Copenhagen, July 22, 1952. Willi Dansgaard carefully collects the rain produced by a storm occurring over the capital of Denmark. He is interested in the oxygen 18 composition of the successive samples. If it is true, as we learned in school, that the formula for water is written H2O, that which we drink, as we have mentioned, contains different isotopic molecules, H216O, H218O, HDO, and so forth. The different isotopes of water were identified in the period between the two world wars, and the variations in their concentration in a natural environment were quickly proven. But Dansgaard’s study opened the path to a systematic exploration of the distribution of water isotopes in precipitations. The article he published in 1964 on his findings,1 and which is the study’s high point, is still often cited. One of the remarkable aspects it reveals is the relationship between the temperature at the site and the isotopic composition of precipitations. The colder it is, the weaker the number of H218O molecules as compared to those of H216O, and inversely. The variations are minor but largely sufficient to be detected thanks to the development of mass spectrometry, which highlights the fact that the H218O molecules (mass 20) are heavier than those of H216O (mass 18). With the help of a simple model Dansgaard explains this observation, but more important, he was one of the first to measure its potential applications, which logically must give access to temperatures that existed in the past, as long as it is possible to have access to past precipitations.
The idea was simple, and it was natural for Dansgaard to propose that it should be applied to Greenland, a Danish land by tradition, but today an autonomous territory. That ice sheet, more than three kilometers thick in the