Can the world’s jet fuel ever be green?

Under pressure to reach net-zero greenhouse gas emissions by 2050, airlines are experimenting with so-called sustainable aviation fuels, or SAF. The most advanced contender is biofuel, mainly made from cooking oil, animal fats, agricultural crops and unused wood, though it is a struggle to use on more than a small fraction of flights because of the lack of supply and the high costs. In the coming decade, different types of alternative fuels could make flying even cleaner.

SAF are imitations of kerosene, the refined petroleum that has burned in jet engines for decades. Different types of SAF exist, but only biofuel is in use today. Biofuel reduces emissions by up to 80% versus conventional jet fuel. International standards allow carriers to burn up to 50% biofuel with kerosene, but a recent test from U.K. engine maker Rolls-Royce and energy company Royal Dutch Shell showed that 100% biofuel is safe.

SAF are in their infancy. From 2016 to 2020, roughly 300,000 out of around 188 million total flights world-wide used SAF—less than 0.2%, according to the International Air Transport Association.

Still, they are key to cutting aviation’s environmental footprint, industry experts say. Aviation has driven 3.5% of the world’s human-made greenhouse gas emissions, according to a September study by Manchester Metropolitan University. Air traffic, the second biggest source of transport emissions after roads, is expected to grow more than threefold by 2045 compared with 2015, though that doesn’t account for the pandemic-related stall in demand, according to the International Civil Aviation Organization.