Carbon sequestration is the natural process of storing carbon in a carbon pool. It is important for managing the global carbon cycle and reducing climate change by lowering the amount of carbon dioxide in the atmosphere. There are two main types of carbon sequestration: biological (also called biosequestration) and geological.
Biological carbon sequestration is a natural part of the carbon cycle. People can help speed up this process through actions and technology. Carbon dioxide (CO₂) is naturally removed from the air through biological, chemical, and physical processes. These processes can be made faster through changes in land use and farming practices, known as carbon farming. Technology is also used to capture and store CO₂ from human activities, a method called carbon capture and storage. This involves putting CO₂ underground or under the ocean floor.
Plants take in carbon dioxide from the air as they grow and store it in their bodies. However, natural storage areas like forests and kelp beds may not keep carbon locked away permanently. Events like wildfires, disease, economic challenges, or changes in government priorities can release stored carbon back into the atmosphere.
Carbon dioxide removed from the air can also be stored in Earth’s crust by injecting it underground or forming insoluble carbonate salts. This process is called mineral sequestration. These methods are considered safe because they remove carbon dioxide from the air and store it for thousands to millions of years.
To improve carbon sequestration in oceans, scientists have proposed methods like ocean fertilization, artificial upwelling, basalt storage, mineralization, deep-sea sediments, and adding bases to neutralize acids. However, these methods are not yet used on a large scale. Large-scale seaweed farming, a biological process, could store significant amounts of carbon. Harvested seaweed might be buried in the deep ocean for long-term storage. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate suggests more research on seaweed farming as a way to reduce carbon emissions.
Terminology
The term carbon sequestration has different meanings in scientific writing and media. The IPCC Sixth Assessment Report describes carbon sequestration as "the process of storing carbon in a carbon pool." Later, a carbon pool is explained as "a storage area in the Earth's system where elements, such as carbon and nitrogen, remain in different forms for some time."
The United States Geological Survey (USGS) defines carbon sequestration as "the process of capturing and storing atmospheric carbon dioxide." Because this definition is very similar to the definition of carbon capture and storage (CCS), carbon sequestration is sometimes confused with CCS. The IPCC defines CCS as "a process in which a clean stream of carbon dioxide (CO₂) from industrial sources is separated, treated, and moved to a long-term storage location."
Roles
Carbon sequestration is a part of the natural carbon cycle. In this cycle, carbon moves between the Earth's living parts, soil, rocks, water, and air. Carbon dioxide is naturally taken from the air through natural processes and stored in long-term places.
Plants take in carbon dioxide from the air as they grow and use it to build their bodies. However, natural storage areas like forests and kelp beds are not always reliable for long-term carbon storage. Events such as wildfires, disease, economic changes, or shifts in political goals can cause stored carbon to return to the air.
Carbon sequestration acts as a carbon sink, which helps reduce the effects of climate change. It slows the buildup of greenhouse gases, such as carbon dioxide, in the air and oceans. These gases are mainly released when fossil fuels are burned.
To help reduce climate change, carbon sequestration can either improve natural carbon storage areas or use technology to capture and store carbon.
In methods that capture and store carbon, sequestration refers to the storage part. Artificial storage methods include putting carbon dioxide into deep underground rock layers filled with saltwater or into old gas fields. Another method is to chemically react carbon dioxide with metal oxides to create stable solid materials.
For carbon to be stored artificially, it must first be captured or its release into the air must be delayed or stopped. This can be done by using carbon-rich materials in long-lasting products, such as buildings, which prevents carbon from being released through burning or decay. Once stored, the carbon can remain in these materials for many years or even centuries. For example, wood harvested from trees can be used in construction or other long-lasting products, keeping its carbon stored for a long time. In factories, engineers often capture carbon dioxide from smoke released by power plants or industrial facilities.
In the United States, Executive Order 13990, passed in 2021 and revoked in January 2025, mentioned carbon sequestration through protecting and restoring natural carbon storage areas like wetlands and forests. The order highlighted the role of farmers, landowners, and coastal communities in storing carbon. It also asked the Treasury Department to support conservation efforts using market-based methods.
A 2025 study noted that Earth's ability to store carbon is limited. It found that using all of Earth's underground storage space could help limit global warming by about 0.7 °C (1.3 °F).
Biological carbon sequestration on land
Biological carbon sequestration, also called biosequestration, is the process of capturing and storing carbon dioxide from the air through natural biological activities. This happens when plants and trees absorb carbon dioxide during photosynthesis, which is the process by which plants use sunlight to grow. Practices like reforestation (planting trees in areas where forests once grew) and managing forests carefully help increase this process. Changing how land is used to protect and restore ecosystems, such as forests, wetlands, and grasslands, can help capture large amounts of carbon dioxide each year. These methods also include improving soil health in agriculture and forestry to store more carbon.
Forests are important in the Earth's carbon cycle because trees and plants take in carbon dioxide from the air through photosynthesis. This helps reduce the amount of carbon dioxide in the atmosphere, which is a major cause of climate change. Forests act as carbon sinks, meaning they store carbon in the form of plant parts like roots, stems, and leaves. Every year, forests absorb about 25% of the carbon dioxide humans produce. Trees continue to store carbon throughout their lives, helping to slow climate change. Practices like managing forests carefully, planting new forests (afforestation), and restoring forests that have been cut down (reforestation) are important ways to reduce carbon emissions.
However, forests can sometimes become sources of carbon instead of sinks. For example, in 2019, forests absorbed about one-third less carbon than they did in the 1990s because of higher temperatures, droughts, and deforestation. Studies from 1999 to 2020 show that some forests are nearing limits where they might stop absorbing carbon and start releasing it. Scientists predict that tropical forests may become carbon sources by the 2060s.
Protecting forests is better than cutting them down and replanting later, because deforestation causes lasting harm to biodiversity and soil health. In younger boreal forests (forests in cold regions), there is a higher chance that carbon stored in the soil will be released. Some boreal forests support fungi that break down tree structures, increasing the risk of carbon release. Damage to tropical rainforests may have caused more greenhouse gas emissions than previously thought. It takes many years for newly planted forests to store as much carbon as older, intact forests. Scientists say that protecting and restoring natural forests is one of the most important ways to fight climate change.
Planting trees on lands that are not ideal for farming or grazing can help store carbon in plant material. For this to work, the carbon must stay in the trees and not return to the air when the trees burn or decay. Some types of Ficus trees, like Ficus wakefieldii, store carbon dioxide as calcium oxalate with the help of bacteria and fungi. This process also makes the soil more alkaline. These trees are being studied for use in agroforestry (farming with trees). A type of tree called the Iroko can store up to one ton of calcium carbonate in the soil over its lifetime. Cacti, like the Saguaro, also store carbon by forming calcium carbonate in the soil.
There is enough land on Earth to plant an additional 0.9 billion hectares of trees, though some experts say the actual area that helps cool the climate may be smaller. If these trees grow to maturity, they could store 205 billion tons of carbon, which is about 20 years of current global carbon emissions (as of 2019). This would be about 25% of the carbon in the atmosphere in 2019.
Forests can store carbon for different lengths of time depending on the type of trees, the environment, and natural events like fires. Some forests store carbon for centuries, while others release it quickly after fires. When forests are cut down before fires, some carbon is stored in products like wood and buildings. However, most of the carbon from logged forests is used for less durable items like paper and pallets. If 90% of new construction used wood instead of materials like concrete or steel, it could store 700 million tons of carbon each year. This would also reduce emissions from producing other materials.
A study found that planting forests with a mix of tree species can increase carbon storage and provide other benefits. While bamboo forests store less total carbon than mature forests, they absorb carbon more quickly. This means growing bamboo for timber could help store carbon effectively.
According to the Food and Agriculture Organization (FAO), the total carbon stored in forests decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020. In Canada's boreal forests, up to 80% of the carbon is stored in the soil as dead organic matter.
The IPCC Sixth Assessment Report states that restoring forests and other ecosystems can help store carbon and improve biodiversity.
The impact of forests on temperature depends on where they are planted. For example, planting forests in cold regions like boreal areas has less effect on climate because forests have a darker surface than snow-covered areas. In contrast, planting forests in tropical regions can help cool the air by forming clouds that reflect sunlight.
Trees in tropical areas with wet seasons grow faster and absorb more carbon because they can grow year-round. Tropical trees tend to have larger and more numerous leaves than trees in other regions. A study of 70,000 trees across Africa found that tropical forests absorb more carbon dioxide than previously thought. Research suggests that forests in Africa, the Amazon, and Asia absorb nearly one-fifth of fossil fuel emissions. Simon Lewis said, "Tropical forest trees absorb about 18% of the carbon dioxide added to the atmosphere each year from burning fossil fuels, which helps slow climate change."
Wetland restoration involves…
Geological carbon sequestration
Geological sequestration is the process of storing carbon dioxide (CO₂) underground in places like old oil and gas reservoirs, salty rock layers, or deep coal beds that are not useful for mining.
After CO₂ is captured from a source, such as a factory, it is compressed to about 100 bar, turning it into a special liquid form called a supercritical fluid. This form can be moved through pipelines to the storage site. Once there, the CO₂ is injected deep underground, usually around 1 kilometer (0.6 miles) below the surface, where it remains stable for hundreds to millions of years. In this state, the density of the CO₂ is between 600 and 800 kilograms per cubic meter.
To choose a good storage site, scientists look at several factors: the rock’s porosity (how much open space is in the rock), permeability (how easily fluids can move through the rock), the absence of cracks or faults in the rock, and the shape of the rock layers. Ideal storage rocks, like sandstone or limestone, have high porosity and permeability. For example, sandstone can have a permeability of 1 to 10 Darcy and a porosity of up to about 30%. The rock must also be covered by a layer with low permeability, called a caprock, which prevents the CO₂ from escaping. Shale is a good example of a caprock, with a permeability of 10⁻¹⁰ to 10⁻⁵ Darcy.
When CO₂ is injected underground, it moves upward because it is less dense than the surrounding rock. When it reaches a caprock, it spreads sideways until it finds a gap. If there are cracks or faults near the injection site, the CO₂ might escape through these cracks, potentially leaking into the atmosphere and harming the environment. Another risk is induced seismicity, which means that injecting CO₂ underground could create pressure that causes the rock to break, leading to small earthquakes.
Structural trapping is the main way CO₂ is stored. Impermeable rocks, such as mudstone, anhydrite, halite, or certain carbonates, act as barriers that stop the CO₂ from rising. Once trapped, the CO₂ can exist as a supercritical fluid, dissolve in groundwater or brine, or react with minerals in the rock to form solid carbonates.
In 2025, research showed that out of about 12,000 gigatons of CO₂ that could theoretically be stored underground, only 1,460 gigatons are considered safe for storage, which is much less than many scientists expected.
Mineral sequestration is a method that traps CO₂ by turning it into solid carbonate salts. This process happens naturally over long periods, like how limestone forms. In nature, carbonic acid in groundwater slowly reacts with minerals in rocks, dissolving calcium, magnesium, and other elements. These dissolved elements then combine with bicarbonate to form calcium and magnesium carbonates, which are used by organisms to make shells. When these organisms die, their shells become sediment and eventually turn into limestone. Over billions of years, this process has stored much of Earth’s carbon in limestone. Scientists are working to speed up similar reactions using alkali carbonates.
Zeolitic imidazolate frameworks (ZIFs) are materials similar to zeolites. Because they are porous, chemically stable, and can withstand high temperatures, ZIFs are being studied for their ability to capture CO₂.
CO₂ reacts with metal oxides, such as those found in olivine, to form stable carbonates like calcite and magnesite. This natural process, called "CO₂-to-stone," is responsible for much of the limestone on Earth’s surface. Rocks rich in metal oxides, such as those found in basalt, can store CO₂ permanently. Scientists are exploring ways to speed up this process, such as using catalysts, increasing pressure, or treating the minerals.
Ultramafic mine tailings, which are finely ground materials from mining, contain metal oxides that can react with CO₂. Using microbes to help break down these minerals and form carbonates could make this process more efficient.
Carbon can be removed from the atmosphere through chemical processes and stored as stable carbonate minerals. This method, called "carbon sequestration by mineral carbonation," involves reacting CO₂ with metal oxides like magnesium oxide (MgO) or calcium oxide (CaO) to form carbonates. These reactions release heat and occur naturally over long periods, such as when rocks weather over time.
Calcium and magnesium in nature are often found as silicates, like forsterite and serpentinite, rather than as simple oxides. When CO₂ reacts with these silicates, it forms carbonates. These reactions are more likely to occur at lower temperatures. While this process happens naturally over millions of years, it can be made faster by increasing temperature or pressure, though this requires extra energy. Alternatively, grinding the minerals to increase their surface area and exposing them to water and abrasion, such as by spreading olivine on beaches, can also help.
Tests have shown that when CO₂ is dissolved in water and injected into hot basaltic rocks underground, it reacts with the basalt to form solid carbonate minerals. In Iceland, a test plant started in 2017 and successfully stored up to 50 tons of CO₂ per year underground in basaltic rock.
Sequestration in oceans
Several start-ups are trying to do this at scale.
The ocean stores carbon through different processes. The solubility pump moves carbon dioxide from the air into the ocean's surface, where it combines with water to form carbonic acid. Cold water holds more carbon dioxide than warm water. Thermohaline circulation moves dissolved carbon dioxide to colder areas, where it becomes more soluble, increasing carbon levels in the deep ocean. The biological pump moves dissolved carbon dioxide from the ocean's surface to the deep ocean by converting inorganic carbon into organic carbon through photosynthesis. Organic matter that survives breakdown can sink to the deep ocean or be carried there by marine life.
In the deep ocean, low temperatures, high pressure, and less oxygen slow down the breakdown of organic matter. This prevents carbon from quickly returning to the atmosphere, acting as a long-term storage place.
Blue carbon refers to carbon stored in marine ecosystems that can be managed to help reduce climate change. These ecosystems include tidal marshes, mangroves, and seagrass meadows. They help store carbon and support climate adaptation. However, if these ecosystems are damaged or destroyed, they release stored carbon back into the atmosphere, increasing greenhouse gas emissions.
Seaweed grows in shallow and coastal areas and captures carbon. Some of this carbon can be carried to the deep ocean, where it remains stored for thousands of years. Growing seaweed offshore and sinking it to the deep ocean has been proposed as a way to store carbon. Seaweed grows quickly and can be harvested to produce biomethane through anaerobic digestion, generate electricity, or replace natural gas. One study suggests that if seaweed farms covered 9% of the ocean, they could produce enough biomethane to meet global energy needs, remove 53 gigatonnes of CO₂ annually, and provide enough fish for 10 billion people. Species like Laminaria digitata, Fucus serratus, and Saccharina latissima are ideal for farming.
Both large seaweed (macroalgae) and tiny algae (microalgae) are being studied for carbon storage. Marine phytoplankton perform about half of the world’s photosynthesis and oxygen production, even though they make up only about 1% of all plant life.
Algae do not have the complex lignin found in land plants, so their carbon is released into the atmosphere more quickly. Algae are being considered as a short-term storage for carbon that can be used to make biogenic fuels.
Large-scale seaweed farming could store significant amounts of carbon. Wild seaweed naturally stores carbon as organic matter sinks to the deep ocean and becomes buried for long periods. For carbon farming, harvested seaweed could be transported to the deep ocean for long-term storage. Seaweed farming is common in coastal areas of the Asian Pacific and has grown rapidly. A report by the IPCC suggests more research is needed on seaweed farming as a climate solution.
Ocean fertilization involves adding nutrients like iron to the ocean to encourage phytoplankton growth. This process mimics natural events like volcanic eruptions or whale activity, which historically helped remove CO₂ from the atmosphere. Adding nutrients increases marine life and reduces atmospheric CO₂ through photosynthesis.
Iron fertilization, a type of ocean fertilization, can boost phytoplankton growth by up to 30 times. Phytoplankton convert dissolved CO₂ into carbohydrates, some of which sink to the deep ocean. Over a dozen open-ocean experiments have confirmed this effect.
Ocean iron fertilization is a well-studied method for removing CO₂ from the atmosphere. However, scientists are unsure how long the stored carbon remains in the ocean. A 2021 study by the National Academies of Science, Engineering, and Medicine suggests this method has high potential for carbon removal.
Artificial upwelling or downwelling involves mixing ocean layers to move nutrients and gases. Large pipes could pump nutrient-rich deep water to the surface, causing algae blooms that store carbon during growth and release it when they die. This process is similar to iron fertilization. However, it may temporarily increase CO₂ levels on the surface, reducing its appeal.
Mixing ocean layers moves cold, dense water from the deep to the surface. As water cools with depth, more CO₂ dissolves in deeper layers. Using large pipes or mixers could reverse this process, bringing nutrient-rich water to the surface. This triggers algae blooms, which absorb CO₂ through photosynthesis. However, this method has not been widely used because algae blooms can harm marine life by blocking sunlight and releasing toxins. The sudden rise in CO₂ may also lower ocean pH, harming coral reefs and disrupting marine ecosystems.
Storing CO₂ in underwater basalt involves injecting CO₂ into deep-sea rock formations. The CO₂ mixes with seawater and reacts with basalt, forming stable carbonate minerals. Basalt is a good storage option because it has multiple natural barriers, such as geochemical reactions, sediment layers, and hydrate formation, that reduce the risk of CO₂ escaping. Injecting CO₂ below 2,700 meters ensures it is denser than seawater, causing it to sink and remain trapped.
Costs
The cost of storing carbon (without capturing or moving it) can be less than $10 per tonne in some places where storage on land is possible. For example, a method called Carbfix costs about $25 per tonne of CO₂. A 2020 report said that storing carbon in forests (including capturing it) costs between $35 and $280 per tonne, depending on how much is needed to keep global warming below 1.5°C. However, there is a risk that forest fires could release the stored carbon. Research shows that making up for the total carbon from the 200 largest fossil fuel companies would cost between 11% and 701% of global GDP, depending on the price of carbon in the market.