This diagrams illustrates Earth's carbon cycle. It shows how carbon atoms 'flow' between various 'reservoirs' in the Earth system. The sizes of reservoirs are in units of gigatons of carbon (GtC). Flows between reservoirs are in units of gigatons of carbon per year (GtC/yr). The values for human influences such as fossil fuel use and cement production represent the state of the carbon cycle in the mid-1980s.