- Critics and researchers caution that pinning aviation’s carbon-cutting hopes on sustainable aviation fuels (SAFs) is problematic. These fuels, derived from liquid biofuels, along with synthetic fuel options such as green hydrogen, have been produced in only miniscule amounts at high cost compared to what’s needed.
- Scaling SAFs up to cover all of the aviation industry’s carbon-reduction goals while avoiding environmental harm will be a mammoth technological and economic challenge, and may not be achievable in the time available as climate change rapidly escalates, say experts.
- Other solutions will almost assuredly be required: Hydrogen-powered or electric planes may be on the horizon for private or short-haul flights. But reducing emissions from commercial, long-haul flights remains a far greater challenge.
- A mixture of technological solutions, increased efficiencies in airplane design, better airport management, and new innovative policies, including controversial ideas to curb customer demand for air travel, are likely needed to cap and significantly bring down commercial aviation’s emissions fast.
The commercial aviation industry hopes to use sustainable aviation fuels (SAFs) to slash its greenhouse gas emissions. However, faced by challenges of SAF availability, production, scaling up, cost, and environmental pitfalls, experts say a range of other technical and policy options are needed now to make deep cuts in the sector’s carbon emissions.
These options range from designing more fuel-efficient planes, to shrinking the carbon footprint of airports. But some analysts say this doesn’t go far enough toward achieving a net-zero emissions goal, and are opting for a more controversial solution: curbing customer demand for air travel.
“It’s inevitable that we need to have demand management now,” says Tim Johnson, director of the NGO Aviation Environment Federation. “There is a role for SAFs and [other] good [emissions-reduction] pathways need to be progressed … But we really need to get real about the fact that [SAFs] aren’t going to deliver at the scale we need, and therefore a conversation around demand is desperately needed now.”
Cutting demand, offering carbon-saving alternatives
With a global warming footprint already the size of some industrialized countries, and an expressed aim to reduce its emissions to net zero by 2050, the aviation industry recognizes its responsibility to combat climate change now. But some experts insist the only way to do that in a timely fashion is to either fly less or change the way we travel.
Flying less, argue these experts, is the best way to reduce transportation emissions quickly; yet how to achieve that is debated. Rail offers one opportunity. A Europe-based study found that rail travel offers carbon reductions over air flight, with single-use car travel, popularly entrenched in the U.S., among the most carbon-intensive per passenger.
“I think one of the short-term goals is to decrease the demand for aviation and increase the use of different modes of transportation,” says researcher Omar Ocampo, with the Washington, D.C.-based Institute for Policy Studies. “One of our policy recommendations is to have increased taxation for short haul flights. Our logic is it will disincentivize the use of [those] flights.”
Some countries are already taking steps to cut back on short air trips; France, for example, has reduced short-haul flights that can be taken by rail. But “[i]n some areas, that can’t be done at all, because rail is not ubiquitous in the rest of the world, as it is in Europe,” notes Steve Csonka, executive director of the Commercial Aviation Alternative Fuels Initiative, an aviation industry coalition.
While a useful policy, cutting down on private and domestic short-haul air travel would still only trim emissions, not eliminate them, because long-haul flights are far more intensive emitters. Another partial solution would be to reduce airport carbon emissions, but these too make up only a fraction of the industry’s carbon footprint.
Some experts put their hopes on continuing technical advancements, such as plane redesign. By “squeezing more efficiency out of the airframe,” aircraft can become lighter, needing less fuel, explains William Crossley, the Uhrig & Vournas head of aeronautics and astronautics at Purdue University. More fuel-efficient planes could also combine with increased SAF efficiency to drive down emissions even further in the long term.
Crossley agrees technology and alternative fuels have a role to play, but only a limited one and he foresees no single “magic bullet” solution. “As an aircraft designer, I would like to see [airlines] continue to be successful because the product is really important for society,” he told Mongabay. “But if we have to somehow suppress demand to get to the carbon targets, that may be where we have to end up.”
Other experts point to another potentially reducible cause of aviation’s climate change footprint: contrails. “Contrail mitigation” is possible, they say, by greatly boosting the amount of SAFs burned, since they contain fewer particulates that contribute to contrails and warming. Contrail formation could also be avoided by flying differently — revising flight paths to avoid cold air pockets and flying at lower altitudes, though more research is required.
That, however, is not a simple solution, says Csonka. Forecasting contrail-forming areas and diverting planes from them would likely necessitate additional fuel usage and cost. It’s a question of potential trade-offs, he says, as the amount of additional CO2 burned must contribute to less overall warming than that caused by the formation of the contrail.
Piers Forster, an atmospheric physicist at the U.K.’s University of Leeds, suggests that reduced contrail formation is a potentially workable solution: “I think the more exciting idea would be to do both SAF and contrail avoidance together,” he says.
Toward carbon-free flight
Csonka notes that, alongside SAFs, multiple routes exist to cut airline emissions, and it’s now up to policymakers to decide what’s best for national economies: “My only hope is that they’re looking at a balanced approach when they try to make those decisions.”
Taking this entire carbon-cutting toolkit into consideration — SAFs, redesigning engines and airframes, creating airport carbon efficiencies, and reducing contrails — Johnson still remains convinced that, if the world wants to significantly shrink aviation’s carbon footprint, policymakers will still need to curb the rapidly growing demand for air travel:
“The zero-emission tech and the genuine SAFs are not immediate solutions,” he says. “[If] you want to decarbonize flight, realistically, because the solutions are a couple of decades out, it’s inevitable that we need to have demand management.”
“It is a huge challenge to decarbonize aviation [and] demand would play a huge role,” agrees Candelaria Bergero, a Ph.D. student in Earth system science at the University of California. “But the deployment of sustainable aviation fuels and conscious carbon dioxide removal can also help us solve this puzzle … policies that incentivize the sustainable deployment of [SAFs] would be beneficial.”
Ogunsina, K., Chao, H., Kolencherry, N. J., Jain, S., Moolchandani, K., DeLaurentis, D., & Crossley, W. (2022). Fleet-level environmental assessments for feasibility of aviation emission reduction goals. arXiv Preprint. Retrieved from https://arxiv.org/pdf/2210.11302.pdf
Gierens, K., Matthes, S., & Rohs, S. (2020). How well can persistent contrails be predicted? Aerospace, 7(12), 169. doi:10.3390/aerospace7120169