A carbon store (reservoir) is anywhere carbon is held. A carbon sink is a store that absorbs more carbon than it releases, so it removes CO2 from the atmosphere. A carbon source releases more carbon than it absorbs.Whether a store acts as a sink depends on the balance between uptake (photosynthesis and dissolving) and release (respiration, decomposition and combustion).
The main natural sinks that exchange carbon with the atmosphere over years to decades are:
• Oceans: CO2 dissolves in surface water, more readily in cold water than in warm water. Phytoplankton fix dissolved CO2 by photosynthesis, and some of the carbon sinks to the deep ocean in dead organisms and faeces. The oceans have absorbed about a quarter of the CO2 released by humans.
• Forests and other vegetation: growing plants fix CO2 into biomass such as wood. A young, growing forest is a sink because photosynthesis is greater than respiration and decomposition. A mature forest in which uptake roughly equals release is a large store but takes up little extra carbon.
• Soils: dead roots, leaf litter and humus add organic carbon. Soils hold more carbon than all the vegetation growing on them.Sinks can change into sources. For example, if warming speeds up decomposition in soil faster than plants can add organic matter, the soil releases more carbon than it takes in. Warmer oceans also dissolve less CO2.Worked example: a forest fixes 12 tonnes of carbon per hectare per year and releases 9 tonnes by respiration and decomposition. It is a sink, removing 12 − 9 = 3 tonnes of carbon per hectare per year.
The main natural sinks that exchange carbon with the atmosphere over years to decades are:• Oceans: CO2 dissolves in surface water, more readily in cold water than in warm water. Phytoplankton fix dissolved CO2 by photosynthesis, and some of the carbon sinks to the deep ocean in dead organisms and faeces. The oceans have absorbed about a quarter of the CO2 released by humans.
• Forests and other vegetation: growing plants fix CO2 into biomass such as wood. A young, growing forest is a sink because photosynthesis is greater than respiration and decomposition. A mature forest in which uptake roughly equals release is a large store but takes up little extra carbon.
• Soils: dead roots, leaf litter and humus add organic carbon. Soils hold more carbon than all the vegetation growing on them.Sinks can change into sources. For example, if warming speeds up decomposition in soil faster than plants can add organic matter, the soil releases more carbon than it takes in. Warmer oceans also dissolve less CO2.Worked example: a forest fixes 12 tonnes of carbon per hectare per year and releases 9 tonnes by respiration and decomposition. It is a sink, removing 12 − 9 = 3 tonnes of carbon per hectare per year.