Carbon Cycle (HL IB Environmental Systems & Societies (ESS))

Carbon Cycle

• Many different materials cycle through the abiotic and biotic components of an ecosystem

  • All materials in the living world are recycled to provide the building blocks for future organisms

• Elements such as carbon are not limitless resources

  • There is a finite amount of each element on the planet

  • Elements need to be recycled in order to allow new organisms to be made and grow

• Carbon is constantly being recycled around the biosphere so that the total amount of carbon in the biosphere is essentially constant

  • Carbon is transferred from one form to another by the various processes in the carbon cycle

Organic and inorganic carbon stores

• Organisms, crude oil and natural gas contain organic stores of carbon

  • Organic stores refer to the carbon-containing compounds found in organisms and fossil fuels

  • For example, carbon in these stores may exist as carbohydrates in organisms or hydrocarbons in fossil fuels

• Inorganic stores exist in the atmosphere, soils and oceans

  • Inorganic stores refer to reservoirs of carbon that exist in other non-living components of the biosphere

  • For example, carbon in these stores may exist as carbon dioxide or carbonates

Equilibrium and residence time

• A carbon store is in equilibrium when absorption (uptake) is balanced by the release

  • For example, the carbon stored in trees through photosynthesis is balanced by the carbon released during respiration

• Residence time is the average time that a carbon atom remains in a store

  • Without human interference like mining, the residence time in fossil fuels would be measured in hundreds of millions of years

Carbon flows in ecosystems

• Carbon flows between stores in ecosystems through various processes

• The main processes include:

  • Photosynthesis (transformation)—plants absorb CO₂ and convert it into organic compounds (carbohydrates)

  • Cellular respiration (transformation)—both plants and animals release CO₂ during respiration

  • Feeding (transfer)—animals consume organic matter, transferring carbon through the food chain

  • Defecation (transfer)—carbon is returned to the soil through waste products

  • Death and decomposition (transfer)—decomposers break down dead organisms, releasing carbon back into the soil

• Other processes include:

  • Fossilisation—if animals and plants die in conditions where decomposing microorganisms are not present, the carbon in their bodies can be converted, over millions of years and significant pressure, into fossil fuels such as 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

  • Combustion—when fossil fuels are burned, the carbon locked within them combines with oxygen to form CO₂, which is released into the atmosphere

Carbon sequestration

• Carbon sequestration is the process of capturing atmospheric CO₂ and storing it in solid or liquid forms

  • For example, trees naturally sequester carbon by absorbing CO₂ during photosynthesis and storing it in their biomass

  • Organic matter can be fossilised over millions of years to form coal, oil and natural gas, resulting in carbon being stored underground

Ecosystems as stores, sinks or sources

• Ecosystems can act as stores, sinks or sources of carbon depending on the balance between inputs and outputs

• Net accumulation of carbon or net release of carbon is determined by the difference between total inputs and outputs

• For example:

  • Young forest ecosystem: acts as a sink, as photosynthesis exceeds respiration, leading to net uptake of CO₂

  • Mature forest ecosystem: acts as a store, with carbon cycling between living organisms, soil and atmosphere

  • Forest destruction (fire or deforestation): acts as a source, releasing stored carbon back into the atmosphere