Sustainability has been a hot topic at this year's Paris Air Show, the world's largest event for the aviation industry, which faces increasing pressure to reduce the climate-changing greenhouse gases that aircraft spew.

The International Air Transport Association, an airline trade group, estimates that sustainable aviation fuel (SAF) could contribute 65 percent of the emissions reductions needed for the industry to hit its 2050 net-zero goal.

Aviation produces 2 to 3 percent of worldwide carbon emissions, but its share is expected to grow as travel increases and other industries become greener.

Sustainable fuel, however, accounts for just 0.1 percent of all jet fuel. Made from sources like used cooking oil and plant waste, SAF can be blended with conventional jet fuel but costs much more.

SAF vs conventional fuel

When it comes to flying, going green may cost you more. And it's going to take a while for the strategy to take off.

Very few flights are powered by SAF because of the limited supply and infrastructure.

Even the massive orders at the show got an emissions-reduction spin. Airlines and manufacturers said the new planes will be more fuel-efficient than the ones they replace.

But most of those planes will burn conventional, kerosene-based jet fuel. Startups are working feverishly on electric-powered aircraft, but they won't catch on as quickly as electric vehicles.

"It's a lot easier to pack a heavy battery into a vehicle if you don't have to lift it off the ground," said Gernot Wagner, a climate economist at Columbia University.

SAF to decarbonize aviation

"Sustainable aviation fuels, they are indeed the biggest technological potential to decarbonize the aviation sector, but the main problem, is that they are not available," said Dimitri Vergne, a senior policy officer at The European Consumer Organisation (BEUC).

"We know that before the end of the next decade, at least, they won't be available in massive quantities" and won't be the main source of fuel for planes, Vergne added.

Producers say SAF reduces greenhouse gas emissions by up to 80 percent, compared with regular jet fuel, over its life cycle.

Airlines have been talking about becoming greener for years. They were rattled by the rise of "flight shaming," a movement that encourages people to find less-polluting forms of transportation, or reduce travel altogether.

Some see sustainable fuel as a bridge to cleaner technologies, including larger electric planes or aircraft powered by hydrogen. But packing enough power to run a large electric plane would require a fantastic leap in battery technology.

Hydrogen must be chilled and stored somewhere, it couldn't be carried in the wings of today's planes, as jet fuel is.

"Hydrogen sounds like a good idea. The problem is the more you look into the details, the more you realize it's an engineering challenge but also an economics challenge," Richard Aboulafia of AeroDynamic Advisory, an aerospace consultancy, said at the Paris Air Show. "It's within the realm of possibility, (but) not for the next few decades."

Pushes on SAF

The issue gained urgency this year when European Union negotiators agreed on new rules requiring airlines to use more sustainable fuel starting in 2025 and rising sharply in later years.

Just before the Paris Air Show opened, President Emmanuel Macron announced that France would contribute 200 million euros ($218 million) toward a 1 billion euro ($1.1 billion) plant to make SAF.

Many airlines have touted investments in SAF producers such as World Energy, which has a plant in Paramount, California, and Finland's Neste.

United Airlines plans to triple its use of SAF this year, to 10 million gallons, but it burned 3.6 billion gallons of fuel last year.

(With input from AP)