Can Flying Be Made Carbon Neutral?

Aviation presents unique challenges to decarbonization. And while there are viable solutions, we are running out of time to scale their deployment.

by Christian Roselund
January 2020

It is a long way from Plymouth, England, to New York; 1500 nautical miles. This summer it took the Malizia II 15 days to make the voyage, a trip that many major airlines complete in less than seven hours. But that wasn’t the point for the 60-foot racing yacht’s world-famous activist passenger, Greta Thunberg. Instead, this was the most dramatic manifestation of her commitment to avoid flying due to the carbon footprint that it imposes.

Mass air travel is one of the marvels of our civilization. It has allowed for more rapid exchange of ideas and cultures, and a global interconnectedness that has shaped our modern world. But this year global aviation will release 1 gigaton of CO2, and will be responsible for somewhere between 3 and 9 percent of total human-made emissions. This includes the effects of radiative forcing, a phenomenon that describes the increased effect of releasing short-lived greenhouse gases like water vapor and NOx in the upper atmosphere.

While this is dwarfed by other sources of emissions such as personal automobiles and the electricity sector, the climate movement has zeroed in on flying as a symbol of wasteful and destructive excess. One factor may be that the wealthiest individuals make up a disproportionate share of air travel.

Another reason is the perception that aviation is unnecessary. This is nuanced. On one side, remote communities rely on airplanes to deliver supplies and connect to the outside world, businesses have found it difficult to replace face-to-face meetings, and economies and trade flows have built up around air travel. On the other side, a large portion of air travel is simply for leisure, and this makes it unlike other sources of emissions such as steel, cement, and commuting in automobiles to work.

In Sweden, a new word has been coined: “flygskam” or “flight shame” to describe the social pressure that is mounting to avoid aviation. And this is having real-world effects, with an increasing number of people taking other forms of transportation such as trains in Thunberg’s native Sweden and Germany.

But can air travel be decarbonized? Even more than other “hard-to-abate” sectors, the answer to this is complex and nuanced. There are several options and at least one very promising technology but it will require an unprecedented rate of change to make serious emissions reductions in aviation before 2030—the date by which we will need to dramatically reduce greenhouse gas emissions from current levels in order to stay below 1.5° Celsius of warming.

Electricity and Hydrogen

At a technical level, aviation isn’t like other forms of transportation. While ground transportation and shipping require sufficient force to overcome inertia and ground and air resistance, the physical feat of getting a container off the ground and through the air presents a relatively unique set of challenges.

Specifically, the challenges of lift (getting off the ground) and drag (moving through the air) can make solutions that work for other forms of transportation impractical, to a greater or lesser degree. Additionally, aviation is often used for distances that exceed that of daily commutes, and these distances present an additional challenge.

Similar to electric vehicles (EVs), battery-electric designs for planes have seen a lot of momentum in the last few years. In December, Greg McDougall, the CEO of North America’s largest seaplane airline, took a five-minute flight in British Columbia in a six-seater seaplane powered solely by a lithium-ion battery. The Harbour Air flight has been seen as a breakthrough for electric aviation, and Bloomberg reports 170 different electric-powered airplane projects in development globally.

Photo: Harbour Air

Harbour and all of the other projects researched by Energy Transition Magazine show batteries powering propellers, in contrast to the jet engines which dominate medium- and long-distance commercial aviation. But beyond this, there is a technical limitation of lithium-ion batteries that is the same whether such batteries are powering automobiles or planes.

Compared to liquid fuels such as gasoline or jet fuel, lithium-ion batteries have a much lower energy density per unit of weight. Despite technical advances in recent years, this still means that there are limits to EV range, even if these are often within the acceptable ranges for most drivers. But limits to the ranges that planes powered by batteries can fly are more of a problem for mass deployment.

The Energy Transition Commission (ETC) estimates that the energy density of lithium-ion batteries would need to increase 5X to accommodate intercontinental flights. “Until and unless there is a major breakthrough in battery gravitational density, battery-powered planes will not be a feasible alternative for long-haul/international flights,” states ETC in its report series Mission Possible. The physics of flying also means that lack of energy density limits the number of passengers that can feasibly be carried.

Hydrogen power, either in the form of fuel cells to power propellers or burning hydrogen in converted jet engines, is another option. Both designs are practical as a means of propulsion. However, while hydrogen has great energy density in terms of weight, hydrogen gas takes up a lot of space per unit of energy delivered.

This isn’t a problem for an automobile or a truck, but for a plane, things are different. Bodies moving through the air encounter resistance relative to the area that they occupy, meaning that bigger objects encounter more drag and end up using more fuel. As such, it is hard to find space for both passengers and/or freight along with fuel in hydrogen-powered airplanes.

Startup ZeroAvia has found a way around these issues, with a “mobile hydrogen refueling vehicle” that allows the company’s planes to be refueled mid-air. ZeroAvia says that it can conduct flights up to 500 miles, but this is still far short of even a mid-range commercial flight.

In its Mission Possible report, ETC states that both battery-electric and hydrogen planes could play a role over shorter distances. This could mean the “bush plane” routes in remote places in Canada, Alaska, Africa, Siberia, the Amazon, and the Australian Outback that are not accessible by road.

Harbour Air is not waiting. Working with technology supplier magniX, the company is beginning the governmental approval process to switch its fleet of 53 planes over to electric drives.

Harbour expects this to take two years, and for smaller companies this delay could be challenging. Adam Klauber, a technical advisor to Rocky Mountain Institute’s (RMI) Industry Program, notes that after the recent issues with Boeing’s 737 MAX, regulators are likely to be even more cautious about new designs. “It will take a while before the certification catches up with the technology,” explains Klauber.

Biofuels and Flight

But while hydrogen and batteries may excite the imagination of technologists and even get supplies to remote villages, they are not going to replace a 150-passenger flight from London to New York any time soon.

In contrast to ground transportation, most of the action in aviation to date has centered not around electric drives, but around biofuels. This is a broad term for any substance that came from a living organism, with feedstocks that include everything from corn to used cooking oil to agricultural wastes and inedible animal fat.

Perhaps the biggest advantage is that they can be made chemically almost identical to petroleum-based jet fuel, dropped in to replace conventional fuel, or mixed in to form a blend. This means little change to manufacturing or the operational practices of airlines.

And they may be better fuels, as well. Biofuel distribution leader SkyNRG estimates that the fuel it supplies is slightly more energy dense, with 90 percent less particulate pollution and none of the sulfides (SOx) that are produced from burning kerosene jet fuel. Overall, SkyNRG advertises a 1.5–3.0 percent improvement in fuel efficiency over conventional fuels.

Most major airlines have conducted at least one test flight with biofuels, but the total amount that is actually burned remains tiny, which is reflected by the limited production capacity. Both the Sustainable Aviation Fuel (SAF) supplied by SkyNRG and all other biofuels for jets comes from only one facility near Los Angeles, California, which is operated by World Energy.

This one plant can only produce 5 million gallons of SAF annually, which equals less than 0.01 percent of the 98 billion gallons of fuel consumed by major airlines in 2019. Misha Valk, the head of Future Fuels at SkyNRG, emphasizes the supply side of the equation, describing his company as a “market maker.” “When SkyNRG started, you were not allowed to fly on SAF,” recalls Valk. “There was no production, there was no demand. Governments didn’t know what it was.”

But there is also price to consider. And while airlines have shown no hesitation to reap the PR value of using SAF in test flights and even a few routes, they have been so far unwilling to pay the higher price for anything more than token amounts of biofuel, which RMI puts at two to three times the current cost of petroleum-based jet fuel. Valk notes that only a few airlines have been responsible for much of the tiny portion of SAF demand that exists, noting the role of KLM and United Airlines.

For biofuels, there are a number of other questions. The predominant feedstock for the SAF produced by World Energy and distributed by SkyNRG is used cooking oil, which does not come with the sustainability issues of other sources such as palm oil. It also avoids the food vs. fuel concerns that have plagued biofuels and biomass more broadly.

However, there is still the question of emissions. Burning carbon-based fuel, whether biofuels, petroleum, coal, or gas-based, releases CO2 and other greenhouse gases into the atmosphere. Proponents of biofuels argue that this CO2 is re-absorbed by the crops that are harvested to make the biofuels, making them effectively carbon neutral.

“The key difference lies in the source of the carbon,” states SkyNRG on its website. “Fossil fuels release additional carbon that was previously stored in reservoirs. SAF recycles CO2 emissions that were emitted previously and subsequently absorbed from the atmosphere during biomass production.”

However, such an analysis avoids the question of what would happen to the feedstock of the fuel if it were not burned, and whether or not it would sequester carbon, or potentially be partially converted to a more potent greenhouse gas such as methane. “The CO2 impact very much depends on what feedstock you’re using,” notes Cate Hight, a principal in RMI’s Industry Practice.

All of this leaves big questions about biofuels, which rely on assumptions about carbon cycles and often complicated calculations to support claims of being carbon neutral.

Fuel from the Air

There is another source of fuel that is clearly carbon neutral and offers similar “drop-in” advantages to biofuels. More than 100 years ago, German Chemist Freidrich Bergius developed a method to turn coal into liquid fuel, which was followed by other German scientists developing methods to convert carbon monoxide and hydrogen into liquid fuel less than 10 years later.

Making “synfuel” is a mature industry based on a well-known process, and today most synfuel starts off with methane as a feedstock in the steam reforming process. This is the dominant form of hydrogen production globally, with the resulting CO2 and H2 then converted to a liquid fuel. However, there is the option of using CO2 straight from the atmosphere, and H2 from electrolysis. If the electrolysis uses electricity from renewable energy, this results in a carbon-neutral process, where CO2 is taken from the air and returned to the air.

There are several plants currently pulling CO2 from the atmosphere, including facilities operated by ClimeWorks in Switzerland, Italy, and Iceland, and a pilot plant operated by Carbon Engineering in British Columbia, Canada. Both companies are currently working on projects to produce synfuel from carbon dioxide captured from the air.

Climeworks is participating in a study with Rotterdam The Hague Airport to study the technical feasibility of this concept, but Carbon Engineering is farther along, and is using some of the CO2 it captures in its Squamish plant to demonstrate the synthesis into fuel.

Geoff Holmes, who works in business development at Carbon Engineering, notes that its strategy attempts to use processes and materials that are well-known and abundant. “Our strategy and approach has been about using industrial precedent, and using equipment from other industries,” states Holmes.

But while the technical angles of this process are well-developed and uncontroversial, the economics are another matter. “As you might imagine, in a world where CO2 trades at $50 per barrel, the challenge here is about finding leading markets willing to pay for environmental benefit as well as for the commodity product itself,” explains Holmes.

Making synfuel from direct air capture is so new that it has not been studied the way that other technologies have. This is likely why Energy Transition Magazine was not able to find the sorts of studies of cost decline via scale that have been useful for solar photovoltaics, lithium-ion batteries, and hydrogen. ETC in its Mission Possible series gives a potentially large role to synfuels in coming decades, but doesn’t expect them to really take off at scale until the latter part of the 2020s.


If we can’t take the CO2 out of flying in the near term, we can at least reduce its intensity. There are several options to do this, and they range in terms of practicality. One of the first things to consider is that airlines have reduced per-passenger carbon intensity over the past few decades through improved engine efficiency, getting more passengers on to flights.

As such, trading in old aircraft is one possible route, and RMI’s Adam Klauber says that a “cash for clunkers” program for airplanes could help airlines to offset the cost of replacing older, less efficient planes. There is also room for operational efficiencies, with ETC identifying a potential to reduce emissions 9 percent through logistics and operations efficiencies, including regional air traffic management. However, RMI has noted that airlines are already packing as many passengers on planes as is feasible, and ultimately efficiency measures alone are limited.

A more radical plan in terms of efficiency is the potential to move to a “blended wing body” (BWB) design, wherein the wings and the main body of the aircraft are one unit. The design of BWB planes allows for increased lift and reduced drag, and NASA estimates that such planes could reduce fuel use by 20 percent over conventional aircraft.

Image: NASA

The novel look of these craft is also likely to excite a futurist aesthetic, but there are multiple barriers to this concept. First, this would mean much larger wingspans for the same numbers of passengers as carried by commercial aircraft today, meaning that airports would need to be reconfigured.

A potentially even more fatal concern is that the shape of BWB planes would require putting passengers farther from the center of the aircraft—essentially over the wing—to accommodate the same number of passengers. And when planes experience turbulence, they tend to rotate along a center axis. If you think you suffer during turbulence now, imagine if your seat was rapidly rising and falling ten feet.

RMI’s Adam Klauber notes that the BWB would be “fantastic for cargo,” however purely cargo planes represent a minority of total air travel, and airlines prefer craft that can be used for both passengers and air freight. With all of these barriers even Boeing, which has done substantial work with BWB aircraft, has stated that the company is unlikely to pursue mass deployment of this design for the time being.

Scaling Biofuels

There are still marginal gains to be made with efficiency, and a switch to battery-electric designs can decarbonize smaller aircraft along shorter routes. But all of this is nibbling around the edges of a bigger problem, and for the bulk of commercial flights most of the near-term action is expected to come from biofuels.

Like any emergent industry, policy has been critical for biofuel deployment in aviation. The most important policies identified by SkyNRG are the Renewable Energy Directive in Europe and the Renewable Fuel Standard (RFS) in the United States. The effects are particularly pronounced in California, where the combination of the federal RFS and the state’s Low Carbon Fuel Standard provides what SkyNRG’s Misha Valk describes as a very strong incentive.

It is no coincidence that many of the large biofuel deployments that have been announced in the United States are at San Francisco International Airport, as SFO has been working with airlines to deploy more biofuels.

However, the role of such incentives could be surpassed by much stronger action at the national level. Norway has mandated that airlines blend in a minimum of 1 percent biofuels in 2020, to increase to 30 percent by 2030. However, as the policy gives double credit to biofuels made from waste—which is the bulk of the current SAF supply—the mandate in practice means 0.5 percent in 2020 and 15 percent by 2030.

Norway’s 5.3 million residents are not going to move the needle much on global carbon emissions from air travel, but this could be the beginning of a bigger trend. Both Sweden and the Netherlands are looking at similar mandates, and both London’s Heathrow and Amsterdam’s Schiphol Airports have made public statements around requirements for biofuels.

“This is shifting from a voluntary market to a more mandated marked,” notes Valk.

Such actions could provide the market certainty to allow biofuel production to scale. Last May, SkyNRG announced plans for its SAF production facility in the Netherlands, which aims to produce 100,000 tons (33 million gallons) of SAF from waste streams (mostly used cooking oil and inedible animal fat) annually, starting in 2022.

Flight Shaming Vs. Demand Growth

But is the social movement to reduce flying working? Shorter-distance and domestic flights in Germany fell in 2019, which experts suggest may be due to flight shaming. However, the effect was not noticeable in international air travel. At the same time, the train business in Germany and Sweden is booming, with Sweden’s rail operator estimating that tickets are sold out six months in advance.

Taken together, these two trends suggest that where there is a viable alternative, some people will choose to take a lower-carbon alternative rather than fly. But while aviation officials have expressed concerns over flight shaming spreading beyond Europe, they have less to worry about in nations where alternatives are less practical. As anyone who has taken Amtrak knows, US train service lags well behind Europe, China, and Japan in terms of not only speed but also basic reliability.

Furthermore, the reduction in air travel due to flight shaming appears to be a smaller trend when set against a much bigger one: rising demand for air travel. Across the developed world more people have been flying every year, and every major analyst expects this to be supercharged as huge numbers of people in the developing world reach a level of affluence that allows for international air travel. Overall, ETC estimates that under a business-as-usual scenario aviation GHG will nearly double to 1.8 gigatons by 2050.

Graphic: Energy Transition Commission

The recent policy developments to mandate biofuels are promising, but the carbon accounting of these fuels still leaves questions as to the efficacy of this approach. Other solutions, including electrification and increased efficiency, are unlikely to make much of a dent in the largest segments of air travel.

“There is no quick fix,” notes Joris Melkert, a researcher at Technology University Delft in the Netherlands.

RMI’s Adam Klauber states that the best near-term solution is a “massive mobilization for investing in alternative fuels.” “Even though actual carbon reduction may largely occur after 2030, we have to mobilize now for 2030,” argues Klauber. Another option is to require airlines and their passengers to cover the costs of lower-carbon fuel—an option that will drive up the cost of flying in the near term, and doubtless encounter resistance from a portion of the most affluent people in every society and even national governments.

With few near-term options, the issue of aviation emissions is a runaway problem, and one that may cause us to rethink our basic assumptions. “Are we willing to pay more for flying, and how can we equip individual leaders who are willing to take on this burden in the short-run?” asks RMI’s Adam Klauber.

SkyNRG’s Misha Valk states that given the social and political pressure, there is no choice but to pursue rapid decarbonization. “In the end, there is no future for aviation if the industry doesn’t become sustainable.”