• A wide range of technologies and innovations are being designed in a bid to tackle aviation's environmental footprint.
• This represents a major task, even if the number of flights last year slumped due to the coronavirus pandemic.

From the Wright brothers' historic flight in 1903 to the development of supersonic aircraft, the history of aviation has been driven by technology and ambition.

Now, as the 21st century progresses, the sector continues to show its appetite for innovation and radical design.

Last September, for instance, a hydrogen fuel-cell plane capable of carrying passengers took to the skies over England for its maiden flight.

The same month also saw Airbus release details of three hydrogen-fueled concept planes, with the European aerospace giant claiming they could enter service by 2035.

More recently, United Airlines announced it had signed a commercial agreement to purchase aircraft from a start-up called Boom Supersonic.

In a statement, United said the Overture aircraft — which is yet to be built — was set to be "optimized to run on 100% sustainable aviation fuel."

These moves are linked by a focus on technologies designed to reduce aviation's environmental footprint — a major task, even if the number of flights last year slumped due to the coronavirus pandemic.

According to the International Energy Agency, carbon dioxide emissions from aviation "have risen rapidly over the past two decades," hitting almost 1 gigatonne in 2019. This, it notes, equates to "about 2.8% of global CO2 emissions from fossil fuel combustion."

Elsewhere, the World Wildlife Fund describes aviation as "one of the fastest-growing sources of the greenhouse gas emissions driving global climate change." It adds that air travel is "currently the most carbon intensive activity an individual can make."

A variety of solutions

In a phone interview, Iain Gray, director of aerospace at the U.K.'s Cranfield University, described zero carbon as "the top priority" for the industry, and he sought to emphasize the importance of developing a range of solutions to tackle the challenge.

"The really big technology driver is around the propulsion system," he said, "but that doesn't take away from the importance of new technologies around … new lightweight materials, enhanced carbon composite materials, and the systems itself."

Expanding on his point, Gray provided an example of why the innovations on planes flying above our heads should not be viewed in isolation. 

"There's a lot of effort goes into reducing the weight on an aeroplane for it only to spend half an hour circling an airport," he said.

"So the whole interaction of air traffic management with the aircraft itself is a … very important development and new technologies on airspace management are emerging all the time."

The power of propulsion

Alongside the development of hydrogen fuel-cell planes there's also been a lot of discussion around electric propulsion in recent years, with firms such as Volocopter and Lilium developing eVTOL, or electric vertical take-off and landing aircraft.

The key with technologies such as these is the types of journeys to which they can be applied.

"If you look at hydrogen fuel cells and you look at batteries, that really is very much aimed at the smaller aircraft, that's the sub 1,000 kilometer range," Gray said.

"You have to do that in a zero carbon way, there's no question," he added. "Is that going to make a big difference to the overall CO2 contributions that aviation makes? No."

"We need to focus on the longer-range flights, flights greater than 1,000 kilometers, flights greater than 3,000 kilometers in particular."

Fueling change

This focus on long-haul trips will be important in the years ahead, even though they make up a small proportion of flights.  

According to a sustainability briefing from Eurocontrol published earlier this year, "some 6% of flights from European airports were long-haul" in 2020 — over 4,000 kilometers (around 2,485 miles).

The intergovernmental organization went on to state that "more than half of European aviation's CO2 emissions were from this tiny proportion of the overall number of flights."

This viewpoint was echoed by Jo Dardenne, aviation manager at Transport & Environment, a campaign group headquartered in Brussels.

"We shouldn't forget that the biggest chunk of aviation emissions are linked to long-haul flights because you fly longer, you fly higher," she told CNBC.

"So all in all you're producing more CO2. … Those long-haul flights can only be decarbonized by replacing the kerosene that they're using."

It's on these longer journeys that sustainable aviation fuel could have a significant role to play in the future.

Although the European Union Aviation Safety Agency says there's "not a single internationally agreed definition" of sustainable aviation fuel, the overarching idea is that it can be used to reduce an aircraft's emissions.

Aircraft-maker Airbus describes sustainable aviation fuel as being "made from renewable raw material." It adds that the most common feedstocks are based on crops or used cooking oil and animal fat.

"Currently, the big challenges of sustainable aviation fuel are producing it in the right volumes that are required, and at the right cost point," Gray said.

The provenance of feedstocks used for sustainable fuel is also important, he said. "If what you're doing … to produce sustainable aviation fuel is transporting fuel right across the world using feedstocks from the other side of the planet, then is it really sustainable?"

"The big effort at the moment is looking at how you can produce sustainable aviation fuels in a …  green way." This could be fuel from waste or local resources, Gray added.

One type of fuel generating interest is e-kerosene, which also goes by the name of synthetic kerosene. According to a briefing from T&E published in February, e-kerosene is produced by combining carbon dioxide and hydrogen.

"What's great about it is that it can be dropped into these jets without any modification of the engine and of the technology of the plane," Dardenne said.  

"It's a carbon-neutral fuel, it's something that can be easily dropped in," she added. "The only problem is that it's very expensive."

Driving cost down will indeed be key in the years ahead, but organizations like T&E are keen to emphasize the potential environmental benefits of using it.

If the CO2 is "captured from the atmosphere" and hydrogen produced using renewables, T&E says "the combustion of e-kerosene will, apart from some residual emissions, be close to CO2 neutral."

The future

While technology may be developing, the world also needs to come up with rules and regulations focused on the environmental footprint of air travel. 

Examples of these efforts include the Carbon Offsetting and Reduction Scheme for International Aviation and the European Union including carbon dioxide emissions from aviation in its emissions trading system since the year 2012.

In her interview with CNBC, T&E's Dardenne stressed the importance of "proper regulation."

"If you price emissions and pollution effectively, then mandate the use of clean technologies, you send the right signals to investors, private and public, to invest in them," she said. 

"The clearer the regulatory framework the more certainty you can provide to the market that these technologies will have a future," she added.

"And that will actually bring added value, financial added value, as well as environmental added value."

Looking at the bigger picture, she said that "proper regulation" would come via effective carbon pricing and fuel mandates, describing the latter as an obligation to use clean fuels. These were, she argued, "the cornerstone of effective aviation decarbonization strategy."