Iron fertilization
Iron fertilization refers to both natural and intentional processes that replenish iron in the upper ocean. Iron can stimulate the growth of phytoplankton, which remove carbon dioxide (CO2) from the atmosphere through photosynthesis. Phytoplankton are the primary producers that feed the rest of the marine food web.
Natural iron distribution by dust storms, volcanic eruptions, hydrothermal vents, upwelling, and whale defecation can all prompt large blooms of phytoplankton and restore local marine life. Plankton photosynthesis can remove large amounts of carbon dioxide (CO2) from the atmosphere and in some circumstances sequester it long term. Natural iron fertilization is accepted as a major force behind the significant drop in CO2 and temperatures that lead to ice ages.
Iron is necessary for all life and for photosynthesis in plants. Yet it is found in such minute concentrations in the upper the ocean, that no one even succeeded in measuring it until the 1980s. Iron is generally insoluble in seawater and sinks readily. In much of the ocean, this element is the limiting nutrient for phytoplankton growth. Recent studies find that iron levels have been decreasing in most of the ocean, along with phytoplankton.
Approximately 25 percent of the ocean surface contains ample macronutrients but little plant biomass (as defined by the presence of chlorophyll). The production in these high-nutrient low-chlorophyll (HNLC) waters is primarily limited by micronutrients, especially iron.
Intentional ocean iron fertilization (OIF) represents deliberate biomimicry of the natural processes that have for eons distributed iron, boosted ocean photosynthesis and ocean life, and removed atmospheric CO2.
Because of its potential to mitigate climate change, intentional OIF has been called "geoengineering." Some activists argue that effective atmospheric CO2 removal would lessen the will to reduce the use of fossil fuels, thus creating a moral hazard. Some critics point to our limited understanding of the complex marine ecosystem and the effect large-scale OIF might have on it, including the risk of potential side effects such as release of nitrogen oxides and potential ecological disruption.
However, OIF researchers stress that ecological risks remain theoretical, as no harm has actually been observed in field trials. Nevertheless, scientists call for vigilant ecological monitoring in future trials.
Since 1990, 13 major large scale experiments have been carried out to evaluate efficiency and possible consequences of iron fertilization in ocean waters. Results varied according to aims and conditions. All found an increase in phytoplankton growth with the addition of trace amounts of iron. In the EIFEX trial, at least half the bloom biomass sank far below a depth of 1,000 meters, and a substantial portion likely reached the seafloor; likely becoming sequestered for hundreds or thousands of years.
In the last few years, interest has markedly increased in the science of and potential for iron fertilization. A white paper by the US National Oceanographic and Atmospheric Administration rated iron fertilization as having "moderate potential for cost, scalability and how long carbon might be stored compared to other marine sequestration ideas".
Research on the effects of volcanic eruptions, particularly Mt. Pinatubo in 1991, suggests that many gigatons of CO2 were removed by natural iron fertilization from volcanic ash, with the implication that biomimicry might produce significant results in similar conditions. The conditions are iron distribution 1) in a downwelling eddy and 2) sufficient to nourish the growth of nitrogen-fixing (self-fertilizing) phytoplankton.