Oxygen and Hydrogen Isotopes
The most widely used method of determining ancient temperatures is based on the relative proportion of isotopes (variant forms) of oxygen and hydrogen in the ice. A molecule of water ice consists of two hydrogen atoms and one oxygen atom. The nucleus of all atoms consists of subatomic particles called protons, neutrons, and electrons. Isotopes of an element have the same number of protons, but a different number of neutrons, in the nucleus. All isotopes of an element exhibit the same chemical properties, but because they have different masses (a greater number of neutrons makes the isotope heavier), isotopes behave differently from one another under certain conditions.
The two most common isotopes of oxygen are oxygen 16 and oxygen 18, with either 8 or 10 neutrons, respectively. Both forms are found in water molecules, but oxygen 16 is far more common. Because oxygen 16 has fewer neutrons in its nucleus than oxygen 18, an oxygen-16 water molecule is lighter than an oxygen-18 water molecule. This difference causes oxygen-16 water molecules to evaporate at a faster rate than oxygen-18 water molecules. This oxygen-16-rich water vapor condenses to form raindrops or snowflakes. The snowflakes may then accumulate to make ice sheets rich in oxygen 16. Thus, when the climate is warmer (and evaporation rates become higher) ice becomes richer in oxygen 16. If a layer in an ice core contains a relatively high proportion of oxygen 16, scientists can generally infer that the layer is composed of ice that formed during a warm period.
After determining the relative proportions of oxygen isotopes in a layer of ice, paleoclimatologists use mathematical formulas to calculate the air temperature in that area when the ice formed. Applying this formula to the isotope record in ice cores indicates that during the Last Glacial Maximum (the farthest advance of the icecaps), about 22,000 years ago, northern Greenland was about 17 °C (30 °F) colder than it is today. Ice-core data from a glacier in the mountains of Peru indicate that temperatures in that area were about 11 °C (20 °F) lower at that time than they are now.