An ice age is a period in which the earth's climate is colder than normal, with ice sheets capping the poles and glaciers dominating higher altitudes. Within an ice age, there are varying pulses of colder and warmer climatic conditions, known as 'glacials' and 'interglacials'. Even within the interglacials, ice continues to cover at least one of the poles. In contrast, outside an ice age temperatures are higher and more stable, and there is far less ice all around. The earth has thus far made it through at least five significant ice ages.
More about: Ice AgeDefinition
Timeline
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c. 2600000 BCE - c. 12000 BCEThe Pleistocene epoch, ranging from c. 2,6 million years ago until c. 12,000 years ago. It is characterised by repeated cycles of glacials and interglacials.
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c. 26500 BCE - c. 19000 BCELast Glacial Maximum - the time during which the ice sheets reached peak growth within the most recent glacial.
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c. 11700 BCEEnd of the most recent glacial episode within the current Quaternary Ice Age.
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1837 CEFirst description of an Ice Age by Louis Agassiz.