Aviation significantly contributes to the global economy and societal mobility needs. However, it also contributes substantially to climate change through CO2 and non-CO2 effects, including contrail-cirrus and ozone formation. This study aims to assess the effectiveness of various policies, regulations, and technological advancements in mitigating aviation's climate impact. The research question focuses on whether current emission reduction targets for the aviation sector are sufficient to meet the goals of the Paris Agreement, considering both CO2 and non-CO2 effects. The study's importance lies in providing a comprehensive and realistic evaluation of the aviation sector's contribution to climate change and the feasibility of achieving ambitious emission reduction targets. It addresses a critical gap in understanding the effectiveness of current policies and the potential of future technological innovations to mitigate aviation's warming effect. This is crucial for informed policymaking and the development of effective strategies for decarbonizing the aviation industry, particularly given the sector's projected growth.

The paper references previous research showing a steady increase in jet aircraft fuel efficiency since the 1960s, attributed to improvements in aerodynamics, materials, and engine technology. Studies like Kharina and Rutherford (2015) highlight historical fuel consumption reduction trends. However, the rapid growth in air passenger transport (exceeding 4 billion passengers annually in 2017) counteracts these efficiency gains. Airbus and Boeing forecasts predict continued substantial growth in revenue passenger kilometers (RPK) in the coming decades. Previous research on aviation's climate impact highlights its contribution to anthropogenic climate change (approximately 5% currently), primarily through CO2 emissions, but also from non-CO2 effects such as contrail-cirrus and ozone formation. The literature also details the targets set by organizations like ACARE (Advisory Council for Aviation Research and Innovation in Europe) in its Flightpath 2050 document and the ICAO (International Civil Aviation Organisation) with its CORSIA (Carbon Offsetting and Reduction Scheme for International Aviation) scheme. These documents outline goals for emission reductions, but their impact on near-surface temperature changes and the significance of non-CO2 effects have not been fully explored. The paper builds upon existing knowledge by using a more comprehensive modelling approach to incorporate the complex interplay between CO2 and non-CO2 effects.

The study employs a combination of top-down and bottom-up approaches to assess aviation's climate impact. Top-down scenarios are developed based on assumptions about future growth in RPK, technological improvements, and sustainable aviation fuel (SAF) availability. Eight top-down scenarios were created, using industry-independent growth rate projections which accounts for worldwide saturation effects of economic growth and considers a decrease in the growth rate down to 1.2%/year in 2050, and extrapolates to 0.8%/year in 2100 (based on Randers scenario 2052). These projections are lower than those of Airbus and Boeing forecasts, but slightly higher than other academic estimates for 2050. The model incorporates advances in airline efficiency, such as changes in aircraft type, number of seats and load factors which leads to a reduced increase in flown kilometers (distance) compared to transport volume (RPKs). Five key scenarios are analyzed: Current Technology (CurTec), Business-as-usual (BAU), CORSIA, Flightpath 2050 (FP2050), and Flightpath 2050 with continuous implementation (FP2050-cont). The CORSIA scenario incorporates an optimistic assessment of SAF availability and price premiums. The FP2050 scenarios incorporate the technology targets outlined in the Flightpath 2050 document. The bottom-up approach involves an expert assessment of feasible technological advancements and their impact on fuel consumption and NOx emissions. This assessment considers various aircraft segments and innovative aircraft designs (conventional, Flying-V, and N3-X), evaluating their potential for fuel efficiency and NOx emission reduction. The study employs the non-linear climate-chemistry response model AirClim to calculate the near-surface temperature change for each scenario, considering CO2, NOx, H2O emissions, and contrail-cirrus effects. The model accounts for uncertainties through a Monte-Carlo analysis with 10,000 simulations, varying atmospheric lifetimes, radiative forcing, and climate sensitivity parameters. COVID-19 impacts are assessed by modifying the BAU scenario to reflect different recovery pathways, including a fast, slow recovery and behavioral changes, to understand the pandemic's potential influence on aviation's climate impact.

The study's key findings indicate that current emission reduction targets are insufficient to meet the Paris Agreement goals. The analysis of near-surface temperature changes reveals that the CurTec, BAU, and CORSIA scenarios all show a continuous temperature increase until 2100. Although the rate of increase slows down in the BAU and CORSIA scenarios due to efficiency improvements and CO2 offsetting, respectively, the temperature increase surpasses the 1.5°C and 2°C targets, suggesting that these strategies are not enough. However, the FP2050 scenarios demonstrate a stabilization of aviation's climate impact after an initial overshoot around 2050, complying with the 2°C target, and FP2050-cont, even with the 1.5°C target. The relative contribution of CO2 to aviation's climate impact increases significantly over time across all scenarios, even with CO2 regulations and caps, primarily because the reduction in NOx emissions leads to a faster decrease in temperature via ozone than reductions in CO2 emissions. The bottom-up analysis using ECATS scenarios, which are based on expert judgment of feasible technology developments, shows a reduction in climate impact compared to the BAU scenario. However, it doesn't achieve the stabilization observed in the FP2050 scenarios, falling between BAU and FP2050. The Monte-Carlo analysis provides probability ranges for surpassing specific temperature thresholds. For example, for the CORSIA scenario, there is a 90% likelihood that the 5% of 1.5°C threshold will be surpassed between 2025 and 2064, and the 5% of 2°C threshold roughly 10 years later. Sensitivity analysis shows that varying growth rates, climate targets, SAF availability, and technology efficiency impacts the timing of threshold exceedance, but the overall conclusions are robust across most variations. The COVID-19 analysis reveals that only sustained reductions in air travel post-pandemic will significantly alter aviation's long-term climate impact.

The findings highlight the insufficient nature of current strategies to meet the Paris Agreement's goals for aviation's climate impact. Although efficiency improvements and carbon offsetting schemes (like CORSIA) reduce the rate of temperature increase, they don't prevent surpassing the temperature targets. The results emphasize the critical need for aggressive technological advancements, as exemplified by the Flightpath 2050 scenarios, to stabilize and eventually reduce aviation's contribution to climate change. The discrepancy between the top-down (policy-driven) and bottom-up (technology-driven) scenarios illustrates the challenges in translating aspirational goals into real-world technological achievements. The robust nature of the conclusions across various sensitivity analyses and COVID-19 scenarios underscores the gravity of the situation. The study's findings are highly relevant for policymakers, aviation stakeholders, and researchers involved in developing and implementing strategies to decarbonize the aviation sector. The significance of non-CO2 effects highlights the need for a holistic approach that considers all aspects of aviation's climate impact. The need for the integration of realistic assessments of technological feasibility with policy targets is underscored.

The study demonstrates that current strategies for mitigating aviation's climate impact are inadequate to achieve the Paris Agreement goals. While technological advancements offer substantial potential for emission reductions, their timely implementation is crucial. The significant role of non-CO2 effects necessitates a holistic approach to emission reduction. Future research should focus on accelerating the development and deployment of innovative technologies, improving the accuracy of climate impact modelling, and exploring effective policy mechanisms to incentivize decarbonization in the aviation sector. The potential impact of sustained shifts in travel behavior should also be investigated further.

The study's limitations include the reliance on model projections for future technological developments and air travel demand. The optimistic assumptions about SAF availability and cost in the CORSIA scenario might overestimate its effectiveness. The choice of specific climate models and parameters introduces uncertainty into the quantitative results, though sensitivity analyses mitigate this to some extent. The focus on scheduled international aviation excludes other aviation segments, potentially underestimating the overall climate impact. The analysis doesn't fully incorporate potential demand-suppressing effects from higher SAF prices.