Iron fertilization

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Iron fertilization is the intentional introduction of iron-containing compounds (like iron sulfate) to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide (CO₂) sequestration from the atmosphere. Iron is a trace element necessary for photosynthesis in plants. It is highly insoluble in sea water and in a variety of locations is the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters. These blooms can nourish other organisms.

Ocean iron fertilization is an example of a geoengineering technique.^[1] Iron fertilization^[2] attempts to encourage phytoplankton growth, which removes carbon from the atmosphere for at least a period of time.^[3][4] This technique is controversial because there is limited understanding of its complete effects on the marine ecosystem,^[5] including side effects and possibly large deviations from expected behavior. Such effects potentially include release of nitrogen oxides,^[6] and disruption of the ocean's nutrient balance.^[1] Controversy remains over the effectiveness of atmospheric CO
₂ sequestration and ecological effects.^[7] Since 1990, 13 major large scale experiments have been carried out to evaluate efficiency and possible consequences of iron fertilization in ocean waters. A study in 2017 considered that the method is unproven; the sequestering efficiency was low and sometimes no effect was seen and the amount of iron deposits needed to make a small cut in the carbon emissions would be in the million tons per year.^[8] However since 2021, interest is renewed in the potential of iron fertilization, among other from a white paper study of NOAA, the US National Oceanographic and Atmospheric Administration, which rated iron fertilization as having "moderate potential for cost, scalability and how long carbon might be stored compared to other marine sequestration ideas" ^[9]

Approximately 25 per cent of the ocean surface has ample macronutrients, with little plant biomass (as defined by chlorophyll). The production in these high-nutrient low-chlorophyll (HNLC) waters is primarily limited by micronutrients, especially iron.^[10] The cost of distributing iron over large ocean areas is large compared with the expected value of carbon credits.^[11] Research in the early 2020s suggested that it could only permanently sequester a small amount of carbon.^[12]