Stratospheric aerosol injection (SAI) is a method being studied to help reduce global warming. This process would involve adding tiny particles into the upper atmosphere to reflect sunlight back into space, which could lower Earth's temperature. This effect is similar to what happens naturally after large volcanic eruptions, which can cause temporary cooling. Some scientists say that using SAI at a moderate level might help reduce temperature changes, affect rainfall patterns, work quickly, cost less to implement, and be reversed if needed. The Intergovernmental Panel on Climate Change says SAI is the most studied method for altering sunlight, and scientists agree it could help keep global warming below 1.5 °C (2.7 °F). However, SAI may not work perfectly and could cause other unexpected effects, especially if used incorrectly.

Scientists have observed that sulfur-based materials can cool the planet after major volcanic eruptions. However, as of 2021, research on this topic is limited, and scientists do not yet know which material is best for this purpose. Other materials, such as alumina, calcite, and salt, are also being studied. The most common way to deliver these materials into the stratosphere is through specially designed aircraft.

Background and mechanism

An aerosol is a mixture of tiny solid particles or liquid droplets in air or another gas. Aerosols can come from natural sources, like wind or volcanoes, or from human activities, such as burning fuel. The term "aerosol" describes the combination of these tiny particles in a gas, not the particles alone. The particles in an aerosol are usually smaller than 1 micrometer in size. Larger particles that settle quickly are called suspensions, but the line between aerosols and suspensions is not always clear.

Natural aerosols include fog, mist, and dust from natural sources like deserts or living things. Some diseases spread through tiny droplets in the air, known as bioaerosols. Certain types of aerosols in the atmosphere, such as those from volcanoes, deserts, and the ocean, affect Earth's climate. Volcanic aerosols form in the stratosphere after eruptions as sulfuric acid droplets that can stay in the air for up to two years. These droplets reflect sunlight, which can lower Earth's temperature. Desert dust, made of mineral particles, absorbs heat and may reduce the formation of storm clouds.

Natural aerosols come from oceans, volcanoes, deserts, and living organisms. Oceans create aerosols in two main ways. First, wind blowing over waves produces sea spray made mostly of salt. Second, tiny ocean organisms, like plankton, release gases such as dimethyl sulfide into the air. These gases react with water vapor in the atmosphere to form sulfate aerosols. Both sea salt and sulfate aerosols help form clouds by acting as "seeds" for water droplets, which affects cloud formation and Earth's energy balance. Scientists are still studying how much these ocean aerosols influence the atmosphere.

Volcanic eruptions release ash and gases into the air. While ash settles quickly, sulfur dioxide can rise into the stratosphere, where it reacts with water vapor to create long-lasting sulfate aerosols. These aerosols reflect sunlight and temporarily cool the planet. After major eruptions, these particles can remain in the air for over a year.

Natural aerosols can cool Earth. Large volcanic eruptions can cause short-term global cooling, sometimes lowering temperatures by about half a degree or more. For example, the 1991 eruption of Mount Pinatubo reduced global temperatures by about 0.5 degrees Celsius for three years. These events have influenced Earth's climate in the past.

Human activities, such as burning fossil fuels and burning biomass, release aerosols directly and indirectly through gases that react in the atmosphere. Common human-made aerosols include sulfates, nitrates, black carbon (soot), and organic carbon. Sulfates are the main cooling agent among these. Organic carbon aerosols reflect light, while black carbon absorbs it, warming the air and darkening snow and ice.

Overall, human-made aerosols have helped slow global warming. Between 1850 and 2014, they reduced the average global temperature by about 0.66°C. This cooling effect is stronger in the more populated Northern Hemisphere. This uneven cooling has changed rainfall patterns, including weakening tropical monsoons.

Air pollution rules have reduced sulfate emissions in Europe, North America, and China since the 1980s. These changes have improved air quality but reduced the cooling effect of aerosols, leading to faster warming.

History

In 1974, Mikhail Budyko proposed the idea of using stratospheric sulfate aerosols to manage solar radiation if global warming became a serious problem. This approach, which involves reducing the amount of sunlight reaching Earth, is sometimes called a "Budyko Blanket."

In 2009, a Russian team used helicopters to test aerosol formation in the lower part of the atmosphere. In 2015, David Keith and Gernot Wagner suggested a field experiment called the Stratospheric Controlled Perturbation Experiment (SCoPEx), which would involve injecting calcium carbonate into the stratosphere. However, as of October 2020, the experiment’s timing and location had not been decided. SCoPEx is partially funded by Bill Gates. Sir David King, a former chief scientific adviser to the UK government, warned that SCoPEx and Gates’ plans to dim the sun with calcium carbonate might cause serious problems.

In 2012, the SPICE project, led by Bristol University, planned a small field test to study a possible delivery system for solar radiation management. The project received £2.1 million in funding from EPSRC, NERC, and STFC, making it one of the first UK projects to provide evidence-based research on this topic. Although the field test was canceled, the team continued laboratory research. A separate project by Cardiff University surveyed public opinions about the SPICE test. Most participants supported the field trial, but few were comfortable with using stratospheric aerosols. The ETC Group, an organization opposing geoengineering, asked for the project to be paused until international agreements were reached, especially ahead of the 2012 Convention on Biological Diversity meeting.

Stardust Solutions was created in 2023–2024 as a for-profit venture focused on solar geoengineering.

Implementation and technical considerations

Various types of sulfur have been suggested as the material to be injected into the atmosphere, as this is one way volcanic eruptions help cool the planet. Gases like sulfur dioxide and hydrogen sulfide have been studied as possible options. According to estimates, "one kilogram of sulfur placed correctly in the stratosphere would roughly balance the warming effect of several hundred thousand kilograms of carbon dioxide." One study examined the effects of injecting sulfate particles, or aerosols, into the stratosphere every one to four years in amounts similar to those released during the 1991 Mount Pinatubo volcanic eruption. However, this study did not address the many technical and political challenges that might arise with solar geoengineering. Using gaseous sulfuric acid may help reduce the problem of aerosol growth. Other materials being considered include photophoretic particles, metal oxides (such as those used in Welsbach seeding and titanium dioxide), and diamond.

Several methods have been proposed to deliver aerosols or precursor gases into the stratosphere. To reach the stratosphere, materials must be sent to the height of the tropopause, which changes depending on location. At the poles, the tropopause is about 11 kilometers (6.8 miles or 36,000 feet), while at the equator, it is approximately 17 kilometers (11 miles or 58,000 feet).

• Civilian aircraft, such as the Boeing 747-400, Gulfstream G550/650, and C-37A, could be modified at a relatively low cost to deliver the required materials, according to one study. However, a later study suggested that new aircraft might be needed, though they could be developed easily.
• Military aircraft, like the F-15-C variant of the F-15 Eagle, can reach the necessary altitude but have limited carrying capacity. Military tanker planes, such as the KC-135 Stratotanker and KC-10 Extender, can reach higher altitudes near the poles and carry more weight.
• Modified artillery could be used, but it would require expensive and polluting propellant charges. Railgun artillery might be a cleaner alternative.
• High-altitude balloons can be used to carry precursor gases in tanks, bladders, or within the balloon itself.

The location and spread of injection points have been discussed by researchers. Injecting particles near the equator allows them to enter the rising part of the Brewer-Dobson circulation. However, studies suggest that injecting particles at higher latitudes may reduce the amount of material needed or improve climate benefits. Focusing injections along a single longitude may help control the size of aerosols by reducing condensation on existing particles. Because carbon dioxide stays in the atmosphere for a very long time, if emissions are not significantly reduced, aerosol injection might need to continue for many centuries.

Welsbach seeding is a patented method to modify solar radiation by adding small metal oxide particles (such as thorium dioxide and aluminum oxide) to the stratosphere. This method aims to reduce warming by changing how heat is radiated from the atmosphere. The particles would be spread by airplanes flying between 7 and 13 kilometers high. The method was patented by Hughes Aircraft Company in 1991 (U.S. Patent 5003186). However, current experts do not consider it a practical option.

A 2020 study examined the cost of solar geoengineering (SAI) through the year 2100. It found that SAI is relatively inexpensive compared to other climate solutions. However, at about $18 billion per year per degree Celsius of warming avoided (in 2020 USD), a large-scale solar geoengineering program would cost more than individuals, small countries, or other non-state actors could afford. The cost of delivering enough sulfur to counteract expected warming is estimated at $5–10 billion annually.

SAI is expected to have low direct costs compared to the costs of uncontrolled climate change or aggressive efforts to reduce emissions. Early studies suggest that stratospheric aerosol injection might be relatively inexpensive. One analysis estimated the annual cost of delivering 5 million tons of an albedo-enhancing aerosol to an altitude of 20 to 30 kilometers to be between $2 billion and $8 billion. This amount, the study suggests, could be enough to offset warming over the next century. In comparison, the annual costs of climate damage or emission reduction are estimated to range from $200 billion to $2 trillion.

A 2016 study found the cost to achieve 1 W/m² of cooling to be between $5–50 billion per year. Larger particles are less effective at cooling and fall from the sky faster, so the cost per unit of cooling is expected to increase over time due to processes like coalescence and Ostwald ripening. Assuming the RCP8.5 scenario, -5.5 W/m² of cooling would be needed by 2100 to maintain 2020 climate conditions. At the dose level required to achieve this cooling, the efficiency of aerosols would drop below 50% compared to lower doses. At a total dose of -5.5 W/m², the cost would be between $55–550 billion per year, making it comparable to other climate mitigation strategies.

Advantages

The advantages of this approach compared to other solar geoengineering methods include:

• Copies a natural process: Sulfur aerosols in the stratosphere are created by natural events like volcanoes, which scientists have studied through observations. Other methods, such as space sunshades, do not have similar natural examples.

• Technological feasibility: Many of the tools needed for this method already exist, such as chemical manufacturing, artillery shells, high-altitude planes, and weather balloons. Challenges remain in how to spread the material evenly and make it reflect sunlight effectively.

• Scalability: Some methods, like cool roofs, can only reduce temperatures slightly because they cannot be used widely. This method may have a strong effect on climate but can be adjusted based on how much cooling is needed.

• Speed: This method can be implemented quickly, giving time for projects that remove carbon dioxide from the air to work over many years and centuries.

Uncertainties

It is unclear how well solar geoengineering methods might work because predicting their effects is difficult, and the global climate system is very complex. Some problems with effectiveness are specific to using aerosols in the stratosphere.

Lifespan of aerosols: Sulfur particles in the lower part of the atmosphere do not stay long. Particles placed in the stratosphere above the Arctic usually remain in the air for only a few weeks or months because air in this area tends to move downward. To keep particles in the air longer, they should be injected higher up, such as in the rising part of the air movement above the tropics. Particles injected here can stay in the air for several years. The size of the particles also affects how long they remain.

Aerosol delivery: Two methods have been proposed to create a stratospheric sulfate aerosol cloud. One involves releasing a gas called sulfur dioxide (SO₂), which turns into sulfuric acid (H₂SO₄) and forms droplets far from where it is released. This method does not allow control over the size of the particles but requires simpler equipment. Simulations suggest that increasing the rate of SO₂ release may not improve cooling because larger particles form, which do not last as long and reflect less sunlight. The other method involves directly releasing sulfuric acid (H₂SO₄), which forms particles quickly. This method could allow control over particle size but requires advanced engineering. If this method were possible, it might improve efficiency compared to using SO₂.

Strength of cooling: Scientists are unsure how much cooling stratospheric aerosols might cause because calculating their effects involves many complex factors. These include how much sunlight is reflected, where and when particles are released, how much sunlight reaches Earth, the brightness of surfaces, geography, how quickly sulfate moves through the atmosphere, the total amount of sulfate in the atmosphere, chemical reactions, mixing with other substances, particle size, humidity, and clouds. The size and ability of particles to absorb moisture are especially uncertain because they depend on interactions with other particles. As of 2021, advanced climate models estimate that current aerosols reduce Earth’s temperature by about 0.1°C to 0.7°C. The IPCC Sixth Assessment Report suggests an average cooling of 0.5°C, but research on how aerosols affect clouds is still unclear, making it hard to predict how much cooling could be achieved.

Hydrological cycle: Past pollution from sulfur particles in the troposphere is known to have reduced rainfall in some areas, weakened the South Asian monsoon, and may have contributed to the 1984 Ethiopian famine. Changes in rainfall patterns are a major concern when considering stratospheric aerosol injection. While these changes might be easier to manage than those caused by global warming, they could affect human health by altering where mosquitoes live, which could change the spread of diseases like malaria. It is unclear whether these changes would be harmful or helpful because mosquito habitats are already widespread.

Risks

Solar geoengineering generally has several problems and risks. Some issues are more common or worse with stratospheric sulfide injection.

• Ozone depletion: Sulfur particles may harm the ozone layer, which protects Earth from harmful sunlight. Computer models support this concern. However, this risk might only happen if large amounts of sulfur particles reach polar stratospheric clouds before chemicals that destroy ozone, like CFCs, naturally decrease to safe levels. Sulfur particles and ozone-destroying chemicals together cause ozone loss. Scientists suggest using other particles, like calcite (a type of limestone), instead. These safer particles could cool Earth, reduce ozone damage, and lower other risks.

• Whitening of the sky: Volcanic eruptions change how sunsets look, as seen in art from 1816 after Mount Tambora’s eruption. Stratospheric aerosol injection would use smaller amounts of particles, likely causing less noticeable changes to sunsets and a slight haze in blue skies. How this might affect clouds is still unknown.

• Stratospheric temperature change: Aerosols can absorb sunlight and heat from Earth and the atmosphere, changing air temperatures. This could alter air movement in the stratosphere, which might affect air movement near Earth’s surface.

• Deposition and acid rain: Sulfur particles injected into the stratosphere could fall to Earth, possibly affecting ecosystems. However, because the particles would spread over a large area, their impact on air pollution and acid rain would be very small.

• Ecological consequences: The effects of stratospheric aerosol injection on ecosystems are unknown. These effects might differ between land and ocean environments.

• Mixed effects on agriculture: A 2018 study found that volcanic eruptions in 1982 and 1991 had mixed effects on crop yields. Research from the IPCC Sixth Assessment Report suggests that crop growth and carbon storage might not change much, or could even slightly increase, because reduced sunlight might be balanced by benefits from increased carbon dioxide and less heat stress. However, scientists are less certain about how specific ecosystems might be affected.

• Inhibition of Solar Energy Technologies: Less sunlight reaching Earth would reduce the efficiency of solar panels by 2–5%, similar to how plants are affected. Scattering of sunlight would also greatly lower the efficiency of solar thermal power systems, which rely on concentrated sunlight for energy production and chemical processes like making cement.

Governance

Most of the rules about stratospheric sulfate aerosols come from those that apply to solar radiation management in general. However, some laws already in place may specifically apply to stratospheric sulfate aerosols. At the international level, the Convention on Long-Range Transboundary Air Pollution (CLRTAP Convention) requires countries that have agreed to it to reduce emissions of air pollutants that travel across borders. Both solar radiation management and climate change (as well as greenhouse gases) could be considered "air pollution" under this agreement, depending on their harmful effects. Specific limits for pollutants like sulfates are set through agreements added to the CLRTAP Convention. Large-scale tests or full use of stratospheric sulfate aerosols might cause countries to go over their pollution limits. However, because these aerosols would spread across the globe instead of being concentrated in a few nearby countries, they might lead to overall reductions in the air pollution the CLRTAP Convention aims to control. This could allow their use.

Stratospheric injection of sulfate aerosols might make the Vienna Convention for the Protection of the Ozone Layer relevant because these aerosols could harm the ozone layer. This treaty requires its members to take actions to prevent activities that harm the ozone layer. The Montreal Protocol, part of the Vienna Convention, bans the production of certain substances that destroy ozone. Sulfates are not currently among these banned substances.

In the United States, the Clean Air Act may give the Environmental Protection Agency the power to regulate stratospheric sulfate aerosols.