As pointed out by Oldfield (this volume) climate change can have important impacts on human society. The two fundamental characteristics of a change are its magnitude and rate. Although the former is obvious, the latter is often not taken seriously enough. Although every change causes problems of adaptation it is much easier to deal with if it occurs slowly.
The climate shift around 850 cal. bc (Sub-boreal-Sub-atlantic transition), one of the most important climate shifts during the Holocene, with evidence for climatic tele-connections is such an example. It had strong socio-economic impacts in areas that were marginal from a hydrologic point of view. In lowland regions in The Netherlands, the climate shift caused a sudden, considerable rise of the groundwater table so that arable land was transformed into wetland, where peat growth started. Farming communities living in such areas were forced to migrate because they could no longer produce sufficient food (van Geel et al. 1996). In Figure 6.9, the landscape development in the northern Netherlands is shown for three successive phases. At the start of the second phase, coincident with an abrupt decline of solar activity, the atmospheric circulation changed, leading to cooler and wetter climate conditions. The rise of the water table forced the farmers to migrate to well-drained areas in the northern Netherlands where salt marshes offered them new fertile land. Magny (2004) showed that over a period of several millennia the presence of lakeside villages in south-eastern France and adjacent Switzerland was strongly linked with lake levels and solar activity. No lakeside villages occurred after 850 cal. bc.
In north-west and central Europe, the climatic shift around 850 cal. bc preceded a considerable rise in palynological indicators of human impact on the landscape. Van Geel and Berglund (2000) suggested a causal link between the climate shift around 850 bc and the evidence for a subsequent increase in human population density. They postulated that the climatic crisis in the first instance caused an environmental and social crisis. A collapse of societies resulted in a weakening of the position of dominating groups, which brought about a change in the social structure of farming communities. This facilitated the introduction of a new technological complex, which again created further social change combined with a leap forward in production, food consumption, and population density.
Figure 6.9 The effects of climate change during the period ca. 950-750 bc in the northern Netherlands. From phase A to phase B, solar activity abruptly declined (the Sub-boreal-Sub-atlantic transition). Atmospheric circulation changes occurred and the Westerlies became more intense. Precipitation in north-west Europe suddenly increased, while temperatures declined. As a consequence, a sudden rise in the water table occurred. Fenlands extended, often at the expense of arable land. Species composition in raised bogs changed as a consequence of cooler, wetter climatic
Figure 6.9 The effects of climate change during the period ca. 950-750 bc in the northern Netherlands. From phase A to phase B, solar activity abruptly declined (the Sub-boreal-Sub-atlantic transition). Atmospheric circulation changes occurred and the Westerlies became more intense. Precipitation in north-west Europe suddenly increased, while temperatures declined. As a consequence, a sudden rise in the water table occurred. Fenlands extended, often at the expense of arable land. Species composition in raised bogs changed as a consequence of cooler, wetter climatic conditions (compare Figure 6.3c). Farmers in marginal areas could no longer produce enough food and they had to migrate to well-drained areas. The thermal contraction of ocean water caused a temporary pause or slowing of the rise in sea level that had been occurring since the end of the last Ice Age. Salt marshes occurred for the first time along the coast ofthe northern Netherlands. These fertile areas were ideal for farmers who had lost their habitat as a consequence of rising groundwater tables. Artificial earth mounds were constructed for protection.
In south-central Siberia, archaeological evidence suggests an acceleration of cultural development and a sudden increase in density and geographic distribution of the nomadic Scythian population after 850 bc. Van Geel et al. (2004a) hypothesized a relationship with an abrupt climatic shift towards increased humidity (equatorward relocation of mid-latitude storm tracks). The hypothesis is supported by pollen-analytic evidence. Areas that initially may have been hostile semi-deserts changed into attractive steppe landscapes with a high biomass production and carrying capacity. Newly available steppe areas could be utilized by herbivores, making them attractive for nomadic tribes. The Central Asian horse-riding Scythian culture expanded, and an increased population density was a stimulus for westward migration towards south-eastern Europe.
There is strong evidence for climate change in the Central African rain forest belt around 850 bc (van Geel et al. 1998). Palynological studies point to a drastic change in the vegetation cover (from predominantly rain forest to a more open savannah landscape) as a consequence of an aridity crisis. The change of climate and vegetation was also important for prehistoric humans. A population of farmers migrated from the south into the area. The contrast between this change to dry-ness in central west Africa and the contemporary increase of precipitation in the temperate zones fits well with the hypothesis that, after a decline of solar activity, there was a decrease in the latitudinal extent of the Hadley Cell circulation and consequently the monsoon decreased in intensity, while the mid-latitude storm tracks in the temperate zones were enhanced and moved in the direction of the Equator (van Geel and Renssen 1998). A dryness crisis caused by a weak monsoon intensity in north-west India after 850 bc also supports this hypothesis (van Geel et al. 2004b). Furthermore, massive glacier advance in the south-central Andes of Chile, probably resulting from an equatorward relocation of mid-latitude storm tracks (like in the Northern Hemisphere), forms part of a wealth of evidence for worldwide climate change around 850 bc (van Geel et al. 2000).
Evidence from paleodata indicates that the climate shift around 850 bc occurred suddenly, probably within a decade. The end of the event, however, seems to have been gradual (a time-transgressive passing of thresholds), so that, given present knowledge, it is not yet possible to pinpoint an end of the event. Changing climatic conditions at 850 bc may have been similar to climatic cooling shifts during the Little Ice Age (Mauquoy et al. 2002).
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