Iron fertilization is the process of adding iron to the upper part of the ocean, either naturally or intentionally. Iron helps phytoplankton grow, and these tiny plants use photosynthesis to take carbon dioxide (CO₂) from the air. Phytoplankton are the main producers in the ocean, providing food for many other sea creatures.

Natural sources of iron, such as dust storms, volcanic eruptions, hydrothermal vents, upwelling, and whale waste, can cause large phytoplankton blooms. These blooms help remove CO₂ from the atmosphere and sometimes store it for a long time. Scientists believe natural iron fertilization played a major role in reducing CO₂ levels and cooling Earth during ice ages.

Iron is essential for all life and for photosynthesis in plants. However, it is very scarce in the ocean’s upper layer. Scientists did not measure iron in the ocean until the 1980s because it dissolves poorly in seawater and quickly sinks to the ocean floor. In many parts of the ocean, iron is the main factor limiting phytoplankton growth. Recent studies show ocean iron levels and phytoplankton populations are both decreasing.

Intentional ocean iron fertilization (OIF) is a human-made effort to copy natural processes that spread iron, boost ocean life, and remove CO₂ from the air. Because of its potential to help fight climate change, OIF is sometimes called "geoengineering" or "climate intervention." Some people worry that using OIF might reduce efforts to cut fossil fuel use, creating a risk. Others are concerned about how OIF might affect the ocean’s complex ecosystems, such as releasing nitrogen oxides or changing nutrient balances.

Scientists say these risks are still theoretical and have not been seen in experiments. Between 1990 and 2012, 13 major OIF experiments were conducted. No harmful effects were observed in these trials. However, scientists stress the need for careful monitoring in future studies.

Since 1990, 13 large-scale experiments have tested OIF’s effectiveness and possible effects. Results varied depending on goals and conditions. One 2017 study said the method is unproven, as it removed little CO₂ and required large amounts of iron. Other studies found higher potential, with some showing that phytoplankton blooms sank deep into the ocean, possibly storing carbon for hundreds or thousands of years.

In recent years, interest in OIF has grown. A report by the US National Oceanographic and Atmospheric Administration said OIF has "moderate potential" compared to other ocean-based methods for storing carbon.

About 25 percent of the ocean has plenty of nutrients but little plant life (as shown by low chlorophyll levels). In these high-nutrient, low-chlorophyll areas, iron is the main limiting factor for growth. Spreading iron over large ocean areas is expensive compared to the value of carbon credits. However, studies of volcanic eruptions, like the 1991 eruption of Mount Pinatubo, suggest that natural iron from volcanic ash removed billions of tons of CO₂. This shows that carefully targeted OIF might have significant benefits.

Process

Iron is a trace element in the ocean, and it is important for photosynthesis in plants like phytoplankton. Adding iron to areas where it is lacking can help phytoplankton grow. Because of this, the "iron hypothesis" was proposed by Martin in the late 1980s. He suggested that changes in iron levels in iron-deficient seawater could cause plankton to grow rapidly and affect the amount of carbon dioxide in the atmosphere by changing how much carbon is stored. Fertilization is a natural process in the ocean. For example, ocean currents can bring nutrient-rich sediments to the surface.

Another example is when iron-rich minerals, dust, or volcanic ash are carried over long distances by rivers, glaciers, or wind. Whales eat iron-rich organisms deep in the ocean and then release iron into the surface water when they defecate, which helps phytoplankton grow. Studies show that a decrease in the number of sperm whales in the Southern Ocean has led to a 200,000 tonnes/year drop in the ocean’s ability to absorb carbon from the atmosphere, possibly because phytoplankton growth was limited.

Phytoplankton uses sunlight and nutrients to grow and takes in carbon dioxide during this process. Plankton can store atmospheric carbon by creating calcium or silicon-carbonate skeletons. When these organisms die, they sink to the ocean floor, where their carbonate skeletons become part of the deep sea’s carbon-rich layers, known as marine snow, located far below areas where plankton blooms occur.

In some cases, much of the carbon-rich material from plankton is eaten by other organisms, such as small fish or zooplankton. Scientists agree that some biocarbon reaches the deep ocean, where it can stay for hundreds or thousands of years—considered permanent for practical purposes. If the biocarbon does not sink deeply enough, it may return to the atmosphere. Therefore, it is important to ensure that biocarbon is transported to the ocean depths.

Supporters of iron fertilization argue that carbon sequestration should be measured over shorter time frames. They claim that carbon stored in the deep ocean is effectively isolated from the atmosphere for hundreds of years, making it a practical method for reducing carbon levels.

Under ideal conditions, the maximum estimated effect of iron fertilization on slowing global warming is about 0.3W/m² of averaged negative forcing, which could offset roughly 15–20% of current human-caused carbon dioxide emissions. This method involves adding iron to nutrient-poor ocean areas to stimulate phytoplankton growth, which could be a scalable way to reduce atmospheric carbon. However, ocean iron fertilization remains controversial because of its potential negative effects on marine ecosystems.

Research shows that adding large amounts of iron-rich dust to the ocean can disrupt nutrient balance, harming the food chain and threatening marine life that depends on stable nutrients. Excessive iron may also change plankton communities, favoring some species over others and reducing biodiversity.

Iron fertilization can cause large phytoplankton blooms. When these blooms decay, they may create areas in the ocean with low oxygen levels, which can harm marine life and biodiversity. In some cases, iron fertilization has led to harmful algal blooms that produce toxins harmful to marine organisms and humans. For example, experiments in the Southern Ocean, such as the SOFeX trials, showed that iron fertilization can cause rapid growth of harmful algae, potentially harming local ecosystems and food chains.

In addition to ecological concerns, there are challenges related to the long-term effectiveness of carbon sequestration through iron fertilization. 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 carbon sequestration varies, influenced by factors like ocean currents and temperature. Feedback mechanisms, such as changes in ocean chemical cycles or marine species populations, may reduce the overall effectiveness of iron fertilization as a strategy to combat climate change.

Methods

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

Experiments that add iron sulfate to the ocean's surface using ships are explained in detail in the experiment section below.

Iron-rich dust that rises into the air is a natural source of iron for the ocean. For example, dust carried by wind from the Sahara desert helps feed 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 breaks down methane and other greenhouse gases, makes clouds brighter, and eventually falls with rain in small amounts across large areas of the world. 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 reduce or stop global warming.

Experiments

John Martin, director of the Moss Landing Marine Laboratories, proposed that low phytoplankton levels in certain ocean regions 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 iron in the ocean could help explain past ice ages.

In 1993, Martin’s colleagues completed a larger experiment called IronEx I near the Galápagos Islands by adding 445 kg of iron to an ocean area. Phytoplankton levels in the treated area increased three times compared to untreated areas. This success inspired further research into using iron to remove carbon dioxide from the atmosphere.

Following IronEx I, scientists conducted 12 international studies to examine the effects of iron fertilization:
– IronEx II (1995)
– SOIREE (1999)
– EisenEx (2000)
– SEEDS (2001)
– SOFeX (2002)
– SERIES (2002)
– SEEDS-II (2004)
– EIFEX (2004) – This experiment in the South Atlantic caused a bloom of diatoms, which sank to the ocean floor after fertilization ended. Unlike LOHAFEX, this area had enough silicon for diatoms to grow.
– CROZEX (2005)
– A Planktos project (2008) was canceled due to lack of funding. The company claimed environmental groups were responsible.
– LOHAFEX (2009) – Conducted in the South Atlantic, this experiment used iron sulfate in an area with low silicon levels. While a phytoplankton bloom occurred, only a small amount of carbon was sequestered because other phytoplankton species were eaten by zooplankton and did not sink quickly. LOHAFEX showed that successful carbon sequestration depends on choosing areas with high silicon levels.
– Haida Salmon Restoration Corporation (2012) – Funded by the Old Massett Haida band, this project added 100 tonnes of iron sulfate into the Pacific Ocean near Haida Gwaii. It caused algae growth over 10,000 square miles. Critics said the action violated international agreements. Scientific data from the project was released publicly in 2014.

In 2000 and 2004, the EisenEx experiment added iron sulfate to the ocean. About 10–20% of the resulting algae bloom sank to the ocean floor.

Planktos, a U.S. company, planned six iron fertilization cruises from 2007 to 2009 but canceled them after its ship, Weatherbird II, was denied entry to a port in the Canary Islands.

In 2007, companies like Climos and GreenSea Ventures planned to use iron fertilization to offset carbon emissions by selling carbon credits.

LOHAFEX, initiated by Germany and India in 2009, involved adding 6 tonnes of ferrous sulfate to 300 square kilometers of ocean. Scientists expected the bloom to trap carbon dioxide and sink it to the ocean floor. However, environmental concerns led to calls for the experiment to stop.

A 2012 study near Antarctica added iron fertilizer to an ocean eddy, creating a bloom that sent carbon into the deep ocean. Nutrient levels, including nitrogen, phosphorus, and silicon, dropped after 24 days, while organic matter from algae sank below 1,000 meters. Each iron atom helped convert at least 13,000 carbon atoms into algae.

In 2012, the Haida Salmon Restoration Corporation added 100 tonnes of iron sulfate to the Pacific Ocean near Haida Gwaii. The project aimed to boost salmon populations by increasing phytoplankton. However, experts questioned the scientific validity of the results, and some claimed the action violated international rules.

In 2014, data from the Haida project was made public.

Science

The best possible result from iron fertilization, if the most favorable conditions are assumed and real-world problems are ignored, is 0.29 W/m² of globally averaged negative forcing. This would reduce human-caused CO₂ emissions by about 1/6. However, research suggests that adding iron to seawater might reduce other important nutrients, which could limit 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. This occurs where ocean currents meet underwater features like banks or seamounts. This natural process creates some of the largest marine habitats on Earth. Fertilization can also happen when wind carries dust over long distances or when iron-rich materials are carried into the ocean by glaciers, rivers, or icebergs.

About 70% of Earth’s surface is covered by oceans. The parts of the ocean where sunlight can reach support algae and other marine life. In some areas, algae growth is limited by the amount of iron available. Iron is a key nutrient for phytoplankton growth and photosynthesis. Historically, dust storms from dry land carried iron to the open ocean. This dust contains 3–5% iron, but its delivery to the ocean has decreased by nearly 25% in recent decades.

The Redfield ratio describes the balance of nutrients needed for plankton growth. 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 to grow. Later research added iron to this ratio, showing that in iron-poor areas, 1 iron atom can help fix 106,000 carbon atoms. In the 2004 EIFEX experiment, scientists found that 1 kilogram of iron could help remove nearly 3,000 kilograms of carbon dioxide from the atmosphere.

In areas with low iron, called HNLC zones, small amounts of iron (measured in parts per trillion) can cause large phytoplankton blooms. For example, 1 kilogram of iron might support 100,000 kilograms of plankton. The size of iron particles matters—those 0.5–1 micrometer in size are most effective because they stay in the sunlit upper ocean longer and are easier for phytoplankton to use. One way to add iron to HNLC zones is through Atmospheric Methane Removal.

Iron reaches the ocean through atmospheric deposition. Satellite images and data, combined with wind pattern analysis, help identify natural sources of iron-rich dust. This dust erodes from soil and is carried by wind. While many dust sources are in the Northern Hemisphere, the largest sources are in northern and southern Africa, North America, central Asia, and Australia.

In the atmosphere, chemical reactions change how iron is structured in dust, which affects how usable it is for marine life. Iron in tiny particles, like those in aerosols, is more soluble than in soil. Some chemical reactions with organic acids increase iron solubility. For example, sunlight can convert iron from a less usable form (Fe(III)) to a more usable form (Fe(II)) through interactions with organic compounds. Studies show that Fe(II) levels are higher during the day than at night.

Volcanic ash is another source of iron for the ocean. It contains glass, minerals, and other materials that release nutrients when they mix with water. Over the past million years, increases in biogenic opal in ocean sediments have been linked to higher iron levels. In 2008, an eruption in the Aleutian Islands released ash into the nutrient-poor Northeast Pacific, causing one of the largest phytoplankton blooms ever observed in the subarctic.

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

About half of the carbon produced by plankton blooms is eaten by other sea life, such as zooplankton, krill, and small fish. However, 20–30% of this carbon sinks below 200 meters into colder, deeper water. Much of this carbon remains in the deep ocean for centuries, isolated from the atmosphere.

To study the effects of iron fertilization, scientists use tools like ship-based sampling, underwater traps, satellite imaging, and buoys. However, unpredictable ocean currents can move iron patches away from study areas, making experiments difficult.

Iron fertilization could help reduce global warming. For example, if phytoplankton in the Antarctic Circumpolar Current used all the nitrate and phosphate in the surface ocean to create organic carbon, it could remove 0.8 to 1.4 gigatonnes of carbon from the atmosphere each year. This is about 1/6 of the carbon released by human fossil fuel use annually. Other areas, like the Galapagos 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 more sunlight and cooling Earth. This idea is part of the CLAW hypothesis, which is related to the Gaia hypothesis. During the SOFeX experiment, DMS levels in fertilized areas increased fourfold. If iron fertilization were used widely in the Southern Ocean, it might cause cooling through both CO₂ removal and increased cloud cover, though the exact cooling effect is uncertain.

Financial opportunities

Beginning with the Kyoto Protocol, several countries and the European Union created carbon offset markets that trade certified emission reduction credits (CERs) and other types of carbon credit instruments. In 2007, CERs were sold for about €15 to €20 per ton of CO₂. Iron fertilization is cheaper than methods like scrubbing, direct injection, and other industrial approaches, and can theoretically store carbon for less than €5 per ton of CO₂, offering a large return.

In August 2010, Russia set a minimum price of €10 per ton for offsets to reduce uncertainty for offset providers. Scientists have reported a 6–12% drop in global plankton production since 1980. A full-scale plankton restoration program could regenerate about 3 to 5 billion tons of sequestration capacity, which is worth €50 to €100 billion in carbon offset value. However, a 2013 study found that the cost and benefits of iron fertilization make it less effective than carbon capture and storage and carbon taxes.

Debate

Ocean iron fertilization might be a powerful way to slow global warming, but scientists are still discussing whether this method works well and if it could cause harm.

The precautionary principle is a rule used in environmental protection. A 2021 article explains that this principle says, "if human activities could possibly cause serious harm, steps should be taken to prevent or reduce that harm, even if there is not complete scientific proof." Because there is not enough information about the effects of iron fertilization, experts believe it is important to avoid harm until more research is done. This idea is one reason some scientists argue against using iron fertilization widely until more data is available.

Some scientists worry that adding iron to the ocean might cause harmful algal blooms (HABs). This is because some types of algae that produce toxins may grow more when iron is added. However, a 2010 study found that in an area of the ocean with high nitrogen and low chlorophyll, a type of diatom called Pseudo-nitzschia began producing high levels of domoic acid, a toxin, even though these diatoms are usually not harmful in open ocean areas. Even short-lived blooms with these toxins could harm marine life.

Most phytoplankton species are harmless or helpful because they are the base of the marine food chain. Iron fertilization increases phytoplankton growth mainly in the open ocean, where iron is scarce. Coastal waters already have enough iron, so adding more does not help. Studies show that iron fertilization can lead to faster breakdown of organic matter, which may not help the environment and could actually increase carbon dioxide levels. A 2023 study found that iron fertilization might make climate change worse instead of helping.

A 2010 study showed that adding iron to high-nitrate, low-chlorophyll areas encourages the growth of toxic diatoms. The researchers said this raises "serious concerns" about whether large-scale iron fertilization is useful or sustainable. Nitrogen from whales and iron chelate are important for the marine food chain and help store carbon for long periods.

A 2009 study used a computer model to test if iron fertilization could reduce carbon dioxide and ocean acidity. The study found that even in extreme scenarios, iron fertilization had little effect on reducing ocean acidification caused by carbon dioxide. The study also said that iron fertilization likely would not significantly change ocean acidity because it has limited effects on carbon dioxide levels.

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 a paper titled "On the phytoplankton of the South-West Atlantic and Bellingshausen Sea, 1929-31," he suggested that certain ocean areas, which seemed rich in nutrients but had little phytoplankton or other sea life, might lack enough iron. He revisited this idea in a 1942 paper, "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 renewed interest in the subject through his research on ocean nutrients. His findings supported Hart’s earlier hypothesis. These nutrient-rich but phytoplankton-poor areas later became known as "high-nutrient, low-chlorophyll regions" (HNLC).

In the 1980s, John Gribbin, a scientist, first publicly proposed that adding large amounts of soluble iron to the ocean could help reduce climate change. Later, in 1988, John Martin made a remark at Woods Hole Oceanographic Institution, saying, "Give me a half a tanker of iron and I will give you an ice age." This statement led to years of research on the topic.

Studies showed that iron shortages limited ocean productivity and offered a possible way to reduce climate change. Strong support for Martin’s idea came after the 1991 eruption of Mount Pinatubo in the Philippines. Environmental scientist Andrew Watson analyzed global data from the eruption and found that it released about 40,000 tons of iron dust into the world’s oceans. This event was followed by a noticeable drop in global atmospheric carbon dioxide levels and a rise in oxygen levels.

In 2008, the parties to the London Dumping Convention adopted a non-binding resolution labeled LC-LP.1(2008). It stated that ocean fertilization activities, except for legitimate scientific research, "should be considered as contrary to the aims of the Convention and Protocol and do not currently 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 waste dumping at sea.

Many ocean labs, scientists, and businesses have studied ocean fertilization. Starting in 1993, thirteen research teams conducted ocean trials showing that adding iron can stimulate phytoplankton growth. However, debates continue about how well this method reduces atmospheric carbon dioxide and its effects on ocean ecosystems. Ocean trials of iron fertilization took place 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).