Found a nice explanation on Yahoo Answers...
There are two types of ice ages and both are caused by different natural cycles. The conventional ice age occurs of it’s own accord at intervals of approx 100,000 years and this is due to the shape of the orbit that Earth takes around the Sun (it’s called eccentricity and relates to the elliptical orbit of the Earth, over time the orbit tends to and from circularity). The last ice age of this nature ended 10,000 years ago, we’re due for another in about 90,000 years.
There are also the great ice ages that occur at intervals of approx 125 million years. Unlike the lesser ice ages that only affect parts of the planet, these great ice ages can turn the entire planet into a ball of ice. These are most likely caused by changes in the orbit of the Sun around the centre of the galaxy.
http://answers.yahoo.com/question/index?qid=20100524160435AAgMjZO
COMPLETELY unrelated, the logical fallacy of the day is:
The Argument from Personal Incredulity: I cannot explain this, therefore it cannot be true.