Periodic Reporting for period 2 - ACACIA (Advancing the Science for Aviation and ClimAte)
Reporting period: 2021-07-01 to 2022-06-30
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.
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.
Samples of real aircraft engine soot could be obtained. Ice nucleation properties have been characterised. The pore freezing mechanism seems to be important. Numerical sensitivity studies indicate that the indirect aerosol effect is weak. A plume-model has been coded and tested. It will provide initial conditions for the study of aerosol processes in large-scale (climate) models. A climatology of cloud-active aviation aerosol has been established for testing the plume model. Simulations show that aviation emissions increase cloud lifetime. The long-range transport of aviation in-cruise emissions has been studied by means of a global circulation model and application of a clustering algorithm.
Southern-hemispheric (low aircraft influence) and northern-hemispheric data (strong influence) have been compared with focus on crystal number densities. Aircraft emissions in pre-existing cirrus clouds lead to increases in crystal number densities a few hundred meters below flight altitude. Properties, occurrence and ambient conditions of contrails and contrail cirrus are analysed IAGOS and colocated reanalysis data have been used to demonstrate the large weather-induced variation of contrail radiative forcing. Dynamical proxies for an improved prediction of contrail persistence have been derived. The impact of NOx emissions on climate is studied using LES and global models. Quality checks by comparison of model output with IAGOS data have been performed.
Measurement campaigns between 1971 and 2018 have been scanned and overviews of each field experiment have been provided to find gaps in data or instrumentation that would be used to better characterise the indirect aviation aerosol effects. A Master’s Thesis on the impact of aviation generated soot on natural cirrus clouds was completed, using a global climate model. In these simulations, aviation soot leads to a weak cooling. The impact of aviation soot on the resulting ice crystal number concentration is statistically insignificant. Targets and tools for a future measurement campaign have been formulated. One target is the large-scale variability of water vapour in the UTLS and its predictability, another is the determination of the ice nucleating properties of aged aircraft soot.
Aviation is projected to cause a total of about 0.1°C of warming by 2050. Its contribution to further warming would be immediately halted by certain measures. ICAO-CAEP-generated scenarios are currently analysed. Unclear definitions of notions like “net zero CO2” etc. open the possibility for different interpretations that lead to different results. A new assessment of the uncertainties in three aviation radiative forcing mechanisms: Induced cirrus, aerosol-cloud interactions, NOx emissions, is underway.
Contributions to IPCC WGI and WGII as well as contributions to ICAO-CAEP have been made. The TAC-5 conference took place in Bad Aibling, Germany, 26-30 June 2022, organized by ACACIA members. Thematic exhange between the partners has taken place with the goal to produce common papers. ACACIA will engage in a project Kommunikationswerkstatt INKOPA.
Southern-hemispheric (low aircraft influence) and northern-hemispheric data (strong influence) have been compared with focus on crystal number densities. Aircraft emissions in pre-existing cirrus clouds lead to increases in crystal number densities a few hundred meters below flight altitude. Properties, occurrence and ambient conditions of contrails and contrail cirrus are analysed IAGOS and colocated reanalysis data have been used to demonstrate the large weather-induced variation of contrail radiative forcing. Dynamical proxies for an improved prediction of contrail persistence have been derived. The impact of NOx emissions on climate is studied using LES and global models. Quality checks by comparison of model output with IAGOS data have been performed.
Measurement campaigns between 1971 and 2018 have been scanned and overviews of each field experiment have been provided to find gaps in data or instrumentation that would be used to better characterise the indirect aviation aerosol effects. A Master’s Thesis on the impact of aviation generated soot on natural cirrus clouds was completed, using a global climate model. In these simulations, aviation soot leads to a weak cooling. The impact of aviation soot on the resulting ice crystal number concentration is statistically insignificant. Targets and tools for a future measurement campaign have been formulated. One target is the large-scale variability of water vapour in the UTLS and its predictability, another is the determination of the ice nucleating properties of aged aircraft soot.
Aviation is projected to cause a total of about 0.1°C of warming by 2050. Its contribution to further warming would be immediately halted by certain measures. ICAO-CAEP-generated scenarios are currently analysed. Unclear definitions of notions like “net zero CO2” etc. open the possibility for different interpretations that lead to different results. A new assessment of the uncertainties in three aviation radiative forcing mechanisms: Induced cirrus, aerosol-cloud interactions, NOx emissions, is underway.
Contributions to IPCC WGI and WGII as well as contributions to ICAO-CAEP have been made. The TAC-5 conference took place in Bad Aibling, Germany, 26-30 June 2022, organized by ACACIA members. Thematic exhange between the partners has taken place with the goal to produce common papers. ACACIA will engage in a project Kommunikationswerkstatt INKOPA.
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.