Carbon Cycle

The Carbon Cycle

The continuous movement of carbon among Earth’s spheres through natural and human processes.

Major Carbon Reservoirs

Rocks (largest), oceans, atmosphere, plants, soil, fossil fuels.

Carbon Reservoir (Definition)

A natural storage location for carbon, such as the atmosphere or ocean.

Processes in the Carbon Cycle

Photosynthesis, atmosphere/ocean exchange, decomposition, respiration, fossil fuel burning, volcanic eruptions, rock formation.

Photosynthesis Process

by which plants absorb CO₂ from the atmosphere and convert it into sugars.

Respiration Process

by which living organisms release CO₂ back into the atmosphere by breaking down sugars.

Decomposition Process

by which bacteria and fungi break down dead material, releasing carbon into the soil and atmosphere.

Atmosphere–Ocean Exchange

Process in which carbon moves between the ocean and atmosphere through gas exchange.

Volcanic Eruptions

Process that releases CO₂ from rocks in the geosphere into the atmosphere.

Fossil Fuel Burning

Process by which humans release stored carbon from fossil fuels into the atmosphere as CO₂.

Rock Formation

Process that locks carbon into solid form in the geosphere as sedimentary or carbonate rock.

Slow Carbon Cycle

Movement of carbon between rocks, soil, ocean, and atmosphere over 100–200 million years.

Fast Carbon Cycle

Movement of carbon through living things on short timescales (days to years).

Example of Slow Carbon Cycle

CO₂ dissolves in rainwater → reacts with rocks → carried to oceans → forms carbonate rock → released by volcanoes.

Example of Fast Carbon Cycle

Plants absorb CO₂ → animals eat plants → respiration releases CO₂ back to atmosphere.

Hydrosphere

Includes rivers, lakes, oceans, and groundwater; exchanges carbon with the atmosphere.

Geosphere

Includes rocks, magma, and sediments; stores carbon as carbonate and fossil fuels.

Biosphere

Includes plants, animals, fungi, and microorganisms; moves carbon through respiration and photosynthesis.

Atmosphere

Layer of gases surrounding Earth; stores carbon as CO₂.

Process Moving Carbon Between Atmosphere and Biosphere

Photosynthesis and respiration.

Process Moving Carbon Between Geosphere and Atmosphere

Volcanic eruptions and fossil fuel burning.

Process Moving Carbon Between Hydrosphere and Atmosphere

Atmosphere–ocean gas exchange.

Sources and Sinks

Sources add carbon to a system

sinks remove carbon from a system.

Carbon Source

Releases more CO₂ than it absorbs (e.g., fossil fuel burning, decomposition).

Carbon Sink

Absorbs more CO₂ than it releases (e.g., oceans, forests).

When Sources > Sinks

Atmospheric CO₂ increases and the planet warms.