The Characteristics of the Main Inputs and Outputs of the Carbon Cycle

The global carbon cycle describes the transfer of carbon between different stores. Carbon is essential to life on Earth, forming the building blocks of organic molecules and influencing the Earth’s climate system through its role in the greenhouse effect.

Although the global carbon cycle is a closed system, carbon is constantly transferred between stores through physical, chemical, and biological processes.

These processes operate at different speeds and scales, forming two interconnected subsystems: the slow carbon cycle and the fast carbon cycle.

The Slow Carbon Cycle

The slow carbon cycle involves the movement of carbon between the atmosphere, oceans, and lithosphere over millions of years.

The total amount of carbon circulated by the slow cycle is around 10–100 million tonnes each year, with typical residence times for lithospheric carbon of about 150 million years.

Key Processes in the Slow Carbon Cycle

1. Carbon Storage in Rocks and Fossil Fuels

• Most of Earth’s carbon is locked up in sedimentary rocks, such as limestone and chalk, and in fossil fuels like coal, oil, and natural gas.
• This carbon is stored for millions of years, forming long-term carbon sinks.

2. Marine Carbonate Formation

• CO₂ diffuses from the atmosphere into the oceans, where marine organisms such as corals and shellfish use it to build shells and skeletons made of calcium carbonate (CaCO₃).
• When these organisms die, their remains accumulate on the ocean floor, eventually forming carbon-rich sedimentary rocks under heat and pressure.

3. Subduction and Volcanic Outgassing

• Over geological time, some carbon-rich rocks are subducted into the mantle at tectonic plate boundaries.
• Carbon is later returned to the atmosphere through volcanic eruptions, releasing CO₂ and other gases.

4. Chemical Weathering

• Carbonation weathering occurs when rainwater absorbs CO₂, forming weak carbonic acid (H₂CO₃).
• This acid reacts with carbonate rocks, breaking them down and releasing carbon to the atmosphere and rivers as dissolved ions.
• These ions are transported to the oceans, where they can form new marine carbonates.

5. Burial of Organic Carbon

• On land, partly decomposed plant material can be buried under sediments, forming fossil fuels (coal, oil, and gas).
  These long-term stores can remain locked away for millions of years until combustion or tectonic uplift releases their carbon.

The Fast Carbon Cycle

The fast carbon cycle circulates carbon rapidly between the atmosphere, biosphere, oceans, and soils.
Transfers in this cycle occur 10 to 1,000 times faster than those in the slow carbon cycle, with carbon typically residing in the atmosphere or biosphere for only a few years or decades.

Key Processes in the Fast Carbon Cycle

1. Photosynthesis

• Plants and phytoplankton absorb CO₂ from the atmosphere (or oceans) and combine it with water using sunlight to produce glucose and oxygen:
• CO₂ + H₂O → C₆H₁₂O₆ + O₂
• This process transfers carbon from the atmosphere into the biosphere, forming the basis of the food chain.

2. Respiration

• Plants, animals, and microorganisms release CO₂ back to the atmosphere through respiration, as glucose is broken down to release energy: C₆H₁₂O₆ + O₂ → CO₂ + H₂O + Energy

3. Decomposition

• When organisms die, bacteria and fungi break down organic matter, releasing CO₂ and methane (CH₄).
• In some environments (e.g. peatlands or permafrost), decomposition is incomplete, leading to temporary carbon storage in soils.

4. Combustion

• Burning of organic matter, including forests or fossil fuels, rapidly releases carbon back to the atmosphere as CO₂ and CH₄.
• This process is a major anthropogenic input to the atmospheric carbon store.

5. Ocean–Atmosphere Exchange

• CO₂ diffuses between the atmosphere and ocean surface waters.
• Warm tropical waters tend to release CO₂, while cold polar waters absorb it, helping regulate global carbon balance.
• Individual carbon atoms remain stored in the oceans for an average of about 350 years before re-entering the atmosphere.