Visualizing the time-varying carbon impact of biofuels

Several years ago I did a post entitled "When do biofuels really balance carbon?" At issue is the belief that biofuels are inherently carbon neutral. That assumption is typically justified in terms of a feedstock offset, i.e., that a biofuel's end-use and other biogenic CO2 emissions are fully balanced by CO2 uptake during photosynthesis when their feedstocks are grown. However, that proposition is not generally true. A more correct understanding is that the CO2 emitted from biofuel use can be partly balanced by gains in net CO2 uptake in several locations (not just where feedstock is grown) over a possibly long period of time.

In the biofuel literature, this understanding of biofuel-related carbon flows was developed through consequential lifecycle assessment (CLCA) modeling, which is quite complicated and hardly transparent. However, the key insights can be seen with relatively simple stock-and-flow modeling. The outcome of such an exercise is described here.

The following figure shows illustrative results for a scenario of biofuel use as shown in the bottom panel (c) of the figure. This model input assumes that corn ethanol use ramps up to 13 billion gallons per year (Ggal/year) over a 10-year period (2005-2015) and then remains constant thereafter. The analysis was done in units of teragrams of carbon (Tgc); 13 Ggal/year of ethanol corresponds to 20 Tgc/year on a carbon (not CO2) mass basis.

Changes over time in (a) atmospheric carbon and (b) net carbon emissions as a result of (c) biofuel use, relative to a fossil-fuel reference case; units of teragrams of carbon (Tgc). 

The middle panel (b) shows the effect on net carbon emissions. Emissions rise sharply as ethanol use ramps up, reflecting the release of terrestrial carbon stocks as natural lands (such as tropical forests or grasslands) are cleared, directly or indirectly, to make way for new cropland. Such land-use change is triggered by the additional crop production needed for biofuels. Once biofuel use stops rising (after 2015 in the scenario modeled), net carbon emissions fall below the fossil fuel reference case. This emission reduction, which amounts to roughly 17 Tgc/year for the case shown here, reflects the net induced offset due to biofuel use. It is less than a full offset of the biofuel's associated biogenic emissions and results from changes in several carbon flows economically induced by biofuel use, as will be explained in a subsequent post.

The top panel (a) shows the resulting change in atmospheric carbon which, as a stock, is the integral of the net emissions flow of panel (b). Atmospheric carbon rises above the fossil fuel reference level as biofuel increases, reflecting the carbon debt from the biofuel-related land-use change. Atmospheric carbon begins decreasing once biofuel use stops increasing, falling below the reference level after 2054. That is 48 years after the first year of modeled biofuel use (2006) in the scenario shown here, reflecting the time it takes to pay back the carbon debt and highlighting the nearly five decade lag before the biofuel use achieves a net CO2 reduction.

This greatly delayed climate mitigation is obscured by most LCA studies, which smooth over important time-varying effects when reducing their results to carbon intensity values (e.g., grams of CO2-equivalent per megajoule of fuel). The system dynamics revealed by stock-and-flow modeling emphasize that, although biofuels may offer a long-term net carbon reduction, they can make matters much worse before they get better.

The latest light truck versus EV emissions gap

CO2 reduction from EVs compared to CO2 emissions increase from higher light truck sales,
light-duty fleet average changes from 2012 through 2021.  

Although U.S. electric vehicle sales have been growing by leaps and bounds, so too have the sales of large light trucks. Last year, my analysis of the then-current EPA data revealed that CO2 emissions due to the market shift to light trucks (mainly pickups and most SUVs) was over five times greater than the emission reduction that EVs would have achieved on their own. This gap has dropped to just under a factor of four, as shown in the chart above. In short, EVs sales still have not made a dent in the overall lifetime carbon burden of the new light-duty vehicle (LDV) fleet. 

Last year's analysis examined data through model year 2019; EPA's most recent Automotive Trends report includes preliminary data through model year 2021. Combining those data with plug-in electric vehicle (PEV) sales reports from Argonne National Laboratory (ANL, sponsored by the U.S. Department of Energy) enables an updated calculation. The methodology used is the same as that described in last year's post.

The market share of EVs (counting pure plug-in battery electric vehicles and plug-in hybrids, but not gasoline-only hybrids) reached 4.2% in 2021, up from the 2% share in 2019. But 2021 also saw the light truck share of the new LDV fleet reaching a record high of 61% (using EPA's classifications, which count small, front-wheel-drive SUVs as cars while counting all other SUVs as well as pickups and minivans as light trucks). 

These data imply a factor of 3.7 for the gap; that is to say, the excess emissions from the increased light truck sales are nearly four times greater than the potential emission reductions from the increased EV sales. So although the gap has narrowed somewhat, there is still quite a way to go before EV sales become high enough to dig us out of the hole caused by adverse market shifts. The EV reductions are termed "potential" because they are not realized on a net basis due to the fleet average nature of clean car standards. 

As in most recent years, the average CO2 emission rate of each vehicle class has declined. However, as the mix of vehicles sold has shifted to higher-emitting light trucks, fleetwide progress has all but ground to a halt. EPA's estimate for 2021 is a LDV average of 348 grams per mile (g/mi), differing only trivially from the 2020 average of 349 g/mi. 

The calculation shown here tracks progress since 2012. That year is selected as the baseline because it was when the Obama Administration finalized strong fuel economy and GHG emission ("clean car") standards. That year also saw a number of significant EV introductions even though plug-in vehicle sales were still quite small. The ANL data indicated that EVs represented just 0.4% of new LDV sales in 2012. Thus, over the nine years through 2021, EV market share has increased by an order of magnitude (to the aforementioned 4.2%). 

The EPA data show the overall fleet-average new LDV CO2 emission rate fell by 7.7%, from 377 g/mi in 2012 to the 348 g/mi preliminary estimate for 2021. That's a decline of only about 1% per year, much less than the 4%-5% annual reduction hoped for with the GHG standards issued in 2012. The reasons for this large shortfall in carbon-cutting progress include the weakening of the standards by the Trump administration as well as the market shift from cars to light trucks and the general upsizing within each class, which relax the effective stringency of the standards that automakers must meet. 

As was the case for last year's version of this analysis, it excludes the effect of the largest pickups. These ever-more-massive and powerful, ¾-ton and 1-ton pickups are omitted from EPA's public reports. Including these large, luxurious personal trucks would make the emissions gap even larger. 

Looking back at the RFS on the eve of its target year

When Congress and the Bush Administration expanded the Renewable Fuel Standard in 2007, they set next year as the target for a full-scale biofuel mandate as then envisioned. The goal was 36 billion (ethanol-equivalent) gallons by 2022, with 16 billion of those to have been cellulosic ethanol or other fuels derived from such non-food feedstocks. 

Just posted on The Conversation is my look-back at the RFS, pointing out that the U.S. biofuel mandate helps farmers, but does little for energy security and harms the environment. It reviews the RFS experience to date in terms of the policy's three main rationales: improving energy security, reducing CO2 emissions and boosting agricultural income. My article's title sums it up ... read more there! 

Cellulosic ethanol's highly subsidized failure

Actual U.S. production of cellulosic ethanol and other cellulosic liquids compared to the targets for cellulosic biofuels specified in the Renewable Fuel Standard

Cellulosic ethanol, once a great green hope for cutting petroleum use and CO2 emissions, has been a bust. The chart above compares the volume of cellulose-based liquid biofuels (largely ethanol) actually used in the United States to the targets for such fuels set by Congress when it expanded the Renewable Fuel Standard (RFS) in 2007. Note the logarithmic scale on the vertical axis; the gap between promise and reality is so large that actual production would be barely visible on a linear scale. 

The fuel has been delivered at levels of no more than about 0.1% of (three orders of magnitude less than) the volumes on which the RFS was premised [1]. This chart does not include cellulosic biogas, which EPA re-classified to qualify for RFS compliance purposes and has seen recent production of around 500 million ethanol-equivalent gallons. However, such "renewable natural gas" is not in the spirit of the law, which envisioned liquid biofuels that could be readily used in motor vehicles. The 2019 RFS target for cellulosic biofuels was 8.5 billion gallons, set to reach 16 billion gallons by 2022. But in 2019, before the pandemic-related drop-off in 2020 for nearly all forms of energy, only 9.8 million gallons of cellulosic ethanol were tallied by EPA.  

Newly proposed auto standards hold promise

The Biden Administration is clearly making good on its pledge to revisit the automobile GHG emission standards weakened by the previous administration. The notice of proposed rulemaking issued by EPA on August 5, 2021 aims to cut model year 2026 car and light truck GHG emission rates 17% compared to the recent level, from 205 grams/mile (g/mi) in 2020 to a nominal target of 171 g/mi in 2026. 

The White House announcement just prior to the EPA proposal also states an ambitious goal for EVs to comprise half of U.S. vehicle sales in 2030. This non-binding target would include plug-in hybrid electric vehicles (PHEVs) as well as pure zero-emission vehicles such as battery electric and fuel cell cars. 

Fleetwide efficiency gains more important than electric cars over the next decade

For at least the next decade, the overall fuel economy of the entire car and light truck fleet will be more crucial for climate protection than the number of electric vehicles sold. That's the message of my recent online article in Scientific American: "Want Greener Cars? Focus on Fuel Efficiency." Read the discussion there (the article was first published in The Conversation under the title "To make the US auto fleet greener, increasing fuel efficiency matters more than selling electric vehicles").   

Personal trucks widen emissions gap over EVs

Excess carbon dioxide emissions from the rising popularity of light trucks, such as the Ram pickup, swamp many times over the potential carbon savings from increased sales of EVs, such as the Tesla Model 3, to date.  

Last fall, I posted an analysis showing that, even as electric vehicle sales had grown significantly over the past several years, the broader market shift to personal trucks (mainly SUVs and pickups) has overwhelmed the potential CO2 reductions from EV use by more than a factor of four. With the new EPA data now released, this ratio has increased to a factor of 5.6, as shown in the chart below. 

GM touts innovation while weakening regulation

My opinion piece published in The Detroit News points out the inconsistency of how some automakers have advocated for weaker fuel economy and GHG emission standards even as they promote their technology innovations. Taking General Motors' recent public statements as an example, it reminds readers that what ultimately matters for reducing emissions is the stringency of the regulations that apply across the entire vehicle fleet. 

Light trucks overwhelm EVs' carbon-cutting benefits to date

Electric vehicle sales have grown rapidly over the past several years. In 2012, only about 53,000 EVs were sold in the United States, counting both battery electric and plug-in hybrid models. By 2018, the annual tally of new EVs sold in the United States reached 361,000. It then tapered to 327,000 in 2019, the last full year of data before the 2020 pandemic. The vast majority of EVs are Teslas, with the big jump in 2018 due to the introduction of the Tesla Model 3. With overall light vehicle sales on the order of 17 million per year (pre-pandemic), EVs comprised about 2% of the U.S. market as of 2019.