Aviation accounts for a rising share of global greenhouse gas emissions – currently around 2 to 3 percent. Cutting that pollution is hard: Planes need energy-dense fuel, aircraft fleets turn over slowly, and electrifying or using hydrogen for long-haul flights still faces major technical hurdles.

Sustainable aviation fuel, or SAF, offers one option because it works in existing engines. The biofuel can be made from waste cooking oil, crop residues, or captured carbon combined with hydrogen. But the source matters: Growing crops just for fuel, for example, can replace natural habitat and compete with land and water needed for food.

For now, SAF supplies only a sliver of the market – less than 1 percent in the United States.

The organic matter in trash presents a promising option, tapping an abundant resource already produced in huge volumes, avoiding the land competition that dogs many biofuels, and diverting from landfills material that can emit planet-heating methane. Powering tomorrow’s flights with yesterday’s trash will not solve aviation emissions by itself, but this reliable, lower-emission feedstock could complement efficiency improvements and other decarbonization strategies.

In a paper this month for Nature Sustainability, my colleagues and I found that city waste could potentially meet about 10% of current U.S. jet fuel demand, according to our analysis of resource availability and conversion performance. Accounting for variation among countries in their potential SAF capacity, we estimate that global SAF production from municipal solid waste could exceed 60 billion liters per year – offering roughly a 16% reduction in aviation greenhouse gas emissions. For comparison, a Boeing 777 flying from Boston to London uses as much as 50,000 liters.

How it works

Even when burned as jet fuel, municipal waste carries a lower lifetime carbon footprint compared with fossil fuels extracted to use as fuel. This is because the organic matter in waste originates from biomass that absorbed carbon from the atmosphere when it was alive and growing. Converted into SAF, it essentially recycles atmospheric carbon, making the combustion process carbon neutral. The remaining emissions mainly come from the energy and materials used during production.

The process works like this: After separating out most plastics, ceramics and metals, a waste-to-fuel facility heats the biogenic waste in a chamber with limited oxygen and steam, breaking the mixed components into synthesis gas (largely carbon monoxide and hydrogen). The gas is then conditioned to adjust the hydrogen-to-carbon monoxide ratio and cleaned to remove remaining impurities. Next, that syngas is converted into liquid hydrocarbons using a catalyst under the Fischer-Tropsch synthesis process. Then it is refined into a jet fuel stock that is compatible with today’s aircraft and fueling infrastructure.

Our life-cycle assessment indicates that waste-derived SAF can cut greenhouse gas intensity by 80-90 percent compared with conventional jet fuel. The main technical challenge is in pre-treating the complex mixture of waste and efficiently converting it into synthesis gas.

Introducing hydrogen can lower emissions further. Gasification produces syngas with a carbon monoxide-to-hydrogen ratio of about 1:1, while Fischer-Tropsch synthesis ideally needs 0.5:1. By adding green hydrogen, the process can boost carbon intake about 150% and cut overall life-cycle emissions.

This isn’t cheap. To make the economics work, waste-to-fuel facilities require a stable feedstock supply, capital-intensive gasification and synthesis systems, and guaranteed demand. Policy support is therefore crucial. In the U.S., programs such as the federal Renewable Fuel Standard, state Low Carbon Fuel Standards, and tax credits targeted at SAF and clean fuels can close the cost gap and make projects investable. Where these incentives are aligned with municipal waste contracts and airline purchase agreements, waste-based SAF can become competitive at scale. Where they are absent, projects struggle to pencil out.

A practical wedge for cleaner skies

Electrification and hydrogen may transform parts of aviation in the long run, especially short routes. For medium- and long-haul flights, drop-in SAF will remain a tool for decarbonization well into the foreseeable future. Waste-based SAF offers a way to grow that fuel supply without expanding cropland, while leveraging a resource available everywhere.

All perspectives expressed in the Harvard Climate Blog are those of the authors and not of Harvard University or the Salata Institute for Climate and Sustainability. Any errors are the authors’ own. The Harvard Climate Blog is edited by an interdisciplinary team of Harvard faculty.