Iron fertilization describes both natural and human-caused processes that add iron to the upper ocean. Iron helps phytoplankton grow, and phytoplankton remove carbon dioxide (CO₂) from the atmosphere through photosynthesis. Phytoplankton are the main food source for marine life and support the entire ocean food web.

Natural processes, such as dust storms, volcanic eruptions, hydrothermal vents, upwelling, and whale waste, can cause large phytoplankton blooms and help restore marine life. Phytoplankton photosynthesis removes large amounts of CO₂ from the atmosphere and, in some cases, stores it for a long time. Scientists agree that natural iron fertilization played a major role in the large drops in CO₂ levels and temperatures that caused ice ages.

Iron is needed for all life and for plant photosynthesis. However, it is present in very small amounts in the upper ocean, and scientists could not measure it until the 1980s. Iron is usually not dissolved in seawater and sinks quickly. In much of the ocean, iron is the main limiting factor for phytoplankton growth. Recent studies show that ocean iron levels have been decreasing, along with phytoplankton.

Intentional ocean iron fertilization (OIF) is a deliberate effort to copy natural processes that have, for a long time, spread iron, boosted ocean life, and removed CO₂ from the atmosphere.

Because of its potential to reduce climate change, intentional OIF is considered a type of "geoengineering" or "climate intervention." Some people argue that using OIF might reduce efforts to cut fossil fuel use, creating a risk. Others worry about the effects of large-scale OIF on the ocean’s complex ecosystem, including possible unintended consequences, such as releasing nitrogen oxides or disrupting ocean nutrient balance.

Scientists say these risks are still theoretical and have not been observed in field tests. Between 1990 and 2012, 13 major OIF experiments were conducted, and no harmful effects were found. Scientists still recommend careful monitoring in future studies.

Since 1990, 13 large-scale experiments have tested the effectiveness and risks of iron fertilization in oceans. Results varied depending on goals and conditions. One 2017 study said the method is not proven, as CO₂ removal was low or nonexistent, and large amounts of iron would be needed to make a small difference in carbon emissions. Other studies suggest the method could be more effective. One found that at least half of the phytoplankton biomass sank below 1,000 meters, possibly storing carbon for hundreds or thousands of years.

In recent years, interest in iron fertilization has grown. A report by the US National Oceanographic and Atmospheric Administration said iron fertilization has "moderate potential" for cost, scalability, and long-term carbon storage compared to other ocean-based methods.

About 25% of the ocean’s surface has enough nutrients but little plant life (as measured by chlorophyll). In these high-nutrient, low-chlorophyll (HNLC) areas, growth is limited by iron. Spreading iron over large ocean areas is expensive compared to the value of carbon credits. However, research on events like the 1991 eruption of Mount Pinatubo suggests that natural iron fertilization from volcanic ash removed billions of tons of CO₂. This shows that carefully targeted human efforts might achieve similar results.

Process

Iron is a small but important element in the ocean. It helps plants like phytoplankton grow through photosynthesis. Adding iron to areas where it is missing can increase phytoplankton growth. In the late 1980s, a scientist named Martin proposed the "iron hypothesis," suggesting that changes in iron levels in seawater could influence plankton growth and affect how much carbon dioxide is stored in the ocean. Natural fertilization happens in the ocean through processes like upwelling, where ocean currents bring nutrient-rich sediments to the surface.

Iron can also reach the ocean through other natural methods, such as wind carrying iron-rich dust or volcanic ash over long distances. Whales eat iron-rich organisms deep in the ocean and release iron into the surface waters when they defecate, which helps phytoplankton grow. Studies have shown that a decrease in sperm whale numbers in the Southern Ocean led to a reduction in how much carbon dioxide was absorbed from the atmosphere, possibly because phytoplankton growth was limited.

Phytoplankton uses sunlight and nutrients to grow and takes in carbon dioxide during this process. Some plankton create calcium or silicon-carbonate skeletons, which can sink to the ocean floor after they die. These skeletons become part of deep-sea deposits called marine snow, located far below where plankton blooms occur. In some cases, carbon from plankton is eaten by other sea creatures, such as small fish and zooplankton. Scientists agree that some of this carbon eventually reaches the deep ocean, where it can remain for hundreds or thousands of years. If the carbon does not sink deeply enough, it may return to the atmosphere. Careful management is needed to ensure carbon is transported to the deep ocean.

Supporters of iron fertilization argue that carbon stored in the deep ocean is effectively removed from the atmosphere for hundreds of years, making it a potential method to reduce carbon dioxide levels. Under ideal conditions, iron fertilization could reduce global warming by about 0.3W/m², which might offset 15–20% of current human-caused carbon emissions. This method involves adding iron to nutrient-poor ocean regions to stimulate phytoplankton growth. However, it remains controversial because it may harm marine ecosystems.

Research shows that adding large amounts of iron to the ocean can disrupt nutrient balance, harming marine life and food chains. Excess iron may also change plankton communities, favoring some species over others and reducing biodiversity. Large phytoplankton blooms can lead to low-oxygen zones in the ocean, which are dangerous for marine life. Some experiments, like SOFeX in the Southern Ocean, found that iron fertilization can cause harmful algae to grow rapidly, producing toxins that harm marine and human health.

In addition to ecological risks, the long-term effectiveness of carbon sequestration through iron fertilization is uncertain. While phytoplankton can capture carbon dioxide and sink to the ocean floor, much of this carbon may return to the atmosphere due to ocean processes. Recent studies suggest that the success of this method depends on factors like ocean currents and temperature. Changes in marine life or ocean chemistry may reduce how well iron fertilization works as a strategy to address climate change.

Methods

There are two main methods for artificial iron fertilization: adding iron directly into the ocean from ships and spreading iron into the atmosphere.

Experiments that add iron sulfate directly to ocean water from ships are described in detail in the experiment section below.

Iron-rich dust that rises into the air is a major natural source of iron for the ocean. For example, wind-blown dust from the Sahara Desert provides iron to the Atlantic Ocean and the Amazon Rainforest. Naturally occurring iron oxide in atmospheric dust reacts with hydrogen chloride from sea spray to create iron chloride. This substance helps reduce methane and other greenhouse gases, makes clouds brighter, and eventually falls as rain in low amounts across large areas of the globe. Unlike ship-based methods, no experiments have tested increasing the natural amount of iron in the atmosphere. Expanding this natural source could work alongside ship-based methods.

One idea is to increase atmospheric iron levels by using iron salt aerosol. Adding iron(III) chloride to the troposphere could strengthen natural cooling effects, such as removing methane, brightening clouds, and fertilizing the ocean. These actions may help slow or reverse global warming.

Experiments

John Martin, director of the Moss Landing Marine Laboratories, proposed that low phytoplankton levels in certain ocean areas were caused by a lack of iron. In 1989, he tested this idea by adding iron to clean water samples from Antarctica. The samples with iron showed much greater phytoplankton growth than those without. This led him to suggest that increasing ocean iron levels might help explain past ice ages.

In 1993, Martin’s colleagues carried out Ironex I near the Galapagos Islands, adding 445 kg of iron to an ocean area. Phytoplankton levels in the treated region increased three times. This success inspired further research into using iron to remove carbon dioxide from the atmosphere.

Between 1995 and 2012, scientists conducted 12 international studies to test the effects of iron fertilization. These included:
– Ironex II (1995)
– SOIREE (1999)
– EisenEx (2000)
– SEEDS (2001)
– SOFeX (2002)
– SERIES (2002)
– SEEDS-II (2004)
– EIFEX (2004)
– CROZEX (2005)
– LOHAFEX (2009)
– Haida Salmon Restoration Corporation (2012)

In 2004, EIFEX found that adding iron to a South Atlantic ocean eddy caused a large diatom bloom. When fertilization ended, many diatoms sank to the ocean floor, storing carbon. This contrasted with LOHAFEX (2009), which took place in low-silica waters. There, other phytoplankton species dominated, and less carbon was stored because they sank slowly or were eaten by zooplankton. LOHAFEX showed that successful carbon storage depends on selecting areas with enough silica for diatoms to grow.

In 2012, the Haida Salmon Restoration Corporation added 100 tonnes of iron sulfate to the Pacific Ocean near Haida Gwaii. This caused algae to grow over 10,000 square miles. Critics claimed this violated international agreements against unapproved geoengineering experiments. Data from the project was released publicly in 2014.

Planktos, a U.S. company, planned to conduct iron fertilization experiments from 2007 to 2009 but canceled them due to funding issues. Environmental groups were blamed for the failure.

In 2000 and 2004, EisenEx experiments showed that 10–20% of algal blooms sank to the ocean floor after iron was added.

LOHAFEX, led by Germany and India in 2009, involved adding 6 tons of iron sulfate to 300 square kilometers of ocean. Scientists expected the bloom to store carbon, but low silica levels limited its effectiveness.

A 2012 study near Antarctica found that iron fertilization in an isolated eddy caused a significant amount of carbon to sink into the deep ocean. Nutrient levels dropped after day 24, and organic matter sank below 1,000 meters. Each iron atom helped convert at least 13,000 carbon atoms into algae.

The Haida project aimed to boost salmon populations by increasing phytoplankton, which would serve as food. However, experts questioned the scientific validity of the results, noting that salmon population increases in 2013 might have been due to other factors.

Science

Iron fertilization, under the best possible conditions and without considering practical challenges, could reduce global warming by about 0.29 W/m². This effect might reduce human-caused carbon dioxide emissions by about one-sixth. However, some studies suggest that adding iron to the ocean might reduce other important nutrients in seawater, which could harm phytoplankton growth in other areas. This means iron may only help phytoplankton grow in specific local areas, not globally.

Ocean fertilization happens naturally when deep, nutrient-rich water rises to the surface, such as where ocean currents meet underwater mountains or banks. This process creates the largest marine habitats on Earth. Fertilization can also occur when wind carries dust over long distances or when iron-rich minerals from glaciers, rivers, or icebergs enter the ocean.

About 70% of Earth’s surface is covered by oceans. In areas where sunlight can reach, algae and other marine life live. In some oceans, the growth of algae is limited by the amount of iron available. Iron is essential for phytoplankton to grow and perform photosynthesis. Historically, iron has been delivered to open oceans by dust storms from dry regions. This dust contains 3–5% iron, but its delivery has decreased by about 25% in recent decades.

The Redfield ratio describes the balance of nutrients in plankton. It is often written as "106 C: 16 N: 1 P," meaning 106 carbon atoms, 16 nitrogen atoms, and 1 phosphorus atom are needed for plankton growth. Later research added iron to this ratio, showing that in areas with little iron, each iron atom can help fix 106,000 carbon atoms. In the 2004 EIFEX experiment, scientists found that 3,000 units of carbon were linked to 1 unit of iron.

Small amounts of iron in specific ocean regions, called HNLC zones, can cause large phytoplankton blooms. For example, 100,000 kilograms of plankton might grow for every kilogram of iron added. The size of iron particles matters: particles smaller than 1 micrometer are most effective because they stay in sunlight-receiving areas longer and are easier for phytoplankton to use. One method to add iron is through Atmospheric Methane Removal.

Iron reaches the ocean through natural processes like wind-blown dust. Satellite images and data, combined with wind direction studies, help identify where dust comes from. Most dust originates in the Northern Hemisphere, but major sources include Africa, North America, central Asia, and Australia.

In the atmosphere, chemical reactions change how iron behaves in dust, affecting how useful it is for life. Iron in tiny particles is more likely to dissolve than in soil. Chemical reactions with organic acids in the air can increase iron solubility. For example, sunlight can convert iron from a less usable form (Fe(III)) into a more usable form (Fe(II)). Studies show that Fe(II) levels rise during the day compared to Fe(III).

Volcanic ash also provides iron to the ocean. Ash contains glass, minerals, and other materials that release nutrients at different rates when they mix with water.

Evidence from ocean sediments shows that increases in biogenic opal (a type of silica made by plankton) are linked to higher iron levels over the past million years. In 2008, volcanic ash from the Aleutian Islands caused a massive phytoplankton bloom in the nutrient-poor Northeast Pacific.

In the past, large phytoplankton blooms helped cool Earth, such as during the Azolla event. Plankton like diatoms, coccolithophores, and foraminifera create calcium or silicon carbonate shells. When these organisms die, their shells sink to the ocean floor, forming "marine snow," which includes organic matter and fecal pellets from fish. This process removes carbon from the atmosphere.

About half of the carbon from plankton blooms is eaten by other ocean life, but 20–30% sinks below 200 meters into colder, deeper waters. Some of this carbon remains trapped in deep ocean currents for centuries, isolating it from the atmosphere.

To measure the effects of plankton blooms, scientists use tools like ship samples, underwater traps, buoys, and satellites. However, unpredictable ocean currents can move iron from test areas, making experiments unreliable.

Iron fertilization could help reduce global warming. For example, if phytoplankton in the Antarctic Circumpolar Current used all available nitrate and phosphate to create organic carbon, it could remove about 0.8 to 1.4 gigatonnes of carbon from the atmosphere each year. This is similar to the amount of carbon released annually by burning fossil fuels. Other areas, like the Galápagos Islands, might also be suitable for iron fertilization.

Some plankton produce dimethyl sulfide (DMS), which enters the atmosphere and forms sulfate particles. These particles may increase cloud cover, reflecting sunlight and cooling Earth. This idea is part of the CLAW hypothesis, which James Lovelock used to support his Gaia hypothesis.

During the SOFeX experiment, DMS levels rose fourfold in fertilized areas. Large-scale iron fertilization in the Southern Ocean might increase cooling through sulfur effects, in addition to reducing carbon dioxide and increasing ocean reflectivity. However, the exact cooling effect of this process is uncertain.

Financial opportunities

The Kyoto Protocol started this, and several countries and the European Union created carbon offset markets that trade certified credits that reduce emissions and other types of carbon credit tools. In 2007, these credits sold for about €15 to €20 per ton of CO2. Iron fertilization is much less expensive than methods like scrubbing, direct injection, and other industrial techniques. It could theoretically store carbon for less than €5 per ton of CO2, which could create a large return.

In August 2010, Russia set a minimum price of €10 per ton for carbon offsets to help reduce uncertainty for those who provide them. Scientists have reported a 6 to 12 percent drop in global plankton production since 1980. A full-scale program to restore plankton populations could rebuild about 3 to 5 billion tons of carbon storage capacity, valued at €50 to €100 billion in carbon offset terms. However, a 2013 study found that the costs and benefits of iron fertilization make it less effective than carbon capture and storage and carbon taxes.

Debate

Ocean iron fertilization might be a way to slow global warming. However, scientists are debating how well it works and if it could cause problems.

The precautionary principle is a rule about protecting the environment. A 2021 article explains that this rule says, "If human actions might cause serious harm, steps should be taken to prevent or reduce that harm. Uncertainty should not stop action." Because there is not enough data about the effects of iron fertilization, experts say leaders should avoid harming the environment until more information is known. This idea is one reason some people oppose using iron fertilization widely until more research is done.

Some scientists worry that adding iron to the ocean might cause harmful algal blooms. These blooms can include toxic algae, which can harm marine life. A 2010 study in an area of the ocean with high nitrogen and low chlorophyll found that a type of diatom called Pseudo-nitzschia began producing large amounts of domoic acid, a toxin. Even short-lived blooms with this toxin could harm marine food webs.

Most phytoplankton are harmless or helpful because they are the base of the ocean food chain. Iron fertilization increases phytoplankton only in open ocean areas where iron is very low. Coastal waters already have plenty of iron, so adding more does not help. Studies show that iron fertilization can increase the breakdown of organic matter, leading to changes in plankton populations. This process may not help the environment and could even increase carbon dioxide levels. A 2023 study found that iron fertilization might worsen climate change instead of helping it.

A 2010 study showed that adding iron to high-nitrate, low-chlorophyll areas can cause toxic diatoms to grow. The researchers said this raises concerns about the usefulness and safety of large-scale iron fertilization. Nitrogen from whales and iron chelate (a chemical that helps iron stay in the water) can help the ocean food chain and store carbon for long periods.

A 2009 study used a computer model to test if iron fertilization could reduce carbon dioxide and ocean acidification. The study found that even if all surface nutrients in the ocean were completely used up, iron fertilization would have little effect on reducing ocean acidification. Since iron fertilization has a small impact on carbon dioxide levels, it may not help reduce ocean acidification.

History

In the 1930s, Dr. Thomas John Hart, a British marine biologist working on the RRS Discovery II in the Southern Ocean, first studied the role of iron in the growth of phytoplankton and photosynthesis. In his 1929–1931 research, he suggested that certain areas of the ocean, which appeared rich in nutrients but had little phytoplankton or other sea life, might lack iron. Hart revisited this idea in a 1942 paper titled "Phytoplankton periodicity in Antarctic surface waters." However, few scientists discussed this topic until the 1980s, when oceanographer John Martin of the Moss Landing Marine Laboratories reignited the debate through his studies of ocean nutrients. His findings supported Hart’s earlier hypothesis. These areas, now called "high-nutrient, low-chlorophyll regions" (HNLC), were identified as places where iron shortages limited phytoplankton growth.

In 1988, John Gribbin proposed that adding large amounts of iron to the ocean could help reduce climate change. This idea was further supported by John Martin’s famous statement at the Woods Hole Oceanographic Institution: "Give me a half a tanker of iron and I will give you an ice age." This statement inspired research over the next decade.

Studies showed that iron shortages limited ocean productivity and could influence climate change. Strong evidence for this idea came in 1991 after the eruption of Mount Pinatubo in the Philippines. Scientist Andrew Watson analyzed data from this event and found that the eruption released about 40,000 tons of iron dust into the world’s oceans. This event was followed by a noticeable drop in atmospheric carbon dioxide and a rise in oxygen levels.

In 2008, the parties to the London Dumping Convention passed a non-binding resolution about ocean fertilization, labeled LC-LP.1(2008). The resolution stated that ocean fertilization, except for legitimate scientific research, "should be considered as contrary to the aims of the Convention and Protocol" and does not qualify for any exemption from the definition of dumping. In 2010, the Contracting Parties to the Convention adopted an Assessment Framework for Scientific Research Involving Ocean Fertilization, labeled LC-LP.2(2010), to regulate the dumping of wastes at sea.

Since the 1990s, multiple ocean labs, scientists, and businesses have tested ocean fertilization. Starting in 1993, thirteen research teams conducted trials showing that adding iron can stimulate phytoplankton growth. However, debates continue about how well this method removes carbon dioxide from the atmosphere and its effects on ocean ecosystems. Ocean trials of iron fertilization occurred in 2009 in the South Atlantic as part of the LOHAFEX project and in July 2012 in the North Pacific near British Columbia, Canada, by the Haida Salmon Restoration Corporation (HSRC).