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 producers of food in the ocean, supporting the rest of the marine food web.
Natural sources of iron, such as dust storms, volcanic eruptions, hydrothermal vents, upwelling, and whale waste, can cause large phytoplankton blooms, which help restore marine life. Phytoplankton photosynthesis removes large amounts of CO₂ from the atmosphere, and in some cases, stores it for a long time. Natural iron fertilization is widely accepted as a major reason for the large drops in CO₂ levels and temperatures that caused ice ages.
Iron is needed for all life and for photosynthesis in plants. However, it is found in very small amounts in the upper ocean, and scientists could not measure it until the 1980s. Iron does not dissolve easily in seawater and tends to sink quickly. In many parts of the ocean, iron is the main nutrient that limits phytoplankton growth. Recent studies show that ocean iron levels have been decreasing, along with phytoplankton.
Intentional ocean iron fertilization (OIF) is a human 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 the effort to cut fossil fuel use, creating a moral hazard. Others worry about the risks of disrupting the ocean’s complex ecosystem, such as releasing nitrogen oxides or changing nutrient balances.
Scientists say these risks are still theoretical, as no major harmful effects have been seen in field tests. Between 1990 and 2012, 13 major OIF experiments were conducted. No significant harm was observed in these trials, though scientists recommend careful monitoring in future tests.
Since 1990, 13 large-scale experiments have tested the effectiveness and risks of ocean iron fertilization. Results varied depending on the goals and conditions of each study. One 2017 study said the method is unproven, as it removes little CO₂ and sometimes has no effect. Other studies found higher potential, with some showing that at least half of the phytoplankton biomass sank below 1,000 meters, possibly storing carbon for hundreds or thousands of years.
Recent years have seen growing interest in iron fertilization research. 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 percent of the ocean surface has plenty of macronutrients but little plant life, as shown by low chlorophyll levels. In these "high-nutrient, low-chlorophyll" (HNLC) waters, phytoplankton growth is limited by micronutrients, especially iron. Spreading iron over large ocean areas is expensive compared to the value of carbon credits. However, studies of volcanic eruptions, like Mount Pinatubo in 1991, suggest that natural iron fertilization from volcanic ash removed many gigatons of CO₂, showing that carefully targeted human efforts might achieve similar results.
Process
Iron is a small amount of a substance found in the ocean, and it is important for plants like phytoplankton to perform photosynthesis. Adding iron to areas where it is missing can help phytoplankton grow better. In the late 1980s, a scientist named Martin proposed the "iron hypothesis," which suggests that changing the amount of iron in seawater that lacks it can cause phytoplankton to grow more, which might affect the amount of carbon dioxide in the atmosphere by changing how much carbon is stored. Natural processes help fertilize the ocean. For example, ocean currents can bring nutrient-rich sediments from the deep to the surface.
Another natural way is when iron-rich materials, such as dust or volcanic ash, are carried by rivers, glaciers, or wind over long distances. 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 led to a loss of 200,000 tonnes of carbon being absorbed from the atmosphere each year, possibly because phytoplankton growth was limited.
Phytoplankton uses sunlight and nutrients to grow and takes in carbon dioxide during this process. These organisms can store atmospheric carbon by forming calcium or silicon-carbonate skeletons. When they die, their skeletons sink to the ocean floor, becoming part of the deep sea's carbon-rich layers, known as marine snow. Some of the carbon from plankton is eaten by other sea creatures, like small fish and zooplankton. Scientists agree that some carbon from plankton reaches the deep ocean, where it can stay for hundreds or thousands of years. If the carbon does not sink deep enough, it may return to the atmosphere. Therefore, it is important to ensure that carbon is carried to the deep ocean.
People who support using iron to fertilize the ocean believe that carbon can be stored effectively for hundreds of years if it stays in the deep ocean, even though it is not permanent. Under ideal conditions, adding iron to the ocean could reduce global warming by about 0.3W/m², which might offset 15–20% of current human-caused carbon dioxide emissions. This method involves adding iron to parts of the ocean that lack nutrients to encourage phytoplankton growth. However, this approach is debated because it may harm marine ecosystems.
Research shows that adding large amounts of iron to the ocean can disrupt the balance of nutrients, causing problems for marine life and threatening species that depend on stable nutrient cycles. Too much iron might change the types of plankton that grow, reducing the variety of species needed for a healthy ocean. Iron fertilization can also cause large phytoplankton blooms. When these blooms die and break down, they may create areas with very low oxygen levels, harming marine life. In some cases, iron fertilization has led to harmful algae blooms that produce toxins dangerous to marine animals and humans. Experiments in the Southern Ocean, such as SOFeX, showed that adding iron can cause harmful algae to grow quickly, which could damage local ecosystems.
In addition to environmental risks, the effectiveness of storing carbon 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 carbon storage varies based on factors like ocean currents and temperature. Changes in ocean chemistry or marine life populations might reduce how well iron fertilization works to fight climate change.
Methods
There are two main methods for artificial iron fertilization: using ships to add iron directly into the ocean and spreading iron into the atmosphere.
Experiments that add iron sulphate directly to the ocean's surface using ships are explained in more detail in the experiment section below.
Iron-rich dust in the air is a main source of iron for the ocean. For example, dust from the Sahara Desert carried by wind helps the Atlantic Ocean and the Amazon Rainforest. Iron oxide in the dust interacts with hydrogen chloride from sea spray to form iron chloride. This substance helps reduce methane and other greenhouse gases, makes clouds brighter, and falls with rain in small amounts over large areas of the world. No experiments have been done to increase the natural level of iron in the atmosphere. Improving this natural source could work alongside ship-based methods.
One idea is to increase atmospheric iron by using iron salt aerosol. Adding Iron(III) chloride to the troposphere might enhance natural cooling effects, such as reducing methane, brightening clouds, and fertilizing the ocean, which could help stop or reverse global warming.
Experiments
John Martin, a scientist, believed that increasing the amount of phytoplankton in the ocean could help reduce global warming by capturing carbon dioxide from the air and storing it in the sea. He died before a test called Ironex I could be completed. This test was later done in 1993 near the Galapagos Islands by scientists from Moss Landing Marine Laboratories. After this, 12 international studies looked into the effects of adding iron to the ocean.
John Martin, who led Moss Landing Marine Laboratories, thought that low levels of phytoplankton in certain areas were caused by a lack of iron. In 1989, he tested this idea by adding iron to samples of clean water from Antarctica. The samples with iron showed much more growth of phytoplankton than those without iron. This made Martin think that adding iron to the ocean might explain why past ice ages happened.
A larger experiment called Ironex I followed. Scientists added 445 kilograms of iron to an area of the ocean near the Galapagos Islands. Phytoplankton levels in this area increased three times compared to areas without iron. The success of this experiment and others led to ideas about using iron to remove carbon dioxide from the air.
In 2000 and 2004, iron sulfate was used in an experiment called EisenEx. About 10 to 20 percent of the algae that grew in the area died and sank to the ocean floor.
A company called Planktos planned to carry out six iron fertilization projects between 2007 and 2009. Each project would have spread up to 100 tons of iron over 10,000 kilometers of ocean. Their ship, Weatherbird II, was not allowed to enter a port in the Canary Islands to get supplies and equipment.
In 2007, companies like Climos, GreenSea Ventures, and Ocean Nourishment Corporation planned to use iron fertilization. These companies asked for financial support from groups that wanted to reduce carbon emissions by offering carbon credits.
An experiment called LOHAFEX was started in 2009 by German scientists with help from India. They added 6 tons of ferrous sulfate to an area of 300 square kilometers in the South Atlantic. Scientists hoped that this would cause an algal bloom, which would capture carbon dioxide and sink to the ocean floor. However, some environmental groups warned that this might harm marine life or cause long-term problems.
In 2012, scientists added iron near Antarctica to test how it affects the ocean. A large amount of carbon was expected to sink to the deep ocean, where it could stay for centuries. After 24 days, levels of nutrients used by algae dropped, and organic matter, including algae, increased in the surface water. Later, much of this organic matter sank to the ocean floor. Each iron atom helped create at least 13,000 carbon atoms in algae. At least half of this organic matter sank below 1,000 meters.
In July 2012, the Haida Salmon Restoration Corporation spread 100 tons of iron sulfate into the Pacific Ocean near Haida Gwaii. The Old Massett Village Council paid for the project with $2.5 million. The goal was to help salmon by increasing phytoplankton, which would act as food for the fish. The project’s leader, Russ George, hoped to sell carbon credits to cover costs. However, some scientists criticized the project for being unscientific and risky. Others said the iron amount was small compared to natural ocean processes.
Some environmental groups called the action a violation of international rules. George claimed that the Old Massett Village Council and its lawyers approved the project, and several Canadian agencies knew about it. George said salmon numbers increased from 50 million to 226 million in 2013, but many experts said other factors might have caused the change. Most data from the experiment are considered unreliable.
In 2014, data from the Haida project were shared publicly.
In 2022, scientists from the UK and India planned to place iron-coated rice husks in the Arabian Sea to test if less iron could create an algal bloom. The rice husks were kept in plastic bags that stretched from the ocean surface to the seafloor. Scientists hoped the iron would help algae grow, which would support the marine food chain and store carbon. However, a storm destroyed the experiment, and the results were unclear.
Science
The maximum possible result from iron fertilization, under the best conditions, is 0.29 W/m² of global cooling. This cooling could reduce 1/6 of today's human-caused CO2 emissions. However, some research suggests that adding iron to the ocean might cause other nutrients in seawater to become scarce, which could reduce the growth of phytoplankton in other areas. This means that iron may only affect phytoplankton growth in specific locations rather than globally.
Ocean fertilization happens naturally when deep, nutrient-rich water rises to the surface. This occurs near ocean banks or underwater mountains. These natural events create the world's largest marine habitats. Fertilization can also happen when wind carries dust over long distances across the ocean or when iron-rich materials from glaciers, rivers, or icebergs enter the ocean.
About 70% of Earth's surface is covered by oceans. The parts of the ocean where sunlight can reach are home to algae and other marine life. In some areas, 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 ocean areas by dust storms from dry regions. This dust contains 3–5% iron, and its spread has decreased by nearly 25% in recent decades.
The Redfield ratio describes the balance of nutrients in plankton. It is usually written as "106 C: 16 N: 1 P." This means 106 carbon atoms, 16 nitrogen atoms, and 1 phosphorus atom are needed for every plankton cell. Research later added iron to this ratio, showing that in areas with little iron, each iron atom can help fix 106,000 carbon atoms. On a mass basis, each kilogram of iron can help remove 83,000 kilograms of carbon dioxide. The 2004 EIFEX experiment found a ratio of nearly 3,000 kilograms of carbon dioxide removed for every 1 kilogram of iron added. The atomic ratio would be approximately "3,000 C: 58,000 N: 3,600 P: 1 Fe."
Small amounts of iron, measured in parts per trillion, in HNLC zones can cause large phytoplankton blooms. For example, 100,000 kilograms of plankton can grow for every kilogram of iron added. The size of iron particles is important. Particles smaller than 0.5–1 micrometer are ideal because they are easier for phytoplankton to use and stay near the ocean's surface where sunlight is available. One way to add iron to HNLC zones is through Atmospheric Methane Removal.
Iron is also delivered to the ocean through atmospheric deposition. Satellite images and data, combined with wind tracking, have identified natural sources of iron-rich dust. This dust comes 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.
Chemical reactions in the air change how iron in dust behaves, which can affect how much iron is available for marine life. The soluble form of iron is much higher in airborne dust than in soil (~0.5%). Some chemical interactions with organic acids in the air increase iron solubility. For example, sunlight can cause iron in certain minerals to change from Fe(III) to Fe(II), which is more easily used by marine life. Studies show that Fe(II) levels are higher during the day than at night.
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. Increases in biogenic opal in ocean sediments are linked to higher iron levels over the last million years. In 2008, an eruption in the Aleutian Islands deposited ash in the nutrient-poor Northeast Pacific, leading to one of the largest phytoplankton blooms observed in the subarctic.
Past events where plankton removed large amounts of carbon from the atmosphere caused significant cooling, such as the Azolla event. Plankton that form calcium or silicon carbonate skeletons, like diatoms, coccolithophores, and foraminifera, play a major role in storing carbon. When these organisms die, their skeletons sink to the deep ocean and become part of marine snow, which includes fish waste and other organic material.
About half of the carbon produced by plankton blooms is eaten by other ocean creatures, such as zooplankton and small fish. However, 20–30% of this carbon sinks below 200 meters into colder, deeper waters. Much of this carbon continues to the ocean floor, but some is broken down and released back into the water. At this depth, the carbon is carried by deep ocean currents and remains isolated from the atmosphere for centuries.
Measuring the effects of plankton blooms and the amount of carbon stored requires many methods, including ship-based sampling, underwater traps, satellite data, and tracking buoys. Unpredictable ocean currents can move iron from experimental areas, making it hard to study the effects.
The potential of iron fertilization to reduce global warming is shown by calculations. If phytoplankton used all the nitrate and phosphate in the surface waters of the Antarctic Circumpolar Current to create organic carbon, the ocean could absorb 0.8 to
Financial opportunities
Starting with the Kyoto Protocol, several countries and the European Union created carbon offset markets that trade certified emission reduction credits (CERs) and other carbon credit tools. In 2007, each CER was worth about €15 to €20 per ton of CO2. Iron fertilization is less expensive than methods like scrubbing, direct injection, and other industrial techniques, and it could theoretically store carbon for less than €5 per ton of CO2, offering a large financial benefit.
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 6–12% drop in global plankton production since 1980. A full-scale plankton restoration program could restore about 3–5 billion tons of carbon storage capacity, valued at €50–100 billion in carbon offset benefits. However, a 2013 study found that the costs of iron fertilization compared to its benefits make it less effective than carbon capture and storage and carbon taxes.
Debate
Ocean iron fertilization might help slow global warming, but scientists are still discussing how well this method works and whether it could cause harm. The precautionary principle is a rule for protecting the environment. A 2021 article explained that this principle says, "If human actions could possibly cause serious harm, steps should be taken to prevent or reduce that harm. Uncertainty should not stop action." 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 people oppose using iron fertilization widely until more data is available.
Some scientists worry that adding iron to the ocean might cause harmful algal blooms. These blooms can happen because certain toxic algae grow better when iron is added. A 2010 study in an area of the ocean with high nitrogen and low chlorophyll found that a type of algae called Pseudo-nitzschia diatom spp. began producing large amounts of a harmful toxin called domoic acid. Even short-lived blooms with this toxin could damage marine life.
Most phytoplankton are not harmful and are important because they form the base of the ocean food chain. Iron fertilization increases phytoplankton growth only in open ocean areas where iron is very limited. Coastal waters already have enough iron, so adding more does not help. Studies show that iron fertilization can increase the breakdown of organic matter, which changes the types of plankton produced. This process does not help reduce carbon dioxide and may even increase it. A 2023 study found that iron fertilization might make climate change worse.
A 2010 study showed that adding iron to high-nitrate, low-chlorophyll areas can increase the production of toxic algae. The researchers said this raises "serious concerns" about whether large-scale iron fertilization is helpful or sustainable. Iron and nitrogen from whales and other marine life also support the ocean food chain and help store carbon for long periods.
A 2009 study used a computer model to test whether iron fertilization could reduce carbon dioxide and ocean acidity. The results showed that even in extreme conditions, iron fertilization had little effect on reducing ocean acidification. Since iron fertilization does not significantly lower carbon dioxide levels, it is unlikely to change ocean acidity much.
History
The importance of iron for phytoplankton growth and photosynthesis was first studied in the 1930s by Dr. Thomas John Hart, a British marine biologist working on the RRS Discovery II in the Southern Ocean. In his 1931 report titled "On the phytoplankton of the South-West Atlantic and Bellingshausen Sea, 1929-31," he suggested that certain ocean areas, which appeared rich in nutrients but had little phytoplankton or sea life, might lack iron. Hart revisited this idea in a 1942 paper, "Phytoplankton periodicity in Antarctic surface waters," but few scientists discussed the topic until the 1980s. At that time, oceanographer John Martin of the Moss Landing Marine Laboratories reignited interest in the subject through his research on ocean nutrients. His findings supported Hart’s earlier idea, and these areas came to be called "high-nutrient, low-chlorophyll regions" (HNLC).
In 1988, John Gribbin, a scientist, proposed that adding iron to the ocean could help reduce climate change. Later that year, John Martin famously said at Woods Hole Oceanographic Institution, "Give me a half a tanker of iron and I will give you an ice age." This statement inspired years of research into the topic.
Studies showed that iron shortages limited ocean productivity and could affect climate change. Strong support for Martin’s theory came in 1991 after Mount Pinatubo, a volcano in the Philippines, erupted. Environmental scientist Andrew Watson analyzed data from the 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 global atmospheric carbon dioxide levels and a rise in oxygen levels.
In 2008, the London Dumping Convention created a non-binding resolution (LC-LP.1(2008)) about ocean fertilization. It stated that activities involving ocean fertilization, except for scientific research, should be avoided as they conflict with the goals of the Convention. In 2010, the Contracting Parties to the Convention adopted an Assessment Framework for Scientific Research Involving Ocean Fertilization (LC-LP.2(2010)) to regulate waste dumping at sea.
Since the 1990s, multiple research teams, labs, and companies have tested ocean fertilization. In 1993, thirteen research groups conducted trials showing that adding iron to the ocean can stimulate phytoplankton growth. However, debates continue about whether this method effectively reduces atmospheric carbon dioxide and its effects on ocean ecosystems. Ocean trials of iron fertilization occurred in 2009 in the South Atlantic (LOHAFEX project) and in July 2012 in the North Pacific near British Columbia, Canada (Haida Salmon Restoration Corporation project).