It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation, which transports warm water from the equator towards the North Pole, and which in turn is thought to have been caused by an influx of fresh cold water from North America into the Atlantic.
The application of alkenone paleothermometers to high-resolution paleotemperature reconstructions of older glacial terminations have found that very similar Younger Dryas-like paleoclimatic oscillations occurred during Terminations II and IV.Measurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40–50 years in three discrete steps, each lasting five years.Other proxy data, such as dust concentration, and snow accumulation, suggest an even more rapid transition, requiring about a 7 °C (13 °F) warming in just a few years.This stadial is often, but not always, known as the Oldest Dryas.The Oldest Dryas occurred approximately 1,770 calendar years before the Younger Dryas and lasted about 400 calendar years.
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In the southern hemisphere, and some areas of the north such as southeastern North America, there was a slight warming.The presence of a distinct cold period at the end of the Late Glacial interval has been known for a long time.The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a).Since 1916 and the onset and later refinement of pollen analytical techniques and a steadily growing number of pollen diagrams, palynologists have concluded that the Younger Dryas represented a distinct period of vegetational change in large parts of Europe during which warmer climate vegetation was replaced by that of a generally cold climate, a glacial plant succession that often contained Dryas octopetala.The analysis of Greenland Summit ice cores as a part of the Greenland Ice Sheet Project-2 (GISP-2) and Greenland Icecore Project (GRIP) estimated that the Younger Dryas started about 12,800 ice (calendar) years BP.
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Depending on the specific ice core analysis consulted, the Younger Dryas is estimated to have lasted 1,150–1,300 years.In northern Scotland the glaciers were thicker and more extensive than during the Younger Dryas.The Older Dryas, in turn, is preceded by another warmer stage, the Bølling oscillation that separates it from a third and even older stadial.Paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, e.g.the Allerød clay pit in Denmark, first recognized and described the Younger Dryas.