Overview
Cirrus Cloud Thinning is a technique which aims at reducing the amount of cirrus clouds in the sky. Cirrus clouds affect the escape of infrared radiation back out to space and so contribute to the net heating of Earth's atmosphere. CCT techniques involve the seeding of cirrus clouds with ice-nucleating particles to increase the sedimentation rate of the ice crystals.
Cirrus Cloud Thinning - the benefits and options
Aviation exhaust emissions release greenhouse gases, particles and water vapour into the atmosphere. At high altitudes this can create linear clouds called contrails and cirrus clouds. These clouds add to cloud cover and indirectly modify properties of existing cirrus.
There may be no perfect natural analogues for CCT, but aviation-relevant cirrus cloud and the involved processes can help to understand how CCT might work in the real atmosphere. Within ACtIon4Cooling, we will use the existing airborne observations to focus on specific clouds where aviation emissions are dense and further derive their optical thicknesses and ice crystal number concentrations. From a statistical perspective, the project will also use satellite data to determine the optical and microphysical properties of cirrus clouds as a function of latitude and longitude as input for Earth system model studies.
The CCT technique is not well studied to date. Gasparini and Lohmann (2016) performed climate model simulations and concluded that cirrus cloud seeding cannot result in a significant cooling due to the large uncertainties of the complex microphysical mechanisms of those clouds. Penner et al., 2015 found that the cirrus cloud seeding cannot be considered as a viable climate intervention technique but they associate their conclusion to the uncertainties in the modelling and observations of cirrus clouds and particularly, the balance between homogeneous and heterogeneous ice nucleation.
Cirrus Cloud Thinning (CCT) Analogues
There are several best-known natural analogues of CCT: Volcanic eruptions inject sulfur dioxide and water vapor into the atmosphere, forming sulfate aerosols and affecting cirrus formation indirectly. During a mineral dust episode (like Saharan dust), a significant amount of mineral dust is lifted into troposphere, which may act as INPs. The consequent transport of dust particles, depending on meteorological factors, from their source regions across large distance will spread the influence into larger scales. Aircraft-emitted particles may also act as INPs, causing heterogeneous nucleation in regions with a favorable atmospheric state. It leads to the formation of contrails and exerts indirect effects on the existing cirrus clouds. In the frame of the current project, we will focus on the changes of cirrus cloud properties responding to aviation impact as a natural analogue of CCT.
Previous studies indicated that the enhanced heterogeneous nucleation caused by aviation exhaust particles can be responsible for the high values of PLDR of cirrus clouds (Urbanek et al., 2018; Li and Groß, 2021, 2022). Furthermore, cirrus clouds with enhanced PLDR exhibit larger effective ice particles and lower number concentrations (Groß et al., 2023). The findings provide strong support that changes in microphysical properties of cirrus clouds depending on aviation emissions can serve as a natural analogue of CCT.
Key scientific gaps addressed in ACtIon4Cooling
Within this context, ACtIon4Cooling addresses key scientific gaps related to CCT:
- Identification and characterization of aviation-induced cirrus modifications as natural analogues for CCT.
- Analysis of cirrus microphysical and optical properties in high-aviation versus pristine regions using airborne measurements and backward trajectory analysis.
- Regional comparison of cirrus optical depth, depolarization ratio, and microphysical parameters across midlatitudes and high latitudes, accounting for meteorological influences using ERA5 reanalysis data.
- Assessment of long-term trends in cirrus properties in regions with increasing aviation activity using CALIPSO observations.
- Evaluation of potential impacts on precipitation patterns and regional atmospheric circulation.
- Quantification of radiative forcing from CCT-like perturbations using radiative transfer modeling and provision of observational constraints for ICON climate model simulations.
Summary & Results
Available airborne measurements during the ML-CIRRUS aircraft campaign were used to trace specific clouds forming in the regions with either dense aviation emissions, which exhibit enhanced PLDR. Furthermore the cloud optical thickness, ice crystal effective diameters and number concentrations were calculated with coordinated in situ instruments and lidar, revealing that high-PLDR-mode cirrus clouds are characterized by larger particles with smaller number concentrations (Groß et al., 2023). From a statistical perspective, the available CAPLISO satellite data have also been exploited to determine the optical and microphysical properties of cirrus clouds temporally (e.g. during the pre-COVID years period and year-to-year variation) and spatially (comparison between midlatitudes and high latitudes) for studying aviation impacts on cirrus cloud properties (Li and Groß, 2021, 2022, 2025).
The derived microphysical and optical parameters of cirrus clouds as a function of latitude and longitude have been provided for model simulation of ICON and RTM. The RTM results clearly demonstrate that reducing COT decreases the cloud reflectance, leading to lower TOA upward irradiance. This corresponds to a positive shortwave radiative forcing (warming), since less solar radiation is reflected back to space. Thus, in the shortwave domain, cirrus cloud thinning produces a warming tendency. In the longwave domain, the radiative effect of cirrus clouds is different. Cirrus clouds act as semi-transparent emitters and absorbers of terrestrial radiation. A reduction in COT decreases the cloud emissivity, allowing more outgoing longwave radiation to escape to space. This leads to a negative longwave radiative forcing (cooling).
In ICON simulations, no clear perturbation to the top-of-atmosphere radiation budget is detected within the region of interest, suggesting that the imposed perturbation is masked by signals arising from cloud adjustments. Changes in the top-of-atmosphere radiation budget induced by cloud adjustments are particularly pronounced in the tropics. A small increase in surface air temperature of approximately 0.2 K is detected within the region of interest, even if the expected signal was a cooling.
Globally, a mean decrease of 0.01 K in surface air temperature is simulated. The magnitude of the change in surface air temperature over land exceeds that over the oceans. Precipitation responses extend beyond the region of imposed perturbation, reflecting the strong coupling between latent heating, large-scale circulation, and atmospheric energy balance. These model-based results point to the large challenge detecting and attributing desired climate effects of CCT – and similarly, of SAI and MCB as well – to occur in field trials or short-term deployment.
References
Gasparini, B. and Lohmann, U. Why cirrus cloud seeding cannot substantially cool the planet. Journal of Geophysical Research: Atmospheres. 121: 4877–4893, https://doi.org/10.1002/2015JD024666 2016.
Groß, S., Jurkat-Witschas, T., Li, Q., Wirth, M., Urbanek, B., Krämer, M., Weigel, R., and Voigt, C.: Investigating an indirect aviation effect on mid-latitude cirrus clouds – linking lidar-derived optical properties to in situ measurements, Atmos. Chem. Phys., 23, 8369–8381, https://doi.org/10.5194/acp-23-8369-2023, 2023.
Li, Q., and Groß, S.: Changes in cirrus cloud properties and occurrence over Europe during the COVID-19-caused air traffic reduction, Atmos. Chem. Phys., 21, 14573-14590, https://doi.org/10.5194/acp-21-14573-2021, 2021.
Li, Q. and Groß, S.: Satellite observations of seasonality and long-term trends in cirrus cloud properties over Europe: investigation of possible aviation impacts, Atmos. Chem. Phys., 22, 15963–15980, https://doi.org/10.5194/acp-22-15963-2022, 2022.
Li, Q. and Groß, S.: Lidar observations of cirrus cloud properties with CALIPSO from midlatitudes towards high-latitudes, Atmos. Chem. Phys., 25, 16657–16677, https://doi.org/10.5194/acp-25-16657-2025, 2025.
Penner, J. E., Zhou, C. and Liu, X. Can cirrus cloud seeding be used for geoengineering? Geophysical Research Letters. 42: 8775–8782, https://doi.org/10.1002/2015GL065992, 2015.
Urbanek, B., Groß, S., Wirth, M., Rolf, C., Krämer, M., and Voigt, C.: High depolarization ratios of naturally occurring cirrus clouds near air traffic regions over Europe, Geophys. Res. Lett., 45, 13,166–13,172, https://doi.org/10.1029/2018GL079345, 2018.