I was staring intently at the variations in Earth temperatures:

Check it out: the variations have a period of some 100,000 years, eh?
What (on Earth) could cause that?
The burning of fossil fuels ... 400,000 years ago? Increased industrial activity ... 400,000 years ago?
Okay, so it's something more ancient, perhaps having to do with the Earth and Sun and other astronomical gyrations.

So I look around for something that has a 100,000 year period ...

>Good luck!
... and discover Milankovitch Cycles

The axis of the Earth wobbles. Now, it points to the North Star, but it'll point elsewhere in a few thousand years. To the Ancient Egyptians, Thuban was the North Star. Eventually the axis will point to the Star Vega. >Is the period of wobble 100,000 years? No, it's more like 23,000 years.

Wobble of the Earth's axis

The orbit of the Earth also rotates, about the Sun sorta like this >And the period is 100,000 years, right? No, it's more like 41,000 years. But there's another Milankovitch Cycle. The orbit of the earth in now pretty circular. However, that changes. It'll become more elliptical ... then, again, back to circular. And the period? About 95,000 years!

Rotation of the earth's orbit.

According to the 2nd of Kepler's Laws, the area swept out in a month will be the same ... for each month of the year.
When it gets more elliptical, we'll be closer to the sun for that fraction of the year.
Of course, that may not be the cause of 100K-year temperature cycles, but ...

>Well, I still think it's the dramatic increase in the number of SUVs. Cavemen, driving SUVs? >Why not? I've seen 'em on cave drawings ...

Burning fossil fuels: ca 10000 BC