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

===== 4.6.3.3.2 Marine cloud brightening =====

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Marine cloud brightening (MCB) involves injecting small aerosols such as sea salt into the base of marine stratocumulus clouds where the aerosols act as cloud condensation nuclei (CCN). In the absence of other changes, an increase in CCN would produce higher cloud droplet number concentration with reduced droplet sizes, increasing cloud albedo. Increased droplet concentration may also increase cloud water content and optical thickness, but recent studies suggest that liquid water path response to anthropogenic aerosols is weak due to the competing effects of suppressed precipitation and enhanced cloud water evaporation ( [[#Toll--2019|Toll et al., 2019]] ). An analogue for MCB are reflective, persistent ‘ship tracks’ observed after the passage of a sea-going vessel emitting combustion aerosols into susceptible clouds (Christensen and Stephens, 2011; [[#Chen--2012|Chen et al., 2012]] ; [[#Gryspeerdt--2019|Gryspeerdt et al., 2019]] ). A recent study ( [[#Diamond--2020|Diamond et al., 2020]] ) found a substantial increase in cloud reflectivity from shipping in south-east Atlantic basin, suggesting that a regional-scale test of MCB in stratocumulus‐dominated regions could be successful.

Modelling studiessuggest that MCB has the potential to achieve a negative forcing of about 1 to 5 W m <sup>–2</sup> , depending on the deployment area and strategies of cloud seeding (Hill and Ming, 2012; [[#Partanen--2012|Partanen et al., 2012]] ; [[#Alterskjær--2013|Alterskjær et al., 2013]] ; [[#Ahlm--2017|Ahlm et al., 2017]] ; [[#Stjern--2018|Stjern et al., 2018]] ). Regional applications of MCB has also been suggested for offsetting severe impacts from tropical cyclones whose genesis is associated with higher SST ( [[#MacCracken--2016|MacCracken, 2016]] ; [[#Latham--2014|Latham et al., 2014]] ) and for protecting coral reefs from higher SST ( [[#Latham--2013|Latham et al., 2013]] ). However, such regional approaches also involve large uncertainties in the magnitude of the responses and consequences.

Several modelling studies suggest that the direct scattering effect by injected particles might also play an important role in the cooling effect of MCB, but the relative contribution of aerosol–cloud and aerosol–cloud–radiation effect is uncertain (Partanen et al., 2012; [[#Kravitz--2013b|Kravitz et al., 2013b]] ; [[#Ahlm--2017|Ahlm et al., 2017]] ). Relative to the high-GHG climate, it is ''likely'' that MCB would increase precipitation over tropical land due to the inhomogeneous forcing pattern of MCB over ocean and land ( ''medium confidence'' ) ( [[#Bala--2011|Bala et al., 2011]] ; [[#Alterskjær--2013|Alterskjær et al., 2013]] ; [[#Niemeier--2013|Niemeier et al., 2013]] ; [[#Ahlm--2017|Ahlm et al., 2017]] ; [[#Muri--2018|Muri et al., 2018]] ; [[#Stjern--2018|Stjern et al., 2018]] ). Because of the high level of uncertainty associated with cloud microphysics and aerosol–cloud–radiation interaction (Section 7.3), the climate response to MCB is as uncertain. Results from global climate models are subject to large uncertainty because of different treatment of cloud microphysics and inadequate representation of sub-grid aerosol and cloud processes (Alterskjær and Kristjánsson, 2013; [[#Stuart--2013|Stuart et al., 2013]] ; [[#Connolly--2014|Connolly et al., 2014]] ; [[#Stjern--2018|Stjern et al., 2018]] ). Sea salt deposition over land ( [[#Muri--2015|Muri et al., 2015]] ) and the effect of sea salt emission on atmospheric chemistry ( [[#Horowitz--2020|Horowitz et al., 2020]] ) are some of the potential side effects of MCB.

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