Carbon Cycle Stores (AQA A Level Geography)

• 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

Diagram of a very simple carbon cycle showing interaction between terrestrial, atmospheric and oceanic sub-systems

• 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

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

The carbon cycle between the 'spheres' of Earth is complex and involves many stores and fluxes

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.