Although the exact cause of ice ages is not known, scientists speculate that they are caused by a combination of the distance of the Earth from the Sun, the position of the continents and solar output. The last ice age happened around 20,000 years ago.
During an ice age, the whole planet is noticeably colder. Ice ages can last for million of years and help to shape continents. Today's climate is actually a warm period between periods of glaciation. The world's climate continues to change and land masses continue to move. This means that the Earth is in the middle of a major cooling period that started more than 3 million years ago.