What is the carbon cycle?
Carbon is in every living thing, in the air as carbon dioxide (CO₂), dissolved in the sea, and locked in rocks such as limestone and in fossil fuels such as coal, oil and gas. The total amount of carbon on Earth stays the same. It just moves around. This movement is the carbon cycle.
It is one of the biogeochemical cycles (like the water, nitrogen and phosphorus cycles): an element moves through living things (bio), rocks (geo) and chemical changes.
We describe it as a system:
- Stores (reservoirs): places where carbon is kept.
- Flows (fluxes): movements of carbon between stores, measured in gigatonnes of carbon per year (GtC/yr). 1 Gt = 1 billion tonnes.
- A sink takes in more carbon than it gives out; a source gives out more than it takes in.
The Earth is a closed system for carbon: almost no carbon enters from space or leaves.
Global carbon stores
| Store | About how much carbon |
|---|---|
| Sedimentary rocks (limestone, chalk) | 60–100 million GtC (the largest by far) |
| Deep ocean | about 37 000 GtC |
| Fossil fuels (coal, oil, gas) | about 1000 GtC in known reserves |
| Soil, including permafrost | about 1500–2400 GtC |
| Atmosphere | about 870 GtC today (about 590 before 1750) |
| Plants and animals (biosphere) | about 450–650 GtC |
Numbers are rounded estimates from science reports; what matters is the order: rocks ≫ ocean ≫ soil > fossil fuels ≈ air > living things.
Carbon can stay in a store for very different times: a few years in the air, decades to centuries in trees and soil, thousands of years in the deep ocean, and millions of years in rocks.
The fast cycle: photosynthesis, respiration and decomposition
Photosynthesis: plants and algae use light energy to turn CO₂ and water into glucose and oxygen. Carbon moves from the air into living things (about 120 GtC/yr on land).
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light, chlorophyll)
Respiration: plants, animals and microbes break down glucose to release energy, giving CO₂ back to the air.
Feeding: carbon passes along food chains when animals eat plants or other animals.
Decomposition: when living things die, bacteria and fungi break them down and respire, releasing CO₂. In waterlogged or very cold places (peat bogs, permafrost) decay is slow, so carbon builds up in the soil.
Combustion: burning wood or fuels joins carbon with oxygen and releases CO₂ quickly.
In nature, photosynthesis and respiration nearly balance. That is why the air's CO₂ stayed close to 280 ppm for thousands of years.
Oceans, rocks and the slow cycle
Ocean exchange: CO₂ dissolves in cold surface water and is carried down to the deep ocean (the physical pump). Tiny ocean plants (phytoplankton) take in CO₂, and when they and shelled animals die, some sink to the sea floor (the biological pump). Warm water holds less CO₂ and releases some.
Sequestration means locking carbon away in a long-term store, such as deep ocean water, sea-floor sediments, rocks, peat or growing forests.
The slow cycle takes millions of years:
- Shells and skeletons pile up on the sea floor and are pressed into limestone and chalk.
- Buried plant and plankton remains, squeezed and heated, become coal, oil and gas.
- Chemical weathering: rain is a weak carbonic acid. It slowly dissolves rocks, and rivers carry the carbon to the sea.
- Volcanoes release CO₂ from deep rocks back into the air (well under 1 GtC/yr).
Human changes, the carbon budget and climate
Natural changes: wildfires, volcanic eruptions and the seasons change the flows. In northern spring, plants grow and CO₂ in the air dips; in autumn it rises again.
Human changes:
- Burning coal, oil and gas releases carbon that took millions of years to store (about 10 GtC/yr).
- Clearing forests, especially tropical rainforests, removes a big store and a sink (about 1 GtC/yr). Rainforests also pump water into the air through transpiration; fewer trees means less rain and drier soils, so the water and carbon cycles are linked.
- Farming, draining peat and making cement add more.
The carbon budget compares inputs and outputs. Today roughly half of our emissions stay in the air, about a quarter goes into the ocean and a quarter into land plants. CO₂ in the air rose from about 280 ppm before 1750 to over 420 ppm now.
More CO₂ strengthens the greenhouse effect, so the Earth warms. Extra CO₂ dissolving in the sea also makes it more acidic, which harms corals and shellfish. Warming can thaw permafrost and release more carbon: a positive feedback.
Cutting carbon
- Use renewable energy (solar, wind, hydro) and save energy.
- Protect and plant forests; restore mangroves and peat bogs.
- Better farming and less food waste; public transport and electric vehicles.
- Carbon capture and storage at power stations and factories.
Try it at home
List the fuels your family uses in one week (LPG, petrol, electricity). Use 1 kg of LPG ≈ 3 kg CO₂ and 1 litre of petrol ≈ 2.3 kg CO₂ to estimate your weekly emissions, then pick one way to cut it.
Key formulas and definitions
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
- Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O
- Net change in a store = inputs − outputs (GtC/yr)
- 1 GtC ≈ 3.67 Gt CO₂; about 2.12 GtC raises air CO₂ by 1 ppm
Worked examples
1. Name the process that moves carbon from the air into plants.
Photosynthesis. Plants take in CO₂ and build it into glucose.
2. A store receives 120 GtC/yr and loses 123 GtC/yr. Is it a sink or a source, and by how much?
Net = 120 − 123 = −3 GtC/yr. It loses carbon, so it is a source of 3 GtC each year.
3. Why is peat a carbon store?
Peat forms in waterlogged ground with little oxygen. Decomposers work slowly there, so dead plant carbon builds up instead of being respired back to the air.
4. People emit 11 GtC/yr. The ocean takes 2.9 GtC/yr and land plants 3.2 GtC/yr. How much stays in the air?
11 − 2.9 − 3.2 = 4.9 GtC/yr stays in the air (about 45%).
5. Using 2.12 GtC per ppm, how much does 4.9 GtC raise the CO₂ level?
4.9 ÷ 2.12 ≈ 2.3 ppm in one year.
6. Explain how cutting down a tropical rainforest affects both the carbon and the water cycle.
Carbon: the trees' store is lost (often burned, releasing CO₂) and the sink that took in carbon each year is gone. Water: less transpiration and interception means less moisture going back into the air, less local rain and more runoff and soil erosion.
Common mistakes
- Thinking plants only photosynthesise. They also respire all the time, day and night.
- Saying the atmosphere is the biggest carbon store. Rocks and the deep ocean hold far more.
- Thinking burning fossil fuels creates new carbon. It moves old, locked carbon back into the air quickly.
- Mixing up the ozone hole with the extra greenhouse effect. More CO₂ traps heat; it does not make the ozone hole.