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Stratospheric Aerosol Injection
Stratospheric Aerosol Injection (SAI) is one of the most-studied forms of climate engineering, but research has almost exclusively been limited to numerical simulations. Most of the studies investigating different strategies look at the effects of introducing sulphur dioxide (SO2) into the stratosphere, which reacts to become sulphuric acid aerosol (H2SO4) and which reflects some of the radiation from the sun back into space increasing the planetary albedo.
This has in part been inspired by observations from volcanic eruptions, especially 1991 Mount Pinatubo which lofted ~20 million tonnes of SO2 into the stratosphere resulting in global cooling by approximately 0.5°C for around a year.
Injection of aerosols into the stratosphere rather than the troposphere reduces the required mass injection rate in part due to longer residence times. Depending on the latitude of injection, the aerosol cloud will be dispersed across the globe within a period of weeks. SAI is considered by a number of scientists as a potential way of actively cooling the earth.
CCR's research effort in Cambridge’s Department of Geography focusses on establishing paleoclimate records, and investigating the Atlantic Meridional Overturning Circulation, as well as where large cluster volcanoes may have had an impact on the climate.
CCR is also working with Harvard University and Imperial College London on a new project to investigate alternative materials for SAI.
Starting 2025: Novel Materials for Stratospheric Aerosol Injection project
From summer 2025, Hugh Hunt will be leading a project on Natural Materials for Stratospheric Aerosol Injection, in collaboration with Harvard University and Imperial College London, and supported by the Advanced Research + Invention Agency (Aria).
Stratospheric Aerosol Injection (SAI) is a widely discussed potential climate cooling method, but the most commonly proposed materials (sulfates) carry significant hazards in this context, including potential ozone depletion and toxicity. Addressing whether safer, alternative materials could ever be feasible or effective for SAI is therefore a critical, unanswered scientific question. This project will undertake fundamental research to investigate the properties and behaviour of innovative, non-toxic, non-sulfate materials in a very controlled manner.
The research combines laboratory studies and computational modelling with unique and contained material exposure experiments. In these experiments, tiny (milligram) amounts of materials that occur in natural mineral dust (such as limestone, dolomite, or corundum) will be secured onto supports inside the gondolas of specially adapted weather balloons. These balloons are likely to be launched from sites in the USA and/or the UK; the specific site will be determined in line with ARIA’s requirements for community engagement. The balloons will carry the samples into the stratosphere for exposure periods ranging from hours to weeks before performing controlled descent for recovery. Crucially, no materials will be released into the stratosphere; this approach effectively brings the stratosphere to the samples. Studying the recovered samples will reveal how stratospheric conditions affect their properties over time. This foundational science is essential to advance understanding of the potential impacts of SAI and for determining if less harmful alternatives to sulfates might exist (and if they might warrant further study in the context of SAI).
Updated 27/08/2025
