Greater fuel efficiency is potentially preferable to reducing NOx emissions for aviation's climate impacts

The paper addresses whether further tightening aircraft engine NOx emission standards yields net climate benefits once technological trade-offs with fuel burn (and CO2) and updated methane radiative forcing are considered. Historically, ICAO-CAEP standards targeted local air quality with an assumed climate co-benefit because aviation NOx tends to warm via short-term O3 formation and cool via longer-term methane (CH4) reductions. However, reducing aviation NOx often incurs a fuel-burn penalty, potentially worsening long-lived CO2-driven warming. The study examines how changing background surface emissions of ozone precursors (NOx, CO, NMVOC) and future RCP scenarios modulate the net radiative forcing (RF) from aviation NOx. The goal is to reassess the balance between NOx reduction and fuel efficiency for aviation climate policy under present and future atmospheric conditions.

The authors synthesize prior findings that aviation NOx drives a net warming under present-day conditions through short-lived O3 increases offset by longer-lived CH4 decreases, with additional CH4-induced O3 and stratospheric water vapour (SWV) cooling terms. They note technological trade-offs between NOx control and fuel efficiency and emphasize the difficulty of trading short-lived climate forcers against long-lived CO2. Previous CTM/CCM studies projected substantial increases in both O3 and CH4-related RFs from aviation by 2050 under RCP4.5, though with large inter-model spread due to differences in chemistry schemes, coupling, and physical process representations. Limited sensitivity studies had explored how surface precursor emission changes affect aviation NOx impacts; this work expands on that gap, especially considering the updated CH4 RF formulation (Etminan et al. 2016) that increases the magnitude of methane forcing relative to IPCC AR5’s simplified formula.

- Atmospheric chemistry modeling: The MOZART-3 3D chemistry transport model (CTM) was used, configured at T42 (~2.8° × 2.8°) with 60 hybrid layers from surface to 0.1 hPa. Transport and physical processes include semi-Lagrangian advection, convection, boundary layer exchanges, and wet/dry deposition. Kinetics/photolysis follow NASA/JPL evaluations. - Meteorology: ECMWF ERA-Interim 6-hourly reanalysis for 2006 was used to drive all simulations (including 2050 runs), i.e., no interactive climate. - Emissions: Aviation NOx for 2006 from REACT4C base case (0.71 Tg(N) yr−1). For 2050, ICAO-CAEP projections provided a low NOx scenario (2.17 Tg(N) yr−1; low traffic, optimistic technology) and a high NOx scenario (5.59 Tg(N) yr−1; high traffic, low technology). Surface anthropogenic and biomass burning emissions (base year ~2005) from IPCC AR5; biogenic from POET. Sensitivity experiments reduced surface precursors globally by 30% individually (NOx, CO, NMVOC) and all together (ALL). Future 2050 surface emissions followed RCP 2.6, 4.5, and 8.5 inventories. Natural emissions (isoprene, lightning, soil NOx, oceanic CO) were kept constant; lightning NOx parameterized from convective cloud tops with a global 4.7 Tg(N) yr−1 source. - Radiative forcing calculations: Short-term O3 RF from MOZART-3 monthly O3 fields was computed with the Edwards–Slingo radiative transfer model (offline UM RTM; 9 LW and 6 SW bands), using ISCCP D2 cloud climatology and ERA-Interim temperature/humidity. Long-lived species for 2050 RF were consistent with each RCP. CH4 concentration change was assumed in steady-state equilibrium with OH changes due to aircraft NOx, using a 1.4 feedback factor. Initial CH4 RF used the IPCC AR5 simplified expression (Myhre et al.) with CH4–N2O overlap; CH4-induced O3 RF was 50% of CH4 RF and SWV RF 15% of CH4 RF. A second set of RFs was recalculated with the updated simplified CH4 RF expression of Etminan et al. (2016), which increases CH4 RF magnitude by ~20–25%. - Diagnostics: Net aviation NOx RF is the sum of short-term O3, CH4, CH4-induced O3, and SWV terms. Additional sensitivity assessed the effect of maintaining present-day surface emissions alongside 2050 aviation emissions. Temperature responses for unit pulses were derived using AGTP methods for NOx and the LinClim simple climate model for CO2, to contrast time scales and cumulative effects.

- Under the IPCC AR5 CH4 forcing parameterization: aviation net NOx RF is highly sensitive to background conditions and aviation emission growth. • 2006 baseline: net NOx RF = 3.5 mW m−2 (SO3 = 14.8; CH4 = −6.9; IO3 = −3.4; SWV = −1.0). • 2050 low-traffic/optimistic tech (2.17 Tg(N) yr−1): net NOx RF ranges 5.8 (RCP2.6) to 7.2 mW m−2 (RCP8.5). • 2050 high-traffic/low tech (5.59 Tg(N) yr−1): net NOx RF ranges 9.2 (RCP2.6) to 12.5 mW m−2 (RCP8.5). • If surface emissions remained at present levels, 2050 high-traffic net NOx RF would be 17.5 mW m−2, 48% greater than under 2050 RCP2.6 and 28% greater than RCP8.5. - Background surface precursor reductions strongly lower aviation net NOx RF for the same aircraft emissions (2006 base): • −30% surface NOx: net NOx RF decreases from 3.5 to 2.0 mW m−2; short-term O3 RF increases by 16% but the magnitude of negative CH4 RF increases by 28%. • −30% surface CO: net NOx RF = 3.0 mW m−2; −30% NMVOC: 2.8 mW m−2; −30% ALL: 2.2 mW m−2. • Any 1% change in surface NOx emissions modifies aviation net NOx RF by ~1.5% (inventory dependent: ~1.4–1.6%). • To reduce global aviation NOx climate impact by 1 mW m−2, a ~17% reduction in surface NOx is needed vs ~35% reduction in aviation NOx emissions. - Using the updated Etminan et al. CH4 RF parameterization markedly increases the negative CH4-related terms, tipping net NOx RF towards zero or negative: • 2006 baseline: net NOx RF = 0.9 mW m−2; with −30% surface NOx the net is −1.6 mW m−2; −30% ALL yields −1.1 mW m−2. • 2050 low-traffic/optimistic tech: net NOx RF becomes negative for all RCPs (−1.4 to −2.4 mW m−2). • 2050 high-traffic/low tech: net NOx RF is strongly negative (−6.3 to −8.5 mW m−2). • With the updated CH4 formula, net NOx RF decreases (becomes more negative) with increasing aviation NOx emissions and with cleaner backgrounds (Fig. 4), opposite to the trend under the older CH4 formulation (Fig. 1). - Overall, cleaner future surface backgrounds (RCP2.6/4.5) reduce aviation NOx climate impacts. Given technology trade-offs, large reductions in aviation NOx could incur fuel penalties leading to greater CO2, risking perverse net climate outcomes. CO2’s cumulative, long-lived warming effect is about twice the RF of aviation NOx in 2018 and dominates long-term temperature response.

The study demonstrates that aviation NOx climate impacts are not intrinsic but depend strongly on background atmospheric composition and the treatment of methane radiative forcing. Reductions in surface precursor emissions, especially NOx, decrease the oxidative capacity, lengthen CH4 lifetime, and make the CH4-mediated negative RF terms from aviation NOx more pronounced relative to the short-term O3 warming. Consequently, for a given level of aviation NOx, cleaner backgrounds reduce net NOx RF; under the updated CH4 RF formulation, the net term can become negative and more negative as aviation NOx increases. These findings challenge the presumption that ever-stricter aircraft NOx standards automatically deliver climate benefits. Because technological NOx reductions can come with fuel-burn penalties, prioritizing NOx reductions may increase CO2 emissions, which have a dominant, long-lived warming effect. The results support focusing on fuel efficiency and CO2 reduction as the more robust climate mitigation strategy for aviation, while acknowledging the clear air quality benefits of NOx control. The analysis also explains the variability in prior literature through model sensitivities and underscores the crucial role of methane forcing representation in quantifying aviation NOx climate effects.

The paper provides a comprehensive reassessment of aviation NOx climate effects under present and future atmospheric conditions, showing that: (i) background surface emission reductions substantially diminish aviation net NOx RF; (ii) updated methane RF (Etminan et al.) increases the magnitude of negative CH4-related forcings, often rendering the net aviation NOx RF near-zero or negative; and (iii) tightening aircraft NOx standards can be counterproductive for climate if they increase fuel burn and CO2. Therefore, in terms of climate protection, greater fuel efficiency and CO2 reductions are generally preferable to pursuing further NOx reductions, though NOx control remains important for local air quality. Future work should develop integrated assessments that jointly optimize climate and air quality outcomes under evolving background conditions, incorporate interactive climate–chemistry feedbacks, and include NOx-related aerosol effects to refine net forcing estimates.

- The CTM is run with fixed-year (2006) meteorology for all scenarios, lacking interactive climate–chemistry feedbacks; climate change effects on oxidizing capacity and O3 are not explicitly simulated. - Natural emissions (e.g., lightning, biogenic, oceanic) were held constant across scenarios. - Methane responses are treated in steady state with a simplified feedback factor; CH4-induced O3 and SWV are parameterized (50% and 15% of CH4 RF, respectively). - Large inter-model differences exist in CH4/O3 sensitivities; results depend on the chosen CH4 RF formulation (IPCC AR5 vs Etminan et al.). - Potential NOx effects on aerosol (nitrate, sulfate via OH) were not included; these likely add negative forcing but are uncertain. - Temperature response analyses used simplified metrics (AGTP, LinClim) rather than fully coupled Earth system models. - Aircraft technology and emission projections carry uncertainties; only bounding 2050 scenarios were explored.