Carbon storage ($C$)

The amount of carbon held in a specified pool or system. It is a stock, distinct from gross uptake or its rate of net change; large old stocks can coexist with small current fluxes.

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 $C$, 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]

Transfer of carbon into storage over a specified time horizon. Net sequestration requires uptake to exceed relevant losses under an explicit boundary; high photosynthesis alone does not establish it.