Younger 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 has been discovered by geological research. This temperature change occurred at the end of what the earth sciences refer to as the Pleistocene epoch and immediately before the current, warmer Holocene epoch. In archaeology, this time frame coincides with the final stages of the Upper Paleolithic in many areas. 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 years BP. The change was relatively sudden, taking place in decades, and it resulted in a decline of 2 to 6 degrees Celsius (3.6 to 10.8 degrees Fahrenheit) 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. 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, like in the Allerød clay pit in Denmark, first recognized and described the Younger Dryas. The Younger Dryas is the youngest and longest of three stadials, which 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. That 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, was preceded by another warmer stage, the Bølling oscillation, that separated it from a third and even older stadial, often 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. In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, and in Great Britain, it has been called the Loch Lomond Stadial. In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). 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 then the refinement of pollen analytical techniques and a steadily-growing number of pollen diagrams, palynologists have concluded that the Younger Dryas was a distinct period of vegetational change in large parts of Europe during which vegetation of a warmer climate was replaced by that of a generally-cold climate, a glacial plant succession that often contained Dryas octopetala. The drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the forest vegetation that had been spreading northward rapidly. The cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna but also led to regional glacial advances in Scandinavia and a lowering of the regional snow line.