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There have been at least five significant ice ages in Earth’s history, with approximately a dozen epochs of glacial expansion occurring in the past 1 million years. Humans developed significantly during the most recent glaciation period, emerging as the dominant land animal afterward as megafauna such as the wooly mammoth went extinct.
During an ice age, colder global temperatures lead to recurring glacial expansion across the Earth’s surface. Capable of lasting hundreds of millions of years, these periods are interspersed with regular warmer interglacial intervals in which at least one major ice sheet is present. Earth is currently in the midst of an ice age, as the Antarctic and Greenland ice sheets remain intact despite moderate temperatures.
These global cooling periods begin when a drop in temperature prevents snow from fully melting in some areas. The bottom layer turns to ice, which becomes a glacier as the weight of accumulated snow causes it to slowly move forward. A cyclical pattern emerges in which the snow and ice traps the Earth’s moisture, fueling the growth of these ice sheets as the sea levels simultaneously drop.
Whereas the first 30 million years of the Late Cenozoic Ice Age mostly involved Antarctica, the Quaternary has seen numerous ice sheets extending over parts of Europe and North America that are currently populated and easily accessible. Early geologists therefore named apparent sequences of glacial and interglacial periods of the Quaternary Ice Age after characteristic geological features, and these names varied from region to region. The marine record preserves all the past glaciations; the land-based evidence is less complete because successive glaciations may wipe out evidence of their predecessors. Ice cores from continental ice accumulations also provide a complete record, but do not go as far back in time as marine data.
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