Grape harvest dates as a proxy for Swiss April to August temperature reconstructions back to AD 1480

We present an annually resolved record of grape harvest dates for Switzerland. The strong negative relationship between grape harvest dates and April-August temperatures allowed a new reconstruction, with associated uncertainties, to be derived back to 1480. Calibration (1928-1979) was performed with monthly data from the Basel and Geneva stations and verified over 1980-2006. Twelve days of grape harvest difference correspond to around 1°C April-August temperature difference. Periods cooler (late grape harvest dates) than the 1961-1990 mean are reconstructed during the 17th century and at the beginning of the 19th century. Warmer conditions were experienced in the early 18th century and during the recent decades, in agreement with grape harvest temperature reconstructions from France and other independent temperature estimates. On decadal (annual) time-scales the earliest harvests were in the 1580s (2003) and the latest vintages in the 1740s (1816). Large tropical volcanic eruptions led to significantly later grape harvest dates (cooling) one to two years after the event.