Danish geophysicist Willi Dansgaard (1922-) discovered that by measuring the levels of oxygen isotopes and deuterium (a hydrogen isotope) in glaciers, as well as dust content and the acidity of ice, it was possible to reconstruct what the Earth’s climate was in the past. He came about his conclusion by studying ice cores drilled out of glaciers in Greenland in the 1960s with Swiss physicist Hans Oeschger (1927–1998), the inventor of the Oeschger counter, a radiation measuring device. Oeschger and Dansgaard drilled ice corps samples dating back some 150,000 years in Earth’s history. Layers in the ice showed that there had, in that time, been 24 abrupt changes in the world’s climate. These are now called Dansgaard-Oeschger events.