The present ice age, known as the Wisconsonian, began about 2.7 million years ago. Since then, Earth has undergone a sequence of glacial advances and interglacial warm periods.

Approximately 1 million years ago, the frequency switched from about every 40,000 years to about every 100,000 years, during which time the cold periods became colder. The present interglacial began about 12,000 years ago and is known as the Holocene Epoch. The previous glacial period had lasted about 112,000 years.

The coldest point is thought to have been about 20,000 years ago when the globally averaged temperature was about 8-10 degrees colder than today. The level of the sea was about 400 feet lower than today due to there being so much water locked in ice. The previous interglacial is known as the Eemian and featured peak temperatures several degrees warmer and sea levels 20-30 feet higher than today. There are at least four other known glacial periods in Earth’s past.

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