The Carbon Cycle

What Is the Carbon Cycle?

The carbon cycle describes the continuous movement of carbon atoms between the atmosphere, living organisms, oceans, soil, and rocks. Carbon is the chemical building block of all known life, found in every living cell, and it also exists in non-living forms such as carbon dioxide gas, dissolved carbon in seawater, and carbon locked inside rock and fossil fuels.

Like the water cycle, no carbon is created or destroyed as it cycles — it is constantly recycled between different reservoirs (places carbon is stored).

Carbon Reservoirs

Carbon is stored in four main reservoirs, though the amount held in each and how long it stays there varies enormously:

Reservoir	Examples	Typical residence time
Atmosphere	Carbon dioxide gas	Years to decades
Biosphere	Plants, animals, soil, dead organic matter	Days to centuries
Hydrosphere	Dissolved CO2 in oceans, lakes, rivers	Years to centuries
Geosphere (lithosphere)	Fossil fuels, limestone, other carbonate rock	Thousands to millions of years

The geosphere holds by far the largest amount of carbon, but it moves the slowest — carbon can stay locked in rock and fossil fuels for millions of years before it re-enters the faster-moving parts of the cycle.

Key Processes

Photosynthesis

Plants, algae, and some bacteria remove carbon dioxide from the atmosphere and use light energy to combine it with water, producing glucose and releasing oxygen. This converts inorganic atmospheric carbon into organic carbon stored in living tissue.

Respiration

Plants, animals, fungi, and microorganisms break down glucose to release the energy they need to live. This process releases carbon dioxide back into the atmosphere (or into water, for aquatic organisms), balancing out photosynthesis.

Decomposition

When organisms die, decomposers (bacteria and fungi) break down their remains. Much of the carbon in dead tissue is released as carbon dioxide through the decomposers' own respiration, while some remains in the soil as organic matter.

Combustion

Burning organic material — wood, or fossil fuels such as coal, oil, and natural gas — releases carbon dioxide that was stored, in some cases, for millions of years. Wildfires and the burning of fossil fuels for energy are both forms of combustion.

Ocean-Atmosphere Exchange

Carbon dioxide dissolves directly into ocean water at the surface, and dissolved CO2 can also be released back into the atmosphere. The oceans absorb roughly a quarter of the carbon dioxide humans emit each year, making them a crucial carbon sink.

Sedimentation and Fossilisation

Marine organisms build shells and skeletons from calcium carbonate. When they die, these hard parts accumulate on the seafloor, and over millions of years, pressure and burial can compress them into carbon-rich sedimentary rock such as limestone. Similarly, ancient plant and animal remains buried under heat and pressure can slowly form fossil fuels like coal, oil, and natural gas.

The Fast Cycle vs the Slow Cycle

Scientists often describe two overlapping carbon cycles:

• The fast (biological) carbon cycle — carbon moves between the atmosphere, living organisms, and surface oceans over years to decades, driven mainly by photosynthesis, respiration, and ocean exchange.
• The slow (geological) carbon cycle — carbon moves between rocks, sediments, and the deep ocean over thousands to millions of years, involving processes like rock weathering, volcanic eruptions, and sedimentation.

The slow cycle acts as Earth's long-term thermostat, gradually removing carbon dioxide from the atmosphere through rock weathering and burial, then returning it slowly through volcanic activity.

Human Impact on the Carbon Cycle

Human activity has significantly sped up the movement of carbon out of long-term storage and into the atmosphere:

• Burning fossil fuels — releases carbon that took millions of years to form in a matter of decades
• Deforestation — removes trees that would otherwise absorb CO2, and burning cleared vegetation releases stored carbon
• Cement production — chemically releases CO2 from limestone during manufacturing
• Agriculture — livestock farming and soil disturbance release both carbon dioxide and methane

Since the industrial revolution, atmospheric carbon dioxide concentrations have risen sharply, strengthening the greenhouse effect and driving climate change. Rising ocean CO2 absorption is also causing ocean acidification, which threatens shell-building marine life.

Why the Carbon Cycle Matters

• Regulates Earth's climate by controlling how much heat-trapping CO2 is in the atmosphere
• Supplies the carbon that makes up every living organism — proteins, carbohydrates, fats, and DNA all contain carbon
• Connects the biosphere, atmosphere, hydrosphere, and geosphere into one interacting system
• Understanding it helps scientists predict and respond to climate change

Worked Example

Q: Trace a carbon atom from the atmosphere, through a food chain, and into long-term storage.

1. A carbon atom exists as part of a carbon dioxide molecule in the atmosphere.
2. A grass plant absorbs it during photosynthesis, converting it into glucose stored in plant tissue.
3. A cow eats the grass; the carbon becomes part of the cow's body through digestion.
4. The cow respires, releasing some of that carbon back to the atmosphere as CO2 through respiration — or the cow dies and decomposers break down its remains, releasing more carbon through decomposition.
5. If instead the cow's remains (or ancient plant matter) are buried under sediment for millions of years without fully decomposing, the carbon can eventually form fossil fuels, locking it away in the geosphere until it is released again by combustion.

Key Terms

Term	Definition
Carbon cycle	The continuous movement of carbon between the atmosphere, living things, oceans, and rocks
Reservoir	A place where carbon is stored, such as the atmosphere, oceans, or rocks
Carbon sink	A reservoir that absorbs more carbon than it releases, such as forests or oceans
Photosynthesis	Process that converts atmospheric CO2 into organic carbon using light energy
Respiration	Process that releases CO2 by breaking down glucose for energy
Combustion	Burning organic material or fossil fuels, releasing stored carbon as CO2
Fossil fuels	Coal, oil, and natural gas formed from ancient buried organic matter over millions of years

Common Mistakes

• Thinking carbon dioxide is only released by burning fossil fuels — respiration in every living organism also releases CO2 constantly, as part of the natural cycle.
• Confusing the fast and slow carbon cycles — the fast cycle moves carbon through living things in years, while the slow cycle moves carbon through rock over millions of years.
• Believing the carbon cycle and the water cycle are the same system — they are separate cycles, though both are driven partly by the same processes, like photosynthesis and respiration.
• Assuming oceans only release carbon dioxide — oceans both absorb and release CO2, currently acting as a net carbon sink.

Tips and Tricks

• Photosynthesis removes CO2 from the air; respiration and combustion put it back — the cycle balances these opposing processes.
• Remember the four reservoirs by thinking "air, life, water, rock" — atmosphere, biosphere, hydrosphere, geosphere.
• The slower a reservoir releases carbon, the longer it typically stores it — rocks and fossil fuels hold carbon the longest by far.
• Human activity is not creating new carbon — it is releasing ancient stored carbon far faster than natural processes would.