Aviation contributes to anthropogenic climate change not only through CO2 emissions, but also via non-CO2 effects, including stratospheric water vapour (SWV). SWV influences the Earth's radiation budget by altering longwave and shortwave radiative fluxes, resulting in a positive radiative forcing. Aviation affects SWV through multiple pathways, including changes in methane oxidation driven by nitrogen oxide emissions, direct emission of water vapour at stratospheric altitudes, hydrogen oxidation, and temperature-driven changes in stratosphere–troposphere exchange. While comprehensive climate chemistry models can represent these processes in detail, their computational cost limits their applicability for rapid scenario analysis.

This thesis develops and evaluates a method to represent aviation-induced changes in stratospheric water vapour within the OpenAirClim (OAC) response model. The novelty of this work lies in the quantification of SWV changes due to methane oxidation within a reduced-form climate response model, enabling fast yet process-consistent scenario analysis. Other potential SWV pathways are assessed but not explicitly implemented due to methodological limitations, overlap with existing OAC modules, or negligible expected impact.

The implementation is verified through consistency checks on fractional release factors, age-of-air distributions, spatial SWV patterns, and mass conservation, and validated against published results. A sensitivity and uncertainty analysis is performed to assess the robustness of the calculated radiative forcing, followed by scenario analyses illustrating the relative magnitude of SWV forcing compared to methane-related effects for different future aviation scenarios. The results demonstrate that aviation-induced reductions in methane lead to a net decrease in SWV and associated radiative forcing, highlighting the importance of including SWV effects for a more complete assessment of aviation's climate impact within simplified climate models. ... Read more

The adoption of hydrogen as an alternative fuel in aircraft has the potential to reduce the climate effect of aviation significantly. However, hydrogen leakage during production, storage, or use can offset these benefits by altering atmospheric chemistry and composition, particularly through interactions with methane, ozone, and stratospheric water vapour. This study employs surrogate models based on recurrent and convolutional neural networks to simulate the climate effects of hydrogen leaks, achieving rapid projections 30,000 times faster than conventional climate models, with an error margin of less than 5%. This efficiency enables the quantification of uncertainties related to hydrogen leakage rates and atmospheric chemistry through Monte Carlo simulations, allowing for an assessment of their contributions to radiative forcing under various Shared Socioeconomic Pathway (SSP) climate scenarios. By 2100, the radiative forcing from aviation-induced hydrogen leaks is projected to reach 43.9 ± 21.2 mW m-2 (±1σ) under the stringent climate change mitigation projection SSP1-2.6, accounting for 35% of aviation’s total radiative forcing. Under the more conservative scenario SSP3-7.0, higher methane levels reduce the oxidative capacity of the atmosphere, lowering the projection to 17.7 ± 6.9 mW m-2 (±1σ) which corresponds to approximately 5% of the aviation’s total radiative forcing for this scenario. These findings demonstrate that hydrogen leaks have the potential to substantially contribute to aviation’s total radiative forcing, with the magnitude of their impact heavily influenced by background climate conditions. It will be important to minimise hydrogen leakage to fully harness the climate benefits of transitioning to hydrogen as a fuel for aircraft. ... Read more