Alameda is not the setting you would think of for a battle over climate change. Situated on a 58-square-kilometer island bordering Oakland, California, this residential suburb of 78,000 is best known for its former naval air station, visible from San Francisco across the bay.

Unwittingly, and above all unknowingly, the municipality has just become embroiled in an experiment involving cloud brightening, one of the – highly controversial – solar geoengineering technologies aimed at artificially lowering the earth's temperature. It's "the first open-air Marine Cloud Brightening project in the US," according to Friends of the Earth.

In early April, the city discovered that it was home to a University of Washington experiment designed to lighten clouds in order to send solar radiation back into space. Researchers had set up their equipment on the deck of the USS Hornet, a former aircraft carrier anchored in Alameda and converted into a museum. The experiment was advertised to visitors as part of a scientific project on climate change. No one objected.

The experiment, which began on April 2, involved spraying tiny particles of sea salt into the air using a kind of snow cannon to determine, over time, the particle size most likely to make clouds brighter. It was to last for months, if not years, with a budget of around $1 million (€924,230) a year, provided by the SilverLining association which campaigns for geoengineering.

Alerted by a New York Times article about the long-term purpose of the project, Alameda's elected officials suspended the tests and commissioned an impact study from a group of consultants. According to a report published by the mayor's administrative director on May 23, this concluded that the experiment posed "no measurable health risk to the surrounding community" or to animals. "The chemical components of the saltwater solution (which is similar to seawater) being sprayed are naturally occurring in the environment," the report added, recommending nevertheless that monitors be installed to measure air quality in the surrounding area.

On June 4, the city council meeting devoted to the project lasted several hours. Throughout the evening, Alameda became a kind of capital of the solar geoengineering debate. "While this is a local decision, it has far-reaching consequences," said Gary Hughes of Hands Off Mother Earth. "There are global climate justice dynamics at stake."

Among the online speakers – scientists from around the world and environmental law specialists – a young Honduran climate leader asked the city to maintain the experiment, given the climate emergency in countries like his.

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