Carbon dioxide in earth’s atmosphere is considered a trace gas currently occurring at an average concentration of about 385 parts per million by volume or 582 parts per million by mass. The mass of the Earth atmosphere is 5.14×1018 kg, so the total mass of atmospheric carbon dioxide is 3.0×1015 kg (3,000 gigatonnes). Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Concentrations also vary considerably on a regional basis: in urban areas it is generally higher and indoors it can reach 10 times the background atmospheric concentration.