Stratospheric Aerosol Injection

Stratospheric Aerosol Injection, also known as SAI, is one of the most widely discussed and studied climate cooling method with research almost exclusively limited to computer modelling. It aims to reflect a portion of the Sun’s energy back into space by dispersing tiny reflective particles into the upper atmosphere. This idea is inspired by natural volcanic eruptions, which have been observed to cool the Earth for short periods.

SAI involves releasing fine particles – often proposed to be sulfate aerosols – into the stratosphere, a part of the atmosphere about 10 – 50 kilometres above the Earth’s surface.

Once dispersed, the particles would reflect incoming solar radiation back into space, which would reduce the amount of solar energy reaching this Earth’s surface, and in turn lowering global temperatures temporarily.

This cooling effect is similar to what occured after volcanic eruptions such as the 1991 eruption of Mount Pinatubo, which led to a global temperature drop of about 0.5 degrees C the following year.

CCR's research effort in Cambridge’s Department of Geography focuses 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).