Ice age

The term ice age refers to a period in a planet's geologic history when the atmosphere gets cold enough that large glaciers can grow and spread over vast regions of the surface.

Planet Earth is believed to have had periods last close to one hundred million years, when its entire surface was covered with ice.

Since ice has a high albedo, glacier growth can accelerate, as more of a planet's surface reflects incoming light from its star.

The very long ice ages where Earth's entire surface froze are believe to have ended due to greenhouse gases from volcanism.

The Laurentian Glaciation, Earth's most recent Glaciation, that reached its peak over tens of thousands of years, is believed to be caused by periodic wobbles in the Earth's orbit. These wobbles are cyclic. Several distinct cycles have different periods. Earth's orbit is elliptical, and due to perturbation from other bodies in the Solar system, the eccentricity of its orbit varies. The axis on which Earth spins is not perpendicular to the ecliptic, the plane in which Earth spins around Sol, our sun. Perturbations from other bodies cause the tilt of the Earth's orbit to undergo small changes, over tens of thousands of years. This grows and shrinks the regions around Earth's poles, which receive of total darkness or 24 hour days.