The Glacial - a period of instability

The figure shows that the glacial climate was much more unstable than the current interglacial, the Holocene. The most prominent climatic event in the Holocene was the 8.2ka event which is barely visible compared to the climate swings of the Glacial.

Profiles of δ18O (reflecting temperature) from the DYE-3, GRIP and NGRIP cores for the past 123,000 years. The end of the Eem interglacial ~120,000 years ago is covered by the NGRIP ice core only, while the late part of the Glacial is seen in all three Greenland records. The most prominent Dansgaard-Oeschger events during the Glacial are numbered.

Dansgaard-Oeschger events

The abrupt shifts from low to high δ18O values during the Glacial are often referred to as Dansgaard-Oeschger events. This name is in honour of two persons who were instrumental in discovering the Glacial instability: Professor Emeritus Willy Dansgaard, who founded the Ice and Climate group at the University of Copenhagen and the late Professor Hans Oeschger from the University of Bern. There are a total of 25 Dansgaard-Oeschger events during the Glacial and they are the most prominent features observed in the glacial parts of all Greenland ice cores.

An oceanic cause?

The Dansgaard-Oeschger events must be due to large scale reorganizations of the atmospheric and oceanic circulation during the Glacial. The coldest conditions are likely related to a drop in the poleward North Atlantic heat transport. The warm Dansgaard-Oeschger events are then caused by the succeeding re-establishment of the oceanic heat transport strength. Changes from cold climatic conditions to the 10-15°C warmer Greenland temperatures during the Dansgaard-Oeschger events are believed to have occurred within a few decades. The Glacial climate is therefore characterized by a degree of instability unparalleled during the Holocene.