United Airlines will buy clean jet fuel made from recycled carbon dioxide under a deal inked today with the Houston, Texas-based startup Cemvita.

Before the decade ends, the company will start supplying United with one billion gallons of clean jet fuel over a 20-year period, or 50 million gallons a year, said Cemvita CEO Moji Karimi.

Sustainable aviation fuel (SAF) is a catch-all term for lower-emitting liquid fuels (usually 60 to 70% fewer greenhouse gas emissions) that can replace or be blended with traditional and dirtier jet fuels made from kerosene in jet engines.

The United-Cemvita agreement is the largest deal to date between an airline and a SAF producer that uses this novel method. The company uses engineered microbes to convert carbon dioxide captured either directly from the air or from industrial plants and a mix of wastes into jet fuel, according to Cemvita .

Currently, most SAF is made with a mixture of water- and land-intensive vegetable oils, waste fats and cooking oils that are in limited supply. The microbes method could offer a more reliable and sustainable feedstock for clean jet fuel, said Karimi.

An analyst at RMI said the United deal sends a clear signal of support for Cemvita that can draw investors that would have otherwise been scared to take the risk on a newer approach.

The aviation sector is responsible for about 10% of greenhouse gas emissions in the U.S. and 2 to 3% of global emissions.

Despite numerous airlines committing to use SAF, it’s currently supplying a miniscule amount: less than 0.1% of global jet fuel consumption, according to the International Energy Agency.

President Biden set a goal of producing 3 billion gallons of SAF a year by 2030. The U.S. is on track to produce an estimated 1.7 billion gallons a year by then, about half of its goal, according to Hartej Singh, aviation and chemicals program manager at nonprofit think tank RMI.

“We need several more plants, such as the one United inked with Cemvita, if we truly want to make a dent in U.S. ambitions by 2030,” said Singh.

For Cemvita, the deal means it is able to re-use CO2 that would otherwise be emitted into the atmosphere, contributing to global warming.

United was the first company to introduce SAF into its normal operations back in 2016. A year ago, Cemvita was among the first companies to be funded by United’s venture fund.

Because 98% of United’s emissions come from jet fuel, the fund focuses on innovative approaches to SAF production. “That’s why we are excited about Cemvita,” Andrew Chang, managing director of United Airlines Ventures, told Cipher. “We love their microbiology approach.”

LanzaJet, a spinoff of Illinois-based LanzaTech, is the only other U.S. company that uses microbes to make SAF and is about to finish building a commercial plant in Georgia. The plant will make up to 9 million gallons of SAF annually for which LanzaJet has already inked a deal with two airlines, British Airways and ANA.

Cemvita is in the process of negotiating agreements with investors to build a commercial demonstration facility and a full-fledged plant no later than 2029, Cemvita Commercial Director Phil Garcia told Cipher.

“The size of the agreement with an investor like United tells the Cemvita story way better than we ever could,” Garcia said.

Jet fuel accounts for 98% of emissions for United Airlines, not 90% as reported earlier.

Editor’s note: LanzaTech received funding through Breakthrough Energy Catalyst, a program of Breakthrough Energy, which also supports Cipher.