Biology · Ecology · Biogeochemical Cycles

Carbon Cycle

How carbon moves through Earth’s systems

This lesson explains how matter moves through Earth's systems, where it is stored, how organisms obtain it, and how biological, chemical, and geological processes return it to other parts of the cycle.

A visual overview of the major reservoirs and transfers in the carbon cycle.

What Is the Carbon Cycle?

The carbon cycle is the movement of carbon among the atmosphere, living organisms, oceans, soil, and long-term geological reservoirs. Carbon is essential because it forms the backbone of carbohydrates, lipids, proteins, nucleic acids, and many other molecules.

Carbon is stored in different reservoirs. Carbon dioxide (CO₂) is found in the atmosphere and dissolved in water. Plants and animals contain carbon in organic molecules. Soil stores carbon in dead material and organic matter, while rocks, sediments, and fossil fuels can store carbon for very long periods.

Carbon can move rapidly through living systems or remain stored in geological reservoirs for millions of years.

Key Processes in the Carbon Cycle

Photosynthesis

Plants, algae, and some bacteria remove CO₂ from the atmosphere or water and use carbon to build organic molecules.

Cellular Respiration

Organisms break down organic molecules for energy and release CO₂ back into the environment.

Feeding

Carbon moves from one organism to another when organisms consume food containing organic carbon.

Decomposition

Decomposers break down dead organisms and waste, returning carbon to soil, water, and the atmosphere.

Combustion

Burning biomass and fossil fuels rapidly transfers stored carbon to the atmosphere as CO₂.

Ocean Exchange

CO₂ moves between the atmosphere and ocean. Dissolved carbon can be stored as carbonic acid, bicarbonate, and carbonate ions.

Carbon Storage and Residence Time

Not all carbon moves through the cycle at the same rate. Carbon in leaves or atmospheric CO₂ can move relatively quickly, while carbon buried in sediments, carbonate rocks, or fossil fuels can remain stored for thousands to millions of years.

Short-Term Reservoirs

Atmosphere, plants, animals, surface water, and soils can exchange carbon relatively quickly.

Long-Term Reservoirs

Deep ocean sediments, carbonate rocks, and fossil fuels can store carbon for geologic timescales.

Why This Matters

Moving carbon into or out of long-term reservoirs changes the rate at which atmospheric CO₂ can accumulate.

Human Impacts on the Carbon Cycle

Human activities can change the balance among carbon reservoirs. Burning fossil fuels transfers carbon that was stored underground into the atmosphere. Deforestation can reduce the amount of carbon removed from the atmosphere by photosynthesis and can release carbon stored in vegetation and soil.

Fossil Fuel Combustion

Coal, oil, and natural gas contain carbon that was stored for very long periods. Combustion converts much of that carbon into atmospheric CO₂.

Deforestation

Removing vegetation can decrease photosynthetic carbon uptake and release carbon through burning and decomposition.

Ocean Uptake

Oceans absorb a portion of atmospheric CO₂, but increased dissolved CO₂ can contribute to ocean acidification.

Climate Feedbacks

Changes in temperature, vegetation, soils, and oceans can alter how much carbon is stored or released.

Common Misconceptions

Misconception: Carbon dioxide is the only form of carbon in the carbon cycle.

Correct: Carbon occurs in many forms, including CO₂, organic molecules, dissolved carbon, carbonate compounds, sediments, and fossil fuels.

Misconception: Photosynthesis adds CO₂ to the atmosphere.

Correct: Photosynthesis generally removes CO₂ from the atmosphere or water and incorporates carbon into organic molecules.

Misconception: Decomposition destroys carbon.

Correct: Decomposition changes where carbon is stored and returns carbon to the environment; matter is recycled.

Misconception: Burning fossil fuels creates carbon from nothing.

Correct: Combustion transfers carbon that was already stored in fossil fuels into the atmosphere, mainly as CO₂.

If You See This on a Test, Think This

CO₂ decreases after a plant-rich ecosystem experiences increased photosynthesis. Why?

Photosynthesis removes CO₂ and incorporates its carbon into organic molecules.

A graph shows atmospheric CO₂ rising after fossil fuel use increases. What process is responsible?

Combustion transfers previously stored carbon into the atmosphere as CO₂.

A question asks which reservoir stores carbon for the longest times. What should you look for?

Geological reservoirs such as rocks, sediments, and fossil fuels store carbon over long timescales.

Quick Practice

A forest experiences increased photosynthesis during a growing season. What happens to atmospheric CO₂?

Atmospheric CO₂ generally decreases because photosynthesis removes CO₂ and stores its carbon in organic molecules.

Why can burning fossil fuels alter the carbon cycle?

It rapidly moves carbon from a long-term geological reservoir into the atmosphere as CO₂.

How can oceans affect atmospheric CO₂?

Oceans exchange CO₂ with the atmosphere and can absorb and store dissolved carbon.

Which process returns CO₂ when cells obtain energy from glucose?

Cellular respiration.

Key Takeaways

Remember These Ideas

Carbon cycles among the atmosphere, organisms, oceans, soil, and geological reservoirs.
Photosynthesis removes CO₂; respiration, decomposition, and combustion can return CO₂.
Feeding transfers organic carbon through food webs.
Fossil fuels and rocks are long-term carbon reservoirs.
Human activities can increase atmospheric CO₂ by moving stored carbon into the atmosphere.

HEY, TEACHERS.

Need a worksheet to go with this lesson?

Find classroom-ready Biology and Chemistry practice designed by a science teacher.

Browse Teacher Resources →

NEED TUTORING?

Sometimes you just need someone to explain it differently.

Get personalized Biology or Chemistry help from an experienced high school science teacher.

Learn About Tutoring →