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 at the Woods Hole Oceanographic Institution in Massachusetts, whose team announced their initial findings at the 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.

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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 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 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.

The centuries-old practice of spreading crushed limestone on farmland can improve crop yields by making soil less acidic. This custom is typically considered a source of greenhouse gas emissions, but new findings suggest “liming” may actually help remove large amounts of carbon dioxide from the atmosphere.

“Liming can be a carbon source or a carbon sink. Empirical measurements suggest this is a pretty efficient carbon sink,” says at Yale University. This could offer new motivation to spread more limestone on the world’s farms – but liming won’t have this effect everywhere.

Currently, most of the millions of tonnes of crushed limestone farmers spread on fields each year are counted as a source of emissions. That is because, as the alkaline rock dissolves in the acidic soil, much of its carbon is released as CO2. But this accounting is incomplete, says , also at Yale.

For example, soils today are very acidic due to the intensive use of fertilisers, as well as pollution from burning fossil fuels. As a result, even without crushed limestone present, other alkaline minerals found in soil will dissolve and release carbon. “Those CO2 emissions are going to occur no matter if you are putting lime into the system or not”, so added acidity, rather than liming, is to blame, says Suhrhoff.

To provide a more accurate picture of this practice’s emissions, argues Suhrhoff, researchers must compare how much CO2 is released from and taken up by the soil in scenarios with and without liming.

As an example of this approach, Suhrhoff, Planavsky and their colleagues looked at the Mississippi river basin, which collects runoff from most of the agricultural land in the US. They calculated the net carbon effect of all of the liming carried out between 1900 and 2015 in this region.

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The researchers used geochemical models, as well as data on how interventions like fertiliser and liming change soil acidity, to estimate emissions from soil. They also compared their modelling results with direct measurements of alkalinity in the Mississippi, since limestone creates alkalinity when it reacts with carbon dioxide.

Using their new approach, the researchers found liming in this region – rather than generating hundreds of millions of tonnes of emissions – actually removed about 300 to 400 million tonnes of CO2, compared to a scenario where no liming was done. Suhrhoff the work at the Goldschmidt Conference on geochemistry in Prague, Czech Republic on 10 July.

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Liming could also be paired with the growing practice of spreading crushed volcanic rocks on farms – called enhanced rock weathering – to remove even more CO2 from the atmosphere, says Planavsky.

at the Australian National University says liming can act as a carbon sink, but what worked in the Mississippi river basin won’t necessarily work everywhere. “There are risks associated with lime application that can make it a net carbon dioxide source in other systems, given strong acidification of agricultural soils,” he says.

The next steps are to identify those places where liming is most needed. “It opens up the possibility that we can incentivise something that will be good for crop yields and will potentially give us billions of tonnes of carbon dioxide removal,” says Planavsky. Such financial incentives could be particularly helpful for low-income farmers who can’t afford to do the optimal amount of liming for their crops.