The Carbon Cycle (HL IB Biology)

• The many processes by which carbon is transferred from one store to another are collectively known as the carbon cycle
  • During the carbon cycle, carbon is present in both organic and inorganic forms
    • Organic carbon is found in the biomass of living organisms, e.g. in carbohydrates and proteins
    • Inorganic carbon is found in the atmosphere as carbon dioxide and in the oceans as, e.g. hydrogen carbonate ions
• The carbon cycle can be represented using a diagram
  • Carbon cycle diagrams show:
    • Carbon stores, known as pools or sinks, e.g. the ocean, fossil fuels, or living organisms
    • Processes of carbon transfer, known as fluxes, e.g. dissolving, combustion, or photosynthesis
• Diagrams can be illustrated, or can be simple, containing just text boxes and arrows
• Diagrams can show terrestrial carbon cycling, marine carbon cycling, or both combined in one diagram

Carbon cycle diagrams

Carbon cycle diagrams can be illustrated, and can show both terrestrial and marine cycling

Carbon cycle diagrams can be simple, and may show only terrestrial carbon cycling

• A carbon sink is a part of the carbon cycle that takes up and stores carbon, e.g.
  • Plants take up carbon dioxide when they photosynthesise and convert it into carbon compounds which they store in their tissues; plants therefore act as carbon sinks
  • Plant material sometimes fails to decompose and forms fossil fuels or peat; these substances act as carbon sinks over very long time periods
  • Carbon dioxide dissolves in the oceans, which form a large carbon sink
• A carbon source is a part of the carbon cycle that releases carbon, e.g.
  • If plant material is burned then the carbon stored within the tissues is released back into the atmosphere
  • The decay of dead or waste material leads to the release of carbon

Net uptake and release of carbon dioxide

• If an organism carries out photosynthesis at a higher rate than respiration, e.g. plants, then there can be said to be a net uptake of carbon dioxide and that organism will function as a carbon sink within its ecosystem
  • The term 'net' refers to the overall direction of movement
• If an organism carries out respiration at a higher rate than photosynthesis, e.g. animals, then there will be a net release of carbon dioxide and that organism will function as a carbon source within its ecosystem

• Carbon can be returned to the atmosphere by the burning of fossil fuels and organic material; a process known as combustion
  • Complete combustion releases carbon dioxide and water as by-products
• Carbon is released during the combustion of:
  • Coal
  • Oil
  • Natural gas
    • Coal, oil and gas are fossil fuels; these have formed over millions of years from the bodies of dead plants and animals; burning them releases carbon that has been locked up for very long time periods
  • Peat
    • This is a material that forms when plant matter does not fully decompose due to waterlogged and acidic conditions; it releases carbon when it is burned, and when it is allowed to dry out and decompose
  • Biomass
    • This refers to plant matter, e.g. wood
• Organic material, or biomass, burns when fires occur in, e.g. forests or grasslands; human activities have increased the burning of biomass
  • Such fires can have natural causes, e.g. lightning hitting hot, dry ground, but can be set by humans, e.g. when clearing land for the purpose of farming
    • Climate change has increased the occurrence of wildfires
  • Biomass can also be burned as a fuel in e.g. wood fires or biomass boilers
• The burning of biomass is considered to have a less significant impact on atmospheric carbon dioxide than the burning of peat and fossil fuels; this is because the carbon contained in plant tissues has been removed from the atmosphere relatively recently (i.e. within the lifetime of the plant), whereas the carbon locked up in peat has been in that form for potentially thousands of years, and the carbon in fossil fuels for millions of years

CC BY-ND 2.0, via Flickr

CC BY 2.0, via Wikimedia Commons

Carbon is released during combustion of peat (left) and combustion of biomass, e.g. during forest fires (right)

• Scientists from the World Meteorological Organisation and research stations have been taking quantitative measurements of the atmospheric carbon dioxide for many years
• The Mauna Loa Observatory, Hawaii, has been recording carbon dioxide levels since 1958
  • The data collection was initially carried out by American scientist Charles Keeling, and the dataset from Mauna Loa is now named after him
• The concentration of carbon in the atmosphere is constantly changing due to seasonal fluctuations in rates of photosynthesis; this is shown on the Keeling curve graph below in red
  • Photosynthesis removes carbon dioxide from the atmosphere, meaning that atmospheric carbon dioxide levels decrease in whichever hemisphere is experiencing spring and summer
  • This seasonal decrease is reversed during autumn and winter when photosynthesis rates decrease and are overtaken by processes such as respiration, decomposition, and combustion
• The overall trend in atmospheric carbon dioxide levels is the result of human activities; the combustion of fossil fuels by humans releases carbon dioxide into the atmosphere faster than photosynthesis is able to remove it, meaning that carbon dioxide levels are slightly higher every year
  
  • The overall trend is shown on the Keeling curve below in blue

Keeling curve graph

The Keeling curve shows changes in atmospheric carbon dioxide levels measured at the Mauna Loa Observatory (ppmv = parts per million by volume)

The yearly fluctuations shown in red are due to seasonal changes in photosynthesis rates, while the overall trend shown in blue is due to human combustion of fossil fuels