Younger Dryas

The Younger Dryas, which occurred circa 12,900 to 11,700 years Before Present (BP),^[2] was a stadial (cooling) event which marked a return to glacial conditions, temporarily reversing the climatic warming of the preceding Late Glacial Interstadial (also known as the Bølling–Allerød interstadial, which spanned from 14,670 to 12,900 BP.).^[3] The Younger Dryas was the most severe and longest lasting of several interruptions to the warming of the Earth's climate. The end of the Younger Dryas marks the beginning of the current Holocene epoch.

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 hypotheses have been put forward about the cause, and the hypothesis historically most 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] However, several issues do exist with this hypothesis, one of which is the lack of a clear geomorphological route for the meltwater. In fact, the originator of the meltwater hypothesis, Wallace Broecker, stated in 2010 that "The long-held scenario that the Younger Dryas was a one-time outlier triggered by a flood of water stored in proglacial Lake Agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape".^[6] A volcanic trigger has been proposed more recently,^[7] and the presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores^[8] and cave deposits.^[9]

The Younger Dryas did not affect the climate equally worldwide, but the average worldwide temperature changed drastically. For example, in the Southern Hemisphere and some areas of the Northern Hemisphere, such as southeastern North America, a slight warming occurred.^[10]

The Younger Dryas is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala, as its leaves are occasionally abundant in late glacial, often minerogenic-rich sediments, such as the lake sediments of Scandinavia.