Younger Dryas

The Younger Dryas is a geological period from c. 12,900 to c. 11,700 calendar years ago (BP). It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala. Leaves of Dryas octopetala are occasionally abundant in the Late Glacial, often minerogenic-rich, like the lake sediments of Scandinavian lakes. The Younger Dryas saw a sharp decline in temperature over most of the northern hemisphere, at the end of the epoch, immediately preceding the current warmer Holocene. It was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum, c. 27,000 to 24,000 calendar years BP. The change was relatively sudden, taking place in decades, and resulted in a decline of 2 to 6 degrees Celsius, advances of glaciers and drier conditions, over much of the temperate northern hemisphere. 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 Younger Dryas was a period of climatic change, but the effects were complex and variable. 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. 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.

The Younger Dryas is the youngest and longest of three stadials that resulted from typically abrupt climatic changes that took place over the last 16,000 calendar years. Within the Blytt-Sernander classification of north European climatic phases, the prefix ‘Younger’ refers to the recognition that this original ‘Dryas’ period was preceded by a warmer stage, the Allerød oscillation, which in turn was preceded by the Older Dryas around 14,000 calendar years BP. This is not securely dated, and estimates vary by 400 years, but it is generally accepted that it lasted around 200 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. 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. According to the GISP2 ice core from Greenland, the Oldest Dryas occurred between about 15,070 and 14,670 calendar years BP.

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