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

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At least five major ice ages have been documented in Earth's history. And guess what? You're living in an ice age right now. But don't expect the planet to chill over from pole to pole.
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.

That's hard to believe in these days of dangerously increasing global temperatures, but ice ages aren't uniformly hard-frozen. Within these major ice ages there are warmer, shorter-term periods, called interglacials, when the ice sheets retreat, and some, or even most, of the planet is free of ice. (By contrast, the periods of time when glaciers advance are called, fittingly, glacials.) Interglacials can last tens of thousands of years. We're in an interglacial now — this one began around 10,000 years ago. That's why we have ice caps at the poles while temperatures are more or less comfortable (for humans) on most of the planet.

What Causes Ice Ages?

Earth goes about its business in a pretty regular way, spinning on its axis and looping around and around the sun. But there are some variations in the pattern. Over time, the tilt of the Earth, its orbit, and its wobble change a bit. These very minor (and regular) adjustments in the angle of the Earth relative to the sun affects the amount of solar radiation, or insolation, that reaches Earth. "Even though the tilt changes by only one degree or two, that's…
Avery Hurt
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