Today’s airplanes -- and the carbon dioxide they produce -- aren’t a good fit with the clean energy future envisioned to fight global warming. Last year, even with Covid-19 travel restrictions, more than 1.5 billion barrels of aviation fuel were burned, a figure set to more than triple by 2050 as more people take to the skies. Today, that fuel is almost exclusively petroleum-based, but the airline industry is backing a potential solution that means we can all travel without “flying shame.” It’s called sustainable aviation fuel, or SAF for short. Right now though, it’s extremely expensive and there isn’t enough being used to make any meaningful impact on emissions.

1. What is SAF?

There are two main types:

• BIOFUEL: This is fuel made from either biomass materials (such as plants or wood), or waste and residue products (such as used cooking oil and animal fat). It can be made through a process known as the Hydroprocessed Esters and Fatty Acids, or HEFA approach, which is likely to be the basis of most SAF production this decade. While some biofuels are controversial because of concerns about deforestation and competition with food crops, the switch to SAF will not require large amounts of land or impact food or water use, according to the Air Transport Action Group, which represents the aviation industry.

• SYNTHETIC FUEL: This is a clean, engineered version of today’s crude-oil based hydrocarbons. They’re made by combining carbon with green hydrogen -- which is hydrogen fuel created with renewable energy -- to create a new fuel. Production of SAF using this technology does not yet exist at scale.

2. How is it used?

Once SAF is made -- both the bio-based and synthetic kind -- it can be mixed with oil-based jet fuel and safely used in today’s planes with a blend ratio limit of up to 50%, set by the American Society for Testing and Materials, a standard-setting body. Planemaker Boeing Co. aims to certify its aircraft to fly on 100% SAF by 2030. Today though, where it’s used at all, SAF blend ratios tend to be tiny. In California, airports are estimated to be blending, on average, 0.1% of SAF into their fuel supply, according to the International Council on Clean Transportation. This year, SAF consumption is estimated to be about 120 million liters, according to the International Air Transport Association, the industry’s global trade group. That’s only about 2,000 barrels a day -- a drop in the ocean of the global jet fuel industry.

3. How much carbon can it really cut?

Using SAF could dramatically reduce emissions, though by precisely how much depends on blend ratios and how it’s made. Finland’s Neste Oyj has invested in plants in Europe and Asia to produce bio-based jet fuel and diesel. The company estimates that, in neat form, its SAF cuts lifecycle greenhouse gas emissions -- which take into account the fuel’s production, as well as its consumption -- by up to 80%. Synthetic SAF can be CO2 neutral. So far though, high prices and limited supply mean SAF’s impact on pollution is negligible. SAF made from biomass costs about three times more than conventional aviation fuel, synthetic versions even eight times more, according to a February report from Eurocontrol, an intergovernmental organization focused on air traffic management. Using SAF could also reduce the wispy clouds planes sometimes leave behind them in the sky -- known as contrails -- which can also contribute to global warming by trapping warmer air in the lower atmosphere.

4. What about other technologies?

Aviation is seen as one of the most difficult industries to decarbonize, though there are several technologies in the works. Electric planes are in development, but the big problem remains the weight-to-energy ratio of batteries, which is much lower than that of jet fuel. Short-range, fully electrified civil aircraft carrying 100-150 passengers are optimistically expected to enter service by 2035-2040. Hydrogen-powered flight is another option. It’s backed by Airbus, though the planemaker has said it only expects to bring a hydrogen aircraft into service by 2035.

5. Is there more to decarbonizing air travel than changing the fuel?

Yes. The industry is focusing on improvements in design, operation and infrastructure. Carbon offsetting and carbon capture are also seen as potentially significant parts of how the industry will hit its target of halving net CO2 emissions by 2050 compared with 2005. The International Civil Aviation Organization, a UN body that develops policies and standards, has also established a carbon offsetting program known as CORSIA. This is currently a voluntary effort though it’s set to become mandatory in the coming years, with some exceptions. Offsets are also controversial, with uncertainty over whether promised reductions are actually happening.

6. Who is betting on SAF?

Energy companies and airlines are lining up to back SAF. Neste is modifying refineries in Rotterdam and Singapore, giving it the capacity to produce about 1.5 millions tons of SAF per year by the end of 2023. TotalEnergies SE has started bio-jet production at its La Mede refinery in southern France. Essar Oil UK Ltd., which operates the Stanlow refinery in northwest England, is part of a group of firms planning to produce about 80,000 tons a year of SAF. Royal Dutch Shell Plc has invested in LanzaJet Inc., which is building an alcohol-to-jet facility in the U.S. state of Georgia. Shell is involved in multiple SAF projects, including one with Rolls-Royce Holdings Plc to develop engines that can run on 100% SAF. On the consumption side, IAG SA, the parent of British Airways, has said it will commit to powering 10% of its flights with SAF by 2030 and has committed to net zero CO2 output by 2050. Southwest Airlines Co. has partnered with Marathon Petroleum Corp. and Phillips 66 to facilitate the development and production of SAF.

7. What’s the target to clean up flying?

Before the pandemic hit, CO2 emisions from aviation were about 2.4% of anthropogenic carbon dioxide emissions -- including land use change. With air passenger numbers set to recover and then surge in the years to come, that figure is all but certain to rise. The 2050 target of IATA could soon be upped to net-zero carbon emissions. ICAO also has a target of carbon neutral growth for international aviation from 2020. Meanwhile, the European Union is pushing for more regulation, with a raft of measures spanning various sectors set to be announced. The U.K. also plans to begin including international aviation for the first time in its carbon budget -- which is a cap on the amount of greenhouse gases emitted over a five-year period.

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