Oxygen isotope dating
From the very deepest ice cores reaching depths of more than three kilometers in the Antarctic ice sheet, we can clearly see the steady pulsing of the ice ages on a period of about 100,000 years.
At a range of sites in the polar regions scientists have measured a near linear relationship between O and D in samples of modern snowfall taken over several years and the mean annual temperature.
Water is made up of molecules comprising two atoms of hydrogen and one atom of oxygen (HO).
But it's not that simple, because there are several isotopes (chemically identical atoms with the same number of protons, but differing numbers of neutrons, and therefore mass) of oxygen, and several isotopes of hydrogen.
The measurement of the gas composition is direct: trapped in deep ice cores are tiny bubbles of ancient air, which we can extract and analyze using mass spectrometers.
Temperature, in contrast, is not measured directly, but is instead inferred from the isotopic composition of the water molecules released by melting the ice cores.