Ocean Carbon Removal: Captura’s marine carbon capture explained

_{If you search up} Hawaii’s Keāhole Point on Google Maps, center it on your screen, and then zoom out until you can see the edges of the globe, one thing will become abundantly clear: The Pacific Ocean is very, very big.

In a few months, on this volcanic headland on Hawaii’s Big Island, marine-tech startup
Captura will begin pumping as much of the mighty Pacific through its pipes and tanks as it can. The company’s plan is to electrochemically strip carbon dioxide out of the ocean, store or use the CO₂, and then return the water to the sea, where it will naturally absorb more CO₂from the air.

Captura is one of a cadre of startups eyeing Earth’s oceans as a carbon sink ready to be harnessed. The
bioengineering strategies it’s deploying aim to accelerate what the oceans already do: absorb carbon emissions on a massive scale. This natural process has helped keep atmospheric CO₂ levels in check for millions of years, but it can’t keep up with present-day industrial emissions. Dozens of field trials and pilot projects have begun, and in 2025, Captura and several other companies will begin scaling up their facilities.

Their approaches are as diverse as they are bold. Some groups are growing
kelp forests or microalgae in the sea. Others propose pumping seawater between shallow and deep layers to move carbon around. Two strategies caught IEEE Spectrum’s gaze—Captura’s ocean carbon dioxide removal approach, which sucks carbon out of the sea, and ocean alkalinity enhancement, which stores carbon in the sea. Both have inspired the engineering of novel, highly efficient electrochemical systems to treat copious amounts of seawater.

Big funding entities support these ideas. The finalists for both the US $100 million
XPrize for Carbon Removal and the $35 million Carbon Dioxide Removal Purchase Pilot Prize from the U.S. Department of Energy include marine-based strategies, alongside atmospheric ones.

But the challenges facing marine carbon companies feel as grand as the companies’ plans. Most of their business models ultimately depend on selling carbon credits on voluntary markets. And to sell carbon credits, they must quantify how much CO₂ they’re causing the oceans to draw down from the air. This can’t be done with physical measurements alone; instead, they must rely on numerical models that come with considerable uncertainty.

On top of that, a lot of environmental monitoring needs to be done to prove that marine carbon-removal strategies aren’t harming aquatic life. And then there’s the issue of scale. To make a dent in the more than
1,000 gigatonnes of excess CO₂ lingering in Earth’s atmosphere, and the few dozen gigatonnes continuing to be emitted each year from human activities, companies would have to process ocean water in biblical proportions.

“If you want to strip out 1 gigatonne of CO₂ from the ocean, you probably have to put the upper few meters of the Atlantic through your machines every…