How many ice ages has Earth had?

We are currently living through the planet's fifth ice age.

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First, what is an ice age? It's when Earth has cold temperatures for a long time - for millions or even tens of millions of years - and that leads to ice sheets and glaciers covering large areas of the planet's surface. We know that Earth has had at least five major ice ages. The first one happened about two billion years ago and lasted about 300 million years. The most recent one started about 2.6 million years ago, and technically we are still in it.

So, why isn't Earth covered in ice right now? It's because we are living in a time known as an "interglacial period". Even in an ice age, temperatures shift between colder and warmer periods. Ice sheets and glaciers melt during warmer phases, which are called interglacials, and expand during colder phases, called glacials. In one dramatic episode between 700 million and 600 million years ago, scientists think ice sheets covered the entire planet from the poles to the equator - a so-called snowball Earth. Right now we are in the most recent ice age's warmer interglacial period, which began about 11,000 years ago.