Approximately 91% of the world’s transportation energy comes from petroleum-based fuels like gasoline and diesel.1 As a result, transportation accounts for 29% of greenhouse gas (GHG) emissions in the United States, and about 20% percent in the world.2 The road transport of people and freight dominate those emissions. Liquid fuels for transportation produced from biomass (biofuels) represent one option for reducing such emissions.
Biofuels are a contentious subject. One advantageous property of renewable fuels is that they can be derived from a variety of feedstocks (crops, algae) and converted to a variety of useful fuels (ethanol, biodiesel) via multiple transformation pathways (chemical, thermal, biological). Biofuels can boost economies by creating additional markets for agricultural and forestry products. In principle, CO2 emissions from biofuel combustion are balanced by the CO2 uptake in growing the biomass feedstocks. Biofuels generally produce fewer air pollutants such as sulfur oxides, particulate matter, and nitrogen oxides than petroleum-based fuels. Countries can reduce oil import dependence if they produce domestic biofuels.
Counterarguments often begin by prioritizing land, fertilizer, and water for human food production over transportation fuels. The intensive cultivation of biofuel feedstock crops can consume lots of water and cause soil erosion, loss of biodiversity, and nutrient depletion. Current methods of producing some biofuels require large fossil fuel inputs and hence generate large GHG emissions, reducing or eliminating the climate benefit.
Regarding GHG emissions, it is essential to perform a thorough and transparent life cycle analysis (LCA) of biofuels to understand their potential to reduce emissions compared to fuels derived from petroleum. GHG emissions associated with biofuels include emissions from producing the fuel, from combusting the fuel, and from the full supply chain for producing and distributing the fuel. GHG emissions also include market effects, including changes in land use, changes in electricity infrastructure and system operations, and changes in the demand for fuel and other products.3
Researchers have used a wide range of data, methods, and assumptions in the LCA of renewables, leading to a wide range of estimates of the climate benefits of biofuels. The lack of consensus in research has led to heated policy debates in which stakeholders often cherry-pick studies to support their positions.
The United States Environmental Protection Agency has produced a comprehensive assessment of renewable fuels to support implementing the Renewable Fuel Standard (RFS) program created under the Energy Policy Act of 2005.4 The EPA reports a very wide in emissions across different combinations of feedstocks, fuels, and technology combinations.
At the high end, emissions from some fuel pathways exceed those from conventional petroleum fuel when coal is used in the production process. Then there are very low or even so-called “negative” emissions from some fuel pathways. What is a negative emission in this context? One example is a perennial grass such as switchgrass grown as a fuel feedstock that can accumulate more carbon in its extensive root systems and the soil compared to the emissions associated with their cultivation, processing, and combustion.5
The use of ethanol as a gasoline additive accelerated after the Clean Air Act amendments of 1990 (and subsequent laws) mandated the sale of oxygenated fuels in areas with unhealthy levels of carbon monoxide. The consumption of ethanol increased from 1.6 billion gallons in 2000 to 10.6 billion gallons in 2023.6 Emissions from ethanol from corn starch became the most well-known comparison with petroleum-based fuels.
The early LCA of corn ethanol suggested it reduced GHG emissions by about 20% compared to gasoline. Scientific and engineering advances have dramatically reduced energy inputs to corn ethanol and have reduced its carbon emissions. Corn yields are higher, the intensity of fertilizer use has declined, the energy efficiency and yield of biorefineries have increased, and electricity purchased from the grid is cleaner. Current research indicates that corn ethanol now has 40% to 50% lower emissions compared to gasoline,7 and that continued reductions are feasible and likely.8
Renewable fuel projects can cause direct and indirect land-use change (LUC) that releases and sequesters carbon. The demand for biofuels can cause land to be converted to feedstock production for other uses, including non-feedstock agricultural lands, forests, and grasslands. This type of LUC is sometimes called direct LUC. The resulting change in crop production levels (e.g., an increase in corn production may cause a decrease in soybean production) and exports may shift land uses domestically and abroad through economic linkages. This latter type of LUC is called indirect LUC and requires the economic assessment of commodity markets and international trade.9
Early research suggested that emissions from LUC caused by biofuels erased any emissions benefit from the farm and ethanol production stages.10 Subsequent research with more sophisticated economic models points to much smaller emissions from LUC. In the case of corn ethanol, recent studies indicate that emissions from LUC are less than 10% of total lifecycle emissions.11
There is strong and growing government support for biofuels in many countries and regions. For example, the Renewable Energy Directive of the European Union (2023) not only mandates increased use of biofuels but also specifically discourages biofuel supply chains that trigger land use change that lower carbon stocks in plants and soils.12 Considerable new investments are flowing toward biofuels for new construction and the conversion of petroleum refineries to biorefineries.13 One report projects a compound annual growth rate of 11.3% from 2024 to 2030 for the global biofuels market.14 Underpinning the expansion is continued technological progress in lowering the cost of biofuels and reducing GHG emissions and other environmental impacts.
The major biofuels in use today produce a significant reduction in GHGs relative to their petroleum counterparts, a benefit that has grown over the past several decades. Biofuels will continue to be scrutinized for the magnitude of their climate benefits, and their environmental impacts, including LUC. Appropriate government policy, ecologically appropriate land use decisions, enhanced R&D of cellulosic feedstocks, and rigorous and transparent lifecycle analysis can mitigate such impacts. These forces are shifting the conversation from “Are biofuel crops beneficial?” to “Which biofuel crops are beneficial in a given location?”5
1 International Energy Agency, “World Energy Balances,” accessed February 28th, 2024, https://www.iea.org/data-and-statistics
2 U.S. Environmental Protection Agency, “Fast Facts on Transportation Greenhouse Gas Emissions,” Link
3 National Academies of Sciences, Engineering, and Medicine. 2022. “Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States,” Washington, DC: The National Academies Press. https://doi.org/10.17226/26402.
4 U.S. Environmental Protection Agency, “Lifecycle Greenhouse Gas Results,” May 2023, Link
5 Davis, Sarah C, William J Parton, Stephen J Del Grosso, Cindy Keough, Ernest Marx, Paul R Adler, and Evan H DeLucia. “Impact of Second-Generation Biofuel Agriculture on Greenhouse-Gas Emissions in the Corn-Growing Regions of the US.” Frontiers in Ecology and the Environment 10, no. 2 (2012): 69–74. https://doi.org/10.1890/110003.
6 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Alternative Fuels Data Center, accessed February 28, 2024, https://afdc.energy.gov/data
7 Lee, Uisung, Hoyoung Kwon, May Wu, and Michael Wang. “Retrospective Analysis of the U.S. Corn Ethanol Industry for 2005–2019: Implications for Greenhouse Gas Emission Reductions.” Biofuels, Bioproducts and Biorefining 15, no. 5 (2021): 1318–31. https://doi.org/10.1002/bbb.2225.
8 Lewandrowski, Jan, Jeffrey Rosenfeld, Diana Pape, Tommy Hendrickson, Kirsten Jaglo, and Katrin Moffroid. “The Greenhouse Gas Benefits of Corn Ethanol – Assessing Recent Evidence.” Biofuels 11, no. 3 (April 2, 2020): 361–75. https://doi.org/10.1080/17597269.2018.1546488.
9 Dunn, Jennifer B., Steffen Mueller, Ho-young Kwon, and Michael Q. Wang. “Land-Use Change and Greenhouse Gas Emissions from Corn and Cellulosic Ethanol.” Biotechnology for Biofuels 6, no. 1 (April 10, 2013): 51. https://doi.org/10.1186/1754-6834-6-51.
10 Searchinger, Timothy, Ralph Heimlich, R. A. Houghton, Fengxia Dong, Amani Elobeid, Jacinto Fabiosa, Simla Tokgoz, Dermot Hayes, and Tun-Hsiang Yu. “Use of U.S. Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land-Use Change.” Science 319, no. 5867 (February 29, 2008): 1238–40. https://doi.org/10.1126/science.1151861.
11 Scully, Melissa J., Gregory A. Norris, Tania M. Alarcon Falconi, and David L. MacIntosh. “Carbon Intensity of Corn Ethanol in the United States: State of the Science.” Environmental Research Letters 16, no. 4 (March 2021): 043001. https://doi.org/10.1088/1748-9326/abde08.
12 Commission Delegated Regulation (EU) 2023/1640 of 5 June 2023 on the methodology to determine the share of biofuel and biogas for transport, produced from biomass being processed with fossil fuels in a common process, http://data.europa.eu/eli/reg_del/2023/1640/oj
13 Braya Renewable Fuels Commences Commercial Operations, February 22, 2024, https://brayafuels.com/news/
14 Grandview Research, “Biofuels Market Size & Trends,” accessed February 28, 2024, https://www.grandviewresearch.com/industry-analysis/biofuels-market