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Winter air temperature variations in western Europe during the Early and High Middle Ages (AD 750–1300)

In this paper an attempt is made to reconstruct air temperature variations in winter (December, January and February) from 2500 documentary data over the period ad 750–1300 for a region comprising the Benelux countries, eastern France, western Germany, Switzerland and northern Italy. Anomalous (warm and cold) winters were classified on the basis of proxy information on frost, freezing of water bodies, duration of snowcover and untimely activity of vegetation using semiquantitative indices. For the most severe winters during the ‘Medieval Warm Period’ (MWP) as well as for the outstanding warm and dry winter ad 1289/90, possible analogue cases from the last 300 years are considered, analysed, synoptically interpreted and compared with each other. It is concluded that severe winters were somewhat less frequent and less extreme during the MWP, ad 900–1300, than in the ninth century and from 1300 to 1900. Mean air temperatures for 30 year. periods were estimated from linear regression models including indices and instrumental measurements. From ad 1090 to 1179 winter temperatures were at the level of the ‘Little Ice Age’ (LIA). From ad 1180 to 1299 they were at that of the twentieth century. The warm and stable winter climate in the thirteenth century supported subtropical plants such as olive trees in the Po valley (northern Italy) and fig trees around Cologne (Germany). The period ad 1300–1329 which marks the transition to the LIA was 1°C colder. It is concluded that the 1961–90 level of winter temperatures in western central Europe is still within the threshold of natural variability of the last thousand years, albeit at its upper boundary.