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Bas van Geel

4 November 2012

Bas van Geel (University of Amsterdam) was a speaker at the 1997 SIS Conference at Fitzwilliam College in Cambridge, 'Natural Catastrophes during Bronze Age Civilisations', and together with Hans Renssen (University of Utrecht) subsequently wrote a paper on the subject of abrupt climate change around 2650BP in NW Europe – between 850 and 760BC (or 2750 and 2450 BP on the radio carbon time scale, which roughly includes tree ring anomalies noted by Mike Baillie). In archaeologically descriptive terminology this occurs at the transition between the Late Bronze and Iron Age in Britain, Ireland, and the Netherlands. In ecological terminology it is the transition from the sub-Boreal to the sub-Atlantic – basically it got very wet and cool after around 800BC and the cool climate system persisted until around 450BC. Abandonment of low lying areas at this time are interpreted as evidence of the effects of a rise in the water table and the growth of marsh, fed and bog. Bas van Geel ent on to correlate palaeoecological data with gelogical and geomorpholological data and in turn, with the archaeology, in order to point out a synchronous change of climate elsewhere in Europe and on other continents too. An oscillation in carbon 14 at around 2650BP  was probably caused by reduced solar activity, they say, which in turn may have been the forcing mechanism behind the continuing cooling phase. Reduced solar activity is thought to lead to a cosmic ray flux that involved higher C14 levels in the atmosphere – and we may note that it is in this general period the radio carbon calibration curve is thought to diverge from raw C14 data. In addition, a reduction in ozone levels, caused by reduced solar radiation, may have triggered climate change etc.

Now, the subject of lower solar activity has caused a rash of articles in various climate orientated journals, and this has been reflected in blog posts on sites such as Tall Bloke's Talk Shop. Are we heading towards a new cooling period? This argument is low key in some places and a trifle over egged in other places. Why would anyone wish for a repeat of the Maunder Minimum? Well, we will find out when it happens – or if it happens, over the next 20 years or so. Solar activity is also a by product of an opaque atmosphere, an atmosphere loaded with 'space dust' (volcanic dust appears to dissipate after a few years). Once again, we might find out. If less solar activity results in no big changes in global climate scientists may have to reconsider and look for an alternative explanation – which might included space dust, which has the ability to reduce temperatures reaching the surface of the Earth.

Basvan Geel and Hans Renssen also mention Polar Cells that they think would have repositioned the main depression tracks, or rainfall belts, at mid-latitude. This moved them southwards in the direction of the equator and that implies a weakening of the monsoon rain system – less rainfall in the semi tropical zone. Basically, it would have led to less rainfall in levels that relied on the monsoon rains, but more importantly it would have led to stress in those countries at the edge of the monsoon track, leading to famine and drought episodes. The most obvious country to be affected, at the margins, would be Egypt, as a result of reduced monsoon rains in Ethiopia and East Africa, therefore some correspondence with flood level records should be expected, between 800 and 450BC in this instance. However, rainfall levels over the Near and Middle East would also be affected and if they were they should be preserved in records of famine and drought in sources which would include the Biblical narrative during the Monarchy period, 9th century onwards. Drought and famine episodes across the Assyrian Empire, and the Babylonian and Persian empires too. In other words, if drought is recorded in the reign of a particular king it is likely that a corresponding drought might be recorded elsewhere, creating a chronological synchronism. This appears to be an under-used resource that historians, including revisionists, might dig into, as some sort of pattern might emerge.

At the same time we may note the article is referring to an episode with parallels to what Steve Mitchell has proposed in the Late Roman/Early Saxon period, both involving a sharp climate change, and archaeologically, both involving a movement of people away from river valleys and the establishment of settlements further up the slope.

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