Oceans Hold Enormous Carbon Potential, But We Are Nowhere Near Ready

On a chilly spring morning off Germany’s Helgoland Island, a researcher leaned over a floating ring of seawater and drew a sample from a 7000 liter mesocosm, a tiny controlled slice of the North Sea that reveals how hard it is to measure carbon in a system that never sits still.

A new European Marine Board report, led by climate scientist Helene Muri of NILU and the Norwegian University of Science and Technology, asks a blunt question: can we trust ocean based carbon removal claims? The authors examine biological, chemical and physical approaches, from plankton fertilization to ocean alkalinity enhancement, and reach a cautious answer. Until rigorous systems for monitoring, reporting and verification are in place, they argue, none of these methods is ready to be scaled.

Inside the Push to Turn the Ocean Into a Carbon Sink

The interventions under consideration are tangible and often sweeping. Some teams test whether a dusting of iron can spark plankton blooms that shuttle carbon into the deep sea. Others investigate grinding alkaline minerals into powder and dispersing them into surface waters to increase the ocean’s natural ability to absorb CO2. Seaweed farms, ocean pumps and enhanced coastal ecosystems are all on the table. What unites them is promise, and what threatens them is uncertainty. The ocean mixes, churns and circulates in ways that make it nearly impossible to track a molecule once it slips below the surface.

“We don’t know all the threats of these immature methods yet, but it’s a bit hard to just take them off the table because they’re uncomfortable to think about.”

Muri and her colleagues note that the hardest part is not running the experiments. It is proving, in defensible numbers, how much extra carbon an intervention removes, how long it will stay locked away, and what ecological side effects come with altering chemistry or biology at scale. Residual emissions from sectors like aviation and shipping will require society to remove billions of tons of CO2 later this century. Yet without a way to verify that carbon is actually leaving the atmosphere for decades or centuries, ocean based removal risks becoming a well intentioned guess rather than a climate solution.

The Ocean Refuses to Behave Like Infrastructure

A forest stays where you plant it. A direct air capture facility can meter what it captures. The ocean, by contrast, carries carbon on currents that stretch across hemispheres. A pulse of alkalinity in the North Sea can disperse into the Norwegian Current within weeks. The turbulence makes baseline measurements slippery and crediting difficult. Some companies have already begun selling ocean removal credits. Researchers argue that this moment, before major deployment, is the last chance to build guardrails fast enough to matter.

“If we want to be serious about figuring out if you can do marine carbon dioxide removal in responsible ways that can make meaningful contributions, then we have to get serious about the monitoring, reporting and verification aspects.”

The report does not dismiss marine carbon removal. It recognizes that nearly every IPCC pathway that limits warming to 1.5 degrees requires removing CO2 later this century. What it challenges is the assumption that the ocean is a blank canvas for quick fixes. The mesocosms off Helgoland are a reminder that even in a controlled enclosure, tracking carbon is painstaking. Scaling that task to the open ocean, where currents tangle and ecosystems react in unpredictable ways, demands rules as dynamic as the water itself. If societies want ocean based removal to become a climate tool rather than a leap of faith, the work of building those rules must begin now.

European Marine Board: 10.5281/zenodo.17435116