On the other end of the spectrum, some crop-based biofuels can actually produce more carbon dioxide emissions overall than fossil fuels, Pavlenko says. That’s frequently the case for biofuels made from palm oil, since growing that crop can decimate rainforests. Even synthetic e-fuels can approach the impact of jet fuel if they’re produced using electricity from fossil fuels.
Today, most commercially available alternative jet fuels are made from fats, oils, and greases. If they’re derived from waste sources like used cooking oils, these fuels reduce carbon dioxide emissions by roughly 70% to 80% compared with fossil fuels.
It’s worth noting that while SAFs can approach net-zero carbon dioxide emissions, burning the fuels still produces other types of pollution, including other greenhouse gases and particulate matter. The fuels can also contribute to formation of contrails, which trap heat in the atmosphere.
What’s next for SAFs?
There are a few other technologies on the table for cutting climate impacts in aviation, including hydrogen– and battery-powered planes. However, without further technical progress, these options may be limited to smaller planes flying shorter routes, and most global carbon dioxide emissions today come from flights over about 900 miles (1,500 kilometers). That’s where SAFs could help. Alternative fuels are attractive for the aviation industry because they’re a drop-in solution, requiring little adjustment of aircraft and airport infrastructure. (Planes might need small adjustments to run on 100% SAFs in the future, depending on the mix of chemicals in the fuel.)
Many aviation net-zero plans, like the one published by the International Air Transport Association, assume that SAFs will make up the majority of the industry’s climate progress in the coming decades. Over the past year, several test flights powered by 100% SAFs have taken off. However, alternative fuels made up less than 0.2% of the global jet fuel supply in 2022. So there’s a lot of progress needed to supply alternative fuels that are actually helpful for the climate.
One of the main challenges to getting SAFs into the skies is expanding the supply. While fats, oils, and greases are the basis of most commercially available SAFs today, the world doesn’t eat enough french fries for used cooking oils to meet global jet fuel demand alone. In fact, even with increased collection, waste fats, oils, and greases probably won’t provide more than 5% of global jet fuel supply, Pavlenko says.
Some new biofuels, like those made from agricultural residue, municipal solid waste, and hardy crops like switchgrass, are starting to enter the market; a few facilities are under construction or producing jet fuel from these sources worldwide, and the carbon dioxide emission savings they achieve can range from 50% to 90%.
Recent policy moves in both the US and the European Union are aimed at boosting the market for alternative fuels. RefuelEU Aviation, a deal finalized in April, requires that fuel supply at EU airports include 2% SAFs by 2025 and 70% by 2050. The EU rule will only count SAFs from waste sources, advanced biofuels, and e-fuels, not crop-derived fuels. It also has a specific target for e-fuels that’s aimed at boosting their production.
The US, on the other hand, recently passed new tax credits for alternative fuels, aimed at helping expensive options reach price parity with fossil fuels. The tax credits last until 2027 and apply to any fuel that reduces carbon dioxide emissions by at least 50% compared with fossil fuels, though the details on how reductions are calculated haven’t been released yet.
Ultimately, alternative fuels present one of the most straightforward pathways to cutting the climate impacts from aviation, but only certain types will end up benefiting the climate. “SAFs are a solution, but they need to be very properly done,” Mirolo says. Otherwise, they risk becoming “a cure that’s worse than the disease.”