Editing Paleoclimatology (section)

=== Paleothermometry (Water Isotopes) ===
The ice itself acts as a historical thermometer. Water molecules (H2O) naturally contain different isotopes of oxygen—specifically, lighter Oxygen-16 and heavier Oxygen-18. Because water molecules containing Oxygen-16 are lighter, they evaporate more easily from the oceans. Conversely, the heavier Oxygen-18 molecules precipitate out more readily as rain or snow as the air mass travels toward the poles.

The ratio of these two isotopes in an ice core layer is highly dependent on the temperature at the time the snow fell. During cold glacial periods, the ice contains even less of the heavy Oxygen-18. By measuring this isotopic ratio, scientists can reconstruct highly accurate localized temperature records spanning up to 800,000 years in Antarctica.