Flying high above the clouds, Lufthansa flight 8400 to Shanghai is like no other. 

It's carrying nearly 100 tons of cargo – including automotive parts, machinery and some pharmaceutical products.

But unlike other air freighters, which have a costly environmental footprint, this plane is adding no CO2 emissions.

Once a week, Lufthansa Cargo operates one of the world's first regular flights powered by Sustainable Aviation Fuel (SAF). It's an expensive but greener alternative to traditional jet fuel.

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Lufthansa's flights use SAF made from bio waste such as leftover cooking oil and vegetables as opposed to the traditional fossil fuels. 

"It has grown as a plant, as a vegetable – taking the CO2 from the atmosphere just a few days ago and now putting it here on the aircraft, it goes back into the atmosphere," explained Lufthansa Cargo Vice President to Germany Achim Martinka.

Unlike other fossil fuel alternatives, SAF doesn't require planes to be modified in order to fly with it.

The dedicated Boeing 777 flying between Frankfurt and Shanghai can fill up with regular fuel or a more sustainable source if the destination airport offers it. 

Costs are high, supplies limited

However, there's a reason the world's airlines aren't rushing to integrate sustainable fuel into their fleets – supplies of SAF are limited and can cost airlines three to five times more than traditional fuel.

In fact, used cooking oil, the most popular SAF base, is trading at commodity prices close to or above the cost of new, unused cooking oil.

"We need many more producers because the more production we have, the better will be the prize at the end," Martinka said.

But scientists say, while increased production of bio waste-powered SAF is a good first step, it is not the final answer.

"We cannot go on with fossil fuels. We need to move to fuel that is sustainable," said Alexander Zschocke, a SAF researcher at CENA Hessen. "But if you look at bio waste, it's limited. If you want to use more, you drive up prices, which gives an incentive to produce waste."

Jet fuel out of thin air?

Zschocke is exploring the possibility of producing sustainable jet fuel out of thin air through a method known as "power to liquid."

Still in early research phases, power to liquid takes carbon out of the air and combines it with hydrogen through electrolyzed water.

"It could certainly meet all the aviation needs. The crucial factor here will be renewable electricity," Zschocke said.

There could eventually be enough of it to power all of today's flights. Yet, production at larger quantities is not expected for at least 10 years.