The Earth's climate changes when the amount of energy stored by the climate system is varied. The most significant changes occur when the global energy balance between incoming energy from the Sun and outgoing heat from the Earth is upset. There are a number of natural mechanisms that can upset this balance, for example fluctuations in the Earth's orbit, variations in ocean circulation and changes in the composition of the Earth's atmosphere. In recent times, the latter has been evident as a consequence not of natural processes but of man-made pollution, through emissions of greenhouse gases. By altering the global energy balance, such mechanisms "force" the climate to change. Consequently, scientists call them "climate forcing" mechanisms.