After being halted over safety concerns, a study related to cloud brightening—a form of geoengineering that could one day be used to cool the Earth—was deemed not to be a health risk to the local community in Alameda, California. 

Beginning in April, University of Washington researchers were conducting an experiment on how particles move through the atmosphere, using a machine that would spray plumes of saltwater into the air. That study was part of the Coastal Aerosol Research and Engagement Program on the USS Hornet, docked in Alameda.

But in early May, Alameda officials halted the study due to health and safety concerns. Officials called the research a “cloud brightening study,” though researchers involved noted it wasn’t actually testing cloud brightening. Cloud brightening involves spraying saltwater into clouds above the ocean; condensation then forms around those particles, making the clouds appear fuller and brighter. Brighter clouds then reflect more sun away from the Earth’s surface, offering a cooling effect. 

The researchers weren’t looking to manipulate the climate with their experiment. Instead, they were spraying a mist of sea salt particles from the deck of the ship to study how they move through the atmosphere, which could later inform scientific models of cloud brightening. (They would measure the effects of that spray between 10 to 200 meters from the nozzles.) It also helps in understanding the effects of pollution.

City officials halted the study in order to do their own assessment of the sea salt particles. Their report, out this week, states that the city’s consultant, Terraphase Engineering, “does not see the experiment as generating a measurable health risk to the surrounding community. . . . The chemical components of the saltwater solution (which is similar to seawater) being sprayed are naturally occurring in the environment.” Seawater, the report continued, “is one of the largest sources of natural aerosols in the atmosphere.” 

The findings were detailed in an agenda item for the next Alameda City Council meeting on June 4, when the Council will review the report and decide whether the experiment can resume. Terraphase Engineering recommended approving the continuation of the study, and also that researchers use air quality monitors during the experiment. The University of Washington team previously noted they would use air monitors at the study area.

The USS Hornet has a Sea, Air, and Space museum on board. Kelly Wanser, cofounder of and senior adviser to the University of Washington Marine Cloud Brightening Program, previously told Fast Company that the USS Hornet was picked for the study in part because of that museum, and the chance to inform both the public and other scientists about this atmospheric research. 

Terraphase Engineering said it does not anticipate that the study—which is expected to operate four days a week over a 20-week period—“would present any adverse effects to museum visitors or staff.”