Carbon cycle
The carbon cycle can seem like a complex process because there are many interacting parts.
Sunflower
Living things are made up of carbon and need carbon to survive.
Carbon is found in both organic (living) and inorganic (non-living) forms. Before we discuss the cycle, there are a few other things you need to know about carbon:
The Earth has a finite amount of carbon.
Living things are made up of carbon (often they are described as being carbon-based) and need carbon to survive.
Carbon is also found in non-living things such as rocks, animal shells, the atmosphere and oceans.
Carbon found in something living is called organic carbon.
Carbon found in something non-living is called inorganic carbon.
Carbon dioxide is an important gas in our atmosphere. It prevents heat from escaping and, in doing so, warms up the Earth’s atmosphere. In a similar way to the glass of a greenhouse, it traps the heat from escaping and, for this reason, it is called a greenhouse gas.
After reading through that list you might ask yourself, “If there is a fixed amount of carbon and living things need carbon to survive, won’t we just use it all up?” The answer to that is no, and this is where the carbon cycle comes in.
Carbon moves through the Earth’s system from living to non-living in many different ways. You could almost think of it as the ultimate in recycling.
Carbon cycle
Explore this interactive diagram to learn more about the global carbon cycle.
Let’s start with how living things get carbon. Plants use carbon dioxide for photosynthesis. By doing so, they remove inorganic carbon from the atmosphere and incorporate it into the plants’ tissues in the form of organic carbon (sugar and starch). Animals get carbon by eating plants or by eating other animals.
Fuels and greenhouse gases
Different products produce differing amounts of greenhouse gases. Common greenhouse gases include carbon dioxide, carbon monoxide and methane. If we compare burning wood and burning coal, we don’t just compare how much greenhouse gases are emitted – a life cycle analysis should also account for the carbon dioxide that trees capture while they are growing. This means that, when we burn wood, it has a much smaller carbon footprint than compared to burning coal, a fossil fuel.
Carbon is returned to an inorganic state in a number of ways. As an animal breathes (respires), it exhales carbon dioxide, returning it back to the atmosphere. When an animal or plant dies, it is broken down by bacteria and fungi and again the carbon is released (this process is called decomposition).
Sometimes, instead of completely decomposing, a plant or animal may be fossilised, leading to its carbon being stored in a rock. After millions of years and under the right conditions, these fossils may turn into fossil fuels (oil, coal and natural gas).
How oil and gas are made
Adam Vonk explains that a source rock contains the raw materials from which hydrocarbons will eventually form.
These raw materials are rich in organic matter and may contain marine algae or leaf material. This material forms what is called kerogen in the source rock – these are the chemical compounds that make up the organic matter. Carbon is released from kerogen when the source rock is buried.
Burying a rock submits it to both pressure and temperature. As the temperature changes, different compounds are released from the kerogen in the source rock. Initially, it is water and carbon dioxide, then oil and, last, natural gas.
Points of interest: What is the average temperature increase per kilometre burial depth?
The carbon stored in fossil fuels is released when they are burned. This release is called carbon dioxide emission, with each different fossil fuel emitting a different amount of carbon dioxide as well as carbon monoxide and soot (carbon particles).
Because people use so much fossil fuel, the emission rate means the amount of carbon dioxide in the atmosphere today is around 30% higher than it was 200 years ago. The carbon levels are important because having more carbon dioxide in the atmosphere has been linked to an increase of heat that is being trapped in the atmosphere (greenhouse effect).
Carbon dioxide sequestration
Adam Vonk explains what happens after we remove the oil and gas from the ground. We are left with a void that may just fill up with water, a space that is no longer useful as there are no hydrocarbons stored, but maybe we can use it. There is an idea that perhaps we can take some of the excess CO2 that is in our environment and pump it into these empty reservoirs and store it there. This idea is called CO2 sequestration.
Related content
Read about Carbon – life’s framework element and then discover more about the exchange of carbon dioxide between the ocean and the atmosphere in the articles Carbon dioxide in the ocean and Carbon dioxide in the atmosphere.
In the Connected article Trees, seas and soil discover what a carbon sink is and why they are so important.
Activity ideas
Carbon cycle – three-level reading guide – read about the carbon cycle using a three-level reading guide.
Carbon dioxide emissions calculator – use this online calculator to calculate and compare the amount of CO2 produced by different energy sources.
Carbon cycle quiz – uses the interactive carbon cycle diagram to explore the global carbon cycle and to answer questions in an online or paper-based quiz.
Why are the egg shells dissolving?
Pinterest board
See our carbon cycle Pinterest board with links to a range of supporting resources.
