Stories
Nov 13, 2020

Greening the Friendly Skies

Aviation could contribute up to one quarter of the world’s entire carbon budget by 2050, but there's hope | by Mark Betancourt
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This story originally appeared in Eos and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.


The fight over Heathrow Airport’s third runway has been going on for decades. On the proposed site in a cluster of villages outside of London, residents fought the demolition of 750 properties and the addition of a lot more aircraft noise.

Climate activists—some of whom went so far as to occupy one of the existing runways—opposed the additional air traffic the new runway would bring to what is already the busiest airport in Europe and all of the carbon emissions that would come with it. By some counts, the new runway at Heathrow would increase emissions associated with the airport by 60%.

Construction was set to begin next year, with the runway becoming operational by 2026. Then, in February, a judge in the United Kingdom’s appeals court halted progress on the new runway on the unprecedented grounds that the project failed to take into account Britain’s obligations under the Paris Agreement, signed by the United Kingdom in 2016. (The United States, originally a party to the agreement, was set to withdraw from it on 4 November.)

Climate activists reveled in the win, but the decision hardly paves a way forward. The world’s goals for reducing aircraft emissions are still fuzzy, and strategies vary by country and region. Even keeping track of emissions, and deciding who is responsible for them, poses significant challenges.

And all of that was before the coronavirus pandemic plunged the aviation industry into a historic crisis. Global flight frequency dipped by as much as 70% by May and remains well below 2019 levels. Aircraft emissions have fallen proportionally, but the hiatus won’t last. When the pandemic ends, airlines, airports, and policymakers will find themselves at a crossroads.

Aviation accounts for 2.4% of the world’s carbon dioxide (CO2) emissions, and demand for flights is projected to double within the next 2 decades. If the industry’s current decarbonization goals aren’t met, aviation could contribute up to one quarter of the world’s entire carbon budget by 2050.

Experts agree that decarbonizing aviation will require investment—and bold leaps forward—in technology. It will also require unified action on the part of the world’s governments to guide market behavior. Even with those developments, aviation likely will be one of the last sectors to go green.

Carbon and Noncarbon Emissions

The first challenge is in measuring how much the aviation industry contributes to climate change. Burning jet fuel produces the same general cocktail of pollutants that burning other fossil fuels does, but aircraft have the added impact of injecting the pollutants directly into the upper troposphere, where they do more damage.

High-flying aircraft also form contrails, the streaks of frozen water vapor that often mark their paths across the sky. Both aerosols and soot produced by the incomplete burning of jet fuel provide surfaces on which water vapor can condense and freeze, creating a translucent barrier that traps heat the same way greenhouse gases do. Contrails in turn can become whole banks of cirrus clouds that further warm the atmosphere.

Contrails are short-lived in comparison with CO2, which can remain in the atmosphere for centuries. The clouds may last for only a few minutes or hours, but given that new ones are constantly appearing, their net effect on radiative forcing is believed to be roughly as large as that of carbon emissions.

Photo by WonHo Sung / Unsplash

But exactly how contrails affect warming is still poorly understood. Further complicating things, burning jet fuel produces non-CO2 chemicals that contribute to both warming and cooling in the atmosphere. The most significant are the nitrogen oxides (nitric oxide (NO) and nitrogen dioxide (NO2)), key ingredients in the formation of ozone, a greenhouse gas that can linger in the upper troposphere for a month or two. Nitrogen oxides produce more ozone at a jet’s cruising altitude than at ground level, which causes more warming. But they also have a cooling effect by helping to destroy methane, an even more potent greenhouse gas.

“The state of the science is too uncertain to really try and pin down all these short-lived effects of aviation,” said Piers Forster, a professor of climate physics at the University of Leeds in the United Kingdom, “and it’s also quite difficult to compare the short-lived effects of aviation to the long-lived effects of carbon dioxide.”

The International Civil Aviation Organization (ICAO), a specialized agency of the United Nations that has recently taken the lead in curbing global aviation emissions, doesn’t yet take noncarbon emissions into account. Forster thinks it should.

“[Non-CO2 aircraft emissions] do have an influence on increasing the surface temperature,” he said. He and his colleagues recently found that aviation emissions have a warming effect 3 times larger than that of the carbon component alone, with contrails and contrail cirrus clouds being the greatest contributors. “We ought to begin to think about ways of intelligently taking those effects into account.”

Who Owns Aircraft Emissions?

Measuring the impact of emissions is one challenge; deciding who is responsible for them is another.

After the Paris Agreement was signed, ICAO was tasked with curbing emissions from international flights, which emit roughly 65% of the sector’s carbon. The agreement leaves it up to signatory countries on how to reduce their total emissions, including emissions from domestic flights.

Some countries lump any portion of international flights that occur within their borders in with their domestic flights, even though the former is included under their commitment to ICAO. “Sometimes there’s double counting. It is not always clear what types of flights are included in the national aviation emissions,” said Jasenka Rakas, deputy director of University of California, Berkeley’s National Center of Excellence for Aviation Operations Research (NEXTOR), a multiuniversity aviation research center.

Individual airports are often owned and operated by local governments and have little power to curb emissions beyond their ground operations. Heathrow, which is privately owned by an international consortium, is one of a few companies and the first airport to receive accreditation from the Carbon Trust for greening its supply chain. But those efforts don’t touch the bulk of emissions at Heathrow.

S. Esenin
Photo by Iwan Shimko / Unsplash

According to Rakas’s current research, about 60% of the emissions associated with busy airports come from the landing and takeoff cycles of aircraft, including taxiing. Air traffic controllers can reduce delays that cause more fuel burn, Rakas said, but curbing overall emissions is mostly left up to airlines. “Airports don’t control those emissions directly because they don’t own those emissions.”

That’s partly because CO2 emissions are not like noise or smog. “The authority to introduce regulations at the local airport is restricted to [regulating] local pollutions that have local effects,” said Anna Elofsson, a doctoral student at Chalmers University of Technology in Sweden who specializes in climate policy for aviation. “The climate problem is not a local problem.”

In terms of jurisdiction and range of regulatory tools, most of the muscle for controlling aircraft emissions is at the national level—if governments know where to apply it.

Can Technology Decarbonize Aviation?

Airlines and aircraft manufacturers are already strongly incentivized to improve the efficiency of jet engines, as burning less fuel saves money (the industry currently gets about 2% more fuel efficient every year). Regulations requiring airlines to carry more passengers and freight per flight, to find more direct routes to destinations, or to swap old, fuel-guzzling aircraft for more efficient ones might squeeze out another half percent per year, according to the International Council on Clean Transportation (ICCT).

Some climate researchers also suggest changing flight patterns and schedules to mitigate contrailing. Jets produce more contrails at higher altitudes and latitudes, and flight paths could be adjusted to avoid those conditions. Contrails also contribute to warming only at night—during the day, they reflect more energy than they absorb—so fewer nighttime flights would help.

Forster suggests that real-time weather data, coupled with a powerful enough algorithm, could plot flight paths that would minimize contrails while also minimizing fuel burn, and therefore CO2 emissions. “From good weather forecasting and good technology, you ought to be able to have a look in real time if your aircraft is contrailing,” he said.

The carbon impact of producing fuel can also be improved, but converting to sustainable jet fuel won’t be easy. “Airplanes are very, very picky about what kinds of fuels they can take,” said Stephanie Searle, who leads the fuels program at ICCT. To power long-distance flights, jet fuel needs to be lightweight but energy dense.

The most immediately available option is carbon-neutral biofuels, in this case, liquid jet fuel made from energy crops or waste products rather than from fossil fuels. The technology to produce them is well established, and they can be used in existing aircraft without modifications. But, said Searle, “biofuels are sometimes a good thing, sometimes a bad thing.”

Food-based biofuels end up displacing actual food production enough to push farmers to clear more carbon-sequestering forests and grasslands to meet food demand. Because they often result in the clearing of high-carbon-stock tropical forest, palm and soy-based biofuels actually create more carbon emissions than fossil jet fuels do.

Biofuel production currently meets less than 0.1% of global jet fuel demand, partly because providing it to other sectors is cheaper. But there may simply never be enough of it. Searle estimates that using every grassland on Earth to grow energy crops would still meet only 8% of the world’s projected energy requirements in 2050.

It’s also possible to synthesize jet fuel, in a process called “power-to-liquids,” by electrolyzing hydrogen and CO2. As long as the electricity used is from renewable sources and the carbon is captured from existing emissions—say, from flue gas streams at a coal-fired power plant—the process can be carbon neutral, taking as much CO2 out of the atmosphere in production as is emitted when the fuel burns. But it’s a very inefficient process, said Searle. “If you’re lucky, you could end up with about half of the energy in your power-to-liquids that you started with in terms of electricity.”

Both carbon capture and renewable electricity are still prohibitively expensive, and Searle said they’re likely to remain so for the foreseeable future. No one is producing electrofuel at the commercial scale yet, but Searle estimates it would cost almost $600 per barrel to make, compared with around $90 for a barrel of fossil jet fuel (and that’s before COVID-19 drove the price down to around $40).

There are other technologies on the horizon. Hydrogen fuels—either for combustion in a jet engine or for use in generating electricity in a fuel cell—offer a lightweight option that emits water vapor instead of CO2. The technology is still in its infancy, however, and is limited by the same high-cost processes as fuel synthesis is. Hydrogen combustion engines would emit nitrogen oxides, leading again to more ozone production at altitude, and Forster points out that water vapor emissions may actually worsen the contrail problem.

Pilot filling up an SUU Aviation fixed-wing aircraft
Photo by Billy Clouse / Unsplash

“We really are in the phase now when we have to look for completely brand-new technology, like electric aircraft,” Forster said.

But the technology to replace more than a sliver of commercial aviation with electric aircraft is mostly theoretical. The main consideration is weight—by one estimate, an electric 747 airliner would require 10 times its weight in today’s best batteries.

That said, roughly one third of air passenger carbon emissions come from flights of 1,500 kilometers or shorter, and many of them are in small aircraft that could conceivably be fitted with electric or hybrid engines using existing technology. The Canadian airline Harbour Air Seaplanes, operator of the largest seaplane fleet in North America, has committed to replacing all of its aircraft engines with battery-powered ones.

Dan Rutherford, program director for marine and aviation at ICCT, said electric aircraft could replace only a fraction of aviation’s global fuel use, and that’s unlikely to happen for decades. “We’re a long way off,” he said. Meanwhile, aviation emissions aren’t even going in the right direction. “The evidence suggests that it’s very, very hard to absolutely reduce emissions when you have traffic growing 5% or 6% per year,” he said.

Air Traffic Control

Ultimately, the world likely won’t be able to meet its climate goals under the Paris Agreement without limiting the number of flights we take. It’s possible to do that physically by not expanding airport capacity. But without regional moratoriums, Rutherford said, individual airports like Heathrow would risk simply driving traffic elsewhere if they chose not to add new runways.

Barring a technological miracle, the most effective way to reduce the climate impacts of aviation emissions may be to disincentivize flying. Forster notes that it’s the only way to reduce all of those impacts in the immediate term. “You actually get 2 times the benefit,” he said, “because by changing behavior, you get rid of the contrails and you get rid of the CO2.”

Governments can change behavior—of airlines and of passengers—by putting a price on carbon. One way to do that is to set up cap-and-trade markets, which place a limit on how much CO2 a company can emit without facing hefty fines. The market allows those who emit less to sell their unused carbon credits—each representing 1 metric ton of carbon emissions—to those who emit more. The cap is then gradually reduced, causing an overall reduction in emissions.

The problem is that when cap-and-trade markets are open to many different sectors, carbon is drained out of sectors that are the least expensive to green. In Europe’s Emissions Trading System, for example, the power sector sells credits for around €25–€30. To earn a credit through flight emissions reductions, an airline would need to invest more than 13 times that amount in biofuel.

“All of the airlines that talk about biofuels, they’re not going to use biofuels, they’re going to buy the cheapest offsets,” said Rutherford. “[Cap-and-trade’s] track record is really using aviation to decarbonize the power sector.”

Meanwhile, as other sectors decarbonize, Forster and his colleagues estimate that aviation likely will contribute an increasingly larger proportion of the world’s anthropogenic radiative forcing.

A much more direct way to limit air traffic is simply to tax carbon emissions, a step many experts believe will be necessary to drive the level of change aviation needs. “The cost will be higher for fuel, and then you will push the efficiency improvements,” said Elofsson. “And also, the price of the tickets will be higher, and then the alternatives, like trains and closer destinations and so on, will also be more attractive in comparison.”

The tax would have to be globally or at least regionally consistent to prevent airlines from refueling only in tax-free countries. It would also be fair, Eloffson said, because only those who take flights, or who profit from them, would bear the cost. “Of course, with the CO2 tax or a fuel tax, it will be like it is right now, that the people with more money can fly more.”

Rutherford noted that two thirds of the flights in most countries are taken by between 12% and 15% of the population. “There’s this really interesting equity problem embedded in aviation emissions,” he said. “Only the globally rich fly.”

That’s particularly relevant given the demographics of the projected uptick in demand for air travel. The majority of new flights will originate in Asia, where the travel boom is partly the result of millions of people emerging from poverty into a burgeoning middle class. But the majority of emissions are coming from passengers who can afford to fly often, and there are more of them in rich countries.

Rutherford recommends a simple solution to that imbalance: a global frequent flyer levy. It’s essentially a progressive carbon tax, one that would charge passengers more the more they fly, while allowing occasional flights to remain affordable.

According to Elofsson, regulators are weighing the social benefits of increased access to aviation as well as its negative impacts on the environment. “It makes the world closer,” she said. “What kind of value is that for democracy, understanding, cultural development?”

The Silver Lining of Staying Home

Aviation is also arguably what got us into the mess of a global pandemic in the first place, scattering infected travelers from Wuhan, China, to the four winds in a matter of weeks. Realizing our mistake, we stopped flying, in turn creating one of the many strains the pandemic has put on the global economy.

But the sudden scarcity of aircraft in the skies provides a windfall for atmospheric scientists: a massive, free experiment in high-altitude contrail cirrus formation. Until now, the phenomenon has been described only by computer models; the pandemic provides an unprecedented opportunity to observe it directly. “We can compare the clouds in 2019, when we did have aviation, with the clouds today, when we do not,” said Forster.

Similar work was done during the eruption of Iceland’s Eyjafjallajökull volcano in 2010, which shut down air travel in Europe, and in the days after the September 11 terrorist attacks in the United States. However, “[those events] weren’t global, and they were only for a short time,” said Forster. “What we have this time is that we can begin to put together far better statistics because the change has been far more pronounced. We can hopefully be more conclusive this time.”

Researchers are currently poring over satellite imagery and launching instrument flights to observe the changes in cloud cover in areas with normally high air traffic, but it won’t be simple to interpret the results. “Clouds are so variable from one day to the next. You have to worry about the effects of local meteorology,” said Forster. “So you have to try and correct for all these confounding different ideas.”

Forster noted that it could also be possible to compare current surface temperature fluctuations from day to night with the same fluctuations last year, to observe whether nighttime contrail cirrus has a measurable effect.

As for regulation, the dip in traffic complicates fledgling efforts to set an emissions baseline for aviation or to predict where emissions are headed, said Eloffson. But because the aviation industry now needs government bailouts to recover from the pandemic, there is an opportunity to rebuild it greener than it was before.

“It’s not only the aviation sector; it’s all sectors. What kind of companies and businesses do we want to recover, and what is kind of belonging to the past? And what technologies are they wanting to support with all this money?” Eloffson asked. “That will also be very important—how brave the political leaders are right now.”

The question may be less one of bravery than one of priorities. “We’ve just seen governments move so fast on the bailouts, primarily with the desire to maintain employment as the major motivator, that environmental conditions largely haven’t gotten in,” said Rutherford.

Airport in the evening
Photo by Ashim D’Silva / Unsplash

At least in the near term, one thing that can change relatively easily is consumer choice. Consumers can simply choose to shrink their flying-related carbon footprints. “We’re working right now to see that ‘emissions by itinerary’ starts to get integrated into travel search engines,” said Rutherford. The idea, called an emissions disclosure, would be much like nutritional labels on food, showing ticket buyers how much carbon each flight will emit.

It may seem that flying is a monolithically bad thing to do, from a climate perspective, but passengers have a surprising amount of choice in the impact they have. Rutherford and his colleagues have found that economy flights between the same two cities can vary in their emissions outputs by as much as 85%.

And Rutherford said that recent cultural stigma around flying has made a measurable difference. “We definitely saw domestic demand for aviation in northern Europe start to fall off purely as a function of the flight shame movement in 2019.”

Seizing the Moment

The upheaval of 2020 presents a new opportunity for individuals, and organizations, to rethink their relationship to air travel. A recent paper in Nature found that attendees at AGU’s 5-day Fall Meeting in 2019 added as much carbon to the atmosphere in getting there as the city of Edinburgh, Scotland, does in a week—about 80,000 metric tons. This year, the pandemic has forced the meeting online, but organizers are considering it a chance to explore ways to shrink the carbon footprint of global science.

As for the locals and climate activists who have fought against Heathrow’s third runway, they’ve been granted a twofold reprieve, first by the court’s decision and then by the coronavirus. Heathrow CEO John Holland-Kaye recently told the British Parliament it may be another 15 years before the aviation industry can recover enough from the pandemic to warrant an additional runway.

In the meantime, the villages adjacent to the airport have been given an unexpected gift: the occasional silence.


Written by Mark Betancourt (@markbetancourt)

Photo by Anton Shuvalov on Unsplash


This story originally appeared in Eos and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.