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

Some countries also want to be able to trade in emission rights to make it possible for one country to buy the use of CO2 sinks in another country. The use of CO2 sinks, however, is not uncontroversial.

Carbon sinks base on an understanding of the carbon cycle. Enormous amounts of carbon are naturally stored in trees and the oceans. As part of the photosynthesis trees absorb CO2 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 CO2. Older trees grow less rapidly and thus have a lower intake of CO2. 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.

Some studies indicate that a forest can be a net source of CO2, the exact circumstances are currently unclear. Moreover, the plantation of new forests may also be a source of CO2 emission when carbon from the soil is released into the atmosphere. Even though the extent of carbon storing is unclear, it seems clear that the use of forests to curb climate change is only a temporary measure. To prevent the stored carbon from being released into the atmosphere, there are suggestions of sinking trees into the ocean.

Such suggestions rise serious questions about feasability, especially since even optimistic estimates come to the conclusion that the plantion of new forests is not enough to counter-balance the curent level of CO2 emissions.

Oceans are important CO2 sinks. Other than forestation plans, there are no means to enlargen the size of the oceans. It has, however, been suggested to pump carbondioxide into the oceans where at least some of the gas will be absorbed. Again, such ideas fail tests of feasability.