Carbon cycle: land, oceans and atmosphere

The key features of the carbon cycle on land, in the oceans and the atmosphere, can be summarised as follows:

The carbon cycle on land (terrestrial)

• Dominated by photosynthesis of plants absorbing CO2 from the atmosphere.
• Carbon is stored within biomass, such as tropical and temperate forests.
• Carbon is transferred to the soil via leaf litter, roots and plant debris upon decomposition.
• Bacterial action in decomposition releases CO2 back to the atmosphere.
• Carbon is cycled quite rapidly through organic (living) systems between the atmosphere, vegetation (dominant biomass) and soils and is called the Fast Carbon Cycle.
• Human impact on this sub-cycle is considerable. The clearing of natural vegetation for urbanisation and agriculture is a major change to the biomass component and affects the carbon exchange between atmosphere and soil. Clearing vegetation by burning releases much stored carbon to the atmosphere very rapidly.

The carbon cycle in the oceans

• Carbon is stored in the oceans as dissolved CO2, as bicarbonate ions in solution, and as the tissues (especially calcium carbonate skeletons and shells) of marine organisms.
• The inputs of carbon are from the atmosphere (dissolved CO2) in a direct exchange with ocean surfaces, as bicarbonate ions brought by rivers as a result of the weathering of carbonate terrestrial rocks, and a small input from subterranean volcanoes.
• Phytoplankton (‘plant’ plankton) in surface waters absorb CO2 in photosynthesis. They are fed on by zooplankton (‘animal’ plankton).
• A carbon pump operates within oceans transferring carbon from upper layers to the sea bed. A constant ‘snow’ of carbon deposits sinks with gravity as a result of marine organisms dying and zooplankton feeding on phytoplankton and discharging excrement.
• Carbon accumulates as/within ocean sediments in shallow seas (in deeper oceans it is often re-dissolved) leading to the natural sequestration of carbon by removing it to a long-term store within ocean-bed deposits.
• Human impacts on the oceans are only now becoming understood, but the warming of oceans as a result of climate change is believed to have a considerable impact on the ocean carbon cycle. Warmer seas are less able to absorb CO2 from the atmosphere and cause a reduction in phytoplankton activity.

The carbon cycle in the atmosphere

• Atmospheric carbon is usually in the form of carbon dioxide (CO2) or methane (CH4). Both are natural greenhouse gases, with methane being over 20 times more powerful in absorbing solar radiation, but much shorter-lived in the atmosphere, than CO2 (about 12 years as opposed to 50).
• Carbon dioxide combines with water molecules in clouds to form carbonic acid, and naturally-acidic rain. This leads to terrestrial weathering and can contribute to ocean acidification.
• Outputs from the atmosphere include absorption by surface vegetation and by oceans in the atmosphere-ocean gas exchange.
• Human impacts on the carbon cycle are most directly implicated in increasing atmospheric CO2 through the burning of fossil fuels.