Saturday, 29 December 2012

8.2 kiloyear event

As we found out recently, it was not until the end of Younger Dryas when the Earth's climate system started to behave more like it does today. The large continental ice sheets shrank, temperatures ameliorated, sea level rose and monsoons grew in strength. However, around 8,200 years ago yet another surprise occured...

Suddenly the temperatures dropped by 4-8°C in central Greenland and 1.5-3°C at marine and terrestrial sites around the northeastern sites around the northeastern North Atlantic Ocean, all happening in just a couple of decades (Barber et al. 1999). Atmospheric methane concentration also dropped by ~80+/-25 ppb over ~40 years (Kobashi et al., 2007). We are going to stick with the assumption from Bond et al. (1997) that this rapid clamitic shift was part of a 1,500-year Bond cycle and correlates with Bond event 5.

Not as impressive as the Yonger Dryas, still more severe than anything to come afterwards, the 8.2 kiloyear event lasted around 150 years with the coldest period occupying roughly 60 years, followed by a subsequent warming in several steps over the next 70 years (Kobashi et al., 2007).

Now let's have a look at what happened around the globe during this oscillation. Unfortunately, the picture is not equally clear for all the parts of the world...

Source: NCDC (2008),

While it is relatively easy to infer about the way European climate was affected, palaeorecords from the rest of the world are more sparse. Some evidence exists from the natural proxy archives that parts of the tropics became drier, such as the 10-15% drop in atmospheric methane recorded in air bubbles of Greenland ice cores (Alley et al., 1997). At present there are too few high-resolution records from the Southern Hemisphere to determine whether climate changed there. Temperatures might have become warmer in Antarctica, but this conclusion is still controversial (NCDC, 2008)

So what can we blame for this dramatic decrease in temperatures? The scientist's opinions are almost uniform here - freshwater forcing.

A paper by Baber et al. (1999) presents the evidence of  a large meltwater pulse from the final collapse of the Laurentide ice sheet and proposes it as a potential trigger of the 8.2 kyr event. This was most likely to have happened when the huge glacial lakes Agassiz and Ojibway, formed by melting waters from the retreating ice sheet, eventually came into contact with Hudson Bay. Due to the difference in elevation once the ice wall separating the lakes from the bay broke, immense volumes of water drained into the ocean, causing global sea level to permanently rise as much as 1.2 metres.

Source: Lewis et al. (2012)

This sudden increase in freshwater flux may have modified North Atlantic thermohaline circulation, reducing northward heat transport in the Atlantic and causing significant circum-North Atlantic cooling.

Since the 8.2 kiloyear event our planet has not experienced anything quite as drastic. Nevertheless even more minor fluctuations proved to be enough to bring some civilisations to collapse, while letting others thrive. In my subsequent posts I shall focus on how more recent Bond events modified the contours of human history in the mid- to late Holocene.

List of references:

Alley, R., P. Mayewski, T. Sowers, M. Stuvier, K. Taylor and P. Clark (1997) 'Holocene Climatic Instability: A Prominent Widespread Event 8200 yr ago', Geology, 25, 6, 483-486.
Barber, D., A. Dyke, C. Hillaire-Marcel, A. Jennings, J. Andrews, M. Kerwin, G. Bilodeau, R. McNeely, J. Southon, M. Morehead and J.-M. Gagnon (1999) 'Forcing of the Cold Event of 8,200 Years Ago by Catastrophic Drainage of Laurentide Lake', Nature, 400, 344-348.
Bond, G., W. Showers, M. Cheseby, R. Lotti, P. Almasi, P. deMenocal, P. Priore, H. Cullen, I. Hajdas and G. Bonani (1997) 'A Pervasive Millennial-Scale Cycle in the North Atlantic Holocene and Glacial Climates', Science, 278, 1257-1266.
Kobashi, T., J. Severinghaus, E. Brook, J.-M. Barnola and A. Grachev (2007) 'Precise timing and Characterisation of Abrupt Climate Change 8,200 Years Ago from Air Trapped in polar Ice', Quaternary Science Reviews, 26, 9-10, 1212-1222.
Lewis, C., A. Miller, E. Levac, D. Piper and G. Sonnichsen (2012) 'Lake Agassiz Outburst Age Routing by Labrador Current and the 8.2 cal ka Cold Event', Quaternary International, 260, 83-97.
National Climatic Data Center (2008) 'A Palaeo Perspective on Abrupt Climate Change: Heinrich and Dansgaard-Oeschger Events' (WWW), National Oceanic and Atmospheric Adimistration (, 29/10/2012.

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