Carbon sequestration

With increasing levels of carbon dioxide in the atmosphere, the removal and storage of atmospheric carbon is an important field of study. Many of the major storage areas—terrestrial, geological, and aquatic—are all interconnected, and continued study is necessary to understand the impact of increased concentrations of carbon dioxide on these systems. New technologies to capture and use carbon dioxide can lead to real economical advantage if implemented correctly.

Introduction

Since the advent of the Industrial Revolution, the input of large amounts of carbon dioxide from human-derived sources into the atmosphere has increased. The annual rate of anthropogenic carbon dioxide release from energy-related production worldwide was around 36.8 billion metric tons in 2022, according to the International Energy Agency (2023), and is expected to grow as more countries industrialize. Scientists claim that this large increase in carbon dioxide in the atmosphere has had a detrimental effect on the climate. Consequently, several international policies have been created to mitigate carbon release, including the Kyoto Protocol, which addresses emissions of carbon dioxide and other greenhouse gases, and the Paris Agreement, which is a binding agreement by participating nations to limit global warming by reducing greenhouse gas emissions. One way to address the large amount of carbon being released into the atmosphere is to capture and store, or sequester, it.

Carbon sequestration is the removal and storage of carbon from the atmosphere. This can be accomplished naturally, as wetlands and forests absorb carbon, or by way of anthropogenic techniques, such as carbon capture and storage (CCS) power plants where the carbon-capture technology is built in. A number of hurdles must be overcome to achieve carbon storage on the scale needed to compensate for the anthropogenic source production. Successful carbon-storage processes are benign and inert to the environment, sufficiently long term, and commercially viable. Various natural and anthropogenic sources of carbon storage exist and are being developed, as are methods of capturing carbon dioxide at the source of production.

Vegetation and Soil as a Source of Carbon Sequestration

In the natural world, there are various reservoirs that provide both long- and short-term storage of carbon. A large reservoir of carbon can be found in plants and soil. Plant mass is estimated to hold about 600 billion metric tons of carbon worldwide; soil is estimated to store slightly more than twice that amount, between 1.4 and 1.65 trillion metric tons. A large percentage of the soil-based reserve of carbon is found in peat, which is rich in carbon-containing compounds. Terrestrial agents of carbon-dioxide removal from the atmosphere, such as crops, forests, and wetlands, account for between 1.5 and 2 billion metric tons of carbon removed per year. The terrestrial storage can be either long or short term, depending on the type of plant and what occurs after the carbon is fixed into the plant matter.

Different ecosystems store carbon at different rates. Young forests tend to store carbon faster, but older-growth forests tend to have more carbon stored in both plant matter and soils. After a few thousand years, most reclaimed forests have equilibrated to a steady state of carbon storage where the input equals the output. With levels of carbon dioxide rising, the impact on forest growth is still under investigation. Studies have been conducted on the impact of higher concentrations of atmospheric carbon dioxide on plant growth, with evidence suggesting that while initial growth is faster in most cases, carbon storage is often limited by other important nutrients such as nitrogen, phosphorus, or iron.

One important consideration of terrestrial carbon storage is how to limit disturbances of previously stored carbon. Much of the terrestrially stored carbon can be found in old-growth forests, both tropical and temperate, and in peat reserves found in wetlands. Limiting disturbances and increasing the size of these carbon sinks (natural or artificial reservoirs that store carbon) can increase, or at least slow down, carbon loss. Large animals, human interactions, and forest fires are some of the most common disturbances to terrestrial carbon storage. For example, changes in agricultural practices can affect carbon storage. Limiting the destruction of old-growth forest to make way for agriculture will maintain the large carbon reservoir, and evidence suggests that retaining crop residue and increasing soil nutrients can increase carbon levels in the soil by 30 to 50 percent in land that is already dedicated to farming. Studies also have shown that limiting oxidation of organic carbon in the soil will increase the overall carbon storage, as oxidized carbon is more likely to degrade back to carbon dioxide. Limiting oxidation can be accomplished by the burial of plant matter, such as trees or crop residues, so they are not in contact with the atmosphere. The burial of the plant matter also allows for the overall carbon content of the soil to increase. Further research is necessary to quantify the best practices for retaining or increasing carbon storage in plant material and soil.

Water as a Source of Carbon Sequestration

Carbon dioxide is very soluble in water under certain conditions, including lower temperature and higher pressure, and large amounts of carbon can be and are stored in water. The stored carbon exists in water as carbon dioxide, carbonic acid, carbonates, and bicarbonate compounds. Two major sources of aquatic carbon storage are being studied: oceans, which account for significant carbon storage by sheer volume; and underground aquifers, which can contain high concentrations of carbon.

The ocean already is a large dynamic reservoir of carbon dioxide from the atmosphere. Researchers estimate that the ocean absorbs about 2.3 billion metric tons of carbon yearly. The dissolved carbon plays an important role in aquatic life, as many marine species depend on soluble carbonate compounds for growth. However, the increase in these acidic carbon compounds has led to an acidification of the ocean. Such acidification can lead to poor solubility of compounds such as calcium carbonate, which is an important compound for marine organisms.

Some ideas have been put forth regarding how to use the ocean to store carbon in a more active manner. One method suggested is to inject the carbon dioxide deep into the ocean. After being injected, the carbon dioxide would theoretically follow a number of different paths. It may dissolve and be absorbed by the seawater, create large deepwater carbon-dioxide lakes, or become trapped in sediments or clathrates. Also called gas hydrates, clathrates are crystallized structures of trapped gases and water that form under certain conditions of temperature and pressure—conditions that can be found deep in the ocean, below about five hundred meters. The effect of such large carbon dioxide injections is still unknown.

Another idea for deep-sea carbon storage is to increase the amount of phytoplankton in the oceans, thus increasing the fixation of carbon dioxide by the phytoplankton and, subsequently, the marine animals that eat them. Often, it is the lack of proper nutrients that retards phytoplankton growth; fertilization of the ocean could be a solution. The carbon would become fixed in the new phytoplankton until they were eaten, at which point some of the fixed carbon would transform into carbon dioxide and dissolve in the deep ocean, while the rest would settle on the sea floor. Researchers at the Lawrence Berkeley National Laboratory have conducted studies on the fate of both organic and inorganic carbon in the ocean.

A second possibility for water-based carbon storage is in deep saline aquifers. Because of the great pressures found in the depths and the secluded nature of these aquifers, researchers postulate that a large amount of carbon dioxide can be stored in them for long periods. Researchers are looking into some of the possible pitfalls of deep aquifer storage. It is known that upon dissolving in the water, there is an acidification and sequential dissolving of carbonate rocks; it is not known if the dissolved compounds remineralize as other carbon-containing compounds and what impact that would have on the flow patterns found in the aquifer. Also, not much is known about where the carbon dioxide goes once it is injected. Many of these questions are being addressed by some of the first injection test sites in Norway and Canada that are capturing carbon dioxide, injecting it into deep saline aquifers, and studying the resulting data.

Reinjection into Oil, Gas, and Coal Fields

Much research has been already done on using depleted oil and gas deposits as carbon sequestration sites. In addition to sequestering the carbon, injecting carbon dioxide into natural gas and oil wells can sometimes make the gas or oil field more productive by displacing the removed material with carbon dioxide. Carbon dioxide has been used in this manner, known as the enhanced oil recovery method, since the late 1970s. The carbon dioxide helps maintain the pressure of the reservoir, which reduces subsidence. It is hypothesized that between 120 and 150 billion metric tons of carbon could be stored in depleted oil and gas fields. The concern with this injection strategy is the feasibility of bringing concentrated carbon dioxide to the site of injection.

Coal mines are also good storage points for carbon sequestration. Carbon dioxide binds strongly to coal and will displace methane when injected into a defunct coal bed. This process could be advantageous, as the displaced methane can be collected and used for energy production and the potential reservoir of coal beds is thought to be between 50 and 200 billion metric tons of carbon.

Anthropogenic Carbon Capture and Storage

An important step in reducing carbon dioxide is reducing emissions from large producers, such as energy plants and industry. The best strategy for carbon capture and storage is to create techniques that can be used with point-source emitters, where the carbon dioxide is produced. A major source of carbon dioxide release is the process of energy production. During power production, the combustion of carbon-rich fuels leads to the release of carbon dioxide. Different types of power plants release different amounts of carbon into the atmosphere. Coal-based plants are the main producers of carbon dioxide because their fuel source is rich in carbon and they are not very efficient in converting the fuel into energy. Other fuel sources, such as natural gas, produce less carbon, but capturing that carbon is more complex. Much of the cost of carbon-capture technology is tied to retrofitting old plants or building new plants to use these advanced carbon-capture techniques.

New power plants can be built that use techniques that are both more energy- and carbon-capture efficient. For example, new coal-gasification plants, known as integrated gasification combined-cycle (IGCC) plants, turn coal into hydrogen gas and carbon monoxide. The produced gas can then be scrubbed of the carbon monoxide, a process that produces carbon dioxide. Next, the two gases can be separated to produce a pure stream of hydrogen gas to be used for energy production and a pure stream of carbon dioxide to be concentrated and stored.

Both old and new point-source emitters use various techniques to capture carbon dioxide. Both pre- and post-combustion carbon capture are important tools in reducing emissions. During pre-combustion capture, the fossil fuels themselves are scrubbed of carbon dioxide; post-combustion processes remove carbon dioxide from the waste stream. The captured carbon dioxide is then concentrated and compressed for storage and transportation. A number of techniques are used to capture carbon dioxide at the source of emission. Amine solvents and cold methane have a high affinity for carbon dioxide and will take it up and remove it from the gas stream. Various membranes and activated carbon are also used. Another method of carbon capture is the mineralization process, starting with a material such as calcium oxide that will react with carbon dioxide to make calcium carbonate (limestone).

These techniques all produce a concentrated mixture of carbon along with the carrier molecule. Many of these mixtures can then be recycled by heating the mixture to drive off a stream of carbon dioxide that is then concentrated and compressed for transport. For example, after calcium carbonate is produced, the compound can be transported to a kiln. At the kiln, the reaction can be reversed, with the carbon dioxide that is released being concentrated for transport and the calcium oxide being reused to capture more carbon dioxide. All these carbon-capture processes have an energy cost, the amount of which depends on the capture technique being used and the concentration of the carbon in the waste stream. Different strategies are being explored to reduce these costs, which should go down as new techniques are discovered.

The same carbon-capturing techniques can be implemented with other point-source emitters. Industry produces about 15 percent of all carbon dioxide during the production process. Because many of these plants produce the carbon dioxide directly from energy production, similar solutions would work on these processes as well.

Transportation and Storage

Carbon-capture technologies are constantly being explored, but also of significant importance is the transportation and subsequent storage or use of the carbon dioxide. Fifty percent of the cost of carbon capture and storage comes from the capture step; the other 50 percent comes from transportation and storage. Finding a way to transport large volumes of liquefied carbon dioxide has sparked ideas such as using old gas pipelines for transport. Having carbon-storage options close to the point of emission would help control costs.

Outside of storage, alternative uses for carbon dioxide need to be found. If carbon-capture techniques are adopted on a large scale, a large amount of liquefied carbon dioxide will be available for use. By creating economical uses for the stored carbon dioxide, the cost of capture will be offset by the economic benefit of the final material. For example, creating plastic from carbon dioxide as a starting material would fix the carbon dioxide into a useful form. Using carbon dioxide to directly create biomass, such as algae farms, would create biomaterials that could be used as fuel or as starting materials for other products.

Summary

For carbon sequestration to be viable, it will need to come in many forms, as both natural and industrial processes. Researchers are continuing to develop ideas for enhancing natural processes by exploring the ability of the ocean, plants, and soil to absorb more carbon. Most important in this quest, though, is to ensure that this increased uptake of carbon does not produce a more dangerous environmental situation. Industrial methods of reducing carbon emissions, as well as recycling and storing the carbon produced, are also integral to this discussion.

Principal Terms

acidification: the increased presence of hydrogen or aluminum in water or soil, which decreases the water's pH

anthropogenic: caused by humans

carbon capture and storage (CCS): the capture of carbon dioxide from industrial processes and its storage in geological formations, biological organisms, bodies of water, or other carbon sinks

carbon sequestration: the removal of carbon from the atmosphere for storage

carbon sink: a reservoir of carbon that has been captured and stored for a short- or long-term period

clathrate hydrates or gas hydrates: crystal structures of gas molecules trapped or co-crystallized with water under conditions of high pressure and low temperature

enhanced oil recovery (EOR) method: the use of carbon dioxide to enhance oil recovery from depleted fields

point-source emitter: a power or industrial plant that produces carbon dioxide during the plant's operation

terrestrial: found on land

Bibliography

Anderson, Soren T., and Richard G. Newell. Prospects for Carbon Capture and Storage Technologies. Washington: Resources for the Fut., 2003. Resources for the Future. Web. 26 Aug. 2014.

"CO2 Emissions in 2022." International Energy Agency, Mar. 2023, www.iea.org/reports/co2-emissions-in-2022. Accessed 6 Feb. 2025.

Khatiwala, S., F. Primeau, and T. Hall. “Reconstruction of the History of Anthropogenic CO₂ Concentrations in the Ocean.” Nature 462.7271 (2009): 346–49. Print.

Lal, Rattan, et al., eds. Ecosystem Services and Carbon Sequestration in the Biosphere. Dordrecht: Springer, 2013. Print.

Lorenz, Klaus, and Rattan Lal. Carbon Sequestration in Forest Ecosystems. Dordrecht: Springer, 2010. Print.

Michael, Karsten, et al. “Geological Storage of CO₂ in Saline Aquifers: A Review of the Experience from Existing Storage Operations.” International Journal of Greenhouse Gas Control 4.4 (2010): 659–67. Print.

National Academy of Engineering and the National Research Council. The Carbon Dioxide Dilemma: Promising Technologies and Policies. Washington: Natl. Acads., 2003. Print.

Rackley, Stephen A. Carbon Capture and Storage. Burlington: Butterworth, 2010. Print.

Smit, Berend, et al. Introduction to Carbon Capture and Sequestration. London: Imperial Coll., 2014. Print.

"What Is the Paris Agreement?" United Nations, Climate Change, 2025, unfccc.int/process-and-meetings/the-paris-agreement. Accessed 6 Feb. 2025.

Wilcox, Jennifer. Carbon Capture. New York: Springer, 2012. Print.