Younger Dryas

Younger Dryas
0.0129 – 0.0117 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Younger Dryas event was associated with significant cooling in the Northern Hemisphere, but there was also warming in the Southern Hemisphere. Precipitation had substantially decreased (brown) or increased (green) in many areas across the globe. Altogether, this indicates large changes in thermohaline circulation as the cause[1]
Etymology
Alternate spelling(s)	YD
Synonym(s)	Loch Lomond Stadial Nahanagan Stadial
Usage information
Celestial body	Earth
Definition
Chronological unit	Chron
Stratigraphic unit	Chronozone
Atmospheric and climatic data
Mean atmospheric CO2 content	c. 240 ppm (0.9 times pre-industrial)
Mean surface temperature	c. 10.5 °C (3 °C below pre-industrial)

The Younger Dryas (YD) was a period in Earth's geologic history which occurred circa 12,900 to 11,700 years Before Present (BP), at the end of the Pleistocene epoch.^[2] It is named after the alpine-tundra wildflower Dryas octopetala, because its fossils are abundant in the European (particularly Scandinavian) sediments dating to this timeframe. The two earlier geologic periods where this flower was abundant in Europe are the Oldest Dryas (approx. 18,500-14,000 BP) and Older Dryas (~14,050–13,900 BP), respectively.^[3] Younger Dryas ended when the entire globe had warmed consistently, which marks the beginning of the current Holocene epoch.^[3]

Younger Dryas was preceded by the Bølling–Allerød interstadial (14,670-12,900 BP), when European temperatures were warm enough to support trees in Scandinavia (i.e. Bølling and Allerød sites in Denmark) and Dryas octopetala was rare.^[4] The abundance of Dryas octopetala and the corresponding absence of plants adapted to warmer climates shows that Europe had reverted to glacial conditions during the YD itself, and the local severity of the cooling approached that of the Last Glacial Maximum (27,000-20,000 years BP).^[5] For instance, temperatures in Greenland declined by 4–10 °C (7.2–18.0 °F).^[6] The climatic changes were sudden or "abrupt" in geological terms, taking place over several decades.^[5]

On the other hand, Southern Hemisphere had experienced warming during the YD.^[5][7] The net global change in temperature was a cooling of about 0.6 °C (1.1 °F), primarily due to the ice-albedo feedback in the north.^[5] During the preceding period, the Bølling–Allerød Interstadial, rapid warming in the Northern Hemisphere^[8]: 677 was offset by the equivalent cooling in the Southern Hemipshere. ^[9][3] The "polar seesaw" pattern which defined both the YD and the B-A interstadial is consistent with changes in thermohaline circulation (particularly the Atlantic meridional overturning circulation or AMOC), which greatly affects how much heat is able to go from the Southern Hemisphere to the North. Southern Hemisphere cools and the Northern Hemisphere warms when the AMOC is strong, and the opposite happens when it is weak.^[9]

The scientific consensus is that severe AMOC weakening explains the climatic effects of Younger Dryas.^[10]: 1148 However, there is some debate over what caused the AMOC to become so weak in the first place. The hypothesis historically most supported by scientists was the interruption from an influx of fresh, cold water from North America into the Atlantic.^[11] Such influx has often been attributed to Lake Agassiz, but the lack of conclusive evidence means that other theories have also emerged.^[12] A volcanic trigger has been proposed more recently,^[13] and the presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores^[14] and cave deposits.^[15]