The world's oceans have quietly been doing humanity a favor for centuries — absorbing roughly a quarter of all the carbon dioxide we pump into the atmosphere. Now, a growing coalition of scientists, entrepreneurs, and governments are asking whether we can dramatically amplify that natural process to pull us back from the climate brink.
Ocean-based carbon removal encompasses a diverse array of approaches, from cultivating vast kelp forests that sequester carbon as they grow, to adding alkaline minerals to seawater to enhance its natural carbon absorption capacity, to directly electrolyzing seawater to capture dissolved CO2. What unites them is the sheer scale they could potentially achieve — and the profound uncertainties that remain about their effects on marine ecosystems.
The Kelp Highway
Giant kelp grows faster than almost any other organism on earth — up to two feet per day under ideal conditions. Startup Running Tide has been cultivating kelp at sea and sinking it to the ocean floor before it can decompose, locking the carbon it absorbed during growth into deep-sea sediments for potentially thousands of years. The company has sold carbon credits to Microsoft, Stripe, and several other major corporations.
The challenge is measurement. Verifying how much carbon actually reaches the deep ocean and stays there — rather than decomposing in shallower water and releasing CO2 back into the atmosphere — requires sophisticated monitoring systems that are still being developed. Without robust measurement, ocean carbon credits risk becoming another form of greenwashing.
The Alkalinity Question
Ocean alkalinity enhancement takes a different approach. By adding crushed silicate rocks or other alkaline materials to seawater, researchers can shift the ocean's chemistry in ways that allow it to absorb more CO2 from the atmosphere — the same principle that makes antacids neutralize stomach acid. Early field trials have shown promising results, with some estimates suggesting the approach could eventually remove gigatons of CO2 per year.
The environmental risks are real and poorly understood. Altering ocean chemistry at scale could affect everything from coral reef calcification to fish behavior. Governance frameworks for large-scale ocean interventions essentially don't exist. And the specter of moral hazard looms — if ocean carbon capture becomes seen as a viable backstop, will it reduce pressure to cut emissions at the source?
The honest answer is that we don't yet know whether ocean carbon capture can be deployed safely at the scale the climate requires. What we do know is that we're running out of time to find out.