Western And Northwestern Europe

The impact of climate changes on lake levels in Europe during the Holocene was investigated using a water balance model (Harrison et al., 1993) in which the relation between the changes in runoff and evaporation could be simulated. Into this model, various constraints were introduced, including insolation anomalies deduced from orbital variations, temperature anomalies inferred from pollen analysis, and cloudiness anomalies deduced from changes in the position of the sub-tropical anticyclone. The simulations of the model show that precipitation was the main factor responsible for changing the water balance, thus affecting lake levels, but that evapo-transpiration did not play an important role in runoff. The data show a difference between northern and southern Europe with regard to the behavior of the lake levels at 9 kaBP. In northern Europe, the levels are low (southern Sweden and Estonia) while they are high in southern Europe. In order to explain this difference, Harrison et al. (1993) suggest a northward shift of the southern tropical anticyclone. This is similar to the

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Levant

History Paleo-cli mates

French Alps and Jura Mountains Magny (1992)

Swiss Alps Aflcr Zoller (1977), interpreted b> Magny (1992)

Swiss Plateau cold phases Modified from Haas ifii/.(l997)

Levels of Swiss Midland Lakes Modified from Jus (1982)

History Paleo-cli mates

Swiss Alps Aflcr Zoller (1977), interpreted b> Magny (1992)

Levels of Swiss Midland Lakes Modified from Jus (1982)

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Fig. 2.5. Paleo-hydrology of the Jura mountains and the Alps.

explanations of Kutzbach and Guetter (1986) and Hastenrath and Kutzbach (1983) with regard to variations in lake levels in east Africa during the Holocene. It is also in agreement with Issar's conclusion (1990) that the climate in the Levant during the PPN B was warm and humid.

Warm periods during the Lower Holocene coincided more or less with the Boreal and Atlantic stages of the classification by Blytt-Sernander (Blytt, 1876, Sernander, 1894), for the western European Holocene, which are:

Boreal period: warm and dry, 9.5 ka to 7 kaBP; Atlantic period: warm and wet, 7 ka to 5 ka BP.

A thermal optimum in Europe, from 8ka to 5kaBP, was also identified from the extension of plants and animals north of their present climatic limits, for example the water chestnut and the pond tortoise (Iversen, 1973; Roberts, 1989). On the basis of evidence from fossil beetles (Coleoptera), studied by Coope (1975), temperatures in Britain rose sharply after the cold Younger Dryas stadialat c. 10kaBP.

The interconnection between climate changes during the Late Quaternary andbiotic changes in the terrestrial and lacustrine environments of northwestern Europe was examined by Birks (1986). After the ice glacial conditions reached a peak at c. 18 ka BP, warmer and more humid conditions followed. At c. 14 ka BP, an abrupt amelioration took place. Lakes became abundant in the deglaciated regions from the melting of ice blocks. By c. 1.5 ka BP, the climate deteriorated, as summer and/or winter temperatures decreased; this was accompanied by decreased snow amounts and by droughts. From c. 13ka to 11 kaBP, a period of strong climate fluctuations occurred. From 11 ka to 10kaBP (Younger Dryas), temperatures decreased, the retreat of Scandinavian and British Isles ice was halted and the ice cover even regained ground. Woodlands became sparse or even disappeared and were replaced by heaths. The landscape became unstable with eolian activity, solifluction and redevelopment of ground-ice formation and extensive flow of minerals into the lakes. The Younger Dryas cold period ended abruptly, and from 10.3 ka to 10 ka BP warm conditions prevailed, bringing renovation of soil-producing processes. Temperate conditions allowed deciduous forest to prevail from 9ka to 4kaBP. From 5 kaBP onwards, the biotic environment, according to Birk's (1986) synthesis, was mainly a function of human interference.

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