Editing IPCC:AR6/WGI/Chapter-4 (section)

===== 4.6.3.3.3 Cirrus cloud thinning =====

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Cirrus clouds trap more outgoing thermal radiation than they reflect incoming solar radiation and thus have an overall warming effect on the climate system ( [[#Mitchell--2009|Mitchell and Finnegan, 2009]] ). The aim of cirrus cloud thinning (CCT) is to reduce cirrus cloud optical depth by increasing the heterogeneous nucleation via seeding cirrus clouds with an optimal concentration of ice nucleating particles, which might cause larger ice crystals and rapid fallout, resulting in reduced lifetime and coverage of cirrus clouds ( [[#Muri--2014|Muri et al., 2014]] ; Gasparini et al., 2017; [[#Lohmann--2017|Lohmann and Gasparini, 2017]] ; [[#Gruber--2019|Gruber et al., 2019]] ). CCT aims to achieve the opposite effect of contrails that increase cirrus cover and cause a small positive ERF (Section 7.3). A high-resolution modelling study of CCT over a limited area of the Arctic suggested that cirrus seeding causes a decrease in ice crystal number concentration and a reduction in mixed-phase cloud cover, both of which cause a cooling effect ( [[#Gruber--2019|Gruber et al., 2019]] ).

Under present-day climate, cirrus clouds exerts a net positive radiative forcing of about 5 W m <sup>–2</sup> ( [[#Gasparini--2016|Gasparini and Lohmann, 2016]] ; [[#Hong--2016|Hong et al., 2016]] ), indicating a maximum cooling potential of the same magnitude if all cirrus cloud were removed from the climate system. However, modelling results show a much smaller cooling effect of CCT. For the optimal ice nuclei seeding concentration and globally non-uniform seeding strategy, a net negative cloud radiative forcing of about 1 to 2 W m <sup>–2</sup> is achieved (Storelvmo and Herger, 2014; [[#Gasparini--2020|Gasparini et al., 2020]] ). A few studies find that no seeding strategy could achieve a significant cooling effect, owing to complex microphysical mechanisms limiting robust climate responses to cirrus seeding ( [[#Penner--2015|Penner et al., 2015]] ; [[#Gasparini--2016|Gasparini and Lohmann, 2016]] ). A higher than optimal concentration of ice nucleating particles could also result in over-seeding that increases rather than decreases cirrus optical thickness ( [[#Storelvmo--2013|Storelvmo et al., 2013]] ; [[#Gasparini--2016|Gasparini and Lohmann, 2016]] ). Thus, there is ''low confidence'' in the cooling effect of CCT, due to limited understanding of cirrus microphysics, its interaction with aerosols, and the complexity of seeding strategy.

Relative to the high-GHG climate and for the same amount of global cooling, CCT is simulated to cause an increase in global precipitation compared to shortwave-based SRM options such as SAI and MCB ( [[#Duan--2018|Duan et al., 2018]] ; Muriet al., 2018) because of the opposing effects of CCT and increased CO <sub>2</sub> on outgoing longwave radiation ( [[#Kristjánsson--2015|Kristjánsson et al., 2015]] ; [[#Jackson--2016|Jackson et al., 2016]] ). Combining SAI and CCT has suggested that GHG-induced changes in global mean temperature and precipitation can be simultaneously offset ( [[#Cao--2017|Cao et al., 2017]] ), but there is ''low confidence'' in the applicability of this result to the real world owing to the large uncertainty in simulating aerosol forcing and the complex cirrus microphysical processes.

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