What is the Carbon Cycle? (SL IB Environmental Systems & Societies (ESS))

What is the carbon cycle?

• Carbon is constantly being recycled around the biosphere so that the number of carbon atoms in the biosphere is essentially constant; carbon atoms merely swap from one compound to another by the various processes in the carbon cycle

Carbon cycle diagram

Storage and Flows in the Carbon Cycle

• Storages in the carbon cycle include:
  • The atmosphere (as CO₂)
  • Sedimentary rocks
  • Fossil fuels like coal, oil, and gas; coal is largely carbon
  • Soil and other organic matter
  • Vegetation (e.g. as cellulose)
  • Animals
  • Dissolved in the oceans (as CO₂)

• Flows in the carbon cycle include:
  
  • Consumption (feeding)
  • Death and decomposition
  • Photosynthesis
  • Respiration
  • Dissolving
  • Fossilisation

Photosynthesis

• Autotrophs use the energy of sunlight to 'fix' carbon dioxide, turning its carbon into sugars and other organic molecules
• This removes carbon from the atmosphere
• Terrestrial plants use gaseous CO₂ directly from the air
• Aquatic organisms use CO₂ dissolved in water
• As much CO₂ is fixed from ocean microorganisms, as from terrestrial plants

Sedimentation

• Plants that die are not fully decomposed by saprobionts; their bodies form layers of sediment that can accumulate over millions of years, locking carbon into the ground
• This sediment is a store of energy and can form fossil fuels like peat and coal
• Aquatic organisms that die also form sediments on the sea bed; these can go on to form other fossil fuels like oil and gas
• Shells and other calcium-containing body parts can form sedimentary rocks such as limestone
• The existence of life forms over billions of years has shaped the biosphere, in that their remains are still being recycled

Respiration

• All life forms respire, including autotrophs
• Heterotrophs rely on respiration for all their energy needs
• Respiration puts CO₂ into the atmosphere, in the opposite direction to photosynthesis
• Anaerobic respiration also releases CO₂ into the atmosphere, via fermentation by yeast, moulds and bacteria

Feeding

• Carbon is passed from autotroph to heterotroph during feeding
• Carbon is also passed from primary consumer to secondary consumer
• Biomass transfer always includes the transfer of carbon, the main element in biomass

Decay & Decomposition

• Dead plants and animals are fed upon by detritivores and decayed by saprophytes
• Releasing carbon into the surroundings
• Supplying carbon to the detritivores
• Supplying carbon to the saprophytes
• Waste matter such as faeces and urine is used by decaying saprobionts
• Such processes can release CO₂ back into the air

What is the human impact on the carbon cycle?

• Human activities such as burning fossil fuels, burning, deforestation, urbanisation and agriculture impact the balance of storages and flow within the carbon cycle 

Burning fossil fuels

• Since the mid 19th century, humans have extracted and burned increasing amounts of fossil fuels from the Earth
• CO₂ is being returned to the atmosphere faster than it can be absorbed by plants and aquatic producers
• The CO₂ level in the atmosphere is approximately double that of 800,000 years ago
• Warmer temperatures mean that less CO₂ can be dissolved in the oceans, so is released into the air
• This has caused dramatic climate change and affected many other species, mainly through changing habitats

Photo by Renaldo Matamoro on Unsplash 

Deforestation reduces the ability of forests to act as carbon sinks

Deforestation

• Mass deforestation is reducing the amount of producers available to take carbon dioxide out of the atmosphere by photosynthesis
• In many areas of the world, deforestation is taking place for land rather than for timber, and as such these forest are simply burnt down, releasing yet more carbon dioxide into the atmosphere

Urbanisation

• As well as increasing human activities such as transportation, energy use, and industrial activities in urban areas, urbanisation also affects the carbon cycle by altering land use patterns
• Forests, wetlands, and other natural ecosystems are often replaced with buildings, roads, and other infrastructure
• This reduces the amount of carbon that can be sequestered in plants and soil, and the carbon storage capacity of the land is decreased.