A carbon dioxide sink or CO₂ sink is a carbon reservoir that is increasing in size, and is the opposite of a carbon "source". The main natural sinks are the oceans and growing vegetation. The concept has become more widely known because of its role in the Kyoto Protocol.

Carbon sequestration is the term describing processes that remove carbon from the atmosphere. A variety of means of artificially capturing and storing carbon, as well as of enhancing natural sequestration processes, are being explored. This is intended to help mitigate global warming.

Natural sinks

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Forests

Enormous amounts of carbon are naturally stored in the forest by trees and other plants, as well as in the forest soil. As part of photosynthesis, plants absorb carbon dioxide from the atmosphere, store the carbon as sugar, starch and cellulose, while oxygen is released back into the atmosphere. A young forest, composed of rapidly growing trees, absorbs carbon dioxide and acts as a sink. Mature forests, made up of a mix of various aged trees as well as dead and decaying matter, may be carbon neutral above ground. In the soil however, the gradual buildup of slowly decaying organic material will continue to accumulate carbon, thereby acting as a sink.

Over the life of an individual tree or other forest plant, the carbon capturing (sequestering) and releasing is neutral. As the plant grows carbon is absorbed from the atmosphere and then released back into the atmosphere as the plant matures, dies, and rots. Most forests are a mix of old and new trees or plants, and carbon is stored and released continuously depending on the plant and the phase of its life at the time. Also, a severe forest fire will quickly release absorbed carbon back into the atmosphere. A more permanent sequestering of carbon from forests comes from the use of wood products such as "stick built" (i.e., with lumber) homebuilding, the predominate form of home building in the US. However most buildings are eventually demolished, and depending upon how the waste is disposed of this may again release the carbon.

The dead trees, plants, and moss in peat bogs undergo slow anaerobic decomposition below the surface of the bog. This process is slow enough that in many cases the bog grows rapidly and fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs inter approximately one-quarter of the carbon stored in land plants and soils ^*.

Under some conditions, forests and peat bogs may become sources of CO₂. This can happen, for example, when a forest is flooded by the construction of a hydroelectric dam. The rotting vegetation is a source of CO₂ and methane comparable in magnitude to the amount of carbon released by a fossil-fuel powered plant of equivalent power.

Oceans

Oceans are natural carbon dioxide sinks, and as the level of carbon dioxide increases in the atmosphere, the level in the oceans also increases, creating potentially disastrous acidic oceans. Ocean water can hold a variable amount of dissolved CO₂ depending on temperature and pressure. Phytoplankton in the oceans, like trees, use photosynthesis to extract carbon from CO₂. They are the starting point of the marine food chain. Plankton and other marine organisms extract CO₂ from the ocean water to build their skeletons and shells of the mineral calcite, CaCO₃. This removes CO₂ from the water and more dissolves in from the atmosphere. These calcite skeletons and shells along with the organic carbon of the organism eventually fall to the bottom of the ocean when the organisms die. The carbon or plankton cells have to sink to the deep water in 2000 to 4000 meter to be sequestered for ca. 1000 years. The sinking can be accelerated orders of magnitude when zooplankton prey on the cells and produce fast sinking fecal pellets or fecal strings, like the Antarctic krill. This process is called the biological pump. It has been theorized that the organic carbon within the accumulating ocean bottom sediments is how fossil fuels are created.

Enhancing natural sequestration

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Forests

Forests are carbon dioxide stores, but the sink effect exists only when they grow in size: it is thus naturally limited. The rate at which forests can sequester carbon, given the available land, is far exceeded by the rate at which it is released by the combustion of fossilised forests (coal, oil and natural gas). It seems clear that the use of forests to curb climate change can only be a temporary measure. Even optimistic estimates come to the conclusion that the planting of new forests is not enough to counter-balance the current level of greenhouse gas emissions To reduce U.S. carbon emissions by 7%, as stipulated in the Kyoto Protocol, would require the planting of "an area the size of Texas every 30 years", according to William H. Schlesinger, dean of the Nicholas School of the environment and earth sciences at Duke University, in Durham, N.C. [http://www.spectrum.ieee.org/WEBONLY/publicfeature/jan04/0104bio2.html.

Furthermore, forests, particularly new ones, may not be straightforward carbon sinks. Although a forest is a net CO₂ sink over time, the plantation of new forests may also initially be a source of carbon dioxide emission when carbon from the soil is released into the atmosphere. Other studies indicate that the cooling effect of removing carbon by forest growth can be counteracted by the effects of the forest on reflection of sunlight, or albedo. Mid-to-high latitude forests have a much lower albedo during snow seasons than flat ground, and this contributes to warming.

The planting of forests provides a number of additional benefits including reduction of erosion, increased water capture, and economic benefits when sustainably harvested.

Oceans

One of the most promising ways to increase the carbon sequestration efficiency of oceans is to add micrometre-sized iron particles called hematite or iron sulfate to the water. This has the effect of stimulating growth of plankton. Iron is an important nutrient for phytoplankton, usually made available via upwelling along the continental shelves, inflows from rivers and streams, as well as deposition of dust suspended in the atmosphere. Natural sources of ocean iron have been declining in recent decades, contributing to an overall decline in ocean productivity (NASA, 2003). Yet in the presence of iron nutrients plankton populations quickly grow, or 'bloom', expanding the base of biomass productivity throughout the region and removing significant quantities of CO₂ from the atmosphere via photosynthesis. A test in 2002 in the Southern Ocean around Antarctica suggests that between 10,000 and 100,000 carbon atoms are sunk for each iron atom added to the water. More recent work in Germany (2005) suggests that any biomass carbon in the oceans, whether exported to depth or recycled in the euphotic zone, represents long term storage of carbon. This means that application of iron nutrients in select parts of the oceans, at appropriate scales, could have the combined effect of restoring ocean productivity while at the same time mitigating the effects of human caused emissions of carbon dioxide to the atmosphere.

Those skeptical of this approach argue that the effect of periodic small scale phytoplankton blooms on ocean ecosystems is unclear, and that more studies would be advantageous. For example, it's known that phytoplankton have a complex effect on cloud formation via the release of substances such as dimethyl sulfide (DMS) which are converted to sulfate aerosols in the atmosphere providing cloud condensation nuclei, or CCN. But the effect of small scale plankton blooms on overall DMS production is unknown. The oceans are the largest active carbon sinks in the world.

Soils

The carbon sequestration potential of soils (by increasing soil organic matter) is substantial; below ground organic carbon storage is more than twice above-ground storage. Soils' organic carbon levels in many agricultural areas have been severely depleted. Improving the humus levels of these soils would both improve soil quality and increase the amount of carbon sequestered in these soils.

Grasslands contribute huge quantities of soil organic matter over time, mostly in the form of roots, and much of this organic matter can remain unoxidized for long periods. Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. No-till agricultural systems can increase the amount of carbon stored in soil, and conversion to pastureland, particularly with good management of grazing, can sequester even more carbon in the soil.

Mechanisms to enhance carbon sequestration in soil include conservation tilling, cover cropping, and crop rotation. Terra preta, an anthropogenic, high-carbon soil, is also being investigated as a sequestration mechanism.

Artificial sequestration

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For carbon to be sequestered artificially (i.e. not using the natural processes of the carbon cycle) it must first be captured. Thereafter it can be stored in a variety of ways.

Natural gas purification plants often already have to remove carbon dioxide, either to avoid dry ice clogging gas tankers or to prevent carbon dioxide concentrations exceeding the 3% maximum permitted on the natural gas distribution grid.

Beyond this, one of the most likely early applications of carbon capture is the capture of carbon dioxide from flue gases at power stations (in the case of coal, this is known as "clean coal"). A typical new 1000-MW coal-fired power station produces around 6m tons of carbon dioxide annually. Adding carbon capture to existing plants can add significantly to the costs of energy production; scrubbing costs aside, a 1000-MW coal plant will require the storage of about 50 million barrels of carbon dioxide a year. However, scrubbing is relatively affordable when added to new plants based on coal gasification technology, where it is estimated to raise energy costs for households in the United States using only coal-fired electricity sources from 10 cents per kWh to 12 Scientific American, July 2005, p42.

Carbon capture

Currently, capture of carbon dioxide is performed on a large scale by absorption of carbon dioxide onto various amine based solvents. Other techniques are currently being investigated such as pressure and temperature swing absorption, gas separation membranes and cryogenics.

In coal-fired power stations, the main alternatives to retro-fitting amine-based absorbers to existing power stations are two new technologies - coal gasification combined-cycle and oxyfuel combustion. Gasification first produces a "syngas" primarily of hydrogen and carbon monoxide, which is burned, with carbon dioxide filtered from the flue gas. Oxyfuel combustion burns the coal in oxygen instead of air, producing only carbon dioxide and water vapour, which are relatively easily separated. Oxyfuel combustion, however, produces very high temperatures, and the materials to withstand its temperatures are still being developed.

Another long term option is carbon capture directly from the air using hydroxides. The air would literally be scrubbed of its CO_{2 content. This idea offers an alternative to non-carbon based fuels for the transportation sector.}

Oceans

Another proposed form of carbon sequestration in the ocean is direct injection. In this method, carbon dioxide is pumped directly into the water at depth, and expected to form "lakes" of liquid CO₂ at the bottom. Experiments carried out in moderate to deep waters (350 - 3600 meters) indicate that the liquid CO₂ reacts to form solid CO₂ clathrate hydrates which gradually dissolve in the surrounding waters.

This method, too, has potentially dangerous environmental consequences. The carbon dioxide does react with the water to form carbonic acid, H₂CO₃; however, most (as much as 99%) remains as dissolved molecular CO₂. The equilibrium would no doubt be quite different under the high pressure conditions in the deep ocean. The resulting environmental effects on benthic life forms of the bathypelagic, abyssopelagic and hadopelagic zones are unknown. Even though life appears to be rather sparse in the deep ocean basins, energy and chemical effects in these deep basins could have far reaching implications. Much more work is needed here to define the extent of the potential problems.

It is not clear whether carbon storage in or under oceans is compatible with the London Convention (Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter) ^*.

An additional method of long term ocean based sequestration is to gather crop residue such as corn stalks or excess hay into large weighted bales of biomass and deposit it in the alluvial fan areas of the deep ocean basin. Dropping these residues in alluvial fans would cause the residues to be quickly buried in silt on the sea floor, sequestering the biomass for very long time spans. Alluvial fans exist in all of the world's oceans and seas where river deltas fall off the edge of the continental shelf such as the Mississippi alluvial fan in the gulf of Mexico and the Nile alluvial fan in the Mediterranean Sea.

Geological sequestration

Also known as geo-sequestration or geological storage, this method involves injecting carbon dioxide directly into underground geological formations. Declining oil fields, saline aquifers, and unminable coal seams have been suggested as storage sites. Caverns and old mines, that are commonly used to store natural gas are not considered, because of a lack of storage safety.

CO_{2 has been injected into declining oil fields for more than 30 years, to increase oil recovery. This option is attractive because the storage cost are offset by the sale of additional oil that is recovered. Further benefits are the existing infrastructure, and the geophysical and geological information about the oil field that is available from the oil exploration. All oil fields have a geological barrier preventing upward migration of buoyant fluids (oil in the past, CO2 in the future).}

Disadvantages of old oil fields are their geographic distribution and their limited capacity. Unminable coal seams can be used to store CO_{2, because CO2 adsorbs to the coal surface, ensuring safe long term storage. In the process it releases methane, that was previously adsorbed to the coal surface, and that may be recovered. Again the sale of the methane can be used to offset the cost of the CO2 storage.}

Saline aquifers contain highly mineralized brines, and have so far been considered of no benefit to humans. Saline aquifers have been used for storage of chemical waste in a few cases. The main advantage of saline aquifers is their large potential storage volume and their common occurrence. This will reduce the distances over which CO_{2 has to be transported. The major disadvantage of saline aquifers is that relatively little is known about them, compared to oil fields. To keep the cost of storage acceptable the geophysical exploration may be limited, resulting in larger uncertainty about the aquifer structure. Unlike storage in oil fields or coal beds no side product will offset the storage cost. Leakage of CO2 back into the atmosphere, may be a problem in saline aquifer storage. However, current research shows that several trapping mechanisms immobilize the CO2 underground, reducing the risk of leakage.}

A major research project examining the geological sequestration of carbon dioxide is currently being performed at an oil field at Weyburn in southeastern Saskatchewan. In the North Sea, Norway's Statoil natural gas platform Sleipner strips carbon dioxide out of the natural gas with amine solvents and disposes of this carbon dioxide by geological sequestration. Sleipner reduces emissions of carbon dioxide by approximately one million tonnes a year. The cost of geological sequestration is minor relative to the overall running costs. As of April 2005, BP is considering a trial of large-scale sequestration of carbon dioxide stripped from power plant emissions in the Miller oilfield as its reserves are depleted.

Mineral sequestration

Mineral sequestration aims to trap carbon by placing it in its thermodynamics groundstate where it will be nonreactive. This occurs naturally and is responsible for much of the surface limestone. Acids are used to convert mineral silicates to mineral carbonates. Ongoing research aims to speed up the kinetics of the reactions.

One proposed reaction is that of the rock dunite, or serpentinite with carbon dioxide to form the carbonate mineral magnesite, plus some silica and magnetite. This is proposed by ZECA Corporation, a consortium aiming to produce a low-emission coal-fired power source.

Serpentinite sequestration is favored because of the non-toxic and predictable nature of magnesium carbonate. However, the ideal reaction (reaction 1) takes place only with extremely magnesium rich olivine or serpentine minerals. The presence of iron in the olivine or serpentine will reduce the efficiency of the circuit and reactions 2 and 3 must take place, producing a slag of silica and iron oxide (magnetite).

Serpentinite reactions

Reaction 1
Mg-Olivine + Water + Carbon dioxide → Serpentine + Magnesite + Silica

$(Mg)\_2SiO\_4\; +\; nH\_2O\; +\; CO\_2\; \backslash rarr\; Mg\_3*+\; MgCO\_3\; +\; SiO\_2\; +\; H\_2O$

Reaction 2
Fe-Olivine + Water + Carbonic acid → Serpentine + Magnetite + Magnesite + Silica

$4(Fe,Mg)\_2SiO\_4\; +\; nH\_2O\; +\; H\_2CO\_3\; \backslash rarr\; 2Mg\_3*+\; 2Fe\_3O\_4\; +\; 2MgCO\_3\; +\; SiO\_2\; +\; H\_2O$

Reaction 3
Serpentine + carbon dioxide → Magnesite + silica + water

$Mg\_3*+\; 3CO\_2\; \backslash rarr\; 3MgCO\_3\; +\; 2SiO\_2\; +\; 2H\_2O$

Carbon sinks and the Kyoto Protocol

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The protocols hold that, since growing vegetation absorbs carbon dioxide, countries that have large areas of forest (or other vegetation) can deduct a certain amount from their emissions, thus making it easier for them to achieve the desired emission levels. The effectiveness of these provisions is controversial.

Some countries want to be able to trade in emission rights in carbon emission markets, to make it possible for one country to buy the benefit of carbon dioxide sinks in another country. It is said that such a market mechanism will help find cost-effective ways to reduce greenhouse emissions. There is as yet no carbon audit regime for all such markets globally, and none is specified in the Kyoto Protocol. Each nation is on its own to verify actual carbon emission reductions, and to account for carbon sequestration using some less formal method.

In the Clean Development Mechanism, only afforestation and reforestation are eligible to produce CERs in the first commitment period of the Kyoto Protocol (2008–2012). Forest conservation activities or activities avoiding deforestation, which would result in emission reduction through the conservation of existing carbon stocks, are not eligible at this time Manguiat, M. S. Z., Verheyen, R., Mackensen, J. & Scholz, G. (2005), Legal aspects in the implementation of CDM forestry projects, number 59 in ‘IUCN Environmental Policy and Law Papers’, IUCN. Available from: http://www.iucn.org/themes/law/pdfdocuments/EPLP59EN.pdf. Also agricultural carbon sequestration is not possible yet Rosenbaum, K. L., Schoene, D. & Mekouar, A. (2004), Climate change and the forest sector. Possible national and subnational legislation, number 144 in ‘FAO Forestry Papers’, FAO. Available from: http://www.fao.org/docrep/007/y5647e/y5647e00.HTM.

Notes

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See also

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• Carbon capture and storage
• Carbon cycle
• Global warming
• Greenhouse gas
• Iron fertilization
• Kyoto Protocol
• North American Carbon Program
• Sleipner Platform

References and external links

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General

• SinksWatch - An initiative to track and scrutinize carbon sink projects
• National Energy Technology Laboratory (NETL) Carbon Sequestration Home Page
• U.S. Department of Energy's Office of Science Carbon Sequestration Research Programs
• U.S. Department of Energy's National Energy Technology Laboratory - Carbon Sequestration Technology Roadmap
• The Carbon Offset Opportunity Program: A Tool for Collaborative Carbon Sequestration Project Development
• Carbon Capture and Sequestration Technologies Program at MIT
• International industry R&D group focussed on CO₂ sequestration
• CO₂ Capture Project
• Carbon Mitigation Initiative
• Southeast Regional Carbon Sequestration Partnership (SECARB)
• Carbon Capture and Storage (CCS) from power plants explained
• 'Catalyst' on Australian science TV on geosequestration
• The U.S. North American Carbon Program
• Priority One: Together we can beat global warming, downloadable book
• Collection of recent news articles on CO₂ capture and storage
• ZECA Corporation carbon sequestration via serpentinite

Research

• FAO (2004) Carbon sequestration in dryland soils
• IEA Reports: Putting carbon back into the ground (pdf) and Ocean storage of CO₂ (pdf)
• Haszeldine (2005) Deep geological CO₂ storage: principles, and prospecting for bio-energy disposal sites (pdf)
• The Role of Carbon in Agricultural Soils in Carbon Sequestration - A Better Alternative for Climate Change? Chapter 1: Agricultural Sinks (1999) University of Maryland pdf format doc format
• Schlesinger, W.H. 1991. Biogeochemistry: An Analysis of Global Change. Academic Press, San Diego.
• Peat bogs may be soaking up 10 to 20% of the excess CO₂ generated by human activity
• DMS and Climate
• Carbon Store in U.S. Forests

Action

• Collection of recent news articles on CO₂ capture and storage
• Britain entertains the idea
• Seattle Times, 20 February 2004, Canada pumps CO₂ underground (Weyburn oil field)
• United States pumps CO₂ underground
• Observer 24 April 2005 Seabed supplies a cure for global warming crisis
• Tyndall Centre - Assessing the potential for geological carbon sequestration in the UK

Carbon dioxide | Climate change | Climate forcing agents | Photosynthesis | Environmental soil science

CO2-Sequestrierung | Puits de carbone