Sustainable aviation fuel is taking off. What is it, exactly?

Late last year, Virgin Atlantic ran a radio spot in the United Kingdom touting a noteworthy environmental achievement: The British airline was about to fly a plane on ​“100 percent sustainable aviation fuel.”

The ad referred to a unique demonstration flight that took place days later on November 28, 2023, in which the company flew a Boeing 787 Dreamliner from London to New York City using only a blend of waste oils and plant-based sugars. In doing so, Virgin Atlantic became the first commercial airline to fly a passenger plane across the Atlantic Ocean without directly burning fossil fuels.

Although the flight did indeed use ​“100 percent sustainable aviation fuel,” the U.K. Advertising Standards Authority deemed the claim to be misleading. Listeners might assume that the fuel itself was ​“100 percent sustainable” and thus had zero environmental impacts, the agency said in August, when it ultimately banned the ad in the country.

The anecdote illustrates a complicated reality about the bid to decarbonize air travel, which today represents nearly 2.5 percent of the world’s annual carbon dioxide emissions. Airlines are increasingly clamoring about SAF, both within the industry and in splashy advertising campaigns. But the exact definition of SAF remains confusing and contested.

Since aviation experts consider SAF to be the ​“only viable solution” to decarbonize air travel in the near term — albeit not a perfect one — you can expect to start hearing this acronym a lot more often. Here in the United States, the Biden administration has invested heavily to build out a domestic SAF industry, and that trend is poised to continue during the second Trump administration.

“There is bipartisan support for biofuels in general, and SAF will be able to take advantage of that,” said Alexander Laska, the deputy director for transportation and innovation at Third Way, a center-left think tank. ​“I think we’re going to see that dynamic continue to play out over the next few years.”

So, with SAF on the rise, let’s review some of the key details that passengers, investors, and other decision-makers should know about this squishy industry term.

1. What is SAF, and where does it come from?

“Sustainable aviation fuel” is an umbrella term for liquid fuels that are made without crude oil. These alternative fuels can ​“drop in” to existing aircraft and engine fuel systems, distribution infrastructure, and storage facilities — unlike electric planes and hydrogen-fueled jets, which are in much earlier stages of development and would require airlines to overhaul operations.

Nearly all of the world’s current SAF supply is made up of a certain type of biofuel: hydroprocessed esters and fatty acids, or HEFA, which can be derived from feedstocks such as virgin vegetable oils, animal fats, and used cooking oil. HEFA is one of seven fuel pathways approved for use by ASTM International, the aviation industry’s standard-setter. 

Other forms of SAF can include biofuels made from corn ethanol, landfill waste, agricultural scraps, algae, and sugar beets, as well as synthetic fuels (or ​“e-fuels”) made with carbon dioxide and hydrogen.

Virgin Atlantic’s fully HEFA-fueled flight was notable because, with a few exceptions, planes generally aren’t allowed to fly using only alternative fuels. SAF must be blended with fossil jet fuel to ensure that everything remains compatible with fuel systems worldwide. ASTM limits how much SAF suppliers can blend in, capping it at 10 to 50 percent of a given fuel mix, depending on the feedstock.

That hasn’t been a problem for the industry yet. Among the airlines that use SAF, most burn the equivalent of a few drops on any given flight, owing to the tiny production volumes. In 2023, global SAF production reached 159 million gallons (600 million liters) — representing just 0.2 percent of total jet fuel use.

A major barrier to wider SAF adoption is the cost. Alternatives are currently two to 10 times more expensive than fossil jet fuel, depending on what they’re made from and how, according to the U.S. Department of Energy.

But airlines, which operate on thin profit margins, are trying out ways to cover those extra costs. A handful of European carriers, including Virgin Atlantic, are starting to raise ticket prices for certain routes, adding surcharges of anywhere from $1 to $78 per flight. Airlines can also sell SAF ​“certificates” to corporations, which allow third parties to count the carbon reductions associated with the fuel toward their own climate goals.

2. How does the aviation industry define ​“sustainable”?

Alternative jet fuels contain carbon molecules, so burning them still generates planet-warming emissions. And making SAF can be quite energy-intensive.

However, regulators generally consider fuels to be sustainable if their lifecycle greenhouse gas emissions are lower than those of fossil jet fuel. That means that the entire process of growing or procuring feedstocks, transporting them to refineries, processing them into SAF, loading fuels onto planes, and burning them in flight ultimately pumps less CO₂ into the atmosphere than does the entire start-to-finish process of using conventional fuel.

When Virgin Atlantic crossed the ocean using only SAF, it produced the same amount of in-flight carbon emissions as any other plane. But on a lifecycle basis, the demo flight reduced net emissions by 64 percent, according to the airline.

More specific criteria for SAF vary slightly by agency or organization.

The Biden administration has stipulated that SAF should reduce lifecycle emissions by a minimum of 50 percent compared to conventional jet fuel. And fuels made from food crops — e.g., corn ethanol — can qualify for federal SAF tax credits under the current guidelines if they adopt additional measures to limit CO₂ emissions during production.

The International Civil Aviation Organization, a specialized United Nations agency, says that SAF should reduce lifecycle emissions by at least 10 percent. The International Air Transport Association, a trade group, doesn’t consider fuels made from food crops to be sustainable.

Meanwhile, the Sustainable Aviation Buyers Alliance says SAF should reduce emissions by at least 60 percent, and it includes all feedstocks except palm oil. The alliance is a joint initiative of the Environmental Defense Fund and RMI, a clean energy think tank. Earlier this year, the groups helped land commitments from private businesses like Meta and Netflix to purchase close to $200 million in SAF certificates, equal to about 50 million gallons of fuel.

“We think 60 percent is enough to keep pushing the market forward, without making the hurdle unattainable,” said Mark Porter, a principal at RMI, whose colleagues joined the Virgin Atlantic demo flight for research purposes.

“It’s trying to strike a balance, and over time ideally we can nudge that up,” Porter said of the emissions threshold.

3. What are other environmental concerns with SAF?

While lifecycle emission data is useful, such figures don’t always reflect the full picture when it comes to assessing climate and environmental impacts.

Consider the greasy goods collected from restaurants and fast-food joints. Until very recently, waste oils and fats were mainly used for other purposes. If a batch of beef tallow would’ve been made into truck fuel but becomes SAF instead, there’s no net benefit for the climate; the emission reductions simply shift from one sector to another. If used cooking oil that could’ve been used to make animal feed is used in planes, then feed-makers might turn to virgin vegetable oil, resulting in more emissions from the farming of corn or soy.

About a third of total U.S. biofuels production came from the remnants of french fries, burgers, and the like in 2022. Nearly all the rest came from soy, corn, and canola oil, and production of those feedstocks is expected to climb this decade. While proponents say that’s good for boosting rural economies, the environmental implications are complex. 

Clearing forests and grasslands to grow crops can destroy habitats and increase emissions throughout the SAF supply chain, as carbon stored in soil and plants is released. There’s also a risk of diverting cropland away from food production, which leads to further environmental destruction as more land is cleared to grow food. Ensuring that crop-based fuels avoid these pitfalls will require rigorous accounting and careful scrutiny at a global scale.

“Some of these alternative fuels are very problematic environmentally,” said Dan Lashof, the U.S. director of World Resources Institute. ​“Other options could be sustainable. So it really depends on the feedstocks and the process that’s used to produce the alternative jet fuel.”

Little of that nuance is reflected in ads or promotional materials about SAF, which is why more skeptical observers say they’re concerned that greenwashing is taking hold within the aviation industry. ​“Airlines at this point are treating anything that is not petroleum jet fuel as [sustainable],” Lashof said.

4. What could be some better long-term options?

Environmental groups note that less intensive approaches for making SAF include using biomass waste, such as sawdust from lumber mills, branches and bark collected during forest-thinning, and the corn stalks and leaves left over after harvest.

Very little, if any, SAF is made today using these feedstocks, though there’s plenty of opportunity. The Department of Energy estimates that the U.S. has around 350 million dry tons of ​“currently available but unused” biomass such as agricultural waste and logging residues. That’s more than all of the biomass (waste or otherwise) that the U.S. uses annually for transportation fuels and industrial heating.

Synthetic fuels are often hailed as the most promising version of SAF. Today, the technology for making these so-called e-fuels remains highly expensive, energy-intensive, and unproven at large scale. But if successful, next-generation SAF could deliver much deeper lifecycle emissions reductions than fuel made using crops or organic waste.

The startup Twelve, for example, is among a growing number of companies working to make SAF from carbon dioxide and hydrogen. Twelve takes CO₂ and splits it into carbon monoxide using electricity. A separate electrolyzer then splits water molecules into H2 and oxygen. The resulting ​“syngas” is converted into a liquid — one that’s chemically identical to jet fuel made from petroleum.

In September, Twelve said it raised $645 million in funding to scale production of its ​“E-Jet” fuel, and it’s working to complete its first commercial-scale facility in Moses Lake, Washington. Today, the company uses CO₂ captured from ethanol refineries and other industrial facilities, though eventually it could pull carbon from the atmosphere through direct air capture.

5. How soon will SAF production scale up?

A solution that represents just 0.2 percent of the world’s jet fuel use won’t do much to reduce the industry’s climate impact — especially not when aviation emissions are projected to soar in coming years as more passengers take more flights.

But fuel companies and governments are pushing to dramatically ramp up both the production and adoption of alternative jet fuels.

In the United States, projects representing more than 3 billion gallons’ worth of SAF production capacity are targeted to come online by 2030 — enough to satisfy over 10 percent of projected U.S. jet fuel demand. The country will need to produce some 35 billion gallons of SAF per year by 2050 in order to meet 100 percent of the U.S. industry’s fuel needs, according to the Department of Energy.

While America’s airlines have adopted voluntary targets, in other regions operators will soon be required to get at least some of their fuel supply from alternative feedstocks.

Fuel blends used at European Union airports must contain a minimum share of 2 percent SAF starting in 2025, rising to 70 percent in 2050. Airlines operating in China, the world’s second-largest aviation market after the U.S., will need to use fuel blends with at least 2 percent SAF by 2025, increasing to 15 percent by 2030.

As airlines and fuel producers double down on SAF, they’ll need to communicate more clearly so that consumers and regulators can understand what this transition does, or doesn’t, mean for tackling CO₂ emissions. Companies must strike a difficult balance between touting their genuine achievements — like crossing the Atlantic Ocean without burning fossil fuels — and misleading customers by overstating their environmental benefits.

“This is a movement, and we’re really still getting our arms around how we’re going to make this thing happen at a massive scale,” said RMI’s Porter. ​“It’s important for consumers to understand that we are charting a flight path, if you will, toward a zero-carbon aviation system, but that’s not going to happen today, and it won’t be tomorrow. It’s going to take a while to get there.”