Path analysis for controlling climate change in global aviation

Summary The aviation industry’s emissions have had a significant impact on global climate change. This study focuses on carbon emission trading schemes, sustainable aviation fuels (SAFs), and hydrogen energy, as vital means for the aviation industry to reduce emissions. To evaluate the climate effects of global routes under four scenarios (24 sub-scenarios) until 2100, this study proposes the Aviation-FAIR (Aviation-Finite Amplitude Impulse Response) method. The findings reveal that while CO2 emissions and concentrations are significant, other emissions, such as N2O and CH4, have a greater effective radiative forcing (ERF) and contribute significantly to climate change. Moreover, SAFs are more effective in mitigating airline pollutant emissions than relying solely on carbon trading schemes. The effectiveness of hydrogen fuel cells may be hindered by technical limitations compared to hydrogen turbine engines. The findings of this study provide reference for the global aviation industry to adopt emission reduction measures.

This supplementary information contains two parts: annual emissions under the scenarios, and detailed MATLAB codes.

Annual emissions under the scenarios
This paper has set four scenarios: the baseline scenario (Scenario 0), the scenario of only Emissions Trading Scheme (ETS) (Scenario 1), the scenario of only Sustainable Aviation Fuels (SAFs) (Scenario 2) and the scenario of hydrogen energy mixed with SAFs.The last one contains two sub-scenarios: hydrogen turbine engine mixed with SAFs (Scenario 3) and hydrogen fuel cell mixed with SAFs (Scenario 4).
For the baseline scenario (Scenario 0), this study assumes that the annual pollutant emissions after 2022 are the average of 2015-2021.The International Air Transport Association (IATA) committed in 2021 that emissions from global aviation would be net-zero by 2050 1 .However, the aviation industry is still developing, and its annual emissions are likely to exceed 2019 for a period.They may decline later due to emission constraints and alternative energy.Similarly, the global aviation emissions have been increasing from 2015 to 2019, but the emissions will decrease in 2020 and 2021 due to the impact of the COVID-19.This process is somewhat like the trend of future global aviation emissions, so it is reasonable to set the average emissions from 2015 to 2021 as the baseline scenario.
Therefore, the annual emissions of CO2, HC, NOx, PM2. 5, and SO2 are 805,428,571.4tons,  636,280.9tons, 6,517,924.97tons, 74,496.4tons, and 987,332.46tons.For the scenario of only ETS (Scenario 1), two sub-scenarios are set: Scenario 1-1 and Scenario 1-2.According to the goal of International Air Transport Association (IATA), if the aviation carbon peak can be achieved by 2035, it is scenario 1-1, 2035-2050.The current carbon emissions will continue until 2035, and the zero-carbon will be achieved by 2050 2 .If current carbon emissions will continue until 2040, the scenario is 1-2, 2040-2055.The zero-carbon will be achieved in 2055.In this scenario, the CO2 and SO2 emissions (the SO2 changes with the change of CO2) will change but the other emissions remain unchanged.
Therefore, for Scenario 1-1, CO2 and SO2 emissions remain 805,428,571.4tons and 987,332.46tons until 2035 and will decrease annually to lead to zero emissions by 2050.For Scenario 1-2, CO2  and SO2 emissions remain 805,428,571.4tons and 987,332.46tons until 2040 and will decrease annually to lead to zero emissions by 2055.
For the scenario of Hydrogen Turbine engines (HT) mixed with SAFs (Scenario According to Airbus's plan, the hydrogen turbine engine is expected to be commercially available in 2035 4 .Still, considering the deployment efficiency of less developed countries such as Africa, the commercial proportion will not be too large.
For example, Scenario 3-1 shows that 50% of sustainable aviation fuel will be used in 2025, 20% of hydrogen turbine engines and 50% of sustainable aviation fuel will be used in 2035, 40% of hydrogen turbine engines and 50% of sustainable aviation fuel will be used in 2045, and 60% of hydrogen turbine engine and 40% of sustainable aviation fuel will be used in 2055.
Compared with ordinary engines, the application of hydrogen turbine engines will reduce NOx by 50%-80%.At the same time, CO2 emission is 0%, HC is 0%, and PM2.5 are reduced by 30-50%.This study assumes that NOx will reduce by 65% (the average value) and the PM2.5 will reduce by 40% when the hydrogen turbine engines are used 6 .
Under For the scenario of Hydrogen Fuel Cells (HFC) mixed with SAFs (Scenario For example, Scenario 4-1 shows that 50% of sustainable aviation fuel will be used in 2025, 20% of hydrogen fuel cells and 50% of sustainable aviation fuel will be used in 2040, 40% of hydrogen fuel cells and 50% of sustainable aviation fuel will be used in 2050, and 60% of hydrogen fuel cells and 40% of sustainable aviation fuel will be used in 2055.
Compared with ordinary engines, hydrogen fuel cells only produce water, and CO2, HC, and NOx emissions are 0%, which can reduce PM2.5 by 60%-80%.This study assumes that PM2.5 will reduce by 70% when hydrogen fuel cells are used 5 .

Detailed MATLAB codes
The results are calculated through MATLAB R2014b, the detailed codes are as follows.