Definition
An ice age is a long period when Earth's average temperature stays much lower for a very long time. Not only the poles but also huge areas of the continents get buried under thick masses of ice (glaciers). It lasts millions of years, with very cold glacial periods and milder interglacial periods taking turns.
The Domino Effect of Cold Ice
Picture a winter day when heavy snow has turned the whole neighborhood white. Sunlight bounces off the snow so brightly that it hurts your eyes, and the air around you feels even colder. The whole Earth goes through the same process.
When Earth cools a little and snow and ice start piling up over wide areas, the white snowfields act like a mirror and bounce sunlight back into space. Scientists call this the albedo (reflectivity) effect. Since sunlight is reflected away, Earth gets colder, and the colder it gets, the more ice freezes over. This is a cold domino effect.
As this keeps going, water that evaporated from the ocean falls as snow and piles up on the continents as giant glaciers. Because that water can't return to the sea, sea level drops by more than 100 meters. The sea floor turns into dry land, connecting continents with land bridges, and big furry animals like mammoths walked along those paths from one continent to another.
To Be More Precise: We're Still in an Ice Age
To be more precise, scientists call even this very moment a kind of ice age in the broad sense, because ice still remains at both poles. Inside an ice age, very cold "glacial periods" and relatively mild "interglacial periods" take turns, like a swinging pendulum.
About 20,000 years ago, when mammoths lived, was the coldest stretch of the last glacial period, when ice spread the widest. The era humans live in now is an interglacial period, when the ice has pulled back for a while. Across Earth's whole history, warm times with no ice at all at the South and North Poles lasted much longer. So with huge ice sheets still sitting at the poles, we are in the middle of an ice age right now.
The biggest cause of this rhythm is that the shape of Earth's path around the Sun and the tilt of its axis change in regular cycles. These changes in Earth's orbit are called Milankovitch cycles, named after the scientist who discovered them.
How Does a Frozen Earth Warm Up Again?
The trigger that ends a cold glacial period and starts a warm interglacial period comes from changes in Earth's orbit. As the tilt of its axis and its path around the Sun change in cycles, there are times when more summer sunlight falls on the high latitudes of the Northern Hemisphere.
When stronger summer sun melts the edges of the glaciers, the dark ocean and land that were hiding under the ice come into view. Dark surfaces don't bounce sunlight back. They soak it up, so temperatures rise even faster. On top of that, the warming ocean releases the carbon dioxide stored in its water into the air, which speeds up Earth's heat-trapping effect.
Warming that starts this way melts ice faster and pulls out even more greenhouse gases, and that brings the cold glacial period to an end. Meanwhile, the "Snowball Earth" hypothesis says that hundreds of millions of years ago the whole planet froze all the way to the equator. In that idea, greenhouse gases that built up from volcanic activity served as the escape route that melted the ice.
Common misconceptions
During an ice age, the whole Earth was frozen solid and every living thing froze to death.
Even during ice ages, areas near the equator stayed warm, and plants and animals thrived there. Also, within an ice age, mild interglacial periods came every tens of thousands of years, giving living things time to adapt.
Where you meet it
When sea level dropped by as much as the ice piled up on the continents during a glacial period, the Bering land bridge (a strip of land between Asia and North America) appeared, and people walked across it.
Huge glaciers carved the land as they moved. They left behind the grand fjords and U-shaped valleys of northern Europe.
In one sentence
An ice age is a climate period when changes in Earth's orbit and the reflection of sunlight by ice make ice spread widely across the poles and continents.