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Much to the surprise of investigators, evidence is mounting that major changes in the earth's climate can take place in a very short time. Data from ice cores and ocean sediments suggest, for example, that 11,650 years ago the climate in Greenland switched from ice-age conditions to the current relatively warm conditions (a warming of 5 to 10 degrees Celsius on average) in only 40 years. The author describes the oceanic currents that influence climate and establish its stability, as well as "triggers" that may perturb changes—including the possibility that "greenhouse" warming could invoke a rapid switch.