How to make climate-neutral aviation fly

Aviation contributes about 2.5% of global CO2 emissions, and despite efficiency gains (~2% annually over two decades), overall aviation CO2 has risen with ~4% annual demand growth. Beyond CO2, aviation’s non-CO2 effects via short-lived climate forcers (SLCFs) such as NOx, water vapor, soot, and contrail-cirrus substantially add to climate forcing, yet are often omitted in policy and roadmaps. The European Commission acknowledges the need to address aviation’s full climate impacts, including non-CO2 effects, but most decarbonization strategies focus on flight CO2 only, risk overestimating mitigation progress, and depend on offsets of uncertain effectiveness. Recent EU regulations require high GHG savings for synthetic fuels and near-exclusive use of additional low-carbon electricity. This study evaluates the full climate impacts (CO2 and non-CO2) of the European aviation fleet from 2018 to 2100 and assesses two mitigation options: (i) CO2 removal via DAC with permanent storage (DACCS) to offset impacts; and (ii) electricity-based synthetic jet fuel (syn-jet) produced from DAC-derived CO2 and electrolytic hydrogen (DACCU), complemented with DACCS for residual impacts. It further quantifies life-cycle costs, energy, land, freshwater, and CO2 storage needs under different demand and climate scenarios, and emphasizes the pivotal role of demand reduction to feasibly achieve climate-neutral aviation.

The study places its assessment alongside recent literature on aviation climate impacts. Prior works (e.g., Grewe et al., Klöwer et al., Brazzola et al., Bergero et al., Planès et al., Dray et al.) often exclude full life-cycle emissions of synthetic fuel production, omit some or all non-CO2 effects, or assume zero net carbon intensity for SAF production. Methods used to equate CO2 and non-CO2 impacts vary (GWP*, GWP/GTP over fixed horizons, AirClim), whereas this study applies the Linear-Warming-Equivalent (LWE) method to time series emissions from a prospective LCA—an exact and metric-independent approach. A comparative overview (Methods: Previous works, Table 1) highlights that excluding the life-cycle climate impact of syn-jet can neglect up to about one-third (or more under growth) of the total impact, and that non-CO2 effects are significant, uncertain, and grow in relative importance as aviation activity increases. This study’s completeness—covering full LCA for aircraft, infrastructure, fuels (including electricity), DACCS, and explicit treatment of SLCF RF—addresses key shortcomings noted in prior analyses.

Study scope: European passenger aviation, 2018–2100. Two mitigation technology pathways are assessed: (1) fossil jet fuel with DACCS to offset total climate impacts; (2) syn-jet fuel (via DACCU: DAC-derived CO2 combined with electrolytic H2, Fischer–Tropsch synthesis) blended per ReFuelEU targets (5% in 2030, 63% in 2050, 100% by 2063) and complemented with DACCS for remaining impacts. Three mitigation scopes are defined: flight-CO2 neutrality (only flight CO2 plus mitigation-chain GHG); warming neutrality (stabilize RF at 2050 level); and climate neutrality (net-zero RF from 2018 onward). Demand trajectories: (i) growth (converging to +1.8% km/year after recovery), (ii) stationary (post-2024 stabilization), and (iii) declining (minimum decline to reach warming neutrality without CDR: up to −0.8%/yr for fossil, −0.2%/yr for syn-jet). Two climate scenarios inform background LCA data: 2 °C (SSP2-RCP2.6) with deep power decarbonization; and 3.5 °C (SSP2 without stringent mitigation). Modeling framework: A prospective life-cycle environmental and cost assessment integrates aircraft production and operation, airport infrastructure, fuel production and distribution (fossil and syn-jet), DACCS operation, and related supply chains (electricity, materials, services). Aircraft sizing and performance evolve to 2050 via improvements (fuel efficiency −1.2%/yr in liters/passenger-km, seating and load factors +0.3%/yr each), then held constant post-2050 (except the electricity mix projected to 2100). Flight emissions are calculated across flight phases (T/C/C/D/L), with altitude-resolved emissions (troposphere vs. stratosphere split by distance flown). Surface emissions include life-cycle CO2 and SLCFs (e.g., methane, hydrogen, refrigerants) from manufacturing, infrastructure, fuel production (including electricity), and DACCS. Flight “non-CO2” forcers modeled include NOx, black carbon, SOx, water vapor, and contrail-cirrus; altitude-dependent effects follow literature and EMEP/EEA factors, with assumed annual reductions in NOx and SOx of 0.6% and black carbon of 0.3% per unit fuel (plus overall fuel efficiency improvement). Syn-jet properties (higher H/C ratio ~2.15 vs. 1.89) reduce soot and ice particle formation; cirrus RF is assumed to decrease by 65% at 100% syn-jet based on literature. Radiative forcing and warming equivalence: RF for surface and flight emissions is computed using a linear impulse-response model (species lifetimes and radiative efficiencies from IPCC AR6 and Lee et al.). Contrail-cirrus RF is estimated via an empirical relationship with kilometers flown. Warming contributions are expressed as Linear-Warming-Equivalent (LWE) CO2 time series; required DACCS to offset SLCF warming is calculated by inverting the impulse-response model. Uncertainty in non-CO2 RF is propagated (5th–95th percentiles), notably large for cirrus. Fuel pathways and DACCS: Syn-jet production modeled via RWGS + Fischer–Tropsch, with DAC-sourced CO/CO2 and PEM electrolytic H2 (55→44 kWh/kg H2 from 2020 to 2050; +3.2 kWh/kg for compression to 700 bar; 1% H2 leakage along the supply chain). Life-cycle burdens for FT co-products allocated by energy content with carbon mass balance correction to match combustion CO2 (3.14 kg CO2/kg fuel). DAC uses grid electricity with heat pump (COP 2.9), includes compression to 100 bar; DACCS includes pipeline transport (400 km), compression/injection to 3000 m, and storage infrastructure. Learning and prospective background: Prospective LCI database (premise/economic pathways) adjusts electricity mixes and sectoral intensities over time under each climate scenario. Learning rates applied: electrolyzers and DAC (scenario-specific), with time-dependent cumulative capacities. Costs: Levelized investment and operating costs for syn-jet components (electrolysis, RWGS, FT upgrading, distribution) and DACCS are modeled with learning; electricity prices reflect scenario mixes. Resource indicators (costs, electricity, land occupation, freshwater abstraction, CO2 storage) are aggregated over 2018–2100 and contextualized against EU-28 reference levels. Outputs: For each combination of mitigation scope, demand trajectory, climate scenario, and fuel pathway, the model quantifies emissions by species, total RF, CDR/DACCS requirements (including additional removal to cover DACCS operations), cumulative costs, electricity, land, freshwater, and geological storage needs.

• Non-CO2 effects dominate aviation climate impacts when demand grows: up to ~80% of climate impacts are unaccounted for if only flight CO2 is targeted.
• Under growing demand with fossil fuel, unmitigated direct CO2 emissions reach ~24 Gt (2018–2100); SLCFs contribute over two-thirds of RF.
• Switching to syn-jet reduces flight CO2 of fossil origin and lowers soot/ice particle formation; at 100% syn-jet, contrail-cirrus RF is reduced by ~65%.
• GMST contribution (2018–2100) under growth: fossil jet unmitigated ~+0.035 °C; syn-jet (with low-carbon power) lowers fleet-induced warming to ~+0.02 °C.
• Mitigating only flight CO2 cuts GMST increase by ~20% versus unmitigated (+0.028 °C vs. +0.035 °C), leaving most impacts (non-CO2 + upstream) unaddressed.
• CDR requirements scale with mitigation scope and fuel: • Climate neutrality (2050–2100, 2 °C scenario, growth): average annual removals ~1.7 Gt CO2/yr (fossil+DACCS) vs. ~1.0 Gt CO2/yr (syn-jet+DACCS); ~20% higher under the 3.5 °C scenario. • Large initial removal needed in 2050 to offset 2018–2049 cumulative RF; earlier ramp-up of CDR would be more practical. • Electricity use for DACCS adds ~13% to CDR needs in 2050, declining to ~5% by 2100 as grids decarbonize.
• Demand management is decisive: • Stationary demand cuts 2100 climate impacts by ~55–57% vs. growth and lowers CDR needs; with syn-jet, warming neutrality can be achieved by 2050 without DACCS (negative CDR requirements thereafter under the warming-neutrality definition). • Declining demand can eliminate CDR for flight-CO2 and warming neutrality: up to −0.8%/yr decline with fossil fuel, or −0.2%/yr with syn-jet. • Climate neutrality still requires substantial CDR even with demand decline, due to pre-2050 legacy RF.
• Life-cycle resource implications (growth, syn-jet+DACCS, 2 °C scenario): • Electricity: cumulative ~180,000 TWh (2018–2100)—~70× EU-28’s 2020 annual electricity; about 1.3× current EU-28 annual output needed each year between 2050–2100. • Land: ~200–250 million hectare-years; Freshwater: approaching the EU-28’s annual abstraction—both dominated by renewable power supply chains. • Using fossil jet + DACCS halves electricity, land, and freshwater vs. syn-jet+DACCS but demands massive CO2 storage, exceeding e.g., proven capacity of the Norwegian shelf, and prolongs fossil dependence.
• Neglecting the LCA footprint of syn-jet production would miss more than one-third of total impacts in a growing fleet, even with rapid power decarbonization.
• Syn-jet viability hinges on very low-carbon electricity (e.g., <~65 gCO2/kWh), consistent with EU delegated regulations; otherwise, emissions shift upstream.
• Uncertainty in non-CO2 RF (especially cirrus) is large, widening uncertainty bands for warming/climate neutrality vs. flight CO2-neutrality.

The study addresses how European aviation could achieve climate-neutrality when both CO2 and non-CO2 effects are considered. It shows that technology options—syn-jet fuels from air-captured CO2 with low-carbon electricity and DACCS—can, in principle, offset aviation’s total climate forcing, but only at very large scales of removals and resource use if demand keeps growing. Three determinants shape feasibility and requirements: (1) mitigation scope—flight CO2 neutrality is insufficient (addresses only ~20% of warming), whereas warming/climate neutrality entail large CDR; (2) technology choice—syn-jet can cut CO2 storage needs up to ~45% vs. fossil+DACCS if supplied by very low-carbon power; and (3) demand trend—stabilizing or reducing air traffic dramatically lowers RF and resource demands, and in declining-demand cases can obviate CDR for warming neutrality due to falling SLCF RF compensating accumulating CO2. Findings highlight the centrality of non-CO2 effects in aviation climate strategies and the necessity of robust low-carbon electricity for syn-jet. They also underscore uncertainties, particularly in contrail-cirrus RF, which propagate into CDR requirements. Policy implications include integrating non-CO2 effects into targets and standards, prioritizing demand-side measures, ensuring additional low-carbon electricity for syn-jet, and planning for limited, contested CO2 storage capacity, alongside broader energy-system material constraints.

From a technological standpoint, European aviation can achieve climate-neutrality by combining electricity-based synthetic jet fuels with comprehensive offsetting via DACCS that covers both CO2 and non-CO2 effects. However, sustaining growth in air traffic renders this pathway highly resource-intensive—requiring vast low-carbon electricity, land, freshwater, and CO2 storage—and prolongs reliance on constrained infrastructures. Demand reduction emerges as the most effective short- to mid-term lever to limit required CDR and resource burdens, buying time to mature complementary options. The study’s main contributions are: a prospective, time-resolved LCA including flight and surface emissions with the LWE method; explicit treatment of non-CO2 effects; and quantified resource and cost implications under multiple demand and climate scenarios. Future research should refine non-CO2 RF estimates (especially contrail-cirrus), explore operational measures (e.g., contrail avoidance), advance low-CO2 aircraft (hydrogen and battery-electric), assess alternative SAF (including biomass and solar routes) with full LCA, and evaluate diversified CDR portfolios and storage availability constraints in system-integrated pathways.

• Large uncertainties in non-CO2 effects, particularly contrail-cirrus RF, lead to wide uncertainty ranges in warming/climate-neutrality estimates.
• Simplified fleet representation: assumes new aircraft reflect each modeled year (e.g., all 2050 aircraft built in 2050), likely underestimating fuel burn by ~5% vs. a realistic age distribution.
• Post-2050 improvements in aircraft technology are not modeled (except electricity mix), potentially overestimating future impacts and resource needs.
• Demand-side rebound effects and modal shifts are excluded; only air travel demand is varied, not total travel behavior or substitution.
• Certain mitigation options are out of scope: improved air traffic management/contrail avoidance, revolutionary aircraft designs, biomass-based SAF, hydrogen-powered aircraft, and battery-electric aircraft.
• Potential inconsistency in DAC learning/efficiency assumptions across climate scenarios (deployment capacities not varied with scenario stringency).
• End-of-life treatment of aircraft is omitted (assumed negligible impacts).
• Syn-jet production is modeled at generic European sites; regional resource yields and footprints elsewhere may differ.
• LCA includes capital goods and hydropower reservoir emissions, unlike some policy methodologies; results are not directly comparable to EU GHG savings thresholds.
• H2 leakage is assumed at 1% along the supply chain; real-world rates may vary and affect RF via methane lifetime impacts.