Ice Ages: What Causes the Earth to Freeze Over Every Few Million Years?

Periodically, global temperatures drop, ice sheets form at the poles, then the ice creeps down to cover the continents. We call these ice ages. There have been five major ice ages in Earth’s 4.5-billion-year history. The last one began about 2.5 to 3 million years ago. And get this: it’s still going on. That’s right, we’re living in an Ice Age.