Aircraft emissions of nitrogen oxides (NOx= NO + NO2), aerosols, and aerosol precursors provide a non-negligible contribution to the climate impact of air traffic, and the uncertainty in their climate Effective Radiative Forcing (ERF) remains significant. This study presents results from a new model intercomparison of the impact of aircraft emissions involving five state-of-the-art global models including both tropospheric and stratospheric chemistry. Aircraft NOx increases ozone photochemical production in the free troposphere throughout the year and decreases ozone chemical loss in the high-latitude lowermost stratosphere during spring–early summer. The models generally agree on the spatial pattern of NOx , ozone, and hydroxyl radical (OH) responses. The NOx net ERF is systematically positive with a model mean of 18.3 mW m−2, ranging from 9.4 to 24.5 mW m−2 among the different models. This net NOx forcing is reduced by 35 % and 43 % accounting for the negative forcing arising from the formation of nitrate and sulfate particles, respectively. Estimates of the aerosol direct ERF are systematically negative and range between -6.5 and -17.8 mW m−2, compensating most of the net NOx ERF albeit with noticeable intermodel differences arising from the diversity in aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NOx -induced ozone response because of a good model agreement. To a lesser extent, some similarities in the results regarding aerosols are also encouraging, given the few existing model intercomparisons on this topic. However, the results also highlight areas where further modeling experiments are needed, both with more models and with dedicated sensitivity simulations to further understand the factors giving rise to the spread in model estimates of aviation emission impacts on atmospheric composition and climate. ... Read more

Commercial supersonic aircraft may return in the near future, offering reduced travel time while flying higher in the atmosphere than subsonic aircraft, thus displacing part of the passenger traffic and associated emissions to higher altitudes. For the first time since 2007, we present a comprehensive multi-model assessment of the atmospheric and radiative effect of this displacement. We use four models (EMAC, GEOS-Chem, LMDz–INCA, and MOZART-3) to evaluate three scenarios in which subsonic aviation is partially replaced with supersonic aircraft. Replacing 4 % of subsonic traffic with Mach 2 aircraft that have a NOx emissions index of 13.8 g (NO2) kg−1 leads to ozone column loss of −0.3 % (−0.9 DU; model range from −0.4 % to −0.1 %), and it increases radiative forcing by 19.1 mW m−2 (model range from 16.7 to 28.1). This forcing is driven by water vapor (18.2 mW m−2), ozone (11.4 mW m−2), and aerosol emissions (−10.5 mW m−2). The use of a Mach 2 concept with low-NOx emissions (4.6 g (NO2) kg−1) reduces the effect on forcing and ozone to 13.4 mW m−2 (model range from 2.4 to 23.4) and −0.1 % (−0.3 DU; model range from −0.2 % to +0.0 %), respectively. If a Mach 1.6 aircraft with a lower cruise altitude and NOx emissions of 4.6 g (NO2) kg−1 is used instead, we find a near-net-zero effect on the ozone column and an increase in the radiative forcing of 3.7 mW m−2 (model range from 0.5 to 7.1). The supersonic concepts have up to 185 % greater radiative effect per passenger kilometer from non-CO2 emissions compared to subsonic aviation (excluding contrail impacts). ... Read more

Aviation is seeking for ways to reduce its climate impact caused by CO₂ emissions and non-CO₂ effects. Operational measures which change overall flight altitude have the potential to reduce climate impact of individual effects, comprising CO₂ but in particular non-CO₂ 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-CO₂ effects together with slightly increased CO₂ emissions and impacts, when cruise speed is not modified. Flying higher increases radiative forcing of non-CO₂ effects by about 10%, together with a slight decrease of CO₂ emissions and impacts. Overall, flying lower decreases aviation-induced temperature change by about 20%, as a decrease of non-CO₂ impacts by about 30% dominates over slightly increasing CO₂ 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. ... Read more