Stratospheric aerosol injection (SAI) 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 has been claimed 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 radiation modification method, with high agreement that it could limit warming to below ." 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 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.
Background and mechanism
Natural aerosols Sources of natural aerosols include oceans, volcanoes, deserts, and living organisms. The ocean produces aerosols in two main ways. First, when wind blows over waves, it creates spray made up mostly of sea salt. Second, tiny ocean organisms—such as plankton—release dimethyl sulfide and other gases into the air which, in turn, react with other substances in the atmosphere, including water vapor, to form gaseous sulfate (sulfuric acid) aerosols. Both sea salt and sulfate aerosols help to form clouds by acting as "seeds" for water droplets, affecting cloud formation and Earth's energy balance. While these ocean aerosols are widespread, there is still uncertainty about exactly how much they affect the atmosphere.
Volcanic eruptions release ash and gases into the air. Although the ash falls out of the atmosphere relatively quickly, sulfur dioxide can rise into the stratosphere, where it reacts with water vapor to form long-lived sulfate aerosols in the upper atmosphere. These reflect sunlight and temporarily cool the planet. After a large eruption, these particles can stay in the air for a year or more.
Natural aerosols cool the Earth. When large volcanic eruptions occur, they can cause short-term global cooling of around half a degree or more, depending on the size of the eruption. For example, the eruption of Mount Pinatubo in 1991 caused global temperatures to drop by about 0.5 degrees Celsius for up to three years. These events have played an important role in past climate variability.
Anthropogenic aerosols Human activities, especially fossil fuel combustion and biomass burning, emit aerosols directly and indirectly via gases that react in the atmosphere. Common anthropogenic aerosols include sulfates, nitrates, black carbon (soot), and organic carbon. Among these, sulfates are the dominant cooling agent. Organic carbon aerosols also reflect light, while black carbon absorbs it, warming the air and darkening snow and ice.
The net effect of anthropogenic aerosols has been to mask global warming. From 1850 to 2014, they reduced global average surface temperature by about 0.66 °C. This cooling is stronger in the more populous Northern Hemisphere. This uneven effect has altered rainfall patterns, including a weakening of tropical monsoons.
Air pollution regulations have reduced sulfate emissions in Europe and North America since the 1980s, and more recently in China. These reductions have improved air quality but diminish the cooling influence of aerosols, contributing to accelerated warming.
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 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 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.
Stardust Solutions was established in 2023-24 as a for-profit venture capital backed SAI venture.
Implementation and technical considerations
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. Use of gaseous sulfuric acid appears to reduce the problem of aerosol growth. Materials such as photophoretic particles, metal oxides (as in Welsbach seeding, and 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, 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 at latitudes closer to the poles 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. While 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.
Welsbach seeding (metal oxide particles)
Welsbach seeding is a patented solar radiation modification 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.
The method was patented by Hughes Aircraft Company in 1991, US patent 5003186. Quote from the patent: "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." This is not considered to be a viable option by current geoengineering experts.
Cost A study in 2020 looked at the cost of SAI through to the year 2100. It found that relative to other climate interventions and solutions, SAI remains inexpensive. However, at about $18 billion per year per degree Celsius of warming avoided (in 2020 USD), a solar geoengineering program with substantial climate impact would lie well beyond the financial reach of individuals, small states, or other non-state potential rogue actors. The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at $5–10 billion US dollars.
SAI is expected to have low direct financial costs of implementation, relative to the expected costs of both unabated climate change and aggressive mitigation.
Early studies suggest that stratospheric aerosol injection might have a relatively low direct cost. One analysis estimated the annual cost of delivering 5 million tons of an albedo enhancing aerosol to an altitude of 20 to 30 km is at US$2 billion to 8 billion, an amount which they suggest would be sufficient to offset the expected warming during the next century. In comparison, the annual cost estimates for climate damage or emission mitigation range from US$200 billion to 2 trillion.