Carbon cycle

Carbon is the chemical backbone of life on Earth. Carbon compounds regulate the Earth’s temperature, make up the food that sustains us, and provide energy that fuels our global economy.

A diagram of the carbon cycle with arrows showing the movement of carbon through a landscape with plants and animals, mountains and a volcano, a river leading to the ocean, and an industrial area. Carbon moves in and out of our atmosphere, ocean, waterways, and soil through burning fossil fuels, precipitation, fires, vegetation, volcanoes, and organic processes.

The carbon cycle. (Image credit: NOAA)

Most of Earth’s carbon is stored in rocks and sediments. The rest is located in the ocean, atmosphere, and in living organisms. These are the reservoirs through which carbon cycles.

Carbon storage and exchange

Carbon moves from one storage reservoir to another through a variety of mechanisms. For example, in the food chain, plants move carbon from the atmosphere into the biosphere through photosynthesis. They use energy from the sun to chemically combine carbon dioxide with hydrogen and oxygen from water to create sugar molecules. Animals that eat plants digest the sugar molecules to get energy for their bodies. Respiration, excretion, and decomposition release the carbon back into the atmosphere or soil, continuing the cycle.

The ocean plays a critical role in carbon storage, as it holds about 50 times more carbon than the atmosphere. Two-way carbon exchange can occur quickly between the ocean’s surface waters and the atmosphere, but carbon may be stored for centuries at the deepest ocean depths.

Rocks like limestone and fossil fuels like coal and oil are storage reservoirs that contain carbon from plants and animals that lived millions of years ago. When these organisms died, slow geologic processes trapped their carbon and transformed it into these natural resources. Processes such as erosion release this carbon back into the atmosphere very slowly, while volcanic activity can release it very quickly. Burning fossil fuels in cars or power plants is another way this carbon can be released into the atmospheric reservoir quickly.

Changes to the carbon cycle

Human activities have a tremendous impact on the carbon cycle. Burning fossil fuels, changing land use, and using limestone to make concrete all transfer significant quantities of carbon into the atmosphere. As a result, the amount of carbon dioxide in the atmosphere is rapidly rising; it is already greater than at any time in the last 3.6 million years. The ocean absorbs much of the carbon dioxide that is released from burning fossil fuels. This extra carbon dioxide is lowering the ocean’s pH, through a process called ocean acidification. Ocean acidification interferes with the ability of marine organisms (including corals, Dungeness crabs, and snails) to build their shells and skeletons.


Take a bite of dinner, breathe in air, or a drive in a car — you are part of the carbon cycle. The resources in this collection provide real world examples of the changes occurring in the cycle. There is much to learn about this essential topic and some of the resources highlight exciting career opportunities in this field of study.