Cooling the planet on purpose

How far are we from using solar geoengineering to cool the planet?

Solar geoengineering would cool the surface by reflecting a little sunlight back to space. The best-known idea is to add reflective particles to the stratosphere. The 1991 Mount Pinatubo eruption shows that such particles can cool the surface, and climate models suggest a sustained programme would do the same, but the sources reviewed show no deliberate use. Two planned outdoor tests were halted in 2024: a Harvard stratospheric balloon experiment and a University of Washington marine cloud test in Alameda, California. The UK's ARIA funds research through its Exploring Climate Cooling programme, which includes five outdoor experiments. As of April 2026, two were underway outside the UK and none had a confirmed UK site. Expert bodies, EU foreign ministers and African environment ministers have called for limits, a moratorium or a non-use agreement.

AI illustration · Illustrative scene, not a photograph of the specific device or experiment.
WHERE WE AREScience unproven

Very far away. The basic science has not been shown to work.

A volcanic eruption and climate models support the idea that stratospheric particles cool the surface, but neither shows a deliberate, sustained use of the technique. The expert review by the Scientific Advice Mechanism and SAPEA (2024) rated the technology readiness for solar radiation modification as very low. The sources reviewed describe no completed outdoor release for cooling purposes and no test flight for this purpose. Two ARIA-funded outdoor projects were underway in April 2026, both outside the UK. The stage is therefore science-unproven for deliberate use, not engineering prototype.

The tech tree

Everything recorded on Vrulum that this future needs, oldest foundations at the top and the goal at the bottom. Lines show what each step requires.

What it takes: Cooling the planet on purpose Unlocked Being researched Locked
  1. 1991Stratospheric aerosol injectionTheory Locked.
  1. 1991Natural analogue: volcanic sulphate coolingDone Unlocked.
  2. 2021Expert reviews of readiness and research needsDone Unlocked.
  3. 2024Stratospheric particle release testNot started Locked.
  4. 2024Permitted outdoor marine cloud brightening testNot started Locked.
  5. 2024International rules for research and deploymentIn progress Being researched.
  6. 2026Climate model studies of sustained injectionIn progress Being researched.
  7. Date not recordedSmall outdoor experiments under oversightIn progress Being researched.
  8. Date not recordedDeployment of stratospheric or marine cloud coolingNot started Locked.
  1. GoalCooling the planet on purposeGoalRequires: Stratospheric aerosol injection, Natural analogue: volcanic sulphate cooling, Climate model studies of sustained injection, Expert reviews of readiness and research needs, Small outdoor experiments under oversight, Stratospheric particle release test, Permitted outdoor marine cloud brightening test, International rules for research and deployment, Deployment of stratospheric or marine cloud cooling. Locked.

The road so far

2 of 8 steps done, 3 in progress.

  1. Done · 1991Natural analogue: volcanic sulphate cooling

    The 1991 eruption of Mount Pinatubo injected about 20 teragrams of sulphur dioxide into the stratosphere. The SAPEA review cites a global-mean surface cooling of about 0.3°C from it, and describes volcanic eruptions as an imperfect natural analogue for sustained injection. [2]

  2. In progress · 10 Sept 2026Climate model studies of sustained injection

    A 2026 study of three climate models (CESM2(WACCM6), MIROC-ES2H and UKESM1.1) simulated sulphur dioxide injection at about 30°N and 30°S under the SSP2-4.5 emissions scenario. For 2045 to 2064 it found global ozone loss of about 4 Dobson units per kelvin of cooling, compared with about 1 to 2 Dobson units per kelvin for 2065 to 2084. The paper's title and focus are on ozone, but it also analyses temperature, circulation, age of air, water vapour and aerosol surface area. [12]

  3. Done · 2021Expert reviews of readiness and research needs

    The US National Academies (2021) recommended a transdisciplinary research programme, with a research code of conduct, a public registry, permitting systems for outdoor experiments and inclusive public and stakeholder engagement. The SAPEA review (December 2024) found that neither the technology nor the governance frameworks for aerosol deployment are ready. [1][2]

  4. In progressSmall outdoor experiments under oversight

    ARIA's Exploring Climate Cooling programme, backed by £56.8m, lists 22 funded research teams. ARIA says it funds five outdoor experiments, which are time-bound, limited in size and scale, and subject to oversight by an independent oversight committee. As of April 2026, two were underway: a Cambridge project pumping seawater onto sea ice in Nunavut, Canada, and a Southern Cross University marine cloud brightening study on the Great Barrier Reef. The other three were in technical design and pre-site selection. No location had been confirmed for any outdoor experiment in the UK. [9][10][11]

  5. Not started · 18 Mar 2024Stratospheric particle release test

    Harvard's SCoPEx balloon project planned to release a few kilograms of material, such as calcium carbonate or sulphuric acid, high in the stratosphere. A hardware test flight from Kiruna, Sweden, planned for summer 2021, was cancelled on the recommendation of the project's advisory committee, which wanted public discussion first. On 18 March 2024 Harvard said that the principal investigator was no longer pursuing the experiment. [6]

  6. Not started · May 2024Permitted outdoor marine cloud brightening test

    A University of Washington team ran a test on the flight deck of the USS Hornet museum in Alameda, California, using a machine that sprayed sea salt droplets. The City of Alameda stopped it and announced on 4 May 2024 that the work had violated the museum's lease. The city said it found no indication that the spray presented a threat to human health or the environment. In June 2024 the city council voted that the experiment would not be resumed. [7][8]

  7. In progress · 9 Dec 2024International rules for research and deployment

    The sources reviewed describe no formal international governance for solar radiation modification research or deployment. The European Commission's Group of Chief Scientific Advisors recommended a Europe-wide moratorium on using these technologies, and a global governance system for future decisions (9 December 2024). In late April 2026 EU foreign ministers called for a moratorium on deployment and for talks on governance, and their conclusions took no position on whether research should continue, as reported by Climate Change News. African environment ministers at AMCEN-20 (July 2025) called for a solar geoengineering non-use agreement, as quoted in a compilation of agency statements. [3][4][5]

  8. Not startedDeployment of stratospheric or marine cloud cooling

    No deployment has been reported in the sources reviewed. ARIA states that it is not funding or deploying these technologies. The SAPEA review found that neither the technology nor the governance frameworks for aerosol deployment are ready. [10][2]

Why it matters

Solar geoengineering would not remove the greenhouse gases that cause warming. Its side effects, such as changes to ozone and to rainfall patterns, are uncertain, and any use would affect the whole planet. Who decides on research, outdoor tests and any future use is therefore as important as the science. The debate is about whether to research the option at all, not only how to build it.

What stands in the way

  • There is no agreed international framework for research or deployment. The positions in the sources reviewed range from research under oversight to a moratorium on deployment and a non-use agreement.
  • Outdoor tests have been cancelled or stopped after concerns about public discussion and transparency. The 2021 Kiruna flight was cancelled on an advisory committee's recommendation for public discussion first, Harvard stopped SCoPEx in 2024, and Alameda stopped its marine cloud test in May 2024.
  • Ozone and regional climate effects are uncertain. The 2026 model study covers three models and focuses on ozone. The SAPEA review notes that injection in one hemisphere could shift the Intertropical Convergence Zone and cause potential droughts in the Sahel.
  • The aircraft needed to carry particles into the tropical stratosphere do not yet exist. The SAPEA review says recent studies consider them feasible to design and build.
  • Cost estimates vary widely with the study and its assumptions. The SAPEA review cites about $10 billion a year for one study and between $34 billion and $107 billion a year for others.

Signals to watch

  • An ARIA outdoor experiment with a confirmed UK site. As of April 2026, ARIA had not confirmed any UK location.
  • A UN Environment Assembly or other intergovernmental decision on a solar geoengineering non-use agreement, which African environment ministers called for in July 2025.
  • EU action that follows the April 2026 call for a moratorium on deployment.
  • A research code of conduct or public registry adopted for outdoor experiments, in line with the 2021 National Academies recommendations.
  • Multi-model studies with larger ensembles that test effects beyond ozone, such as rainfall and regional climate.

Sources

  1. other
    Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance
    National Academies of Sciences, Engineering, and Medicine, 2021.
  2. review article
    Solar radiation modification: evidence review report
    SAPEA working group. Scientific Advice Mechanism (SAM) and SAPEA, 9 Dec 2024.
  3. other
    Solar radiation modification technologies cannot fully address climate change, and responsible research on impacts is needed - advisors tell the European Commission
    European Commission, Research and Innovation, 9 Dec 2024.
  4. news
    EU warns on solar geoengineering but research debate grinds on
    Joe Lo. Climate Change News, 8 May 2026.
  5. other
    Statements of Public Agencies (quoting AMCEN/20/Dec.9, July 2025)
    Solar Geoengineering Non-Use Agreement (compilation), Jul 2025.
  6. news
    Harvard has halted its long-planned atmospheric geoengineering experiment
    James Temple. MIT Technology Review, 18 Mar 2024.
  7. news
    City Halts Climate Experiment
    Ken Der. Alameda Post, 10 May 2024.
  8. news
    Alameda nixes climate geoengineering experiment to test technology that could mitigate global warming
    CBS News San Francisco, Jun 2024.
  9. grant
    Funded projects: Exploring Climate Cooling
    Advanced Research and Invention Agency (ARIA), 2026.
  10. other
    Exploring Climate Cooling: FAQs
    Advanced Research and Invention Agency (ARIA), 2026.
  11. other
    Exploring Climate Cooling: One year in
    Advanced Research and Invention Agency (ARIA), 10 Apr 2026.
  12. 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.

Last reviewed 21 Oct 2026.