Image courtesy USGS
Distribution of ice in the northern hemisphere during the Ice Age.

Durin­g its history, the Earth has gone through warm periods and cold periods. During its earliest days as a planet, it was extremely hot and largely volcanic. In some of the warmer periods, rainforests grew much farther to the north and south than they do now. Between 52 and 57 million years ago, for example, trees grew on both poles.

An ice age is usually defined as a cool period in which large glaciers cover parts of the Earth's surface. When people talk about "the Ice Age," they generally mean the most recent one, which peaked about 20,000 years ago. Ice ages generally last from tens of millions to hundreds of millions of years. The Earth has experienced them off and on for the last 600 million years. The Earth is in a relatively cool period now, with permanent ice on the polar caps.

Ice ages can have several causes, including:

  • How much heat the sun produces - The sun's output is always changing. During a prolonged weaker phase, the Earth can get a lot cooler.
  • The position and height of the continents - In the Earth's distant past, the continents looked very different than they do today. This affected the circulation of air, water and heat, as well as the amount of land located in polar regions.
  • Ocean and atmospheric currents - Ocean currents circulate warm and cold water. Changes in these currents can significantly alter a region's climate.
  • Atmospheric composition - Greenhouse gases reflect infrared light -- or heat -- back to the Earth's surface. Without these gases present, the planet can become significantly colder. One study claims that the Black Death caused a "little ice age" when numerous trees began growing in abandoned fields, changing the composition of the atmosphere [ref].
Not One Big Ball of Ice
Many people imagine the Earth during the Ice Age as a solid sheet of ice. But in spite of the increased presence of glaciers, much of the Earth was still green. The climate of the area now known as California, for example, was much the same as it is today. See the La Brea Exploration Guide to learn about the flora and fauna that lived in parts of North America during the Ice Age.

In addition, ice ages can become more pronounced as they progress. Ice reflects sunlight back into space, and it can't warm the Earth. In other words, ice makes more ice.

Even though it could cool the earth a little, reducing greenhouse emissions isn't likely to spark an ice age. Scientists can't really predict ice ages, but most suspect that one will occur sometime far in the future.

Learn more about the planet Earth, the environment and ice ages by following the links on the next page.­