Sustainable Aviation Fuel

Sustainable aviation fuel, or SAF, is aviation fuel made from alternative feedstocks that can reduce greenhouse-gas emissions compared with petroleum-derived jet fuel. Its climate advantage is assessed through lifecycle emissions, covering feedstock production or collection, fuel manufacture, transport and final combustion, rather than aircraft exhaust alone.

Why the carbon source matters

Conventional jet fuel transfers carbon from underground petroleum reserves into the atmosphere. SAF can reduce this addition of fossil carbon by using biomass, suitable wastes or captured atmospheric carbon as its carbon source.

• For biomass-derived fuel, plants absorb atmospheric carbon dioxide while growing; this uptake can offset the carbon dioxide subsequently released when the fuel burns.
• Fuels made from suitable wastes and residues can use existing material streams rather than require dedicated energy crops, although collection and processing still generate emissions.
• Synthetic fuels made using captured atmospheric carbon dioxide and low-emission hydrogen can recycle atmospheric carbon, provided their energy-intensive production uses sufficiently low-carbon energy.

How lifecycle savings are calculated

A lifecycle assessment compares emissions from the complete SAF supply chain with those from conventional jet fuel on a consistent basis, usually per unit of energy. SAF combustion still releases carbon dioxide; the potential saving arises mainly from its carbon source and upstream production pathway.

• The calculation includes feedstock cultivation or collection, processing, fuel conversion, transport and combustion.
• Agricultural pathways must account for fertiliser-related nitrous oxide emissions, farm energy use and changes in soil carbon.
• Waste-based pathways must consider what would otherwise have happened to the waste, because avoided disposal emissions depend on that alternative.
• The reduction is the difference between conventional fuel and SAF lifecycle emissions, expressed relative to the conventional-fuel baseline.

Why the reduction is not automatic

The label SAF does not imply zero emissions or a uniform percentage reduction. Results depend on the feedstock, production technology, energy supply and accounting assumptions.

• Forest clearance or other land-use change can release stored carbon and substantially erode the benefits of crop-based fuels.
• Fossil-powered processing, carbon-intensive hydrogen and long transport chains can reduce lifecycle savings.
• Strong sustainability safeguards and credible lifecycle accounting are therefore essential to establish a genuine benefit.
• Lower lifecycle greenhouse-gas emissions do not eliminate aviation's wider climate effects, including those associated with contrails.