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: biologic (also called biosequestration) and geologic.

Biologic carbon sequestration is a natural part of the carbon cycle. Humans can help it by making intentional changes and using technology. Carbon dioxide (CO₂) is naturally removed from the atmosphere through biological, chemical, and physical processes. These processes can be made faster by changing land use or farming practices, a method called carbon farming. Technology can also be used to capture and store CO₂ from human activities, a process called carbon capture and storage. This involves storing CO₂ underground or under the sea bed.

Plants take in carbon dioxide from the air as they grow and store it in their biomass. However, natural storage areas like forests and kelp beds may only hold carbon temporarily. Events such as wildfires, disease, or changes in land use can release stored carbon back into the atmosphere.

Carbon dioxide can also be stored in Earth’s crust by injecting it underground or by forming insoluble carbonate salts. This method is called mineral sequestration. These methods are considered safe because they store carbon permanently, locking it away for thousands to millions of years.

To improve carbon sequestration in the ocean, scientists have proposed methods like ocean fertilization, artificial upwelling, basalt storage, mineralization, deep-sea sediments, and adding bases to neutralize acids. However, these methods have not been used on a large scale yet. Large-scale seaweed farming is a biological process that 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 that more research is needed on seaweed farming as a way to reduce carbon emissions.

Terminology

The term carbon sequestration has different meanings in books and articles. The IPCC Sixth Assessment Report says carbon sequestration is "the process of storing carbon in a carbon pool." A carbon pool is described as "a place in Earth's systems where elements like carbon and nitrogen stay in different chemical forms for some time."

The United States Geological Survey (USGS) defines carbon sequestration as "the process of capturing and storing atmospheric carbon dioxide." This definition is very similar to the definition of carbon capture and storage (CCS). The IPCC defines CCS as "a process in which a pure stream of carbon dioxide (CO₂) from industrial sources is separated, treated, and moved to a long-term storage location." Because of these similarities, carbon sequestration is sometimes confused with CCS.

Roles

Carbon sequestration is a natural process that moves carbon between Earth's atmosphere, oceans, soil, rocks, and living things. Carbon dioxide is 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 store it in their bodies. However, carbon stored in living things like forests and underwater plants can be released back into the air if events like fires, disease, or human actions happen.

Carbon sequestration helps reduce the effects of climate change by slowing 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 fight climate change, carbon sequestration can either strengthen natural carbon storage areas or use technology to capture and store carbon.

In carbon capture and storage methods, sequestration means the part where carbon is stored. Artificial storage can include putting carbon dioxide in deep underground areas, like saltwater layers or old gas fields, or combining it with materials to form stable solids.

For carbon to be stored artificially, it must first be captured or its release into the air must be stopped. This can happen by using carbon-rich materials in long-lasting products, like buildings, to avoid releasing carbon through burning or decay. These materials can be stored for many years or used in products over time. For example, wood from trees can be used in construction, storing its carbon for decades or even centuries. In factories, engineers often capture carbon dioxide from smoke or emissions.

In the United States, a 2021 rule called "Protecting Public Health and the Environment…" included steps to improve carbon storage by protecting areas like wetlands and forests. The rule highlighted the role of farmers, landowners, and coastal communities in storing carbon. It also asked the Treasury Department to support carbon storage through market-based methods.

A 2025 study found that Earth's ability to store carbon is limited. Using all of Earth's underground storage areas could help reduce global warming by about 0.7 degrees Celsius.

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 take in carbon dioxide during photosynthesis. Practices like reforestation and managing forests in a way that helps them grow more can increase how much carbon is stored. Changing how land is used, such as by protecting forests, wetlands, and grasslands, can help capture large amounts of carbon each year. These efforts also include improving soil health in farming and forestry to store more carbon.

Forests are important in the global carbon cycle because trees and plants absorb carbon dioxide through photosynthesis. This helps reduce the amount of greenhouse gases in the atmosphere, which is important for slowing climate change. Forests act as carbon sinks, meaning they store carbon in the form of plant parts like roots, stems, and leaves. Each year, forests store about 25% of human-made carbon emissions. Trees continue to store carbon throughout their lives, helping to reduce the effects of climate change. Managing forests carefully, planting new trees, and restoring forests are important ways to help the environment.

A key challenge is that forests can sometimes stop storing carbon and instead release it. 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 show that some forests are nearing a point where they may start releasing more carbon than they store. Scientists predict that tropical forests could become a source of carbon emissions by the 2060s.

Protecting forests is more effective than allowing them to be cut down and later regrown. Deforestation can cause long-term damage to soil and reduce biodiversity, and younger forests may release more carbon from the soil. In boreal forests, certain fungi can break down tree parts, increasing the chance of carbon being released. 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 mature forests, so protecting existing forests is a major solution for reducing climate change.

Planting trees on lands that are not ideal for farming can help store carbon in plant matter. To be effective, this carbon must stay in the plants and not return to the air when the trees die or burn. Some types of Ficus trees, like Ficus wakefieldii, store carbon in the form of 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. Another example is the Iroko tree, which can store up to one ton of calcium carbonate in the soil over its lifetime. Cacti, like the Saguaro, also help move carbon from living plants to the soil by forming calcium carbonate.

There is enough land available to plant about 0.9 billion hectares of trees, though some experts say the actual area that helps cool the climate is smaller. If these trees survive and grow to maturity, they could store 205 billion tons of carbon. This is about 20 years of global carbon emissions as of 2019 and would cover about 25% of the atmosphere's carbon in 2019.

Forests around the world have different lifespans, depending on the tree species and environmental conditions. Some forests store carbon for hundreds of years, while others release it quickly due to fires. When forests are cut down before major fires occur, some carbon is stored in products like wood and buildings. However, much of the carbon from logged forests ends up in products like paper and pallets. If construction used more wood products, especially mass timber, it could store 700 million tons of carbon each year. This would also reduce emissions from using materials like steel or concrete, which are carbon-heavy to produce.

Studies show that planting forests with a mix of tree species can increase carbon storage and provide other benefits. While bamboo forests store less carbon than mature forests, they absorb carbon more quickly. This makes bamboo a promising option for carbon sequestration.

The Food and Agriculture Organization (FAO) reported that 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 more carbon and support biodiversity.

The impact of forests on temperature depends on where they are planted. For example, planting trees in cold regions like boreal forests has less effect on climate because the dark forest canopy absorbs more sunlight than the snowy, reflective ground. In contrast, planting trees in tropical regions can create more clouds, which reflect sunlight and lower temperatures.

In tropical areas with wet seasons, trees grow faster and absorb more carbon because they can grow all year. Tropical trees typically have larger, brighter leaves than trees in other regions. A study of 70,000 trees in Africa found that tropical forests absorb more carbon dioxide than previously thought. Research suggests that tropical forests in Africa, the Amazon, and Asia absorb about 18% of the carbon dioxide from burning fossil fuels each year, helping to slow climate change.

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 into a special liquid-like form called a supercritical fluid. This form can be sent through pipes to the storage site. Once there, the CO₂ is injected deep underground, usually about 1 kilometer (0.6 miles) below the surface. At this depth, the CO₂ remains stable for hundreds to millions of years. The density of this supercritical CO₂ is between 600 and 800 kilograms per cubic meter.

To choose a good storage site, scientists look at several factors: the ability of rocks to hold space (porosity), how easily fluids can flow through rocks (permeability), the absence of cracks or faults in the rock layers, 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 up to about 30%. These rocks must be covered by a layer with very low permeability, such as shale, which acts as a seal to keep the CO₂ trapped. If CO₂ is injected into a rock layer, it will rise due to its lower density until it hits the seal. If there are cracks or faults near the injection site, CO₂ might escape through them and reach the surface, which could be dangerous. Another risk is induced seismicity, where high underground pressure from CO₂ injection could cause rocks to break, possibly leading to earthquakes.

The main way CO₂ is stored underground is through structural trapping. Impermeable rocks, such as mudstone, anhydrite, halite, or certain carbonates, block CO₂ from rising. Once trapped, CO₂ can exist as a supercritical fluid, dissolve in groundwater or brine, or react with minerals in the rock to form carbonates.

Research in 2025 found that out of about 12,000 gigatonnes of CO₂ that could theoretically be stored underground, only 1,460 gigatonnes are considered safe for storage, which is much less than earlier estimates.

Mineral sequestration involves trapping CO₂ in solid carbonate salts. This process happens slowly in nature, like how limestone forms over millions of years. Carbonic acid in groundwater reacts with minerals in rocks, dissolving some materials and leaving behind clay. The dissolved calcium and magnesium then combine with bicarbonate to form carbonates, such as those found in shells. When organisms die, their shells become sediment and eventually turn into limestone. Scientists are trying to speed up these reactions using materials like alkali carbonates.

Zeolitic imidazolate frameworks (ZIFs) are materials similar to zeolites. Because they are porous, chemically stable, and heat-resistant, they are being studied for their ability to capture CO₂.

CO₂ reacts with metal oxides, such as those in olivine, to form stable carbonates like calcite or magnesite. This process, called "CO₂-to-stone," happens naturally over time and is responsible for much of Earth's surface limestone. Rocks rich in metal oxides, like those found in basalt, can store CO₂ effectively. Scientists are exploring ways to speed up this process using catalysts, higher pressures, or special treatments, though these methods may require extra energy.

Ultramafic mine tailings, which are rich in metal oxides, could be used for this purpose. Microbial processes that help break down minerals and form carbonates might also be used to speed up natural CO₂ storage.

Carbon dioxide can be removed from the air through chemical reactions and stored as stable carbonate minerals. This process, called mineral sequestration, involves reacting CO₂ with metal oxides like magnesium oxide (MgO) or calcium oxide (CaO) to form carbonates. These reactions release heat and occur naturally, such as when rocks weather over time.

Calcium and magnesium are often found in minerals like forsterite and serpentine, not as pure oxides. When these minerals react with CO₂, they form carbonates. These reactions are easier at lower temperatures and occur naturally over long periods, forming much of Earth's limestone. Scientists can speed up the process by using higher temperatures or pressures, though this requires extra energy. Alternatively, grinding minerals into smaller pieces and exposing them to water and abrasion, like what happens on beaches, can also help.

When CO₂ is dissolved in water and injected into hot basaltic rocks underground, it reacts with the basalt to form solid carbonate minerals. A test project in Iceland, started in October 2017, captures up to 50 tons of CO₂ each year from the air and stores it in basaltic rock underground.

Sequestration in oceans

Several new companies are trying to do this on a large scale.

The ocean stores carbon through many processes. One process, called the solubility pump, moves carbon dioxide from the air into the ocean’s surface. There, it reacts with water to form carbonic acid. Carbon dioxide dissolves more easily in colder water. Another process, thermohaline circulation, moves dissolved carbon dioxide to cooler, deeper waters where it becomes more soluble, increasing carbon levels in the ocean’s interior. A third process, the biological pump, moves dissolved carbon dioxide from the ocean’s surface to its interior. This happens when photosynthesis converts inorganic carbon into organic carbon. Organic matter that survives breakdown can sink to the deep ocean through particles or the movement of organisms.

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 and helps store it for long periods.

Blue carbon refers to carbon stored in marine ecosystems that can be managed to help reduce climate change. It often involves tidal marshes, mangroves, and seagrass meadows, which capture carbon. These ecosystems help fight climate change, but if they are destroyed, they release stored carbon 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 stays buried for thousands of years. Growing seaweed offshore and sinking it into the deep ocean to store carbon has been suggested. Seaweed grows quickly and can be harvested to make biomethane through digestion, used to generate electricity, or replace natural gas. One study suggests that if seaweed farms covered 9% of the ocean, they could supply Earth’s energy needs, remove 53 gigatonnes of CO₂ yearly, and provide enough fish for 10 billion people. Ideal seaweed species for farming include Laminaria digitata, Fucus serratus, and Saccharina latissima.

Both large and tiny algae are being studied for carbon storage. Marine phytoplankton, though making up only 1% of global plant life, perform half of the world’s photosynthesis and oxygen production. Algae lack the complex structures found in land plants, so their carbon is released into the atmosphere faster than land-based carbon. Algae are being explored as a short-term carbon storage option for producing biogenic fuels.

Large-scale seaweed farming could store significant carbon. Natural seaweed also stores carbon as organic matter sinks to the deep ocean floor, where it remains buried for long periods. For carbon farming, harvested seaweed could be transported to the deep ocean for long-term burial. Seaweed farming is growing rapidly in Asian Pacific coastal areas. The IPCC recommends further research on seaweed farming as a climate solution.

Ocean fertilization involves adding nutrients like iron to the ocean’s surface to help phytoplankton grow. This process mimics natural events like volcanic eruptions or whale activity. Adding nutrients increases marine life and removes carbon dioxide from the atmosphere through photosynthesis. Experiments show that adding iron can boost phytoplankton growth by up to 30 times.

Ocean iron fertilization is a well-studied method for removing carbon dioxide from the atmosphere. However, scientists are unsure how long the stored carbon remains in the ocean. A 2021 study suggests this method has high potential for reducing carbon.

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. These blooms store carbon while alive and release it when they die. However, this method temporarily increases surface CO₂ levels, which can harm marine life and lower seawater pH, affecting coral reefs and ocean ecosystems.

Storing carbon in underwater basalt involves injecting CO₂ into deep ocean rock. The CO₂ reacts with seawater and basalt, forming stable minerals. Basalt offers strong protection against CO₂ leakage through chemical, sediment, and gravitational barriers. Injecting CO₂ below 2,700 meters ensures it sinks due to its higher density than seawater.

Costs

The cost to store carbon (not including capturing or moving it) varies, but is sometimes less than US$10 per tonne when onshore storage is available. For example, a method called Carbfix costs about US$25 per tonne of CO₂. A 2020 report estimated that storing carbon in forests (which includes capturing it) costs between US$35 and US$280 per tonne, depending on how much is needed to help limit warming to 1.5°C. However, there is a risk that forest fires could release stored carbon. Research suggests that compensating for the total carbon reserves of the 200 largest fossil fuel companies would cost between 11% and 701% of global GDP, depending on the price of carbon in the market.