Editing IPCC:AR6/WGI/Chapter-5 (section)

=== 5.6.3 Biogeochemical Responses to Solar Radiation Modification (SRM) ===

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This section assesses the possible consequences of solar radiation modification (SRM) on the biosphere and global biogeochemical cycles. The SRM options and the physical climate response to SRM is assessed in detail in [[IPCC:Wg1:Chapter:Chapter-4#4.6.3|Section 4.6.3]] and Table 4.7. Section 6.3.6 assesses the potential effective radiative forcing of aerosol-based SRM options and [[IPCC:Wg1:Chapter:Chapter-8#8.6.3|Section 8.6.3]] assesses the abrupt water cycle changes in response to initiation or termination of SRM. Most literature on the biogeochemical responses to SRM focuses on stratospheric aerosol injection (SAI), and only a few studies have investigated the biogeochemical responses to marine cloud brightening (MCB) and cirrus cloud thinning (CCT). At the time of AR5, there were only a few studies on the biogeochemical responses to SRM. The main assessment of AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ) was that SRM will not interfere with the direct biogeochemical effects of increased CO <sub>2</sub> , such as ocean acidification and CO <sub>2</sub> fertilization, but could affect the carbon cycle through climate–carbon feedbacks. Overall, AR5 concluded that the level of confidence on the effects of SRM on carbon and other biogeochemical cycles is ''very low'' ( [[#Ciais--2013|Ciais et al., 2013]] ). Since AR5, more modelling work has been conducted to examine various aspects of the global biogeochemical cycle responses to SRM.

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[[File:3009735603b3df72dabc344e575ea39c IPCC_AR6_WGI_Figure_5_37.png]]

'''Figure 5.37 |''' '''Cumulative carbon dioxide (CO''' <sub>2</sub> ''') uptake by land and ocean carbon sinks in response to stratospheric sulphur dioxide (SO2) injection.''' Results are shown for a scenario with 50-year (2020−2069) continuous stratospheric SO <sub>2</sub> injection at a rate of 5 Tg yr <sup>–1</sup> appplied to a RCP4.5 baseline scenario (GeoMIP experiment G4; [[#Kravitz--2011|Kravitz et al., 2011]] ), followed by termination in year 2070. Anomalies are shown relative to RCP4.5 for the multi-model ensemble mean and for each of six Earth system models (ESMs) over the 50-year period of stratospheric SO <sub>2</sub> injection (left-hand side), and over 20 years after termination of SO <sub>2</sub> injection (right-hand side). Adapted from [[#Plazzotta--2019|Plazzotta et al. (2019)]] . Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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==== 5.6.3.1 Effects of SRM on the Carbon Cycle ====

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Relative to a high-greenhouse gas (GHG) world without solar radiation modification (SRM), SRM would affect the carbon cycles through changes in sunlight, climate (e.g., temperature, precipitation, soil moisture, ocean circulation), and atmospheric chemistry (e.g., ozone; [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] ; [[#Cao--2018|Cao, 2018]] ). Net SRM effects on the carbon cycle, relative to a world without SRM, depend on the change of individual factors, and interactions among them.

SRM-mediated sunlight changes directly affect the carbon cycle. In particular, SAI would reduce the sunlight reaching the Earth’s surface, but also increase the fraction of sunlight that is diffuse. These changes in the quantity and quality of the sunlight have opposing effects on the photosynthesis of land plants. On their own, reductions in photosynthetically active radiation (PAR) will reduce photosynthesis. However, diffuse light is more effective than direct light in accessing the light-limited leaves within plant canopies, leading to the so-called ‘diffuse-radiation’ fertilization effect ( [[#Mercado--2009|Mercado et al., 2009]] ). The estimated balance between the negative impacts of reducing PAR and the positive impacts of increasing diffuse fraction differ between models ( [[#Kalidindi--2015|Kalidindi et al., 2015]] ; [[#Xia--2016|Xia et al., 2016]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ) and across different ecosystems. The change in the absolute amount of direct and diffuse radiation could also depend on the height of the additional sulphate aerosol layer in the stratosphere and the hygroscopic growth of aerosols ( [[#Krishnamohan--2019|Krishnamohan et al., 2019]] , 2020).

SRM-mediated cooling also affects the terrestrial carbon cycle. Relative to a high-GHG world without SRM, the simulated responses of net primary production (NPP) to SRM differ widely between models, such that even the sign of global mean change is uncertain ( [[#Glienke--2015|Glienke et al., 2015]] ). SRM-induced cooling would decrease NPP at high latitudes by reducing the length of the growing season ( [[#Glienke--2015|Glienke et al., 2015]] ). At low latitudes, the NPP response to SRM-induced cooling is sensitive to the effect of nitrogen limitation (Glienke et al., 2015; [[#Duan--2020|Duan et al., 2020]] ). SRM-induced cooling tends to increase NPP in models without the nitrogen cycle because of reduced heat stress. However, in models including the nitrogen cycle, this is counteracted by reductions in NPP because of reductions in nitrogen mineralization and nitrogen availability (Glienke et al., 2015). SRM-induced changes in the hydrological cycle ( [[IPCC:Wg1:Chapter:Chapter-8#8.6.3|Section 8.6.3]] ), including changes in evapotranspiration, precipitation, and soil moisture, also pose strong constraints on the vegetation response (Dagon and Schrag, 2019). For the same amount of global mean cooling, different SRM options, such as SAI, MCB, and CCT, would have different effects on gross primary production (GPP) and NPP because of different spatial patterns of temperature, available sunlight and hydrological cycle changes ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] ) ( [[#Duan--2020|Duan et al., 2020]] ). Modelling studies show that SRM-induced cooling would reduce plant and soil respiration ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Cao--2017|Cao and Jiang, 2017]] ; [[#Muri--2018|Muri et al., 2018]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). Despite the large uncertainty in modelled NPP response, existing modelling studies consistently show that SRM would increase the global land carbon sink relative to a high-CO <sub>2</sub> world without SRM ( ''hi'' ''gh confidence'' ).

Based on available evidence, SRM with elevated CO <sub>2</sub> would increase global mean NPP and carbon storage on land relative to an unperturbed climate, mainly because of CO <sub>2</sub> fertilization of photosynthesis ( ''high confidence'' ) ( [[#Glienke--2015|Glienke et al., 2015]] ; [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Dagon--2019|Dagon and Schrag, 2019]] ; [[#Duan--2020|Duan et al., 2020]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). However, the amount of increase is uncertain as it depends on the extent to which CO <sub>2</sub> fertilization of land plants is limited by nutrient availability.

Relative to a high-CO <sub>2</sub> world without SRM, SRM would also have compensating effects on crop yields. SRM is expected to have a positive impact on crop yields by diminishing heat stress ( [[#Pongratz--2012|Pongratz et al., 2012]] ). However, reductions in light availability will produce a counteracting reduction in crop yields, especially if the crop type does not benefit appreciably from diffuse-light fertilization ( [[#Proctor--2018|Proctor et al., 2018]] ). The balance between these effects varies markedly across crop types and regions, from projected increases in maize production in China ( [[#Xia--2014|Xia et al., 2014]] ) to reductions in groundnut yields in parts of India ( [[#Yang--2016|Yang et al., 2016]] ). Because of these diverging results from a limited set of studies, there is overall ''low confidence'' in the effect of SRM on crop yields.

Consistent with the AR5 assessment, there is ''high confidence'' that SRM would not mitigate CO <sub>2</sub> -induced ocean acidification ( [[#Ciais--2013|Ciais et al., 2013]] ). Some studies even suggest an acceleration of deep-ocean acidification as a result of ocean circulation change ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Lauvset--2017|Lauvset et al., 2017]] ). There are large differences in the simulated spatial pattern of ocean NPP change in response to SRM, which depends strongly on the SRM method that is considered ( [[#Partanen--2016|Partanen et al., 2016]] ; [[#Lauvset--2017|Lauvset et al., 2017]] ).

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==== 5.6.3.2 Consequences of SRM and its Termination on Atmospheric CO <sub>2</sub> burden ====

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Modelling studies consistently show that, relative to a high-CO <sub>2</sub> world without SRM, SRM-induced cooling ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] ) would reduce plant and soil respiration, and also reduce the negative effects of warming on ocean carbon uptake ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Xia--2016|Xia et al., 2016]] ; [[#Cao--2017|Cao and Jiang, 2017]] ; [[#Jiang--2018|Jiang et al., 2018]] ; [[#Muri--2018|Muri et al., 2018]] ; [[#Sonntag--2018|Sonntag et al., 2018]] ; [[#Plazzotta--2019|Plazzotta et al., 2019]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). A multi-model study ( [[#Plazzotta--2019|Plazzotta et al., 2019]] ) indicates that, relative to a high-CO <sub>2</sub> concentration world without SRM, stratospheric sulphur dioxide (SO <sub>2</sub> ) injection increases the allowable CO <sub>2</sub> emissions by enhancing CO <sub>2</sub> uptake by both land and ocean (Figure 5.37). As a result of enhanced global carbon uptake, SRM would reduce the burden of atmospheric CO <sub>2</sub> ( ''high confidence'' ). However, the amount of SRM-induced reduction in atmospheric CO <sub>2</sub> depends on the future emissions scenario and modelled oceanic and terrestrial carbon sinks, which differ widely between models ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Xia--2016|Xia et al., 2016]] ; [[#Cao--2017|Cao and Jiang, 2017]] ; [[#Muri--2018|Muri et al., 2018]] ). Models that include the terrestrial nitrogen cycle usually report a much smaller reduction of atmospheric CO <sub>2</sub> in response to SRM than models without the nitrogen cycle, mainly because nitrogen limitation leads to a weaker terrestrial carbon sink ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Muri--2018|Muri et al., 2018]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). Large-scale application of SAI is found to reduce the rate of release of CO <sub>2</sub> and CH <sub>4</sub> from permafrost thaw ( [[#Lee--2019|Lee et al., 2019]] ; [[#Chen--2020|Chen et al., 2020]] ).

A hypothetical sudden and sustained termination of SRM would cause a rapid increase in global warming that would pose great risks to biodiversity ( [[#Jones--2013|]] [[#Jones--2013|A. Jones et al., 2013]] ; [[#McCusker--2014|McCusker et al., 2014]] ; [[#Trisos--2018|Trisos et al., 2018]] ) ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] ). It would also weaken carbon sinks, accelerating atmospheric CO <sub>2</sub> accumulation and inducing further warming (Figure 5.37; [[#Matthews--2007|Matthews and Caldeira, 2007]] ; [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Muri--2018|Muri et al., 2018]] ; [[#Plazzotta--2019|Plazzotta et al., 2019]] ). However, a scenario with gradual phase-out of SRM under emissions reduction could reduce the large negative effect of sudden SRM termination ( [[#MacMartin--2014|MacMartin et al., 2014]] ; [[#Keith--2015|Keith and MacMartin, 2015]] ; [[#Tilmes--2016|Tilmes et al., 2016]] ),though this would be limited by how rapidly emissions reductions can be scaled-up ( [[#Ekholm--2016|Ekholm and Korhonen, 2016]] ).

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==== 5.6.3.3 Consequences of SRM on other Biogeochemical Cycles ====

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SAI is found to reduce global average surface ozone concentration ( [[#Xia--2017|Xia et al., 2017]] ) mainly as a result of aerosol-induced reduction in stratospheric ozone at polar regions, resulting in reduced transport of ozone from the stratosphere ( [[#Pitari--2014|Pitari et al., 2014]] ; [[#Tilmes--2018|Tilmes et al., 2018]] ). The reduction in surface ozone, together with alteration to ultraviolet (UV) radiation, would have important implications for vegetation response ( [[#Xia--2017|Xia et al., 2017]] ). A modelling study shows that sea salt aerosol injection for MCB would reduce hydroxyl (OH) concentration and increase CH <sub>4</sub> lifetime, and hence, have potential implications for surface ozone pollution ( [[#Horowitz--2020|Horowitz et al., 2020]] ). It has also been reported that the use of SAI to limit global mean warming from 2°C to 1.5°C would reduce fire weather in many areas ( [[#Burton--2018|Burton et al., 2018]] ).

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==== 5.6.3.4 Synthesis of Biogeochemical Responses to SRM ====

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SRM would alter the global carbon cycle through SRM-induced climate effect, such as changes in sunlight, temperature, precipitation, and ocean circulation. Compared to a high-CO <sub>2</sub> world without SRM, SRM would enhance the net uptake of CO <sub>2</sub> by the terrestrial biosphere and ocean, thus acting to reduce atmospheric CO <sub>2</sub> ( ''high confidence'' ). SRM would also affect surface ozone, UV radiation, and atmospheric chemistry. Due to complex interplays between SRM-induced changes in direct and diffuse sunlight, temperature, the coupling of water-carbon-nitrogen cycles, and atmospheric chemistry, there is ''large uncertainty'' in the overall response of the terrestrial biosphere response to SRM. Thus, the level of ''confidence'' on the effect of SRM on carbon and other biogeochemical cycles is ''low'' .

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