ENVIRONMENT

The 8.2 ka Cooling Event

~8,200 years ago

Neolithic · Neolithic · Holocene · Northgrippian

The 8.2 ka event is a short cold interval in the first third of the Holocene, the warm period that follows the last ice age. Four high-resolution records from the Greenland ice cap show it as a 160-year stretch of below-average isotope values, holding a sharper core of 69 years, with snow accumulating more slowly. How cold it was depends on where the reading is taken: 1 to 3 °C over large parts of the Northern Hemisphere, 4 to 8 °C in central Greenland, 1.5 to 3 °C at sites around the north-eastern North Atlantic. Two causes compete. The older account is the catastrophic draining of glacial lakes Agassiz and Ojibway, dammed by a remnant of the Laurentide ice sheet, which sent at least a hundred thousand cubic kilometres of fresh water into the Labrador Sea and slowed the ocean circulation that carries heat northwards. The other is the collapse of the ice saddle linking the domes over Hudson Bay, whose meltwater would have run for a century instead of a few years.

Why it matters

A cooling this large interrupts a period otherwise treated as calm, and that is its use: it serves as a reference case for testing how climate models answer a large input of fresh water to the North Atlantic. One published simulation had to swap the brief lake outburst for a century-long pulse of meltwater before the recorded duration and amplitude came out right. The interval also carries a formal boundary. When the Holocene was cut into three stages in 2018, the middle one, the Northgrippian, was anchored in the Greenland NGRIP1 ice core at 8,236 years before 2000, so a whole division of geological time begins inside this cold spell.

Dating & uncertainty

The event is dated between 8,400 and 8,000 calendar years ago in marine and ice records, and the interval of below-average values lasts about 160 years, so a single figure of ~8,200 stands for a span and not for a year. The lake outburst is placed at about 8,470 calendar years ago, earlier than the cooling it is held to have caused, and that gap is the heart of the argument: if the drainage came too soon and ran too briefly, the cause has to be sought in a slower source of meltwater, such as the collapsing ice saddle over Hudson Bay. Greenland ice cores give only weak evidence for effects outside the North Atlantic region, so consequences claimed further afield do not rest on them.

Sources

1. Barber et al. (1999), Nature · Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakesdoi:10.1038/22504
2. Thomas et al. (2007), Quaternary Science Reviews · The 8.2ka event from Greenland ice coresdoi:10.1016/j.quascirev.2006.07.017
3. Matero et al. (2017), Earth and Planetary Science Letters · The 8.2 ka cooling event caused by Laurentide ice saddle collapsedoi:10.1016/j.epsl.2017.06.011
4. Walker et al. (2019), Journal of Quaternary Science · Subdividing the Holocene Series/Epoch: formalization of stages/ages and subseries/subepochs, and designation of GSSPs and auxiliary stratotypesdoi:10.1002/jqs.3097

Our species’ history as one day

23:22:3037 min 29 s before midnight

if the 315,000 years of Homo sapiens were compressed into a single day