Wondering whether ocean iron fertilization could really help the climate? Here is the short answer: adding iron to the sea may boost plankton and carbon storage, but the evidence is thin and the risks are debated. You deserve the facts.
This explainer sits within the wider topic of ocean climate solutions, and it covers the science, the past trials, the risks and the rules.
| Item | Value |
|---|---|
| Open-ocean iron experiments, 1993 to 2009 | 13 experiments |
| Experiments that documented more carbon in the deep ocean | 1 of the 13 |
| Modeled storage time for removed carbon dioxide | Hundreds to thousands of years |
| Modeled carbon drawdown by 2100 (one study) | 45 billion metric tons |
| Proposed trial area in the northeastern Pacific | Up to 10,000 square kilometers |
| Fundraising target for the proposed research program | $160 million |
These figures come from the Congressional Research Service report dated July 7, 2022 and a Scientific American article of September 12, 2024. Figures and plans can change, so check the source before you rely on them.
How does ocean iron fertilization work?
Phytoplankton are microscopic organisms that take up carbon dioxide as they grow. NOAA’s Ocean Acidification Program describes iron as a critical limiting nutrient for phytoplankton in the ocean, which is why scientists see it as a lever (NOAA).
The Congressional Research Service says the technique aims to raise the rate at which carbon dioxide moves to the deep ocean by stimulating the biological pump with added iron (CRS, July 7, 2022). In many parts of the ocean iron is rare, so a small addition can trigger a large bloom.
The chain of events, as described by Scientific American, runs like this:
- Iron arrives, either naturally as windblown dust or volcanic ash, or from a ship pumping out an iron sulfate solution.
- Phytoplankton grow and multiply in the iron-poor water.
- When they die, or are eaten and excreted, some of the carbon sinks to deep, slow-moving waters as “marine snow”.
- That carbon stays out of the atmosphere for decades or centuries.
The CRS report cites modeling studies that put the potential at hundreds to thousands of years. The two sources give different timescales, which is one sign of how uncertain the numbers still are.
What did the past experiments find?
The CRS report counts 13 experiments in regions of the open ocean from 1993 to 2009. Researchers documented increased carbon levels in the deep ocean in only 1 of the 13. The other 12 did not observe it, or were too short-lived to make measurements (CRS, July 7, 2022).
Two individual trials show why results were mixed. In a 2009 experiment in the southwest Atlantic by German and Indian scientists, larger zooplankton ate the smaller phytoplankton, and little carbon actually reached the deep sea.
In a 2006 experiment in the northeastern Pacific by U.S. and Canadian researchers, toxic phytoplankton species flourished (Scientific American, September 12, 2024).
Location matters too. The CRS report notes that most studies took place in international waters, where iron tends to be scarcer than in coastal waters, and that adding iron to regions with existing high productivity may not amplify the biological pump (CRS, July 7, 2022).
Risks and trade-offs in the debate
The debate is not only about whether the method removes carbon. It is also about what else changes when you feed an ecosystem. Scientific American lists several concerns:
- Dead zones: fears that rampant algal blooms could consume the oxygen in the water and snuff out other life.
- Nutrient robbing: blooms could use up phosphorus and nitrogen that would otherwise be available to organisms elsewhere.
- Unknown deep-sea effects: deep-sea expert Lisa Levin of the Scripps Institution of Oceanography says iron fertilization will most likely affect something we do not yet understand.
A modeling study by British, American and French researchers found that adding one million to two million metric tons of iron a year could draw down 45 billion metric tons of carbon by 2100. It would also rob nutrients from other sea life.
Along with an estimated 15 percent reduction in marine biomass caused by warming, another 5 percent could be lost because of iron fertilization, particularly in fishing areas. Co-lead author Alessandro Tagliabue of the University of Liverpool said he had not seen the new research consortium explain what was wrong with previous work.
Supporters see it differently. Ken Buesseler of the Woods Hole Oceanographic Institution calls it “a small change in biology, relative to doing nothing and watching this planet boil” (Scientific American, September 12, 2024).
How ocean iron fertilization is regulated
As of the September 12, 2024 Scientific American report, the London Protocol had set an international ban on ocean iron fertilization for commercial purposes in 2013. The same report says the convention allows fertilization for research if it is monitored and does not harm the environment (Scientific American).
The Congressional Research Service, in its July 7, 2022 report, said international agreements generally categorize ocean fertilization to mitigate climate change as marine scientific research. That allows small-scale activities that comply with agreements to protect the marine environment and limit marine pollution (CRS).
The same report says activities in U.S. waters might fall under existing federal laws such as the Marine Protection, Research, and Sanctuaries Act. It also notes that Congress may consider the geopolitical consequences, including whether others view the practice as transboundary pollution.
What research is planned next?
A not-for-profit, noncommercial consortium called Exploring Ocean Iron Solutions, or ExOIS, laid out a research program in Frontiers in Climate. As of the Scientific American report of September 12, 2024, it hoped to start trials across as much as 10,000 square kilometers of the northeastern Pacific as soon as 2026.
The consortium was trying to raise $160 million for the whole program and had received a $2-million NOAA grant for computer modeling. It plans to apply to the U.S. Environmental Protection Agency for permission to run trials under the London Protocol (Scientific American).
The plan includes monitoring. Scientists would add a nonreactive tracer such as sulfur hexafluoride to track the fertilized water, and measure carbon dioxide using ships, floats and underwater drones.
NOAA’s Ocean Acidification Program lists a related modeling project with an award of $1,983,731, funded in fiscal years 2023, 2024 and 2025 and lasting three years. It uses simulation experiments to target field trials in the Southern Ocean, the Equatorial Pacific and the Subarctic Pacific (NOAA).
The sources used here do not say whether any new trial has started, so check recent reporting before you assume one has.
If you want to compare approaches, you can read about seaweed farming carbon sequestration and about direct ocean capture, which are covered on separate pages.
Frequently asked questions
Does ocean iron fertilization actually remove carbon?
Show answer
The evidence is mixed. Of 13 open-ocean experiments from 1993 to 2009, researchers documented increased deep-ocean carbon in only 1, according to the Congressional Research Service’s July 7, 2022 report. Modeling studies suggest it could work over long periods, but models are not field proof.
Is ocean iron fertilization legal?
Show answer
As of the September 12, 2024 Scientific American report, the London Protocol banned it for commercial purposes (set in 2013) while allowing monitored research that does not harm the environment. Rules for specific projects depend on where they take place.
What are the main risks?
Show answer
Critics point to toxic phytoplankton, oxygen-poor dead zones, nutrient robbing and unknown effects on deep-sea ecosystems. One modeling study estimated an extra 5 percent loss of marine biomass on top of an estimated 15 percent from warming.
Who is planning new trials?
Show answer
The ExOIS consortium, described as not-for-profit and noncommercial, laid out a program in Frontiers in Climate. As of the September 12, 2024 report, it hoped to begin trials as soon as 2026, pending permission and funding.
