stratospheric aerosol injection
Stratospheric aerosol injection is a proposed method of solar geoengineering (or solar radiation modification) to reduce global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter. It appears that stratospheric aerosol injection, at a moderate intensity, could counter most changes to temperature and precipitation, take effect rapidly, have low direct implementation costs, and be reversible in its direct climatic effects. The Intergovernmental Panel on Climate Change concludes that it "is the most-researched [solar geoengineering] method, with high agreement that it could limit warming to below 1.5 °C (2.7 °F)." However, like other solar geoengineering approaches, stratospheric aerosol injection would do so imperfectly and other effects are possible, particularly if used in a suboptimal manner.Various forms of sulfur have been shown to cool the planet after large volcanic eruptions. However, as of 2021, there has been little research and existing natural aerosols in the stratosphere are not well understood. so there is no leading candidate material. Alumina, calcite and salt are also under consideration. The leading proposed method of delivery is custom aircraft.

Scientific basis
Natural and anthropogenic sulfates
There is a wide range of particulate matter suspended in the atmosphere at various height and in various sizes. By far the best-studied are the various sulfur compounds collectively referred to sulfate aerosols. This group includes inorganic sulfates (SO42-),HSO4- and H2SO4-: organic sulfur compounds are sometimes included as well, but are of lower importance. Sulfate aerosols can be anthropogenic (through the combustion of fossil fuels with a high sulfur content, primarily coal and certain less-refined fuels, like aviation and bunker fuel),  biogenic from hydrosphere and biosphere, geological via volcanoes or weather-driven from wildfires and other natural combustion events.Inorganic aerosols are mainly produced when sulfur dioxide reacts with water vapor to form gaseous sulfuric acid and various salts (often through an oxidation reaction in the clouds), which are then thought to experience hygroscopic growth and coagulation and then shrink through evaporation. as microscopic liquid droplets or fine (diameter of about 0.1 to 1.0 micrometre) sulfate solid particles in a colloidal suspension, with smaller particles at times coagulating into larger ones.The other major source are chemical reactions with dimethyl sulfide (DMS), predominantly sourced from marine plankton, with a smaller contribution from swamps and other such wetlands. And sometimes, aerosols are produced from photochemical decomposition of COS (carbonyl sulfide), or when solid sulfates in the sea salt spray can react with gypsum dust particles).

Pollution controls and the discovery of radiative effects
The discovery of these negative effects spurred the rush to reduce atmospheric sulfate pollution, typically through flue-gas desulfurization installations at power plants, such as wet scrubbers or fluidized bed combustion. In the United States, this began with the passage of the Clean Air Act in 1970, which was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including sulfates, dropped by 53% in the US. By 2010, it valued the healthcare savings from these reductions at $50 billion annually. In Europe, it was estimated in 2021 that the 18 coal-fired power plants in the western Balkans which lack controls on sulfur dioxide pollution have emitted two-and-half times more of it than all 221 coal plants in the European Union which are fitted with these technologies. Globally, the uptake of treaties such as the 1985 Helsinki Protocol on the Reduction of Sulfur Emissions and its successors had gradually spread from the developed to the developing countries. While China and India have seen decades in rapid growth of sulfur emissions while they declined in the U.S. and Europe, they have also peaked in the recent years. In 2005, China was the largest polluter, with its estimated 25,490,000 short tons (23.1 Mt) emissions increasing by 27% since 2000 alone and roughly matching the U.S. emissions in 1980. That year was also the peak, and a consistent decline was recorded since then. Similarly, India's sulfur dioxide emissions appear to have been largely flat in the 2010s, as more coal-fired power plants were fitted with pollution controls even as the newer ones were still coming online.
Yet, around the time these treaties and technology improvements were taking place, evidence was coming in that sulfate aerosols were affecting both the visible light received by the Earth and its surface temperature. On one hand, the study of volcanic eruptions, notably 1991 eruption of Mount Pinatubo in the Philippines,  had shown that the mass formation of sulfate aerosols by these eruptions formed a subtle whitish haze in the sky, reducing the amount of Sun's radiation reaching the Earth's surface and rapidly losing the heat they absorb back to space, as well increasing clouds' albedo (i.e. making them more reflective) by changing their consistency to a larger amount of smaller droplets, which was the principal reason for a clear drop in global temperatures for several years in their wake. On the other hand, multiple studies have shown that between 1950s and 1980s, the amount of sunlight reaching the surface declined by around 4–5% per decade, even though the changes in solar radiation at the top of the atmosphere were never more than 0.1-0.3%. Yet, this trend (commonly described as global dimming) began to reverse in the 1990s, consistent with the reductions in anthropogenic sulfate pollution, while at the same time, climate change accelerated. Areas like eastern United States went from seeing cooling in contrast to the global trend to becoming global warming hotspots as their enormous levels of air pollution were reduced, even as sulfate particles still accounted for around 25% of all particulates.
As the real world had shown the importance of sulfate aerosol concentrations to the global climate, research into the subject accelerated.  Formation of the aerosols and their effects on the atmosphere can be studied in the lab, with methods like ion-chromatography and mass spectrometry Samples of actual particles can be recovered from the stratosphere using balloons or aircraft,  and remote satellites were also used for observation. This data is fed into the climate models, as the necessity of accounting for aerosol cooling to truly understand the rate and evolution of warming had long been apparent, with the IPCC Second Assessment Report being the first to include an estimate of their impact on climate, and every major model able to simulate them by the time IPCC Fourth Assessment Report was published in 2007. Many scientists also see the other side of this research, which is learning how to cause the same effect artificially. While discussed around the 1990s, if not earlier, stratospheric aerosol injection as a solar geoengineering method is best associated with Paul Crutzen's detailed 2006 proposal. Deploying in the stratosphere ensures that the aerosols are at their most effective, and that the progress of clean air measures would not be reversed: more recent research estimated that even under the highest-emission scenario RCP 8.5,  the addition of stratospheric sulfur required to avoid 4 °C (7.2 °F) relative to now (and 5 °C (9.0 °F) relative to the preindustrial) would be effectively offset by the future controls on tropospheric sulfate pollution, and the amount required would be even less for less drastic warming scenarios. This spurred a detailed look at its costs and benefits, but even with hundreds of studies into the subject completed by the early 2020s, some notable uncertainties remain.

Methods
Materials
Various forms of sulfur were proposed as the injected substance, as this is  in part how volcanic eruptions cool the planet. Precursor gases such as sulfur dioxide and hydrogen sulfide have been considered. According to estimates, "one kilogram of well placed sulfur in the stratosphere would roughly offset the warming effect of several hundred thousand kilograms of carbon dioxide." One study calculated the impact of injecting sulfate particles, or aerosols, every one to four years into the stratosphere in amounts equal to those lofted by the volcanic eruption of Mount Pinatubo in 1991, but did not address the many technical and political challenges involved in potential solar geoengineering efforts. Use of gaseous sulfuric acid appears to reduce the problem of aerosol growth. Materials such as photophoretic particles, titanium dioxide, and diamond are also under consideration.

Delivery
Various techniques have been proposed for delivering the aerosol or precursor gases.  The required altitude to enter the stratosphere is the height of the tropopause, which varies from 11 kilometres (6.8 mi/36,000 ft) at the poles to 17 kilometers (11 mi/58,000 ft) at the equator.

Civilian aircraft including the Boeing 747–400 and Gulfstream G550/650, C-37A could be modified at relatively low cost to deliver sufficient amounts of required material according to one study, but a later metastudy suggests a new aircraft would be needed but easy to develop.
Military aircraft such as the F15-C variant of the F-15 Eagle have the necessary flight ceiling, but limited payload. Military tanker aircraft such as the KC-135 Stratotanker and KC-10 Extender also have the necessary ceiling and have greater payload capacity.
Modified artillery might have the necessary capability, but requires a polluting and expensive propellant charge to loft the payload. Railgun artillery could be a non-polluting alternative.
High-altitude balloons can be used to lift precursor gases, in tanks, bladders or in the balloons' envelope.

Injection system
The latitude and distribution of injection locations has been discussed by various authors.  Whilst a near-equatorial injection regime will allow particles to enter the rising leg of the Brewer-Dobson circulation, several studies have concluded that a broader, and higher-latitude, injection regime will reduce injection mass flow rates and/or yield climatic benefits.  Concentration of precursor injection in a single longitude appears to be beneficial, with condensation onto existing particles reduced, giving better control of the size distribution of aerosols resulting.  The long residence time of carbon dioxide in the atmosphere may require a millennium-timescale commitment to aerosol injection if aggressive emissions abatement is not pursued simultaneously.

Advantages of the technique
The advantages of this approach in comparison to other possible means of solar geoengineering are:

Mimics a natural process: Stratospheric sulfur aerosols are created by existing natural processes (especially volcanoes), whose impacts have been studied via observations. This contrasts with other, more speculative solar geoengineering techniques which do not have natural analogs (e.g., space sunshade).
Technological feasibility: In contrast to other proposed solar geoengineering techniques, such as marine cloud brightening, much of the required technology is pre-existing: chemical manufacturing, artillery shells, high-altitude aircraft, weather balloons, etc.  Unsolved technical challenges include methods to deliver the material in controlled diameter with good scattering properties.
Scalability: Some solar geoengineering techniques, such as cool roofs and ice protection, can only provide a limited intervention in the climate due to insufficient scale—one cannot reduce the temperature by more than a certain amount with each technique. Research has suggested that this technique may have a high radiative 'forcing potential'., yet can be finely tuned according to how much cooling is needed.
Speed: A common argument is that stratospheric aerosol injection can take place quickly, and would be able to buy time for carbon sequestration projects such as carbon dioxide air capture to be implemented and start acting over decades and centuries.

Uncertainties
It is uncertain how effective any solar geoengineering technique would be, due to the difficulties modeling their impacts and the complex nature of the global climate system.  Certain efficacy issues are specific to stratospheric aerosols.

Lifespan of aerosols: Tropospheric sulfur aerosols are short-lived.  Delivery of particles into the lower stratosphere in the arctic will typically ensure that they remain aloft only for a few weeks or months, as air in this region is predominantly descending. To ensure endurance, higher-altitude delivery is needed, ensuring a typical endurance of several years by enabling injection into the rising leg of the Brewer-Dobson circulation above the tropical tropopause. Further, sizing of particles is crucial to their endurance.
Aerosol delivery: There are two proposals for how to create a stratospheric sulfate aerosol cloud, either through the release of a precursor gas (SO2) or the direct release of sulfuric acid (H2SO4) and these face different challenges. If SO2 gas is released it will oxidize to form H2SO4 and then condense to form droplets far from the injection site. Releasing SO2 would not allow control over the size of the particles that are formed but would not require a sophisticated release mechanism. Simulations suggest that as the SO2 release rate is increased there would be diminishing returns on the cooling effect, as larger particles would be formed which have a shorter lifetime and are less effective scatterers of light. If H2SO4 is released directly then the aerosol particles would form very quickly and in principle the particle size could be controlled although the engineering requirements for this are uncertain.  Assuming a technology for direct H2SO4 release could be conceived and developed, it would allow control over the particle size to possibly alleviate some of the inefficiencies associated with SO2 release.
Strength of cooling: The magnitude of the effect of forcing from aerosols by decreasing insolation received at the surface is not completely certain, as its scientific modelling involves complex calculations due to different confounding factors and parameters such as optical properties, spatial and temporal distribution of emission or injection, albedo, geography, loading, rate of transport of sulfate, global burden, atmospheric chemistry, mixing and reactions with other compounds and aerosols, particle size, relative humidity, and clouds. Along with others, aerosol size distribution and hygroscopicity have particularly high uncertainty due to being closely related to sulfate aerosol interactions with other aerosols which affects the amount of radiation reflected. As of 2021, state-of-the-art CMIP6 models estimate that total cooling from the currently present aerosols is between 0.1 °C (0.18 °F) to 0.7 °C (1.3 °F); the IPCC Sixth Assessment Report uses the best estimate of  0.5 °C (0.90 °F), but there's still a lot of contradictory research on the impacts of aerosols of clouds which can alter this estimate of aerosol cooling, and consequently, our knowledge of how many millions of tons must be deployed annually to achieve the desired effect.Hydrological cycle: Since the historical global dimming from tropospheric sulfate pollution is already well-known to have reduced rainfall in certain areas, and is likely to have weakened Monsoon of South Asia and contributed to or even outright caused the 1984 Ethiopian famine, the impact on the hydrological cycle and patterns is one of the most-discussed uncertainties of the different stratospheric aerosol injection proposals. It has been suggested that while changes in precipitation from stratospheric aerosol injection are likely to be more manageable than the changes expected under future warming, one of the main impacts it would have on mortality is by shifting the habitat of mosquitoes and thus substantially affecting the distribution and spread of vector-borne diseases. Considering the already-extensive present-day mosquito habitat, it is currently unclear whether those changes are likely to be positive or negative.

Cost
Early studies suggest that stratospheric aerosol injection might have a relatively low direct cost. The annual cost of delivering 5 million tons of an albedo enhancing aerosol (sufficient to offset the expected warming over the next century) to an altitude of 20 to 30 km is estimated at US$2 billion to 8 billion. In comparison, the annual cost estimates for climate damage or emission mitigation range from US$200 billion to 2 trillion.A 2016 study finds the cost per 1 W/m2 of cooling to be between 5–50 billion USD/yr.  Because larger particles are less efficient at cooling and drop out of the sky faster, the unit-cooling cost is expected to increase over time as increased dose leads to larger, but less efficient, particles by mechanism such as coalescence and Ostwald ripening.   Assume RCP8.5, -5.5 W/m2 of cooling would be required by 2100 to maintain 2020 climate. At the dose level required to provide this cooling, the net efficiency per mass of injected aerosols would reduce to below 50% compared to low-level deployment (below 1W/m2).    At a total dose of -5.5 W/m2, the cost would be between 55-550 billion USD/yr when efficiency reduction is also taken into account, bringing annual expenditure to levels comparable to other mitigation alternatives.

Other possible side effects
Solar geoengineering in general poses various problems and risks. However, certain problems are specific to or more pronounced with stratospheric sulfide injection.
Ozone depletion: a potential side effect of sulfur aerosols; and these concerns have been supported by modelling. However, this may only occur if high enough quantities of aerosols drift to, or are deposited in, polar stratospheric clouds before the levels of CFCs and other ozone destroying gases fall naturally to safe levels because stratospheric aerosols, together with the ozone destroying gases, are responsible for ozone depletion. The injection of other aerosols that may be safer such as calcite has therefore been proposed. The injection of non-sulfide aerosols like calcite (limestone) would also have a cooling effect while counteracting ozone depletion and would be expected to reduce other side effects.
Whitening of the sky: Volcanic eruptions are known to affect the appearance of sunsets significantly, and a change in sky appearance after the eruption of Mount Tambora in 1816 "The Year Without A Summer" was the inspiration for the paintings of J. M. W. Turner. Since stratospheric aerosol injection would involve smaller quantities of aerosols, it is expected to cause a subtler change to sunsets and a slight hazing of blue skies. How stratospheric aerosol injection may affect clouds remains uncertain.
Stratospheric temperature change: Aerosols can also absorb some radiation from the Sun, the Earth, and the surrounding atmosphere. This changes the surrounding air temperature and could potentially impact the stratospheric circulation, which in turn may impact the surface circulation.
Deposition and acid rain: The surface deposition of sulfate injected into the stratosphere may also have an impact on ecosystems. However, the amount and wide dispersal of injected aerosols means that their impact on particulate concentrations and acidity of precipitation would be very small.
Ecological consequences: The consequences of stratospheric aerosol injection on ecological systems are unknown and potentially vary by ecosystem with differing impacts on marine versus terrestrial biomes.
Mixed effects on agriculture: A historical study in 2018 found that stratospheric sulfate aerosols injected by the volcanic eruptions of Chicón (1982) and Mount Pinatubo (1991) had mixed effects on global crop yields of certain major crops. Based on several studies, the IPCC Sixth Assessment Report suggests that crop yields and carbon sinks would be largely unaffected or may even increase slightly, because reduced photosynthesis due to lower sunlight would be offset by CO2 fertilization effect and the reduction in thermal stress, but there's less confidence about how the specific ecosystems may be affected.
Inhibition of Solar Energy Technologies: Uniformly reduced net shortwave radiation would hurt solar photovoltaics by the same 2-5% as for plants. the increased scattering of collimated incoming sunlight would more drastically reduce the efficiencies (by 11% for RCP8.5) of concentrating solar thermal power for both electricity production  and chemical reactions, such as solar cement production.

Outdoors research
Almost all work to date on stratospheric sulfate injection has been limited to modeling and laboratory work. In 2009, a Russian team tested aerosol formation in the lower troposphere using helicopters.  In 2015, David Keith and Gernot Wagner described a potential field experiment, the Stratospheric Controlled Perturbation Experiment (SCoPEx), using stratospheric calcium carbonate injection, but as of October 2020 the time and place had not yet been determined. SCoPEx is in part funded by Bill Gates. Sir David King, a former chief scientific adviser to the government of the United Kingdom, stated that SCoPEX and Gates' plans to dim the sun with calcium carbonate could have disastrous effects.In 2012, the Bristol University-led Stratospheric Particle Injection for Climate Engineering (SPICE) project planned on a limited field test in order to evaluate a potential delivery system. The group received support from the EPSRC, NERC and STFC to the tune of £2.1 million and was one of the first UK projects aimed at providing evidence-based knowledge about solar radiation management. Although the field testing was cancelled, the project panel decided to continue the lab-based elements of the project. Furthermore, a consultation exercise was undertaken with members of the
public in a parallel project by Cardiff University, with specific exploration of
attitudes to the SPICE test. This research found that almost all of the participants in the poll were willing to allow the field trial to proceed, but very few were comfortable with the actual use of stratospheric aerosols. A campaign opposing geoengineering led by the ETC Group drafted an open letter calling for the project to be suspended until international agreement is reached, specifically pointing to the upcoming convention of parties to the Convention on Biological Diversity in 2012.

Governance
Most of the existing governance of stratospheric sulfate aerosols is from that which is applicable to solar radiation management more broadly. However, some existing legal instruments would be relevant to stratospheric sulfate aerosols specifically. At the international level, the Convention on Long-Range Transboundary Air Pollution (CLRTAP Convention) obligates those countries which have ratified it to reduce their emissions of particular transboundary air pollutants. Notably, both solar radiation management and climate change (as well as greenhouse gases) could satisfy the definition of "air pollution" which the signatories commit to reduce, depending on their actual negative effects. Commitments to specific values of the pollutants, including sulfates, are made through protocols to the CLRTAP Convention. Full implementation or large scale climate response field tests of stratospheric sulfate aerosols could cause countries to exceed their limits. However, because stratospheric injections would be spread across the globe instead of concentrated in a few nearby countries, and could lead to net reductions in the "air pollution" which the CLRTAP Convention is to reduce.
The stratospheric injection of sulfate aerosols would cause the Vienna Convention for the Protection of the Ozone Layer to be applicable due to their possible deleterious effects on stratospheric ozone. That treaty generally obligates its Parties to enact policies to control activities which "have or are likely to have adverse effects resulting from modification or likely modification of the ozone layer." The Montreal Protocol to the Vienna Convention prohibits the production of certain ozone depleting substances, via phase outs. Sulfates are presently not among the prohibited substances.
In the United States, the Clean Air Act might give the United States Environmental Protection Agency authority to regulate stratospheric sulfate aerosols.

Welsbach seeding
Welsbach seeding is a patented climate engineering method, involving seeding the stratosphere with small (10 to 100 micron) metal oxide particles (thorium dioxide, aluminium oxide). The purpose of the Welsbach seeding would be to "(reduce) atmospheric warming due to the greenhouse effect resulting from a greenhouse gases layer," by converting radiative energy at near-infrared wavelengths into radiation at far-infrared wavelengths, permitting some of the converted radiation to escape into space, thus cooling the atmosphere.  The seeding as described would be performed by airplanes at altitudes between 7 and 13 kilometres.

Patent
The method was patented by Hughes Aircraft Company in 1991,  US patent 5003186. Quote from the patent: 

"Global warming has been a great concern of many environmental scientists. Scientists believe that the greenhouse effect is responsible for global warming. Greatly increased amounts of heat-trapping gases have been generated since the Industrial Revolution. These gases, such as CO2, CFC, and methane, accumulate in the atmosphere and allow sunlight to stream in freely but block heat from escaping (greenhouse effect). These gases are relatively transparent to sunshine but absorb strongly the long-wavelength infrared radiation released by the earth."
"This invention relates to a method for the reduction of global warming resulting from the greenhouse effect, and in particular to a method which involves the seeding of the earth's stratosphere with Welsbach-like materials."

Feasibility
The method has never been implemented, and is not considered to be a viable option by current geoengineering experts; in fact the proposed mechanism is considered to violate the second law of thermodynamics. Currently proposed atmospheric geoengineering methods would instead use other aerosols, at considerably higher altitudes.

History
Mikhail Budyko is believed to have been the first, in 1974, to put forth the concept of artificial solar radiation management with stratospheric sulfate aerosols if global warming ever became a pressing issue. Such controversial climate engineering proposals for global dimming have sometimes been called a "Budyko Blanket".

In popular-culture
In the film Snowpiercer, as well as in the television spin-off, an apocalyptic global ice-age is caused by the introduction of a fictional substance, dubbed, CW-7 into the atmosphere, with the intention of preventing global-warming by blocking out the light of the sun. In the Novel The Ministry for the Future by Kim Stanley Robinson, stratospheric aerosol injection is used by the Indian Government as a climate mitigation measure following a catastrophic and deadly heatwave.

See also
References
External links
What can we do about climate change?, Oceanography magazine
Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change, Lawrence Livermore National Laboratory
The Geoengineering Option:A Last Resort Against Global Warming?, Council on Foreign Relations
Geo-Engineering Climate Change with Sulfate Aerosols, Pacific Northwest National Laboratory
Geo-Engineering Research, Parliamentary Office of Science and Technology
Geo-engineering Options for Mitigating Climate Change, Department of Energy and Climate Change
Unilateral Geoengineering, Council on Foreign Relations
Rasch, Philip J; Tilmes, Simone; Turco, Richard P; Robock, Alan; Oman, Luke; Chen, Chih-Chieh (Jack); Stenchikov, Georgiy L; Garcia, Rolando R (13 November 2008). "An overview of geoengineering of climate using stratospheric sulphate aerosols". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 366 (1882): 4007–4037. Bibcode:2008RSPTA.366.4007R. doi:10.1098/rsta.2008.0131. PMID 18757276. S2CID 9869660.
US 5003186  "Stratospheric Welsbach seeding for reduction of global warming"
As planet warms, scientists explore 'far out' ways to reduce atmospheric CO2 on YouTube PBS NewsHour published on March 27, 2019 animation of SCoPEx