“What caused the ice age?”
There have actually been numerous ice ages throughout the history of the Earth. An ice age occurs when thick sheets of ice build up over much of the surface of the Earth in a process called glaciation. There are still places on the Earth where ice sheets cover the landscape year-round, most notably in Antarctica and Greenland. During a glacial period — informally known as an ice age — these ice sheets grow over the course of many thousands of years, until they cover much of the land and sea.
No one knows for certain what actually causes this to happen, though scientists have many theories. Most agree that ice ages are caused by a combination of factors, including the precession of the Earth’s equinoxes (its “wobbly axis”), volcanic activity, the movement of tectonic plates, sunspot activity, and the amount of greenhouse gases (such as water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons) in the atmosphere. Any of these factors can affect the amount of sunlight that reaches the Earth, or escapes its atmosphere, preventing ice sheets from melting.
The more ice that covers the face of the Earth, the harder it is for the Sun to melt it. Large sheets of ice reflect much of the Sun’s light and heat away from the ground and back into the atmosphere.
At one point in time, hundreds of millions of years ago, the entire face of the Earth, including the Equator, may have been completely encased in ice. Scientists call this theory, “snowball Earth.” Most ice ages aren’t quite that extreme, and may affect only one hemisphere or the other, rather than both at the same time.
The last major period of glaciation ended between 15,000 and 20,000 years ago.