Aviation’s green fuel plans push Earth past its safe limits, study finds

The aviation industry has spent the past decade promoting a reassuring message: keep flying, and swap fossil kerosene for sustainable fuels. A new study published in Nature Communications now delivers a stark challenge to that narrative. Researchers from the University of Toulouse evaluated the decarbonization roadmaps proposed by the International Civil Aviation Organization, the Air Transport Action Group and the European Union’s ReFuelEU Aviation initiative, and found that none of them brings commercial aviation within the safe operating space defined by the planetary boundaries framework. Even the most ambitious scenarios, which phase out fossil kerosene almost entirely by 2045, persistently transgress the two most critical boundaries: climate change and biosphere integrity.

The planetary boundaries framework, first introduced in 2009 and refined since, identifies nine Earth-system processes with quantifiable limits beyond which humanity risks destabilizing the conditions that have allowed civilizations to flourish. Seven of the nine boundaries are already crossed at the global level. What makes the new analysis distinctive is that it does not treat aviation’s environmental footprint as a static snapshot. Instead, the team built time-dependent trajectories spanning 1940 to 2050, combining prospective life-cycle inventories with the planetary boundaries framework to assess whether each roadmap would keep the sector within its allocated share of Earth’s safe operating space.

The researchers modeled nine scenarios. Four mirrored the institutional roadmaps, ranging from Roadmap-70, in which fossil kerosene still supplies 73 percent of the sector’s energy in 2050, to Roadmap-0, which eliminates fossil kerosene by 2045 and relies on electricity-based fuels and liquid hydrogen for nearly 60 percent of demand. They also tested four mono-type scenarios in which the entire 2050 fuel demand is met either with biofuels or with synthetic e-fuels, plus a fossil baseline. Fuel demand was assumed to grow at 2.1 percent per year between 2023 and 2050, reflecting a traffic growth rate of 3.7 percent annually offset by efficiency gains of 1.5 percent per year, assumptions drawn from the ICAO and Waypoint 2050 scenarios themselves.

A crucial methodological choice involved how to allocate aviation’s share of each planetary boundary. The team applied two contrasting principles. The first, an economic-based allocation, grants the sector 2.9 percent of each safe operating space, matching aviation’s direct, indirect and induced contribution to global gross domestic product in 2018. The second, an ethics-based allocation rooted in the fulfillment of human needs principle, computes shares from the situation of reference countries deemed socially and environmentally sustainable. Under this approach, aviation’s allocated shares shrink dramatically, ranging from as little as 0.002 percent for the phosphorus cycle to no more than 2.2 percent for freshwater use, reflecting the fact that flying satisfies needs that are, in ethical terms, far from essential.

The verdict was unambiguous under both allocation principles. Climate change and biosphere integrity, the two core boundaries whose transgression threatens the stability of the entire Earth system, were exceeded in every scenario. Stratospheric ozone depletion and freshwater use remained respected, while the nitrogen and phosphorus cycles fell in between, respected under the economic allocation but transgressed under the ethics-based one, and by margins so large that even a tenfold increase in aviation’s allocated shares would not change the outcome. Notably, historical emissions between 1940 and 2022 alone account for 103 percent of aviation’s climate budget and 52 percent of its biosphere integrity budget under the economic allocation, meaning the sector has effectively exhausted its share of the safe operating space before any future traffic is counted.

The study also exposes a troubling pattern of burden-shifting. Every scenario reduces climate impacts relative to the fossil baseline, but it does so at the expense of increasing impacts on all the other boundaries. The most climate-ambitious roadmap, Roadmap-0, cuts climate impacts by 25 percent compared with the other roadmaps, yet raises impacts across the remaining boundaries by an average of 64 percent. The relative merits of e-fuels versus biofuels proved highly sensitive to production details. E-fuels performed slightly worse on climate but roughly 10 percent better on the other boundaries when biofuel pathways were chosen for climate optimality; when biofuel pathways were selected to balance all boundaries, the two options became nearly equivalent, differing mainly in how their impacts were distributed.

Perhaps the most sobering findings concern raw resource requirements. By 2050, the alternative roadmaps would increase aviation’s electricity demand by a factor of 20 to 120 and its land occupation by a factor of 60 to 200 relative to the fossil baseline. The most ambitious scenario would consume 16 percent of the world’s projected electricity production, around 10 percent of bioenergy and freshwater withdrawal for electricity, a land area 1.5 times larger than the total global area dedicated to energy crops, and three times the projected global capacity for direct air carbon capture and storage. In the mono-type e-fuel scenarios, aviation alone would need roughly a quarter of all electricity produced worldwide and almost six times the available direct air capture capacity. Because these resources were never budgeted for aviation in the underlying integrated assessment scenarios, the roadmaps implicitly demand a massive, unaccounted expansion of global infrastructure.

Demand-side measures emerged as the only lever capable of reducing the number of transgressed boundaries. The sensitivity analysis showed that respecting additional planetary boundaries would require air traffic to decline by at least 7.2 percent per year, equivalent to a tenfold reduction in traffic by 2050. That rate is comparable to the annual reduction in global carbon dioxide emissions needed to hold warming to 1.5 degrees Celsius, and the authors acknowledge it appears politically unlikely. Under the economic allocation, more moderate degrowth rates between 0.9 and 2.3 percent per year would suffice to bring biosphere integrity back within bounds, but no realistic growth scenario, positive or negative, eliminated the transgression of the core boundaries.

The authors conclude that aligning aviation with Earth’s safe operating space cannot be achieved through fuel substitution alone. They outline four priorities for genuinely sustainable roadmaps: prioritizing renewable electricity production, carefully selecting biofuel feedstocks to balance impacts across boundaries, deploying carbon capture and storage not just for e-fuel production but to compensate for historical emissions, and incorporating explicit demand-side measures. They also note that their analysis likely underestimates the problem, since aviation’s non-carbon-dioxide climate effects, such as contrails and nitrogen oxide emissions, were excluded. As international aviation sits outside the Paris Agreement and depends on distinct governance mechanisms, the study provides critical scientific evidence for policymakers weighing whether technological optimism can substitute for honest conversations about how much, and how far, humanity can afford to fly.

Subject of Research: Absolute environmental sustainability assessment of aviation decarbonization roadmaps against the planetary boundaries framework

Article Title: Current aviation roadmaps are not within planetary boundaries

Article References: Current aviation roadmaps are not within planetary boundaries. (n.d.). https://doi.org/10.1038/s41467-026-77472-6

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77472-6

Keywords: aviation, planetary boundaries, sustainable aviation fuels, biofuels, e-fuels, climate change, biosphere integrity, life-cycle assessment, ICAO, ReFuelEU, demand reduction, carbon capture