David, You need to update your lightbulb. There is sooo much reasoned explanation of the phenom you're referring to, you must be THE laziest person on Earth not to have found any easily found online info on why there were Major Ice Ages in the past. Knock your socks off by doing a simple online search.
Hint: Search on Milankovitch cycles.  Of course that's not the only issue, but it's a start to opening your mind and see.