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CO2 sink
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CO2 sink

A carbon dioxide (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. They say 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.

Furthermore, some studies indicate that a forest can be a net source of carbon dioxide, the exact circumstances of which are currently unclear. Moreover, the plantation of new forests may also 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 affects of the forest on albedo. Mid-to-high latitude forests have a much larger 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 rise serious questions about feasibility.


Oceans are also natural carbon dioxide sinks. Planktons in the oceans, like trees, use photosynthesis to extract carbon from CO2. They are the starting point of the sea food chain, and it is believed that the carbon eventually falls in the bottom of the ocean when the animals die. According to one theory, this is how the fossil fuels we know were created.

One of the most promising ways to increase the efficiency of this sink is to fertilize the water with iron sulphate: 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.

Skeptics argue that the final effect of plankton blooming on the ecosystem is not clear, and that more studies are needed.

Another form of carbon sequestration in the ocean is direct injection. In this method, carbon dioxide is pumped directly into the water, and expected to form "lakes" at the bottom.

This method, too, has potentially dangerous environmental consequences. The carbon dioxide reacts with the water to form carbon acid. The resulting energy costs for proton expulsion can divert energy from other cellular life processes, creating metabolic shutdown.

Geological Sequestration

This method involves directly injecting carbon dioxide directly into underground geological formations. Such formation may be natural such a caverns and pourous rock structures. They may also be man made, just as unused mines,expended oil and gas fields.

See also

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