower petroleum input and lower overall fuel demand. GHG emissions from fuel processing increase due to the greater amounts of energy and other inputs needed for producing biofuels compared to petroleum fuels. As seen in Fig. 5, there was a loss of biomass carbon output from the system. Therefore, although the system’s net GHG emissions fell, the decrease is only partly from a gain in carbon uptake tied to biofuel use. In gross terms, the 20 TgC/yr increase in NEP explains just over half of the 38 TgC/yr GHG reduction, but that is before considering other important effects such as reduced fuel demand.

Displacement effects

Changes in flows of material carbon across the system boundary change the amount of carbon available to the rest of the economy nationally and internationally. Many effects are indirect, as changes in supply and demand cause changes in price that affect petroleum fuels, grains and other farm products as well as their coproducts, substitutes and other items, affecting GHG emissions the associated markets. These effects include:

Substitution of agricultural products (including co-products)

Deprivation of agricultural products (reduced feed and food consumption)

Intensification of agriculture (increased yield)

Expansion of agriculture (direct and indirect land-use change)

Petroleum market rebound (higher demand in non-regulated fuel markets)

Substitution, deprivation and intensification decrease net GHG emissions due to biofuel use while expansion and rebound effects increase net emissions. Because it involves a release of carbon stocks, agricultural expansion can have a very large impact. The other effects involve marginal changes but have magnitudes significant relative to the direct impacts of the vehicle-fuel system. Modeling displacement effects is beyond the scope of this study and so we use estimates from the literature, acknowledging their very high uncertainty due to market behavior, differences in modeling methods and data limitations.

Substitution effects are captured by EPA’s RFS analysis and so are reflected in the process emissions estimates (Table A2). Evaluating deprivation effects is a new area of research; they may be on the order of one-third of biogenic end-use emissions (Searchinger et al. 2015). Agricultural intensification on U.S. cropland is reflected in the harvest data and so are reflected in our carbon uptake results; we did not attempt to estimate intensification internationally. Petroleum market rebound can amount to as much as one-half of the petroleum fuel displaced by biofuel, raising CO 2 emissions in other markets (Chen et al. 2014). The net impact of these interactions is highly uncertain and so it is difficult to ascertain whether their combined effect is either positive or negative.

The displacement effects that clearly increase biofuel-related carbon emissions are direct and indirect land-use change (DLUC and ILUC). For the RFS, EPA projected no significant DLUC-induced release of carbon stocks and a small gain in soil carbon by 2022. Nevertheless, the available evidence does not support a gain in soil carbon to date [A4]. For DLUC, Lark et al. (2015) examined the 2008–12 subset of our 2005–13 analysis period and estimated a cumulative release of 36 TgC associated with the biofuel-related expansion of U.S. cropland [A5(a)].