Editing IPCC:AR6/SR15/Chapter-4 (section)

== 4.3.8 Solar Radiation Modification (SRM) ==

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This report refrains from using the term ‘geoengineering’ and separates SRM from CDR and other mitigation options (see Chapter 1, Section 1.4.1 and Glossary).

Table 4.7 gives an overview of SRM methods and characteristics. For a more comprehensive discussion of currently proposed SRM methods, and their implications for geophysical quantities and sustainable development, also see Cross-Chapter Box 10 in this Chapter. This section assesses the feasibility, from an institutional, technological, economic and social-cultural viewpoint, focusing on stratospheric aerosol injection (SAI) unless otherwise indicated, as most available literature is about SAI.

Some of the literature on SRM appears in the forms of commentaries, policy briefs, viewpoints and opinions (e.g., (Horton et al., 2016; Keith et al., 2017; Parson, 2017) <sup>[[#fn:r737|737]]</sup> . This assessment covers original research rather than viewpoints, even if the latter appear in peer-reviewed journals.

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====== Table 4.7 ======

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==== Overview of the main characteristics of the most-studied SRM methods ====

{| class="wikitable"
|-
! SRM indicator
! Stratospheric Aerosol injection (SAI)
! Marine Cloud Brightening (MCB)
! Cirrus Cloud<br />
Thinning (CCT)
! Ground-Based Albedo Modification (GBAM)
|-
| Description of SRM method
| Injection of a gas in the stratosphere, which then converts to aerosols. Injection of other particles also considered.
| Spraying sea salt or other particles into marine clouds, making them more reflective.
| Seeding to promote nucleation, reducing optical thickness and cloud lifetime, to allow more outgoing longwave radiation to escape into space.
| Whitening roofs, changes in land use management (e.g., no-till farming), change of albedo at a larger scale (covering glaciers or deserts with reflective sheeting and changes in ocean albedo).
|-
| Radiative forcing efficiencies
| 1–4 TgS W <sup>−1</sup> m <sup>2</sup> yr <sup>−1</sup>
| 100–295 Tg dry sea salt W <sup>−1</sup> m <sup>2</sup> yr <sup>−1</sup>
| Not known
| Small on global scale, up to 1°C–3°C on regional scale
|-
| Amount needed for 1°C overshoot
| 2–8 TgS yr <sup>−1</sup>
| 70 Tg dry sea salt yr <sup>−1</sup>
| Not known
| 0.04–0.1 albedo change in agricultural and urban areas
|-
| SRM specific impacts on climate variables
| Changes in precipitation patterns and circulation regimes; in case of SO <sub>2</sub> injection, disruption to stratospheric chemistry (for instance NOx depletion and changes in methane lifetime); increase in stratospheric water vapour and tropospheric-stratospheric ice formation affecting cloud microphysics
| Regional rainfall responses; reduction in hurricane intensity
| Low-level cloud changes; tropospheric drying; intensification of the hydrological cycle
| Impacts on precipitation in monsoon areas; could target hot extremes
|-
| SRM specific impacts on human/natural systems
| In case of SO <sub>2</sub> injection, stratospheric ozone loss (which could also have a positive effect – a net reduction in global mortality due to competing health impact pathways) and significant increase of surface UV
| Reduction in the number of mild crop failures
| Not known
|-
| Maturity of science
| Volcanic analogues; ''high agreement'' amongst simulations;<br />
''robust evidence'' on ethical, governance and sustainable development limitations
| Observed in ships tracks;<br />
several simulations confirm mechanism;<br />
regionally limited
| No clear physical mechanism;<br />
''limited evidence'' and ''low agreement'' ;<br />
several simulations
| Natural and land-use analogues;<br />
several simulations confirm mechanism;<br />
''high agreement'' to influence on regional temperature; land use costly
|-
| Key references
| Robock et al., 2008;<br />
Heckendorn et al., 2009;<br />
Tilmes et al., 2012, 2016;<br />
Pitari et al., 2014;<br />
Crook et al., 2015;<br />
C.J. Smith et al., 2017;<br />
Visioni et al., 2017a, b;<br />
Eastham et al., 2018; Plazzotta et al., 2018 <sup>[[#fn:r738|738]]</sup>
| Salter et al., 2008;<br />
Alterskjær et al., 2012;<br />
Jones and Haywood, 2012; Latham et al., 2012, 2013;<br />
Kravitz et al., 2013;<br />
Crook et al., 2015;<br />
Parkes et al., 2015; Ahlm et al., 2017 <sup>[[#fn:r739|739]]</sup>
| Storelvmo et al., 2014;<br />
Kristjánsson et al., 2015;<br />
Jackson et al., 2016;<br />
Kärcher, 2017;<br />
Lohmann and Gasparini, 2017 <sup>[[#fn:r740|740]]</sup>
| Irvine et al., 2011;<br />
Akbari et al., 2012;<br />
Jacobson and Ten Hoeve, 2012;<br />
Davin et al., 2014;<br />
Crook et al., 2015, 2016;<br />
Seneviratne et al., 2018 <sup>[[#fn:r741|741]]</sup>
|}

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SRM could reduce some of the global risks of climate change related to temperature rise (Izrael et al., 2014; MacMartin et al., 2014) <sup>[[#fn:r742|742]]</sup> , rate of sea level rise (Moore et al., 2010) <sup>[[#fn:r743|743]]</sup> , sea-ice loss (Berdahl et al., 2014) <sup>[[#fn:r744|744]]</sup> and frequency of extreme storms in the North Atlantic and heatwaves in Europe (Jones et al., 2018) <sup>[[#fn:r745|745]]</sup> . SRM also holds risks of changing precipitation and ozone concentrations and potentially reductions in biodiversity (Pitari et al., 2014; Visioni et al., 2017a; Trisos et al., 2018) <sup>[[#fn:r746|746]]</sup> . Literature only supports SRM as a supplement to deep mitigation, for example in overshoot scenarios (Smith and Rasch, 2013; MacMartin et al., 2018) <sup>[[#fn:r747|747]]</sup> .

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