Ask a Nerd

AskANerd:  February 25, 1999

Cindy Champion of Bloomington, IN asks:

Do you know why we had an ice age? Is it the whole meteor
theory? Was there really ice all the way down in Mexico and stuff? How
long did it last?

The Nerd Responds:

Weather can be unpredictable, but ice ages are actually fairly predictable.  The meteorite (or comet, depending on which theory you choose) that probably wiped out the dinosaurs probably did cause an ice age, but there have been many many ice ages to come and go since then.  In fact, ice ages occur on this planet just about every 100,000 years (of varying intensity depending on other factors).

The major cause of ice ages is the precession of Earth's axis.  The reason we have seasons on Earth is because of the 23.5 degree tilt that the Earth has with respect to the Sun:

As you can see from this picture, the Earth has a tilt that causes seasons.  In this picture, it is winter in the northern hemisphere, summer in the southern hemisphere. You can see that the southern hemisphere is getting more direct sunlight than the northern.  When the Earth moves halfway through its orbit, the situation will be reversed and it will be summer in the northern hemisphere. This is the way matters stand at the moment, but the Earth doesn't stay this way.  The gravitational pull of the moon forces the earth to "wobble" like a top as it rotates and orbits the Sun.  In fact,  20,000 years ago the seasons were completely the reverse of what they are now because the Earth was tilted exactly the opposite direction that is showed in the picture above.  Click the Watch the Wobble button above to see an animation of this wobbling effect.

Realize, of course, that this is just a 2-diminsional view of the wobbling. In reality, the wobbling actually occurs in three diminsions, much like a a top that is starting to slow down. The same thing that happens to a top is slowly happening to the Earth. A top starts to wobble because gravity works against its spinning action to try to get it to topple over. When the top's circular momentem can no longer fight gravity, it starts to wobble and finally topples over. The same thing is happening to the Earth, only it is the gravitational force from the moon that is forcing the Earth to wobble in its spin.

Now, in addition to this wobbling motion, the Earth also orbits the Sun in a slightly eliptical orbit, so sometimes in the course of a year, the Earth is farther away from the Sun than others.  Normally, this doesn't have much affect on the Seasons.   Perhaps for some decades, during the summer months the Earth is farthest from the Sun in its orbits.  Perhaps a century later the summer months will occur when the Earth is closest to the Sun.  Because these two cases flip-flop with some regularity, the potential effects of each case is canceled out.  However, every 100,000 years or so, the Earth's orbit and its tilt coincide and stay in synch for a long long period of time.  Millinea may pass where the Summers always occur when the Earth is farthest from the Sun, and winters when it is closest:

nsummer.gif (2774 bytes)
Summer occuring when Earth is farthest from the Sun
nwinter_out.gif (2698 bytes)
Winter occurring when Earth is closest to the Sun

When this situation occurs, we are likely to have an ice age.  Because Summers occur farthest from the Sun, they are not quite as warm as they could be.  As such, ice that would eventually melt in the warmer summers, doesn't quite all melt.  Over time, this unmelted ice accumulates and accumulates until it starts to form huge ice sheets accross the northern hemisphere.  This process will continue until the Earth finally falls out of synch with the particular tilt.

To answer the second part of your question:  "How far did the ice reach?" we should notice that the tilt of the Earth affects the northern and southern hemispheres, but doesn't greatly alter the Equatorial regions (just as these regions don't typically experience much seasonal change).  So during an ice age, the equatorial regions are likely to be relatively ice-free.  Not surprisingly, the further south you go (in the northern hemisphere), the less glacially-designed landscape you will see.

Living in Bloomington, IN, you can actually see a dramatic example of this.   Bloomington sits just about on the edge of a climate zone that is typically a few degrees warmer than anything to the north.  In fact, the glacial advance halted just about at Martinsville.  When you drive from Indianapolis to Bloomington sometime, notice how the land is rather flat until you get past Martinsville.  Then, rather suddenly, you sart seeing large hills.  This is because the glaciers reached the warmer climate and melted and never advanced past Martinsville.  All the land north of this was smoothed out by the glaciers.  This line of advance is pretty similar throughout the country and so landscape altering glaciers never got as far a Mexico.