Cirrus cloud thinning

There is a lack of research on CCT, and to date no outdoor experiments have been conducted. This means that there are many open questions around CCT. The individual (groups of) scholars working on CCT are mainly conducting modeling studies. Tully et al (2022) however recently recommended that the high uncertainties around CCT require ‘more observational evidence … on cirrus formation mechanisms and the impact that natural as well as anthropogenic aerosol have on cirrus properties before further modeling studies proceed.' Research is still continuing, with Smith et al (2023) announcing preliminary results from their modeling of CCT at a conference in early 2023.

It is equally not clear how clouds would be seeded. It has been suggested that already existing commercial jets could have their fuel modified (Mitchell & Finnegan, 2009), or perhaps fleets of drones could be used (Mitchell et al, 2011).

Some studies (Muri et al 2014; Lohmann & Gasparini 2017; Lawrence et al 2018; Gasparini et al 2020) found that CCT could have a significant cooling effect. Muri et al (2014) for instance state that global CCT could have a −1.55 W m−2 forcing effect and change global mean temperatures by −0.94 K. Lawrence et al. (2018) estimate a potential 2 to 3,5 W/m2 forcing effect. However, there are many uncertainties around these numbers (Lohmann and Gasparini, 2017). The underdeveloped state of research on CCT is highlighted in the IPCC AR6 Wg1 report in the comment that they have ‘low confidence in the cooling effect of CCT’ (2021, chapter 4). They write that even though all current cirrus clouds have a net positive radiative forcing effect of around 5 W m–2, maximum CCT cooling potential would only be about 1 to 2 W m–2, and that some model studies found CCT was entirely ineffective and could even have an opposite effect if “over-seeded”. One such adverse effect is described by Liu and Shi (2021), as they find that the seeding of cirrus clouds can also influence other clouds, and thereby eventually have a warming effect.

The complexities around cloud physics would also require further research into many potential side effects and interactions with other suggested measures, as Kuebbeler et al (2012) for example found that SAI could have an effect on cirrus clouds.

There have been very few studies on potential environmental effects of CCT. An important issue would be the material that would be used to thin the clouds, as the use of toxic substances would obviously have greater environmental impact. The IPCC AR6 WG1 report also mentions a possible impact due to an increased amount of radiation reaching the surface (2021, chapter 4). There have been some studies on the potential effect of CCT on the hydrological cycle. Kristjánsson et al (2015) and Muri et al. (2018) found CCT would enhance this cycle and lead to increased global precipitation, although regional variation would have to be further studied.

Cloud seeding is already frequently done to enhance precipitation (see Cloud seeding). Such seeding falls under national or local legislative and governance structures, and CCT could potentially be considered similarly. However, difficulties emerge with regards to deployment over the High Seas.

Despite some positive estimates around the potential of CCT and its listing alongside other SRM climate interventions in major climate reports (see for example the IPCC AR6 reports of WG1 & WG3), public and academic interest remains minimal. The IPCC AR6 reports clearly state that the lack of research activity on the potential of CCT is a major source of uncertainty (WG1 chapter 6 & WG3 chapter 14).