Upper Dryas

The Younger Dryas (c. 12,900 to c. 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum started receding around 20,000 BP. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala, as its leaves are occasionally abundant in the Late Glacial, often minerogenic-rich, like the lake sediments of Scandinavian lakes.

Physical evidence of a sharp decline in temperature over most of the Northern Hemisphere, discovered by geological research, has been the key physical evidence found for the Younger Dryas. This temperature change occurred at the end of what the earth sciences refer to as the epoch and immediately before the current, warmer Holocene epoch. In the social sciences, this time frame coincides with the final stages of the Upper Paleolithic.

The Younger Dryas 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 it resulted in a decline of 2 to 6 degrees Celsius and 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, in turn thought to have been caused by an influx of fresh cold water from North America to 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 Northern Hemisphere, such as southeastern North America, there was a slight warming.

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