We are in an Ice Age that started around 34 million years ago and then became more intense and oscillatory around 3 million years ago. Evidence for the modern Ice Age and the timing of glacial maxima and minima includes erratic boulders, glacial till, glacial moraines, glacially formed lakes, lowering of sea level evident by the migration of ocean margins in the rock record, & migration of species across land bridges exposed during low sea level intervals. This ice age, however, is not only characterizd by cooling but also drying, causing a global expansion of deserts and shrinking of forests. Tectonics in the southern hemisphere ~34 million years ago isolated Antarctica and led to the cooling of Antarctic waters. Connection of North and South America by the Isthmus of Panama ~3 million years ago changed ocean circulation which led to the cooling of the northern hemisphere. The interglacial cycles which began around 3 million years ago and characterize the modern Ice Age are caused by astronomical factors, namely Earth's tilt or obliquity and orbit or eccentricity. Positive feedback mechanisms associated with C release and burial during the warming (glacial minima) and cooling (glacial maxima) periods amplify the temperature changes during the interglacial cycles.

References: Earth System History: https://amzn.to/3v1Iy0G

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Brief history of Earth by A. Knoll: https://amzn.to/3w3hC1I
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0:00 We are in an Ice Age
1:14 Evidence for Ice Age
4:50 Effects of Cooling & Drying
6:36 Effects of Interglacial Cycles
8:30 Forest vs Tree Migration
9:31 The cause of Ice Age cooling
11:34 The cause of Ice Age cycles
12:57 Positive feedbacks amplify glacial cycles
14:26 related videos & references

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