Phytoplankton-Based Carbon Dioxide Removal

To meet global climate goals, the world must both cut greenhouse gas emissions and remove enormous amounts of carbon dioxide (CO₂) already in the atmosphere. The U.S. National Academies of Sciences estimates that by 2050 the world will need to remove up to 10 gigatons of CO₂ every year, and even more beyond that. Reaching the 2050 goal will require a 10,000x scale-up from where we are currently.

Why the Ocean Matters

The ocean plays a central role in Earth’s carbon cycle. It already absorbs roughly a quarter of the CO₂ humans emit each year, making it the largest active carbon sink on the planet.

Because of its size and natural ability to store carbon, the ocean will almost certainly need to play a substantial role in large-scale carbon dioxide removal. However, marine carbon dioxide removal (mCDR) approaches remain relatively unexplored compared to those that are land-based.

What is Phytoplankton-Based Carbon Dioxide Removal?

Phytoplankton-Based Carbon Dioxide Removal is a group of mCDR approaches that would enhance the ocean’s natural carbon sequestering processes through the biological carbon pump, the process being characterized by:

Microscopic marine plants called phytoplankton absorb CO₂ as they grow
Phytoplankton are the base of the marine food web and are mostly consumed by larger marine life, but a portion of phytoplankton escape consumption and sink into the deep ocean
Once in the deep ocean, carbon can be stored for hundreds to thousands of years

Phytoplankton-based approaches seek to increase how much carbon is absorbed by phytoplankton and stored in the deep ocean.

A Well-Known Example: Ocean Iron Fertilization

The most widely studied phytoplankton-based approach is ocean iron fertilization, whereby small amounts of iron are added to the ocean to stimulate phytoplankton growth—with the intent to sequester additional phytoplankton biomass in the deep ocean. During the 1990s and 2000s, more than a dozen field trials showed that iron additions do increase phytoplankton productivity. However, major questions remain related to the fate of the newly produced phytoplankton, additional uptake of CO₂ from the atmosphere, and environmental impacts of the process at scale.

Could Phytoplankton-Based Carbon Dioxide Removal Matter at Climate Scale?

If key risks and uncertainties can be addressed, phytoplankton-based approaches may be capable of removing on the order of one gigaton of CO₂ per year—a meaningful contribution to addressing climate change.

Critical open questions include:

Effectiveness – How much additional CO₂ can phytoplankton-based approaches really remove?
Durability – How long is that carbon stored?
Acceptability – Are environmental and social impacts acceptable and equitable?

Answering these questions requires coordinated, transparent, and ethically grounded research.

Ocean Visions’ Phytoplankton-Based Carbon Dioxide Removal Project

In 2025 Ocean Visions launched the Phytoplankton-Based Carbon Dioxide Removal project to assess the state of knowledge, identify remaining knowledge gaps, and recommend priority actions to close those gaps. The project is part of a larger mCDR effort, Advancing Marine Carbon Sequestration (AMCS), supported financially by the Ocean Resilience and Climate Alliance (ORCA) and led by the Grantham Environmental Trust as a re-granter.

Purpose

Determine what is known about phytoplankton-based carbon dioxide removal
Identify what remains unknown
Recommend how to close critical knowledge gaps

Elements

Year-long investigation of all known phytoplankton-based pathways
Focus on approaches with potential to reach climate-relevant scale
Extensive literature review and desktop research
Multiple rounds of expert interviews and workshops
Guided by an international advisory board
Draft report for public comment

Deliverables:

Identified and prioritized key scientific, environmental, and social questions that must be addressed
Defined the core elements of a responsible research, development, and demonstration (RD&D) program
Assessed potential benefits and risks of priority phytoplankton-based pathways
Recommended specific research designs to evaluate those risks and benefits

Takeaways:

There is strong evidence that phytoplankton-based approaches could meaningfully contribute to closing the global CDR gap
Continued research and testing are appropriate to determine whether phytoplankton-based carbon dioxide removal should be pursued at scale
Any decisions on deployment must be guided by evidence, ethics, and public engagement
Across all phytoplankton-based pathways, priority needs include:
- Improving quantification of net CDR via phytoplankton-based pathways by identifying the largest sources of uncertainty in these estimates and setting clear targets to reduce this uncertainty
- Improving ocean biogeochemical models to understand long-term storage and large-scale impacts
- Strengthening understanding of the ocean’s biological carbon pump, including how climate change is already altering it
- Advancing carefully designed field trials that maximize learning while addressing environmental and social concerns
Pathway-specific insights include:
- Models suggest Southern Ocean iron fertilization may be the most scalable, but real-world feasibility and impacts remain unclear
- New iron-based approaches that stimulate nitrogen fixation in tropical regions could expand scalability, but are very early stage
- Approaches that help phytoplankton carbon reach the deep ocean more efficiently may improve viability as a climate solution, yet remain underexplored

The Case for an International Phytoplankton-Based Carbon Dioxide Removal Research Program

A dedicated phytoplankton-based carbon dioxide removal RD&D program could:

Coordinate research to ensure funding is used strategically
Apply a stage-gate framework, with clear decision points to advance, refine, pause, or stop work
Ensure transparency, inclusivity, and ethical standards
Mobilize sustained funding and global collaboration

Work Already Underway

Ocean Visions supports related efforts, including:

A co-designed South Pacific research partnership led by AltaSea’s Growing Oceans team to assess the carbon sequestration and ecosystem benefits of naturally stimulated phytoplankton growth, while building trusted scientific and sovereign partnerships—beginning with Tonga—to inform the viability and governance of phytoplankton-based approaches.
The Exploring Ocean Iron Solutions (ExIOS) program, hosted at Woods Hole Oceanographic Institution, investigates the feasibility, impacts, and governance of ocean iron fertilization. Its research focuses on stimulating phytoplankton blooms to enhance the biological bump and increase the long-term storage of atmospheric carbon dioxide in the deep ocean.
A partnership between NASA’s Jet Propulsion Laboratory at Caltech and San Jose State University is using satellite observations of marine ecosystems to improve ocean biogeochemical models, with an eye towards improving CDR measurements for phytoplankton-based pathways.

Project Advisory Board

Mattias Cape, Marine Biogeochemical Scientist, Environmental Defense Fund
Leticia Cotrim da Cunha, Associate Professor and Coordinator of Laboratório de Oceanografia Química, Universidade do Estado do Rio de Janeiro
Anna-Maria Hubert, Assistant Professor, University of Calgary Faculty of Law
Tom Lawton, Senior Staff Scientist, John Hopkins Applied Physics Laboratory
Terre Satterfield, Professor of Culture, Risk and the Environment, The University of British Columbia
Brad Warren, Chief Executive Officer, Global Ocean Health
Angelicque White, Associate Professor, University of Hawaiʻi at Mānoa

Project Team Leads

Eric Schwaab, Senior Fellow, Ocean Visions, formally Senior Vice President at Environmental Defense Fund and Assistant Administrator at NOAA
Lydia Kapsenberg, Senior Associate, CEA Consulting, PhD, Marine Ecology, Evolution, and Marine Biology