UK facility starts sucking CO2 out of seawater to help the climate (2025)

At the back of the Sea Life aquarium in Weymouth on England’s south coast, a hangar resounds with the rush of water into tanks. Three reef sharks and a nurse shark are cruising around in one. A stingray is hiding somewhere in another. But a closed rectangular tank of stainless steel holds an entirely different beast: carbon dioxide bubbling out of seawater as part of a process scientists hope could someday help reverse climate change. A red line on a computer screen shows the rising CO2 concentration in the tank.

“It’s been increasing like this for quite a while, and it still hasn’t levelled out,” says Tom Bell at Plymouth Marine Laboratory, UK.

The ocean absorbs almost a third of humanity’s carbon emissions. In January, this project, called SeaCURE, became the first in the UK — and one of a handful worldwide — to begin stripping CO2 out of seawater. Pumped back into Weymouth Bay, the resulting low-carbon water can absorb more CO2 from the air.

“There are now [weather] events occurring that physically couldn’t have happened without climate change,” project leader Paul Halloran at the University of Exeter, UK, told two dozen guests invited to tour the plant last week. “We’ve sort of left this so late, we’ve got to do something dramatic.”

In addition to slashing emissions at the source, that means removing billions of tonnes of carbon emissions from the atmosphere annually by 2050, according to United Nations modelling — far more than could be achieved by simply planting trees.

SeaCURE, which is funded by a £3 million grant from the UK government, can only remove a few dozen tonnes per year. And it is currently venting that back into the atmosphere, since two UK sites where CO2 might be injected underground for long-term storage are still in development. But if this pilot is successful, the scientists could move on to a demonstration plant that can remove several hundred tonnes a year.

A growing number of carbon removal facilities are stripping CO2 from the air with fans and chemicals. The ocean, however, holds around 150 times more CO2 by volume than the atmosphere, making it “a much richer, much more concentrated starting point for CO2 removal,” says Halloran.

When CO2 dissolves in seawater, 99 per cent of it becomes bicarbonate and carbonate ions. SeaCURE essentially breaks down these carbonates with acid, allowing the CO2 to bubble out for capture.

For now, the team is dosing seawater siphoned off from Sea Life’s intake pipes with off-the-shelf hydrochloric acid. But once fully operational, the plant will generate its own acid by pumping a portion of the seawater through a membrane, where an electric current will split it into acidic and alkaline streams.

The acidic water will be added to the main seawater stream until the pH reaches 4, similar in strength to tomato juice. This will cascade through tiny plastic rings in the stainless steel tank, releasing CO2 as it comes into contact with air. Then the alkaline stream will be added back in, raising the pH of the water before it is piped into the sea to absorb more atmospheric CO2.

The process consumes up to 10 megawatt-hours of electricity per tonne of CO2. At that rate, removing 500 tonnes of CO2 per year would use as much electricity as 1850 UK households. The team has to reduce that consumption to below 1 megawatt-hour or “this approach has got no legs”, says Halloran.

Environmental impacts are also a concern after a carbon removal trial in the UK that added magnesium hydroxide to seawater off the coast of Cornwall sparked protests in 2023 and 2024. SeaCURE discharges only seawater, but until that water absorbs more CO2, it has an elevated pH, which, in experiments, has caused mussels to start spawning, a sign of potential stress.

Pumping seawater through power plant cooling systems kills fish larvae, crustaceans and other vital parts of the food web, and carbon removal facilities could have similar effects, warns James Kerry, a marine scientist at conservation organisation OceanCare. “There really are fundamental issues that you’re facing if you start creating large volumes of dead water,” he says.

The SeaCURE pilot only draws from Sea Life’s existing water intake, and its alkaline discharge is quickly diluted, according to Bell. “It’s designed to help us think through these potential implications without operating at a scale where you’d be concerned about unintended consequences,” he says.