“...why is the 100,000 cycle limited to the past 4 million years, and not seen before for the prior billions of years?”
A lot of that has to do with the changing locations of continents and such things as the closing of the Isthmus of Panama, which affect ocean currents and the amount of land in more polar regions.
People forget the Earth is a living breathing planet that has managed to outlast Lordy knows how many ancient civilizations and cosmic disasters long before Al Gore was ever a tot..and it has done just fine.
Mankind seeks to make itself relevant.. Politics makes people believe they can make a difference. On a cosmic timetable, Mother Nature proves them wrong every time.
I agree. My question was rhetorical. The closing of the Isthmus of Panama coincided with the beginning of the current Ice Age.
My problem is with easy, simple answers to complex questions.