The carbon in your body has probably been in many other places before. Some carbon atoms may once have been carbon dioxide in the atmosphere, then became part of a plant during photosynthesis, passed into an animal through feeding, and eventually returned to the environment.
This continuous movement is called the carbon cycle. The important idea is that carbon is recycled. Individual carbon atoms move between different carbon stores, but they are not used up.
Understanding this cycle also helps us see why burning fossil fuels and removing forests can affect Earth's climate. Human activity does not create new carbon atoms. Instead, it changes where carbon is stored and how quickly it moves between stores.
Try these five questions before reading the chapter. They check whether you can recognise the main processes that move carbon and connect atmospheric carbon dioxide with global warming.
- 5 diagnostic questions
- Carbon-cycle and climate reasoning
- Instant explanation for every answer option
What is the carbon cycle?
Carbon occurs in many different forms. In the atmosphere, much of the carbon relevant to this topic is present as carbon dioxide, CO₂. In living organisms, carbon is found in substances such as carbohydrates, lipids and proteins. Large amounts are also stored in soil, oceans, rocks and fossil fuels.
A place in which carbon is held is called a carbon store or carbon reservoir. Carbon continually moves between these stores. The collection of processes that causes this movement forms the carbon cycle.
The central idea: carbon atoms are recycled. Photosynthesis moves carbon into living organisms, feeding moves it through food chains, and respiration, decomposition and combustion can return carbon to the atmosphere.
Photosynthesis moves atmospheric carbon into living organisms
Green plants take in carbon dioxide from the atmosphere. During photosynthesis, carbon dioxide and water are used to manufacture glucose.
The carbon atoms that were originally part of atmospheric CO₂ therefore become part of organic molecules in the plant.
Atmospheric CO₂ → plant biomass
This transfer occurs through photosynthesis.
Some of the glucose formed may be used immediately in respiration. Other glucose molecules may be converted into substances such as starch, cellulose, lipids or amino acids, so the carbon becomes part of the plant's tissues.
Feeding transfers carbon through food chains
Once carbon has entered a plant, it can pass to other organisms.
When a herbivore eats a plant, carbon-containing molecules from the plant enter the herbivore. If the herbivore is later eaten by a predator, some of that carbon is transferred again.
Plant → herbivore → predator
At each feeding stage, carbon-containing organic substances move from one organism to another.
Not all ingested carbon becomes part of the consumer's body. Some material may remain undigested and leave the body in faeces, while some absorbed organic substances are broken down during respiration.
Respiration returns carbon dioxide to the environment
Plants are sometimes incorrectly thought of as organisms that only remove carbon dioxide. In fact, plants respire as well as photosynthesise.
Plants, animals and many microorganisms carry out respiration. During aerobic respiration, organic substances are broken down and carbon dioxide is produced.
Organic carbon in organisms → atmospheric CO₂
This transfer occurs through respiration.
Photosynthesis and respiration therefore move carbon in opposite overall directions. Photosynthesis transfers carbon from atmospheric carbon dioxide into organic compounds, while respiration transfers carbon from organic compounds back into carbon dioxide.
Decomposition recycles carbon from dead organisms and waste
Carbon does not stop moving when an organism dies. Dead organisms, fallen leaves and waste products contain organic carbon.
Decomposers, including many bacteria and fungi, break down this dead organic material. They obtain nutrients and organic substances from it and carry out respiration.
Their respiration releases carbon dioxide back into the environment.
A useful distinction is that decomposition breaks down the dead material, while it is largely respiration by decomposers that releases its carbon as carbon dioxide.
Combustion returns stored carbon to the atmosphere
Wood and fossil fuels contain carbon. When these materials undergo combustion, their carbon reacts with oxygen and carbon dioxide is produced.
Burning wood transfers recently stored biological carbon back into the atmosphere. Burning fossil fuels is especially important because coal, petroleum and natural gas contain carbon that has been stored underground for very long periods.
Humans can burn this fuel in minutes even though the carbon-containing deposits took millions of years to form. The rate of carbon transfer into the atmosphere can therefore greatly exceed the rate at which comparable long-term stores are formed.
Putting the major carbon transfers together
The carbon cycle is easier to understand when each arrow is treated as a named process. Do not simply memorise a circle of arrows. Ask what is happening to the carbon at each stage.
The diagram is deliberately simplified. In nature, carbon can move through many additional pathways. For O-Level Biology, the important skill is to connect each major transfer with the correct biological or human process.
| Process | Main carbon transfer | What happens? |
|---|---|---|
| Photosynthesis | Atmosphere → plants | CO₂ is used to manufacture organic substances. |
| Feeding | One organism → another organism | Carbon-containing food molecules are transferred through a food chain. |
| Respiration | Living organisms → atmosphere | Organic substances are broken down and carbon dioxide is produced. |
| Decomposition | Dead material → decomposers | Decomposers obtain organic material from dead organisms and waste. |
| Decomposer respiration | Decomposers → atmosphere | Carbon dioxide is released as decomposers respire. |
| Combustion | Fuel → atmosphere | Carbon in wood or fossil fuels is oxidised and carbon dioxide is released. |
Carbon stores and carbon sinks are not exactly the same thing
A carbon store is simply somewhere carbon is held. Forest biomass, soil, oceans and fossil fuels are all examples of stores.
A carbon sink is a store that, over a particular period of time, absorbs more carbon than it releases.
Growing forests can act as carbon sinks because photosynthesis transfers carbon dioxide into increasing amounts of plant biomass. The oceans can also take up substantial amounts of carbon dioxide from the atmosphere.
Carbon store: holds carbon.
Carbon sink: takes in more carbon than it releases over a given period.
This distinction matters because a forest is not automatically an unlimited carbon sink. Plants also respire, organisms die and decompose, and fires can return stored carbon to the atmosphere.
How humans change the carbon cycle
The carbon cycle has operated for a very long time, but human activities can change the rates of particular carbon transfers.
Two especially important changes are the large-scale combustion of fossil fuels and deforestation.
Burning fossil fuels
Coal, petroleum and natural gas contain carbon that was stored underground over geological timescales.
When these fuels are burned for electricity generation, industry or transport, combustion converts much of this carbon into carbon dioxide.
This creates a rapid transfer:
Long-term underground carbon store → atmospheric CO₂
Deforestation
Deforestation can affect atmospheric carbon dioxide in more than one way.
First, removing trees reduces the amount of photosynthesising biomass available to take carbon dioxide out of the atmosphere.
Second, the carbon already stored in the vegetation may be returned to the atmosphere if the wood is burned or if dead material decomposes.
Deforestation can therefore both reduce carbon dioxide uptake and increase carbon dioxide release.
Why increasing carbon dioxide affects Earth’s temperature
Carbon dioxide is a greenhouse gas. To understand its effect, it helps to separate the natural greenhouse effect from the enhanced greenhouse effect.
The natural greenhouse effect
Energy from the Sun reaches Earth, and Earth's surface absorbs part of this energy. The warmed surface then emits energy mainly as infrared radiation.
Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation and re-emit it in different directions. Some of the energy is therefore returned towards Earth's surface and lower atmosphere instead of immediately escaping to space.
This natural greenhouse effect keeps Earth warmer than it would otherwise be and is essential for maintaining conditions suitable for life.
The enhanced greenhouse effect
Increasing the concentration of greenhouse gases such as carbon dioxide strengthens this effect. More outgoing infrared radiation is absorbed and re-emitted within the atmosphere.
This changes Earth's energy balance and contributes to an increase in average global temperature.
More atmospheric greenhouse gases → stronger greenhouse effect → higher average global temperature
What is global warming?
Global warming refers to the long-term increase in Earth's average surface temperature.
It is closely linked to the enhanced greenhouse effect caused by increasing concentrations of greenhouse gases. Carbon dioxide is particularly important because human activities such as fossil-fuel combustion and deforestation have increased the amount of CO₂ in the atmosphere.
Other greenhouse gases, including methane, also contribute to the greenhouse effect, but the carbon cycle provides the direct link between this topic and changing atmospheric CO₂.
Possible effects of global warming
A rise in global average temperature does not mean every place becomes uniformly warmer every day. Climate responses vary from region to region.
However, continued global warming can produce several important changes.
| Change | How global warming contributes | Possible biological or environmental effect |
|---|---|---|
| Rising sea level | Ocean water expands as it warms, while melting glaciers and land ice add water to the oceans. | Low-lying coastal habitats and human settlements may be affected. |
| Changing rainfall patterns | A warmer climate alters evaporation, atmospheric circulation and rainfall. | Some regions may face greater risks of drought or flooding. |
| More severe heat events | Higher average temperatures increase the likelihood of extreme heat. | Organisms may experience heat stress and changes in survival or reproduction. |
| Changes in habitats | Temperature and rainfall conditions suitable for particular species can shift. | Species distributions may change, and some populations may decline if they cannot adapt or move. |
| Changes in ecosystems | Different species respond differently to altered climate conditions. | Food webs and interactions between organisms may be disrupted. |
How can the increase in atmospheric carbon dioxide be reduced?
There are two broad approaches: reduce the rate at which carbon dioxide is added to the atmosphere and protect or increase processes that remove it.
Reduce fossil-fuel combustion
Using less coal, petroleum and natural gas reduces the transfer of carbon from long-term underground stores into the atmosphere.
This may involve greater use of lower-carbon or renewable energy sources such as solar, wind and hydroelectric power, together with improved energy efficiency.
Use energy more efficiently
Reducing unnecessary energy use can reduce the amount of fuel that needs to be burned. More energy-efficient buildings, appliances, transport and industrial processes can therefore reduce carbon dioxide emissions.
Reduce deforestation
Protecting forests helps retain carbon already stored in plant biomass and maintains photosynthetic uptake of carbon dioxide.
Reforestation and afforestation
Establishing additional forest can increase the amount of carbon held in plant biomass as growing trees carry out photosynthesis.
However, planting trees should not be viewed as permission to release unlimited carbon dioxide elsewhere. Forests have finite storage capacity and take time to grow.
Reduce the source: burn less fossil fuel and use energy more efficiently.
Protect or increase uptake: conserve forests and increase suitable vegetation where appropriate.
Common mistakes
| Common mistake | Why it is incorrect | Better way to think about it |
|---|---|---|
| Saying plants only take in carbon dioxide and never release it | Plants carry out respiration as well as photosynthesis. | Photosynthesis removes CO₂ overall, while plant respiration releases CO₂. |
| Saying animals obtain carbon directly from atmospheric carbon dioxide | Animals cannot normally fix atmospheric CO₂ by photosynthesis. | Animals obtain carbon mainly by feeding on other organisms. |
| Saying decomposition itself is exactly the same as respiration | Decomposition involves the breakdown of dead organic material; decomposers also respire. | Decomposers break down dead matter and release CO₂ through respiration. |
| Saying only animals respire | Plants and decomposers also respire. | Respiration occurs in living cells of plants, animals and many microorganisms. |
| Saying a carbon sink is simply any place containing carbon | That describes a carbon store. | A sink takes in more carbon than it releases over a given period. |
| Saying deforestation only matters because fewer trees produce oxygen | The important carbon-cycle effects involve carbon dioxide uptake and release. | Fewer trees means less photosynthetic CO₂ uptake, and stored carbon may also be released. |
| Saying the greenhouse effect itself is harmful and should disappear completely | The natural greenhouse effect keeps Earth warm enough for life. | The concern is the enhanced greenhouse effect caused by increased greenhouse-gas concentrations. |
| Saying global warming is caused by a hole in the ozone layer | Ozone depletion and global warming are different environmental issues. | Global warming is linked to the enhanced greenhouse effect caused by increased greenhouse gases. |
| Saying melting sea ice is the only cause of sea-level rise | Floating sea ice contributes little directly when it melts. | Thermal expansion of seawater and melting land ice are major contributors to rising sea level. |
| Saying fossil-fuel burning creates new carbon atoms | The carbon atoms already existed in the fuel. | Combustion transfers carbon from a long-term store into the atmosphere. |
What you should be able to do
Describe the carbon cycle
Explain how carbon moves between the atmosphere, organisms, decomposers and long-term stores.
Explain photosynthesis in the carbon cycle
State that photosynthesis transfers carbon dioxide from the atmosphere into organic substances in plants.
Explain feeding
Describe how carbon-containing organic compounds pass from one organism to another through food chains.
Explain respiration
State that plants, animals and decomposers can release carbon dioxide during respiration.
Explain decomposition
Describe how decomposers break down dead organisms and waste and return carbon to the cycle.
Explain combustion
Relate the burning of fossil fuels to the transfer of long-stored carbon into atmospheric CO₂.
Explain the effects of deforestation
Link forest removal to reduced CO₂ uptake and possible release of carbon stored in vegetation.
Distinguish stores and sinks
Recognise that a carbon store holds carbon, whereas a carbon sink absorbs more carbon than it releases over a period.
Explain the greenhouse effect
Describe how greenhouse gases absorb and re-emit outgoing infrared radiation.
Explain global warming
Link increased greenhouse-gas concentrations to the enhanced greenhouse effect and rising global average temperature.
Describe possible consequences
Relate global warming to sea-level rise, altered rainfall, heat events and ecosystem changes.
Describe ways to reduce the impact
Explain how reducing fossil-fuel use, improving energy efficiency and protecting forests can affect atmospheric CO₂.
These ten questions test whether you can trace carbon through unfamiliar situations and explain how changes in the carbon cycle can influence global warming.
- 10 mastery questions
- Carbon-cycle and climate applications
- Instant explanation for every answer option
Frequently asked questions
Carbon cycle and global warming: questions students often ask
Most mistakes in this topic come from treating the carbon cycle as a diagram to memorise. It becomes much easier when you trace one carbon atom at a time and identify the process responsible for each transfer.
What is the carbon cycle?
The carbon cycle is the continuous movement of carbon between different stores such as the atmosphere, living organisms, dead organic material, oceans and long-term geological stores. Important processes include photosynthesis, feeding, respiration, decomposition and combustion.
Does photosynthesis remove carbon permanently from the atmosphere?
No. Photosynthesis transfers carbon from atmospheric CO₂ into organic substances, but that carbon may later return to the atmosphere through respiration, decomposition or combustion. Carbon is continually recycled.
Do plants release carbon dioxide?
Yes. Plants carry out respiration and therefore produce carbon dioxide. During periods of active photosynthesis, however, a plant may remove CO₂ faster than respiration releases it, giving a net uptake of carbon dioxide.
How does carbon enter an animal?
Animals obtain carbon mainly through feeding. The food they eat contains organic molecules such as carbohydrates, lipids and proteins, all of which contain carbon.
How does decomposition return carbon to the atmosphere?
Decomposers such as bacteria and fungi break down dead organisms and waste. They use some of the organic material in respiration, producing carbon dioxide that can return to the atmosphere.
What is the difference between a carbon store and a carbon sink?
A carbon store is any reservoir that contains carbon. A carbon sink is a store that absorbs more carbon than it releases over a particular period. Therefore, every sink stores carbon, but a carbon store is not necessarily acting as a sink.
Why are fossil fuels important in the carbon cycle?
Fossil fuels contain carbon that has remained in long-term geological stores. Burning them transfers this carbon rapidly into the atmosphere as carbon dioxide. This can increase atmospheric CO₂ when the additional release is greater than the amount removed by other processes.
Why does deforestation increase atmospheric carbon dioxide?
Removing forests reduces the amount of plant biomass available for photosynthesis, so less carbon dioxide may be removed from the atmosphere. Carbon stored in the removed vegetation may also be released if it is burned or decomposes.
Is the greenhouse effect always harmful?
No. The natural greenhouse effect is essential because it keeps Earth warm enough for life. The concern is the enhanced greenhouse effect, in which increased concentrations of greenhouse gases cause additional warming.
How does carbon dioxide cause warming?
Earth's surface emits infrared radiation. Carbon dioxide can absorb some of this outgoing infrared radiation and re-emit it. Increasing the concentration of CO₂ strengthens this greenhouse effect and changes Earth's energy balance, contributing to warming.
What is the difference between the greenhouse effect and global warming?
The greenhouse effect is the process by which greenhouse gases absorb and re-emit outgoing infrared radiation. Global warming refers to the long-term rise in Earth's average surface temperature. An enhanced greenhouse effect contributes to global warming.
Is global warming caused by the ozone hole?
No. Ozone depletion and global warming are different environmental issues. Global warming is mainly associated with the enhanced greenhouse effect caused by increased concentrations of greenhouse gases.
Why does global warming cause sea level to rise?
Warmer seawater expands, increasing its volume. Melting glaciers and large ice sheets on land also add water to the oceans. Both processes contribute to rising global sea level.
Does planting trees completely solve global warming?
No. Growing trees can store carbon and remove some CO₂ through photosynthesis, but forests have limited storage capacity and take time to develop. Reducing greenhouse-gas emissions, especially from fossil-fuel combustion, remains important.
What is the best way to answer a carbon-cycle question in an examination?
Identify where the carbon starts, where it ends, and then name the process that causes the transfer. For example, atmospheric CO₂ to plant biomass is photosynthesis, plant to animal is feeding, and organic carbon to atmospheric CO₂ may occur through respiration or combustion.
The carbon cycle becomes much easier once you stop treating it as a collection of arrows and instead follow the carbon itself. Photosynthesis brings carbon into living biomass, feeding moves it through food chains, and respiration, decomposition and combustion return carbon to the environment. Human activities can alter the rates of these transfers, allowing carbon dioxide to accumulate in the atmosphere. That provides the crucial link between the carbon cycle, the enhanced greenhouse effect and global warming.