Introduction | A LONG LONG LONG time ago... | A LONG LONG time ago...
A LONG time ago... | Not so long ago...

A LONG time ago...

Around 3 million years ago, the amount of carbon dioxide in the atmosphere decreased and Earth became cooler. Large ice sheets called glaciers formed across many of the continents. So much water was used to make the ice that the ocean level fell. The climate didn’t just stay cool. It went through several cycles when it was cool and then warm and then cool again. Ice sheets grew, then melted, and then grew again. These cycles are called the Ice Ages. During the last Ice Age, much of North America was covered with an ice sheet that was more than a mile thick.

Antarctica is covered with ice. Some of the ice is very new and some of it is very old. In fact, the oldest ice in the world is 420,000 years old and is in Antarctica. The ice has thin layers in it, one layer for each season. The layers of ice contain tiny bubbles full of air from the ancient atmosphere. Ash and dust from volcanoes, forest fires, and dust storms is also found within the ice. Scientists look closely at the ice to understand ancient climates.