Carbon Cycle

Carbon Cycle Definition

The carbon cycle is the cycle by which the element carbon moves through our Earth’s various systems. It is a fascinating and complex process because living things, atmospheric changes, ocean chemistry, and geologic activity are all part of this cycle.

The Earth is a closed system; that is, it cannot gain or lose materials or resources. Instead, materials must be used and re-used many times over for life to be sustainable. This applies to materials including carbon, oxygen, nitrogen, and water.

Carbon is an essential element for life as we know it because of its ability to form multiple, stable bonds with other molecules.

This is why nucleotides, amino acids, sugars, and lipids all depend on carbon backbones: carbon provides a stable structure that allows the chemistry of life to happen. Without carbon, none of these molecules could exist and function in the ways that permit the chemistry of life to occur.

Our type of life is referred to as “carbon-based life” by scientists. Some scientists speculate that life on other planets could be based on a different chemistry, but no evidence such life has yet been found.

Originally, carbon was created through nuclear fusion in the hearts of stars. When those stars exploded, materials including carbon were scattered through space. And when the Earth was formed – it was formed of some of those carbon-containing materials.

In time, living things developed as a result of carbon and other atoms reacting in the chemically volatile atmosphere of ancient Earth. When some of these molecules became self-replicating, they began to take carbon from the atmosphere and even from rocks and use it to build materials for life such as sugars, proteins, and lipids.

Early photosynthesizers such as cyanobacteria transformed Earth’s atmosphere by turning huge amounts of carbon dioxide gas into oxygen gas. This was not their goal – but rather a happy side effect of these first life forms removing the “C” from CO2. This left O2 in the atmosphere – and countless molecules of carbon serving as part of the machinery for life.

In time, other forms of life discovered that the new O2 in the atmosphere could be used to power a highly efficient method of liberating the energy stored in carbon-based organic molecules like sugars. This allowed these life forms to use the energy plants used to make these molecule for their own metabolism.

Using this process of “cellular respiration,” animals and other oxygen-breathers started turning O2 back into CO2 – effectively spitting out the carbon atoms once contained in sugars, proteins, and lipids after extracting all of their energy.

This happy balance of plants turning carbon dioxide into living matter while animals reduce it back into gases for the plants to consume has existed for billions of years. In the process, new steps became incorporated – such as the formation of fossil fuels, which occurs when organic matter such as dead plants and animals become trapped underground by geologic processes.

The graphic below illustrates some common ways in which carbon moves through the ecosystem:

Carbon CycleCarbon Cycle (click to play animated GIF)

Recently, humans have made some big changes to the Earth’s carbon cycle.

By burning huge amounts of fossil fuels and cutting down roughly half of the Earth’s forests, humans have decreased the Earth’s ability to take carbon out of the atmosphere, while releasing large amounts of carbon into the atmosphere that had been stored in solid form as plant matter and fossil fuels.

This means more carbon dioxide in Earth’s atmosphere – which is particularly dangerous since carbon dioxide is a “greenhouse gas” that plays a role in regulating the Earth’s temperature and weather patterns.

Function of Carbon Cycle

The carbon cycle, under normal circumstances, works to ensure the stability of variables such as the Earth’s atmosphere, the acidity of the ocean, and the availability of carbon for use by living things.

Each of its components is of crucial importance to the health of all living things – especially humans, who rely on many food crops and animals to feed our large population.

Carbon dioxide in the atmosphere prevents the sun’s eat from escaping into space, very much like the glass walls of a greenhouse. This isn’t always a bad thing – some carbon dioxide in the atmosphere is good for keeping the Earth warm and its temperature stable.

But Earth has experienced catastrophic warming cycles in the past, such as the Permian extinction, which is thought to have been caused by a drastic increase in the atmosphere’s level of greenhouse gases.

No one is sure what caused the change that brought about the Permian extinction. Greenhouse gases may have been added to an atmosphere by an asteroid impact, volcanic activity, or even massive forest fires.

Whatever the cause, during this warming episode, temperatures rose drastically. Much of the Earth became desert, and over 90% of all species living at that time went extinct.

This is a good example of what can happen if our planet’s essential cycles experience a big change.

Another important variable effected by the carbon cycle is the acidity of the ocean. Carbon dioxide can react with ocean water to form carbonic acid. This has been an important stabilizing force of of the carbon cycle over the years, since the chemical equilibrium between carbon dioxide and carbonic acid means that the ocean can absorb or release carbon dioxide as atmospheric levels rise and fall.

However, as you might guess, increasing ocean acidity can mean trouble for sea life – and this might eventually pose a problem for other parts of the carbon system. Many forms of sea life that have shells, for example, can take carbon out of the water to create the calcium carbonate that they make their shells out of. If these species suffer, the ocean may lose some of its ability to remove carbon from the atmosphere.

Lastly, of course, there is the role of living things in the carbon cycle. The activity of plants and animals has been one of the major forces affecting changes to the carbon cycle in the past several billion years. Photosynthesizers have changed Earth’s atmosphere and climate drastically by taking huge amounts of carbon out of the atmosphere and turning that carbon into cellular materials.

Those activities created free oxygen and the ozone layer, and generally set the stage for the evolution of animals that obtain their energy by breaking down the organic materials created by photosynthesizers and extracting the energy that the photosynthesizers used to make those molecules.

With one particular species of animals – humans – making big changes, the future of the Earth’s carbon cycle is uncertain.

All such cycles in closed systems eventually correct themselves – but sometimes this happens through drastic population reduction of the offending species through starvation.

Steps of Carbon Cycle

Carbon in the Atmosphere

To become part of the carbon cycle, carbon atoms start out in gaseous form. Carbon dioxide gas – CO2 – can be produced by inorganic processes, or by the metabolisms of living things.

Before Earth had life on it, carbon dioxide gas likely came from volcanic activity and asteroid impacts.

Today, carbon is also released into the atmosphere through the activities of living things, such the exhalations of animals, the actions of decomposer organisms, and the burning of wood and fossil fuels by humans.

However carbon dioxide gets into the atmosphere, CO2 gas is the starting point of the carbon cycle. The next step is…

Producers Absorb Carbon

“Producers” – organisms that produce food from sunlight, such as plants – absorb carbon dioxide from the atmosphere and use it to build sugars, lipids, proteins, and other essential building blocks of life.

For plants, CO2 is absorbed through pores in their leaves called “stomata.” Carbon dioxide enters the plant through the stomata and is incorporated into containing carbon compounds with the help of energy from sunlight.

Plants and other producer organisms such as cyanobacteria are crucial to life on Earth, because they can turn atmospheric carbon into living matter. Next…

Producers are Eaten

“Consumers” are organisms that eat other living things. Animals are the most visible type of consumer in our ecosystems, though many types of microbes also fall into this category.

Consumers incorporate the carbon compounds from plants and other food sources when they eat them.
They use some of these carbon compounds from food to build their own bodies – but much of the food they eat is broken down to release energy, in a process that is almost the reverse of what producers do.

While producers use energy from sunlight to make bonds between carbon atoms – animals break these bonds to release the energy they contain, ultimately turning sugars, lipids, and other carbon compounds into single-carbon units. These are ultimately released into the atmosphere in the form of CO2.

This process of “cellular respiration” – where oxygen gas is inhaled and carbon dioxide is exhaled – is a major source of carbon release back into the atmosphere.

But it’s not always the last step of the carbon cycle. What about the carbon compounds that don’t get eaten, or broken down by animals?

Decomposers Release Carbon

Plants and animals that die without being eaten by other animals are broken down by other organisms, called “decomposers.”

Decomposers include many bacteria and some fungi. They usually only break down matter that is already dead, rather than catching and eating a living animal or plant.

Just like animals, decomposers break down the chemical bonds in their food molecules. They create many chemical produces, including in some cases CO2.

Carbon that isn’t released back into the atmosphere in this way can also be released by…

Human Combustion

Humans are the only animals we know of who can create fire on purpose. And we set fire to things a lot.

Our cars are driven by burning fossil fuels – oil and gasoline, which are made of dead plant and animal material that spent millions of years buried deep in the Earth.

Many of our electrical power plants are powered by burning fossil fuels as well, including coal, which is another form of dead plant matter that was buried underground and transformed by geologic heat.

Lastly, humans also burn a lot of wood. We no longer burn wood to power our machines as we did in the 19th century, but now we often burn forests in order to clear land for agriculture, mining, and other purposes. About half of Earth’s forests have been burned or otherwise destroyed by human activity to date.

The scientific community has raised alarms that by making significant changes to the Earth’s carbon cycle, we may end up changing our climate or other important aspects of the ecosystem we rely upon to survive.

As a result, many scientists advocate to decrease the amount of carbon burned by humans by reducing car trips and electricity consumption, and investing in non-burning sources of energy such as solar power and wind power.

Examples of the Carbon Cycle

The carbon cycle consists of many parallel systems which can either absorb or release carbon. Together, these systems work to keep Earth’s carbon cycle – and subsequently its climate and biosphere – relatively stable.

Here are some examples of parts of Earth’s ecosystems that can absorb carbon, turn carbon into living matter, or release carbon back into the atmosphere.


One major repository of carbon is the carbon dioxide in the Earth’s atmosphere. Carbon forms a stable, gaseous molecule in combination with two atoms of oxygen.

In nature, this gas is released by volcanic activity, and by the respiration of animals who affix carbon molecules from the food they eat to molecules of oxygen before exhaling it.

Humans also release carbon dioxide into the atmosphere by burning organic matter such as wood and fossil fuels.

Carbon dioxide can be removed from the atmosphere by plants, which take the atmospheric carbon and turn it into sugars, proteins, lipids, and other essential molecules for life.

It can also be removed from the atmosphere by absorption into the ocean, whose water molecules can bond with carbon dioxide to form carbonic acid.

In recent years, scientists have raised concerns that by cutting down about half of Earth’s forests, humans may be decreasing the Earth’s ability to remove carbon dioxide from the atmosphere at the same time they’re adding new sources by burning wood and fossil fuels.

Many organizations that hope to fight man-made climate change now plant trees which can remove carbon dioxide from the atmosphere, as well as advocating for alternative energy sources and less burning of fossil fuels such as gasoline, oil, and coal.


The Earth’s crust – called the “lithosphere” from the Greek word “litho” for “stone” and “sphere” for globe – can also release carbon dioxide into Earth’s atmosphere. This gas can be created by chemical reactions in the Earth’s crust and mantel.

Volcanic activity can result in natural releases of carbon dioxide. Some scientists believe that widespread volcanic activity may be to blame for the warming of the Earth that caused the Permian extinction.

While the Earth’s crust can add carbon to the atmosphere, it can also remove it. Movements of the Earth’s crust can bury carbon-containing chemicals such as dead plants and animals deep underground, where their carbon cannot escape back into the atmosphere.

Over millions of years, these underground reservoirs of organic matter liquefy and become coal, oil, and gasoline. In recent years, humans have begun releasing much of this sequestered carbon back into the atmosphere by burning these materials to power cars, power plants, and other human equipment.


Among living things, some remove carbon from the atmosphere, while others release it back. The most noticeable participants in this system are plants and animals.

Plants remove carbon from the atmosphere. They don’t do this as a charitable act; atmospheric carbon is actually the “food” which plants use to make sugars, proteins, lipids, and other essential molecules for life.

Plants use the energy of sunlight, harvested through photosynthesis, to build these organic compounds out of carbon dioxide and other trace elements. Indeed, the term “photosynthesis” comes from the Greek words “photo” for “light” and “synthesis” for “to put together.”

Here lies arguably the most important part of the carbon cycle: all life is made of carbon. Without plants or other organisms that could turn inorganic carbon compounds like carbon dioxide into organic compounds, life could not exist.

Indeed, none of the building materials for our cells, from our DNA to our cell membranes, could exist without this ability of photosynthesizers to turn carbon dioxide into life!

In a gracefully balanced set of chemical reactions, animals eat plants (and other animals), and take these synthesized molecules apart again. Animals get their fuel from the chemical energy plants have stored in the bonds between carbon atoms and other atoms during photosynthesis.

In order to do that, animal cells dissemble complex molecules such as sugars, fats, and proteins all the way down to single-carbon units – molecules of carbon dioxide, which are produced by reacting carbon-containing food molecules with oxygen from the air.

In this way, most of the carbon eaten by animals ends up back where it started before it was absorbed by a plant – as part of a carbon dioxide molecule in the atmosphere.

Carbon in plants and animals that is not consumed by other animals can be broken down by other living things until it becomes carbon dioxide again, or can be sequestered deep in the Earth as fossil fuels.


The Earth’s oceans have the ability to both absorb and release carbon dioxide. When carbon dioxide from the atmosphere comes into contact with ocean water, it can react with the water molecules to form carbonic acid – a dissolved liquid form of carbon.

Like most chemical reactions, the rate of this reaction is determined by the equilibrium between the products and the reactants.

When there is more carbonic acid in the ocean compared to carbon dioxide in the atmosphere, some carbonic acid may be released into the atmosphere as carbon dioxide.

On the other hand, when there is more carbon dioxide in the atmosphere, more carbon dioxide will be converted to carbonic acid, and ocean acidity levels will rise.

Some scientists have raised concerns that acidity is rising in some parts of the ocean, possibly as a result of increased carbon dioxide in the atmosphere due to human activity.

Although these changes in ocean acidity may sound small by human standards, many types of sea life depend on chemical reactions that need a highly specific acidity level to survive.

As a result, there is concern that increasing ocean acidity due to carbonic acid may contribute to the die-offs of some marine ecosystems, and even to extinctions of marine species.


1. Which of the following is NOT a vital component of the carbon cycle?
A. Photosynthesizers take carbon from the atmosphere and turn it into sugars, proteins, lipids, and other vital materials for life.
B. Oxygen-breathers break down organic materials into energy and carbon dioxide, which they release back into the atmosphere.
C. Geologic activity releases carbon in the form of volcanic gases.
D. The ocean absorbs carbon in the form of carbonic acid or calcium carbonate.
E. None of the above.

Answer to Question #1

2. Which of the following is NOT true of carbon levels in Earth’s atmosphere?
A. The composition of Earth’s atmosphere has changed drastically over time.
B. Drastic changes to the temperature of Earth’s surface have happened as a result of changes in the atmosphere’s carbon levels.
C. Mass extinctions have occurred as a result of changes to the atmosphere’s carbon levels.
D. It is impossible to significantly change the carbon levels in Earth’s atmosphere.

Answer to Question #2

3. Which of the following would NOT be a possible outcome if the carbon cycle were severely disrupted?
A. Severe global cooling as a result of fewer greenhouse gases in the Earth’s atmosphere.
B. Severe global warming as a result of more greenhouse gases in the Earth’s atmosphere.
C. Drastic changes to ocean ecosystems due to changing ocean acidity.
D. Drastic changes to land ecosystems as a result of changing temperatures and weather patterns.
E. None of the above.

Answer to Question #3


  • Falkowski, P. (2000). The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System. Science, 290(5490), 291-296. doi:10.1126/science.290.5490.291
  • Houghton, J. T. (2001). Climate change 2001: the scientific basis: contribution of Working Group I to the third assessment report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press.
  • Hoffman, W. B. (n.d.). Permian Extinction Article, Mass Extinction Information, Park Tourism Facts. Retrieved May 10, 2017, from

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