Ocean fertilization

Ocean fertilization or ocean nourishment refers to both natural and intentional processes that replenish iron and other nutrients in the upper ocean, which in turn stimulate the growth of phytoplankton and in some circumstances draw down large amounts of carbon dioxide (CO2) through photosynthesis. Intentional ocean fertilization is biomimicry of natural processes that have removed atmospheric CO2 before ice ages as well as after volcanic eruptions, whale defecation, and near hydrothermal vents. The introduction of nutrients to the upper ocean increases marine food production as well as removing carbon dioxide from the atmosphere.

A number of techniques, including fertilization by the micronutrient iron (called "ocean iron fertilization" or OIF) or with nitrogen and phosphorus (both macronutrients), have been proposed. A NOAA special report rated OIF as having "a moderate potential for cost, scalability and how long carbon might be stored compared to other marine sequestration ideas." Ocean iron fertilization can stimulate photosynthesis in phytoplankton. The phytoplankton converts the ocean's dissolved carbon dioxide into carbohydrate, some of which has been shown to sink into the deeper ocean. More than a dozen open-sea experiments confirmed that adding iron to the ocean increases photosynthesis in phytoplankton by up to 30 times. OIF is one of the more well-researched carbon dioxide removal (CDR) approaches, and supported by climate restoration proponents. Critics cite uncertainty about this approach regarding the duration of the effective oceanic carbon sequestration. But a landmark field trial found that, more than half the phytoplankton biomass sank below 1,000 meters, where it is likely to remain sequestered for hundreds or thousands of years. The National Academies of Science, Engineering and Medicine (NASEM) 2021 study on marine CDR (mCDR) concludes that OIF has among the highest potential of mCDR approaches. The report indicates that there is medium-high confidence that the technique could be efficient and scalable at low cost, with medium environmental risks. "This biotic approach has relatively high scalability and low costs for deployment, though challenges would include verifiable C accounting and, as for most ocean CDR at scale, careful monitoring of intended and unexpected ecological effects up and down the food chain."

Peter Fiekowsky and Carole Douglis write, "I consider iron fertilization an important item on our list of potential climate restoration solutions. Given the fact that iron fertilization is a natural process that has taken place on a massive scale for millions of years, it is likely that most of the side effects are familiar ones that pose no major threat."

Rationale The marine food chain is based on photosynthesis by marine phytoplankton that combine carbon with inorganic nutrients to produce organic matter. Production is limited by the availability of nutrients, most commonly nitrogen or iron. Numerous experiments have demonstrated how iron fertilization can increase phytoplankton productivity. Nitrogen is a limiting nutrient over much of the ocean and can be supplied from various sources, including fixation by cyanobacteria. Carbon-to-iron ratios in phytoplankton are much larger than carbon-to-nitrogen or carbon-to-phosphorus ratios, so iron has the highest potential for sequestration per unit mass added. Oceanic carbon naturally cycles between the surface and the deep via two "pumps" of similar scale. The "solubility" pump is driven by ocean circulation and the solubility of CO2 in seawater. The "biological" pump is driven by phytoplankton and subsequent settling of detrital particles or dispersion of dissolved organic carbon. The former has increased as a result of increasing atmospheric CO2 concentration. This CO2 sink is estimated to be approximately 2 GtC/yr. The global phytoplankton population fell about 40 percent between 1950 and 2008 or about 1 percent per year. The most notable declines took place in polar waters and in the tropics. The decline is attributed to sea surface temperature increases. A separate study found that diatoms, the largest type of phytoplankton, declined more than 1 percent per year from 1998 to 2012, particularly in the North Pacific, North Indian and Equatorial Indian oceans. The decline appears to reduce pytoplankton's ability to sequester carbon in the deep ocean. Fertilization offers the prospect of both reducing the concentration of atmospheric greenhouse gases with the aim of slowing climate change and at the same time increasing fish stocks via increasing primary production. The reduction reduces the ocean's rate of carbon sequestration in the deep ocean. Each area of the ocean has a base sequestration rate on some timescale, e.g., annual. Fertilization must increase that rate, but must do so on a scale beyond the natural scale. Otherwise, fertilization changes the timing, but not the total amount sequestered. However, accelerated timing may have beneficial effects for primary production separate from those from sequestration. Biomass production inherently depletes all resources (save for sun and water). Either they must all be subject to fertilization or sequestration will eventually be limited by the one mostly slowly replenished (after some number of cycles) unless the ultimate limiting resource is sunlight and/or surface area. Generally, phosphate is the ultimate limiting nutrient. As oceanic phosphorus is depleted (via sequestration) it would have to be included in the fertilization cocktail supplied from terrestrial sources.

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