The Processes of the Carbon Cycle

The carbon cycle describes the continuous movement of carbon between the atmosphere, biosphere, hydrosphere, lithosphere, and cryosphere. Carbon is essential for all living organisms and plays a key role in regulating the Earth’s climate through the greenhouse effect.

Although the global carbon cycle is a closed system, carbon moves rapidly between stores via biological, chemical, and physical processes. These processes operate at different speeds within two overlapping subsystems, the fast and slow carbon cycles.

Photosynthesis

Photosynthesis transfers carbon from the atmosphere to the biosphere.

• Plants, algae, and phytoplankton absorb carbon dioxide (CO₂) from the atmosphere (or dissolved CO₂ from the oceans) and combine it with water using sunlight to produce glucose (C₆H₁₂O₆) and oxygen.
• This process can be summarised as:
  CO₂ + H₂O → C₆H₁₂O₆ + O₂
• Photosynthesis removes large quantities of CO₂ each year, locking it into organic matter and forming the foundation of food webs.
• Forests and oceans act as major carbon sinks through this process, sequestering carbon in vegetation and marine biomass.

Respiration

Respiration returns carbon from living organisms to the atmosphere.

• Plants, animals, and microorganisms break down glucose to release energy for growth and repair.
• The process releases CO₂ and water vapour:
  C₆H₁₂O₆ + O₂ → CO₂ + H₂O + Energy
• Respiration balances photosynthesis, ensuring continuous carbon exchange between the biosphere and atmosphere.

Decomposition

Decomposition recycles carbon stored in dead plants and animals.

• Bacteria and fungi break down organic material, releasing CO₂ (in aerobic conditions) or methane (CH₄) (in anaerobic conditions).
• This process transfers carbon from the biosphere to the atmosphere and soils.
• Decomposition rates vary with temperature, moisture, and oxygen availability; they are fastest in warm, humid environments and slowest in cold or waterlogged regions such as peatlands or permafrost, where carbon can be trapped for centuries.

Combustion

Combustion rapidly releases carbon stored in organic matter back to the atmosphere.

There are two main forms:

1. Natural Combustion

• Occurs during wildfires, particularly in dry tropical and temperate forests.
• CO₂ and CH₄ are emitted, but wildfires also play a role in renewing vegetation, enabling future carbon sequestration as regrowth absorbs new CO₂.

2. Human (Fossil Fuel) Combustion

• The burning of coal, oil, and natural gas transfers carbon from long-term geological stores in the lithosphere to the atmosphere.
• This process has significantly increased atmospheric CO₂ since the Industrial Revolution, contributing to global warming.
• The reaction can be represented as:
  CH₄ + 2O₂ → CO₂ + 2H₂O

Natural Carbon Sequestration

Carbon sequestration is the process by which carbon is captured and stored in long-term natural reservoirs. It helps balance carbon inputs and outputs, preventing excessive accumulation of CO₂ in the atmosphere.

Sequestration in Oceans

• CO₂ diffuses from the atmosphere into ocean surface waters, where it reacts to form carbonic acid, bicarbonate, and carbonate ions.
• Marine organisms such as corals, shellfish, and plankton use these ions to form calcium carbonate (CaCO₃) shells and skeletons.
• When these organisms die, their remains sink to the seafloor, storing carbon in marine sediments.
• The oceans hold around 38,000 GtC, making them the largest active carbon sink.

Sequestration in Vegetation

• Forests, grasslands, and other ecosystems store carbon through photosynthesis, with trees acting as long-term carbon reservoirs.
• Some carbon is stored in wood and roots for decades, while a portion enters the soil through leaf litter and organic decay.
• Land-use changes such as deforestation and afforestation greatly influence the size of this store.

Sequestration in Sediments

• Carbon that reaches the ocean floor is buried and compacted over millions of years, forming carbonate rocks (e.g. limestone) and organic-rich shales.
• This geological storage is a central part of the slow carbon cycle, removing carbon from active circulation for very long timescales.

Weathering

Weathering is a key process in the slow carbon cycle, transferring carbon between the atmosphere, hydrosphere, and lithosphere over geological timescales.

Chemical Weathering

• Carbonation: Rainwater absorbs atmospheric CO₂ to form carbonic acid (H₂CO₃).
  This weak acid reacts with carbonate rocks such as limestone and chalk, producing calcium bicarbonate (Ca(HCO₃)₂) that is carried to the oceans in solution.
  CaCO₃ + H₂CO₃ → Ca(HCO₃)₂
• Hydrolysis: Carbonic acid reacts with silicate minerals (e.g. feldspar), producing clay minerals and dissolved compounds that also transport carbon to the ocean.
• Oxidation: Oxygen reacts with minerals (especially those containing iron and organic matter), sometimes releasing CO₂ to the atmosphere.

Through these reactions, chemical weathering removes CO₂ from the atmosphere, storing it in oceans and sediments — helping to regulate Earth’s long-term climate.