United Airlines said on Tuesday it has partnered with global firms including Nike and Siemens in an "Eco-Skies Alliance" to finance use this year of about 3.4 million gallons of low-carbon, sustainable aviation fuel derived from trash.

Though tiny compared with the 4.3 billion gallons of jet fuel that United consumed in 2019 prior to the start of the COVID-19 pandemic, the amount triples the roughly 1 million gallons of sustainable fuel it has used each year since 2016.

Airlines have used sustainable fuel since 2008 as part of efforts to reduce outright emissions, but so far this represents barely 1% of the fuel used worldwide, industry groups say.

Chicago-based United named 11 of more than a dozen global partners for the plan but did not disclose the cost, or how much each would contribute.

Air transport accounts for 2%-3% of greenhouse gas emissions, the French aerospace association said on Tuesday. Environmental groups argue the sector's overall contribution is higher.

Partners include companies with corporate or cargo deals with United, like Nike, Siemens, Palantir and Japan's Takeda Pharmaceutical Co.

United said the project gives customers a way to help reduce the environmental impact of flying beyond buying carbon offsets and could help create more of a market for sustainable aviation fuels.

"We'll see how it develops," Chief Executive Scott Kirby told reporters. "I think there's a huge appetite for it."

The airline industry has focused more broadly on the purchase of carbon offsets to reduce the environmental impact of flying, pending the arrival of new technology to meet the sector's goal of halving net emissions by 2050 versus 2005.

Environmental critics say offsets do not directly address climate goals and mask the problem of ongoing jet emissions.

United, which along with some other carriers has said it wants to cut net emissions more aggressively by 100% by 2050, has criticized offsets and announced a recent investment in "carbon-capture" technology. It has invested in a sustainable aviation fuel producer called Fulcrum BioEnergy.

"While we know that aircraft are never going to be completely decarbonized, we are not going to use offsets as the way to get to 100% green," Kirby said.

Airline association IATA says life cycle greenhouse emissions from sustainable fuel can be at least 80% lower than normal fuel and are the only medium-term option for curbing emissions growth, since airlines cannot yet switch to electric planes.

Delta Air Lines has said it plans to replace 10% of its jet fuel, currently refined from fossil fuel, with sustainable aviation fuel by the end of 2030.

While using waste avoids taking land from food production, environmental groups like Transport & Environment say such supplies are limited and face competition from other sectors.

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