Carbon cycle

The carbon cycle can seem like a complex process because there are many interacting parts.

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.

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.

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).

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).