Robert Mulvaney, a glaciologist with the British Antarctic Survey, explains.
The cornerstone of the success achieved by ice core scientists reconstructing climate change over many thousands of years is the ability to measure past changes in both atmospheric greenhouse gas concentrations and temperature. 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.
Water is made up of molecules comprising two atoms of hydrogen and one atom of oxygen (H2O). 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 isotopes of particular interest for climate studies are 16O (with 8 protons and 8 neutrons that makes up 99.76 percent of the oxygen in water) and 18O (8 protons and 10 neutrons), together with 1H (with one proton and no neutrons, which is 99.985 percent of the hydrogen in water) and 2H (also known as deuterium (D), which has one proton and one neutron). All of these isotopes are termed 'stable' because they do not undergo radioactive decay.
Using sensitive mass spectrometers, researchers are able to measure the ratio of the isotopes of both oxygen and hydrogen in samples taken from ice cores, and compare the result with the isotopic ratio of an average ocean water standard known as SMOW (Standard Mean Ocean Water). The water molecules in ice cores are always depleted in the heavier isotopes (that is, the isotopes with the larger number of neutrons) and the difference compared to the standard is expressed as either 18O or D. Both of these values tell essentially the same story--namely, that there is less 18O and D during cold periods than there is in warm. Why is this? Simply put, it takes more energy to evaporate the water molecules containing a heavy isotope from the surface of the ocean, and, as the moist air is transported polewards and cools, the water molecules containing heavier isotopes are preferentially lost in precipitation. Both of these processes, known as fractionation, are temperature dependent.
At a range of sites in the polar regions scientists have measured a near linear relationship between 18O and D in samples of modern snowfall taken over several years and the mean annual temperature. This relationship can be used to calibrate the isotope ratio thermometer, although the calibration changes a little during ice age climates. Plotting either 18O or D with depth along the length of an ice core reveals the seasonal oscillations in temperature and researchers can also count annual layers in order to date them. 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. From a site called Dome C in Antarctica, we have recently reconstructed the climate spanning the last three quarters of a million years, and have shown seven ice ages, each interspersed with a warm interglacial climate such as the one we are living in today.