Carbon Cycle Stores (AQA A Level Geography) : Revision Note

The Carbon Cycle as a System

• Carbon is an essential building block for all life on Earth

• It plays a major role in regulating global climate, particularly temperature and the acidity of rain, rivers, and oceans

• The key carbon cycles operate at a global 'sphere' level - lithosphere, hydrosphere, biosphere etc. 

• Carbon cycles have inputs, stores, fluxes/flows and outputs that transfer carbon from one place to another and either deplete or build carbon stores 

• Carbon is found in many forms (it bonds easily with other molecules) and the major compounds are;

  • Carbon dioxide (CO₂) is a gas found in oceans, soils and the atmosphere and as a waste product in respiration (animal and human)

  • Methane (CH₄) is a greenhouse gas found in rocks, oceans, permafrost, soils, etc. 

  • Hydrocarbons (fossil fuels) found in sedimentary rocks in gas, liquid, or solid form

  • Calcium carbonate (CaCO₃) common substance found in limestone rock, shells, eggs, etc. 

  • Carbon biomolecules are organic molecules including carbohydrates, fats, proteins etc., and form 50% of the total dry mass of living things

• The global carbon system can be subdivided into systems operating on land, oceans and atmosphere, which are inter-related through fluxes/flows, but also as distinct sub-systems in their own right

• Carbon flows/fluxes between the major stores as two systems:

  • Long-term or slow carbon cycle: The movement of carbon between the atmospheric, oceanic and lithospheric stores

  • Short-term or fast carbon cycle: The movement of carbon from living things to the atmosphere and oceans

• The atmosphere, oceans and land are linked together transferring carbon in a giant slow-moving system which takes between 100 and 200 million years for carbon to flow through it

• The short-term or fast cycle through the biosphere moves up to a thousand times more carbon in a shorter space of time

Main Stores in The Carbon Cycle

• The main stores of carbon are located in, and transferred between the biosphere, lithosphere, pedosphere, cryosphere, atmosphere and hydrosphere

Main Stores of the Carbon Cycle and Residence Time

• GtC = Gigatonnes of carbon dioxide equivalent

• One GtC is one billion tonnes

• Different forms of carbon include gaseous carbon such as methane and carbon dioxide

• Hydrocarbons within the lithosphere are the largest forms of carbon stored on Earth 

Changes to the size of stores over time and location

• Global distribution of vegetation changes the amount of stored carbon - the Arctic and the Sahara Desert have virtually no plant storage, whereas the Amazon rainforest has all-year-round storage

• Carbon uptake is higher in the middle/high latitudes of the northern hemisphere but less in the southern hemisphere (less land mass)

• There is a seasonal change in the amount of carbon in the terrestrial biosphere because plants grow and decay differently during the summer compared to the winter

• CO2 emissions change with the seasons - as plants grow they intake more CO2, but during the dormant stage, less CO2 is needed

• Different terrestrial ecosystems also store different amounts of carbon - large tropical trees will store more carbon than a small bramble etc.