SEED Science

Global Climate Change and Energy
The Carbon Cycle

Carbon is constantly entering the atmosphere in the form of carbon dioxide, methane, and other gases. At the same time, it is being removed by green plants, the oceans, and in other ways. This is the carbon cycle. The balance in the cycle is critical in determining the Earth's climate.

Carbon is an essential component of our bodies, the food we eat, the clothes we wear, most of the fuel we burn, and many other materials that we use. Over 90% of the known chemical compounds contain carbon. This is not surprising since carbon combines very easily with other elements and with itself.

Most of the carbon on Earth is in compounds found in sediments and sedimentary rocks. Comparatively little is in the atmosphere.
  Billion Metric Tons
Underwater sediments and sedimentary rocks 80,000,000
Ocean water, shells, and organisms 40,000
Fossil fuels (oil, gas, and coal) 4,000
Organic material in soil 1,500
Atmosphere 825
Land plants 580


Living Systems

Carbon atoms are continuously being exchanged between living and dead organisms, the atmosphere, oceans, rocks, and soil. With every outward breath, we release CO2 from our lungs into the atmosphere, containing atoms of carbon from plants and animals that we have eaten. Atoms of carbon in our bodies today might previously have been in many different plants and animals – perhaps including dinosaurs and other extinct creatures.

The distribution of carbon among atmosphere, organisms, land, and oceans has changed over time. About 550 million years ago the concentration of CO2 in the atmosphere was 7,000 parts per million – more than 18 times what it is today. Where did all that atmospheric carbon go? For the most part it ended up as sedimentary rocks such as limestone. How that happened is part of the larger story of the carbon cycle.

The carbon cycle is a combination of many biological, chemical, and physical processes that move carbon around.

The image shows some of the processes by which carbon is moved between living and dead organisms, the atmosphere, oceans, rocks, and soil. Human interventions are not included here. Click on the image for an animation that shows the chemical processes underlying these transfers of carbon.


Carbon Dioxide Sinks and Sources

A carbon dioxide sink is something that removes CO2 from the atmosphere. For example, green plants consume CO2 during the process of photosynthesis. Burning wood and fossil fuels are sources of CO2. The oceans are both a source of CO2 and a CO2 sink. This is because CO2 in air that is in contact with the surface of the ocean dissolves in water and is therefore removed from the atmosphere. At the same time, dissolved CO2 is released into the atmosphere. The balance between these two processes depends on many factors and is changeable over time. Presently there is more CO2 dissolving into the oceans than is being released. This means that right now the oceans are a CO2 sink.

Let's look more closely at the major CO2 sinks and CO2 sources that make up the carbon cycle.