Stratospheric aerosol injection

Also known as SAI, Stratospheric geoengineering, Solar radiation modification using stratospheric aerosols

Stratospheric aerosol injection is a proposal to cool the Earth by putting reflective particles, or the gases that form them, into the stratosphere. Volcanic eruptions and climate models support the idea, but the sources reviewed describe no outdoor release for this purpose, and the best-known planned test was halted in 2024.

TheoryProposedReviewed 21 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

Stratospheric aerosol injection, often shortened to SAI, is an idea for cooling the planet by copying something volcanoes already do. Large eruptions throw sulfur dioxide high into the stratosphere, the calm layer of air above the weather. There the gas turns into tiny droplets that reflect some sunlight back to space, and the surface cools for a while. The proposal is to add similar particles on purpose, for example by flying aircraft or balloons through the stratosphere.

The idea is controversial. Climate models suggest it could lower temperatures, but it could also change ozone levels and regional climates, and those effects are uncertain. It would not remove the greenhouse gases that cause warming. A planned Harvard balloon test called SCoPEx was halted in 2024, after a planned 2021 test flight in Sweden was cancelled and drew strong criticism. The sources reviewed describe no outdoor release of particles for this purpose.

Technical explanation

Stratospheric aerosol injection would add reflective particles to the stratosphere, where they can stay for months and spread around the globe. Sulfur dioxide is the most discussed precursor, because it oxidises into sulfate aerosols that scatter incoming sunlight. The 1991 eruption of Mount Pinatubo injected nearly 20 million tons of sulfur dioxide into the stratosphere, and the US Geological Survey reports that global temperatures dropped temporarily by about 0.5°C over 1991 to 1993. The eruption is the main real-world evidence that stratospheric sulfate cools the surface, but it was a single pulse rather than a sustained programme, so it does not show the effects of long-term injection.

Climate models give the main evidence for how a sustained programme would behave. A 2026 peer-reviewed study of three models, the G6-1.5K-SAI experiment, injects sulfur dioxide at 30 degrees north and 30 degrees south under the SSP2-4.5 emissions pathway. In these simulations, ozone loss per degree of surface cooling is roughly twice as large in 2045 to 2064 as in 2065 to 2084. The result covers ozone only, not the wider effects on rainfall or regional climate.

The main policy questions are governance and the risk of a sudden stop. The 2021 National Academies report recommends a US-led research programme, run with other countries, with a research code of conduct, a public registry, permitting for outdoor experiments and inclusive public engagement. Parker and Irvine (2018) argue that termination shock, a fast rebound in temperature if injection stops suddenly, is much less of a risk than previously assumed. This profile therefore treats the size of that risk as disputed rather than settled.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

The 1991 eruption of Mount Pinatubo injected nearly 20 million tons of sulfur dioxide into the stratosphere, and global temperatures dropped temporarily by about 0.5°C over 1991 to 1993.

Sources: [1] · Fact sheet 113-97, text on the 1991 eruption

Harvard announced on 18 March 2024 that it was no longer pursuing the SCoPEx stratospheric experiment, and the 2021 Kiruna test flight it had planned had already been cancelled.

Sources: [5]

The Saami Council and environmental groups heavily criticised the planned 2021 Kiruna test flight, which was cancelled on an advisory committee's recommendation that the researchers first hold public discussions.

Sources: [5]

Proposed A design or plan that has not yet been built or tested.

The SCoPEx design was a high-altitude balloon that would release a few kilograms of material, such as calcium carbonate or sulfuric acid, and then fly back through the plume to measure how the particles spread and how much sunlight they reflected.

Sources: [5]

In the G6-1.5K-SAI experiment, which injects sulfur dioxide at 30 degrees north and 30 degrees south under SSP2-4.5 and was run with three climate models, the paper gives ozone loss per degree of surface cooling as about 4 Dobson units per kelvin in 2045 to 2064 and 1 to 2 Dobson units per kelvin in 2065 to 2084.

Sources: [3] · Abstract and results on ozone projections · medium confidence

The 2021 National Academies report recommends a US-led research programme, run with other countries, supported by governance elements: a research code of conduct, a public registry, permitting for outdoor experiments, guidance on intellectual property, and inclusive public and stakeholder engagement.

Sources: [4]

Disputed Contested by other credible sources.

Termination shock is disputed. Parker and Irvine (2018) argue that the risk of termination shock is much less likely, and so much less of a risk, than previously assumed.

Sources: [2] · medium confidence

Limitations and open problems

  • No outdoor release of particles for this purpose has been carried out in the sources reviewed, and the best-known planned test, SCoPEx, was halted in March 2024.
  • The evidence for cooling comes from one volcanic event and from climate models. Pinatubo's cooling lasted about 1991 to 1993 and was not a sustained programme.
  • Modelled ozone loss and regional climate effects are uncertain. The model study reviewed covers ozone only, and no source reviewed quantifies the wider regional effects.
  • Termination shock is disputed. Parker and Irvine (2018) argue the risk is much smaller than previously assumed, so its size remains an open question.
  • The governance measures proposed, such as permitting for outdoor experiments, are recommendations from expert bodies rather than established international rules.
  • Community opposition is a barrier as well as a technical one. The Saami Council and environmental groups heavily criticised the planned 2021 Kiruna test flight, which was cancelled on an advisory committee's recommendation.

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • A temporary cooling bridge that would buy time while emissions are cut. It would not reduce greenhouse gas levels.
  • Regional cooling aimed at reducing heat stress in one area, which would need tests of its side effects on neighbouring regions.

Timeline

  1. 1991The eruption of Mount Pinatubo injects nearly 20 million tons of sulfur dioxide into the stratosphere, and global temperatures drop temporarily by about 0.5°C over 1991 to 1993. Sources: [1]
  2. 2018Parker and Irvine publish an analysis in Earth's Future arguing that termination shock is much less of a risk than previously assumed. Sources: [2]
  3. 2021The US National Academies publish Reflecting Sunlight, recommending a research programme and governance safeguards for solar geoengineering. Sources: [4]
  4. 2021A balloon test of SCoPEx hardware from Kiruna, Sweden, planned for summer 2021, is cancelled on an advisory committee's recommendation after criticism from the Saami Council and environmental groups. The source does not give the date of the cancellation. Sources: [5]
  5. 18 Mar 2024Harvard announces that it is no longer pursuing the SCoPEx stratospheric experiment. Sources: [5]

Futures this could help bring about

AI illustration
How far away?

Cooling the planet on purpose

Science unproven · 2 of 8 steps done

Glossary

Stratosphere
The layer of the atmosphere above the weather, where aerosol particles can stay for months and spread around the globe.
Termination shock
A fast rise in temperature that could follow a sudden stop to stratospheric aerosol injection, as the cooling effect disappears.
Dobson unit (DU)
A measure of the total amount of ozone in a column of air. Ozone loss per degree of cooling is expressed in DU per kelvin.
Solar radiation modification
Methods that reduce the amount of sunlight absorbed by the Earth, rather than cutting the greenhouse gases that cause warming.

Sources

  1. government report
    The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines
    Chris Newhall, James W. Hendley II, Peter H. Stauffer. U.S. Geological Survey, 1997. Accessed 21 Oct 2026.
  2. peer reviewed paper
    The risk of termination shock from solar geoengineering
    Andy Parker, Peter J. Irvine. Earth's Future, 2018. DOI: 10.1002/2017EF000735. Accessed 21 Oct 2026.
  3. peer reviewed paper
    Stratospheric ozone projections under sulfur-based stratospheric aerosol injection: Insights from the multi-model G6-1.5K-SAI experiment
    Ewa M. Bednarz, Amy H. Butler, James M. Haywood et al.. Atmospheric Chemistry and Physics, 10 Sept 2026. DOI: 10.5194/acp-26-12751-2026. Accessed 21 Oct 2026.
  4. other
    Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance
    National Academies of Sciences, Engineering, and Medicine, 2021. DOI: 10.17226/25762. Accessed 21 Oct 2026.
  5. news
    Harvard has halted its long-planned atmospheric geoengineering experiment
    MIT Technology Review, 18 Mar 2024. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 21 Oct 2026
Published on owner instruction (content loop iteration 21)f437d78
Sources and claims re-checked against originalsfbb434c
Initial profile drafted from primary sources7e66249