Carbon resides in many forms on Earth, as diamond, graphite, a compound with other elements, limestone, marble, and other rocks. Carbon is also bound in trees and other plants as part of their respiration cycle. Carbon also exists in dissolved minerals in ocean waters as well as in groundwater. One of the chief repositories of terrestrial carbon is in CO2, also known as carbon dioxide.
Just as H2O circulates between the atmosphere, the ground, and ocean waters in the water cycle, CO2 also circulates between these three reservoirs, solidifing as rock, dissolving out of rock into the water, baked out of rock into the air, or being absorbed directly into the waters at the Earth's surface. The transfer of CO2 between the air, water, and the ground is called the carbon cycle.