Researchers conducted a trial in the Gulf of Maine, pouring 65,000 litres of sodium hydroxide into the ocean in August 2025, which removed up to 10 tonnes of carbon dioxide from the atmosphere. The experiment, the first ship-based alkalinity enhancement test, showed no significant impact on marine life. Initial findings were announced at the Ocean Sciences Meeting in Glasgow on 25 February.
In August 2025, a team led by Adam Subhas at the Woods Hole Oceanographic Institution in Massachusetts carried out an ocean alkalinity enhancement experiment in the Gulf of Maine off the US East Coast. Three ships dispersed 65,000 litres of alkaline sodium hydroxide, mixed with trace amounts of rhodamine dye for tracking. The goal was to counteract ocean acidification by increasing the water's alkalinity, enabling it to absorb more CO2 from the atmosphere.
Monitoring involved satellites, floating sensors, and ocean gliders. The team measured concentrations of microbes, plankton, fish larvae, and lobster larvae, along with photosynthetic activity. Rachel Davitt at Rutgers University in New Jersey reported, “There was no significant impact of our field trial on the biological community.”
Over the following four days, the ocean absorbed between 2 and 10 tonnes of CO2, with estimates suggesting up to 50 tonnes in total. Subhas stated, “We can definitely say that there was additional CO2 uptake as a result of this experiment.” The absorbed carbon converts to bicarbonate ions, which Subhas described as “locked away for tens of thousands of years,” making it a durable form of carbon removal achieved in a single step.
Before the trial, the team engaged local communities, particularly fishers, through two-way dialogue. Kristin Kleisner of the Environmental Defense Fund emphasized, “Two-way dialogue is really critical.”
Oceans hold 40 times more carbon than the atmosphere and have absorbed over a quarter of human-emitted excess CO2, leading to acidification that forms carbonic acid and threatens marine organisms by dissolving carbonate shells. Other proposed methods include adding magnesium hydroxide to wastewater or spreading ground-up olivine along coasts. Subhas noted that some companies are already selling carbon credits for such enhancements, underscoring the need for independent trials.
The team has not yet calculated emissions from producing and transporting the sodium hydroxide. Subhas acknowledged, “That’s going to be a really critical area of research moving forward,” as it determines if the process achieves net CO2 removal.