Ice ages

Ice Ages are periods lasting millions of years when Earth is so cold that the polar ice caps grow huge. There are various theories about why they occur. There have been four Ice Ages in the last 1000 million years, including one which lasted 100 million years.

The most recent Ice Age - called the Pleistocene Ice Age - began about 2 million years ago.

In an Ice Age the weather varies between cold spells called glacials and warm spells called interglacials. There were 17 glacials and interglacials in the last 1.6 million years of the Pleistocene Ice Age.

The last glacial, called the Holocene glacial, peaked about 18,000 years ago and ended 10,000 years ago. Ice covered 40% of the world 18,000 years ago.

Glaciers spread over much of Europe and North America 18,000 years ago. Ice caps grew in Tasmania and New Zealand, and in Hawaii were glaciers.