It was midsummer. The skies were clear, and the sun's rays fell over most of North America. Nearly everywhere the wind was blowing. The day was icy cold even though it was summertime. The coldest winds blew off the mountains of ice that covered much of the land.
It was a good day to hunt. At the shore near New York, a polar bear and her cubs leaped from an ice raft and swam after seals. In Illinois, a pack of hungry arctic wolves trailed a musk ox along the edge of the ice. On the ice in Iowa, a furry arctic fox raced after a snow-white hare. And in Ohio, two men wearing animal skins walked the grasslands with heavy clubs, in search of reindeer.
Surely this is not a scene from last summer. Rather, it is a picture of a day 12,000 years ago--a time long before civilization, when cold-weather animals and early man roamed our land. Twelve thousand years ago was a time when a large part of the Earth was covered by thick blankets of ice known as glaciers.
Frozen Earth: Explaining the Ice Ages, R. V. Fodor
We are all descendants of the ice age. Periods of glaciation have spanned the whole of human existence for the past 2 million years. The rapid melting of the continental glaciers at the end of the last ice age spurred one of the most dramatic climate changes in the history of the planet.
The next ice age is already overdue. Its onset depends on how man alters the climate by adding to the greenhouse effect. Perhaps global warming with man's assistance, might be able to hold off the next ice age for a little while longer.
Ice Ages: Past and Future, Jon Erickson