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Climate reconstructions based on stable isotopes in tree rings rely on the assumption that fractionation controlling processes are strongly linked to meteorological variables. In this context we investigated the climate sensitivity of 350 years of carbon and oxygen isotope ratios of tree ring cellulose from European larch obtained at a high elevation site in the Swiss Alps (similar to 2100 m above sea level). Unlike tree ring width and maximum latewood density which contain only summer temperature information at this site we found that our stable isotope series reveal additionally to temperature a striking sensitivity to precipitation (mainly for carbon) and sunshine duration (mainly for oxygen) during July and August. A drought index reflecting the combined temperature and precipitation influence provided the most stable correlations over time for the carbon isotope series. All of these climate isotope relationships are preserved in the isotope series obtained from younger trees at the same site while strong intertree correlations further emphasize the high climate sensitivity. We thus present the first carbon isotope based summer drought reconstruction for the Swiss Alps which provides new evidence for interannual to long term changes in summer regional moisture variability from 1650 to 2004 in Europe revealing extreme drought summers in the second half of the 18th century and throughout the 20th century.

We present a 1200 year drought reconstruction for the European Alpine region based on carbon isotope variations of tree rings from living larch trees and historic timber. The carbon isotope fractionation at the study site is sensitive to summer precipitation temperature and irradiance resulting in a stable and high correlation with a droughtindex for inter annual to decadal frequencies and possibly beyond (r2=0.58 for 1901 2004 July/August). When combining this information with maximum latewood density derived summer temperature a strongly reduced occurrence of summer droughts during the warm AD 900 1200 period is evident coinciding with the Medieval Climate Anomaly (MCA) with a shift to colder and drier conditions for the subsequent centuries. The warm wet MCA contrasts strongly with the climate of the drought prone warm phase of the recent decades indicating different forcing mechanism for these two warm periods and pointing to beneficial conditions for agriculture and human well being during the MCA in this region.