Carbon Dioxide Sink

A carbon dioxide sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period. The process by which carbon sinks remove carbon dioxide (CO₂) from the atmosphere is known as carbon sequestration. Public awareness of the significance of CO₂ sinks has grown since passage of the Kyoto Protocol, which promotes their use as a form of carbon offset.

The main natural sinks are:

• Absorption of carbon dioxide by the oceans via physicochemical and biological processes
• Photosynthesis by terrestrial plants

Natural sinks are typically much larger than artificial sinks. The main artificial sinks are:

• Landfills
• Carbon capture and storage proposals

Carbon sources include:

• Fossil fuels
• Farmland; there are proposals for improvements in farming practices to reverse this.

Because growing vegetation absorbs carbon dioxide, the Kyoto Protocol allows Annex I countries with large areas of growing forests to issue Removal Units to recognise the sequestration of carbon. The additional units make it easier for them to achieve their target emission levels.

Some countries seek to trade emission rights in carbon emission markets, purchasing the unused carbon emission allowances of other countries. If overall limits on greenhouse gas emission are put into place, cap and trade market mechanisms are purported to find cost-effective ways to reduce emissions. There is as yet no carbon audit regime for all such markets globally, and none is specified in the Kyoto Protocol. National carbon emissions are self-declared.

In the Clean Development Mechanism, only afforestation and reforestation are eligible to produce certified emission reductions (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. Also, agricultural carbon sequestration is not possible yet.