The glacial history of the earth is multifarious and extends back in geological time to the Proterozoic and perhaps the Archean. Global glaciations have occurred during every geological period except for the Jurassic period. This perseverance of global glaciations, in addition to recurring and ongoing glaciations in high mountainous areas, everywhere altitudes exceed the local snowline, is so constant that one might make a case that the Earth is fundamentally a glacial planet. Even though the dearth of precise time correlations remains a limiting factor, climatic data on a regional basis propose that Pleistocene climates differed in different areas, and cooling and warming trends were not uniform. Evidence suggests that glacial epochs have repeatedly occurred almost every 100,000 to 150,000 years.
The Pleistocene period (beginning 2.58 million years ago) saw quite a few phases in which Earth’s mean temperature dropped and the polar icecaps expanded towards the tropics. These are clustered into four broad ice ages—periods with cool temperatures and a significant continental ice sheet (at times, up to one-third of Earth’s land surface was covered), separated by interglacial periods of 10-15 millennia.
Aside from the palpable impact of glacier ice in eroding, transporting, and depositing vast volumes of sediments, glaciation has led to many secondary effects on the landscape and life. Nomadic humans, with fire, weapons and language, were able to hunt across the tundra and secure caves in which to shelter. Their prey needed bigger areas to scavenge, causing humanity to spread widely in pursuit. While geologists and glaciologists think widespread glaciation typifies the Pleistocene Epoch, once upon a time anthropologists considered the emergence of modern humans as a defining criterion, and vertebrate paleontologists used the fossiliferous episodes of the elephant, modern horse, and cattle in their definition of this interval.
Falling sea levels opened up land bridges across today’s oceans—most considerably, the Americas were populated by humans spreading from Mongolia, while horses evolved in America but migrated east before becoming extinct in their native land. The last noteworthy glaciations ended 10,000 years ago, even though a climatic cooling of about 500 years from c. CE 1300 has been noted.
Glacial history is so wrapped up in the evolution and development of human civilization that it is crucial we understand historical glacial history to forecast probable future global environmental change.