Oceanic Carbon Sequestration: The Role of Oceans in Climate Mitigation

Oceans cover more than 70% of Earth’s surface and play a critical role in regulating our planet’s climate. They serve as the largest active carbon sink on Earth, absorbing roughly 30% of the carbon dioxide released into the atmosphere. This natural process has significantly buffered the impacts of human-caused climate change, but it comes with trade-offs-including ocean acidification. As scientists and policymakers explore ways to enhance the ocean’s carbon storage capacity, questions about effectiveness, environmental risks, and ethics take center stage.

How oceans naturally absorb carbon dioxide 🔗

The ocean’s ability to absorb CO₂ from the atmosphere depends on a combination of physical, chemical, and biological processes. When atmospheric carbon dioxide dissolves into seawater, it triggers a series of chemical reactions. Water and CO₂ combine to form carbonic acid, which then breaks apart into hydrogen ions and bicarbonate ions. This process effectively transfers carbon from the air into the ocean.

Beyond surface chemistry, the ocean relies on what scientists call the biological carbon pump. Photosynthesizing organisms like phytoplankton take up CO₂, converting it to biomass. When these organisms die, some of their carbon-rich remains sink to the deep ocean, where the carbon can remain stored for centuries or even millennia. This continuous movement of carbon from surface waters to the ocean depths is fundamental to the global carbon cycle.

The ocean currently holds around 42 times more carbon than the atmosphere. It has also absorbed approximately 90% of the excess heat generated by human activities. While this buffering effect has slowed the pace of atmospheric warming, the ocean itself is paying a heavy price.

The chemical cost: ocean acidification 🔗

As the ocean absorbs more CO₂, its chemistry changes in ways that threaten marine life. Since the industrial revolution, surface ocean pH has dropped by 0.1 units. This may seem small, but the pH scale is logarithmic-a 0.1 unit decrease represents a 30% increase in acidity.

This phenomenon, known as ocean acidification, fundamentally disrupts marine ecosystems. The process is sometimes called the “osteoporosis of the sea” because it erodes the minerals that shellfish, corals, and other marine organisms need to build and maintain their shells and skeletons. The chemical reactions that make the ocean more acidic also reduce the availability of carbonate ions-essential building blocks for calcifying organisms.

The consequences extend across the marine food web. Pteropods, tiny sea snails that serve as food for fish, krill, and whales, are particularly vulnerable. Research shows pteropod shells dissolved within 45 days when exposed to pH and carbonate levels expected by 2100. Scientists have already documented severe shell dissolution in pteropods in the Southern Ocean.

Deliberate ocean carbon sequestration methods 🔗

Given the ocean’s natural capacity to store carbon, researchers have proposed various methods to enhance this process deliberately. These approaches fall into two broad categories: those that leverage chemical or physical properties (abiotic) and those that harness biological processes (biotic).

Direct injection into deep ocean waters 🔗

One of the earliest proposed methods involves capturing CO₂ from industrial sources and injecting it directly into deep ocean waters. At depths below approximately 3,000 meters, liquid CO₂ becomes denser than seawater and would theoretically sink to the ocean floor, forming stable pools. The concept was first proposed by Italian physicist Cesare Marchetti in the 1970s.

Several injection techniques have been explored. These include piping liquid CO₂ from shore-based facilities, releasing it from ships through trailing pipes, and using clathrate hydrates to slow the dissolution of injected carbon dioxide. The hydrates give CO₂ negative buoyancy, potentially allowing injection at shallower depths.

However, the IPCC noted in 2005 that deep ocean storage would help reduce surface ocean impacts but at the expense of effects on deep-ocean biology. Subsequent IPCC reports have moved away from including direct ocean injection in their carbon dioxide removal taxonomies, reflecting growing concerns about environmental impacts and public acceptability.

Ocean alkalinity enhancement 🔗

A different approach involves adding alkaline minerals to seawater to increase its capacity to absorb atmospheric CO₂. When these minerals dissolve, they trigger chemical reactions that enable more carbon dioxide to be converted to stable bicarbonate ions. This process can simultaneously reduce ocean acidity, potentially providing co-benefits for marine organisms.

Companies like Ebb Carbon are developing technologies to separate seawater into acidic and alkaline solutions. The alkaline solution is returned to the ocean, where it binds CO₂ to form bicarbonate-a stable form of storage lasting potentially 10,000 years.

Ocean fertilization and biological approaches 🔗

Biotic methods aim to boost the ocean’s biological carbon pump. Ocean fertilization involves adding nutrients like iron to stimulate phytoplankton growth. As phytoplankton absorb CO₂ through photosynthesis and then die, some of their carbon-rich biomass sinks to the deep ocean.

Other biological approaches include cultivating macroalgae (seaweed) and then sinking it to deep waters, and restoring coastal wetlands like mangroves and seagrass beds that naturally capture and store carbon. Coastal wetland restoration has been practiced for decades, though primarily for ecosystem benefits rather than carbon sequestration.

Impacts on marine life 🔗

Both natural ocean acidification and deliberate sequestration activities pose significant risks to marine ecosystems. The effects ripple through food webs and threaten species that humans depend on for food and livelihoods.

Threats to calcifying organisms 🔗

Reef-building corals are among the most vulnerable species. These organisms construct their skeletons from calcium carbonate, and acidification makes this process increasingly difficult. Research predicts that coral skeletons could become so weakened that reefs will erode faster than they can rebuild-potentially by around 2080 under business-as-usual emissions scenarios.

Mussels and oysters are expected to grow 25% and 10% less shell respectively by century’s end. Sea urchins and starfish, which build their protective structures from high-magnesium calcite-a form of calcium carbonate that dissolves even more readily-face particular challenges.

The commercial implications are substantial. Alaska’s fisheries, which account for nearly 60% of U.S. commercial fish catch and support over 100,000 jobs, are at significant risk. The U.S. shellfish industry, valued at over $1 billion, is already experiencing impacts in vulnerable regions like the Pacific Northwest and Chesapeake Bay.

Risks from direct injection 🔗

Research into direct deep-sea CO₂ injection has revealed troubling impacts on seafloor communities. Field experiments off California found that exposure to dissolution plumes from liquid CO₂ caused high mortality rates among flagellates, amoebae, and nematodes in nearby sediments. These microscopic organisms form the base of deep-sea food webs.

Effects of increased acidity and CO₂ concentration on marine organisms include respiratory distress and reduced habitat as calcium carbonate stability zones shrink. The spatial scale of the ocean and the long timeframes involved make it difficult to detect and monitor these effects precisely.

Ecosystem-wide consequences 🔗

Observations at natural CO₂ seeps worldwide show that shallow biogenic reefs are particularly sensitive to acidification. These natural laboratories demonstrate that ecosystem diversity, species richness, and spatial complexity all decline as CO₂ levels increase. In tropical, subtropical, and temperate sites alike, complex ecosystems give way to simplified communities dominated by algae.

The cascade effects extend beyond shell-building organisms. Ocean acidification can trigger chain reactions through marine food webs, beginning with larval fish, shellfish, and corals, and affecting valuable ecosystem services including food security, tourism, shoreline protection, and biodiversity.

Ethical and regulatory concerns 🔗

Deliberate ocean carbon sequestration raises profound ethical questions and faces significant regulatory challenges. The governance landscape remains fragmented and often inadequate for managing these emerging technologies.

International regulatory framework 🔗

The London Convention and London Protocol provide the primary international legal framework for regulating ocean-based carbon sequestration. These treaties were originally designed to prevent marine pollution from dumping, not to manage climate interventions.

In 2008, parties to the Protocol determined that ocean fertilization activities are against the Convention’s aims unless conducted for legitimate scientific purposes. A 2013 resolution extended this regulatory framework to other marine geoengineering activities, including alkalinity enhancement. However, this resolution is non-binding, meaning parties could choose not to follow it without clear consequences.

Marine geoengineering is defined under these treaties as deliberate intervention in the marine environment to manipulate natural processes with potential for widespread, long-lasting, or severe deleterious effects. The amendments establishing this framework have not yet entered into force.

The challenge of incomplete knowledge 🔗

A central ethical concern is that scientists and policymakers must make decisions about ocean interventions without fully understanding their consequences. Very limited knowledge exists about deep-sea organisms and ecosystems and their interdependencies. The spatial range of the ocean and the time scales involved make comprehensive assessment extremely challenging.

Recent research has revealed previously unknown factors affecting ocean carbon sequestration, including microscopic mucus structures that slow the sinking of carbon-rich particles. Such discoveries suggest that current models may significantly overestimate or underestimate the ocean’s carbon storage potential.

Balancing risks and responsibilities 🔗

The precautionary principle holds that if an action might cause severe harm, proponents must demonstrate safety rather than opponents proving danger. Applying this principle to ocean sequestration creates tension with the urgency of addressing climate change.

Fragmented governance systems make it difficult to determine whether research is conducted responsibly, halt research if negative impacts outweigh benefits, or decide which authorities have power to make these assessments. Projects conducted within national waters could still have far-reaching transboundary impacts.

There are also equity concerns. Who benefits from ocean carbon sequestration, and who bears the risks? Coastal communities that depend on healthy marine ecosystems for their livelihoods may face disproportionate impacts from interventions designed to address emissions generated primarily by industrialized nations.

The path forward 🔗

Experts call for an iterative and adaptive approach that includes more research to understand which approaches are viable, governance frameworks ensuring responsible research, and eventually a new international framework. Any such framework would need to ensure broad stakeholder engagement, scientifically rigorous processes for assessing risks and benefits, and mechanisms for addressing equity issues around intellectual property and benefit sharing.

The Intergovernmental Panel on Climate Change has made clear that large-scale carbon dioxide removal-between 100 and 1,000 gigatons this century-will likely be needed to hold temperatures to 1.5°C above pre-industrial levels. Yet the ocean’s role in this effort remains deeply uncertain.

What do you think? Given the risks of both unmitigated climate change and ocean-based carbon removal interventions, how should societies weigh these trade-offs? Should the precautionary principle prevent ocean interventions until their impacts are fully understood, or does climate urgency justify proceeding with careful research?

References

1. https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
2. https://www.wri.org/insights/ocean-based-carbon-dioxide-removal
3. https://www.fisheries.noaa.gov/insight/understanding-ocean-acidification
4. https://en.wikipedia.org/wiki/Direct_deep-sea_carbon_dioxide_injection
5. https://www.intechopen.com/chapters/46327
6. https://www.weforum.org/stories/2024/10/direct-ocean-capture-carbon-removal-technology/
7. https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification
8. https://link.springer.com/article/10.1007/s10872-004-5768-8
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC7289009/
10. https://sanctuaries.noaa.gov/science/sentinel-site-program/florida-keys/climate-change-ocean-acidification.html
11. https://www.epa.gov/marine-protection-permitting/london-convention-and-london-protocol-international-treaties-prevent
12. https://tracker.carbongap.org/policy/london-protocol/
13. https://news.stanford.edu/stories/2024/10/hidden-biological-processes-can-affect-how-the-ocean-stores-carbon
14. https://blogs.law.columbia.edu/climatechange/2025/09/17/climate-governance-under-the-london-convention-and-protocol-precaution-and-prevention/
15. https://oceanvisions.org/ocean-based-carbon-dioxide-removal/