| Summary: | Aviation is seeking for ways to reduce its climate impact caused by CO<sub>2</sub> emissions and non-CO<sub>2</sub> effects. Operational measures which change overall flight altitude have the potential to reduce climate impact of individual effects, comprising CO<sub>2</sub> but in particular non-CO<sub>2</sub> effects. We study the impact of changes of flight altitude, specifically aircraft flying 2000 feet higher and lower, with a set of global models comprising chemistry-transport, chemistry-climate and general circulation models integrating distinct aviation emission inventories representing such alternative flight altitudes, estimating changes in climate impact of aviation by quantifying radiative forcing and induced temperature change. We find in our sensitivity study that flying lower leads to a reduction of radiative forcing of non-CO<sub>2</sub> effects together with slightly increased CO<sub>2</sub> emissions and impacts, when cruise speed is not modified. Flying higher increases radiative forcing of non-CO<sub>2</sub> effects by about 10%, together with a slight decrease of CO<sub>2</sub> emissions and impacts. Overall, flying lower decreases aviation-induced temperature change by about 20%, as a decrease of non-CO<sub>2</sub> impacts by about 30% dominates over slightly increasing CO<sub>2</sub> impacts assuming a sustained emissions scenario. Those estimates are connected with a large but unquantified uncertainty. To improve the understanding of mechanisms controlling the aviation climate impact, we study the geographical distributions of aviation-induced modifications in the atmosphere, together with changes in global radiative forcing and suggest further efforts in order to reduce long standing uncertainties.
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