Why do ice ages come and go?

1. You start with variations in the Earth's orbit. You see, the Earth's orbit is not a perfect, unchanging thing, but varies over tens of thousands of years.

2. These variations change the amount of sunlight falling on the Earth as well as how it's distributed. This is what paces the ice ages, and you can find nearly exact correspondence between the ice age timing and these variations.

As described in "Ice Ages: Solving the Mystery" by Imbrie and Imbrie

But these changes are not enough to give you an ice age. You also need:

3. Changes in carbon dioxide. The small changes in climate caused by orbital variations change the climate, and this in turn changes atmospheric CO2 in ways that we don't fully understand. These CO2 changes then *amplify* the original change.

Yes, it's a feedback loop:

The fact that CO2 is acting as a *feedback* during ice age cycles explains why it tends to lag changes in temperature, which is an evergreen denier argument.

So ice ages are caused by orbital variations and amplified by changes in CO2.

I should emphasize that, while changes in CO2 are caused by changes in temperature during ice age cycles, human activities (mainly fossil fuel combustion) is what's increasing it today.

@andrewdessler When coming out of an ice age, the decrease of albedo is although a feedback loop, isn’t it? How does it compare to the CO2 feedback loop in terms of contribution to warming?