A notable series of rapid fluctuations of temperature that occurred during the last ice age is called the Dansgaard-Oeschger events. Through the analysis of ice cores and the reconstruction of temperatures, it has been determined that there were 23 fluctuations between 110,000 and 23,000 years ago. The cyclic warming periods in the Northern Hemisphere all show a distinctive pattern: The atmosphere warmed rapidly (generally in a few decades) then gradually cooled over a longer period of time. Although a multitude of ice cores have been studied for this time period, scientists still are not sure what caused the cyclic behavior of the atmosphere. The behavior of the cycles was different in the Southern Hemisphere, however. The warming cycles tended to be much more gradual with less temperature variation.
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