[nickjarboe]

In recent geologic time CO2 levels seem to mostly be influenced by temperature instead of increasing CO2 levels causing temperature rise like today (there does not seem to be animals in past all of a sudden excavating and burning gigatons of carbon annually). Over million year timescales the biggest cause in the change in Earth's temperature is Milankovitch cycles[1] which is short hand for changes in the shape Earth's orbit and changes in the position of the Earth's spin axis. These changes interact to produce various climate conditions. The biggest factor is that sometimes these cycles cause the northern hemisphere to have very cold winters and other times mild winters. When the winters are very cold, continental glaciers can form on North America and Eurasia. They reflect more sunlight than land and the Earth gets colder. This is an ice age. Eventually the orbital and spin axis parameters change (over tens of thousands of years) and, with warmer winters in the northern hemisphere, the ice sheets begin to melt. With this positive feedback loop, the ice age ends quickly, the Earth gets warmer, and the Earth enters what is called an inter-glacial (we are in one now).

This is, of course, a very simplified explanation. As one goes back hundreds of millions to billions of years, other large scale changes like plate tectonics, atmospheric composition/mass, sun brightness, continental weathering, etc. will cause the average surface temperature to vary considerably.

[1]https://en.wikipedia.org/wiki/Milankovitch_cycles