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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<span id="table-4.7"></span>
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'''Table 4.7'''

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

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{| 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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==== 4.3.8.1 Governance and institutional feasibility ====

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There is ''robust evidence'' but ''medium agreement'' for unilateral action potentially becoming a serious SRM governance issue (Weitzman, 2015; Rabitz, 2016) <sup>[[#fn:r748|748]]</sup> , as some argue that enhanced collaboration might emerge around SRM (Horton, 2011) <sup>[[#fn:r749|749]]</sup> . An equitable institutional or governance arrangement around SRM would have to reflect views of different countries (Heyen et al., 2015) <sup>[[#fn:r750|750]]</sup> and be multilateral because of the risk of termination, and risks that implementation or unilateral action by one country or organization will produce negative precipitation or extreme weather effects across borders (Lempert and Prosnitz, 2011; Dilling and Hauser, 2013; NRC, 2015b) <sup>[[#fn:r751|751]]</sup> . Some have suggested that the governance of research and field experimentation can help clarify uncertainties surrounding deployment of SRM (Long and Shepherd, 2014; Parker, 2014; NRC, 2015c; Caldeira and Bala, 2017; Lawrence and Crutzen, 2017) <sup>[[#fn:r752|752]]</sup> , and that SRM is compatible with democratic processes (Horton et al., 2018) <sup>[[#fn:r753|753]]</sup> or not (Szerszynski et al., 2013; Owen, 2014) <sup>[[#fn:r754|754]]</sup> .

Several possible institutional arrangements have been considered for SRM governance: under the UNFCCC (in particular under the Subsidiary Body on Scientific and Technological Advice (SBSTA)) or the United Nations Convention on Biological Diversity (UNCBD) (Honegger et al., 2013; Nicholson et al., 2018) <sup>[[#fn:r755|755]]</sup> , or through a consortium of states (Bodansky, 2013; Sandler, 2017) <sup>[[#fn:r756|756]]</sup> . Reasons for states to join an international governance framework for SRM include having a voice in SRM diplomacy, prevention of unilateral action by others and benefits from research collaboration (Lloyd and Oppenheimer, 2014) <sup>[[#fn:r757|757]]</sup> .

Alongside SBSTA, the WMO, UNESCO and UN Environment could play a role in governance of SRM (Nicholson et al., 2018) <sup>[[#fn:r758|758]]</sup> . Each of these organizations has relevance with respect to the regulatory framework (Bodle et al., 2012; Williamson and Bodle, 2016) <sup>[[#fn:r759|759]]</sup> . The UNCBD gives guidance that ‘that no climate-related geo-engineering activities that may affect biodiversity take place’ (CBD, 2010) <sup>[[#fn:r760|760]]</sup> .

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==== 4.3.8.2 Economic and technological feasibility ====

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The literature on the engineering costs of SRM is limited and may be unreliable in the absence of testing or deployment. There is ''high agreement'' that costs of SAI (not taking into account indirect and social costs, research and development costs and monitoring expenses) may be in the range of 1–10 billion USD yr <sup>−1</sup> for injection of 1–5 MtS to achieve cooling of 1–2 W m <sup>−</sup> <sup>2</sup> (Robock et al., 2009; McClellan et al., 2012; Ryaboshapko and Revokatova, 2015; Moriyama et al., 2016) <sup>[[#fn:r761|761]]</sup> , suggesting that cost-effectiveness may be high if side-effects are low or neglected (McClellan et al., 2012) <sup>[[#fn:r762|762]]</sup> . The overall economic feasibility of SRM also depends on externalities and social costs (Moreno-Cruz and Keith, 2013; Mackerron, 2014) <sup>[[#fn:r763|763]]</sup> , climate sensitivity (Kosugi, 2013) <sup>[[#fn:r764|764]]</sup> , option value (Arino et al., 2016) <sup>[[#fn:r765|765]]</sup> , presence of climate tipping points (Eric Bickel, 2013) <sup>[[#fn:r766|766]]</sup>   and damage costs as a function of the level of SRM (Bahn et al., 2015; Heutel et al., 2018) <sup>[[#fn:r767|767]]</sup> . Modelling of game-theoretic, strategic interactions of states under heterogeneous climatic impacts shows ''low agreement'' on the outcome and viability of a cost-benefit analysis for SRM (Ricke et al., 2015; Weitzman, 2015) <sup>[[#fn:r768|768]]</sup> .

For SAI, there is ''high agreement'' that aircrafts could, after some modifications, inject millions of tons of SO <sub>2</sub> in the lower stratosphere (at approximately 20 km; (Davidson et al., 2012; McClellan et al., 2012; Irvine et al., 2016) <sup>[[#fn:r769|769]]</sup> .

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==== 4.3.8.3 Social acceptability and ethics ====

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Ethical questions around SRM include those of international responsibilities for implementation, financing, compensation for negative effects, the procedural justice questions of who is involved in decisions, privatization and patenting, welfare, informed consent by affected publics, intergenerational ethics (because SRM requires sustained action in order to avoid termination hazards), and the so-called ‘moral hazard’ (Burns, 2011; Whyte, 2012; Gardiner, 2013; Lin, 2013; Buck et al., 2014; Klepper and Rickels, 2014; Morrow, 2014; Wong, 2014; Reynolds, 2015; Lockley and Coffman, 2016; McLaren, 2016; Suarez and van Aalst, 2017; Reynolds et al., 2018) <sup>[[#fn:r770|770]]</sup> . The literature shows ''low agreement'' on whether SRM research and deployment may lead policy-makers to reduce mitigation efforts and thus imply a moral hazard (Linnér and Wibeck, 2015) <sup>[[#fn:r771|771]]</sup> . SRM might motivate individuals (as opposed to policymakers) to reduce their GHG emissions, but even a subtle difference in the articulation of information about SRM can influence subsequent judgements of favourability (Merk et al., 2016) <sup>[[#fn:r772|772]]</sup> . The argument that SRM research increases the likelihood of deployment (the ‘slippery slope’ argument), is also made (Quaas et al., 2017) <sup>[[#fn:r773|773]]</sup> , but some also found an opposite effect (Bellamy and Healey, 2018) <sup>[[#fn:r774|774]]</sup> .

Unequal representation and deliberate exclusion are plausible in decision-making on SRM, given diverging regional interests and the anticipated low resource requirements to deploy SRM (Ricke et al., 2013) <sup>[[#fn:r775|775]]</sup> . Whyte (2012) <sup>[[#fn:r776|776]]</sup> argues that the concerns, sovereignties, and experiences of indigenous peoples may particularly be at risk.

The general public can be characterized as oblivious to and worried about SRM (Carr et al., 2013; Parkhill et al., 2013; Wibeck et al., 2017) <sup>[[#fn:r777|777]]</sup> . An emerging literature discusses public perception of SRM, showing a lack of knowledge and unstable  opinions (Scheer and Renn, 2014) <sup>[[#fn:r778|778]]</sup> . The perception of controllability affects legitimacy and public acceptability of SRM experiments (Bellamy et al., 2017) <sup>[[#fn:r779|779]]</sup> . In Germany, laboratory work on SRM is generally approved of, field research much less so, and immediate deployment is largely rejected (Merk et al., 2015; Braun et al., 2017) <sup>[[#fn:r780|780]]</sup> . Various factors could explain variations in the degree of rejection of SRM between Canada, China, Germany, Switzerland, the United Kingdom, and the United States (Visschers et al., 2017) <sup>[[#fn:r781|781]]</sup> .

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