New Knowledge From the Ice
The study of ice cores has led to great advances in understanding the history of Earth's climate. For the first time, it has become possible to study climatic shifts on a very small scale—periods of 10 years to 3,000 years. Although paleoclimatologists had found scattered evidence of considerable climatic variability over relatively short periods, they could not confirm that such shifts had occurred until ice-core studies made it possible to construct a sequence of past climatic events. Perhaps the most intriguing discovery stemming from ice-core analysis is the fact that global warming has occurred many times over the ages.
This realization stems, in part, from the work of two European paleoclimatologists, Willi Dansgaard of the University of Copenhagen in Denmark and Hans A. Oeschger of the University of Bern in Switzerland. In 1989, Dansgaard and Oeschger reviewed a number of earlier studies of oxygen-isotope ratios in ice cores from Greenland. By sifting through all the data, they discovered numerous episodes of global (or in some cases, regional) warming that began suddenly and lasted only 1,000 to 3,000 years. Later studies identified at least 24 of these rapid shifts, now known as Dansgaard-Oeschger events, between 100,000 and 11,500 years ago. The temperature in Greenland rose by as much as 15 °C (27 °F) during these episodes. In each case, the warming took only a few decades, but the return to cold conditions took place more gradually, over a few hundred years. In 2000, it was still unclear what caused these large, rapid, but short-lived climatic shifts.