On a slice of the ocean front in west Singapore, a startup is building a plant to turn carbon dioxide from air and seawater into the same material as seashells, in a process that will also produce “green” hydrogen — a much-hyped clean fuel.

The cluster of low-slung buildings starting to take shape in Tuas will become the “world’s largest” ocean-based carbon dioxide removal plant when completed later this year, according to Equatic, the startup behind it that was spun out of the University of California at Los Angeles.

The idea is that the plant will pull water from the ocean, zap it with an electric current and run air through it to produce a series of chemical reactions to trap and store carbon dioxide as minerals, which can be put back in the sea or used on land.

It’s a compelling vision of a potential solution in the face of a worsening climate crisis that has fueled unprecedented heat and devastating extreme weather. Efforts to rein in global heating remain hugely off track, and many scientists are now warning that, in addition to rapidly reducing fossil fuels, the world will also need to remove carbon pollution humans have already pumped into the atmosphere.

This Singapore plant is one example of a slew of recent projects that are looking to the oceans, which already absorb almost 30% of humanity’s planet-heating pollution, as a tool to do this. Other projects include sprinkling iron particles into the ocean to stimulate CO2-absorbing phytoplankton, sinking seaweed into the depths to lock up carbon and spraying particles into marine clouds to reflect away some of the sun’s energy.

But carbon-removal projects are controversial, criticized for being expensive, unproven at scale and a distraction from policies to cut fossil fuels. And when they involve the oceans — complex ecosystems already under huge strain from global warming — criticisms can get even louder.

There are “big knowledge gaps” when it comes to ocean geoengineering generally, said Jean-Pierre Gatusso, an ocean scientist at the Sorbonne University in France. “I am very concerned with the fact that science lags behind the industry,” he told CNN.

How it works

Equatic’s technology, which has been tested in small pilot projects in LA and Singapore, requires three main ingredients: seawater, rocks and electricity.

It works like this: The plant pumps in seawater and runs an electric current through it, separating it into four components: two liquids, one acid and one alkaline, and two gases, hydrogen and oxygen.

The acidic water will be mixed with crushed rocks to get the pH back to the same levels as seawater and then sent back into the ocean.

Fans will pump air through the alkaline stream, which causes carbon dioxide to form solid calcium carbonate, the material from which seashells are formed, which will look like a fine sand, as well as dissolved bicarbonate.

The solid and dissolved minerals, which Equatic says can lock up carbon for at least 10,000 years, will be returned to the ocean or used on land. The seawater will also be sent back into the sea, ready to absorb more carbon dioxide from the atmosphere.

None of the processing happens out in the open ocean, said Gaurav Sant, an Equatic founder and professor of sustainability at UCLA, “this is important because it allows you to measure everything that you’re doing perfectly.” This includes the electricity the plant uses as well as the amount of carbon locked away.

The $20 million facility will be fully operational by the end of the year and able to remove 3,650 metric tons of carbon dioxide annually, said Edward Sanders, chief operating officer of Equatic, which has partnered with Singapore’s National Water Agency to construct the plant. That amount is equivalent to taking roughly 870 average passenger cars off the road.

The ambition is to scale up to 100,000 metric tons of CO2 removal a year by the end of 2026, and from there to millions of metric tons over the next few decades, Sanders told CNN. The plant can be replicated pretty much anywhere, he said, stacked up in modules “like lego blocks.”

As the plant scales up, they will need to make sure there aren’t negative impacts, Sant told CNN. “There’s a big difference whether you do this on a scale of one ton, versus a million tons, versus a billion tons. You need to measure, you need to monitor, you need to adapt.”

The upfront costs are high but the company says it plans to make money by selling carbon credits to polluters to offset their pollution, as well as selling the hydrogen produced during the process.

Equatic has already signed a deal with Boeing to sell it 2,100 metric tons of hydrogen, which it plans to use to create green fuel, and to fund the removal of 62,000 metric tons of CO2.

Fragile oceans

For some critics, however, the risks far outweigh the benefits.

Lili Fuhr, the director of the fossil economy program at the Center for International Environmental Law, criticized the use of “speculative technology” at a time when “climate change is already killing our oceans.”

Processing vast amounts of seawater could kill marine life, Fuhr told CNN. “We already know that power plants kill fish larvae and other marine life. Equatic would process much more seawater per plant than a power plant,” she told CNN, “And thousands of such plants would be needed to make any meaningful impact on the global climate.”

The impacts would need to be closely monitored, said James Niffenegger, a researcher at the National Renewable Energy Laboratory, who wrote a report on ocean carbon removal. The “artificial movement of water at this scale is going to be massive,” he told CNN.

The process could also potentially disrupt the delicate balance of ocean chemistry, Niffenegger said. While there is plenty of research into the negative impacts of ocean acidification, there is much less on what might happen if the ocean becomes more alkaline.

There are indications the impacts may be low, he added, but “we’re not going to be able to understand the potential of this solution until we start actually deploying this kind of thing.”

Then there are the energy requirements — as the world moves away from fossil fuels, the demand for clean energy is increasing  exponentially. “Moving seawater would require vast amounts of renewable energy that would be better used to displace fossil fuels in the first place,” Fuhr said.

It’s a point echoed by Gatusso, who said “justice must be considered when precious electricity is used to remove carbon rather than providing it to populations in need.”

Equatic said it filters the seawater to ensure marine life does not enter the system, and that water discharged back into the ocean will have the same composition as normal seawater and will comply with Singapore’s environmental guidelines.

The company also said the production of hydrogen, which could be used to power the process, means its overall energy use is lower than other carbon removal methods.

Equatic’s project encapsulates a broader debate among those who fear rushing into geoengineering could unleash untold damage on ecosystems, versus those who believe the climate crisis is so acute this technology must be an option.

It’s clear which side Equatic is on.

“Inaction is not a climate strategy,” said UCLA’s Sant, adding that it’s vital to be able to make decisions with uncertainty. “If we’re serious about mitigating the trajectory of climate change,” Sant said, “we need to be willing to move, and move quickly at scale.”