We cannot directly measure past atmospheric temperatures from ice cores, but we can obtain data that acts as a proxy measurement for global temperatures. The most commonly used proxy data from ice cores are isotopic ratios.

Elements with the same atomic number but different mass numbers are called isotopes. Most water molecules are made of two atoms of hydrogen (with an atomic mass of 1u) and one atom of oxygen (with an atomic mass of 16u). However, approximately one in every five hundred water molecules contains the heavier isotope of oxygen (¹⁸O); an even smaller fraction of molecules contain the heavier isotope of hydrogen (deuterium). These heavier molecules have lower vapour pressures, which means that they evaporate less readily than regular water molecules. This principle allows isotope ratios from ancient ice to be used as a proxy for temperature data.

Open the IRMS learning tool to learn how the study of isotopes can be applied to determine past temperatures. Answer the following questions as you work through the learning tool:

• Over what part of the world would you expect to find a higher ¹⁸O/¹⁶O ratio in clouds? Why?
• Think about how reliable this method of temperature detection is. What are two advantages and two disadvantages of this technique?