Ocean fertilization, or ocean nourishment, is a process that adds iron and other nutrients to the upper ocean. This helps phytoplankton grow, and in some cases, it can reduce carbon dioxide (CO₂) in the atmosphere through photosynthesis. Intentional ocean fertilization copies natural processes that have removed CO₂ from the air before ice ages, after volcanic eruptions, whale waste, and near underwater vents. Adding nutrients to the ocean increases marine life production and removes CO₂ from the air.

For example, adding iron (a type of ocean fertilization) can help phytoplankton use dissolved CO₂ to make carbohydrates. Some of these carbohydrates sink to the deep ocean. More than a dozen experiments in open seas showed that adding iron can increase phytoplankton photosynthesis by up to 30 times.

Ocean iron fertilization is one of the most studied methods to remove CO₂ from the atmosphere. Supporters of climate restoration believe it could help. However, scientists are unsure how long the carbon stored in the ocean will remain there. A 2021 study by the National Academies of Science, Engineering, and Medicine (NASEM) said ocean iron fertilization has high potential among methods to remove carbon from the ocean.

The study estimated the cost of ocean iron fertilization at 40 cents per ton of CO₂ removed, though more research would add to the cost. The report said there is a medium to high chance the method could be efficient, large-scale, and low-cost, with medium environmental risks. It noted that this method could be scaled up easily and is less expensive, but challenges include tracking carbon removal and carefully monitoring effects on marine life at all levels of the food chain.

Peter Fiekowsky and Carole Douglis wrote that iron fertilization is a key idea for climate restoration. They said iron fertilization is a natural process that has occurred for millions of years, so most side effects are likely familiar and not major threats.

Several methods, such as adding iron or other nutrients like nitrogen and phosphorus, have been suggested. Early research in the 2020s suggested this might only store small amounts of carbon permanently. However, more recent studies show promise. A NOAA report said iron fertilization has "moderate potential" for cost, scalability, and how long carbon might be stored compared to other ocean-based carbon removal ideas.

Rationale

The marine food chain begins with marine phytoplankton using photosynthesis to create organic matter by combining carbon with inorganic nutrients. Production is limited by the availability of nutrients, most often nitrogen or iron. Experiments show that adding iron can increase phytoplankton growth. Nitrogen is a major limiting nutrient across much of the ocean and is supplied from sources like cyanobacteria. Carbon-to-iron ratios in phytoplankton are much higher than carbon-to-nitrogen or carbon-to-phosphorus ratios, so iron has the greatest potential for storing carbon per unit mass added.

Oceanic carbon naturally moves between the surface and deep ocean through two processes called "pumps." The "solubility" pump is driven by ocean currents and the ability of CO₂ to dissolve in seawater. The "biological" pump is driven by phytoplankton and the sinking of organic particles or the spread of dissolved organic carbon. The solubility pump has increased due to higher atmospheric CO₂ levels. This process removes about 2 billion tons of carbon each year.

Global phytoplankton populations decreased by about 40% between 1950 and 2008, or about 1% per year. The largest declines occurred in polar and tropical regions. These declines are linked to rising sea surface temperatures. A separate study found that diatoms, the largest type of phytoplankton, declined by more than 1% per year from 1998 to 2012, especially in the North Pacific, North Indian, and Equatorial Indian oceans. This decline reduces the ocean’s ability to store carbon in the deep ocean.

Fertilization could reduce atmospheric greenhouse gases, helping slow climate change, and increase fish populations by boosting primary production. However, it might also slow the ocean’s rate of storing carbon in the deep ocean.

Each ocean area has a basic carbon storage rate on a specific timescale, such as yearly. Fertilization must increase this rate beyond natural levels. If not, it may only change the timing of carbon storage, not the total amount. However, faster timing might benefit primary production independently of carbon storage.

Biomass production uses up resources, except sunlight and water. If all resources are not replenished through fertilization, carbon storage will eventually be limited by the slowest-replenished resource. Usually, this is phosphate. As oceanic phosphorus is used up by storage, it must be added from land sources in fertilization mixtures.

Approaches

Phytoplankton need many different nutrients to grow. These include main nutrients like nitrate and phosphate, which are needed in larger amounts, and smaller nutrients like iron and zinc, which are needed in much smaller amounts. Different types of phytoplankton may need different nutrients, such as silicon for diatoms. However, no single nutrient alone usually limits how much phytoplankton can grow overall. Sometimes, multiple nutrients are needed together, and one nutrient might help reduce the effect of a shortage in another. Silicon does not affect the total amount of phytoplankton, but it can change when and how different types of phytoplankton grow, which affects how nutrients are distributed in the ocean.

High-nutrient, low-chlorophyll (HNLC) waters are found in subtropical gyres, which cover about 40% of the ocean’s surface. These areas have strong winds and a thick layer of warm water that prevent nutrients from deeper water from reaching the surface. Cyanobacteria fix nitrogen, which provides a major source of nitrogen for the ocean. This process helps keep nitrogen in the ocean for use in photosynthesis. Phosphorus has no major way to be supplied to the ocean, making it the most important limiting nutrient for growth. The main sources of nutrients for ocean life are deep water and runoff or dust from land.

Iron fertilization is the process of adding iron to the ocean, either naturally or by humans. Iron helps phytoplankton grow, which removes carbon dioxide from the atmosphere through photosynthesis. Phytoplankton are the first step in the ocean food chain, supporting all other marine life.

Natural sources of iron, such as dust storms, volcanic eruptions, hydrothermal vents, upwelling, and whale waste, can cause large phytoplankton blooms and improve marine life. Phytoplankton photosynthesis can remove large amounts of carbon dioxide from the atmosphere and, in some cases, store it for a long time. Natural iron fertilization is believed to have played a major role in reducing carbon dioxide and cooling the Earth during ice ages.

Phosphorus is often considered the most important limiting nutrient in the ocean because it has a very slow natural cycle. When phosphate is the limiting nutrient in the sunlit part of the ocean, adding phosphate can increase phytoplankton growth. This method could reduce global warming caused by human carbon emissions. One type of fertilizer used is diammonium phosphate (DAP), which costs about $1,700 per tonne of phosphorus in 2008. Using this cost and the Redfield ratio (106:1 for carbon to phosphorus), the cost to store carbon would be about $45 per tonne of carbon, much less than the cost of buying carbon credits.

Another method involves adding urea, a nitrogen-rich substance, to the ocean to encourage phytoplankton growth. The amount of nutrients per ocean area would be similar to natural upwelling events. Once carbon is removed from the surface, it stays stored for a long time.

An Australian company, Ocean Nourishment Corporation (ONC), planned to add hundreds of tonnes of urea to the ocean to increase phytoplankton growth and reduce carbon dioxide. In 2007, ONC tested this method by adding one tonne of nitrogen to the Sulu Sea near the Philippines. This experiment was criticized by groups like the European Commission because the effects on marine life were not fully understood.

Adding main nutrients can reduce global warming caused by human carbon emissions. The two biggest costs are producing the nutrients and delivering them to the ocean.

In areas where iron is available but nitrogen is limited, urea is a better choice for growing algae. Urea is the most widely used fertilizer because it has a lot of nitrogen, is inexpensive, and reacts quickly with water. In the ocean, phytoplankton use enzymes called urease to break down urea into ammonia.

CO(NH₂)₂ + H₂O → urease → NH₃ + NH₂COOH
NH₂COOH + H₂O → NH₃ + H₂CO₃

The intermediate product, carbamate, also reacts with water to produce two ammonia molecules.

A concern is that much more urea is needed to capture the same amount of carbon as iron fertilization. In algae cells, the ratio of nitrogen to iron is 16:0.0001, meaning that adding nitrogen captures less carbon than adding iron. Scientists warn that adding urea might lower oxygen levels and increase harmful algae, which could harm fish populations. Some argue that fish might benefit from eating healthy phytoplankton.

Local wave energy could be used to bring nutrient-rich water from deep in the ocean to the sunlit surface. However, this process might return dissolved carbon dioxide to the atmosphere.

The supply of dissolved inorganic carbon (DIC) in upwelled water is usually enough for photosynthesis, without needing carbon dioxide from the air. Other effects include differences in the composition of upwelled water compared to sinking particles. More nitrogen than carbon is released from sinking organic material, allowing more carbon to sink than is in the upwelled water. This could let some carbon dioxide from the air be absorbed. However, the exact amount of this effect is unclear. Early calculations suggest that 1,000 square kilometers of ocean could store 1 gigatonne of carbon each year.

How much carbon is stored depends on how quickly nutrients rise to the surface and how quickly surface water mixes with deeper water.

Volcanic ash adds nutrients to the ocean, especially in areas where nutrients are already limited. Studies show that areas with low nutrients benefit most from a mix of nutrients from human activity, wind-blown dust, and volcanic ash. Some ocean areas are limited by more than one nutrient, so adding all needed nutrients increases the chance of success. Volcanic ash provides multiple nutrients, but too much metal can harm marine life. The benefits of volcanic ash might be outweighed by its risks.

Evidence shows that volcanic ash can make up to 45% of deep ocean sediments by weight. In the Pacific Ocean, volcanic ash from the air is estimated to be as common as desert dust over long time periods. This suggests that volcanic ash could be a major source of iron.

In August 2008, the Kasatochi volcano in Alaska erupted, sending ash into the nutrient-poor northeast Pacific. This ash, including iron, caused one of the largest phytoplankton blooms ever recorded in the subarctic. Scientists in Canada linked this

Complications

Ocean fertilization, which involves adding nutrients to the sea to boost the growth of tiny ocean plants called phytoplankton, has not been widely accepted as a solution for reducing carbon dioxide in the atmosphere. Scientists have raised several concerns about its effectiveness and risks.

Lisa Speer of the Natural Resources Defense Council explains that limited time and money make it important to focus on solutions that are proven to work and avoid unintended harm. In 2009, scientists Aaron Strong, Sallie Chisholm, Charles Miller, and John Cullen wrote in Nature that adding iron to the ocean to grow phytoplankton and remove carbon dioxide should not be continued.

Warren Cornwall, writing in Science, notes that while adding iron can increase plankton growth, it is unclear how much of the carbon absorbed by phytoplankton reaches the deep ocean. Wil Burns, an ocean law expert, says investing in iron fertilization research is not wise because past experiments showed only one out of 13 tests increased deep ocean carbon levels.

Phytoplankton growth often follows a specific balance of nutrients, including 106 parts carbon, 16 parts nitrogen, 1 part phosphorus, and a tiny amount of iron (known as the Redfield ratio). In areas where iron is scarce, each iron atom helps capture over a million carbon atoms. However, in regions where iron is not enough, nitrogen may not be the main limiting factor.

In some ocean areas, adding too much iron can lead to poor results because the iron is removed from the water before it can be used. This means less organic material is produced than if the nutrient balance were perfect. When nutrients like nitrogen and phosphorus are added to certain waters, they can be used more efficiently than iron.

The success of ocean fertilization depends on many factors, including changes in nutrient ratios and how gases move between the ocean and air. However, these projects may not create long-term carbon storage. Some scientists suggest adding phosphorus might be more helpful than iron or nitrogen over long periods.

Ocean fertilization can also affect the physical environment. Phytoplankton blooms in shallow waters might block sunlight and heat from reaching deeper ocean life, harming corals and kelp. When these blooms end, they can release nitrous oxide, a gas that may reduce the benefits of carbon removal.

Fertilization can cause harmful algal blooms, which may create "dead zones" with low oxygen levels, like the one in the Gulf of Mexico. Adding nutrients like urea can increase fish food sources, but if certain algae dominate, fish populations may not grow as expected.

Sperm whales help move iron from the deep ocean to the surface through their waste, which supports phytoplankton growth and carbon removal. Overfishing whales has reduced this process, leading to more carbon staying in the atmosphere.

Fertilization must be avoided in areas with high biodiversity, such as the Tubbataha Reef, to protect ecosystems from harm. Adding nutrients to coral reefs can lead to algae overgrowth, damaging coral communities.

As phytoplankton sink to the ocean floor, they use oxygen and release gases like methane and nitrous oxide. These processes can affect ocean circulation and warming. Some phytoplankton release a gas called dimethyl sulfide, which may help form clouds and reduce warming. However, too much of this gas could lower rainfall and slow temperature increases by 2100.

Reactions

In 2007, Working Group III of the United Nations Intergovernmental Panel on Climate Change studied ocean fertilization methods in its fourth assessment report. It found that estimates of how much carbon was removed per ton of iron in field studies were likely too high. The report also said that possible harmful effects had not been fully studied.

In June 2007, the London Dumping Convention released a statement expressing concern about the possible harm to the ocean and human health from large-scale ocean iron fertilization. However, the statement did not explain what "large scale" meant. It is believed that this term included large operations.

In 2008, the London Convention/London Protocol stated in resolution LC-LP.1 that knowledge about the effectiveness and environmental impacts of ocean fertilization was not enough to support activities other than research. This recommendation was not legally required. It said that fertilization, except for research, "should be seen as not following the goals of the Convention and Protocol" and did not qualify for any exceptions related to dumping.

In May 2008, at the Convention on Biological Diversity, 191 countries urged a ban on ocean fertilization until scientists learn more about its effects.

In August 2018, Germany banned the sale of ocean seeding as a carbon sequestration system while experts in the EU and EASAC discussed the issue.

International law

International law creates challenges for ocean fertilization. The United Nations Framework Convention on Climate Change (UNFCCC 1992) agreed to actions that reduce the effects of climate change.

The United Nations Convention on the Law of the Sea (LOSC 1982) states that all countries must take steps to stop, reduce, and control pollution in the ocean. It also requires countries to stop moving harm or dangers from one area to another and to prevent changing one type of pollution into another. It is not clear how these rules apply to ocean fertilization.

Solar radiation management

Fertilization can create tiny particles called sulfate aerosols that reflect sunlight. This changes how much sunlight Earth reflects, which can lower temperatures and lessen some effects of climate change. Adding iron to the Southern Ocean helps the natural sulfur cycle by increasing the production of dimethyl sulfide. This process can improve how clouds reflect sunlight, helping to cool the planet.