How do scientists estimate past temperatures from ice cores?

Oxygen isotopic analysis as a proxy for temperature.

Glacial ice formation…

Glacial ice forms through the accumulation and compression of snow over long periods of time. It begins with the transformation of snow into firn, a granular, partially compacted snow. Over time, additional layers of snow accumulate on top of the firn, and the weight of the overlying snow compresses the underlying layers. With cont…