The Younger Dryas, which occurred circa 12,900 to 11,700 years BP,[2] was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM),[3] which lasted from circa 27,000 to 20,000 years BP. The Younger Dryas was the last stage of the Pleistocene epoch that spanned from 2,580,000 to 11,700 years BP and it preceded the current, warmer Holocene epoch. The Younger Dryas was the most severe and longest lasting of several interruptions to the warming of the Earth's climate, and it was preceded by the Late Glacial Interstadial, an interval of relative warmth that lasted from 14,670 to 12,900 BP.


Evolution of temperatures in the postglacial period, after the Last Glacial Maximum (LGM), showing very low temperatures for the most part of the Younger Dryas, rapidly rising afterwards to reach the level of the warm Holocene, based on Greenland ice cores.[1]
The change was relatively sudden, took place over decades, and resulted in a decline of temperatures in Greenland by 4~10 °C (7.2~18 °F),[4] and advances of glaciers and drier conditions over much of the temperate Northern Hemisphere. A number of theories have been put forward about the cause, and the most widely supported by scientists is that the Atlantic meridional overturning circulation, which transports warm water from the Equator towards the North Pole, was interrupted by an influx of fresh, cold water from North America into the Atlantic.[5]


===================

The Younger Dryas is thought to have been caused primarily by significant reduction or shutdown of the North Atlantic "Conveyor" – which circulates warm tropical waters northward – as the consequence of deglaciation in North America and a sudden influx of fresh water from Lake Agassiz. The precise mechanism for the rapid melting is still the subject of active research. The lack of geological evidence for such an event[107] stimulated further exploration that has now identified a pathway along the Mackenzie River that would have spilled fresh water into the Arctic and thence into the Atlantic.[108][109] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic. However, simulations indicated that a one-time-flood could not likely cause the new state to be locked for 1,000 years. Once the flood ceased, the AMOC would recover and the Younger Dryas would stop in less than 100 years. Therefore, continuous freshwater input was necessary to maintain a weak AMOC for more than 1,000 years. A 2018 study proposed that the snowfall could be a source of continuous freshwater resulting in a prolonged weakened state of the AMOC.[110]

Wikipedia wrote: