Carbon cycle

What is the carbon cycle?

Carbon is one of the most abundant elements on Earth, which exists in many forms. This carbon is stored in reservoirs across the globe, including:

• the atmosphere
• oceans
• living things (also known as the ‘biosphere’)
• rocks and soils (also known as the ‘lithosphere’)

The carbon cycle is the movement of carbon between these reservoirs.

Carbon dioxide is very important in the carbon cycle. Most carbon in the atmosphere is carbon dioxide and it is important in many exchange processes between reservoirs. 

Methane is another important part of the carbon cycle, naturally emitted from wetlands and artificially from fossil fuels and agriculture. 

More complex molecules, such as volatile organic compounds, are also part of the carbon cycle, from both natural and artificial sources.

The carbon cycle is mostly a closed system, so it doesn’t lose or receive carbon from space. This means that if one reservoir loses carbon, other reservoirs will receive it.

There are many processes that move carbon between reservoirs.

This simplified schematic of the global carbon cycle illustrates the main reservoirs of carbon and the processes moving carbon between them.

Why is the carbon cycle important?

The carbon cycle plays a key role in regulating Earth’s climate and making the planet habitable. By moving carbon out of the lithosphere, it becomes available to living things, making life possible.

We also know that carbon dioxide is the most important greenhouse gas produced by human activity, so its fate in the climate system is very important.

The movement of carbon from one reservoir to another can also slow down and spread out climatic changes caused by changes in carbon dioxide levels.

For example, human activity has released large quantities of carbon dioxide in the atmosphere, but oceans absorb some of this.

This reduces the impact on global temperatures, but has also led to ocean acidification, which threatens certain marine creatures and ecosystems.

Atmospheric carbon dioxide levels are now the highest they have been for at least 2 million years and still rising rapidly. Continued emissions of carbon dioxide into the atmosphere because of human activity risk disrupting the balance between different reservoirs in the carbon cycle.

This would cause changes to the processes of carbon transfer, leading to some of the many known impacts of climate change.

Carbon cycle process

Many key processes happen at the same time to move carbon between reservoirs.

Naturally, these reservoirs are balanced and maintain a steady exchange of carbon, with little change between them.

Human activity, however, has changed some of these processes.

Oceanic gas exchange

Carbon dioxide in the atmosphere dissolves into the upper layer of the ocean and ventilates out at the same rate under natural conditions.

Much slower cycling also moves this carbon in and out of the deep ocean.

Photosynthesis

Plants absorb carbon dioxide and convert it into sugars during photosynthesis.

Photosynthesis moves carbon from the atmosphere (or the ocean with algae) into the biosphere. Animals that eat the plants take in this carbon.

Carbon can also be deposited in soils and fossil carbon deposits, such as coal, oil and peat, which were once living plants.

Respiration

The vast majority of living things release carbon dioxide when they breathe. This is a by-product of producing energy in cells.

This moves carbon from the biosphere to the atmosphere.

Decomposition

When microbes, fungi or other decomposers digest dead animals and plants, they also respire.

This releases carbon and returns it to the atmosphere as carbon dioxide.

Combustion

Carbon dioxide is a major product of something burning.

This moves carbon from the biosphere (and sometimes soils or peat) into the atmosphere.

The burning of fossil fuels by humans has dramatically increased carbon emissions from combustion, becoming a major source of carbon dioxide.

Rock weathering

Atmospheric carbon dioxide dissolves in rainwater, making it slightly acidic.

This dissolves rocks, allowing the carbon to flow down rivers and into the ocean, deposited in new rocks — usually calcum carbonate.

This moves carbon from the atmosphere into the lithosphere.

Volcanism

Erupting volcanoes release carbon dioxide from molten rocks in the Earth’s crust.

This moves carbon from the lithosphere into the atmosphere.

Oceans in the carbon cycle

The oceans are key to the carbon cycle, with many processes that move carbon between oceans and other reservoirs.

One of the most simple interactions is gas exchange at the ocean surface.

Carbon dioxide in the atmosphere can dissolve in water, and naturally releases at a similar rate. This produces a store of carbon in surface waters, which interacts with photosynthesis and respiration as part of aquatic life.

The carbon store on the water’s surface is separated from the deep ocean by the thermocline.

The thermocline is a layer at roughly 1,000 metres down, which separates the turbulent, well-mixed surface waters from the calmer waters in the deep sea.

Carbon exchange between these zones is slower than with the atmosphere, but the deep sea is still a larger carbon reservoir because of its size.

Carbon in the ocean, including the deep ocean, can also react with sediment washed into the ocean from the land. This creates new rock deposits, largely of calcium carbonate. This moves carbon from the oceans into the lithosphere.

An increase in atmospheric carbon dioxide is resulting in greater absorption of carbon by the oceans, altering its chemistry.

Photosynthesis in the carbon cycle

Plants take in carbon dioxide as part of photosynthesis, converting it into sugars to use as food. This process removes a large amount of carbon from the atmosphere and locks it into plant biomass.

The absorption of carbon by photosynthesis forms the basis of most food chains, as animals often eat these plants.

Carbon stored in plants that don’t get eaten contributes to the richness of the soil, which can be stored for even longer.

Acidic soil with little oxygen slows the decay of organic matter, especially if it is waterlogged. This can become peat, which consists almost entirely of organic matter, storing the carbon it absorbed underground.

Over millions of years, this carbon-rich soil can become fossil fuels, such as coal, oil and gas, when it is buried by more sediment and subjected to heat and pressure underground.

Burning these fuels releases carbon that was originally absorbed from the atmosphere — when plants take in carbon dioxide through photosynthesis — often many hundreds of millions of years ago.

Respiration in the carbon cycle

While photosynthesis takes carbon dioxide from the atmosphere, aerobic respiration does the opposite. It returns carbon dioxide to the atmosphere by breaking down sugars.

Most organisms, including plants, emit carbon dioxide to the atmosphere as part of their normal life processes.

Plants originally capture this carbon from the atmosphere, moving it up the food chain into other organisms.

The decomposition of dead organisms also involves the respiration of animals and microbes that break down organic matter.

This returns large amounts of carbon to the atmosphere, though this can vary based on environmental conditions.

Combustion in the carbon cycle

Combustion (or burning) is a very important part of the carbon cycle and one that human activity has significantly altered.

Natural wildfires are often a result of lightning strikes. They are an important part of the carbon cycle, transferring carbon dioxide from the biosphere into the atmosphere.

Fires can also burn in below-ground carbon deposits, such as rich soil or peat. This can release long-stored carbon back into the atmosphere and potentially smoulder unnoticed before causing a more obvious fire above ground.

Climate change increases the risk of wildfires, which may also increase this source of carbon emissions.

The combustion of fossil fuels (which store carbon from ancient plants), has also become a very large source of carbon emissions.

Together, these sources represent a significant change to the carbon cycle caused by humans.

Slow carbon cycle

We can place the processes and reservoirs in the carbon cycle into groups: the slow carbon cycle, and the fast carbon cycle.

These cycles intergrade with one another and share some components, but take place over very different timescales.

The slow carbon cycle moves carbon between the atmosphere, lithosphere and oceans.

Rock weathering moves carbon from the atmosphere into the lithosphere by dissolving rocks, washing the constituents into the oceans, and depositing it in new sediment on the sea floor.

Over millions of years, the sea floor gets recycled into the Earth’s crust at fault lines between tectonic plates. Erupting volcanoes then return this rock to the surface, transferring carbon back into the atmosphere.

On average, the slow carbon cycle moves around 10 to 100 millions tonnes of carbon every year. It takes between 100 and 200 million years for carbon to move through this cycle.

Complex chemical feedbacks regulate this process.

For example, an increase in atmospheric carbon dioxide raises global temperature, leading to more rain and dissolving more rock.

Over hundreds of thousands of years, the slow carbon cycle rebalances after a disturbance.

The slow carbon cycle has been important over geological time but does not impact the current climate changes that we have seen over a few centuries.

Fast carbon cycle

The fast carbon cycle moves carbon much faster.

Instead of 100 to 200 million years, the fast carbon cycle happens over years or decades. The increased speed is because the fast carbon cycle moves carbon through living things.

As we’ve already seen, plants absorb atmospheric carbon dioxide through photosynthesis, which is then moved through the food chain and released through respiration.

Before this, organisms use carbon to build cells, proteins and DNA molecules. All known life is carbon-based and uses carbon molecules as its basic building block.

The fast carbon cycle has a major effect on atmospheric carbon dioxide levels.

More plant growth absorbs more carbon dioxide, which we see during spring and summer months.

Atmospheric carbon dioxide levels actually drop during the northern hemisphere growing season because it contains most of the Earth’s land.

The fast carbon cycle moves around 1000 times more carbon per year than the slow carbon cycle.

The two can also interact with each other — an increase in atmospheric carbon dioxide from lower plant growth might encourage more rainfall and improve growing conditions.