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.
Approaches Phytoplankton require a variety of nutrients. These include macronutrients such as nitrate and phosphate (in relatively high concentrations) and micronutrients such as iron and zinc (in much smaller quantities). Nutrient requirements vary across phylogenetic groups (e.g., diatoms require silicon) but may not individually limit total biomass production. Co-limitation (among multiple nutrients) may also mean that one nutrient can partially compensate for a shortage of another. Silicon does not affect total production, but can change the timing and community structure with follow-on effects on remineralization times and subsequent mesopelagic nutrient vertical distribution.
High-nutrient, low-chlorophyll (HNLC) waters occupy the oceans' subtropical gyre systems, approximately 40 per cent of the surface, where wind-driven downwelling and a strong thermocline impede nutrient resupply from deeper water. Nitrogen fixation by cyanobacteria provides a major source of N. In effect, it ultimately prevents the ocean from losing the N required for photosynthesis. Phosphorus has no substantial supply route, making it the ultimate limiting macronutrient. The sources that fuel primary production are deep water stocks and runoff or dust-based.
Iron
Phosphorus In the very long term, phosphorus "is often considered to be the ultimate limiting macronutrient in marine ecosystems" and has a slow natural cycle. Where phosphate is the limiting nutrient in the photic zone, addition of phosphate is expected to increase primary phytoplankton production. This technique can give 0.83 W/m2 of globally averaged negative forcing, which is sufficient to reverse the warming effect of about half the current levels of anthropogenic emissions. One water-soluble fertilizer is diammonium phosphate (DAP), , that as of 2008 had a market price of 1700/tonne−1 of phosphorus. Using that price and the C : P Redfield ratio of 106 : 1 produces a sequestration cost (excluding preparation and injection costs) of some $45 /tonne of carbon (2008), substantially less than the trading price for carbon emissions.
Nitrogen (urea)
This technique proposes to fertilize the ocean with urea, a nitrogen rich substance, to encourage phytoplankton growth. Note typo in paper Fig. 1. Concentrations of macronutrients per area of ocean surface would be similar to large natural upwellings. Once exported from the surface, the carbon remains sequestered for a long time.
An Australian company, Ocean Nourishment Corporation (ONC), planned to inject hundreds of tonnes of urea into the ocean, in order to boost the growth of -absorbing phytoplankton, as a way to combat climate change. In 2007, Sydney-based ONC completed an experiment involving one tonne of nitrogen in the Sulu Sea off the Philippines. This project was criticized by many institutions, including the European Commission, due to lack of knowledge of side effects on the marine ecosystem.
Macronutrient nourishment can give 0.38 W/m2 of globally averaged negative forcing, which is sufficient to reverse the warming effect of current levels of around a quarter of anthropogenic emissions.
The two dominant costs are manufacturing the nitrogen and nutrient delivery.
In waters with sufficient iron micro nutrients, but a deficit of nitrogen, urea fertilization is the better choice for algae growth. Urea is the most used fertilizer in the world, due to its high content of nitrogen, low cost and high reactivity towards water. When exposed to ocean waters, urea is metabolized by phytoplankton via urease enzymes to produce ammonia.
CO(NH_2)_2 + H_2O ->[urease] NH_3 + NH_2COOH
NH_2COOH + H_2O -> NH_3 + H_2CO_3
The intermediate product carbamate also reacts with water to produce a total of two ammonia molecules.
Another cause of concern is the sheer amount of urea needed to capture the same amount of carbon as eq. iron fertilization. The nitrogen to iron ratio in a typical algae cell is 16:0.0001, meaning that for every iron atom added to the ocean a substantial larger amount of carbon is captured compared to adding one atom of nitrogen.Caldeira, Ken, et al. "IPCC Special Report on Carbon Dioxide Capture and Storage: Ocean Storage." International Panel on Climate Change, 2005. Scientists also emphasize that adding urea to ocean waters could reduce oxygen content and result in a rise of toxic marine algae. This could potentially have devastating effects on fish populations, which others argue would be benefiting from the urea fertilization (the argument being that fish populations would feed on healthy phytoplankton).
Pelagic pumping Local wave power could be used to pump nutrient-rich water from hundred- metre-plus depths to the euphotic zone. However, deep water concentrations of dissolved CO2 could be returned to the atmosphere.
The supply of DIC in upwelled water is generally sufficient for photosynthesis permitted by upwelled nutrients, without requiring atmospheric CO2. Second-order effects include how the composition of upwelled water differs from that of settling particles. More nitrogen than carbon is remineralized from sinking organic material. Upwelling of this water allows more carbon to sink than that in the upwelled water, which would make room for at least some atmospheric CO2 to be absorbed. the magnitude of this difference is unclear. No comprehensive studies have yet resolved this question. Preliminary calculations using upper limit assumptions indicate a low value. 1,000 square kilometres (390 sq mi) could sequester 1 gigatonne/year.
Sequestration thus depends on the upward flux and the rate of lateral surface mixing of the surface water with denser pumped water.
Volcanic ash Volcanic ash adds nutrients to the surface ocean. This is most apparent in nutrient-limited areas. Research on the effects of anthropogenic and aeolian iron addition to the ocean surface suggests that nutrient-limited areas benefit most from a combination of nutrients provided by anthropogenic, eolian and volcanic deposition. Some oceanic areas are comparably limited in more than one nutrient, so fertilization regimes that includes all limited nutrients is more likely to succeed. Volcanic ash supplies multiple nutrients to the system, but excess metal ions can be harmful.