Ocean geoengineering trial finds no evidence of harm to marine life

Can we safely remove carbon dioxide from the atmosphere by counteracting ocean acidification? Maybe, suggests a trial in which ships poured 65,000 litres of alkaline sodium hydroxide into the Gulf of Maine off the East Coast of the US in August 2025.

“We’re the first group to do a ship-based alkalinity enhancement experiment,” says Adam Subhas at 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 tonnes of CO2 was removed from the atmosphere in the following four days, Subhas says, and the team estimates that up to 50 tonnes could be removed altogether. What’s more, no significant effect on marine life was seen.

However, when asked by New Scientist, Subhas acknowledged that the team has yet to estimate the emissions required to manufacture the sodium hydroxide and transport it to the trial site. This means it is unclear whether the trial resulted in a net removal of CO2.

“It’s a really good question,” said Subhas. “That’s going to be a really critical area of research moving forward.”

The oceans store 40 times as much carbon as the atmosphere and have soaked up more than a quarter of the excess CO2 we have been pumping into the atmosphere. This extra CO2 reacts with water to form carbonic acid, meaning that the oceans are becoming more acidic.

Ocean acidification could have a major impact on many marine organisms, for instance, by dissolving their carbonate shells. It also reduces the ability of the seas to take up more CO2.

Researchers are exploring a number of methods to counteract ocean acidification, including adding magnesium hydroxide to wastewater that goes into the ocean, adding ground-up olivine to coasts and pumping seawater through land-based treatment plants. Some companies are already selling carbon credits based on alkalinity enhancement.

“This is something that the private sector is moving forward with right now,” says Subhas, which is why there is a need for non-commercial trials like the one his team did.

Because of the controversial nature of these kinds of trials, the team started by engaging with local people, particularly in the fishing community, says team member Kristin Kleisner of the Environmental Defense Fund, a non-profit organisation based in New York. “Two-way dialogue is really critical,” she says.

The trial itself involved three ships and was monitored in several different ways, ranging from satellites to floating sensors to ocean gliders that zigzag up and down. The sodium hydroxide was mixed with trace quantities of a dye called rhodamine, to help accurately track its dispersal.

The team measured the concentrations of microbes, plankton, fish larvae and lobster larvae, and also the level of photosynthetic activity, says Rachel Davitt at 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 is turned into bicarbonate ions, or dissolved baking soda, Subhas says. “We expect that this carbon is locked away 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, Subhas says. This is an advantage over some other approaches, where CO2 is first removed from the atmosphere and then has to be permanently stored in some form.