An ice age is a period of long-term reduction in the temperature of the Earth’s surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Ice ages are characterized by cooling periods during which the Earth’s climate becomes colder and glacial periods during which extensive ice sheets cover portions of continents.
The most recent ice age, called the Quaternary glaciation, began about 2.6 million years ago and included several cycles of glacial advance and retreat that affected much of the Northern Hemisphere. During this time, there were four major glaciations, with consequent advances of continental ice sheets covering vast areas of North America, Europe, Asia, Australia and Antarctica. The last major advance occurred during the late Pleistocene epoch (about 115 thousand to 11 thousand years ago), when ice sheets grew to their greatest extent before receding at the end of that period.