Can we tap the ocean’s power to capture carbon?

The oceans have to play a role in helping humanity remove carbon dioxide from the atmosphere to curb dangerous climate warming. 

But are we ready to scale up the technologies that will do the job? 

The answer, according to an expert group reporting to the European Union, is no.

At least, not yet.

Not until there are measures in place to ensure these technologies, called marine carbon dioxide removal technologies, are doing what they are supposed to do and won’t do more harm than good.

How the ocean can help capture carbon

Marine carbon dioxide removal technologies build on the ocean’s ability to absorb carbon. 

They can be biological, like encouraging the growth of plankton or seaweed that take up carbon dioxide as they grow. Or they can be chemical or physical, such as directly removing carbon dioxide from the ocean.

After these technologies remove the carbon from the air, it can be stored at the ocean floor and sediments, or the deep ocean, or in geological reservoirs.

“This is about safeguarding the oceans for a common good. The oceans can be part of the climate solution, but we need to strengthen the way we safeguard them before we scale things up,” says Helene Muri. 

She is a researcher at NILU and the Norwegian University of Science and Technology (NTNU). Muri was chair of an expert group commissioned by the European Marine Board to study the issue.

Emissions cuts are the first priority

The Earth is getting warmer, and much faster than the nations of the world had hoped a decade ago when they pledged to limit global temperature increases to 1.5°C above pre-industrial levels.

In his opening remarks to the COP30 Leaders’ Summit on November 6, UN General Secretary António Guterres confronted his audience with the urgency of the situation.

“Science now tells us that a temporary overshoot beyond the 1.5 limit – starting at the latest in the early 2030s – is inevitable,” he said. “Let us be clear: the 1.5°C limit is a red line for humanity. It must be kept within reach. And scientists also tell us that this is still possible.”

The European Marine Board report underscores the need to act now with tools that are known to work – namely cutting emissions.

“We know how to cut emissions, and we have lots of methods that work. That has to take top priority,” says Muri.

Net zero by 2050

So why talk about removing carbon dioxide from the ocean at all, if the goal is to cut carbon dioxide emissions to zero?

Here’s where reality comes in. 

Cutting emissions from fossil-fuel energy is challenging, but possible, because we have other energy sources – like solar and wind – that can take over the job.

But some of the things we depend on are much harder to make carbon-free.

Air travel is one example. Researchers are working to cut emissions, but truly carbon-free flying remains out of reach. And even if people are encouraged to fly less, there are still times when air travel is the only option.

Societies across the globe need to achieve something called net zero by 2050. That’s when all the CO₂ emissions are zeroed out by removing the exact same amount of emissions.

Why cutting emissions isn't enough

Reaching the 1.5°C target means moving into net negative emissions. That happens when we cut everything we can, and then find ways to compensate for the remaining 'residual' emissions, those that simply can’t be eliminated.

Muri explains that we need to remove carbon dioxide from the atmosphere to reach 1.5°C because sectors like shipping, aviation, and some industries will always have some residual emissions.

She adds that there will also be large-scale carbon removal too – about five to ten gigatonnes of CO₂ a year by the end of the century, according to IPCC scenarios.

To put those numbers into context: Total global CO₂ emissions were 42.4 gigatonnes of CO₂ in 2024, according to CICERO, the Oslo-based Center for International Climate Research.

Land-based ways to remove this residual carbon are already in use, mainly through planting new forests. Another example is the Climeworks direct air capture plant in Iceland, where giant fans pull in air, captures the CO₂, and mix it with water before it's injected into bedrock, where it turns into stone.

Many types of marine carbon removal have been tested in the field, but most of them are still in the early stages. Others are gaining more traction. That's why it's important to set clear standards now for how these efforts are monitored, reported, and verified.

From mangroves to plankton blooms

Some marine-based methods for removing carbon dioxide work much like land-based options.

Planting lots of trees and protecting forests helps absorb on land, and similarly, certain marine approaches focus on protecting or restoring coastal areas, such as mangrove swamps, which also store large amounts of carbon.

Other approaches involve more direct intervention, such as adding iron or other nutrients to the ocean to boost plankton growth. These huge plankton blooms absorb carbon dioxide. When they die, they carry the carbon into the deep ocean, far from the atmosphere.

The challenge of tracking carbon in oceans

That’s the theory, at least. The problem, Muri says, is knowing how well these different technologies actually work.

For example, how does a company actually prove how much excess carbon dioxide is being removed by the technology in question?

If we send carbon to the deep ocean, do we know how long it will stay there?

And while many government bodies, international agencies, and global agreements exist, it's unclear which ones should take the lead – or how they should verify what is actually being done.

Muri explains that, ideally, you would first measure the ocean's baseline carbon levels, then carry out the project and track how much carbon is removed and how long it stays out of the atmosphere.

“And then you report that to some independent party and then it verifies that what you’re saying is correct,” she says.

The twist?

“If you’re storing it in the ocean, in some form or another, not in a geological reservoir, it’s a lot harder to govern it and also monitor it. The ocean doesn’t stay put,” she says.

Credits and environmental impacts

Addressing these issues will be essential as the technologies advance and governments or companies begin using them to claim credit for removing carbon dioxide. Some companies are already doing this, Muri says.

“None of these methods are mature to use if you cannot verify impacts or where the carbon goes, or how long it stays away from the atmosphere,” she says.

Muri adds that being serious about marine carbon dioxide removal means being equally serious about the monitoring, reporting, and verification needed to show it can be done responsible and make a real difference.

“The credit part of it also has to work right. You have to have reliable and transparent and scientifically defensible crediting systems," she says.

Reporting will also have to include any environmental impacts, Muri adds.

The way forward

Muri says that despite the many uncertainties around marine carbon dioxide removal, all future scenarios indicate that carbon dioxide removal will be necessary to reach the most ambitious temperature targets.

That’s the conclusion of the IPCC from any number of the organiaation’s reports, but particularly in a special report from 2018 on Global Warming of 1.5°C.

“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,” she says.

Nevertheless, Muri says that marine carbon dioxide removal will not be a miracle ocean fix to climate change. 

“Some people are really hoping to find an answer in the ocean, but in our opinion, we’re not there yet. And there’s a question of whether it can be a scientifically governed climate solution, and we don’t have the answer to that yet. But if we want to go in that direction, then we need to clear up all of these standards and establish these properly before we can scale things up,” she says.

Reference:

Muri et al. Monitoring, Reporting and Verification for Marine Carbon Dioxide Removal, Zenodo, 2025. DOI: 10.5281/zenodo.17435116

Marine carbon dixode removal technologies

Two main categories are currently being explored to remove carbon dioxide from the ocean.

Biotic methods rely on photosynthesis, where marine organisms – especially phytoplankton – absorb CO₂ and turn it into biomass. When they die, some of this material sinks, taking the carbon with it. This is a process that removes carbon from the atmosphere and transports it to the ocean’s depths, where it can be stored long term.

Six approaches fall under this category:

1. Preexisting marine biomass removal: Relocating naturally occurring marine algae to the deep ocean where the biomass and degradation products can be stored out of contact with the atmosphere for more than a century. A key candidate for this is Sargassum, a genus of free-floating, fast-growing seaweed, that grows in temperate and tropical seas.
2. Marine biomass cultivation: Marine macroalgae (kelp and seaweed) are intentionally grown to absorb CO₂ through photosynthesis. They are then harvested and sunk to the deep ocean.
3. Marine bioproducts for durable products and energy: Seaweed can be made into bioplastics and construction composites. Seaweed can also be converted into bioenergy where the CO₂ produced can be captured and stored underground.
4. Ocean fertilisation: Nutrients are supplied to phytoplankton to enhance their growth and thus their CO₂ uptake. This carbon is sequestered in the deep ocean when the phytoplankton die and sink.
5. Artificial upwelling: Relies on the ocean’s natural nutrient cycles by pumping cold, nutrient-rich deep water to the surface to stimulate phytoplankton growth. This approach is not technically feasible yet.
6. Coastal blue carbon management: Replanting and restoring coastal ecosystems such as mangroves, salt marshes, and sea grasses.

Geochemical methods capture and store atmospheric CO₂ through chemical processes rather than biological processes. These methods are inspired by natural carbon cycles and the weathering of rocks.

1. Ocean alkalinity enhancement: Increases the ocean’s capacity to absorb CO₂ from the atmosphere by changing the dissolved CO₂ into bicarbonates or carbonates. This allows the ocean to take up even more CO₂ from the air.
2. Artificial downwelling: Creates a downward flow of upper ocean waters to enhance the transport of both dissolved and particulate carbon deeper into the ocean to enhance long-term storage
3. Ocean carbon removal: Removes carbon from seawater in a designated facility by using electrochemical processes powered by renewable energy. This CO₂ is stored underground in geological formations, mineralised into stable carbonate rocks, or used in products with a long lifetime.

(excerpted from Monitoring, Reporting and Verification for Marine Carbon Dioxide Removal, Future Science Brief No 13, Nov. 2025)

See also: National Academies of Sciences, Engineering, and Medicine. 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.

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