What is Ice age?
{{otheruses}} [[Image:Vostok Petit data.svg|thumb|300px|Variations in temperature, CO<sub>2</sub>, and dust from the Vostok, Antarctica [[ice core]] over the last 400,000 years]] [[File:20191021 Temperature from 20,000 to 10,000 years ago - recovery from ice age.png|thumb|300px|Rise of temperature at the end of the last ice age]] An '''ice age''' is a period when for a long time the [[temperature]] of [[Earth]]'s [[climate]] is very low. This leads to an expansion of the continental [[ice sheet]]s, polar ice sheets and mountain [[glacier]]s. 'Ice age' is a term used in [[palaeoclimatology]] for the period of extensive ice sheets in the recent [[Pleistocene]] period. We now know that ice ages have happened a number of times in the past, the greatest and longest of which took place in the [[Proterozoic]] era, before multi-cellular [[eukaryote]]s evolved.<ref>Imbrie J. & Imbri, K.P. 1979. ''Ice ages: solving the mystery''. Short Hills NJ: Enslow. {{ISBN|978-0-89490-015-0}}</ref><ref>Alley R.B. 2000. ''The two-mile time machine: ice cores, abrupt climate change, and our future''. Princeton University Press. {{ISBN|0-691-10296-1}}</ref> During ice ages the [[sea level]] drops as water is held in the great ice sheets at the [[geographical pole|poles]]. How much it drops depends on several factors, such as the length of time that a cold period lasts. == Stages == Within an ice age, there are [[Stage (geology)|stages]]. The longer cold stages are called '''glacials''' or glacial periods. The shorter warm periods are called '''interglacials'''. The last glacial ended about 11,000 years ago when the present interglacial started. The [[Greenland]] and [[Antarctic]] ice sheets still exist. The last two million years have been the Pleistocene ice age. During glacials, large and thick ice sheets cover much of the [[North America]]n and [[Eurasia]]n continents. Many glacial periods that have occurred during the last few million years are initially at 40,000-year frequency, but more recently ice ages have occurred at 10