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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.

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.

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.