There are several lines of evidence that allow us to recognize the occurance of these ice ages. A handful in recent times (10-100 thousand years ago), there were a few large scale ice ages some 1 billion years ago-700 million years ago. As well as plenty in between. For very specific details, you would have to ask a geologist who is a bit more well versed than I, but I could still give basic concepts a run.
Some of the more simple evidence can be visibly seen. Glaciers, when they move, they deposit drop stones, and they make very unique scouring features in the earth.
Right. So, you can go to places like...the mountains of New York, for example. And you can find these marks, indicating that ice once resided over northern parts of the US.
And there is glacial till and morains that exist on and within the earth, that allow us to see that ice has been there. So it can be pretty straight forward evidence.
Then there are temperature records that coincide with the ages of these striations. When discussion temperature records in reference to ice ages, people often refer to the varying weights of oxygen isotopes. Within water molecules, you can have water, less likely to move by means of evaporation, due to its heavier weight and greater energy needed to sustain its evaporated state, and to retain its ability to move through our atmosphere.
So, you would find a greater number of heavier oxygen isotopes, trapped in northern regions, in times of warm weather. Becuase these isotopes arent sapped of their energy by cold temperatures. And vise versa. When its really cold out, your heavier isotopes are going to have less energy needed to put them into motion, so they wont travel as far north. And these molecules freeze up in the ice record.
Moving on...ah, this post is taking more than the 5 minutes i had expected to type it up for...
There are milankovich cycles. Just as the earth transitions from summer to winter, based on its distance from the sun, slight changes in the tilt of axis of the earth, and the way the planets orbit alters and how the planet "wobbles". These are all things that effect our planets temperature. And there are times in which these three alterations, coincide in pushing for cold temperatures. And they make our planet really cold.
And all of these different lines of evidence I am mentioning, all coincide here, more or less.
Then there are...there are many lines of evidence really. Living organisms and their fossils transition in location on the planet along side these temperature changes.
If you are familiar with plate tectonics, you can...basically backtrack and remap the motion of continents over time, and you can determine where the continents were during these ice ages and you can...with that knowledge determine the extent of the ice. An example for this. People who proposed snowball earth, recognized that continents once located on the equator, have straitions. Thats only possible if earth were, just about completely covered in ice.
There are a decent number of evidences for these ice ages. I can make brief mention of them, but each individual evidence is overloaded with its own research and you could study each one for years in its own right.
And just to wrap this post up so i can move along. We are actually currently within an ice age, we are within an interglacial period of our current ice age. And its really only a matter of time before everything freezes up again. Or perhaps we can transform histories trend with our release of CO2. I guess time will tell.
Edited by Belial, 04 July 2012 - 09:22 AM.