How do scientists determine how much CO2 was in the atmosphere thousands of years ago, and to what level of certainty are they able to perform these calculations?

Question

How do scientists determine how much CO2 was in the atmosphere thousands of years ago, and to what level of certainty are they able to perform these calculations?

In: Earth Science

Answer 1

Ice cores is a common one, but there are many others.
For example, a friend of mine did her PHD on using sheep (or one very specific kind of sheep) teeth to figure out climate at the time the sheep was growing their teeth. Based on the chemical makeup of the teeth (the ratio of different isotopes), she was able to tell what the chemical makeup of f the atmosphere was like, and from that tell what the climate was like.
Doing so with these teeth was very useful because they did not decay quickly over time, and locked in the climate data like ice does, but without needing it to stay frozen. So you could get climate data from places other than the Arctic.
My knowledge of this is limited, as it’s her science not mine, but it was super cool.