Bølling–Allerød Interstadial
Interglacial period about 14,000 years ago / From Wikipedia, the free encyclopedia
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The Bølling–Allerød Interstadial (Danish: [ˈpøle̝ŋ ˈæləˌʁœðˀ]), also called the Late Glacial Interstadial (LGI), was an interstadial period which occurred from 14,690 to c. 12,890 years Before Present, during the final stages of the Last Glacial Period.[2] It was defined by abrupt warming in the Northern Hemisphere, and a corresponding cooling in the Southern Hemisphere,[3] as well as a period of major ice sheet collapse and corresponding sea level rise known as Meltwater Pulse 1A.[4] This period was named after two sites in Denmark where paleoclimate evidence for it was first found, in the form of vegetation fossils that could have only survived during a comparatively warm period in Northern Europe.[3] It is also referred to as Interstadial 1 or Dansgaard-Oeschger event 1.[3]
This interstadial followed the Oldest Dryas period, which lasted from ~18,000 to 14,700 BP.[5] While Oldest Dryas was still significantly colder than the current era, the Holocene, globally it was a period of warming from the very cold Last Glacial Maximum, caused by a gradual increase in CO2 concentrations. A warming of around 2 °C (36 °F) had occurred during this period, nearly of half of which had taken place during its last couple of centuries.[6] In contrast, the entire Bølling–Allerød Interstadial experienced very little change in global temperature.[6] Instead, the rapid warming was limited to the Northern Hemisphere,[7]: 677 while the Southern Hemipshere had experienced equivalent cooling.[1][6] This "polar seesaw" pattern had occurred due to the strengthening of the Atlantic meridional overturning circulation (and the corresponding weakening of the Southern Ocean overturning circulation). These changes in thermohaline circulation had caused far more heat to be transferred from the Southern Hemisphere to the North.[1]
This interstadial is commonly divided into three stages. The initial, Bølling, stage had experienced the largest hemispheric temperature change, and it is also the stage when Meltwater Pulse 1A had occurred. It was then interrupted by a geologically brief (around two centuries) stage known as the Older Dryas (after Dryas octopetala, the Arctic plant widespread during such cold periods in the Northern Hemisphere), before NH warming returned during the Allerød stage.[3] In the Northern Hemisphere, Allerød is likely to have been less warm yet more wet than the Bølling. There may have also been another brief cold stage during Allerød.[3] The entire interstadial had abruptly with the onset of the Younger Dryas (YD), when the AMOC pattern had reversed, and the Northern Hemisphere temperatures fell back to near-glacial levels within a decade.[8] Global temperatures declined only slightly during YD, and they had steadily climbed alongside the CO2 concentrations once that period had transitioned to Holocene.[6]
For human populations of the Northern Hemisphere, Bølling–Allerød Interstadial had represented the first pronounced warming since the end of the Last Glacial Maximum (LGM). The cold had previously forced them into refuge areas, but the warming of the interstadial enabled them to begin repopulating the Eurasian landmass.[9]