On the non-CO2 to CO2 ratio of aviation emissions and associated uncertainties

Aviation non-CO₂ emissions influence atmospheric chemistry and physics mainly through NO_x, H₂O, aerosols and contrail-induced cirrus cloudiness (CiC). These processes alter Earth's radiation budget and thus near-surface temperatures. Studies report that all aviation emissions together currently warm the climate at approximately three times the rate of that associated with aviation CO₂ emissions alone. However, this factor is by no means universal, as the various natures of the different climate effects and in particular the different time scales they act on, make blending the effects into one number not straightforward. Climatologically, the CO₂ to non-CO₂ climate effect factor ranges between 0.5 and 10.5 as it depends on numerous decisions, including climate metric, time horizon and if pulse or continuous emissions are considered. We here explain the influence of some of these decisions on the calculated climate effect and discuss implications. The factor is additionally associated with large uncertainties and for individual flights, it strongly depends on meteorological conditions and location.

For this study, we first develop traffic scenarios with representative flight missions covering a wide range of flight regions, altitudes, distances and aircraft types to calculate air traffic emissions. To analyse how much the contribution of non-CO₂ effects to the total climate impact varies for different trajectory types, climate metrics and time horizons, CO₂ and non-CO₂ climate effects are calculated for these trajectories using the AirClim model. Moreover, we identify the uncertainties of aviation non-CO₂ effects, assess their ranges and derive probability distributions of these uncertainties, in particular with regard to lifetimes, radiative forcing and efficacies. To assess the influence of these input data to the uncertainties, we then conduct Monte Carlo simulations with the uncertainty distributions and analyse the confidence intervals for non-CO₂ climate effects to be larger than that of CO₂. Further, the relative climate effect differences through mitigation measures are calculated and the risk that a measure leads to unintentional climate warming is estimated. This work shall deepen the understanding of aviation non-CO₂ uncertainties and help paving the way for their incorporation in operational application.