Spring-summer temperatures reconstructed for northern Switzerland and southwestern Germany from winter rye harvest dates, 1454–1970

This paper presents a unique 517-yr long
documentary data-based reconstruction of spring-summer
(MAMJJ) temperatures for northern Switzerland and southwestern
Germany from 1454 to 1970. It is composed of 25
partial series of winter grain (secale cereale) harvest starting
dates (WGHD) that are partly based on harvest related bookkeeping
of institutions (hospitals, municipalities), partly on
(early) phenological observations. The resulting main Basel
WGHD series was homogenised with regard to dating style,
data type and altitude. The calibration and verification approach
was applied using the homogenous HISTALP temperature
series from 1774–1824 for calibration (r = 0.78) and
from 1920–1970 for verification (r = 0.75). The latter result
even suffers from the weak data base available for 1870–
1950. Temperature reconstructions based on WGHD are
more influenced by spring temperatures than those based on
grape harvest dates (GHD), because rye in contrast to vines
already begins to grow as soon as sunlight brings the plant
to above freezing. The earliest and latest harvest dates were
checked for consistency with narrative documentary weather
reports. Comparisons with other European documentarybased
GHD and WGHD temperature reconstructions generally
reveal significant correlations decreasing with the distance
from Switzerland. The new Basel WGHD series
shows better skills in representing highly climate change
sensitive variations of Swiss Alpine glaciers than available
GHD series.