Although science can't say for sure, ice ages are thought to be triggered by the interaction between the height of the continents, the circulation of the ocean, the distance of Earth from the sun, the atmosphere's composition and solar output. Scientists say that, as of 2014, the Earth is in the midst of a major cooling period. This cooling period began about 3 million years ago.
The Earth has gone through warm and cold periods during the billions of years of its existence. During ice ages, the polar regions of the Earth are cold, and there are large glaciers covering vast regions of the planet. Ice ages tend to occur for millions or even tens of millions of years, and generally happen every 200 million years or so.
Scientists say that the present ice age that is being experienced by the planet has caused the retreating and advancement of glaciers more than 20 times. The existing climate is due to a warm interval between the periods of glacial advancement. What most people refer to as "The Ice Age" was actually a period of glacial advancement that occurred about 20,000 years in the past as part of a larger cycle.