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

===== 4.6.3.3.1 Stratospheric aerosol injection =====

<div id="h4-14-siblings" class="h4-siblings"></div>

Most SRM research has focused on stratospheric aerosol injection (SAI) and most SAI studies have assessed the effects of injection. Most research has focused on stratospheric aerosol injection (SAI): the injection of sulphate particles or its precursor gases such as SO <sub>2</sub> , which would then be oxidized to H <sub>2</sub> SO <sub>4</sub> . Injection of other types of aerosol particles, such as calcite (CaCO <sub>3</sub> ), titanium dioxide (TiO <sub>2</sub> ), aluminium oxide (Al <sub>2</sub> O <sub>3</sub> ), and engineered nanoparticles has also been proposed (Keith, 2010; [[#Ferraro--2011|Ferraro et al., 2011]] ; [[#Pope--2012|Pope et al., 2012]] ; [[#Weisenstein--2015|Weisenstein et al., 2015]] ; [[#Jones--2016|A.C. Jones et al., 2016]] ; [[#Keith--2016|Keith et al., 2016]] ), but are much less studied compared to sulphate injection. The natural analogue for sulphate aerosol injection is major volcanic eruptions ( [[#cross-chapter-box-4.1|Cross-Chapter Box 4.1]] ). While volcanic eruptions are not perfect analogues for SAI ( [[#Robock--2013|Robock et al., 2013]] ; [[#Plazzotta--2018|Plazzotta et al., 2018]] ; [[#Duan--2019|Duan et al., 2019]] ), studies on climate impacts of past volcanic eruptions can inform on the potential impact of stratospheric sulphate injection. For example, emergent constraints (Chapters 1 and 5) that relate the climate system response to volcanic eruptions can be used to reduce uncertainty of the land surface temperature response to SAI ( [[#Plazzotta--2018|Plazzotta et al., 2018]] ).

The cooling potential of SAI using sulphate aerosols depends on many factors ( [[#Visioni--2017|Visioni et al., 2017]] ) including the amount of injection ( [[#Niemeier--2015|Niemeier and Timmreck, 2015]] ), aerosol microphysics ( [[#Krishnamohan--2020|Krishnamohan et al., 2020]] ), the spatial and temporal pattern of injection ( [[#Tilmes--2017|Tilmes et al., 2017]] ), response of stratospheric dynamics and chemistry ( [[#Richter%20Jadwiga--2018|Richter Jadwiga et al., 2018]] ), and aerosol effect on cirrus clouds ( [[#Visioni--2018|Visioni et al., 2018]] ). A negative radiative forcing of a few W m <sup>–2</sup> (ranging from one to eight W m <sup>–2</sup> ) could be achieved depending on the amount and location of SO <sub>2</sub> injected into the stratosphere ( [[#Aquila--2014|Aquila et al., 2014]] ; [[#Pitari--2014|Pitari et al., 2014]] ; [[#Niemeier--2015|Niemeier and Timmreck, 2015]] ; [[#Kravitz--2017|Kravitz et al., 2017]] ; [[#Kleinschmitt--2018|Kleinschmitt et al., 2018]] ; [[#Tilmes--2018a|Tilmes et al., 2018a]] ). The simulated efficacy of SAI by emission of SO <sub>2</sub> (radiative forcing per mass of injection rate) generally decreases with the increase in injection rate because of the growth of larger particles (about 0.5 microns) through condensation and coagulation reducing the mass scattering efficiency ( [[#Niemeier--2015|Niemeier and Timmreck, 2015]] ; [[#Kleinschmitt--2018|Kleinschmitt et al., 2018]] ). However, efficacy changes little for total injection rate up to about 25 Tg sulphur per year when SO <sub>2</sub> is injected at multiple locations simultaneously ( [[#Kravitz--2017|Kravitz et al., 2017]] ; [[#Tilmes--2018a|Tilmes et al., 2018a]] ). Differences in model representation of aerosol microphysics, evolution of particle size, stratospheric dynamics and chemistry, and aerosol microphysics–radiation–circulation interactions all contribute to the uncertainty in simulated cooling efficiency of SAI. Compared to sulphate aerosols, injection of non-sulphate particles would result in different cooling efficacy, but understanding is limited (Pope et al.,2012; [[#Weisenstein--2015|Weisenstein et al., 2015]] ; [[#Jones--2016|A.C. Jones et al., 2016]] ).

Earlier modelling studies focused on the effect of equatorial sulphate injection that tends to overcool the tropics and undercool the poles. Compared to equatorial injection, off-equatorial injection at multiple locations shows a closer resemblance to the baseline climate in many aspects, including temperature, precipitation, and sea ice coverage ( [[#Kravitz--2019|Kravitz et al., 2019]] ). However, significant regional and seasonal residual and overcompensating climate change is reported, including regional shifts in precipitation, continued warming of polar oceans, and shifts in the seasonal cycle of snow depth and sea ice cover ( [[#Fasullo--2018|Fasullo et al., 2018]] ; [[#Jiang--2019|Jiang et al., 2019]] ; [[#Simpson--2019b|Simpson et al., 2019b]] ). By appropriately adjusting the amount, latitude, altitude, and timing of the aerosol injection, modelling studies suggest that SAI is conceptually able to achieve some desired combination of radiative forcing and climate response ( ''medium confidence'' ) ( [[#MacMartin--2017|MacMartin et al., 2017]] ; [[#Dai--2018|Dai et al., 2018]] ; [[#Lee--2020|Lee et al., 2020]] ; [[#Visioni--2020b|Visioni et al., 2020b]] ).

There is large uncertainty in the stratospheric response to SAI, and the change in stratospheric dynamics and chemistry would depend on the amount, size, type, location, and timing of injection. There is ''high confidence'' that aerosol-induced stratospheric heating will play an important role in surface climate change ( [[#Simpson--2019b|Simpson et al., 2019b]] ) by altering the effective radiative forcing ( [[#Krishnamohan--2019|Krishnamohan et al., 2019]] ), lower stratosphere stability ( [[#Ferraro--2016|Ferraro and Griffiths, 2016]] ), quasi-biennial oscillation (QBO) ( [[#Aquila--2014|Aquila et al., 2014]] ; [[#Niemeier--2017|Niemeier and Schmidt, 2017]] ; [[#Kleinschmitt--2018|Kleinschmitt et al., 2018]] ), polar vortexes ( [[#Visioni--2020a|Visioni et al., 2020a]] ), and North Atlantic Oscillation ( [[#Jones--2021|Jones et al., 2021]] ). Model simulations indicate stronger polar jets and weaker storm tracks and a poleward shift of the tropospheric mid-latitude jets in response to stratospheric sulphate injections in the tropics ( [[#Ferraro--2015|Ferraro et al., 2015]] ; [[#Richter%20Jadwiga--2018|Richter Jadwiga et al., 2018]] ), as the meridional temperature gradient is increased in the lower stratosphere by the aerosol-induced heating. The aerosol-induced warming would also offset some of the GHG-induced stratospheric cooling. Compared to equatorial injection, off-equatorial injection is ''likely'' to result in reduced change in stratospheric heating, circulation, and QBO ( [[#Richter%20Jadwiga--2018|Richter Jadwiga et al., 2018]] ; [[#Kravitz--2019|Kravitz et al., 2019]] ). Stratospheric ozone response to sulphate injection is uncertain depending on the amount, altitude, and location of injection ( [[#WMO--2018|WMO, 2018]] ). It is ''likely'' that sulphate injection would cause a reduction in polar column ozone concentration and delay the recovery of Antarctic ozone hole ( [[#Pitari--2014|Pitari et al., 2014]] ; [[#Richter%20Jadwiga--2018|Richter Jadwiga et al., 2018]] ; [[#Tilmes--2018b|Tilmes et al., 2018b]] ), which would have implications for UV radiation and surface ozone ( [[#Pitari--2014|Pitari et al., 2014]] ; [[#Xia--2017|Xia et al., 2017]] ; [[#Richter%20Jadwiga--2018|Richter Jadwiga et al., 2018]] ; [[#Tilmes--2018b|Tilmes et al., 2018b]] ). Injection of non-sulphate aerosols is ''likely'' to result in less stratospheric heating and ozone loss ( [[#Pope--2012|Pope et al., 2012]] ; [[#Weisenstein--2015|Weisenstein et al., 2015]] ; [[#Keith--2016|Keith et al., 2016]] ). One side effect of SAI is increased sulphate deposition at surface. A recent modelling study indicates that to maintain global temperature at 2020 levels under RCP 8.5, increased sulphate deposition from stratospheric sulphate injection could be globally balanced by the projected decrease in tropospheric anthropogenic SO <sub>2</sub> emissions, but the spatial distribution of sulphate deposition would move from low to high latitudes ( [[#Visioni--2020c|Visioni et al., 2020c]] ).

<div id="4.6.3.3.2" class="h4-container"></div>

<span id="marine-cloud-brightening"></span>