Could adding iron to the sea pull carbon out of the air?

Iron makes the sea bloom, and a bloom takes carbon dioxide out of the air. Whether that can cool the planet is the open question. Ocean fertilization means natural or intentional processes that replenish iron and other nutrients in the upper ocean, stimulating the growth of phytoplankton, which in some circumstances draws down large amounts of CO2 through photosynthesis.

The key is that life in the sea is limited by nutrients, most commonly nitrogen or iron. The oceanographer John Martin studied iron as a phytoplankton micronutrient and advocated iron fertilization. He is quoted, from a lecture at Woods Hole, as saying: "Give me a half tanker of iron, and I will give you an ice age." The arithmetic explains his enthusiasm. Assuming the Redfield ratio, each atom of iron in an iron-limited environment helps capture 1,060,000 atoms of carbon, while a nitrogen atom in a nitrogen-limited environment would capture only 6.

More than a dozen open-sea experiments confirmed that adding iron increases phytoplankton photosynthesis by up to 30 times. The disagreement is about what happens next. The European Iron Fertilization Experiment (EIFEX) found that at least half the bloom biomass sank below 1,000 metres, where it will likely remain sequestered for centuries. But Dave Siegel, a marine scientist on the National Academies panel, noted that tests have shown iron stimulates plankton growth, while how much of the absorbed carbon reaches the deep ocean is uncertain. In 2009 four scientists wrote in Nature that fertilizing the oceans with iron should be abandoned. In 2021 the National Academies report concluded that iron fertilization has among the highest potential of marine carbon-removal approaches, with medium-high confidence in its scalability and relatively low costs.

In 2008, the London Convention and Protocol noted that knowledge of its effectiveness and impacts was insufficient to justify anything but research.