Stratospheric aerosol injection
Stratospheric aerosol injection is a proposed method of solar geoengineering (or solar radiation modification) to reduce human-induced global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming, which occurs naturally from volcanic eruptions. 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." 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. 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. 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 SRM if aggressive emissions abatement is not pursued simultaneously. Aerosol formation. Primary aerosol formation, also known as homogeneous aerosol formation, results when gaseous combines with oxygen and water to form aqueous sulfuric acid (HSO). This acidic liquid solution is in the form of a vapor and condenses onto particles of solid matter, either meteoritic in origin or from dust carried from the surface to the stratosphere. Secondary or heterogeneous aerosol formation occurs when HSO vapor condenses onto existing aerosol particles. Existing aerosol particles or  droplets also run into each other, creating larger particles or droplets in a process known as coagulation. Warmer atmospheric temperatures also lead to larger particles. These larger particles would be less effective at scattering sunlight because the peak light scattering is achieved by particles with a diameter of 0.3 μm. Advantages of the technique. The advantages of this approach in comparison to other possible means of solar geoengineering are: 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. Cost. Early studies suggest that stratospheric aerosol injection might have 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/m 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/m of cooling would be required by 2100 to maintain 2020 climate. At 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/m). At a total dose of -5.5 W/m, 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. Outdoors research. Almost all work to date on stratospheric sulfate injection has been limited to modelling 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. 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 CO, 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".