A carbon sink takes up more carbon than it releases over a specified interval. It is a statement about a net transfer, whereas carbon storage describes an amount already held. If a reservoir's carbon stock is , its balance isA positive balance makes it a carbon sink relative to the stated boundary. A large, nearly unchanged stock can provide substantial carbon storage without acting as a strong current carbon sink.
Forests acquire carbon through photosynthesis and lose it through cellular respiration, decay, fire and removal of biomass. Forest inventories and long-term ecosystem measurements therefore have to account for both accumulation and losses: the carbon sink in growing forests is not automatically the net effect of all changes in forested land.[ref]
A large and persistent carbon sink in the world's forests by Yude Pan and others. Science 333, pp. 988–993; separate forest uptake, regrowth and land-use-change emissions for 1990–2007
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