Welcome back to the ice age

The first Holocene Bond event or the last great stab of cold from the Pleistocene? You might guess what I shall be talking about today: the one and only... Younger Dryas!

Once upon a time 13,000 years ago just as the Earth was recovering from the last ice age and things were looking quite promising, it was again almost at once thrown back into the glacial conditions for more than a thousand years. The cold period received its name from a pretty little flower Dryas Octopetala, which became very common in Europe during that time. Being quite a surprise, the Younger Dryas ended even more abruptly 10,700 years ago as the temperatures rose by 7 degrees and precipitation increased by 50% to be replaced with the period known as Pre-Boreal (Dansgaard et al., 1989).

The Yonger Dryas is clearly observable in palaeoclimatic records from many parts of the world. The Cariaco Basin record north of Venezuela shows an approximate 3°C decrease in temperatures, in many parts of the Northern Hemisphere tropics conditions also became drier (NCDC, 2008). Perhaps the most extreme change can be seen in Greenland as reflected in the GISP2 ice core:

Source: Greenland Ice Sheet Project 2, (http://www.gisp2.sr.unh.edu/)

As I have already mentioned, the climatologists are still arguing about the causes of the Bond Cycles, and the origin of the Younger Dryas is likely to remain an enigma for a while aswell. There is an interesting notion by Carlson (2008) that previous glacial terminations probably did not have Younger Dryas-like events, suggesting that its trigger may have had a random component. Apart from the already outlined proposals of solar or volcanic forcing, there are a couple of more interesting ones:

Lake Agassiz flood

Most climate scientists support the idea of a large meltwater lake at the edge of the Laurentide Ice Sheet bursting through an ice dam and discharging thousands of cubic kilometres of fresh water each year into surrounding oceans, and at times suppressing the Atlantic meridional overturning circulation and triggering the abrupt climate change event (Broecker, 2006). Geological evidence of this event has been far lacking due to the inability to identify the path taken by the flood. However, a recent study by Murton et al. (2010) claims to have identified the missing flood path, running through the Mackenzie River system into the Arctic Ocean.

Clovis Comet Hypothesis

Another team argues for the extraterrestrial cause. Firestone et al. (2007) suggested an evidence for object(s) from outer space that contributed to the Younger Drays rapid cooling around 12,900 years ago. However, the proposed extraterrestrial explanation has been largerly questioned as subsequent research has shown there to be misinterpretation of data and lack of confirmatory evidence. Pinter et al. (2011) called the hypothesis "self-contradictory and defying the laws of physics" stating that it "provides a cautionary tale for researchers, the scientific community, the press, and the broader public".

There is a continuous attempt to link the Younger Dryas event to rapid human behavioural shifts at the end of the Clovis period. One of the first studies drawing a parallel between the abrupt climate change events and the adoption of the agriculture was Childe’s (1951) oasis propinquity theory which proposed that a dessication trend forced humans into close contact with plants and animals surrounding permanent water sources. Since then there have been plenty of climatic explanations for agricultural origins and Munro (2003), summarising previous research, argued that climatic instability introduced by the Younger Dryas reduced the distribution of the cereal crops, lowered carrying capacity, and therefore around 14.5-12.8 thousand years ago uprooted the sedentary way of life in the Early Natufian cultural phase. Human hunters were forced to return to a  more mobile settlement strategy in order to optimise their use of increasingly dispersed resources. Further stress induced by the cold and dry Younger Dryas climate eventually encouraged the Natufians to begin cultivating wild cereals and set humans on the path to the agriculture, the process also known as the Neolithic Revolution (Munro, 2003).

List of references:

Broecker, W. (2006) 'Was the Younger Dryas Triggered by a Flood?', Science, 312, 5777, 1146-1148.
Carlson, A. (2008) 'Why Was There Not a Younger Dryas-like Event during the Penultimate Glaciation', Quaternary Science Reviews, 27, 9-10, 882-887.
Dansgaard, W., J. White and S. Johnsen (1989) 'The Abrupt Termination of the Younger Dryas Event', Nature, 339, 532-534.
Firestone, R., A. West, J. Kennet, L. Becker, T. Bunch, Z. Revay, P. Schultz, T. Belgya, D. Kennett, A. Goodyear and R. Harris (2007) 'Evidence for an Extraterrestrial Impact 12,900 Years Ago that Contributed to the Megafaunal Extinctions and the Younger Dryas Cooling', Proceeding of the National Academy of Sciences of the United States of America, 104, 41, 16016-16021.

Munro, N. (2003) 'Small Game, the Younger Dryas, and the Transition to the Agriculture in the Sounthern Levant', Mitteilungen der Gesellschaft für Urgeschichte, 12, 47-72.
Murton, J, M. Bateman, S. Dallimore, J. Teller and Z. Yang (2010) 'Identification of Younger Dryas Outburst Flood Path from Lake Agassiz to the Arctic Ocean', Nature, 464, 740-743.
National Climatic Data Center (2008) 'A Paleo Perspective on Abrupt Climate Change: The Younger Dryas', (WWW), National Oceanic and Atmospheric Adimistration (http://www.ncdc.noaa.gov/paleo/abrupt/data4.html), 21/12/2012.
Pinter, N., A. Scott, T. Daulton, A. Podoll, C. Koeberl, R. Anderson and S. Ishman (2011) 'The Younger Dryas Impact Hypothesis: A Requiem', Earth Science Reviews, 106, 3-4, 247-264.