The Carbon Cycle: Sources and Sinks

0:00Living things take in and give out carbon all the time. Why does it never run out?

0:06Carbon is held in places called stores: the air, living things, soil, oceans, rocks and fossil fuels.

0:15The carbon cycle is how carbon moves between these stores.

0:20In the air, most carbon is in a gas called carbon dioxide.

0:26Plants take in carbon dioxide and use light to make sugar. This is photosynthesis.

0:33Plants and animals both respire, and respiration releases carbon dioxide into the air.

0:39When living things die, their remains and waste add carbon to the soil.

0:45Decomposers, like bacteria and fungi, break down the remains. This decay releases carbon dioxide.

0:53Carbon dioxide also dissolves in the oceans, and some is released back into the air.

0:59Shells of sea creatures can form limestone rock, locking carbon away for millions of years.

1:06Over millions of years, buried remains can become fossil fuels: coal, oil and natural gas.

1:14Burning fossil fuels, called combustion, releases their carbon as carbon dioxide.

1:21A carbon source releases more carbon than it takes in, like burning fossil fuels.

1:27A carbon sink takes in more carbon than it releases, like the oceans today.

1:34Carbon is never used up: the same atoms keep moving between stores, so the total stays about the same.

1:42So carbon is never used up: it cycles between stores, released by sources and taken in by sinks.