A research team poured 65,000 litres of alkaline sodium hydroxide into the Gulf of Maine in August 2025, becoming the first group to conduct a ship-based ocean alkalinity enhancement experiment. Initial findings, announced at the Ocean Sciences Meeting in Glasgow on 25 February, showed measurable carbon dioxide uptake and no significant harm to marine life.
Adam Subhas at the Woods Hole Oceanographic Institution in Massachusetts led the trial. “We can definitely say that there was additional CO2 uptake as a result of this experiment,” he said. Between 2 and 10 tonnes of CO2 were removed from the atmosphere in the four days following the release, with the team estimating that up to 50 tonnes could be removed in total.
What the trial measured
Three ships conducted the operation, monitored through satellites, floating sensors, and ocean gliders that tracked the dispersal of the sodium hydroxide. The team mixed the solution with trace quantities of rhodamine, a dye, to follow its spread through the water accurately.
Researchers measured concentrations of microbes, plankton, fish larvae, and lobster larvae, and tracked photosynthetic activity. Rachel Davitt at Rutgers University in New Jersey reported a clear outcome: “There was no significant impact of our field trial on the biological community.”
A critical gap in the data
The most significant open question the trial left unanswered is whether it produced a net reduction in atmospheric CO2 at all. Subhas acknowledged the team has not yet estimated the emissions generated by manufacturing the sodium hydroxide and transporting it to the trial site. Without that accounting, the true climate benefit remains unknown.
“It’s a really good question,” Subhas said. “That’s going to be a really critical area of research moving forward.”
Why ocean alkalinity enhancement matters
The oceans store 40 times as much carbon as the atmosphere and have absorbed more than a quarter of the excess CO2 humans have produced. That absorption comes at a cost: the CO2 reacts with seawater to form carbonic acid, accelerating ocean acidification. This process threatens marine organisms that rely on carbonate shells and reduces the ocean’s capacity to absorb further CO2.
Adding alkaline substances counteracts that acidification. When ocean alkalinity rises, CO2 is drawn from the atmosphere and converted into bicarbonate ions. Subhas described the result as “dissolved baking soda,” stored in the ocean for tens of thousands of years. The process also removes and stores carbon in a single step, which distinguishes it from approaches that capture CO2 first and then require separate permanent storage.
Community engagement and commercial context
Because of the contentious nature of ocean geoengineering trials, the team invested heavily in outreach before the experiment. Kristin Kleisner of the Environmental Defense Fund said the team prioritised the fishing community in particular. “Two-way dialogue is really critical,” she said.
The commercial dimension adds pressure to establish independent baselines. Some companies are already selling carbon credits based on alkalinity enhancement. “This is something that the private sector is moving forward with right now,” Subhas noted, citing this as a core reason non-commercial research trials are necessary.
Other methods under investigation include adding magnesium hydroxide to coastal wastewater and processing seawater through land-based treatment plants. The Gulf of Maine trial represents the largest field test of ship-based delivery to date, though the net-emissions question will need to be resolved before its practical value can be assessed.
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