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Advancing the Science for Aviation and ClimAte

Periodic Reporting for period 1 - ACACIA (Advancing the Science for Aviation and ClimAte)

Reporting period: 2020-01-01 to 2021-06-30

Global aviation significantly contributes to global warming. The combustion of kerosene in aircraft jet engines leads to emission of gases and particles that alter the chemical composition of the atmosphere, lead to formation of condensation trails and that may perturb natural cloud formation processes. Some of these effects contribute to global warming, others to cooling, but overall the warming contribution predominates. Due to the strong growth in demand, aviation's contribution to climate change will potentially also grow.
The global society is suffering from the impacts of climate change and global warming. Measures and methods have to be developed and implemented to reduce the anthropogenic climate footprint, including aviation’s share. On the way forward to a greener aviation system huge investments are needed. In order to take the best and most effective decisions, they must be based on robust scientific results.
A particularity of aviation is its major share of non-CO2 effects to climate impact; in fact, non-CO2 effects contribute about two thirds, carbon dioxide itself only about one third, according to a recent assessment. However, while the climate impact from CO2 emissions can be estimated from the fuel burn, the impact of the non-CO2 effects is associated with larger uncertainties.
Non-CO2 effects comprise i. a. the emission of nitrogen oxides (NOx) which modifies the abundances of the greenhouse gases ozone (O3) and methane (CH4). The generation of contrails and contrail cirrus is an obvious effect of aviation. The emitted particles (soot and liquid aerosol droplets) might perturb cloud formation and thus cloud properties far away from the place of their emission. The latter effects are called indirect cloud effects, and while they might have a strong cooling contribution to climate change, almost nothing is currently known about the involved processes.
The ACACIA project aims at improving scientific understanding of those impacts that have the largest uncertainty. It formulates concepts for international measurement campaigns with the goal to constrain numerical models and theories with data. Implementation work is performed on putting all aviation effects on a common scale which will eventually allow providing an updated climate impact assessment. Uncertainties are treated in a transparent way, such that trade-offs between different mitigation strategies can be evaluated explicitly. Finally, the project strives for the knowledge basis necessary to allow strategic guidance for future implementation of mitigation options, that is, to give robust recommendations of no-regret strategies for achieving reduced climate impact of aviation.
ACACIA will further be exploring how changes to aviation might help to bring emissions and impacts in line with the goals of the Paris Agreement. Due to a better understanding of aviation's non-CO2 climate effects, the project will during a later phase deliver necessary input for an eco-efficient planning of flight trajectories, which will allow a substantial reduction of the aviation-induced climate change at the same transport capacity. The project will then also deliver guidelines for the design of future aircraft, which are more adapted to a climate friendly transport.
For modelling, an observational climatology of cloud-active aviation aerosol was set-up. The aerosol properties are sampled from the IAGOS-CARIBIC aircraft. Further optimization of the plume detection and matching algorithm has started to better identify multiple matches and determination of peak area and background. The validation of the set-up by reproducing ice nucleation experiments with known aerosol particles and organic carbon has started. A literature overview has been conducted on soot properties detected in aircraft. The models used are developed and updated. Comparing meteorological conditions from the spring 2020 (Covid-19 global lockdown) period to similar ones in previous years helped to extract a clear signature of air traffic in cloud observations to quantify the radiative impact of contrail-induced cirrus.
Regarding non-CO2 effects, the models have been evaluated against the IAGOS/MOZAIC dataset using an improved methodology. The thermodynamic condition for contrail formation can be predicted quite reliably. But underestimation of frequency and degree of ice supersaturation in weather and climate models leads to very low reliability in forecasting of contrail persistence. Subsequent work with the same data showed that the forecast of ice supersaturation is improved considerably if the threshold for saturation is reduced. The evaluation of the various participating global models for ozone, carbon monoxide, water vapour and NOy is underway. The preliminary assessment shows encouraging results.
The work of searching in the database JULIA for atmospheric conditions that favour the formation of contrails/contrail cirrus was conducted. Literature on past measurement campaigns studying aviation and aerosol/cloud effects, cirrus clouds and mineral dust was used to set up a matrix for aerosol and cirrus parameters. Atmospheric situations favourable for aviation-induced effects were identified in simulations with the global aerosol-climate model ECHAM6-HAM. The effect of soot activation on the radiative forcing appears insignificant in these simulations.
Several important new assessments of aviation climate impact have appeared with major contributions and leadership of authors from the ACACIA consortium. The project also started to analyse different types and sources of uncertainty in modelling results including the monitoring of literature for substantially new insights that could be translated into reduced uncertainties.
For dissemination and communication, a project webpage was created ( an internal team site, and emailing lists. Several scientific presentations at international conferences and workshops were performed. The project clustered with the partner EU projects Great, ClimOp, and Alternate. Exchange and collaboration are ongoing. Thematic stakeholder exchange involving industry and regulation takes place inter alia with the members of the Advisory Board. Several scientific papers were published including contributions to the IPCC report. The project was also invited to an Expert Discussion in the German Federal Chancellery related to the topic 'Green Deal’ in Jan 2021.
ACACIA aims at a breakthrough in the understanding and quantification of the overall impact of aviation aerosol on both ice and liquid clouds. This will be achieved by assessing newly available modelling capabilities across scales, from the aircraft -plume and cloud-resolving to the global scale. This takes advantage of high-quality observational data from in situ measurements and dedicated laboratory experiments. ACACIA takes a game-changing step towards the use of climatological results from long-term observations for the design of proof-of-concept studies for aviation climate impact mitigation. This requires the application of both long-term and global-scale atmospheric airborne databases from research infrastructures combined with data from field campaigns. This will lead to the design of novel large-scale field experiments. ACACIA will finally explore mechanisms for how international aviation can align with the temperature goal as well as the greenhouse gas balance goal of the Paris agreement.
Work-package structure of the ACACIA project