During its lifespan the Earth has experienced a number of the ice ages, also called glacial ages.
The were times of extreme cooling of the climate where ice sheets expanded to cover large areas of land. Between ice ages there were warmer interglacial periods and we are now living during such a time.
Many factors have altered the Earths climate over time, including tiny periodic variations in its orbit, the orientation of its spin axis and the movements of the continents. Surprisingly, throughout much of the Earth’s history global temperatures were on average 5 ºC warmer than today and the poles remained ice free.