Impact of future aircraft NOx emissions on atmospheric composition and climate: dependence on background conditions

Abstract. Aviation emissions are predicted to have caused 4 % of anthropogenic warming to date. While aviation CO₂ climate effects are well known, the magnitude of non-CO₂ effects of aviation are highly uncertain. Nitrogen oxide (NOx) emissions from aircraft affect greenhouse gases: local production of ozone in the short term, and long-term impacts on methane, stratospheric water vapour and ozone. Ozone production is non-linear and depends on the background concentrations of NOx and volatile organic compounds (VOCs). Previous single-model studies have found an increased sensitivity of NOx-induced response to aviation emissions in high-mitigation scenarios compared to low-mitigation scenarios. Here we extend this to a multi-model study, using three models to explore the dependence of aviation NOx effects on background conditions in two future scenarios. We calculate the ozone radiative forcing from a 20 % change in aviation NOx emissions for two different future aviation emission scenarios, running each scenario in a high and low mitigation background. We do not find a consistent sensitivity of ozone response to NOx background between the models used. The inter-model variability in ozone response is larger than the effect of different background scenarios. We calculate a positive net NOx forcing in both future scenarios; in the high mitigation scenario in two of three models the long-term methane forcing is sufficiently negative to make the net NOx forcing negative. There is continued uncertainty in the climate impacts of aviation NOx, and we suggest that more model consensus is required to enable parametrisations of these NOx impacts into simplified models.