4.2 Stopping the ocean conveyor belt

One way to cool the planet, as occurred in the Younger Dryas or the 8.2 ka event, is to stop the ocean conveyor carrying the heat northwards. It is believed that this indeed happened as a result of large quantities of melt water from the North American continental ice sheets flooding into the North Atlantic and changing the surface density of the ocean. Once the conveyor was stopped, the climate was plunged into a cold period.

Although similar events seem to have occurred further back in time, the Younger Dryas and the 8.2 ka events may have been particularly significant for human civilisation. The earliest dated farming settlements are in the Mediterranean about 13 000 years ago – around the time of the Younger Dryas. It is interesting to compare the spread of these settlements across the Middle East and Europe (Figure 23) with the temperature data of Figure 19.

During the first 5000 years of human agriculture, from 13 ka BP to 8.4 ka BP, farming settlements are concentrated on the shores of the Mediterranean and Black Sea. However, after the 8.2 ka event and the collapse of the North American ice sheets, the flooding of fresh water into the Atlantic that stopped the conveyor also caused a rapid sea-level rise of around 1.4 m and large-scale flooding. After this date, the settlements rapidly spread northwards.

Maximise

Figure 23 Locations and dates of sites of Neolithic farming settlements across the Middle East and Europe. The coloured dots indicate new sites that were established during each time period; grey dots represent pre-existing sites established during earlier time periods. The final panel shows the full time period (13–5.5 ka BP). (Turney and Brown, 2007)

Show description|Hide description

This is a series of six Mercator maps of the Mediterranean region from North Africa in the south to southern Norway in the north, Britain, Ireland and Spain on the west to the Black Sea in the east. Each of the six maps are dated and have coloured dots representing the oldest dates that human settlements were established. In each successive panel the dots from the previous panels are shown in grey. Panel 1 is dated 13–11.5 ka BP and has fewer than ten purple coloured dots in the Levant region of the Mediterranean (modern day Israel, Jordan, Lebanon and Syria). Panel 2 is dated 11.5–10 ka BP and has approximately twice as many dots as the previous panel and coloured pink, in an expanded geographic range, but again only in the Levant. Panel 3 is dated 10–8.4 ka BP and shows more many yellow-coloured dots in the Levant but now there are approximately 10 dots on the east coast of Greece. The fourth panel is labelled 8.4–7 ka BP and shows an apparent sudden rapid expansion of human settlements. There are more than 100 green-coloured dots extending from the Levant all the way across mainland Europe, with one settlement in Britain. Panel 5 is labelled 7–5.5 ka BP and shows many more blue dots expanding across the whole of northern Europe as far as Britain and Ireland. The final panel is an amalgamation of the previous five panels giving the impression that human settlements have expanded from the Levant across Europe to Britain in the time period 13–5.5 ka BP.

Figure 23 Locations and dates of sites of Neolithic farming settlements across the Middle East and Europe. The coloured dots indicate new...

The exact driving factor for this human migration is impossible to determine, but it is interesting that it seemed to begin after the 8.2 ka climate event. This is an example of the kind of climate change that society will have to cope with.