Carbon Cycle

• Stores in the carbon cycle include the lithosphere, hydrosphere, cryosphere, atmosphere and biosphere
• Carbon Sink = a store that absorbs more carbon than it releases
• Carbon Source = a store that releases more carbon than it absorbs
• Transfers in the carbon cycle include burning fossil fuels, photosynthesis, weathering and erosion, diffusion and volcanic eruptions
• The amount of carbon held in each store is subject to change over timescales ranging from a few minutes to millions of years. These changes are known as fluxes.
• Carbon stores have a geographical component in that they are not evenly distributed across the world. This largely comes from the distrubution of land and sea
• Marine sediments and sedimentary rocks store 100,000 GtC, a long term store, with rocks taking millions of years to form
• The ocean stores 38,000 GtC, CO2 is absorbed from the air and carried in rivers to reach the ocean
• Fossil fuels store 4000 GtC in hydrocarbons such as coal, oil and gas. Since the industrial revolution, these have been used for heat and power
• Soil organic matter stores around 1500 Gtc
• The atmosphere stores around 750 GtC
• Scientists have identified that carbon uptake is increasing in middle and high latitudes of the northern hemisphere, but less carbon is being absorbed in the tropics and southern hemisphere. A major cause of this decrease is thought to be drought (possibly linked to climate change), impacting crop yields, timber production and expanses of natural vegetation.
• When a rock is exposed, it is vulnerable to weathering. Over time, lichen and moss grow on the rock and carbon exchange begins to take place. As organic matter is added to broken fragments of rock, a soil develops that can support a wide range of plants
• Soil can absorb and store carbon for moderate time periods, and also provide carbon for diverse plant species
• Vegetation succession relating to a specific environment is called a sere and each stage in the succession can be referred to as a seral stage. A lithosere is a vegetation succession that occurs on bare rock. Other seres include hydrosere (water – freshwater pond), halosere (salt – coastal salt marsh) and psammosere (coastal sand – sand dunes)
• The final stage of a sere is when environmental equilibrium is achieved and is refered to as the climatic climax
• The climax vegetation for a lithosere in the UK is usually a deciduous woodlad
• Photosynthesis is where plants use light energy to produce carbohydrates in the form of glucose

  • Green plants absorb light using chlorophyll
  • Light energy convert CO2 from the air and water in the soil into glucose, releasing oxygen
  • Some glucose is used for respiration and the rest is converted into starch.
• Respiration is a chemical process that happens in all cells and is common to both plants and animals. Glucose is converted into energy that can be used for growth and repair, movement and control of body temperature in mammals. Carbon dioxide is then returned to the atmosphere, mostly by exhaled air.
• When organisms die they are consumed by decomposers such as bacteria, fungi and earthworms. During this process of decomposition, carbon from their bodies is returned to the atmosphere as carbon dioxide
• Combustion is when organic material is burnt in the presence of oxygen and converted into energy, CO2 and water
• Burial and compaction is where organic matter is buried by sediments and becomes compacted. Over millions of years, these organic sediments containing carbon may form hydrocarbons such as coal and oil.
• Carbon sequestration is an umbrella term used to describe the transfer of carbon from the atmosphere to plants, soils, rock formations and oceans. Sequestration is both a natural and human process. Carbon capture and storage (CCS) is a recent term used to describe the technological ‘capturing’ of carbon emitted from power stations
• Weathering involves the breakdown or decay of rocks in their original place at or close to the surface. When carbon dioxide is absorbed by rainwater it forms a mildly acidic carbonic acid. Through a series of complex chemical reactions, rocks will slowly dissolve with the carbon being held in solution. This is then transported to oceans where the carbon is used to build the shells of marine organisms
• Physical causes of change in the carbon cycle:

  • Natural climate change
  • Cold conditions
  • Warm conditions
  • Volcanic eruptions
• During the Quaternary geological period, the climate fluctuated greatly between warm (interglacial) and cold (glacial) periods.
• The trends for temperature and CO2 mirror each other, higher temperatures are associated with higher levels of CO2 in the atmopshere
• Impacts of cold conditions on carbon stores

  • Chemical weathering is more active, as cold water holds more CO2
  • Forest coverage would be difference and affect photosynthesis
  • Decomposers are less effective
  • Less water flowing into the ocean, so less sediment transfer
  • More permafrost
• In warm conditions, carbon stored within the permafrost – together with other gases such as methane – is now being released into the atmosphere where it further enhances the greenhouse effect, leading to increased warming
• Volcanic activity returns to the atmosphere carbon that has been trapped for millions of years in the Earth's crust. In the Palaezoic era, volcanoes were more active and a vast amount of carbon dioxide was released.
• Human causes of change in the carbon cycle

  • Burning fossil fuels
  • Land use changes
  • Urbanisation
  • Deforestation
• According to the IPCC, about 90% of anthropogenic carbon release comes from the combustion of fossil duels
• Global concentrations of carbon dioxide have increased from about 320ppm to over 400ppm since the 1960s when the 'great acceleration' began
• Most of the gas and oil today is extracted from rocks that are 70-100 million years old, and the carbon has remained locked up for that amount of time
• Fossil fuels are composed of carbon and hydrogen, and so termed hydrocarbons
• Methane is the main component of natural gas, and also one of the most potent of all greenhouse gases
• Since the Industrial Revolution more fossil fuels are being burnt, releasing carbon dioxide into the atmosphere. This has enhanced the greenhouse effect and subsequently increased global temperatures
• Measuring at Mauna Loa in Hawaii has shown an alarming rise in CO2 levels since 1958 mapped on the Keeling Curve
• Land-use change is responsible for about 10% of carbon release globally, which impacts short term stores and has direct links to climate change and global warming.
• Many farms emit large amounts of carbon

  • Methane is a potent greenhouse gas produced by enteric fermentation through cows.
  • Methane is also produced from the cultivation of rice, in fact, rice farms may contribute up to 20% of global methan emissions