If you’ve caught an ad for an airline lately on TV, a podcast, or the entertainment display on your flight, you’ve probably heard the company brag about what it wants to do about climate change.
Major airlines like American, Delta, Southwest, and United have all set targets of achieving net-zero greenhouse gas emissions by 2050. They’re using a suite of tactics including buying more fuel-efficient aircraft, electrifying their ground vehicles, and increasing the efficiency of their operations. They’re also testing the winds on battery- and hydrogen-powered planes, as well as some radically different aircraft designs.
However, in terms of where they expect to make their biggest gains, airlines have begun to arrive at a common strategy: Classic carbon offsets are out, and sustainable aviation fuel is in. But the long haul toward sustainability is just beginning.
Offsets were once the default way airlines cleaned up their greenhouse gas emissions. Some even let you check a box and pay an extra fee to offset your emissions when you booked your flight. But they’ve found out the hard way that trying to negate their emissions by simply paying someone else to plant trees or install solar panels doesn’t deliver results. In 2020, United Airlines became one of the first carriers to cut these types of offsets out of its climate plans. United Airlines CEO Scott Kirby told Politico in May that “the majority of them are fraud.”
Passengers aren’t buying it, either. Over the summer, a group of flyers filed a class-action lawsuit against Delta. The airline said in 2020 that it had become the “the first carbon neutral airline on a global basis.” But plaintiffs say that claim is based on buying carbon offsets that failed to deliver. Environmental activists this summer filed a similar suit against KLM alleging greenwashing. The United Kingdom’s Advertising Standards Authority censured Ryanair, Lufthansa, and Etihad for misleading or overstated sustainability claims. Some airlines have since dropped their environmental ad campaigns.
Now the new hot ticket is sustainable aviation fuel, or SAF. The aim is to develop a fuel that can easily swap in for conventional hydrocarbons, but that is produced with sources like crop waste that emit little to no carbon dioxide on balance. “SAF is the most critical lever in achieving our net zero goal,” Southwest writes on its website. In 2021, United flew the first flight, from Chicago to Washington, DC, with one engine fully powered by SAF. Boeing, NASA, and United recently announced they’ve begun measuring the pollution from burning SAF. The Inflation Reduction Act is giving SAF a boost with nearly $250 million in competitive grants and tax credits.
However, demand is high and supplies are minuscule, making SAF upward of four times as expensive as conventional fuel. For most airlines, fuel is already their biggest or second-biggest expense.
So airlines are stuck in a holding pattern where their cheapest and easiest option for decarbonization doesn’t really work while their best bet is still wildly expensive. The consulting firm McKinsey estimated that decarbonizing aviation would require $175 billion in investments each year until 2050, almost $5 trillion in total. At the same time, demand for flights is poised to climb, and the window for keeping warming in check across the planet is sliding shut. Companies are also bracing for more emissions regulations as governments turn their attention to the climate impacts of aviation.
Air travel is thus one of the most difficult climate change problems, and some airlines are candid that they don’t quite know how to land at net zero. “We do not have the solutions we need at scale in a commercial capacity to really change the way we fly without focused, concerted engagement by the industry to demand a different way of flying,” said Lauren Riley, United’s chief sustainability officer. “Our emissions will go up before they go down.”
That’s why they’re trying out a suite of tactics across both incremental and breakthrough advances. But technology alone won’t clean up the skies; it will also take pressure from policymakers and passengers to get to the destination of net-zero.
Why airlines are banking on sustainable aviation fuel
Aviation produces about 2.5 percent of global carbon dioxide emissions. If you add in the heat-trapping effects of water vapor in contrails and other pollutants, air travel accounts for about 4 percent of the warming across the planet induced by humans to date.
It may seem like a small slice of the climate change problem, but if left unchecked, aviation’s carbon dioxide emissions are poised to double by 2050. “The biggest problem is that demand is rising,” said Gökçin Çınar, an assistant professor of aerospace engineering at the University of Michigan. Even as aircraft grow more fuel-efficient per passenger, more flyers will lead to more greenhouse gases.
However, decarbonizing aircraft is a huge technical challenge. To get a 400,000-pound airliner off the ground, up to 35,000 feet, and across an ocean, you need to squeeze an enormous amount of energy into a tiny space at the lowest weight possible. This trait is called specific energy, which is measured in watt-hours per kilogram or megajoules per kilogram. The best lithium-ion batteries top out around 300 watt-hours per kilogram. Jet fuel has a specific energy around 12,000 watt-hours per kilogram. And as an airplane burns fuel, it gets lighter, increasing its efficiency and range, whereas a dead battery weighs just as much as a live one. Swapping a gasoline engine for an electric motor in a car is trivial in comparison.
So it makes sense that manufacturers and airlines are starting out by imitating conventional fuels with SAF. The added advantage is that it can almost seamlessly drop into existing fuel tanks or blend into conventional fuel. It lets airlines continue using their existing planes, which is great for them because aircraft have operating lifetimes measured in decades.
What is it that makes SAF sustainable?
Instead of making fuel from fossil sources that increase the overall amount of carbon dioxide in the air when you burn it, you can harvest carbon that’s already in circulation. Plants, for instance, take in carbon dioxide from the air as they grow, so if you burn a fuel made from a plant, it merely returns its carbon to the atmosphere without increasing the overall total.
Right now, SAF is made from agricultural waste, lumber, algae — even used cooking oil. These feedstocks are in short supply, though, which is a big reason why SAF is so expensive. Over the long term, manufacturers want to make SAF with a technology called power to liquids. Using renewable electricity, they will take captured carbon dioxide and renewable hydrogen to assemble synthetic jet fuel. One perk is that this variety of raw materials expands the fuel supply chain. Power to liquids are particularly versatile since carbon and hydrogen can be harvested anywhere instead of being extracted and refined in a handful of locations like most fossil fuels. That’s why armed forces, including the US military, are also investing in sustainable fuels.
There are some important caveats to SAF. “It is not ‘zero emissions’ but ‘net-zero emissions’ when you look at the life cycle,” Çınar said. “It’s still the same combustion process.” Burning SAF produces carbon dioxide, as well many of the same pollutants as conventional fuels like nitrogen oxides and soot.
The carbon accounting can also get tricky. It takes energy to refine, process, and transport SAF. If that energy comes from burning fossil fuels, SAF will lead to a net increase in greenhouse gas emissions. And if you clear wilderness to plant biofuel crops to make SAF, that also increases its carbon footprint. Nevertheless, the overall climate impact is still far smaller than conventional fuels.
SAF needs a lot more investment to get off the ground
The focus with SAF now is to get the supplies up and the costs down. Right now, SAF accounts for less than 0.1 percent of global jet fuel and is on track to grow to 4 percent by 2030.
United Airlines is casting lines across the SAF supply chain. It’s invested in companies working on power to liquids, ethanol-based SAF, fuels from microalgae, biofuel refining, and converting trash to fuel. It’s also looking for ways to scale up SAF with existing technologies. Last year, United procured about 3 million gallons of SAF, roughly 0.8 percent of its total fuel use. “I don’t need the 10 million gallon sustainable aviation fuel plant. I need 5 billion gallons of sustainable aviation fuel for United — and we’re one airline,” Riley said.
Incentives and regulations make up the other side of the equation. “Our heads are down on the policy discussions right now,” Riley said. “The market will not get kicked off without the right policies.”
In the US, lawmakers have responded with carrots. The Inflation Reduction Act awards a $1.25-per-gallon credit for SAF blends that reduce emissions by at least 50 percent relative to existing fuels. (Currently, regulators limit SAF to a maximum of 50 percent of a fuel blend.) But in Europe, airlines are responding to sticks. The European Union earlier this year set a mandate requiring SAF to make up at least 6 percent of fuel pumped at airports inside the bloc by 2030, with the share rising to 70 percent by 2050.
So while US airlines are thirsty for SAF, European carriers are downright parched. Air France-KLM already uses more than four times as much SAF as United. Even airlines that don’t have commitments to decarbonize have their eyes on SAF because they expect to fly in the airspace of a country that does impose emissions restrictions.
This heated competition for a new fuel that’s currently very scarce is part of why airlines are also looking at alternative propulsion technologies likely to bear fruit decades from now. United is betting on companies developing batteries, hydrogen engines, and electric vertical takeoff aircraft. And while they aren’t relying on standard carbon offsets, they are investing in companies capturing carbon dioxide directly from the air as a way to balance their books on emissions.
Yet United is also investing in high-speed air travel, which burns vastly more fuel per mile than slower flights. In 2021, the company became the first US airline to order aircraft from Boom Supersonic, a company developing jets that fly faster than the speed of sound. United placed an order for 15 of the dart-shaped aircraft, with an option for 35 more, and aims to begin carrying passengers by 2029. High fuel costs were a major reason why the last supersonic airliner, the Concorde, left service.
Can United then advance on its climate goals and fly faster, thirstier jets? “I’m asking the same thing myself,” said Çınar, the Michigan professor.
The company says its investment in high-speed flight is predicated on using SAF. “Yes, it will require a decent amount of sustainable aviation fuel to power supersonic, but we can’t afford to do it any other way,” said Riley, the United executive.
Cloudy skies lie ahead for clean aviation
The airline industry operates on single-digit profit margins. Air travel often goes through boom and bust cycles, like what happened during the Covid-19 pandemic. Carriers also have shareholders who want to see returns every quarter. Rising interest rates are making it harder to borrow money to make big investments in sustainability. So it’s hard for airlines to keep their eye on climate targets decades away when the business climate en route is so turbulent.
The attention on SAF isn’t new, either. “Airlines have been talking about SAF for 15 years or so and we’re still stuck at one-tenth of 1 percent,” said Dan Rutherford, the aviation program director at the International Council on Clean Transportation.
Getting more SAF into fuel tanks requires more than airlines and governments pushing for it; major oil companies have to get involved and make commitments as well. Only then will the supplies increase, prices drop, and the dirtier alternatives become less appealing. “I’m personally looking for an economic breakthrough,” Rutherford said.
The other complication is the passenger. Air travel demand is somewhat elastic, with price being the major determinant. But a growing number of vocal travelers have also expressed shame about flying, even sacrificing their careers to avoid booking a ticket. For most people, air travel forms the largest share of their carbon footprint. Some travelers are looking for alternatives, and as the recent lawsuits show, they’re taking airlines to task for their commitments.
That’s why airlines are not just investing in sustainability but also working hard to convince you, the traveler, not to feel so guilty about your flight. It’s a reminder that passengers often do have a choice in how they fly, and their decisions can sway the market. But it will take the whole crew — airlines, manufacturers, investors, fuel companies, regulators, travelers — to ensure that sustainable aviation takes off.