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Marine Cloud Brightening (MCB)

• Experimentation
  • Since 2016, researchers have been testing the use of MCB to cool the water surrounding the Great Barrier Reef and slow coral bleaching, which occurs when high water temperatures force coral to expel its outer algae layer, eventually killing the coral. Researchers are spraying salt water into the air over the reef from a barge to create low-lying clouds that reflect sunlight.19 Despite some criticism that the experiments are costly, impractical, and distract from climate mitigation and adaptation, the project is ongoing as of late 2025.20 The program is funded primarily by the Australian government.21
  • In early 2024, researchers from the University of Washington’s MCB Research Program retrofitted a decommissioned aircraft carrier to spray saltwater particles over the sea off the coast of Alameda, California.22 The Alameda city council, which was not aware of the project before it was announced in The New York Times, voted unanimously to halt the experiment in May 2024.23 The University of Washington has not announced plans to continue outdoor MCB research.24
     

Stratospheric Aerosol Injection (SAI)

• Experimentation
  • Harvard professors initiated the Stratospheric Controlled Perturbation Experiment (SCoPEx) in 2015 and abandoned the project in 2024.25 Funded largely by foundations and philanthropists, the researchers intended to release chalk calcite particles from a high-altitude balloon to assess their dispersal and reflective behavior.26 A test flight over Sweden was ultimately abandoned after organized resistance from Indigenous groups and environmental nongovernmental organizations (NGOs). In a 2024 statement celebrating the project’s end, a representative from the Indigenous Environmental Network described SRM as a violation of Indigenous rights and a continuation of “racism, colonialism, and white supremacy.”27
• Private Sector Start-Ups
  • In 2022, the U.S.-based start-up Make Sunsets launched its first atmospheric sulfur dioxide (SO₂) SAI balloons in Mexico. The company is funded by venture capital and “cooling credits that can be purchased on the company’s website.”28 After a second round of balloon launches in 2023, the Mexican government announced its intention to ban SRM deployment.29 Make Sunsets then pivoted to the United States, launching SO₂ balloons in Nevada and California over the next two years.30 In April 2025, the U.S. EPA submitted a demand for information about the company’s activities, and Make Sunsets’ response disclosed that their balloons had released approximately 0.1 tons of SO₂. A July 2025 EPA statement said the agency is aware of the company’s activities.31 Make Sunsets has launched SO₂ balloons as recently as February 2026, according to the company’s X account.32 Researchers and environmentalists are largely opposed to Make Sunsets’ activities, which they say detract from and delegitimize more “responsible” SRM research.33
  • In October 2025, a U.S.-Israeli start-up called Stardust Solutions announced that it had raised $60 million to conduct outdoor SAI experiments. Whether outdoor tests have or will be conducted is unclear.34 The company, funded by Israeli-Canadian venture capital fund AWD and Israeli green energy company SolarEdge, reportedly employs twenty-five scientists and engineers.35 While investors praise the company’s staff of experienced scientists, others criticize its lack of transparency.36
     

Computer Modeling Experiments

Studies are briefly summarized below. Please use the endnote sources to learn more about each experiment’s inputs and findings.

• Initiated in 2011, the Geoengineering Model Intercomparison Project (GeoMIP) is a coordinated effort among multiple international modeling groups to understand how the climate would respond to SRM deployment in idealized conditions. The models predicted that SRM could reduce warming but not control temperature and precipitation at the same time. Models showed that if carbon dioxide (CO₂) continued to increase and some SRM were deployed, temperatures could be kept at their current level, but global average precipitation would be reduced. Evapotranspiration would not increase, but the CO₂ would make the atmosphere more stable, reducing convection and precipitation.37 GeoMIP, which held its sixteenth annual workshop in Tokyo in March 2026, has produced 170 peer-reviewed publications as of this writing.
• In 2017, the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) project produced simulations using a feedback controller to keep global temperatures and large-scale temperature gradients at 2020 values under a high-emissions scenario using aerosol injection at multiple latitudes, whose selection was informed by sensitivity experiments. Researchers found that the modeled SAI successfully offset projected greenhouse warming, though the models also predicted extreme adverse effects on regional rainfall and temperature.38
• In 2022, an international group of researchers released results from the Assessing Responses and Impacts of Solar Climate Intervention on the Earth System with Stratospheric Aerosol Injection (ARISE-SAI) simulations. The initial ARISE-SAI simulations used a feedback-based control algorithm to adjust SO₂ injections into the stratosphere to maintain global mean surface temperatures near 1.5°C above preindustrial levels under a moderate greenhouse gas emissions future. This scenario has been described as more policy-relevant and adaptable than many earlier models. The simulations also provided extensive outputs for atmosphere, ocean, land, and sea-ice components and were designed to be reproducible across Earth system models. Among other results, the models showed that, if deployed, SAI could produce inconsistent cooling effects, including a possible “warming hole” over the North Atlantic Ocean.39
• In 2025, the British government’s Advanced Research + Invention Agency (ARIA) gave £57 million (USD 78 million) to researchers studying SRM. From 2026 to 2029, ARIA will fund twenty-six projects “from computer modelling, to ethical frameworks, and observations of natural analogues of climate cooling approaches (like volcanoes). Where essential questions cannot be answered by models, [ARIA will fund] a limited number of small-scale, carefully controlled outdoor experiments.” One of ARIA’s key stated aims is to “better understand localized perspectives and governance approaches to SRM.”40

For a comprehensive list of SRM and weather modification experiments, visit the Geoengineering Monitor Map.41