The carbon cycle

Carbon is the atom that builds the backbones of life's molecules: sugars, fats, proteins and DNA. It is never created or destroyed on Earth; the same atoms keep moving around. The carbon cycle is the road map of those travels, between the air, living things, the oceans, rocks and soil. Follow the carbon and you are following the planet's whole chemistry.

The air holds carbon as carbon dioxide gas, a tiny share of the atmosphere. Photosynthesis takes it in: plants and algae use the carbon to build glucose and everything made from it. That carbon climbs the food web as animals eat plants. Growing forests, seaweed meadows and plankton blooms are all pulling carbon out of the air and storing it in living matter.

Carbon returns to the air through several doors. Cellular respiration in every living cell breaks carbon molecules down and releases carbon dioxide. Decomposers do the same work on dead plants and animals. And burning wood, coal, oil or gas joins stored carbon quickly with oxygen, releasing carbon dioxide all at once. All three paths send the same atoms back to the atmosphere.

Some carbon takes the slow road. Shells and tiny plankton skeletons sink and become limestone, and the remains of ancient forests and plankton became coal and petroleum. Over millions of years these rocks and fuels lock huge amounts of carbon away from the air. Volcanoes release the carbon of old sea floor, and burning fossil fuels releases the carbon of ancient plants, returning it to the sky in seconds.