Discover

A 'carbon dioxide (CO₂) sink' is a carbon reservoir that is increasing in size, and is the opposite of a carbon dioxide "source". The main natural sinks are (1) the oceans and (2) plants and other organisms that use photosynthesis to remove carbon from the atmosphere by incorporating it into biomass and release oxygen into the atmosphere. This concept of CO₂ sinks has become more widely known because the Kyoto Protocol allows the use of carbon dioxide sinks as a form of carbon offset.
'Carbon sequestration' is the term describing processes that remove carbon dioxide from the atmosphere. To help mitigate global warming, a variety of means of artificially capturing and storing carbon (while releasing oxygen) — as well as of enhancing natural sequestration processes — are being explored.

Contents

Natural sinks

Forests

Oceans

Soils

Enhancing natural sequestration

Forests

Oceans

Soils

Artificial sequestration

Carbon capture

Oceans

Geological sequestration

Mineral sequestration

Serpentinite reactions

Carbon sinks and the Kyoto Protocol

References

See also

External links

General

Research

Action

Natural sinks

Forests

Carbon dioxide is incorporated into forests and forest soils by trees and other plants. Through photosynthesis, plants absorb carbon dioxide from the atmosphere, store the carbon in sugars, starch and cellulose, and release the oxygen into the atmosphere. A young forest, composed of 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 build-up of slowly decaying organic material will continue to accumulate carbon, but at a slower rate than an immature forest. Organic material in the form of humus in the forest floor accumulates in greater quantity in cooler regions such as the boreal and taiga forests. At warmer temperatures humus is oxidized rapidly; this, in addition to high rainfall levels, is the reason why tropical jungles have very thin organic soils. The forest eco-system may eventually become carbon neutral. Forest fires release absorbed carbon back into the atmosphere, as does deforestation due to rapidly increased oxidation of soil organic matter.
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.^[1]
Under some conditions, forests and peat bogs may become sources of CO₂, such as when a forest is flooded by the construction of a hydroelectric dam. Unless the forests and peat are harvested before flooding, 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.^[2]

Oceans

Oceans are natural CO₂ sinks, and represent the largest active carbon sink on Earth. This role as a sink for CO₂ is driven by two processes, the solubility pump and the biological pump.^[3] The former is primarily a function of differential CO₂ solubility in seawater and the thermohaline circulation, while the latter is the sum of a series of biological processes that transport carbon (in organic and inorganic forms) from the surface euphotic zone to the ocean's interior. A small fraction of the organic carbon transported by the biological pump to the seafloor is buried in anoxic conditions under sediments and ultimately forms fossil fuels such as oil and natural gas.
At the present time, approximately one third^[4] of anthropogenic emissions are estimated to be entering the ocean. The solubility pump is the primary mechanism driving this, with the biological pump playing a negligible role. This stems from the limitation of the biological pump by ambient light and nutrients required by the phytoplankton that ultimately drive it. Total inorganic carbon is not believed to limit primary production in the oceans, so its increasing availability in the ocean does not directly affect production (the situation on land is different, since enhanced atmospheric levels of CO₂ essentially "fertilize" land plant growth). However, ocean acidification by invading anthropogenic CO₂ may affect the biological pump by negatively impacting calcifying organisms such as coccolithophores, foraminiferans and pteropods. Climate change may also affect the biological pump in the future by warming and stratifying the surface ocean, thus reducing the supply of limiting nutrients to surface waters. Although the buffering capacity of sea water is keeping the pH nearly constant at present, eventually pH will drop. At this point, the dissruption of life in the sea may turn it into a carbon source rather than a carbon sink. The characteristic of buffered systems is to hold the pH reasonably constant over a large introduction of acid and then drop suddenly with a small additional amount.

Soils

Carbon as plant organic matter is sequestered in soils: Soils contain more carbon than is contained in vegetation and the atmosphere combined.^[5] Soils' organic carbon (humus) levels in many agricultural areas have been severely depleted. Organic material in the form of humus accumulates below about 25 degrees Celsius. Above this temperature, humus is oxidized much more rapidly. This is part of the reason why tropical soils under jungles are so thin, despite the rapid accumulation of organic material on the jungle floor (the other being extensive rainfall leaching soluble components vital to organic soil structure). Areas where shifting cultivation or "hack-and-slash" agriculture are practised are generally only fertile for 2-3 years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert.
Grasslands contribute to soil organic matter, mostly in the form of their extensive fibrous root mats. Much of this organic matter can remain unoxidized for long periods of time, depending on rainfall conditions, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires necessary to recycle inorganic compounds from existing plant material. While these fires release carbon dioxide, they improve the quality of the grass-lands overall, in turn increasing the amount of carbon retained in the retained humic material. They also desposit carbon directly to the soil in the form of char that does not significantly degrade back to carbon dioxide

Enhancing natural sequestration

Forests

Forests are carbon stores, and they are carbon dioxide sinks when they are increasing in density or area. Tropical reforestation can mitigate global warming until all available land has been reforested with mature forests.^[6][7][8][9]. In the United States in 2004 (the most recent year for which EPA statistics^[10] are available), forests sequestered 10.6% (637 teragrams Land Use, Land-Use Change, and Forestry ) of the carbon dioxide released in the United States by the combustion of fossil fuels (coal, oil and natural gas; 5657 teragrams Executive Summary ). Urban trees sequestered another 1.5% (88 teragrams). To further reduce U.S. carbon dioxide emissions by 7%, as stipulated by the Kyoto Protocol, would require the planting of "an area the size of Texas [8% of the area of Brazil] 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.. Carbon offset programs are planting millions of fast-growing trees per year to reforest tropical lands, for as little as $0.10 per tree; over their typical 40-year lifetime, one million of these trees will fix 0.9 teragrams of carbon dioxide^[11].
The global cooling effect of carbon sequestration by forests is partially counterbalanced: For example, the planting of new forests may initially be a source of carbon dioxide emission when carbon from the soil is released into the atmosphere. Also, reforestation can decrease the reflection of sunlight (albedo): Mid-to-high latitude forests have a much lower albedo during snow seasons than flat ground, thus contributing to warming.
A long-term sequestration of carbon from forests comes from the use of wood products such as "stick built" (i.e., with lumber) homebuilding, the predominant form of home building in the US. Because most buildings are eventually demolished, the carbon may be released into the atmosphere, depending upon the fate of the scrap lumber. Reusing the lumber, or using it as fuel to replace a fossil fuel, avoids an increase in atmospheric carbon. (In addition to the global cooling effect of tropical reforestation, planting forests reduces erosion, increases water capture, and provides valuable timber which may be sustainably harvested.)

Oceans

:''See also: {{#if:|