Ice Ages – What are they and what causes them?

What is an ice age? An ice age is a long interval of time (millions to tens of millions of years) when global temperatures are relatively cold and large areas of the Earth are covered by continental ice sheets and alpine glaciers. Within an ice age are multiple shorter-term periods of warmer temperatures when glaciers retreat (called interglacials or interglacial cycles) and colder temperatures when glaciers advance (called glacials or glacial cycles).

At least five major ice ages have occurred throughout Earth’s history: the earliest was over 2 billion years ago, and the most recent one began approximately 3 million years ago and continues today (yes, we live in an ice age!).

Currently, we are in a warm interglacial that began about 11,000 years ago. The last period of glaciation, which is often informally called the “Ice Age,” peaked about 20,000 years ago. At that time, the world was on average probably about 10°F (5°C) colder than today, and locally as much as 40°F (22°C) colder.