Alkaline sodium hydroxide was dumped into the Gulf of Maine to test its effect on carbon uptake and marine life
Daniel Cojanu, Undercurrent Productions, ©Woods Hole Oceanographic Institution
Can we safely remove carbon dioxide from the atmosphere by counteracting ocean acidification? Maybe, suggests a lawsuit where ships spilled 65,000 gallons of alkaline sodium hydroxide into the Gulf of Maine off the east coast of the United States in August 2025.
“We are the first group to do a ship-based alkalinity enhancement experiment,” says Adam Subhas of the Woods Hole Oceanographic Institution in Massachusetts, whose team announced their initial findings at the Ocean Sciences Meeting in Glasgow, UK, on 25 February. “We can definitely say that there was additional CO2 uptake as a result of this experiment.”
Between 2 and 10 tons of CO2 were removed from the atmosphere over the following four days, Subhas says, and the team estimates that up to 50 tons could be removed in total. Furthermore, no significant effect on marine life was seen.
But on questions from New ScientistSubhar acknowledged that the team has not yet estimated the emissions required to produce the sodium hydroxide and transport it to the test site. This means that it is unclear whether the experiment resulted in a net removal of CO2.
“That’s a very good question,” said Subhas. “It’s going to be a very critical area of research going forward.”
The ocean stores 40 times as much carbon as the atmosphere and has absorbed more than a quarter of the excess CO2 we have pumped into the atmosphere. This extra CO2 reacts with water to form carbonic acid, which means the oceans are becoming more acidic.
Ocean acidification can have a major impact on many marine organisms, for example by dissolving their carbonate shells. It also reduces the ocean’s ability to absorb more CO2.
Scientists are exploring a number of methods to counteract ocean acidification, including adding magnesium hydroxide to the waste water that goes into the sea, adding ground olivine to the coast and pumping seawater through land-based treatment plants. Some companies already sell carbon credits based on alkalinity improvement.
“This is something the private sector is moving forward with right now,” says Subhas, which is why non-commercial trials like the one his team did are needed.
Because of the controversial nature of this type of experiment, the team started by engaging with local people, particularly the fishing community, says team member Kristin Kleisner of the Environmental Defense Fund, a nonprofit organization based in New York. “Two-way dialogue is very important,” she says.
The trial itself involved three ships and was monitored in several different ways, from satellites to floating sensors to ocean gliders zigzagging up and down. The sodium hydroxide was mixed with trace amounts of a dye called rhodamine to help accurately track the spread.
The team measured the concentrations of microbes, plankton, fish larvae and lobster larvae, and also the level of photosynthetic activity, says Rachel Davitt of Rutgers University in New Jersey. “There was no significant impact of our field trial on the biological community,” she says.
The extra carbon taken up by the ocean as a result of the increased alkalinity turns into bicarbonate ions, or dissolved baking soda, says Subhas. “We expect this carbon to be locked up for tens of thousands of years. It’s one of the most durable forms of carbon removal.”
The nature of the process means that CO2 is removed and stored in a single step, says Subhas. This is an advantage over some other approaches, where CO2 is first removed from the atmosphere and then must be permanently stored in some form.
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