In climatology, the 8.2 kiloyear event was a sudden decrease in global temperatures that happened about 8,200 years ago. This event lasted between two and four centuries. It marks the start of the Northgrippian Age during the Holocene epoch. Although this cooling period was less severe than the earlier Younger Dryas event, which occurred before the Holocene began, it was still important. During the 8.2 kiloyear event, methane levels in the atmosphere dropped by 80 parts per billion, a 15% decrease. This suggests a widespread cooling and drying trend across the Northern Hemisphere.
Identification
A sudden cooling event around 8,200 years ago was first discovered by Swiss botanist Heinrich Zoller in 1960. He named the event the Misox oscillation, after the Val Mesolcina region in Switzerland. This event is also called the Finse event in Norway. Scientists have found evidence of this cooling in speleothem records across Eurasia, the Mediterranean, South America, and southern Africa. These findings show the event happened at the same time worldwide. The clearest evidence comes from the North Atlantic region, where signs of climate change are visible in Greenland ice cores, sedimentary records, and other North Atlantic records. Fewer signs of the event are found in Antarctic ice cores and South American records. The sudden temperature drop had global effects, most clearly seen in changes to sea levels.
Cooling event
The event might have been caused by a large release of meltwater, likely from the final collapse of the Laurentide Ice Sheet in northeastern North America. This happened when glacial lakes Ojibway and Agassiz suddenly drained into the North Atlantic Ocean. A similar process created the Missoula floods, which shaped the Channeled Scablands in the Columbia River basin. The meltwater may have disrupted the North Atlantic thermohaline circulation, reducing heat transport in the Atlantic and causing significant cooling in the region. The Atlantic meridional overturning circulation (AMOC) weakened by about half to two-thirds. Cooling estimates vary, but some areas experienced drops of 1 to 5 degrees Celsius (1.8 to 9.0 degrees Fahrenheit). In Greenland, the event began 8,175 years ago, with cooling 3.3 degrees Celsius below the average over less than 20 years. The coldest period lasted about 60 years, and the entire event lasted roughly 150 years. However, the meltwater explanation is considered uncertain because of unclear details about when it started and where it occurred.
Researchers suggest the meltwater release likely occurred during a longer period of cooler weather that lasted up to 600 years and was one of several factors contributing to the event.
In tropical regions, some records show a 3-degree Celsius (5.4-degree Fahrenheit) cooling, based on studies of ancient coral reefs in Indonesia. The event also caused a global drop in carbon dioxide levels of about 25 parts per million over 300 years. However, data from other tropical areas is less clear than information from the North Atlantic. Climate models show that both the amount of meltwater and its path into the ocean were important in affecting the North Atlantic thermohaline circulation.
The initial meltwater release caused sea levels to rise by 0.5 to 4 meters (1 foot 8 inches to 13 feet 1 inch). Estimates based on lake volume and ice melt suggest a rise of 0.4 to 1.2 meters (1 foot 4 inches to 3 feet 11 inches). Data from the Mississippi Delta indicates the drainage of Lake Agassiz–Ojibway ended between 8,310 and 8,180 years ago, with a sea-level rise of 0.8 to 2.2 meters. Data from the Rhine–Meuse Delta shows a sudden rise of 2 to 4 meters (6 feet 7 inches to 13 feet 1 inch) between 8,540 and 8,200 years ago, in addition to normal post-glacial sea-level rise. Sea-level rise from the meltwater pulse was most noticeable far from the release area. Gravity and land rebound effects reduced the rise near Hudson Bay. The Mississippi Delta recorded about 20% of the global average rise, Northwestern Europe 70%, and Asia 105%. The cooling from the 8.2-kiloyear event was temporary, but the sea-level rise from the meltwater pulse was permanent.
In 2003, the Office of Net Assessment (ONA) at the United States Department of Defense studied the potential effects of modern climate change. The study, led by ONA head Andrew Marshall, compared possible climate changes to the 8.2-kiloyear event because it represented a middle case between the colder Younger Dryas and the milder Little Ice Age.
Effects
The 8.2 kiloyear event caused drier weather in many parts of the world. In the Northern Hemisphere, monsoon rains decreased by 12.4% for each degree Celsius of temperature change, while in the Southern Hemisphere, they increased by 4.2% per degree. The event also led to higher ocean salt levels and more dust being carried from land to sea.
Drier conditions were especially noticeable in North Africa, where the area near the Charef River in eastern Morocco experienced extreme dryness around 8,200 years before present. East Africa faced drought for about 500 years. In West Asia, particularly Mesopotamia, the event caused a 300-year period of dry and cold weather. This may have encouraged the development of irrigation agriculture and surplus food production, which were important for the formation of social classes and cities. However, changes over long periods are hard to directly link to the sudden 100-year climate event recorded in Greenland ice cores.
In Tell Sabi Abyad, Syria, major cultural changes occurred around 6200 BC, but the settlement was not abandoned at that time. In northwestern Madagascar, the event was linked to changes in oxygen levels and increased calcite deposits, showing wetter conditions caused by the movement of the ITCZ southward. Southern Hemisphere summer monsoons likely became stronger, increasing rainfall. Wet conditions happened in two stages, with a smaller event 20 years before the main 8.2 kiloyear event.
Sediment records from the Fram Strait show a brief cooling during the 8.2 kiloyear event, which occurred during a longer warm period. In western Scotland, the event caused a sharp drop in Mesolithic population. Near Lake Mondsee in Austria, deciduous forests decreased, and boreal plants became more common. In the Iberian Peninsula, the event led to drier summers, more fires, and the spread of fire-resistant evergreen oaks. In northeastern Greece, winter temperatures dropped by over 4°C, likely due to stronger influence from the Siberian High.
Lacustrine sediment records show Western Siberia became wetter during the event, while southeastern Siberia had less rainfall. Carbonates from the Riwasa Palaeolake indicate the Indian Summer Monsoon weakened during the event. Stalagmites from Kotumsar Cave and Socotra and Oman confirm this weakening.
A sediment core from Lop Nur in the Tarim Basin shows a major dry period during the event. The Korean Peninsula’s forests suffered greatly, with reduced pollen production. It took about 400 years for forests to recover.
Evidence from the Gulf of Thailand shows sea levels dropped during the event, and runoff increased. Rainfall in the Indo-Pacific Warm Pool decreased for about 200 years, but this lagged behind North Atlantic cooling by 100 years.
In Greenland, ice cores show a sharp drop in oxygen levels. Elk Lake in Minnesota shifted from being layered to mixed, and the area changed from boreal forest to open prairie. Waters near Cape Hatteras became saltier. Bat guano in the Grand Canyon showed lower carbon and oxygen levels. Southwestern Mexico became drier, with stalagmite growth interrupted. In the Gulf of Mexico, bay-head deltas moved inland as sea levels rose. Mustang Island was breached, losing its role as a salinity barrier. Gulf of Mexico seawater oxygen levels dropped by 0.8%.
In the Western Cordillera of Colombia, the event caused a major dry period. Sediment records from Juréia Paleolagoon show the South American Summer Monsoon intensified during the event.