Evaluating the climate impact of aviation emission scenarios towards the Paris agreement including COVID-19 effects

The study addresses whether current and proposed aviation emission targets and measures are sufficient to align the sector’s climate impact with the Paris Agreement temperature goals. Despite long-term improvements in fuel efficiency and rapid growth in air transport demand, aviation’s net warming influence continues to rise through both CO2 and non-CO2 effects (contrail-cirrus and NOx-driven ozone changes). Policy frameworks include ACARE Flightpath 2050 technology targets (−75% CO2/passenger-km and −90% NOx by 2050 relative to 2000) and ICAO’s CORSIA scheme aiming for carbon-neutral growth from 2020 through offsets and sustainable aviation fuels (SAF). Two key gaps motivate this work: translating sectoral emission targets into temperature outcomes, and quantifying the role of non-CO2 effects. The purpose is to evaluate top-down policy scenarios and bottom-up technology pathways to 2100, including COVID-19 recovery variants, using a climate-chemistry response model to estimate near-surface temperature change and assess feasibility against 1.5 °C/2 °C-consistent thresholds for aviation’s share of warming.

Prior work documents steady historical improvements in aircraft fuel efficiency (~1.3%/yr globally from 1960–2014) and projects continued demand growth (~4–5%/yr RPK) from industry forecasts. Aviation’s non-CO2 climate effects are well established: persistent contrails and contrail-cirrus exert a net positive radiative forcing; NOx emissions increase ozone and reduce methane, both affecting radiative forcing. Recent advances refine estimates of contrail-cirrus microphysics (Bock & Burkhardt, 2019) and NOx attribution methodologies (Grewe et al., 2019), generally indicating larger ozone-related effects than earlier assessments (e.g., Lee et al., 2009). Policy targets include ACARE’s Flightpath 2050 technology goals and ICAO’s CORSIA offsetting framework. Literature also explores prospects for SAF to reduce soot particle emissions (thereby altering contrail-cirrus properties) and provides comparative scenario analyses of aviation’s radiative forcing and temperature impacts. This study builds on and integrates these strands by linking sectoral emission scenarios to temperature outcomes, explicitly including updated non-CO2 effects and uncertainties.

The analysis combines top-down scenario modeling, bottom-up technology assessment, climate impact modeling, and uncertainty quantification. - Top-down scenarios: Five sector pathways share a common transport volume trajectory derived from ICAO historical RPK (1971–2017) and extrapolated with decreasing growth (from ~6%/yr to ~1.2%/yr by 2050 and 0.8%/yr by 2100) based on the Randers 2052 socio-economic scenario and WeCare modeling. Fuel efficiency improvements follow ICAO Environmental Report assumptions (1%/yr in 2018 declining to 0.25%/yr by 2100). Scenarios are: CurTec (2012 technology fixed), BAU (evolutionary efficiency improvements), CORSIA (as BAU plus carbon-neutral growth from 2020 via offsets and SAF), FP2050 and FP2050-cont (both meeting ACARE Flightpath 2050 targets; FP2050 introduces technologies post-2050, FP2050-cont introduces continuously). For CORSIA, an optimistic SAF availability is assumed with lifecycle CO2 reductions improving from ~65% (2020) to ~80% (2100), yielding about one-third SAF share by 2100; about 53% of the required CO2 reduction to meet carbon-neutral growth comes from SAF and the remainder from offsets. - Bottom-up (ECATS) technology scenarios: Expert judgment (TU Delft, Chalmers, DLR, TU Hamburg) evaluates feasible technologies for single-aisle and twin-aisle segments (about 95% of ASK). Reference types are A320neo and A350 (EIS ~2015). Next generations: conventional tube-and-wing in 2035; in 2050, options for twin-aisle include conventional tube-and-wing (TW), Flying-V or multi-fuel blended wing body (MF-BWB), and NASA N3-X blended wing body with turbo-electric distributed propulsion. Estimated fuel burn improvements: 18–22% (2035 vs 2015), 34–44% (2050 vs 2015). Mission NOx reductions range roughly from 18–26% (2035) to 30–44% (2050), depending on configuration. Fleet diffusion assumes ~95% market penetration within ~15 years after EIS. Emission inventories are produced using DLR’s GRIDLAB methodology and scaled by improvement factors; inventories are generated in 5-year steps for 2015–2070, and combined with the same transport and SAF settings as the top-down cases to form nine ECATS scenarios (three configurations with ±10% pessimistic/optimistic variants). - Climate modeling: The AirClim non-linear climate-chemistry response model converts emissions to changes in concentrations (CO2, H2O, O3, CH4) and contrail-cirrus, and then to near-surface temperature. Emission-location sensitivity reflects lifetimes (weeks–months for NOx/O3; short for contrails; globally mixed for CO2). Background climate follows RCP2.6. SAF impacts on contrail-cirrus are modeled via a linear scaling of soot number particle reduction with blend fraction (e.g., 50-50 blend → ~50% fewer soot particles) and a parameterization of contrail-cirrus RF change consistent with Burkhardt et al. (2018), relating relative particle number change to relative RF change. - Uncertainty and Monte Carlo: Eleven uncertainty parameters are sampled: atmospheric residence times (±20%); RF strengths (±5% CO2, ±10% CH4, ±50% H2O, O3 (incl. PM10), contrail-cirrus); climate sensitivity factors (±5% CO2, ±10% CH4 and contrail-cirrus, ±30% H2O and O3). For top-down scenarios, 10,000 simulations produce distributions of temperature outcomes and the timing when aviation surpasses thresholds defined as 5% of global warming targets (1.5 °C and 2 °C). For ECATS scenarios, 30,600 simulations are performed (3400 per scenario across nine variants). Sensitivity analyses examine growth rate ±50%, target shares (3.5%, 5%, 6.5%), SAF availability (−50%), and technology efficiency variations. - COVID-19 scenarios: Three parametric variants modify BAU transport volumes to reflect a fast recovery (~3 years), slow recovery (~15 years), and sustained behavioral change (permanent RPK reduction), and their temperature implications are evaluated.

- Temperature outcomes: CurTec, BAU, and CORSIA all show continued warming through 2100, with slowed rates over time. Relative to CurTec, BAU reduces aviation-induced temperature in 2100 by roughly 25%; CORSIA achieves a larger reduction (~35–40%) due to effective CO2 caps and SAF-induced reductions in contrail-cirrus forcing. FP2050 and FP2050-cont stabilize aviation’s climate impact, with an overshoot around 2050, then declining temperatures thereafter. - Paris-aligned thresholds: Using aviation’s share as 5% of global targets, FP2050 and especially FP2050-cont meet the 2 °C consistency, and FP2050-cont is consistent with 1.5 °C. CORSIA, despite improvements, very likely surpasses the 5% of 1.5 °C threshold between 2025 and 2064 (mid-90% range), with the 5% of 2 °C threshold about a decade later. CurTec and BAU surpass both thresholds well before 2050 in most realizations. - Changing importance of climate agents: As technology improves and NOx effects respond quickly, CO2’s relative share of aviation warming rises from ~25% (2005) to 33–56% (2100 across scenarios). Contrail-cirrus contributions decline relatively (to ~20–30% by 2100), while NOx contributions drop markedly under FP2050 (to ~16%). In BAU/CORSIA, contrail-cirrus remains ~20–24% by 2100. H2O remains ~1% throughout. Updated NOx and contrail-cirrus estimates align with newer literature indicating larger ozone-related effects than older assessments. - CORSIA mechanics: Approximately 53% of CORSIA’s CO2 reduction to achieve net carbon-neutral growth is attributed to SAF, which also diminishes contrail-cirrus forcing via soot reductions, yielding a larger total climate benefit than offsets alone. - Bottom-up feasibility (ECATS): Feasible technology improvements yield 18–22% fuel burn reduction by 2035 and 34–44% by 2050 relative to 2015 baselines, with substantial NOx reductions. These pathways reduce warming versus BAU but do not achieve the stabilization seen in the FP2050 top-down targets, implying ACARE Flightpath 2050 goals are unlikely to be fully met by realistic technology diffusion alone by 2050. - COVID-19 effects: Temporary traffic collapses (fast or slow recovery) have minor long-term temperature effects; only a sustained post-pandemic behavioral shift (persistent RPK reduction) yields a noticeable long-term climate benefit relative to BAU. - Sensitivities: Modifying growth rates by ±50% shifts BAU 2100 fuel by about ±20% and changes median threshold-surpass years by only a few years. Adjusting aviation’s allowed share from 5% to 3.5% or 6.5% shifts the median surpass year by roughly 1–2 decades. Reducing SAF availability by 50% has negligible effect on the distribution of surpass years under CORSIA.

The study bridges sectoral emission targets and Paris-consistent temperature outcomes, showing that while ACARE Flightpath 2050 targets could stabilize aviation’s temperature contribution, realistic technology adoption likely falls short without additional measures. ICAO’s CORSIA, even with optimistic SAF use, is insufficient to keep aviation’s share of warming within a 1.5 °C-consistent threshold over the century in most cases. Non-CO2 effects remain a substantial portion of aviation’s warming, and their relative contributions evolve as NOx decreases and CO2 becomes more dominant. Policy implications include the need to account for non-CO2 effects (particularly contrail-cirrus) in sectoral goals and mitigation strategies, to accelerate technology deployment beyond evolutionary gains, and to expand SAF with attention to soot reduction benefits. Sensitivity analyses indicate that reasonable variations in demand, technology efficiency, and SAF availability slightly shift outcomes but do not change the central conclusion: without deeper, earlier, and broader mitigation—including operational measures, contrail management, NOx reductions, and potentially demand-side measures—the sector will likely exceed Paris-consistent temperature shares under business-as-usual or offset-only approaches. The COVID-19 shock provides limited long-term relief unless it leads to lasting changes in travel behavior.

This work quantifies aviation’s climate impact across policy and technology scenarios up to 2100, explicitly linking emission pathways—including non-CO2 effects—to temperature outcomes. It finds that: (1) CORSIA’s carbon-neutral growth is unlikely to keep aviation’s warming within a 1.5 °C-consistent share, (2) Flightpath 2050 targets would stabilize and potentially reduce aviation’s temperature contribution, but achieving them appears improbable based on bottom-up technology assessments, and (3) non-CO2 effects remain material and must be addressed. The analysis is robust across a range of uncertainties and COVID-19 recovery paths. Future research and action should focus on: integrating non-CO2 effects into policy (e.g., contrail avoidance, soot reductions), accelerating breakthrough aircraft and engine technologies with earlier fleet penetration, scaling SAF with low lifecycle CO2 and low soot emissions, refining demand management and operational measures, and improving modeling of uncertainties and interactions (e.g., cost-demand feedbacks, regional routing to minimize climate impact).

Key limitations include: reliance on a surrogate climate-chemistry model (AirClim) rather than full coupled Earth system models; substantial uncertainties in non-CO2 effects (contrail-cirrus microphysics, NOx-ozone-methane chemistry) addressed via Monte Carlo but still influential; assumptions about future demand growth, technology improvement rates, and SAF availability and lifecycle performance; use of RCP2.6 as the background climate; not distinguishing domestic vs international aviation; bottom-up scenarios exclude general aviation/regional/business jets (~5–6% of CO2); limited modeling of cost-induced demand feedbacks (approximated via open-loop growth-rate sensitivities); and potential delays in technology diffusion affecting near- to mid-century outcomes.