by Robert Marcos
Stratospheric aerosol injection, (SAI), is a theoretical solar geoengineering proposal that involves dispersing sulfate (or other reflective particles) into the stratosphere to reflect a portion of incoming sunlight back into space. Research into delivery methods focuses on platforms capable of reaching the stratosphere, which begins at varying altitudes depending on the latitude. Proposals range from spraying reflective particles, such as sulfur dioxides, finely powdered salt or calcium carbonate, from aircraft or high-flying balloons. None of these solar geo-engineering strategies address the underlying causes of climate change. Instead, they aim to control the amount of incoming solar radiation by emulating the sulfur-rich dust cloud that remains in the atmosphere after large volcanic eruptions.1
Proof of Concept provided by Mt. Pinatubo
The 1991 eruption of Mount Pinatubo injected approximately 17 million tons of sulfur dioxide into the stratosphere, creating a global layer of sulfuric acid haze that significantly increased the Earth’s albedo. This aerosol veil reflected incoming solar radiation back into space, resulting in a measurable drop in global mean temperatures of approximately 0.5°C (0.9°F) between 1992 and 1993. This transient cooling effect temporarily offset the trend of anthropogenic global warming and disrupted global precipitation patterns, demonstrating the profound impact that volcanic stratospheric aerosols can have on the Earth’s energy balance.2
According to one study, by sending specially designed high-altitude airplanes on roughly 4,000 total sulfate injection missions a year, humans could replicate this same level of cooling. This has the potential to offset half of the warming expected over the study’s 15-year period and counteract billions of metric tons of CO2 emissions each year. At a cost of around $2 billion annually, even medium-sized economies could afford such a program. This price tag would also be far less expensive than the potential impacts of climate change. Take the United States: the 2018 US National Climate Assessment Report estimates the impacts of climate change damages will amount to “hundreds of billions of dollars annually” by 2090, making atmospheric sulfate injection an appealing solution.3
Aerial platforms under consideration
Large commercial or military transport aircraft: These could potentially be retrofitted with specialized tanks and nozzle systems. However, most standard aircraft have flight ceilings that only reach the lower stratosphere, particularly near the poles.
Specialized Research Planes: Aircraft designed for high-altitude atmospheric research, such as those used by space agencies, can reach the higher altitudes (around 20 km) often cited as optimal for SAI. These generally have limited payload capacities.
Purpose-Built High-Altitude Jets: Many researchers suggest that a new class of specialized aircraft would be necessary for efficient, large-scale delivery. These designs would require high-lift wings and engines capable of sustained operation in thin air while carrying heavy payloads of aerosol precursors.
High-Altitude Balloons: Tethered or free-floating balloons have been proposed as a lower-cost method to loft materials into the stratosphere, though they face challenges related to stability and large-scale operational control.4
Potential benefits
Rapid Global Cooling: SAI can lower global average temperatures much faster than carbon removal methods. Historical volcanic eruptions, like Mount Pinatubo in 1991, have proven that atmospheric sulfur can cool the planet by roughly 0.5°C within a year.
Cost-Effectiveness: Compared to the trillions needed for a full green energy transition, SAI is estimated to cost between $18 billion and $27 billion per year using modified aircraft.
Life-Saving Potential: Some studies suggest SAI could save up to 400,000 lives annually by reducing heat-related mortality in the world’s hottest regions.
Glacial Preservation: By lowering surface temperatures, it could slow sea-level rise and prevent the melting of land-based glaciers and sea ice.
Reversibility: Unlike permanent carbon storage, SAI effects are temporary; if stopped, the aerosols naturally fall out of the atmosphere within 1–2 years.5
Potential risks
Termination Shock: If SAI is suddenly stopped (due to war, terrorism, or political collapse) while greenhouse gases are still high, the planet would experience a catastrophic and rapid temperature spike.
Ozone Depletion: Injecting sulfates can damage the stratospheric ozone layer, increasing harmful UV radiation and risks of skin cancer.
Disrupted Weather Patterns: Models indicate it could cause regional droughts, specifically by weakening the South Asian monsoon and reducing tropical rainfall.
Ocean Acidification: SAI only masks temperature; it does not reduce CO2 levels. The oceans would continue to absorb carbon, leading to acidification that destroys coral reefs and marine life.
Moral Hazard: The availability of a “quick fix” might reduce the political and corporate incentive to actually cut greenhouse gas emissions.
Geopolitical Conflict: There is no international governance for SAI. A single country could “control the thermostat,” potentially leading to global conflict if their actions cause weather disasters elsewhere.
Ecological Impacts: Reduced direct sunlight could decrease crop yields and interfere with solar power generation.6