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 determined that the method is unproven; sequestering efficiency is low and sometimes no effect was seen and the amount of iron deposits that is needed to make a small cut in the carbon emissions is in the million tons per year.^[8]

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.^[9] The cost of distributing iron over large ocean areas is large compared with the expected value of carbon credits.^[10] Research in the early 2020s suggested that it could only permanently sequester a small amount of carbon.^[11]