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.