Younger Dryas
The Younger Dryas was a period in Earth's geologic history that occurred circa 12,900 to 11,700 years Before Present. It is primarily known for the sudden or "abrupt" cooling in the Northern Hemisphere, when the North Atlantic Ocean cooled and annual air temperatures decreased by ~ over North America, in Europe and up to in Greenland, in a few decades. Cooling in Greenland was particularly rapid, taking place over just 3 years or less. At the same time, the Southern Hemisphere experienced warming. This period ended as rapidly as it began, with dramatic warming over ~50 years, the transition from the glacial Pleistocene epoch into the current Holocene.
The Younger Dryas onset was not fully synchronized; in the tropics, the cooling was spread out over several centuries, and the same was true of the early-Holocene warming. Even in the Northern Hemisphere, temperature change was highly seasonal, with much colder winters, cooler springs, yet no change or even slight warming during the summer. Substantial changes in precipitation also took place, with cooler areas experiencing substantially lower rainfall, while warmer areas received more of it. In the Northern Hemisphere, the length of the growing season declined. Land ice cover experienced little net change, but sea ice extent had increased, contributing to ice–albedo feedback. This increase in albedo was the main reason for net global cooling of.
During the preceding period, the Bølling–Allerød Interstadial, rapid warming in the Northern Hemisphere was offset by the equivalent cooling in the Southern Hemisphere. This "polar seesaw" pattern is consistent with changes in thermohaline circulation, which greatly affects how much heat is able to go from the Southern Hemisphere to the North. The Southern Hemisphere cools and the Northern Hemisphere warms when the AMOC is strong, and the opposite happens when it is weak. The scientific consensus is that severe AMOC weakening explains the climatic effects of the Younger Dryas. It also explains why the Holocene warming had proceeded so rapidly once the AMOC change was no longer counteracting the increase in carbon dioxide levels.
AMOC weakening causing polar seesaw effects is also consistent with the accepted explanation for Dansgaard–Oeschger events, with YD likely to have been the last and the strongest of these events. 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 an interruption from an influx of fresh, cold water from North America's Lake Agassiz into the Atlantic Ocean. While there is evidence of meltwater travelling via the Mackenzie River, this hypothesis may not be consistent with the lack of sea level rise during this period, so other theories have also emerged. Another proposed explanation is an extraterrestrial impact, but this is rejected by most experts. A volcanic eruption as an initial trigger for cooling and sea ice growth has been proposed more recently, and the presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores and cave deposits.
Etymology
The Younger Dryas is named after the alpine–tundra wildflower Dryas octopetala, because its fossils are abundant in the European sediments dating to this timeframe. The two earlier geologic time intervals where this flower was abundant in Europe are the Oldest Dryas and Older Dryas, respectively. On the contrary, Dryas octopetala was rare during the Bølling–Allerød Interstadial. Instead, European temperatures were warm enough to support trees in Scandinavia, as seen at the Bølling and Allerød sites in Denmark.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. The preceding Allerød warm period is subdivided into three events: Greenland Interstadial-1c to 1a.
Climate
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. Collectively, this evidence shows that significant cooling across the Northern Hemisphere began around 12,870 ± 30 years BP. It was particularly severe in Greenland, where temperatures declined by, in an abrupt fashion. Temperatures at the Greenland summit were up to colder than at the start of the 21st century.Strong cooling of around had also taken place in Europe. Icefields and glaciers formed in upland areas of Great Britain, while many lowland areas developed permafrost, implying a cooling of and a mean annual temperature no higher than. North America also became colder, particularly in the eastern and central areas. While the Pacific Northwest region cooled by, cooling in western North America was generally less intense. While the Orca Basin in the Gulf of Mexico still experienced a drop in sea surface temperature of 2.4 ± 0.6°C, land areas closer to it, such as Texas, the Grand Canyon area and New Mexico, ultimately did not cool as much as the continental interior. The Southeastern United States became warmer and wetter than before. There was warming in and around the Caribbean Sea, and in West Africa.
It was once believed that the Younger Dryas cooling started at around the same time across the Northern Hemisphere. However, varve analysis carried out in 2015 suggested that the cooling proceeded in two stages: first along latitude 56–54°N, 12,900–13,100 years ago, and then further north, 12,600–12,750 years ago. 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. Further, the cooling was uniform throughout the year, but had a distinct seasonal pattern. In most places in the Northern Hemisphere, winters became much colder than before, but springs cooled by less, while there was either no temperature change or even slight warming during the summer. An exception appears to have taken place in what is now Maine, where winter temperatures remained stable, yet summer temperatures decreased by up to.
While the Northern Hemisphere cooled, considerable warming occurred in the Southern Hemisphere. Sea surface temperatures were warmer by, and Antarctica, South America and New Zealand all experienced warming. The net temperature change was a relatively modest cooling of. Temperature changes of the Younger Dryas lasted 1,150–1,300 years. According to the International Commission on Stratigraphy, the Younger Dryas ended around 11,700 years ago, although some research places it closer to 11,550 years ago.
The end of Younger Dryas was also abrupt: in previously cooled areas, warming to previous levels took place over 50–60 years. The tropics experienced more gradual temperature recovery over several centuries; the exception was in tropical Atlantic areas such as Costa Rica, where temperature change was similar to Greenland's. The Holocene warming then proceeded across the globe, following an increase in carbon dioxide levels during the YD period.
Ice cover
Younger Dryas cooling was often accompanied by glacier advance and lowering of the regional snow line, with evidence found in areas such as Scandinavia, the Swiss Alps and the Dinaric Alps in the Balkans, northern ranges of North America's Rocky Mountains, Two Creeks Buried Forest in Wisconsin and western parts of the New York State, and in the Pacific Northwest, including the Cascade Range. The entire Laurentide ice sheet had advanced between west Lake Superior and southeast Quebec, leaving behind a layer of rock debris dated to this period. Southeastern Alaska appears to have escaped glaciation; speleothem calcite deposition continued in the region despite being retarded, indicating the absence of permafrost and glaciation.On the other hand, the warming of the Southern Hemisphere led to ice loss in Antarctica, South America and New Zealand. Moreover, while Greenland as a whole had cooled, glaciers had only grown in the north of the island, and they had retreated from the rest of Greenland's coasts. This was likely driven by the strengthened Irminger Current. 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. Unlike now, the glaciers were still present in northern Scotland, but they had thinned during the Younger Dryas.
The amount of water contained within glaciers directly influences global sea levels - sea level rise occurs if the glaciers retreat, and it drops if glaciers grow. Altogether, there appears to have been little change in sea level throughout the Younger Dryas. This is in contrast to rapid increases before and after, such as the Meltwater Pulse 1A. On the coasts, glacier advance and retreat also affects relative sea level. Western Norway experienced a relative sea level rise of as the Scandinavian ice sheet advanced. Notably, ice sheet advance in this area appears to have begun about 600 years before the global onset of the Younger Dryas. Underwater, the deposits of methane clathrate - methane frozen into ice - remained stable throughout the Younger Dryas, including during the rapid warming as it ended.
Weather systems
As the Northern Hemisphere cooled and the Southern Hemisphere warmed, the thermal equator would have shifted to the south. Because trade winds from either hemisphere cancel each other out above the thermal equator in a calm, heavily clouded area known as the Intertropical Convergence Zone, a change in its position affects wind patterns elsewhere. For instance, in East Africa, the sediments of Lake Tanganyika were mixed less strongly during this period, indicating weaker wind systems in this area. Shifts in atmospheric patterns are believed to be the main reason why Northern Hemisphere summers generally did not cool during the Younger Dryas.Since winds carry moisture in the form of clouds, these changes also affect precipitation. Thus, evidence from the pollen record shows that some areas have become very arid, including Scotland, the North American Midwest, Anatolia and southern China. As North Africa, including the Sahara Desert, became drier, the amount of dust blown by wind had also increased. Other areas became wetter including northern China