A carbon dioxide sink or CO₂ sink is the opposite of a carbon source. The main sinks are the oceans and growing vegetation. The concept has become widely known through the Kyoto protocol. The idea is that growing vegetation absorbs carbon dioxide, so that countries that have large areas of forest (or other vegetation) can deduct a certain amount from their emissions, thus making it easier to achieve the desired emission levels. However, the effectiveness of the proposed sinks 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.

Growing trees

The idea of carbon sinks based on growing trees rests on an understanding of the carbon cycle. Enormous amounts of carbon are naturally stored in trees. As part of the photosynthesis trees absorb carbon dioxide from the atmosphere and store it as carbon while oxygen is released back into the atmosphere. Young trees which grow more rapidly absorb a larger amount of carbon dioxide. Older trees grow less rapidly and thus have a lower intake of carbon dioxide. With trees living up to 700 years, for instance in Scandinavia, trees can store a considerable amount of carbon. Eventually, however, all trees die and rot, releasing most of the stored carbon back to the atmosphere. This process is accelerated when burning the wood.

In effect, forests are carbon dioxide stores, and the sink effect exists only when they grow in size: it is thus naturally limited. 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.

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 albedo. Mid-to-high latitude forests have a much lower albedo during snow seasons than flat ground, and this contributes to warming.

To prevent the stored carbon from being released into the atmosphere when the trees die, there have been suggestions of sinking the trees into the ocean. Such suggestions raise serious questions about feasibility.

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Oceans

An image of liquid carbon dioxide spilling from a beaker on the ocean floor. The image was taken from video of a carbon sequestration experiment done by the Monterey Bay Aquarium Research Institute.

Oceans are also natural carbon dioxide sinks. 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 falls in the bottom of the ocean when the organisms die. One theory is that the organic carbon within the accumulating ocean bottom sediments is how fossil fuels were created.

One of the most promising ways to increase the efficiency of this sink is to fertilize the water with iron sulfate: this has the effect of stimulating the growth of the plankton. 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. Advocates of this technique estimate that a large use of it could make a significant dent in the greenhouse effect.

Those skeptical of this approach argue that the final effect of phytoplankton blooming on the ecosystem and its consumption by krill is not clear, and that more studies are needed. Phytoplankton do 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 (CCN).

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 needed here to define the extent of the potential problems.

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Geological sequestration

Also known as geo-sequestration, this method involves injecting carbon dioxide directly into underground geological formations. Such formations may be natural such caverns or porous rock structures. They may also be man-made, such as unused mines and expended petroleum fields.

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, the 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.

Another aspect of geological sequestration is the capture of carbon dioxide from flue gases. This adds significantly to the costs. 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.

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Soil sequestration

The carbon sequestration potential of soils is substantial. 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.

- TAKEN FROM THE WIKIPEDIA