Editing IPCC:AR6/WGIII/Chapter-14 (section)

=== Cross-Working Group Box 4 | Solar Radiation Modification ===

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'''Authors:''' Govindasamy Bala (India), Heleen de Coninck (the Netherlands), Oliver Geden (Germany), Veronika Ginzburg (the Russian Federation), Katharine J. Mach (the United States of America), Anthony Patt (Switzerland), Sonia I. Seneviratne (Switzerland), Masahiro Sugiyama (Japan), Christopher H. Trisos (South Africa), Maarten van Aalst (the Netherlands)

Proposed Solar Radiation Modification schemes

This cross-working group box assesses Solar Radiation Modification (SRM) proposals, their potential contribution to reducing or increasing climate risk, as well as other risks they may pose (categorised as risks from responses to climate change in the IPCC AR6 risk definition in 1.2.1.1), and related perception, ethics and governance questions.

SRM refers to proposals to increase the reflection of shortwave radiation (sunlight) back to space to counteract anthropogenic warming and some of its harmful impacts ( [[#de%20Coninck--2018|de Coninck et al. 2018]] ) (AR6 WGI Chapters 4 and 5). A number of SRM options have been proposed, including: stratospheric aerosol interventions (SAI), marine cloud brightening (MCB), ground-based albedo modifications (GBAM), and ocean albedo change (OAC). Although not strictly a form of SRM, cirrus cloud thinning (CCT) has been proposed to cool the planet by increasing the escape of longwave thermal radiation to space and is included here for consistency with previous assessments ( [[#de%20Coninck--2018|de Coninck et al. 2018]] ). SAI is the most-researched proposal. Modelling studies show SRM could reduce surface temperatures and potentially ameliorate some climate change risks (with more confidence for SAI than other options), but SRM could also introduce a range of new risks.

There is high agreement in the literature that for addressing climate change risks, SRM cannot be the main policy response to climate change and is, at best, a supplement to achieving sustained net zero or net negative CO 2 emission levels globally ( [[#de%20Coninck--2018|de Coninck et al. 2018]] ; [[#MacMartin--2018|MacMartin et al. 2018]] ; [[#Buck--2020|Buck et al. 2020]] ; [[#National%20Academies%20of%20Sciences%20Engineering%20and%20Medecine--2021|National Academies of Sciences Engineering and Medecine 2021]] ). SRM contrasts with climate change mitigation activities, such as emissions reductions and CDR, as it introduces a ‘mask’ to the climate change problem by altering the Earth’s radiation budget, rather than attempting to address the root cause of the problem, which is the increase in GHGs in the atmosphere. In addition, the effects of proposed SRM options would only last as long as a deployment is maintained – for example, requiring a yearly injection of aerosols in the case of SAI as the lifetime of aerosols in the stratosphere is one to three years ( [[#Niemeier--2011|Niemeier et al. 2011]] ) or continuous spraying of sea salt in the case of MCB as the lifetime of sea salt aerosols in the atmosphere is only about 10 days – which contrasts with the long lifetime of CO 2 and its climate effects, with global warming resulting from CO 2 emissions likely remaining at a similar level for a hundred years or more ( [[#MacDougall--2020|MacDougall et al. 2020]] ) and long-term climate effects of emitted CO 2 remaining for several hundreds to thousands of years ( [[#Solomon--2009|Solomon et al. 2009]] ).

Which scenarios?

The choice of SRM deployment scenarios and reference scenarios is crucial in assessment of SRM risks and its effectiveness in attenuating climate change risks ( [[#Keith--2015|Keith and MacMartin 2015]] ; [[#Honegger--2021a|Honegger et al. 2021a]] ). Most climate model simulations have used scenarios with highly stylised large SRM forcing to fully counteract large amounts of warming in order to enhance the signal-to-noise ratio of climate responses to SRM ( [[#Kravitz--2015|Kravitz et al. 2015]] ; [[#Sugiyama--2018a|Sugiyama et al. 2018a]] ; [[#Krishnamohan--2019|Krishnamohan et al. 2019]] ).

The effects of SRM fundamentally depend on a variety of choices about deployment ( [[#Sugiyama--2018b|Sugiyama et al. 2018b]] ), including: its position in the portfolio of human responses to climate change (e.g., the magnitude of SRM used against the background radiative forcing), governance of research and potential deployment strategies, and technical details (latitude, materials, and season, among others, see AR6 WGI Chapter 4.6.3.3). The plausibility of many SRM scenarios is highly contested and not all scenarios are equally plausible because of socio-political considerations ( [[#Talberg--2018|Talberg et al. 2018]] ), as with, for example, CDR ( [[#Fuss--2014|Fuss et al. 2014]] , 2018). Development of scenarios and their selection in assessments should reflect a diverse set of societal values with public and stakeholder inputs ( [[#Sugiyama--2018a|Sugiyama et al. 2018a]] ; [[#Low--2020|Low and Honegger 2020]] ), as depending on the focus of a limited climate model simulation, SRM could look grossly risky or highly beneficial ( [[#Pereira--2021|Pereira et al. 2021]] ).

In the context of reaching the long-term global temperature goal of the Paris Agreement, there are different hypothetical scenarios of SRM deployment: early, substantial mitigation with no SRM, more limited or delayed mitigation with moderate SRM, unchecked emissions with total reliance on SRM, and regionally heterogeneous SRM. Each scenario presents different levels and distributions of SRM benefits, side effects, and risks. The more intense the SRM deployment, the larger is the likelihood for the risks of side effects and environmental risks (e.g., [[#Heutel--2018|Heutel et al., 2018]] ). Regional disparities in climate hazards may result from both regionally-deployed SRM options such as GBAM, and more globally uniform SRM such as SAI ( [[#Jones--2018|Jones et al. 2018]] ; [[#Seneviratne--2018|Seneviratne et al. 2018]] ). There is an emerging literature on smaller forcings of SAI to reduce global average warming, for instance, to hold global warming to 1.5°C or 2°C alongside ambitious conventional mitigation ( [[#Jones--2018|Jones et al. 2018]] ; [[#MacMartin--2018|MacMartin et al. 2018]] ), or bring down temperature after an overshoot

( [[#Tilmes--2020|Tilmes et al. 2020]] ). If emissions reductions and CDR are deemed insufficient, SRM may be seen by some as the only option left to ensure the achievement of the Paris Agreement’s temperature goal by 2100.

SRM risks to human and natural systems and potential for risk reduction

Since AR5, hundreds of climate modelling studies have simulated effects of SRM on climate hazards ( [[#Kravitz--2015|Kravitz et al. 2015]] ; [[#Tilmes--2018|Tilmes et al. 2018]] ). Modelling studies have shown SRM has the potential to offset some effects of increasing GHGs on the global and regional climate, including the increase in frequency and intensity of extremes of temperature and precipitation, melting of Arctic sea ice and mountain glaciers, weakening of Atlantic meridional overturning circulation, changes in frequency and intensity of tropical cyclones, and decrease in soil moisture (AR6 WGI, Chapter 4). However, while SRM may be effective in alleviating anthropogenic climate

'''Cross-Working Group Box 4, Table 1 | SRM options and their potential climate and non-climate impacts.''' '''Description, potential climate impacts, potential impacts on human and natural systems, and termination effects of a number of SRM options: stratospheric aerosol interventions (SAI), marine cloud brightening (MCB), ocean albedo change (OAC), ground-based albedo modifications (GBAM), and cirrus cloud thinning (CCT).'''

{| class="wikitable"
|-
| SRM option

| SAI

| MCB

| OAC

| GBAM

| CCT

|-
| Description

| Injection of reflective aerosol particles directly into the stratosphere or a gas which then

converts to aerosols that reflect sunlight

| Spraying sea salt or other particles in marine clouds, making them more reflective

| Increase surface albedo of the ocean (e.g., by creating microbubbles or placing reflective foam on the surface)

| Whitening roofs, changes in land use management (e.g., no-till farming, bioengineering to make crop leaves more reflective), desert albedo enhancement, covering glaciers with reflective sheeting

| Seeding to promote nucleation of cirrus clouds, reducing optical thickness and cloud lifetime to allow more outgoing longwave radiation to escape to space

|-
| Potential climate impacts ''other than reduced warming''

| Change precipitation and runoff pattern; reduced temperature and precipitation extremes; precipitation reduction in some monsoon regions; decrease in direct and increase in diffuse sunlight at surface; changes to stratospheric dynamics and chemistry; potential

delay in ozone hole recovery; changes in surface ozone and UV radiation

| Change in land–sea contrast in temperature and precipitation, regional precipitation and runoff changes

| Change in land–sea contrast in temperature and precipitation, regional precipitation and runoff changes.

| Changes in regional precipitation pattern, regional extremes and regional circulation

| Changes in temperature and precipitation pattern, altered regional water cycle, increase in sunlight reaching the surface

|-
| Potential impacts on human and natural systems

| Changes in crop yields, changes in land and ocean ecosystem productivity, acid rain (if using sulphate), reduced risk of heat stress to corals

| Changes in regional ocean productivity, changes in crop yields, reduced heat stress for corals, changes in ecosystem productivity on land, sea salt deposition over land

| Unresearched

| Altered photosynthesis and carbon uptake and side effects on biodiversity

| Altered photosynthesis and carbon uptake

|-
| Termination effects

| Sudden and sustained termination would result in rapid warming, and abrupt changes to water cycle. Magnitude of termination depends on the degree of warming offset.

| Sudden and sustained

termination would result in rapid warming, and abrupt changes to water cycle. Magnitude of termination depends on the degree of warming offset.

| Sudden and sustained

termination would result in rapid warming. Magnitude of termination depends on the degree of warming offset.

| GBAM can be maintained over several years without major termination effects because of its regional scale of application.

Magnitude of termination depends on the degree of warming offset.

| Sudden and sustained

termination would result in rapid warming. Magnitude of termination depends on the degree of warming offset.

|-
| References (also see main text of this box)

| [[#Visioni--2017|Visioni et al. (2017)]]

[[#Tilmes--2018|Tilmes et al. (2018)]]

[[#Simpson--2019|Simpson et al. (2019)]]

| [[#Latham--2012|Latham et al. (2012)]]

[[#Ahlm--2017|Ahlm et al. (2017)]]

[[#Stjern--2018|Stjern et al. (2018)]]

| [[#Evans--2010|Evans et al. (2010)]]

[[#Crook--2015|Crook et al. (2015)]]

| [[#Davin--2014|Davin et al. (2014)]]

[[#Crook--2015|Crook et al. (2015)]]

[[#Zhang--2016|Zhang et al. (2016)]]

[[#Field--2018|Field et al. (2018)]]

[[#Seneviratne--2018|Seneviratne et al. (2018)]]

| [[#Storelvmo--2014|Storelvmo and Herger (2014)]]

[[#Crook--2015|Crook et al. (2015)]]

[[#Jackson--2016|Jackson et al. (2016)]]

[[#Duan--2020|Duan et al. (2020)]]

[[#Gasparini--2020|Gasparini et al. (2020)]]

|}

warming either locally or globally, it would not maintain the climate in a present-day state nor return the climate to a pre-industrial state (climate averaged over 1850–1900) (AR6 WGI, Box 1.2) in all regions and in all seasons even when used to fully offset the global mean warming ( ''high confidence'' ) (AR6 WGI Chapter 4). This is because the climate forcing and response to SRM options are different from the forcing and response to GHG increase. Because of these differences in climate forcing and response patterns, the regional and seasonal climates of a world with a global mean warming of 1.5°C or 2°C achieved via SRM would be different from a world with similar global mean warming but achieved through mitigation ( [[#MacMartin--2018|MacMartin et al. 2018]] ). At the regional scale and seasonal timescale there could be considerable residual climate change and/or overcompensating change (e.g., more cooling, wetting or drying than just what’s needed to offset warming, drying or wetting due to anthropogenic greenhouse gas emissions), and there is ''low confidence'' in understanding of the climate response to SRM at the regional scale (AR6 WGI, Chapter 4).

SAI implemented to partially offset warming (e.g., offsetting half of global warming) may have potential to ameliorate hazards in multiple regions and reduce negative residual change, such as drying compared to present-day climate, that are associated with fully offsetting global mean warming ( [[#Irvine--2020|Irvine and Keith 2020]] ), but may also increase flood and drought risk in Europe compared to unmitigated warming ( [[#Jones--2021|Jones et al. 2021]] ). Recent modelling studies suggest it is conceptually possible to meet multiple climate objectives through optimally designed SRM strategies (WGI, Chapter 4). Nevertheless, large uncertainties still exist for climate processes associated with SRM options (e.g., aerosol-cloud-radiation interaction) (AR6 WGI, Chapter 4) ( [[#Kravitz--2020|Kravitz and MacMartin 2020]] ).

Compared with climate hazards, many fewer studies have examined SRM risks – the potential adverse consequences to people and ecosystems from the combination of climate hazards, exposure and vulnerability – or the potential for SRM to reduce risk ( [[#Curry--2014|Curry et al. 2014]] ; [[#Irvine--2017|Irvine et al. 2017]] ). Risk analyses have often used inputs from climate models forced with stylised representations of SRM, such as dimming the sun. Fewer have used inputs from climate models that explicitly simulated injection of gases or aerosols into the atmosphere, which include more complex cloud-radiative feedbacks. Most studies have used scenarios where SAI is deployed to hold average global temperature constant despite high emissions.

There is ''low confidence'' and large uncertainty in projected impacts of SRM on crop yields due in part to a limited number of studies. Because SRM would result in only a slight reduction in CO 2 concentrations relative to the emissions scenario without SRM (AR6 WGI, Chapter 5), the CO 2 fertilisation effect on plant productivity is nearly the same in emissions scenarios with and without SRM. Nevertheless, changes in climate due to SRM are likely to have some impacts on crop yields. A single study indicates MCB may reduce crop failure rates compared to climate change from a doubling of CO 2 pre-industrial concentrations ( [[#Parkes--2015|Parkes et al. 2015]] ). Models suggest SAI cooling would reduce crop productivity at higher latitudes compared to a scenario without SRM by reducing the growing season length, but benefit crop productivity in lower latitudes by reducing heat stress ( [[#Pongratz--2012|Pongratz et al. 2012]] ; [[#Xia--2014|Xia et al. 2014]] ; [[#Zhan--2019|Zhan et al. 2019]] ). Crop productivity is also projected to be reduced where SAI reduces rainfall relative to the scenario without SRM, including a case where reduced Asian summer monsoon rainfall causes a reduction in groundnut yields ( [[#Xia--2014|Xia et al. 2014]] ; [[#Yang--2016|Yang et al. 2016]] ). SAI will increase the fraction of diffuse sunlight, which is projected to increase photosynthesis in forested canopy, but will reduce the direct and total available sunlight, which tends to reduce photosynthesis. As total sunlight is reduced, there is a net reduction in crop photosynthesis with the result that any benefits to crops from avoided heat stress may be offset by reduced photosynthesis, as indicated by a single statistical modelling study ( [[#Proctor--2018|Proctor et al. 2018]] ). SAI would reduce average surface ozone concentration ( [[#Xia--2017|Xia et al. 2017]] ) mainly as a result of aerosol-induced reduction in stratospheric ozone in polar regions, resulting in reduced downward transport of ozone to the troposphere ( [[#Pitari--2014|Pitari et al. 2014]] ; [[#Tilmes--2018|Tilmes et al. 2018]] ). The reduction in stratospheric ozone also allows more UV radiation to reach the surface. The reduction in surface ozone, together with an increase in surface UV radiation, would have important implications for crop yields but there is ''low confidence'' in our understanding of the net impact.

Few studies have assessed potential SRM impacts on human health and well-being. SAI using sulfate aerosols is projected to deplete the ozone layer, increasing mortality from skin cancer, and SAI could increase particulate matter due to offsetting warming, reduced precipitation and deposition of SAI aerosols, which would increase mortality, but SAI also reduces surface-level ozone exposure, which would reduce mortality from air pollution, with net changes in mortality uncertain and depending on aerosol type and deployment scenario ( [[#Effiong--2016|Effiong and Neitzel 2016]] ; [[#Eastham--2018|Eastham et al. 2018]] ; [[#Dai--2020|Dai et al. 2020]] ). However, these effects may be small compared to changes in risk from infectious disease (e.g., mosquito-borne illnesses) or food security due to SRM influences on climate (Carlson et al. 2022). Using volcanic eruptions as a natural analogue, a sudden implementation of SAI that forced the El Niño–Southern Oscillation (ENSO) system may increase risk of severe cholera outbreaks in Bengal ( [[#Trisos--2018|Trisos et al. 2018]] ; [[#Pinke--2019|Pinke et al. 2019]] ). Considering only mean annual temperature and precipitation, SAI that stabilises global temperature at its present-day level is projected to reduce income inequality between countries compared to the highest warming pathway (RCP8.5) ( [[#Harding--2020|Harding et al. 2020]] ). Some integrated assessment model

scenarios have included SAI ( [[#Arino--2016|Arino et al. 2016]] ; [[#Emmerling--2018|Emmerling and Tavoni 2018]] ; [[#Heutel--2018|Heutel et al. 2018]] ; [[#Helwegen--2019|Helwegen et al. 2019]] ; [[#Rickels--2020|Rickels et al. 2020]] ) showing the indirect costs and benefits to welfare dominate, since the direct economic cost of SAI itself is expected to be relatively low ( [[#Moriyama--2017|Moriyama et al. 2017]] ; [[#Smith--2018|Smith and Wagner 2018]] ). There is a general lack of research on the wide scope of potential risk or risk reduction to human health, well-being and sustainable development from SRM and on their distribution across countries and vulnerable groups ( [[#Honegger--2021a|Honegger et al. 2021a]] ; Carlson et al. 2022).

SRM may also introduce novel risks for international collaboration and peace. Conflicting temperature preferences between countries may lead to counter-geoengineering measures such as deliberate release of warming agents or destruction of deployment equipment ( [[#Parker--2018|Parker et al. 2018]] ). Game-theoretic models and laboratory experiments indicate a powerful actor or group with a higher preference for SRM may use SAI to cool the planet beyond what is socially optimal, imposing welfare losses on others although this cooling does not necessarily imply excluded countries would be worse off relative to a world of unmitigated warming ( [[#Ricke--2013|Ricke et al. 2013]] ; [[#Weitzman--2015|Weitzman 2015]] ; [[#Abatayo--2020|Abatayo et al. 2020]] ). In this context, counter-geoengineering may promote international cooperation or lead to large welfare losses ( [[#Helwegen--2019|Helwegen et al. 2019]] ; [[#Abatayo--2020|Abatayo et al. 2020]] ).

Cooling caused by SRM would increase the global land and ocean CO 2 sinks ( ''medium confidence'' ), but this would not stop CO 2 from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions ( ''high confidence'' ) (AR6 WGI, Chapter 5).

Few studies have assessed potential SRM impacts on ecosystems. SAI and MCB may reduce risk of coral reef bleaching compared to global warming with no SAI ( [[#Latham--2013|Latham et al. 2013]] ; [[#Kwiatkowski--2015|Kwiatkowski et al. 2015]] ), but risks to marine life from ocean acidification would remain, because SRM proposals do not reduce elevated anthropogenic atmospheric CO 2 concentrations. MCB could cause changes in marine net primary productivity by reducing light availability in deployment regions, with important fishing regions off the west coast of South America showing both large increases and decreases in productivity ( [[#Partanen--2016|Partanen et al. 2016]] ; [[#Keller--2018|Keller 2018]] ).

There is large uncertainty in terrestrial ecosystem responses to SRM. By decoupling increases in atmospheric greenhouse gas concentrations and temperature, SAI could generate substantial impacts on large-scale biogeochemical cycles, with feedbacks to regional and global climate variability and change ( [[#Zarnetske--2021|Zarnetske et al. 2021]] ). Compared to a high CO 2 world without SRM, global-scale SRM simulations indicate reducing heat stress in low latitudes would increase plant productivity, but cooling would also slow down the process of nitrogen mineralisation, which could decrease plant productivity ( [[#Glienke--2015|Glienke et al. 2015]] ; [[#Duan--2020|Duan et al. 2020]] ). In high latitude and polar regions SRM may limit vegetation growth compared to a high CO 2 world without SRM, but net primary productivity may still be higher than pre-industrial climate ( [[#Glienke--2015|Glienke et al. 2015]] ). Tropical forests cycle more carbon and water than other terrestrial biomes but large areas of the tropics may tip between savanna and tropical forest depending on rainfall and fire ( [[#Beer--2010|Beer et al. 2010]] ; [[#Staver--2011|Staver et al. 2011]] ). Thus, SAI-induced reductions in precipitation in Amazonia and central Africa are expected to change the biogeography of tropical ecosystems in ways different both from present-day climate and global warming without SAI ( [[#Simpson--2019|Simpson et al. 2019]] ; [[#Zarnetske--2021|Zarnetske et al. 2021]] ). This would have potentially large consequences for ecosystem services (AR6 WGII, Chapters 2 and 9). When designing and evaluating SAI scenarios, biome-specific responses need to be considered if SAI approaches are to benefit rather than harm ecosystems. Regional precipitation change and sea salt deposition over land from MCB may increase or decrease primary productivity in tropical rainforests ( [[#Muri--2015|Muri et al. 2015]] ). SRM that fully offsets warming could reduce the dispersal velocity required for species to track shifting temperature niches whereas partially offsetting warming with SAI would not reduce this risk unless rates of warming were also reduced ( [[#Trisos--2018|Trisos et al. 2018]] ; [[#Dagon--2019|Dagon and Schrag 2019]] ). SAI may reduce high fire-risk weather in Australia, Europe and parts of the Americas, compared to global warming without SAI ( [[#Burton--2018|Burton et al. 2018]] ). Yet SAI using sulphur injection could shift the spatial distribution of acid-induced aluminium soil toxicity into relatively undisturbed ecosystems in Europe and North America ( [[#Visioni--2020|Visioni et al. 2020]] ). For the same amount of global mean cooling, SAI, MCB, and CCT would have different effects on gross and net primary productivity because of different spatial patterns of temperature, available sunlight, and hydrological cycle changes ( [[#Duan--2020|Duan et al. 2020]] ). Large-scale modification of land surfaces for GBAM may have strong trade-offs with biodiversity and other ecosystem services, including food security ( [[#Seneviratne--2018|Seneviratne et al. 2018]] ). Although existing studies indicate SRM will have widespread impacts on ecosystems, risks and potential for risk reduction for marine and terrestrial ecosystems and biodiversity remain largely unknown.

A sudden and sustained termination of SRM in a high CO 2 emissions scenario would cause rapid climate change ( ''high confidence'' ) (AR6 WGI, Chapter 4). More scenario analysis is needed on the potential likelihood of sudden termination ( [[#Kosugi--2013|Kosugi 2013]] ; [[#Irvine--2020|Irvine and Keith 2020]] ). A gradual phase-out of SRM combined with emissions reduction and CDR could avoid these termination effects ( ''medium confidence'' ) (MacMartin et al. 2014; [[#Keith--2015|Keith and MacMartin 2015]] ; [[#Tilmes--2016|Tilmes et al. 2016]] ). Several studies find that large and extremely rapid warming and abrupt changes to the water cycle would occur within a decade if a sudden termination of SAI occurred ( [[#McCusker--2014|McCusker et al. 2014]] ; [[#Crook--2015|Crook et al. 2015]] ). The size of this ‘termination shock’ is proportional to the amount of radiative forcing being masked by SAI. A sudden termination of SAI could place many thousands of species at risk of extinction, because the resulting rapid warming would be too fast for species to track the changing climate ( [[#Trisos--2018|Trisos et al. 2018]] ).

Public perceptions of SRM

Studies on the public perception of SRM have used multiple methods: questionnaire surveys, workshops, and focus group interviews ( [[#Burns--2016|Burns et al. 2016]] ; [[#Cummings--2017|Cummings et al. 2017]] ). Most studies have been limited to Western societies with some exceptions. Studies have repeatedly found that respondents are largely unaware of SRM ( [[#Merk--2015|Merk et al. 2015]] ). In the context of this general lack of familiarity, the publics prefer carbon dioxide removal (CDR) to SRM ( [[#Pidgeon--2012|Pidgeon et al. 2012]] ), are very cautious about SRM deployment because of potential environmental side effects and governance concerns, and mostly reject deployment for the foreseeable future. Studies also suggest conditional and reluctant support for research, including proposed field experiments, with conditions of proper governance ( [[#Sugiyama--2020|Sugiyama et al. 2020]] ). Recent studies show that the perception varies with the intensity of deliberation ( [[#Merk--2019|Merk et al. 2019]] ), and that the public distinguishes different funding sources ( [[#Nelson--2021|Nelson et al. 2021]] ). Limited studies for developing countries show a tendency for respondents to be more open to SRM ( [[#Visschers--2017|Visschers et al. 2017]] ; [[#Sugiyama--2020|Sugiyama et al. 2020]] ), perhaps because they experience climate change more directly ( [[#Carr--2018|Carr and Yung 2018]] ). In some Anglophone countries, a small portion of the public believes in chemtrail conspiracy theories, which are easily found in social media ( [[#Tingley--2017|Tingley and Wagner 2017]] ; [[#Allgaier--2019|Allgaier 2019]] ). Since researchers rarely distinguish different SRM options in engagement studies, there remains uncertainty in public perception.

Ethics

There is broad literature on ethical considerations around SRM, mainly stemming from philosophy or political theory, and mainly focused on SAI ( [[#Flegal--2019|Flegal et al. 2019]] ). There is concern that publicly debating, researching and potentially deploying SAI could involve a ‘moral hazard’, with potential to obstruct ongoing and future mitigation efforts ( [[#Morrow--2014|Morrow 2014]] ; [[#Baatz--2016|Baatz 2016]] ; [[#McLaren--2016|McLaren 2016]] ), while empirical evidence is limited and mostly at the individual, not societal, level ( [[#Burns--2016|Burns et al. 2016]] ; [[#Merk--2016|Merk et al. 2016]] ; [[#Merk--2019|Merk et al. 2019]] ). There is low agreement whether research and outdoors experimentation will create a ‘slippery slope’ toward eventual deployment, leading to a lock-in to long-term SRM, or whether it can be effectively regulated at a later stage to avoid undesirable outcomes ( [[#Hulme--2014|Hulme 2014]] ; [[#Parker--2014|Parker 2014]] ; [[#Callies--2019|Callies 2019]] ; [[#McKinnon--2019|McKinnon 2019]] ). Regarding potential deployment of SRM, procedural, distributive and recognitional conceptions of justice are being explored ( [[#Svoboda--2014|Svoboda and Irvine 2014]] ; [[#Svoboda--2017|Svoboda 2017]] ; [[#Preston--2018|Preston and Carr 2018]] ; [[#Hourdequin--2019|Hourdequin 2019]] ). With the SRM research community’s increasing focus on distributional impacts of SAI, researchers have started more explicitly considering inequality in participation and inclusion of vulnerable countries and marginalised social groups ( [[#Flegal--2018|Flegal and Gupta 2018]] ; [[#Whyte--2018|Whyte 2018]] ; [[#Táíwò--2021|Táíwò and Talati 2021]] ), including considering stopping research ( [[#Stephens--2020|Stephens and Surprise 2020]] ; [[#National%20Academies%20of%20Sciences%20Engineering%20and%20Medecine--2021|National Academies of Sciences Engineering and Medecine 2021]] ). There is recognition that SRM research has been conducted predominantly by a relatively small number of experts in the Global North, and that more can be done to enable participation from diverse peoples and geographies in setting research agendas and research governance priorities, and undertaking research, with initial efforts to this effect ( [[#Rahman--2018|Rahman et al. 2018]] ), noting that unequal power relations in participation could influence SRM research governance and have potential implications for policy ( [[#Winickoff--2015|Winickoff et al. 2015]] ; [[#Frumhoff--2018|Frumhoff and Stephens 2018]] ; [[#Whyte--2018|Whyte 2018]] ; [[#Biermann--2019|Biermann and Möller 2019]] ; [[#McLaren--2021|McLaren and Corry 2021]] ; [[#National%20Academies%20of%20Sciences%20Engineering%20and%20Medecine--2021|National Academies of Sciences Engineering and Medecine 2021]] ; [[#Táíwò--2021|Táíwò and Talati 2021]] ).

Governance of research and of deployment

Currently, there is no dedicated, formal international SRM governance for research, development, demonstration, or deployment (AR6 WGIII, Chapter 14). Some multilateral agreements – such as the UN Convention on Biological Diversity or the Vienna Convention on the Protection of the Ozone Layer – indirectly and partially cover SRM, but none is comprehensive and the lack of robust and formal SRM governance poses risks ( [[#Ricke--2013|Ricke et al. 2013]] ; [[#Talberg--2018|Talberg et al. 2018]] ; [[#Reynolds--2019a|Reynolds 2019a]] ). While governance objectives range broadly, from prohibition to enabling research and potentially deployment ( [[#Sugiyama--2018b|Sugiyama et al. 2018b]] ; [[#Gupta--2020|Gupta et al. 2020]] ), there is agreement that SRM governance should cover all interacting stages of research through to any potential, eventual deployment with rules, institutions, and norms ( [[#Reynolds--2019b|Reynolds 2019b]] ). Accordingly, governance arrangements are co-evolving with respective SRM technologies across the interacting stages of research, development, demonstration, and – potentially – deployment ( [[#Rayner--2013|Rayner et al. 2013]] ; [[#Parker--2014|Parker 2014]] ; [[#Parson--2014|Parson 2014]] ). Stakeholders are developing governance already in outdoors research; for example, for MCB and OAC experiments on the

Great Barrier Reef ( [[#McDonald--2019|McDonald et al. 2019]] ). Co-evolution of governance and SRM research provides a chance for responsibly developing SRM technologies with broader public participation and political legitimacy, guarding against potential risks and harms relevant across a full range of scenarios, and ensuring that SRM is considered only as a part of a broader portfolio of responses to climate change ( [[#Stilgoe--2015|Stilgoe 2015]] ; [[#Nicholson--2018|Nicholson et al. 2018]] ). For SAI, large-scale outdoor experiments even with low radiative forcing could be transboundary and those with deployment-scale radiative forcing may not be distinguished from deployment, such that [[#MacMartin--2019|MacMartin and Kravitz (2019)]] argue for continued reliance on modelling until a decision on whether and how to deploy is made, with modelling helping governance development.

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<span id="global-governance-of-solar-radiation-modification-and-associated-risks"></span>
==== 14.4.5.1 Global Governance of Solar Radiation Modification and Associated Risks ====

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Solar radiation modification, in the literature also referred to as ‘solar geoengineering’, refers to the intentional modification of the Earth’s shortwave radiative budget, such as by increasing the reflection of sunlight back to space, with the aim of reducing warming. Several SRM options have been proposed, including stratospheric aerosol injection (SAI), marine cloud brightening (MCB), ground-based albedo modifications (GBAM), and ocean albedo change (OAC). SRM has been discussed as a potential response option within a broader climate risk management strategy, as a supplement to emissions reduction, carbon dioxide removal and adaptation ( [[#Crutzen--2006|Crutzen 2006]] ; [[#Shepherd--2009|Shepherd 2009]] ; [[#Caldeira--2017|Caldeira and Bala 2017]] ; [[#Buck--2020|Buck et al. 2020]] ), for example as a temporary measure to slow the rate of warming ( [[#Keith--2015|Keith and MacMartin 2015]] ) or address temperature overshoot ( [[#MacMartin--2018|MacMartin et al. 2018]] ; [[#Tilmes--2020|Tilmes et al. 2020]] ). SRM assessments of potential benefits and risks still primarily rely on modelling efforts and their underlying scenario assumptions ( [[#Sugiyama--2018a|Sugiyama et al. 2018a]] ), for example in the context of the Geoengineering Model Intercomparison Project GeoMIP6 ( [[#Kravitz--2015|Kravitz et al. 2015]] ). Recently, small-scale MCB and OAC experiments started to take place on the Great Barrier Reef ( [[#McDonald--2019|McDonald et al. 2019]] ).

SAI – the most researched SRM method – poses significant international governance challenges since it could potentially be deployed uni- or minilaterally and alter the global mean temperature much faster than any other climate policy measure, at comparatively low direct costs ( [[#Parson--2014|Parson 2014]] ; [[#Nicholson--2018|Nicholson et al. 2018]] ; [[#Smith--2018|Smith and Wagner 2018]] ; [[#Sugiyama--2018b|Sugiyama et al. 2018b]] ; [[#Reynolds--2019a|Reynolds 2019a]] ). While being dependent on the design of deployment systems, both geophysical benefits and adverse effects would potentially be unevenly distributed (AR6 WGI, Chapter 4). Perceived local harm could exacerbate geopolitical conflicts, not least depending on which countries are part of a deployment coalition ( [[#Maas--2012|Maas and Scheffran 2012]] ; [[#Zürn--2013|Zürn and Schäfer 2013]] ), but also because immediate attribution of climatic impacts to detected SAI deployment would not be possible. Uncoordinated or poorly researched deployment by a limited number of states, triggered by perceived climate emergencies, could create international tensions ( [[#Corry--2017|Corry 2017]] ; [[#Lederer--2018|Lederer and Kreuter 2018]] ). An additional risk is that of rapid temperature rise following an abrupt end of SAI activities ( [[#Parker--2018|Parker and Irvine 2018]] ; [[#Rabitz--2019|Rabitz 2019]] ).

While there is room for national and even sub-national governance of SAI – for example on research (differentiating indoor from open-air) ( [[#Jinnah--2018|Jinnah et al. 2018]] ; [[#Hubert--2020|Hubert 2020]] ) and public engagement ( [[#Bellamy--2017|Bellamy and Lezaun 2017]] ; [[#Flegal--2019|Flegal et al. 2019]] ) – international governance of SAI faces the challenge that comprehensive institutional architectures designed too far in advance could prove either too restrictive or too permissive in light of subsequent political, institutional, geophysical and technological developments ( [[#Sugiyama--2018a|Sugiyama et al. 2018a]] ; [[#Reynolds--2019a|Reynolds 2019a]] ). Views on governance encompass a broad range, from aiming to restrict to wanting to enable research and potentially deployment; in between these poles, other authors stress the operationalisation of the precautionary approach: preventing deployment until specific criteria regarding scientific consensus, impact assessments and governance issues are met ( [[#Tedsen--2013|Tedsen and Homann 2013]] ; [[#Wieding--2020|Wieding et al. 2020]] ). Many scholars suggest that governance arrangements ought to co-evolve with respective SRM technologies ( [[#Parker--2014|Parker 2014]] ), including that it stay at least one step ahead of research, development, demonstration, and – potentially – deployment ( [[#Rayner--2013|Rayner et al. 2013]] ; [[#Parson--2014|Parson 2014]] ). With the modelling community’s increasing focus on showing that, and in what ways, SAI could help to minimise climate change impacts in the Global South, the SRM governance literature has come to include considerations of how SAI could contribute to global equity ( [[#Horton--2016|Horton and Keith 2016]] ; [[#Flegal--2018|Flegal and Gupta 2018]] ; [[#Hourdequin--2018|Hourdequin 2018]] ).

Given that risks and potential benefits of SRM proposals differ substantially and their large-scale deployment is highly speculative, there is a wide array of concrete proposals for near-term anticipatory or adaptive governance. Numerous authors suggest a wide range of governance principles [[#Nicholson--2018|Nicholson et al. (2018)]] encapsulate most of these in suggesting a list of four: (i) Guard against potential risks and harm; (ii) Enable appropriate research and development of scientific knowledge; (iii) Legitimise any future research or policymaking through active and informed public and expert community engagement; (iv) Ensure that SRM is considered only as a part of a broader, mitigation-centred portfolio of responses to climate change. Regarding international institutionalisation, options range from formal integration into existing UN bodies like the UNFCCC ( [[#Nicholson--2018|Nicholson et al. 2018]] ) or the Convention on Biological Diversity (CBD) ( [[#Bodle--2014|Bodle et al. 2014]] ) to the creation of specific, but less formalised global fora ( [[#Parson--2013|Parson and Ernst 2013]] ) to forms of club governance ( [[#Bodansky--2013|Bodansky 2013]] ; [[#Lloyd--2014|Lloyd and Oppenheimer 2014]] ). Recent years have also seen the emergence of transnational non-state actors focusing on SRM governance, primarily expert networks and NGOs ( [[#Horton--2020|Horton and Koremenos 2020]] ).

Currently, there is no targeted international law relating to SRM, although some multilateral agreements – such as the Convention on Biological Diversity, the UN Convention on the Law of the Sea, the Environmental Modification Convention, and the Vienna Convention on the Protection of the Ozone Layer and its Montreal Protocol – contain provisions applicable to SRM ( [[#Bodansky--2013|Bodansky 2013]] ; [[#Jinnah--2019|Jinnah and Nicholson 2019]] ; [[#Reynolds--2019a|Reynolds 2019a]] ).

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==== 14.4.5.2 Carbon Dioxide Removal ====

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Carbon dioxide removal (CDR) refers to a cluster of technologies, practices, and approaches that remove and sequester carbon dioxide from the ocean and atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products (Table 12.6). In contrast to SRM, CDR does not necessarily impose transboundary risks, except insofar as misleading accounting of its use and deployment could give a false picture of countries’ overall mitigation efforts. CDR is clearly a form of climate change mitigation, and as described in [[IPCC:Wg3:Chapter:Chapter-12|Chapter 12]] is needed to counterbalance residual GHG emissions that may prove hard to abate (e.g., from industry, aviation or agriculture) in the context of reaching net zero emissions both globally – in the context of Article 4 of the Paris Agreement – and nationally. CDR could also later be used for reducing atmospheric CO 2 concentrations by providing net negative emissions at the global level ( [[#Fuglestvedt--2018|Fuglestvedt et al. 2018]] ; [[#Bellamy--2019|Bellamy and Geden 2019]] ). Despite the common feature of removing carbon dioxide, technologies like afforestation/reforestation, soil carbon sequestration, bioenergy with carbon capture and storage, direct air capture with carbon storage, enhanced weathering, ocean alkalinity enhancement or ocean fertilisation are very different, as are the governance challenges. [[IPCC:Wg3:Chapter:Chapter-12|Chapter 12]] highlights the sustainable development risks associated with land and water use that are connected to the biological approaches to CDR. As a public good which largely lacks incentives to be pursued as a business case, most types of CDR require a suite of dedicated policy instruments that address both near-term needs as well as long-term continuity at scale ( [[#Honegger--2021b|Honegger et al. 2021b]] ).

CDR methods other than afforestation/reforestation and soil carbon sequestration have only played a minor role in UNFCCC negotiations so far ( [[#Fridahl--2017|Fridahl 2017]] ; [[#Rumpel--2020|Rumpel et al. 2020]] ). To accelerate, and indeed better manage CDR globally, stringent rules and practices regarding emissions accounting, measuring, reporting and verifying and project-based market mechanisms have been proposed ( [[#Honegger--2018|Honegger and Reiner 2018]] ; [[#Mace--2018|Mace et al. 2018]] ). Given their historic responsibility, it can be expected that developed countries would carry the main burden of researching, developing, demonstrating and deploying CDR, or finance such projects in other countries ( [[#Fyson--2020|Fyson et al. 2020]] ; [[#Pozo--2020|Pozo et al. 2020]] ). [[#McLaren--2019|McLaren et al. (2019)]] suggest that there is a rationale for separating the international commitments for net negative emissions from those for emissions reductions.

Specific regulations on CDR options have been limited to those posing transboundary risks, namely the use of ocean fertilisation. In a series of separate decisions from 2008 to 2013, Parties to the London Convention and Protocol limited ocean fertilisation activities to only those of a research character, and in 2012 the CBD made a non-legally-binding decision to do the same, further requiring such research activities to be limited scale, and carried out under controlled conditions, until more knowledge is gained to be able to assess the risks ( [[#GESAMP--2019|GESAMP 2019]] ; [[#Burns--2020|Burns and Corbett 2020]] ). In doing so they have taken a precautionary approach ( [[#Sands--2018|Sands and Peel, 2018]] ). The London Convention and Protocol has also developed an Assessment Framework for Scientific Research Involving Ocean Fertilisation ( [[#London%20Convention/Protocol--2010|London Convention/Protocol 2010]] ) and in 2013 adopted amendments (which are not yet in force) to regulate marine carbon dioxide removal activities, including ocean fertilisation.

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