Science FAQs

Question 5:

What is the carbon cycle? How does human activity contribute to the carbon cycle?

Carbon, in various forms, continuously circulates between the living world, the atmosphere, oceans and the Earth's crust. There are many different processes by which carbon is exchanged between these locations. Events, such as fires, which release carbon dioxide into the atmosphere, are known as ‘sources'. The oceans and growing trees remove carbon dioxide from the atmosphere and are known as ‘sinks'.

Each year human activity adds several billions of tonnes of carbon in the form of carbon dioxide to the atmosphere. A little over half of this carbon dioxide remains there, while the rest is absorbed by plants and the oceans (and ultimately some of this is returned to the Earth's crust).

More than 120 billion tonnes of carbon are exchanged each year between all living things during photosynthesis and respiration. Plants absorb about 61 billion tonnes of carbon and respire about 60 billion tonnes. Plants grow by absorbing carbon dioxide from the air or water and converting it to plant tissue through photosynthesis. Some of this carbon is used to supply the plant with energy. This process, known as respiration, releases carbon dioxide back into the atmosphere. The carbon from carbon dioxide absorbed by a tree may be stored as wood for hundreds of years. Or the carbon may become part of a leaf that dies and decomposes, with the carbon returning to the atmosphere relatively quickly.

The surfaces of the oceans release about 90 billion tonnes of carbon to the atmosphere and absorb about 92 billion tonnes each year. This absorption occurs when carbon dioxide in the air dissolves in the top layer of sea water and through photosynthesis by marine plants.

The amount of carbon dioxide that people add to the atmosphere may seem very small in comparison to the amounts being added and absorbed by natural processes, but it only takes a small change to upset the balance. The burning of fossil fuels by humans adds about 6.5 billion tonnes of carbon each year in the form of carbon dioxide. Land clearing, reduced soil humus and the erosion of topsoil account for one to two billion tonnes of carbon a year.

Proof that more carbon dioxide is being added to the atmosphere than removed is the fact that concentrations of the gas continue to rise. Furthermore, scientific techniques reveal that this additional carbon dioxide originates from fossil fuel combustion. However, most of the observed warming over the past 50 years is very likely to have been due to the human-induced increase in greenhouse gas concentrations.

The carbon cycle