Just to sum up...

I came across this table today in Alley et al. (1997), which compares the 8.2 kiloyear event and Yonger Dryas with present climate, showing the percentage deviations of various environmental parameters. Thought it would help you lot to better imagine the scope of those two very dramatic events.

List of references:

Alley, R., P. Mayewski, T. Sowers, M. Stuvier, K. Taylor and U. Clark (1997) 'Holocene Climatic Instability: A Prominent Widespread Event 8200 Years Ago', Geology, 25, 6, 483-486.

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), http://www.ncdc.noaa.gov/paleo/abrupt/data5.html.

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 (http://www.ncdc.noaa.gov/paleo/abrupt/data5.html), 29/10/2012.

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.

The Holocene timeline

Congratulations, we have finally nailed all the theoretical background behind the "mini D-O cycles"! Probably not exactly the sort of knowledge that will help you in a pub quiz, but I would like to believe that at least some of you got intrigued by the great forces that operate on our planet and beyond.

From now on I shall leave all the general concepts behind and will rather focus on the individual events. For that purpose I have put together a small table of all the Bond oscillations that took place in the Holocene.

No.	Time (BP)	Corresponding Events
0	~0.5 ka	The Little Ice Age
1	~1.4 ka	The Migration Period
2	~2.8 ka	Drought in the Eastern Mediterranean, possibly triggering the collapse of Late Bronze Age cultures
3	~4.2 ka	4.2 kiloyear event; collapse of the Akkadian Empire and the end of the Egyptian Old Kingdom
4	~5.9 ka	5.9. kiloyear event
5	~8.2 ka	8.2. kiloyear event
6	~9.4 ka	Erdalen event in Norway; a cold event in China
7	~10.3 ka
8	~11.1 ka	Ttansition from the Younger Dryas to the boreal; drought in the Levant, possibly leading to the development of agriculture

It is interesting to note that almost all of the abrupt climate change events correspond with some major cultural shift, be it a collapse of a civilisation or a period of intense human migration. The influence of rapid climate change on human populations has not been fully established yet and there is a lot of exciting research going on in that area. In a few posts to follow the discussion will actually be centred around real people and not ice cores and sediment samples for once.