Around 35 million years ago global cooling of a few degrees centigrade began the development of permanent icesheets in Antarctica.
Icesheets also began developing in the Arctic around two and a half million years ago. A cycle of advancing and retreating icesheets began.
Evidence from deep sea sediments provides a continuous record of climatic fluctuation showing that numerous ice ages have occurred, especially in the last 900,000 years.
During periods of glacial advance, icesheets covered northern latitudes and mountains. Much of the world's moisture was locked up in the glaciers: sea-levels fell, exposing land, and rainfall was reduced in the unglaciated regions, much of which became desert.
Glacial periods, which were relatively short, occurred about every 100,000 years. They were separated by long periods of intermediate temperature and occasional short warm phases when temperatures were as high as now or higher.
The ice retreated; sea-levels rose, flooding land, and rainfall increased, bringing life again to desert areas and creating lakes and rivers.
