Lilium featured in latest edition: How green can we make air travel? And how soon?

A fact and a figure keep popping to mind as I talk with aviation experts about whether commercial flight can ever go green. The fact is this: Everything you can think of that's spurring a green revolution on the ground will be of little help in the sky anytime soon. Solar panels, wind turbines, electric engines, high-storage batteries, hydrogen fuel cells, magnetic levitation-they are all, bluntly put, useless at present when it comes to the technological challenge of launching a few hundred people into the stratosphere and carrying them thousands of miles. Here's the figure: More than 80 percent of humanity has never flown at all.

How this fact and this figure relate to each other is the crux of the problem facing airlines and aircraft manufacturers as they take on the critical task of decarbonizing flight. Aviation can go green, but not soon and not as comprehensively as ground-bound transport. Gravity is a very stubborn thing. Yet how quickly the air travel industry does proceed could affect its image-and bottom line. As environmental advocates warn that flying makes an intolerably large contribution to climate change, the pace of progress in greening the sky may well lead travelers to question whether it's ethical to fly at all.

"Look, we simply have to get there," says Jennifer Holmgren, chief executive officer of LanzaTech, a company pioneering the development of aviation fuel from unorthodox sources such as waste to replace the standard kerosene jet fuel. "Everyone agrees: Airplanes simply can't keep flying around on fossil kerosene. But there is no magic solution to this problem."

To be clear, promising developments with zero-emission, battery-powered electric engines are already happening in one area of aviation, involving trips of limited duration and distance. George Jetson's flying car is indeed on the way, albeit with AI and not George at the controls. Airlines specializing in short hops with small planes will lead the way to electric flight.

But no battery yet invented can power, say, a Boeing 747 from New York to London. My favorite expert calculation comes from David Alexander, director of aerospace standards at SAE International, a transportation engineering association. He estimates it would take the juice of 4.4 million laptop batteries just to get airborne. Except that the jumbo jet could never leave the ground; the batteries would weigh seven times as much as the plane. Pound for pound, liquid fuel contains vastly more energy than even the most sophisticated battery in use today.

To the industry's credit, the average commercial flight will get greener every year, as it has consistently since the dawn of the jet age. "Evolution, not revolution" is a phrase I keep hearing, but the small improvements do add up. Today's jetliners are twice as fuel efficient and several times cleaner burning than their venerable ancestors. But that bright side obscures a darker one. The reality is that increasing air traffic outstrips such gains. On average, carbon emissions from flight keep contributing more to the problem of climate change, not less.

This is where the 80-plus percent figure comes in-an estimate by Boeing commonly cited in aviation circles. For the industry, it represents a huge untapped market and the hope that, as the pandemic abates, air traffic will resume its historical growth of about 5 percent a year. For so many of the more than 80 percent, affordable flights pose an opportunity for exploration and connection unthinkable not long ago. As someone who loves to fly and never tires of looking at landmarks below, clouds alongside, or stars above, I can't begrudge anyone the joy of flight.

At the same time, any journey in the skies warms the planet. Commercial aviation generally accounts for about 2.5 percent of all human-induced carbon dioxide emissions, but its true impact is far greater because of the warming effects of other pollutants and aircraft contrails, and the complex way these emissions linger and interact in the atmosphere. Some experts peg air travel as the source of up to 5 percent of the human contribution to global warming today.

That figure will likely climb as passenger and freight air traffic grows while ground transportation and other activities such as construction become far more energy efficient. All this has led to a movement urging people not to fly or at least to fly a lot less, a campaign with a name that has caught on in Europe and is becoming familiar elsewhere: flygskam, a Swedish term best translated as "flight shame." For teenage activist Greta Thunberg and others doing the flygskam-ing, the case is simple but compelling.

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"Hour for hour, there is just about nothing you as an individual can do that's worse for the health of the planet than to sit on an airplane," says Peter Kalmus, an astrophysicist turned NASA climate scientist who hasn't flown since 2012 and is the founder of noflyclimatesci.org, which showcases testimonies from scientists and others to fly less or not at all. "The hard fact that most people haven't accepted yet is that we don't need to fly, and if you truly accept that we are in a climate emergency, you shouldn't fly."

In July, France adopted a ban on all domestic air trips that can be made by train in less than two and a half hours. In the United Kingdom, the official Committee on Climate Change jolted the elite world of the most active fliers by proposing "a ban on air miles and frequent flyer loyalty schemes that incentivise excessive flying."

But trains versus planes is a straw man: Three-quarters of aviation fuel is used for trips of longer than 1,000 miles. At those distances, most people will opt to fly. Thunberg spent 15 days sailing across the Atlantic to make a point before she addressed the United Nations, but most people who need or want to travel across an ocean will do so by airplane-or not at all.

In that sense, flygskam is as much about the decision to travel as it is about the decision to fly. Aviation leaders contend that shaming flight is not the answer-greening it is.

"Aviation is an essential part of the global economy, so our challenge is reducing emissions and decarbonizing aviation, not preventing people who want to travel from traveling," says Sean Newsum, the director of aviation sustainability strategy for Boeing. "That's really our foundational belief as an industry at this point."

Among the potential paths to green salvation for air travel, the quickest might be down a gravel road deep in the woods of central Georgia, leading to a hulking complex of pipes, pumps, tubes, tanks, and steel girders called the Freedom Pines Biorefinery. There I meet Curt Studebaker, a lanky, friendly young chemical engineer who is in the business of turning waste-all kinds of waste-into aviation fuel.

"The amazing thing is, once you get it right, it's really a better fuel even than Jet A," the standard kerosene fuel in U.S. aviation, Studebaker tells me. "It's actually cleaner."

Studebaker's employer, LanzaTech, could impress mad scientists anywhere: By capturing carbon-heavy emissions from a Chinese steel mill, mixing them with fast-chomping microbes originally discovered in the guts of a rabbit, adding water and nutrients, then allowing the mash to ferment like a vat of beer, it created ethanol, which it refined at the Georgia plant.

The result: so-called sustainable aviation fuel, or SAF. Blended with Jet A, LanzaTech's invention powered a Virgin Atlantic Boeing 747 from Orlando, Florida, to London in 2018.

For now, SAFs, as just about everyone in the industry calls them, are still blended with standard fuel. But they are cast as the giant first step toward shrinking aviation's carbon footprint. The reason is simple: The tube-and-wing planes of today are expected by the airlines to last for two or possibly three decades. Their business plans depend on it. So even as engineers are thinking ahead to future generations of aircraft that may have radically different design features, or run on a different energy source, the thousands of planes already in the skies will be up there for a long time-and they will fly on liquid fuel.

Those jetliners will get somewhat more efficient as engines are replaced by newer models, or when mileage-improving enhancements are installed, such as the winglets, "sharklets," or "scimitars" that now adorn the wingtips of many jets. But the most effective way to make them fly cleaner? Change the fuel.

With SAFs, carbon savings occur over their life cycle. Whether derived from agricultural by-products such as sugarcane stalks, or from industrial waste, or even municipal landfills, the SAF sequesters or consumes carbon early in the cycle, ultimately making it a much lower net emitter of carbon than fossil fuel.

And because it's a "drop-in" fuel, with relatively minor engine modifications needed, it wouldn't require new airport infrastructure, as alternatives to liquid fuel would.

The challenges? One, it's very expensive. Although more flights are using SAFs, it's a veritable drop in the bucket, well under 0.1 percent of the 95 billion gallons of fuel the industry used in 2019. This alternative fuel costs two to six times more than kerosene. Second, the industry can't rely on the easiest, cheapest sources for conversion: crops. If fuel producers were to gobble up land and water more urgently needed for food, air travel would simply trade one environmental black eye for another.

So the industry has zeroed in on other promising sources, such as the waste LanzaTech converts to energy and halophytes, salt-tolerant plants that can be irrigated with seawater.

In a patch of desert along the Persian Gulf in the United Arab Emirates, oilseed plants known as Salicornia bigelovii feast on waste generated by fish and shrimp at an aquaculture farm. When ready for harvest, Salicornia seeds are separated and crushed to oil, then refined and mixed with kerosene to become alternative jet fuel.

Proponents contend that if SAF production were built to the scale needed to serve the bulk of aviation needs, the price would drop precipitously, becoming competitive with kerosene. But getting to scale is a classic chicken-or-egg dilemma. Unless there's demand, supply won't grow; but because the current supply is so small and costly, it's hard to stimulate demand. That's where the problem becomes political: The solution could be a carbon tax on kerosene or a requirement that SAFs account for a percentage of all aviation fuel.

"Basically, there has to be a humongous ramp-up to SAFs," says Paul Stein, chief technology officer of Rolls-Royce, the British manufacturer whose next-generation UltraFan, the biggest and one of the most efficient jet engines ever, is designed to use the alternative fuel. "But industry is generally behind a SAF mandate. And certainly our position as a company is, yes! We need more SAFs. It would be a huge contribution to the planet."

At Airbus headquarters in the south of France there is a flying machine made of composite materials resembling no airliner that has ever taken to the skies, at least outside of science fiction movies or UFO sightings. In broad shape it resembles a bulbous manta ray. For a future passenger, a voyage in this Star Trek-like machine would be akin to flying in a movie theater.

No one will do so soon, though. The plane, known as Maveric, is a model aircraft with a 10.5-foot wingspan. For Airbus, the European consortium, Maveric's design could hold the answer to this intriguing question: Is there a more efficient-greener-way to design an airliner?

For all kinds of reasons, the modern aircraft manufacturing industry does not easily lend itself to the disruption that can so suddenly upend conventional thinking in other industries. A true game changer of an airliner will take many, many years to develop and more years to weather the gauntlet of safety tests involved in certification for commercial service.

Yet the so-called blended wing body design employed by Maveric-although with major technical challenges to overcome-could yield as much as a 40 percent reduction in carbon emissions compared with today's planes. The main advantage of the streamlined design is that the entire aircraft functions much like a wing, reducing drag and making it much easier to generate lift. In the Netherlands, researchers at Delft University of Technology used similar principles in designing Flying-V, an aircraft that looks very much like a boomerang.

Airbus created a major stir in the industry last year by announcing it was working on a line of aircraft with a 15-year timeline to service and a stunning capability: zero-emission flight. A Maveric variant and two smaller tube-and-wing airliners, it said, would operate on hydrogen fuel. The main by-product? Water vapor.

As is true with electric automobiles, zero emission doesn't necessarily mean zero pollution. Just as it matters where the electricity comes from to charge the car's battery, Airbus's approach begs the question of how to create and store hydrogen fuel.

Most hydrogen used today comes from fossil fuels. But so-called green hydrogen, in which electricity is used to separate water into hydrogen and oxygen, is the holy grail. Advocates say that technological progress and scaling up will bring green hydrogen its day in the sky.

But there's another complication: Liquid hydrogen, such as that used in the U.S. space program, needs to be super-compressed and kept at cryogenic temperatures of minus 423°F to remain a liquid, which obviously requires significant energy. Conversely, in gaseous form, hydrogen would take up a huge amount of space in an airplane because the fuel tank would need to be much larger to yield the same amount of power as kerosene.

In either instance, a hydrogen-fueled airliner would be significantly different from today's planes, and airports would need new infrastructure to deal with it. Airbus acknowledges the hurdles but remains upbeat about the prospects.

"We strongly believe hydrogen is the necessary clean fuel to develop for aviation, because it's not just a question of reducing carbon dioxide emissions, it's about eliminating them altogether," says Amanda Simpson, vice president for research and technology at Airbus Americas. "If you get it from green hydrogen produced with electricity from sustainable sources-that's about as clean as you can possibly get!"

Boeing, which flew the world's first fully hydrogen-powered airplane in 2008, an experimental two-seater, is publicly less bullish. This is not because it questions hydrogen's potential but because the fuel is not the answer for many years to come.

"Our analysis is very clear that for commercial jets, sustainable aviation fuels are the only viable solution to really decarbonize over the near term or even the middle term," says Brian Yutko, Boeing's chief engineer for sustainability and future mobility. "We need to keep our eye on the ball and influence the outcomes there."

The central California farm town of Hollister claims fame as the site of an annual motorcycle rally that inspired The Wild One, an early Marlon Brando hit. But these days it may be less notable for any roar in the streets than for a whisper-quiet device in the sky above the airport.Whirling around is a stubby, banana yellow aerial vehicle with 13 rotors-three on the front and back of each wing plus a larger pusher propeller in back. One more thing: There's no pilot.

The self-flying electric plane may be an oddity today, but its inventors expect it to be a commonplace of tomorrow-the aerial taxi. As more than one evangelist for the urban air mobility industry puts it, "Think: Uber meets Tesla in the sky."

Their company, called Wisk, is just one of many aspiring entrants, although with major chops: It has financial backing from Boeing and Kitty Hawk, the aviation start-up founded by Google's Larry Page. Its vision is a world in which taking a flying taxi will be as easy and affordable as an automobile ride is today-and safer to boot.

"This is not the Wild West," Gary Gysin, Wisk's chief executive, tells me when I visit the company's hangar. "We will absolutely meet the incredibly stringent safety standards already set for the aviation industry. We have to-nobody's flying anywhere until the FAA says so."

Gysin is referring to the Federal Aviation Administration's regulatory process, but his broader contention is that remarkable advances in battery technology and lightweight composite materials, as well as the low operating costs of electric engines, make the vision feasible.

Just when this particular industry might take off is, well, up in the air. But it's serious business: Wisk's plane is one of more than 475 electric vertical takeoff and landing aircraft under development, according to the Vertical Flight Society.

Wisk's plane and other rival flying taxis can do what a helicopter does: pick people up and put them down in places fixed-wing aircraft can't. Aside from being much cheaper to operate, they are much, much quieter-a critical advantage given that helicopter services often have been stymied by noise complaints.

Gysin says the industry likely will start by shuttling people among airports and "vertiports," which might be a landing pad atop a Manhattan apartment building or a parking lot in a Los Angeles suburb. But as time goes on and people grow comfortable with safe, quiet, cheap air mobility?

"We'll be picking you up on your front lawn," Gysin says with a grin.

I joke that I might book a round-trip flight to nowhere just to admire the bird's-eye view.

"Absolutely!" Gysin shoots back with genuine enthusiasm. "The fun is part of the plan. That's a share of the market." In fact, he adds, he was a bit taken aback when initial marketing research disclosed that along with assurances of a safe, smooth, quiet ride, one feature most requested by potential customers was good Wi-Fi.

"And I'm like, Really? Are you kidding me?" he says, miming a smack to his head. "Put the dang phone away and look out the window!"

Just how strong a public backlash to the idea of air taxis there might be, I can't say. But electric-powered flight, while still severely limited by battery weight and capacity, is happening on another front. One intriguing approach is in British Columbia, Canada, where a commuter seaplane operator is retrofitting its workhorse fleet of 60-year-old de Havilland Beavers and Otters, swapping out gas-fired piston engines for electric motors.

Greg McDougall, Harbour Air's founder and chief executive, piloted the December 2019 initial test run on the first such plane. (The company's application for electric operation is making its way through the labyrinthine regulatory process.)

"We're proud to be the first airline in the world to offer completely clean electric flight, fueled by our province's sustainable hydropower," McDougall tells me. "But I'm not doing this just because I'm some wild-eyed environmentalist hippie. I am a businessman. This is going to lower my costs, which is going to lower the cost of everyone's tickets."

Harbour Air is an ideal candidate for electric conversion: Its flights typically take less than 35 minutes, while the batteries can last at least an hour on a charge, offering ample reserve power.

Across the continent, Massachusetts-based Cape Air has embarked on a different milestone: It was the first airline in the world to announce plans to fly a brand-new, all-electric aircraft.

This plane, named Alice, is a streamlined, T-tailed, nine-passenger aircraft with twin propellers being built in Washington State by Eviation. The company's CEO, Omer Bar-Yohay, boldly if perhaps a tad hyperbolically predicts the plane will stake a claim to aviation fame right up there with the Wright Flyer and the Boeing 707.

"First we had powered flight, with propellers," he says. "Then we had the jet age. And now flight is entering the electric age."

Both Harbour Air's retrofit electric motors and Alice's new ones are built by magniX, a company in Everett, Washington. As with Harbour Air, Cape Air's routes, connecting Cape Cod with Boston and the nearby islands of Martha's Vineyard and Nantucket, are short hops. Dan Wolf, the airline's chief executive, says the electric power needed to charge Alice's batteries could come from Vineyard Wind, an offshore project under construction. That would make Cape Air's flights as clean as Harbour Air's.

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Yet Alice is also an illustration of the challenges of electric flight: At 8,200 pounds, the batteries alone account for 60 percent of the plane's empty weight.

Electric-powered flight may hold out hope that flight can go green. Enthusiasts say that within 15 to 20 years, electric airliners could be carrying as many as 50 people a few hundred miles.

But for major air carriers flying much longer distances with lots more passengers, electric flight will be a chimera for many years to come. Someday, our descendants will take zero-emission flight for granted. But the problem of what we do between now and then remains a hugely vexing one, because that day isn't coming anytime soon.

The National Geographic Society is committed to illuminating and protecting the wonder of our world. Learn more about the Society's support of its Explorers.

Sam Howe Verhovek is a Seattle-based writer and frequent contributor. Davide Monteleone is a photographer based in Zurich, Switzerland. This is his first feature for the magazine.

This story appears in the October 2021 issue of National Geographic magazine.