The Carbon Cycle

• Carbon Cycle


  The movement and storage of carbon between the land, ocean and the atmosphere

  
• Forms of carbon in the Carbon Cycle


  • Inorganic - Found in rocks as bicarbonates and carbonates
  • Organic - Found in plant material and living organisms
  • Gaseous - Found as CO2 and CH4 (methane)

  
• There is generally a balance between production and absorption (or sources and sinks) of carbon in the natural carbon cycle

  
• Stores


  Terrestrial, oceanic or atmospheric
• Fluxes

  The movement/transfer of carbon between stores
• Carbon sink


  Any store which takes in more carbon than it emits
• Carbon source


  Any store that emits more carbon than it stores
• Stores of carbon


  • The atmosphere as CO2 and methane
  • The hydrosphere as dissolved CO2
  • The lithosphere as carbonates in limestone and fossil fuels like coal, gas and oil
  • The biosphere in living and dead organisms

  
• Carbon Sequestration


  The transfer of carbon from the atmosphere to other stores, can be both natural and artificial

  
• Main Carbon Stores (In order of magnitude)


  • Marine Sediments and Sedimentary Rocks - Lithosphere - Long-term
  • Oceans - Hydrosphere - Dynamic
  • Fossil Fuel Deposits - Lithosphere - Long-term but currently dynamic
  • Soil Organic Matter - Lithosphere - Mid-term
  • Atmosphere - Dynamic
  • Terrestrial Plants - Biosphere - Mid-term but very dynamic

  
• The lithosphere is the main store of carbon, with global stores unevenly distributed

  
• Photosynthesis


  Living organisms convert Carbon Dioxide from the atmosphere and Water from the soil, into Oxygen and Glucose using Light Energy

  
• Respiration


  Plants and animals convert oxygen and glucose into energy which then produces the waste products of water and CO₂
• Overall, plants absorb more CO₂ than they emit, so are net carbon dioxide absorbers (from the atmosphere) and net oxygen producers (to the atmosphere)
• Combustion


  When fossil fuels and organic matter such as trees are burnt, they emit CO₂ into the atmosphere, that was previously locked inside of them
• Decomposition


  When living organisms die, they are broken down by decomposers (such as bacteria and detritivores) which respire, returning CO₂ into the atmosphere
• Diffusion


  The oceans can absorb CO₂ from the atmosphere, which has increased ocean acidity by 30% since pre-industrial times
• Sedimentation


  When shelled marine organisms die, their shell fragments fall to the ocean floor and become compacted over time to form limestone. Organic matter from vegetation and decaying marine organisms is compacted over time, whether on land or in the sea, to form fossil fuel deposits
• Weathering and Erosion


  Inorganic carbon is released slowly through weathering: rocks are eroded on land or broken down by carbonation weathering
• Metamorphosis


  Extreme heat and pressure forms metamorphic rock, during which some carbon is released and some becomes trapped
• Volcanic outgassing


  There are pockets of CO2 found in the Earth's crust. During a volcanic eruption or from a fissure in the Earth's crust, this CO2 can be released
• The quickest cycle is completed in seconds as plants absorb carbon for photosynthesis and then they release carbon when they respire
• Dead organic material in soil may hold carbon for hundreds of years
• Ocean Sequestration


  The transfer of CO2 into the sea
• Small changes in oceanic carbon levels can have significant global impacts

  
• Biological Carbon Pump


  Phytoplankton photosynthesise and take in carbon, turning it into organic matter. When they get eaten, carbon is passed through the food chain. Some organisms like Plankton sequester CO2, turning the carbon into their hard outer shells and inner skeletons. When these organisms die, some of their shells dissolve into the ocean water meaning the carbon becomes part of deep ocean currents

  
• Physical Pump


  Oceanic circulation provides a constant source of new water on the surface while transferring surface water into the deep ocean, enabling the ocean to store so much carbon

  
• Thermohaline Circulation


  An ocean current that produces both vertical and horizontal circulation of cold and warm water around the world's oceans

  
• As ocean temperatures increase, the oceans will absorb less CO2 (possibly even emitting some of its stored carbon dioxide), accelerating Climate Change and leading to further ocean warming (positive feedback mechanism)
• Terrestrial Sequestration


  Primary producers sequester carbon through the process of photosynthesis. All living things either release or intake carbon

  
• Diurnally, carbon fluxes are positive from the atmosphere to the ecosystem during the day, but negative from the atmosphere to the ecosystem at night

  
• Seasonally, in the northern hemisphere during winter, plants die and decay leading to high atmospheric CO2 concentrations but during spring when plants begin to grow, CO2 levels in the atmosphere begin to drop
• Tropical areas such as Brazil and Indonesia have seen a decrease in carbon stocks of around 5 Gigatons of Carbon (GtC) in the last 25 years, but Russia, USA and China have seen increases of around 0.3, 2.9 and 2.3 GtC respectively

  
• Non-tropical forests have seen an increase in carbon sequestration in recent years, especially in Europe and Eastern Asia, due to conversion of agricultural land and plantations to new forests

  
• Forests in industrialised regions are expected to increase by 2050 but in the global south, forests are declining
• CO2 levels in the atmosphere begin to drop during spring when plants begin to grow
• The map shows how forests are declining in the tropical areas in the southern hemisphere and growing in the northern hemisphere

  
• Non-tropical forests


  • Have seen an increase in carbon sequestration in recent years, especially in Europe and Eastern Asia, due to conversion of agricultural land and plantations to new forests

  
• Forests in industrialised regions are expected to increase by 2050 but in the global south, forested areas will decrease
• Rate of forest loss has decreased from 9.5 million hectares per year in the 1990's to 5.5 million hectares per year in 2010-15