Iron fertilization

DEFINITION: Purposeful introduction of iron into the oceans on a large scale to encourage plankton blooms as a measure to decrease global warming by removing carbon dioxide from the atmosphere

Small-scale experiments have tested the technique of iron fertilization, but the results were not conclusive, and conducting experiments on a larger scale is problematic for several reasons. How much of a role iron fertilization may eventually play in efforts to remove large amounts of carbon from the atmosphere remains unknown.

Phytoplankton are microscopic plants (algae) that can be found throughout the upper layers of the world’s oceans. These tiny plants require light as well as inorganic material such as phosphate, nitrate, ammonium, carbon dioxide, or carbonate, along with some trace metals such as iron or zinc. Phytoplankton reproduce asexually, with a typical ocean doubling time of a population from a few hours to a few days. Under the right conditions and with access to abundant nutrients, plankton form blooms that can cover hundreds of square kilometers of ocean and are readily visible to satellites orbiting the Earth. Such blooms can remove from the atmosphere large amounts of carbon dioxide, a greenhouse gas that has been linked to global warming. Among the natural causes of such large algal blooms are hurricanes, which bring up nutrients from lower ocean depths into the upper reaches of the oceans.

Since the late 1980s, scientists have known that depositing particles of iron in warm ocean waters can trigger plankton blooms, and by the early 1990s, twelve small-scale experiments involving the introduction of a ton or more of iron dust into a few hundred square kilometers of ocean had shown that massive blooms could be stimulated artificially. However, these experiments also raised a number of problems and complicated earlier views. First, the rate at which carbon was taken up by the phytoplankton was many times lower than had been predicted based on laboratory experiments. Some of the difference was the result of the chaotic mixing that occurs in the oceans, as well as the difficulty of placing iron dust in the oceans under just the right conditions to prevent the dust from sinking too quickly into lower depths. Second, the degree to which the carbon remained locked up in the plankton—which, upon death, would sink to the seafloor, thus relatively permanently sequestering the carbon—was shown to vary considerably. Much plankton would be consumed, and the carbon would begin a journey through the food chains of the ocean; ultimately, much of it would reenter the atmosphere as carbon dioxide.

A few large-scale iron fertilization experiments have been undertaken, but no evidence has been produced yet that this method could substantially reduce the amount of carbon dioxide present in the atmosphere. Critics have argued that no large-scale experiment will be able to fully determine the effectiveness of this technique because of the very long timeframe within which such an experiment would require, as well as the potent technical difficulties involved in effectively monitoring the impacts of such experiments even in the short term over the vast reaches and depths of the world’s oceans. According to estimates from Bigelow Laboratory for Ocean Sciences, iron fertilization could potentially remove many gigatons per year of atmospheric CO2 and help combat climate change. In 2023, scientists with the Woods Hole Oceanographic Institute received $2 million in US government funding to pursue research on the idea using computer modeling.

Bibliography

Emerson, David. "A Cost Model for Ocean Iron Fertilization as a Means of Carbon Dioxide Removal That Compares Ship- and Aerial-Based Delivery and Estimates Verification Costs." Advancing Earth and Space Science, vol. 12, no. 4, Apr. 2024, doi:10.1029/2023EF003732. Accessed 24 Sept. 2025.

Goodell, Jeff. How to Cool the Planet: Geoengineering and the Audacious Quest to Fix Earth’s Climate. Houghton Mifflin, 2010.

Hance, Jeremy. "Is Ocean Iron Fertilization Back from the Dead as a CO₂ Removal Tool?" Mongabay, 14 Nov. 2023, news.mongabay.com/2023/11/is-ocean-iron-fertilization-back-from-the-dead-as-a-co%E2%82%82-removal-tool/. Accessed 24 Sept. 2025.

"How Fertilising the Oceans With Iron Could Help Fight the Climate Crisis." Chartered Institution of Water and Environmental Management, 1 Aug. 2025, www.ciwem.org/news/how-fertilising-the-oceans-with-iron-could-help-fight-the-climate-crisis. Accessed 24 Sept. 2025.