You have probably all heard of ice ages.

And no, I don’t mean the Ice Age movies…

Although, Ice Age is actually a pretty good example of what happens during a real-life ice age. I haven’t seen enough of the movies to really talk about how accurate they are, but I know there’s a lot of ice.

And a lot of breaking of ice.

These movies take place during a time when much of the northern and southern regions of the Earth were covered in glaciers. The world looked a lot like the satellite image up above. Whether mammoths and smilodons (sabre-toothed cats) actually lived then is another question entirely.

For the record, dinosaurs were definitely not still alive back then. Even deep under the ice. The quote from the third movie basically sums it up:

“I thought those guys were extinct!”

“Then that is one angry fossil…”

Yeah, they were extinct. But fiction can do whatever it wants.

But why do ice ages happen…and why isn’t the world covered in glaciers now?

We all know the Earth orbits around the sun, right?

Okay, let’s start with that.

You’ll notice in this image that the Earth is tilted a bit. That was done on purpose. As I’ve described before, the tilt of the earth is in charge of the seasons.

But the tilt doesn’t stay the same. It wobbles a little bit over about 41,000 years.

What does this mean for Earth’s climate? A more dramatic tilt will mean more severe seasons because the northern and southern hemispheres are exposed to dramatic differences in the sun’s light and energy.

Now…what if I told you that the Earth doesn’t stay tilted in the same direction?

That’s right—it wobbles around, a bit like a top. And this happens really slowly, over about 26,000 years. You’d never notice it in your lifetime.

This wobbling is called precession, and it changes when during the year the seasons happen. Right now, we get northern hemisphere summers when we’re as far from the sun as our orbit takes us. But in about 13,000 years, those same summers will happen on the other side of our orbit, when we’re a bit closer to the sun.

This also changes the severity of the seasons. Because summers will happen when we’re a little bit closer to the sun, they’ll be a little bit warmer.

There’s a third motion that affects Earth’s climate, as well. It’s called eccentricity. Basically, the Earth’s orbit actually changes shape a bit.

In this diagram, you’ll see what I mean by the Earth’s orbit carrying us a little bit closer or farther away from the sun during northern hemisphere summers.

Earth’s orbit is an ellipse, not a perfect circle. One one side of its orbit, it swings in a bit closer to the sun. On the other side—half a year later—it swings out a bit farther from the sun. When it’s close to the sun, it’s at perihelion. And when it’s farther, it’s at aphelion.

Note that Earth’s position on its orbit has nothing to do with the seasons. The seasons happen entirely because of the Earth’s tilt.

However…like I said up above, being closer to or farther from the sun does have an effect on the climate. Depending on when the seasons happen along our orbit, they’ll be more mild or more severe.

Naturally, then, if the shape of our orbit changes, seasons will be affected.

Our orbit doesn’t change shape much. But if it becomes more elliptical, we’ll swing out farther from the sun during aphelion. Whatever seasons we have then will just be a little bit cooler. Summers might be too cool to melt all of the snow and ice from the previous winter.

And when that happens, glaciers start to grow larger.

At one point, our planet looked much like the image up above, with glaciers covering about half of the northern hemisphere—all of Canada and Russia, and large parts of the United States and Europe. Our orbit may have been more elliptical then. The tilt may have made winters more severe. And precession may have made northern summers happen closer to aphelion.

Right now, we’re in an ice age. It began about 3 million years ago, and the Earth is slowly growing slightly warmer. This is a natural part of any ice age—the global climate gets a bit warmer and ice melts back to the poles, to look much more like this.

It may seem, with the lack of ice on the globe and the rise of global warming, that we’re not in an ice age at all—that we’re in an interglacial, the periods between ice ages. But if that were true, there would be no ice sheets at all. Not even at the poles.

There have been a number of ice ages in Earth’s past. It’s hard to find evidence of more ancient ones because more recent ones tend to erase the evidence. But we do know that a major one occurred about 2.5 billion years ago.

All the rest that we know of have been in the last billion years, and there have probably been many others. And all ice ages aren’t the same. Remember how many different factors of Earth’s orbit affect the climate. Depending on how those factors cycle against one another, we could get more or less severe ice ages.

So when do these ice ages happen, anyway?

Evidence tells us that there’s probably one every 250 million years or so. But that doesn’t mean that the world gets covered in ice for a while and then it takes 250 million years to melt before everything starts to freeze over again.

Ice ages are complex…and glaciers tend to go and come back during just one cycle. There might be thousands of little mini ice ages within each large one. What we know is that glaciers tend to melt and come back over about 40,000-100,000 years.

But how to we know all these things? I don’t know about you, but I don’t think today’s scientists were alive to take measurements and observe the climate that long ago.

Well…that’s what the next astronomy post will be all about.