Carbon is an element that is essential to all life on Earth. Carbon makes up the fats and carbohydrates of our food and is part of the molecules, like DNA and protein, that make up our bodies. Carbon, in the form of carbon dioxide, is even a part of the air we breathe. It is also stored in places like the ocean, rocks, fossil fuels, and plants.

The carbon cycle describes the flow of carbon between each of these places. For example, carbon continually flows in and out of the atmosphere and also living things. As plants photosynthesize, they absorb carbon dioxide from the atmosphere. When plants die, the carbon goes into the soil, and microbes can release the carbon back into the atmosphere through decomposition.

Forests are typically carbon sinks, places that absorb more carbon than they release. They continually take carbon out of the atmosphere through the process of photosynthesis. The ocean is another example of a carbon sink, absorbing a large amount of carbon dioxide from the atmosphere.

Some processes release more carbon dioxide into the atmosphere than they absorb. Any process that uses fossil fuels—such as burning coal to make electricity—releases a lot of carbon into the atmosphere. Raising cattle for food also releases a lot of carbon into the atmosphere. These processes that release carbon into the atmosphere are known as carbon sources.

Ideally, the carbon cycle would keep Earth’s carbon concentrations in balance, moving the carbon from place to place and keeping atmospheric carbon dioxide levels steady. However, the carbon cycle is changing because of human activity. People are releasing more carbon into the atmosphere by using fossil fuels and maintaining large livestock operations. Deforestation is depleting Earth’s supply of carbon sinks. As a result, the amount of carbon in the atmosphere is rising.

Carbon Sources and Sinks

With their abundance of plants, forests (like Alaska's Tongass National Forest) often absorb more carbon than they release. They continually take carbon out of the atmosphere through the process of photosynthesis. 


layers of gases surrounding a planet or other celestial body.


series of processes in which carbon (C) atoms circulate through Earth's land, ocean, atmosphere, and interior.

carbon dioxide

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

carbon sink

area or ecosystem that absorbs more carbon dioxide than it releases.

carbon source

process, area, or ecosystem that releases more carbon dioxide than it absorbs.


separation of a chemical compound into elements or simpler compounds.

fossil fuel

coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.


process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.