The Younger Dryas (YD, Greenland Stadial GS-1)[2] was a period in Earth's geologic history that occurred circa 12,900 to 11,700 years Before Present (BP).

The hypothesis historically most supported by scientists was an interruption from an influx of fresh, cold water from North America's Lake Agassiz into the Atlantic Ocean.

[14] While there is evidence of meltwater travelling via the Mackenzie River,[15] this hypothesis may not be consistent with the lack of sea level rise during this period,[16] so other theories have also emerged.

[25] As with the other geologic periods, paleoclimate during the Younger Dryas is reconstructed through proxy data such as traces of pollen, ice cores and layers of marine and lake sediments.

[46] Evidence from Lake Suigetsu cores in Japan and the Puerto Princesa cave complex in the Philippines shows that the onset of the Younger Dryas in East Asia was delayed by several hundred years relative to the North Atlantic.

Younger Dryas cooling was often accompanied by glacier advance and lowering of the regional snow line, with evidence found in areas such as Scandinavia,[50] the Swiss Alps[4] and the Dinaric Alps in the Balkans,[61] northern ranges of North America's Rocky Mountains,[62][63][64] Two Creeks Buried Forest in Wisconsin and western parts of the New York State,[65] and in the Pacific Northwest,[66] including the Cascade Range.

[67] The entire Laurentide ice sheet had advanced between west Lake Superior and southeast Quebec, leaving behind a layer of rock debris (moraine) dated to this period.

[72] The Jabllanica mountain range in the Balkans also experienced ice loss and glacial retreat: this was likely caused by the drop in annual precipitation, which would have otherwise frozen and helped to maintain the glaciers.

For instance, in East Africa, the sediments of Lake Tanganyika were mixed less strongly during this period, indicating weaker wind systems in this area.

Thus, evidence from the pollen record shows that some areas have become very arid, including Scotland,[79] the North American Midwest, [80] Anatolia and southern China.

[4] Other areas became wetter including northern China[83] (possibly excepting the Shanxi region)[84] The Younger Dryas was initially discovered around the start of the 20th century, through paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, particularly the Allerød clay pit in Denmark.

[85][51][86][87] The analysis of fossilized pollen had consistently shown how Dryas octopetala, a plant which only thrives in glacial conditions, began to dominate where forests were able to grow during the preceding B-A Interstadial.

[88] For instance, in what is now New England,[89][90][91] cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the boreal forests shifted north.

[95] On the Olympic Peninsula, a mid-elevation site recorded a decrease in fire, but forest persisted and erosion increased during the Younger Dryas, which suggests cool and wet conditions.

[98] Pollen record from the Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range.

[103][102][104][105] That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation[103][106] as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons.

[108][109] The cold and dry Younger Dryas arguably lowered the carrying capacity of the area and forced the sedentary early Natufian population into a more mobile subsistence pattern.

While relative consensus exists regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated.

[4][12]: 1148  This is consistent with climate model simulations,[1] as well as a range of proxy evidence, such as the decreased ventilation (exposure to oxygen from the surface) of the lowest layers of North Atlantic water.

[113] Further, the otherwise anomalous warming of the southeastern United States matches the hypothesis that as the AMOC weakened and transported less heat from the Caribbean towards Europe through the North Atlantic Gyre, more of it would stay trapped in the coastal waters.

[114] It was originally hypothesized that the massive outburst from paleohistorical Lake Agassiz had flooded the North Atlantic via the Saint Lawrence Seaway, but little geological evidence had been found.

[119] Climate models also indicate that a single freshwater outburst, no matter how large, would not have been able to weaken the AMOC for over 1,000 years, as required by the Younger Dryas timeline, unless other factors were also involved.

[120] Some modelling explains this by showing that the melting of Laurentide Ice Sheet led to greater rainfall over the Atlantic Ocean, freshening it and so helping to weaken the AMOC.

[120] Other modelling shows that sea ice in the Arctic Ocean could have been tens of meters thick by the onset of the Younger Dryas, so that it would have been able to shed icebergs into the North Atlantic, which would have been able to weaken the circulation consistently.

[122] Cooling from a high latitude volcanic eruption could have accelerated North Atlantic sea ice growth, finally tipping the AMOC sufficiently to cause the Younger Dryas.

[129] For instance, mineral inclusions from YD-period sediments in Hall's Cave, Texas, have been interpreted by YDIH proponents as extraterrestrial in origin, but a paper published in 2020 argues that they are more likely to be volcanic.

[133][134] Events similar to the Younger Dryas appear to have occurred during the other terminations - a term used to describe a comparatively rapid transition from cold glacial conditions to warm interglacials.