Advancing understanding on aviation's non-CO2 climate effects through combination of numerical modelling and observations: ACACIA

Non-CO2 emissions contribute to climate effects from aviation in the same order of magnitude as carbon dioxide (CO2) emissions. However, the non-CO2 effects, comprising e.g., ozone and methane induced from NOx emissions, together with contrails, or the indirect aerosol effects, are associated with much larger uncertainties. The EU Aeronautics project ACACIA (Advancing the SCience for Aviation and ClimAte) explored the climate impacts of non-CO2 effects which show a strong dependence on atmospheric conditions and synoptic situation. While CO2 and non-CO2 effects in general introduce a warming effect for climate change, some indirect effects might result in a relatively large cooling. ACACIA investigated indirect aerosol effects comprising formation and properties of clouds. Atmospheric conditions for the formation of long-lived contrails have been investigated, with the aim to improve their predictability. Indirect effects of nitrogen oxide emissions on atmospheric ozone and methane have been estimated, using a set of global chemistry-climate models. These different effects have been brought to a common scale by various physical climate metrics. A dedicated study on prevailing uncertainties has been performed, with the goal to provide robust recommendations considering uncertainties of individual estimates. Together with the numerical studies a dedicated analysis of existing measurement data has been completed in order to identify needs and requirements for atmospheric observations. To this end, ACACIA brought together research across scales, from plume to global scale, from laboratory experiments to global models resulting in a series of scientific publications, and it proceeds from fundamental physics and chemistry to the provision of recommendations for policy, regulatory bodies, and other stakeholders in the aviation business.  Acknowledgements This project ACACIA (Advancing the scienCe for Aviation and ClImAte) receives funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 875036. High performance supercomputing resources were used from the German DKRZ Deutsches Klimarechenzentrum Hamburg.