At the peak of the last glacial cycle, giant ice sheets a mile thick covered half of North America.

What is an ice age?

An ice age is a period of time—usually millions or tens of millions of years—when vast glaciers, called ice sheets, cover much of the Earth’s land surface. Several ice ages have occurred throughout our planet's history. The latest ice age began about 2.5 million years ago. During this time, giant ice sheets have advanced and retreated many times in North America and Europe.

Recent cycles of advancing and retreating ice sheets have occurred every 100,000 years. Each cycle consists of a long, generally cold period during which the ice sheets slowly reach their maximum extent over tens of thousands of years, and a relatively short warm period during which the ice sheets rapidly retreat.

We are now in a warm period that has lasted more than 10,000 years, which is longer than many of the previous warm intervals. If the pattern of glacial cycles still holds true, we should be about due for the beginning of the next cold phase. Indeed, several hundred years ago, global temperatures began decreasing during a period known as the Little Ice Age.

Over the last century, however, average global temperatures have instead started to rise. Scientists have recently concluded that much of the recent warming is due to the release of greenhouse gases from human activities such as the burning of fossil fuels. A sobering possibility is that continued human-caused global warming could disrupt or override the natural climate cycle of the ice age.

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