Past and future reorganizations in the climate system

High-resolution records of past climatic changes during the last glacial have revealed a number of abrupt changes on time scales of decades or less. Climate models suggest that the deep ocean circulation has the potential to act as a pacemaker of such changes. Based on results from ice cores from both polar regions, and the reference to a common time scale based on the methane record, it is suggested that the ocean is involved in the 24 Dansgaard-Oeschger events. For the longer events, northern and southern hemispheres are strongly coupled and exhibit climate changes of opposite sign. For the shorter events, the hemispheres are not coupled. The specific global response depends upon the forcing, and probably, the state of the ocean prior to the onset of these events. While such abrupt climate changes appear to be caused by a unique mechanism (changes in the sea surface freshwater balance), models suggest that the response of the ocean circulation depends on the amplitude and temporal evolution of the perturbation. (C) 1999 Elsevier Science Ltd. All rights reserved.