Ice ages occur due to continental movement and changes in ocean and atmospheric patterns. When the plates beneath the continents shift, warm ocean water is obstructed and can no longer reach the poles. This causes glaciers to grow.
The Earth has experienced at least five ice ages, which last for millions of years apiece. However, these ice ages fluctuate. There are warmer and cooler periods during an ice age known as glacial and interglacial periods. These glacial-interglacial cycles are characterized by long periods when glaciers advance of between 70,000 and 90,000 years, and by much shorter periods of warmer weather when glaciers retreat, from 10,000 to 30,000 years. The cycles are caused by variations in the Earth's orbit and the tilt and wobble of its axis.
Currently the world is in an ice age during an interglacial period that has lasted for the last 11,000 years. Scientists hypothesize that it began when the land connecting North and South America prevented tropical currents from flowing between the oceans.
Researchers study ice sheets in Greenland and Antarctica to better understand ice ages. The ice cores and landforms give many hints as to what the last ice ages were like and how long they lasted. In order to obtain these important ice cores, scientists drill deep into glaciers where the ice is old.