8.2 Kiloyear Event

The 8.2 kiloyear event is the term that climatologists have adopted for a sudden decrease in global temperatures that occurred approximately 8,200 years before the present, or c. 6,200 BCE, and which lasted for the next two to four centuries. Milder than the Younger Dryas cold spell that preceded it, but more severe than the Little Ice Age that would follow, the 8.2 kiloyear cooling was a significant exception to general trends of the Holocene climatic optimum. During the event, atmospheric methane concentration decreased by 80 ppb or 15% emission reduction by cooling and drying at a hemispheric scale.

A rapid cooling around 6200 BCE was first identified by Swiss botanist Heinrich Zoller in 1960, who named the event Misox oscillation (for the Val Mesolcina). It is also known as Finse event in Norway. Bond et al. argued that the origin of the 8.2 kiloyear event is linked to a 1,500-year climate cycle; it correlates with Bond event 5.

The strongest evidence for the event comes from the North Atlantic region; the disruption in climate shows clearly in Greenland ice cores and in sedimentary and other records of the temporal and tropical North Atlantic. It is less evident in ice cores from Antarctica and in South American indices. The effects of the cold snap were global, however, most notably in changes in sea level during the relevant era.

The 8.2 Ka cooling event may have been caused by a large meltwater pulse from the final collapse of the Laurentide ice sheet of northeastern North America—most likely when the glacial lakes Ojibway and Agassiz suddenly drained into the North Atlantic Ocean. (The same type of action produced the Missoula floods that created the Channeled scablands of the Columbia River basin.) The meltwater pulse may have affected the North Atlantic thermohaline circulation, reducing northward heat transport in the Atlantic and causing significant circum-North Atlantic cooling. Estimates of the cooling vary and depend somewhat on the interpretation of the proxy data, but drops of around 1 to 5 °C (1 to 11 °F) have been reported. In Greenland, the event started at 8175 Before Present, and the cooling was 3.3 °C (decadal average) in less than ~20 years, and the coldest period lasted for about 60 years, and the total duration was about 150 years. Further afield, some tropical records report a 3 °C (5 °F) cooling from cores drilled into an ancient coral reef in Indonesia. The event also caused a global CO₂ decline of ~ 25 ppm over ~ 300 years. However, the dating and interpretation of this and other tropical sites are more ambiguous than the North Atlantic sites.

Drier conditions were notable in North Africa, while East Africa suffered five centuries of general drought. In West Asia and especially Mesopotamia, the 8.2ky event was a three-hundred year aridification and cooling episode, which provided the natural force for Mesopotamian irrigation agriculture and surplus production that were essential for the earliest class-formation and urban life. However multi-centennial changes around the same period are difficult to link specifically to the approximately 100-year abrupt event as recorded most clearly in the Greenland ice cores.

The initial meltwater pulse has caused between 0.5 and 4 m (1 ft 8 in and 13 ft 1 in) of sea-level rise. Based on estimates of lake volume and decaying ice cap size, values of 0.4–1.2 m (1 ft 4 in–3 ft 10 in) circulate. Based on sea-level data from below modern deltas 2–4 m (6 ft 7 in–13 ft 1 in) of near-instantaneous rise is estimated, recorded superimposed on background 'normal' post-glacial sea-level rise. Meltwater pulse sea level rise was experienced fully at great distance from the release area. Gravity and rebound effects associated to the shifting of watermasses mean that the sea-level fingerprint is smaller in areas closer to the Hudson Bay. The Mississippi delta records ~20%, NW Europe records ~70% and Asia records ~105% of the global averaged amount. The cooling of the 8200 event was a temporary feature; the sea-level rise of the meltwater pulse was permanent.

In 2003, the Office of Net Assessment at the United States Department of Defense was commissioned to produce a study on the likely and potential effects of a modern climate change. The study, conducted under ONA head Andrew Marshall, modelled its prospective climate change on the 8.2 kiloyear event, precisely because it was the middle alternative between the Younger Dryas and the Little Ice Age.