Under New Guidance, “Sustainable” Aviation Fuel in the US Could Be Anything But

The U.S. aviation industry has set a target to reach net-zero carbon emissions by 2050, primarily through the use of so-called “sustainable aviation fuels” (SAF). One of its key levers to achieve this goal is a generous tax credit in the Inflation Reduction Act for sustainable fuels that reduce greenhouse gas emissions by at least 50% compared to petroleum jet fuel.

But new guidance laying out how fuel providers will measure emissions reductions in order to receive this tax credit flies in the face of stated climate goals.

The guidance, released on December 15, 2023, may extend tax credits to fuel made from corn ethanol or vegetable oils — despite strong evidence that these crop-based “biofuels” actually increase net emissions while diverting valuable cropland away from food production.

This would be a sharp turn in the wrong direction. U.S. domestic flights were responsible for 150 million metric tons of CO2 emissions (Mt CO2) in 2019, representing almost 3% of the country’s total emissions. With air travel projected to grow rapidly, aviation emissions are expected to roughly double by 2050, both in the U.S. and globally. And that’s before factoring in a heavier reliance on unsustainable biofuels.

As the Biden administration works to finalize guidance for the SAF tax credit, it must ensure that this rule relies on the best available science for modeling lifecycle airline emissions. If the administration fulfills this commitment — rather than bowing to pressure from the biofuels industry — it will find that instead of being a climate solution, crop-based aviation fuels are even worse than their fossil fuel alternatives and could increase hunger and habitat destruction.

Powering Airplanes with Crops Is Not Sustainable

Many scientists and international regulatory bodies have concluded that growing crops to make aviation fuel does not reduce emissions on a full lifecycle basis (from crop production through to processing and consumption). This is because it displaces food crops, which drives the expansion of cropland into forests and grasslands both in the U.S. and globally to compensate for lost food production. Converting forest or grassland to cropland releases stored carbon and severely reduces carbon sequestration on that land in the future.

Because powering airplanes with crops is also extremely inefficient, the effects on food security and global forests would be severe. For example, 1.7 gallons of corn ethanol are needed to make 1 gallon of sustainable aviation fuel. If the U.S. were to reach its stated goal of 35 billion gallons of SAF using ethanol — which is currently its leading approach — this would require 114 million acres¹ of corn. That’s 20% more than the total area currently planted with corn in the United States for all purposes.

Ethanol production does generate some useful by products, such as animal feed, which would otherwise be made with purpose-grown crops. But even after accounting for this, the overall increase in corn demand driven by biofuel production would ultimately drive up food prices and increase hunger. It would also foreclose almost 500 Mt of land-based carbon sequestration due to displaced food production, which can replace natural lands with agriculture or prevent using land for activities like reforestation. When accounting for total emissions, including production emissions and loss of soil organic carbon due to tillage, the impact would be even higher: 782 Mt CO2.

This represents a net increase in emissions of approximately 340 Mt when compared to burning the same amount of petroleum-based jet fuel, equivalent to the emissions from 75 million gas-powered passenger vehicles.

SAF made from soy oil would likely have an even bigger impact because soy is less efficient than corn at producing energy. Further, there is one interconnected global market for vegetable oil and new production overwhelmingly comes from the expansion of oil palm and soybeans in the tropics, where they are major drivers of deforestation. If just one-quarter of the world’s aviation fuel likely needed in 2050 were to come from vegetable oil, its production would need to double globally.

Perhaps surprisingly, scaling up carbon removal technology to compensate for the emissions from burning 35 billion gallons of petroleum-based jet fuel would require much less land than replacing it gallon-for-gallon with crop-based fuel. If, for example, direct air capture (DAC) technology powered by solar energy were used to remove 434 million Mt of CO2 per year — the amount that would result from 35 billion gallons of petroleum jet fuel — it would require around 3.7 million acres of land. The amount of land needed to replace the same volume of fuel with corn ethanol is around 30 times higher in this scenario.

These types of carbon removal technologies are still nascent and should not be considered a silver bullet; reaching net-zero aviation emissions will require a multifaceted approach. But carbon removal will be needed to compensate for emissions that can’t be avoided and to remove excess carbon dioxide that is already in the atmosphere.

Where Do Emissions Accounting Models Get It Wrong?

Developing more sustainable aviation fuels depends on being able to accurately compare emissions across various fuel types, including from both their use and production.

The International Air Transport Association (IATA) defines sustainable aviation fuels as those that reduce CO2 emissions by up to 80% compared to conventional jet fuel and are made from feedstocks that “do not compete with food crops or output, nor require incremental resource usage such as water or land clearing.” The IRA’s SAF tax credit also offers emissions-reduction guidance: It starts at $1.25 per gallon for SAF that reduces lifecycle greenhouse gas emissions by at least 50% compared to conventional jet fuel, increasing by an additional 1 cent per gallon for each percentage point of emissions reductions beyond this threshold.

This begs the question: How should the full lifecycle emissions from producing and burning aviation fuel be calculated?

The IRA refers to a model adopted by the International Civil Aviation Organization (ICAO) or any similar methodology that satisfies criteria established by the U.S. Clean Air Act. Following an intense lobbying campaign by the ethanol industry, the Treasury’s recent guidance allows for the use of an alternative model, GREET, which could open the door for corn ethanol and other crop-based biofuels to qualify for the credit. Major U.S. airlines supported the ethanol industry’s push despite previously agreeing that SAF production should not compete with food production.

While much of the discussion surrounding the SAF tax credit has focused on which model should be used to estimate airline emissions, more important are the assumptions incorporated into a given model — in particular, how it accounts for the impact of dedicating farmland to fuel production rather than food production.

Models which suggest that these “indirect land-use change” emissions are small compared with petroleum emissions often underestimate the amount of new land needed to grow food, implicitly assuming that poor people will eat less because food prices rise when crops are diverted to fuel production. Shockingly, these models count this as a climate benefit and ignore the huge harm that underlies it. Other models make arbitrary assumptions which imply that no forests will be cleared to replace the cropland used for fuel, despite the fact that an average of 3.3 million acres of forests are already being cleared each year to make room for expanded palm and soybean oil production. Whether or not crop-based biofuels qualify for IRA tax credits will depend on the assumptions used in an updated version of the GREET model, which the Biden administration has committed to release by March 2024.

Besides relying on opaque and unverifiable assumptions, these complex global models can produce dramatically different results. A much more straightforward approach is to consider the following thought experiment: If there really are millions of acres of prime farmland that aren’t needed for food production, what would be the best use of that land to mitigate climate change? The commonsense answer (supported by rigorous analysis) is that avoiding additional deforestation and restoring forests to previously cleared land is 2-4 times more effective at curbing climate change than turning food into fuel over a 30-year period.

How Should the U.S. Approach the Challenge of Decarbonizing Aviation?

Both airlines and policymakers have conveyed a commitment to decarbonize aviation, but crop-based biofuels are clearly not the way to achieve this goal. More research, investment and innovation are needed to understand the best pathways to net-zero aviation emissions, and there are already ideas and technologies in play that offer promising places to start addressing this challenge.

• Invest in rail infrastructure: High-speed rail can be more convenient than flying for short- to medium-length trips (less than 500 miles) and can be powered by zero-emissions electricity. U.S. rail infrastructure is far behind that in Europe and parts of Asia, although a few projects are under development.
• Restrict biofuel production to waste biomass: Plant material left over after harvesting crops and lumber, as well as municipal wastes, have the potential to supply a truly carbon-negative biogenic fuel, particularly when the processing of these fuels is coupled with carbon capture and storage technology. However, the amount of waste biomass available would only produce a portion of total aviation fuel. Estimates of waste biofuel SAF capacity range from supplying 5.5% of European aviation fuel demand in 2030 to up to about 20% of U.S.’s aviation fuel demand in 2050.
• Explore synthetic e-fuels: Also known as “power-to-liquid fuels,” synthetic fuels like e-kerosene can be produced from captured carbon and hydrogen obtained from water and renewable energy. Europe’s SAF mandate requires that at least 2% of aviation fuel come from synthetic fuels by 2030. Current challenges to expanding the use of synthetic fuels include the high cost and obstacles in scaling production.
• Use batteries to power short flights: The batteries that would be needed for long-haul flights are currently too heavy to be used, but battery-powered short-range flights have already been proven viable. Companies in the U.S. plan to produce battery-powered planes that can serve commercial flights ranging from 100-150 miles in the coming years.
• Support development of hydrogen-powered flights: Hydrogen, which can be made from water using renewable electricity, could be the ultimate solution for aviation if fuel handling and storage challenges can be resolved. Several companies are developing planes powered by hydrogen fuel cells for use in short- to medium-haul flights, similar to battery-powered planes. Hydrogen could also be burned in modified jet engines, a concept that Airbus is hoping to commercialize by 2035.

These pathways will require time for development and scaling. In the meantime, permanent carbon removal can compensate for emissions from burning petroleum jet fuel and would require far less land than crop-based biofuels. Carbon removal should not be seen as enabling airlines to continue burning petroleum indefinitely, and offsets claimed using carbon removal should adhere to rigorous standards to ensure their efficacy. Concerns over greenwashing have already led to a class-action lawsuit against Delta Airlines questioning the effectiveness of their emissions offset program.

Although it is not yet clear what will be the most effective way to decarbonize aviation by mid-century, it is certain that crop-based aviation fuels are not the answer. To ensure real reductions in emissions, incentives should be reserved for solutions that are actually sustainable.

^{1Assumes 2022 average corn yields of 173.3 bushels per acre, as reported by USDA, and a production rate of 1 bushel of corn needed to produce 3 gallons of ethanol, based on EIA and USDA data.}