How many extreme shifts in climate has Earth gone through in its existence?
As I finished writing this question I did find one answer. I'll ask anyway and see if anyone has more info to contribute.

From the Wikipedia page on Ice Ages, there's this Timeline of life including a list of ice ages on left side.



Based on this, there were 7 ice ages in the past 4.5 billion years (apx age of the Earth). Were there also 7 major periods of high volcanic activity?