Analysis of the Greenland ice cores and the North Atlantic marine sediment cores performed in the early 1990s revealed that climate changes in this region were everything but gradual. Reconstruction of Greenland temperature during the last glacial cycle shown in Figure 1c reveals pronounced instability of the climate system on millennial timescale. Most of this temperature record with a notable exception for the last 10 000 years is punctuated by numerous rapid warming events known as Dansgaard-Oeschger events. These events are characterized by extremely rapid warming with the magnitude exceeding 10 °C, that is, more than a half of the glacial-interglacial temperature variations observed in Greenland. Although the strongest climate signal associated with Dansgaard-Oeschger events were recorded in Greenland and in the Northern Atlantic, synchronous climate variations have been found in many other places around the world. Analysis of paleoclimate data and model simulations suggests that these abrupt climate changes are related to the rapid reorganizations of the Atlantic thermohaline circulation and events of massive iceberg discharge into the Atlantic Ocean from surrounding ice sheets.
Due to very different climate conditions of the glacial age as compared to the present ones, abrupt climate changes which occurred in the past cannot be considered as a direct analog for the future 'greenhouse' world but they represent an important evidence for the potential instability of two components of the climate system - the thermohaline ocean circulation and the ice sheets. These two components of the climate system are considered by many experts as the prime suspects for dangerous and irreversible future climate changes.
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